Patent Publication Number: US-2012044749-A1

Title: Variable resistance nonvolatile storage device and method of forming memory cell

Description:
TECHNICAL FIELD 
     The present invention relates to a variable resistance nonvolatile storage device having memory cells each of which includes a transistor and a variable resistance element whose resistance value reversibly changes based on electrical signals, and a method of forming such memory cells. 
     BACKGROUND ART 
     In recent years, research and development of nonvolatile storage devices having memory cells formed with variable resistance elements has moved forward. A variable resistance element is an element with a property that its resistance value reversibly changes based on electrical signals, and can store data corresponding to the resistance value in a nonvolatile manner. 
     Commonly known as a nonvolatile storage device that includes variable resistance elements is a nonvolatile storage device which includes memory cells known as 1T1R memory cells arranged in a matrix at positions at which bit lines intersect with word lines and source lines that are arranged orthogonally to the bit lines. Here, the 1T1R memory cells refer to memory cells each of which is formed by connecting in series a MOS (metal oxide semiconductor) transistor and a variable resistance element. 
     Patent Literature 1 discloses a nonvolatile storage device including 1T1R memory cells in which oxides having a perovskite-type crystalline structure are used as variable resistance elements. 
       FIG. 38  is a schematic cross-sectional view of a memory cell described in Patent Literature 1. 
     A memory cell  1011  is formed by electrically connecting a transistor  1006  and a variable resistance element  1010  in series. 
     The transistor  1006  includes, on a semiconductor substrate  1001 , a source region  1002  serving as a first diffusion layer region, a drain region  1003  serving as a second diffusion layer region, and a gate electrode  1005  formed on a gate oxide film  1004 . 
     The variable resistance element  1010  is formed by having a variable resistance layer  1008  between a lower electrode  1007  and an upper electrode  1009 . Here, the variable resistance layer  1008  is a layer whose resistance value changes upon voltage application. 
     The drain region  1003  and the lower electrode  1007  are electrically connected. 
     The upper electrode  1009  is connected to a metal line serving as a bit line  1012 , the gate electrode  1005  is connected to a word line, and the source region  1002  is connected to a metal line serving as a source line  1013 . 
     Here, although Patent Literature 1 discloses Pr 1-x Ca x MnO 3  (PCMO), La 1-x Ca x MnO 3  (LCMO) and so on as the materials of the variable resistance layer  1008 , there is no particular mention of electrode materials. 
     Furthermore, as to a method for writing into the memory cell  1011 , PATENT LITERATURE 1 discloses that: application of a pulse voltage Vpp, a pulse voltage Vss, and a pulse voltage having a predetermined voltage amplitude Vwp to the upper electrode  1009 , the source region  1002 , and the gate electrode, respectively, changes a state of the memory cell  1011  from a low resistance state to a high resistance state; and inversely, application of the pulse voltage Vss, the pulse voltage Vpp, and the pulse voltage having a predetermined voltage amplitude Vwe to the upper electrode  1009 , the source region  1002 , and the gate electrode, respectively, changes the state from the high resistance state to the low resistance state. 
     Patent Literature 2 discloses a nonvolatile storage device which includes 1T1R memory cells having variable resistance elements that exhibit changes in the resistance based on a principle that is different from that of the above-mentioned variable resistance elements that exhibit changes in the resistance according to electrical signals. This storage device is called a phase-change memory. 
     The phase-change memory stores data by making use of the fact that a phase-change material called a chalcogenide material has different resistance between the crystalline state and the amorphous state. Data is rewritten by changing the state of the phase-change material through passage of a current therein so as to generate heat near its melting point. A resistance change to high resistance state (amorphization) called a reset operation is performed by control of maintaining the phase-change material at a relatively high temperature, and a resistance change to low resistance state (crystallization) called a set operation is performed by control of maintaining the phase-change material at a relatively low temperature for a sufficient period. 
     Moreover, Patent Literature 2 discloses that with the phase-change memory, a current necessary for data rewriting is different between the reset operation and the set operation, and the reset operation requires a relatively large current. 
       FIG. 39  is a cross-sectional view of the phase-change memory disclosed in Patent Literature 2. 
     A memory cell  1021  has a 1T1R structure in which a storage unit  1022  and an NMOS transistor  1027  are used. The NMOS transistor  1027  includes N-type diffusion layer regions  1029  and  1030  respectively corresponding to a source and a drain, and a gate electrode  1031  located between the N-type diffusion layer regions  1029  and  1030 . 
     The storage unit  1022  includes a phase change element  1024 , a second metal line layer  1023  above the phase change element  1024 , and a contact via  1025  and a first metal line layer  1026  below the phase change element  1024 , and is connected to the N-type diffusion layer region  1029  of the NMOS transistor  1027 . 
     The other N-type diffusion layer region  1030  of the NMOS transistor  1027  is connected to a third metal line layer  1028  via each of line layers. 
     Here, the second metal line layer  1023 , the third metal line layer  1028 , and the gate electrode  1031  of the NMOS transistor  1027  correspond to a source line, a bit line, and a word line, respectively. 
     Patent Literature 2 discloses adopting into a phase-change memory device a mechanism for controlling the source line and switching the current passage direction in a set operation and a reset operation. 
     In the reset operation requiring passage of a relatively large current, the source line is set to a predetermined high level and the bit line to a low level, and in the set operation requiring only a relatively small current, the bit line is set to a predetermined high level and the source line to a low level. 
     With such settings, the current direction in the reset operation is a direction in which the electric potential at the source of the NMOS transistor  1027  of the memory cell (in this case, corresponding to the electric potential at the N-type diffusion layer region  1030 ) is maintained at a low level almost equal to the electric potential at the semiconductor substrate. This reduces the influence of a so-called substrate bias effect in the MOS transistor, and thus the reset operation is performed with a driving capability of the transistor being high (a large current can be obtained). 
     On the other hand, the current direction in the set operation is a direction in which the electric potential at the source of the NMOS transistor  1027  of the memory cell (in this case, corresponding to the electric potential at the N-type diffusion layer region  1029 ) rises to a voltage value that is determined according to a divided voltage relationship between an on-resistance value of the NMOS transistor  1027  and a resistance value of the phase-change element  1024 . This increases the influence of the substrate bias effect in the MOS transistor, and thus the set operation is performed with the current flowing in the transistor kept relatively small. 
     The above structure makes it easier to separately supply a current of different magnitudes suitable for each of the set operation and the reset operation, thereby allowing the outcome of each operation to be stably obtained. 
     Generally, to form a high density memory cell array, memory cells need to be formed in the smallest possible areas, and thus it is important to form a transistor, as well as a variable resistance element which is another structural element of each memory cell, in the smallest possible areas. 
     An effective way of forming a transistor in the smallest possible area is to make a gate length L of the transistor as short as possible and a gate width W of the transistor as small as possible in an efficient manner. 
     The following considers application of the above structure to the nonvolatile storage device disclosed in PATENT LITERATURE 1. 
     According to PATENT LITERATURE 1, the nonvolatile storage device shown in  FIG. 38  changes the state of the memory cell  1011  from a low resistance state to a high resistance state (resistance change to high resistance state) by applying a positive voltage to the upper electrode  1009  with reference to the lower electrode  1007 , that is, by setting the bit line  1012  to Vpp and the source line  1013  to 0 V. 
     Here, the electric potential at the source region  1002  serving as the first diffusion layer region of the transistor  1006  (in this case, the source region  1002  functions as the source of the transistor  1006 ) is almost equal to the electric potential at the semiconductor substrate  1001 , that is, 0 V, and the substrate bias effect in the transistor  1006  small. 
     On the other hand, the nonvolatile storage device changes the state of the memory cell  1011  from the high resistance state to the low resistance state (resistance change to low resistance state) by setting the bit line  1012  to 0 V and the source line  1013  to Vpp. 
     Here, the electric potential at the drain region  1003  that is the second diffusion layer region (in this case, the drain region  1003  functions as the source of the transistor  1006 ) rises to a voltage determined according to a divided voltage relationship between the resistance value of the variable resistance element  1010  and the on-resistance of the transistor  1006 , and the substrate bias effect in the transistor  1006  becomes greater than in the case of the resistance change to high resistance state. 
     As described above, it is rational, in terms of forming the transistor of the memory cell in an optimum size, to perform the resistance change to high resistance state, which requires a current larger than for the resistance change to low resistance state, by using a current flowing in the direction in which the substrate bias effect in the transistor is smaller, because that way the transistor does not need to have a driving capability with an unnecessary margin. 
     It is to be noted that the semiconductor device disclosed in Patent Literature 2 also adopts the same concept that the reset operation requiring a larger current is performed using a current flowing in a direction in which the substrate bias effect in the transistor is smaller. 
     CITATION LIST 
     Patent Literature 
     [PTL 1] 
     
         
         [Patent Literature 1] Japanese Unexamined Patent Application Publication No. 2005-25914 (FIG. 2) 
       
    
     [PTL 2] 
     
         
         [Patent Literature 2] Japanese Unexamined Patent Application Publication No. 2005-267837 (FIGS. 7 and 8) 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     The inventors of the present application have examined, as one of variable resistance nonvolatile storage devices, a variable resistance nonvolatile storage device which includes 1T1R memory cells each having a variable resistance layer comprising an oxygen-deficient transition metal oxide. 
     Here, the oxygen-deficient oxide is an oxide whose composition is deficient in oxygen compared to its stoichiometric composition. Taking tantalum, one of transition metals, as an example, Ta 2 O 5  is an oxide having a stoichiometric composition. Ta 2 O 5  contains oxygen 2.5 times more than tantalum, and has an oxygen content percentage of 71.4%. An oxide having an oxygen content percentage lower than 71.4%, that is, a tantalum oxide having a non-stoichiometric composition which satisfies 0&lt;x&lt;2.5 when expressed as TaO x , is called an oxygen-deficient tantalum oxide. 
     To facilitate description of problems to be solved, the following describes some properties which have been measured of a variable resistance element having an oxygen-deficient tantalum oxide as a variable resistance layer. 
       FIG. 1  is a schematic diagram showing a basic structure of a variable resistance element used for the measurement. The variable resistance element includes a variable resistance layer  3302  comprising an oxygen-deficient tantalum oxide, and the variable resistance layer  3302  is located between a lower electrode  3301  comprising Pt and an upper electrode  3303  comprising Pt, so as to create a top-bottom symmetry. 
       FIG. 2  is a graph illustrating hysteretic current-voltage characteristics which show an example of how the resistance of the variable resistance element changes. The horizontal axis represents the voltage applied to the upper electrode  3303  with reference to the lower electrode  3301 , and the vertical axis represents the value of a current flowing in the variable resistance element. 
     As  FIG. 2  shows, gradually applying a positive voltage to the upper electrode  3303  with reference to the lower electrode  3301  increases the current almost in proportion to the voltage, and the current rapidly decreases when the positive voltage exceeds the voltage at the point A. That is, it shows a change from a low resistance state to a high resistance state (resistance change to high resistance state). 
     On the other hand, in gradual application of a negative voltage to the upper electrode  3303  with reference to the lower electrode  3301  (equivalent to application of a positive voltage to the lower electrode  3301  with reference to the upper electrode  3303 ) in the high resistance state, the current rapidly increases when the negative voltage exceeds the voltage at the point B. That is, it shows a change from the high resistance state to a low resistance state (resistance change to low resistance state). 
     The variable resistance element exhibiting the characteristics shown in  FIG. 2  and the variable resistance element disclosed in Patent Literature 1 differ in material of the variable resistance layer. However, they are the same in that they both: perform a bipolar operation, that is, switch between the high resistance state and the low resistance state upon bidirectional voltage application; perform the resistance change to high resistance state upon application of a positive voltage to the upper electrode with reference to the lower electrode; and perform the resistance change to low resistance state upon application of a negative voltage to the upper electrode with reference to the lower electrode. 
     Further, the characteristics shown in  FIG. 2  indicate that the resistance change to high resistance state does not occur before the point A and the resistance change to low resistance state does not occur before the point B. These characteristics show that the resistance change to high resistance state of the variable resistance element according to the present invention requires a current larger than that required for the resistance change to low resistance state. 
     As described above, it is rational, in terms of forming the transistor of the memory cell in an optimum size, to perform the resistance change to high resistance state, which requires a larger current, by applying a voltage of a polarity that reduces the substrate bias effect in the transistor, and to perform the resistance change to low resistance state, which requires a smaller current, by applying a voltage of the reversed polarity. 
     However, the inventors of the present application have discovered during the examination that the voltage application direction (driving polarity) stably causing a resistance change in one direction (resistance change to low resistance state or resistance change to high resistance state) is not always the same, and that the driving polarity is different in some of the variable resistance elements which are made of the same materials, that is, Pt for the upper and lower electrodes and an oxygen-deficient tantalum oxide for the variable resistance layer. 
     For instance, it has been verified that a variable resistance element undergoes the resistance change to low resistance state upon application of a pulse voltage of +2.0 V in amplitude and 100 ns in width between the upper electrode  3303  and the lower electrode  3301 , and the resistance change to high resistance state upon application of a pulse voltage of −2.6 V in amplitude and 100 ns in width between the upper electrode  3303  and the lower electrode  3301 , given that a voltage applied between the upper electrode  3303  and the lower electrode  3301  is positive when the voltage at the higher electrode  3304  is higher than the voltage at the lower electrode  3301 . 
     It has also been verified that another variable resistance element undergoes the resistance change to low resistance state upon application of a pulse voltage of −2.0 V in amplitude and 100 ns in width between the upper electrode  3303  and the lower electrode  3301 , and the resistance change to high resistance state upon application of a pulse voltage of +2.7 V in amplitude and 100 ns in width between the upper electrode  3303  and the lower electrode  3301 , given that a voltage applied between the upper electrode  3303  and the lower electrode  3304  is positive when the voltage at the upper electrode  3303  is higher than the voltage at the lower electrode  3301 . 
       FIGS. 3  ( a ) and ( b ) are graphs each showing the resistance value of such a variable resistance element in the case of continuously applying a pulse voltage which causes the resistance change to low resistance state and a pulse voltage which causes the resistance change to high resistance state alternately. The horizontal axis represents the number of electrical pulses, and the vertical axis represents the resistance value. 
     As shown in  FIG. 3  ( a ), a variable resistance element which is initially in a high resistance state of approximately 33 kΩ changes into a low resistance of approximately 500Ω upon application of a pulse voltage of +2.0 V, and then changes into a high resistance state of approximately 40 kΩ upon application of a pulse voltage of −2.6 V. After that, the variable resistance element repeats the resistance change to low resistance state upon application of a positive pulse voltage to the upper electrode  3303  with reference to the lower electrode  3301  and the resistance change to high resistance state upon application of a negative pulse voltage to the upper electrode  3303  with reference to the lower electrode  3301 . 
     This relationship between the direction of the resistance change and the polarity of the applied voltage is called A mode for descriptive purposes. 
     As shown in  FIG. 3  ( b ), another variable resistance element which is initially in a high resistance state of approximately 42 kΩ changes into a low resistance state of approximately 600Ω upon application of a pulse voltage of −2.0 V, and then changes into a high resistance state of approximately 40 kΩ upon application of a pulse voltage of +2.7 V. After that, the variable resistance element repeats the resistance change to low resistance state upon application of a negative pulse voltage to the upper electrode  3303  with reference to the lower electrode  3301  and the resistance change to high resistance state upon application of a positive pulse voltage to the upper electrode  3303  with reference to the lower electrode  3301 . 
     This relationship between the direction of the resistance change and the polarity of the applied voltage is called B mode for descriptive purposes. The hysteretic current-voltage characteristics shown in  FIG. 2  correspond to this B mode. 
     It is to be noted that the above pulse voltage values denote output voltage values set for a pulse generator, and that an effective voltage value applied between the end terminals of the variable resistance element is considered to be a voltage value smaller than a corresponding one of the pulse voltage values due to a voltage drop across a measurement system. 
     In the variable resistance element showing such a result, the upper electrode  3303  and the lower electrode  3301  both comprise Pt, and the variable resistance layer  3302  which comprises the oxygen-deficient tantalum oxide and is located between the upper and lower electrodes has a top-bottom symmetrical relationship with the electrodes electrically. 
     Accordingly, it is not always self-evident as to which of the A mode and the B mode appears as the resistance change characteristics, and the mode is determined based on empirical rules or experimental measurement results. It is expected that these phenomena are determined by some sort of anisotropy factors unexplained in the resistance change mechanism. 
     In the case where the A mode and the B mode appear indeterminately, the following problems are expected in forming a 1T1R memory device. 
     The first problem is that the size of the transistor cannot be optimized. 
     If the resistance change characteristics can be determined as either the A mode or the B mode, the transistor can be formed in a minimum size that allows driving of a current necessary for the resistance change to high resistance state assuming that the transistor operates under a condition that causes a small substrate bias effect according to a conventionally-known concept. 
     However, if the mode is indeterminate, it is necessary to form the transistor in a size sufficient for driving the current necessary for the resistance change to high resistance state, considering also that the transistor operates under a condition that causes a large substrate bias effect. For this reason, unlike in the case where the mode is determined, it is necessary to make the gate width W of the transistor wide. This is not desirable as the wide gate width is a significant impediment to miniaturization of memory cells. 
     The second problem is that it is necessary to manage information identifying the mode of the resistance change characteristics. 
     When the mode is indeterminate, the correspondence is indeterminate between the polarity of a voltage to be applied for changing a resistance state, and a resistance state (high resistance state or low resistance state) to be read after the voltage application. This creates a need of information which identifies the mode in order to actually use the variable resistance element as a storage element. 
     For example, in the case where the same mode appears in a lot or a slice, a storage element dedicated for management is provided in a chip. Then, in the manufacturing stage, identification information indicating in which one of the A mode and the B mode the variable resistance element undergoes a resistance change is recorded in the storage element dedicated for management. Further, in the use stage, according to the identification information, the polarity of the applied voltage is reversed in a write operation or the polarity of output data is reversed in a read operation. 
     It appears that this allows the actual use of the variable resistance element as the storage element. However, it is not desirable because this complicates the circuit configuration and the control method. Further, in the case where different modes appear across smaller units in a lot or a slice, such as memory cells, it is practically impossible to provide, for each memory cell, a storage element dedicated for management and record the identification information of the mode in the storage element. 
     The present invention has been conceived in consideration of the above problems, and it is an object of the present invention to provide a technique that enables a 1T1R nonvolatile storage device including a variable resistance element to control the appearance of the A mode and the B mode of the resistance change characteristics of the variable resistance element and that enables designing of a memory cell with an optimum transistor size. 
     Solution to Problem 
     In order to solve the above problems, the variable resistance nonvolatile storage device according to an aspect of the present invention is a variable resistance nonvolatile storage device including: a semiconductor substrate; a variable resistance element including: a first electrode; a second electrode; and a variable resistance layer which is provided between the first electrode and the second electrode and in contact with the first electrode and the second electrode and which has a resistance value that reversibly varies based on each of voltage signals which have different polarities and are applied between the first electrode and the second electrode; and a MOS transistor formed on a main surface of the semiconductor substrate, wherein the first electrode comprises one of tantalum nitride and tungsten, the second electrode comprises at least one metal selected from platinum, iridium, and palladium, the variable resistance layer includes: a first region which is in contact with the first electrode and contains a first oxygen-deficient transition metal oxide having a composition MOx; and a second region which is in contact with the second electrode and contains a second oxygen-deficient transition metal oxide having a composition MOy (where x&lt;y), and a drain of the MOS transistor is connected with either the first electrode or the second electrode included in the variable resistance element to form a memory cell so that a substrate bias effect in the MOS transistor is smaller when one of the voltage signals having a polarity for changing the variable resistance layer into a high resistance state is applied to the MOS transistor and the variable resistance element, than when an other one of the voltage signals having a polarity for changing the variable resistance layer into a low resistance state is applied to the MOS transistor and the variable resistance element. 
     Further, the second electrode may comprise a material having a standard electrode potential which is higher than a standard electrode potential of a transition metal of the first and second oxygen-deficient transition metal oxides, and the first electrode may comprise a material having a standard electrode potential which is lower than the standard electrode potential of the second electrode. 
     Furthermore, the MOS transistor may be an N-type MOS transistor including, on the main surface of the semiconductor substrate: a first N-type diffusion layer region; a gate; and a second N-type diffusion layer region formed on a side of the gate opposite the first N-type diffusion layer region, and the first electrode may be connected with the first N-type diffusion layer region of the N-type MOS transistor to form the memory cell. 
     Moreover, the MOS transistor may be a P-type MOS transistor including: an N well formed on the main surface of the semiconductor substrate; a first P-type diffusion layer region; a gate; a second P-type diffusion layer region formed on a side of the gate opposite the first P-type diffusion layer region, the first P-type diffusion layer region, the gate, and the second P-type diffusion layer region being formed in the N well, and the second electrode may be connected with the first P-type diffusion layer region of the P-type MOS transistor to form the memory cell. 
     Advantageous Effects of Invention 
     According to the nonvolatile storage device according to an aspect of the present invention, a variable resistance nonvolatile storage element is used in which: a first region containing an oxygen-deficient transition metal oxide which does not easily exhibit a resistance change due to a lower oxygen content percentage is provided in contact with a first electrode; and a second region containing an oxygen-deficient transition metal oxide which easily exhibits a resistance change due to a higher oxygen content percentage is provided in contact with a second electrode. Thus, the voltage application direction (driving polarity) which causes resistance changes can be uniquely determined for each of memory cells such that: the resistance change to high resistance state is performed upon application of a positive voltage to the second electrode with reference to the first electrode; and the resistance change to low resistance state is performed upon application of a positive voltage to the first electrode with reference to the second electrode. 
     Generally, the resistance change to high resistance state of the variable resistance element requires a larger driving current compared to the resistance change to low resistance state, because a voltage needs to be generated to cause a resistance change in the variable resistance element in a low resistance state. 
     Thus, in the case of forming each memory cell with the variable resistance element and an N-type MOS transistor, the first electrode of the variable resistance element is connected with a first N-type diffusion layer region of the N-type MOS transistor. With this connection, when the variable resistance element is to perform the resistance change to high resistance state, a second N-type diffusion layer region of the N-type MOS transistor is grounded and thus it is possible to supply the driving current to the variable resistance element using a ground bias which causes a smaller substrate bias effect in the N-type MOS transistor. 
     Moreover, when forming each memory cell with the variable resistance element and a P-type MOS transistor, the second electrode of the variable resistance element is connected with a first P-type diffusion layer region of the P-type MOS transistor. With this connection, when the variable resistance element is to perform the resistance change to high resistance state, a second N-type diffusion layer region of the P-type MOS transistor is connected to a power source and thus it is possible to supply the driving current to the variable resistance element using a power bias which causes a smaller substrate bias effect in the P-type MOS transistor. 
     As a result, it is no longer necessary to form the transistor in a size sufficient for driving the current necessary for the resistance change to high resistance state considering that the transistor operates under a condition that causes a large substrate bias effect. As a result, it is possible to design the memory cells with an optimum transistor size. 
     In other words, it is possible to provide, in a smaller layout area, a variable resistance nonvolatile storage device in which 1T1R memory cells are used, and to achieve increase in the integration degree and cost reduction. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram showing, as basic data of the present invention, a basic structure of a nonvolatile storage element. 
         FIG. 2  is a graph showing, as basic data of the present invention, an example of hysteretic current-voltage characteristics observed in resistance changes of a nonvolatile storage element. 
         FIGS. 3  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, an example of a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 4  is a cross-sectional view showing, as basic data of the present invention, a structure of a nonvolatile storage element. 
         FIGS. 5  ( a ) to ( c ) are explanatory diagrams each showing, as basic data of the present invention, a manufacturing process of a nonvolatile storage element. 
         FIGS. 6  ( a ) to ( c ) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 7  is a graph showing, as basic data of the present invention, an X-ray diffraction spectrum of a nonvolatile storage element. 
         FIGS. 8  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a result of measuring the X-ray reflectivity of a nonvolatile storage element. 
         FIGS. 9  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 10  is a graph showing, as basic data of the present invention, an example of hysteretic current-voltage characteristics observed in resistance changes of a nonvolatile storage element. 
         FIGS. 11  ( a ) and ( b ) are diagrams showing, as basic data of the present invention, a result of cross-section observation of a nonvolatile storage element. 
         FIG. 12  is a graph showing, as basic data of the present invention, a result of analyzing a composition of a tantalum oxide layer included in a nonvolatile storage element. 
         FIGS. 13  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 14  is a cross-sectional view showing, as basic data of the present invention, a structure of a nonvolatile storage element. 
         FIG. 15  ( a ) to ( c ) are explanatory diagrams each showing, as basic data of the present invention, a manufacturing process of a nonvolatile storage element. 
         FIG. 16  is a graph showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 17  is a graph showing, as basic data of the present invention, an example of hysteretic current-voltage characteristics observed in resistance changes of a nonvolatile storage element. 
         FIG. 18  is a structural diagram of a variable resistance nonvolatile storage device according to an embodiment of the present invention. 
         FIG. 19  is a cross-sectional view showing a structural example of memory cells of a variable resistance nonvolatile storage device according to an embodiment of the present invention. 
         FIG. 20  ( a ) to ( c ) are operational timing charts of a variable resistance nonvolatile storage device according to an embodiment of the present invention. 
         FIG. 21  is a diagram showing simulations of memory cell characteristics of a variable resistance nonvolatile storage device according to an embodiment of the present invention. 
         FIG. 22  ( a ) to ( f ) are circuit diagrams each showing a circuit configuration of a memory cell according to an embodiment of the present invention. 
         FIG. 23  ( a ) to ( f ) are diagrams each showing a connection between a variable resistance element and a transistor for realizing a memory cell according to an embodiment of the present invention. 
         FIG. 24  is a cross-sectional view showing a structural example of memory cells of a variable resistance nonvolatile storage device according to an embodiment of the present invention. 
         FIG. 25  is a cross-sectional view showing, as basic data of the present invention, a structure of a nonvolatile storage element. 
         FIG. 26  is a graph showing, as basic data of the present invention, a result of analyzing compositions of hafnium oxide layers of a nonvolatile storage element. 
         FIGS. 27  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIGS. 28  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a result of measuring the X-ray reflectivity of a nonvolatile storage element. 
         FIGS. 29  ( a ) and ( b ) are cross-sectional views each showing, as basic data of the present invention, a structure of a nonvolatile storage element. 
         FIG. 30  ( a ) to (c) are explanatory diagrams, as basic data of the present invention, of a structure and a manufacturing process of a nonvolatile storage element. 
         FIGS. 31  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a result of measuring the X-ray reflectivity of a nonvolatile storage element. 
         FIG. 32  is a graph showing, as basic data of the present invention, a result of measuring the X-ray reflectivity of a nonvolatile storage element. 
         FIGS. 33  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 34  is a cross-sectional view showing, as basic data of the present invention, a structure of a nonvolatile storage element. 
         FIGS. 35  ( a ) and ( b ) are graphs each showing, as basic data of the present invention, an example of hysteretic resistance-voltage characteristics of a nonvolatile storage element. 
         FIG. 36  ( a ) to (d) are graphs each showing, as basic data of the present invention, a relationship between the resistance value of a nonvolatile storage element and the number of electrical pulses applied. 
         FIG. 37  is a graph showing, as basic data of the present invention, an example of distribution of resistance values of a nonvolatile storage element that are changed with application of electrical pulses. 
         FIG. 38  is a schematic cross-sectional view of a memory cell included in a conventional variable resistance nonvolatile storage device. 
         FIG. 39  is a cross-sectional view of a semiconductor device in which a conventional phase-change memory is used. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The following describes in detail an embodiment of the present invention with reference to the drawings. 
     The variable resistance nonvolatile storage device according to an embodiment of the present invention is a 1T1R nonvolatile storage device in which a variable resistance element and a MOS transistor are connected in series, and which fixes the mode of the resistance change characteristics of the variable resistance element and optimizes the structure of the MOS transistor according to the mode fixed. 
     (Basic Data of Present Invention) 
     The following describes, as preparation, basic data regarding three types of variable resistance elements used for the variable resistance nonvolatile storage device according to the present invention. 
     These variable resistance elements are formed by providing, between two electrodes, a variable resistance layer comprising one of the following oxides: an oxygen-deficient tantalum oxide; an oxygen-deficient hafnium oxide; and an oxygen-deficient zirconium oxide. 
     These variable resistance elements were invented by the inventors of the present application for the purpose of obtaining a nonvolatile storage element which has reversible and stable rewriting characteristics and which makes use of a resistance change phenomenon. Such variable resistance elements are described in detail in the following related patent applications: Japanese Patent Application No. 2007-149032 (Patent Literature); Japanese Patent Application No. 2008-180946 (Patent Literature); and Japanese Patent Application No. 2008-180944 (Patent Literature). 
     A feature of these variable resistance elements that the resistance change characteristics can be fixed to either the aforementioned A mode or B mode is applied to the variable resistance nonvolatile storage device according to the present invention. Hereinafter, the above-mentioned related patent applications are partially cited for purposes of illustration. 
     It is to be noted that the term “variable resistance element” and the term “variable resistance nonvolatile storage element” (or briefly, “nonvolatile storage element”) are used synonymously in the present DESCRIPTION. 
     (Variable Resistance Element Using Oxygen-Deficient Tantalum Oxide as Variable Resistance Layer) 
     First, the following describes the first experiment regarding a variable resistance nonvolatile storage element which uses an oxygen-deficient tantalum oxide as the variable resistance layer and which performs a bipolar operation. 
     (Structure of Variable Resistance Element) 
       FIG. 4  is a cross-sectional view showing a structural example of a variable resistance element according to the first experiment. 
     As shown in  FIG. 4 , a variable resistance element  100  used in the first experiment includes: a substrate  101 ; an oxide layer  102  formed on the substrate  101 ; a lower electrode  103  formed on the oxide layer  102 ; an upper electrode  105 ; and a variable resistance layer  104  located between the lower electrode  103  and the upper electrode  105 . 
     Here, the variable resistance layer  104  includes: a first tantalum containing layer (hereinafter referred to as “first tantalum oxide layer”)  104   a  having a lower oxygen content percentage; and a second tantalum containing layer  104   b  (hereinafter referred to as “second tantalum oxide layer”) formed on the first tantalum oxide layer  104   a  and having a higher oxygen content percentage. 
     To drive the variable resistance element  100 , a voltage satisfying a predetermined condition is applied between the lower electrode  103  and the upper electrode  105  by an external power source. According to the voltage application direction, the resistance value of the variable resistance layer  104  of the variable resistance element  100  increases or decreases reversibly. For example, when a pulse voltage larger than a predetermined threshold voltage is applied, the resistance value of the variable resistance layer  104  increases or decreases, whereas when a pulse voltage smaller than the predetermined threshold voltage is applied, there is no change in the resistance value of the variable resistance layer  104 . 
     The materials of the lower electrode  103  and the upper electrode  105  are, for example, Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), and Cu (copper). 
     Although the substrate  101  may be a single-crystal silicon substrate or a semiconductor substrate, it is not limited to these substrates. The variable resistance layer  104  can be formed at a relatively low substrate temperature and thus can be formed on a resin material and the like. 
     (Manufacturing Method of Variable Resistance Element). 
     Next, a manufacturing method of the variable resistance element  100  used in the present experiment is described with reference to  FIG. 5  ( a ) to (c). 
     First, as shown in  FIG. 5  ( a ), thermal oxidation produces the oxide layer  102  having a thickness of 200 nm on the substrate  101  which is a single-crystal silicon. Then, by sputtering, a Pt thin film having a thickness of 100 nm is formed on the oxide layer  102  as the lower electrode  103 . After that, the first tantalum oxide layer  104   a  is formed on the lower electrode  103  by reactive sputtering, with tantalum used as the target. 
     Next, as shown in  FIG. 5  ( b ), the top surface of the first tantalum oxide layer  104   a  is oxidized to modify the surface property. This produces, on the surface of the first tantalum oxide layer  104   a , the second tantalum oxide layer  104   b  having an oxygen content percentage higher than that of the first tantalum oxide layer  104   a . The layered structure of the first tantalum oxide layer  104   a  and the second tantalum oxide layer  104   b  is the structure of the variable resistance layer  104 . 
     Subsequently, by sputtering, a Pt thin film having a thickness of 150 nm is formed on the second tantalum oxide layer  104   b  as the upper electrode  105 . 
     Lastly, photolithography processing creates a pattern  106  using a photoresist, and dry etching produces an element region  107  as shown in  FIG. 5  ( c ). 
     With the above-described manufacturing method, elements A to C were manufactured. The following describes the details. 
     First, the layered structure of the substrate  101 , the oxide layer  102 , and the lower electrode  103  comprising Pt was formed in the above-described manner. After that, the first tantalum oxide layer  104   a  was formed on the lower electrode  103  by reactive sputtering, that is, a process of sputtering a tantalum target in an argon gas and an oxygen gas. 
     The film forming condition here was as follows: vacuum (back pressure) in the sputtering apparatus was approximately 7×10 −4  Pa before sputtering; power was 250 W when sputtering; the total gas pressure of the argon gas and the oxygen gas was 3.3 Pa; the flow ratio of the oxygen gas was 3.4%; the preset temperature of the substrate was 30° C.; and the time period for film formation was seven minutes. With this, the first tantalum oxide layer  104   a  having an oxygen content percentage of about 58 at %, i.e., TaO 1.4 , was deposited with a thickness of 30 nm. 
     The manufacturing of the elements A to C involved the formation of the first tantalum oxide layer  104   a  and the second tantalum oxide layer  104   b , followed by the formation of the upper electrode  105 , both performed in the sputtering apparatus. More specifically, after depositing the first tantalum oxide layer  104   a , a shutter was inserted between the tantalum target and the oppositely placed substrate  101  under the same sputtering condition, including the condition of the gas pressure and the power, for example, and it was kept in this state for a predetermined time period. 
     This caused oxidation of the top surface of the first tantalum oxide layer  104   a  due to oxygen plasma. As a result, the second tantalum oxide layer  104   b  having an oxygen content percentage higher than that of the first tantalum oxide layer  104   a  was formed on the surface of the first tantalum oxide layer  104   a.    
     Thereafter, the upper electrode  105  comprising Pt was formed on the second tantalum oxide layer  104   b  in the above-described manner. 
     Then, the element region  107  was formed through the photolithography processing. It is to be noted that the element region  107  of the elements A to C has a circular pattern with a diameter of 3 μm. 
     In this experiment, the elements A to C were manufactured with different time periods for the oxidation using the oxygen plasma (oxygen plasma exposure time). Table 1 summarizes the initial resistance value of each element manufactured, and the thickness and the oxygen content percentage x of the first tantalum oxide layer (TaO x  layer) as well as the thickness and the oxygen content percentage y of the second tantalum oxide layer (TaO y  layer) which were measured through X-ray reflectivity measurement (described later). 
     It is to be noted that the oxygen plasma exposure time of the element A being zero minute means that the deposition of the first tantalum oxide layer  104   a  was immediately followed by the deposition of Pt as the upper electrode  105 , without exposure to the oxygen plasma. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Element A 
                 Element B 
                 Element C 
               
               
                   
                 (Sample A) 
                 (Sample B) 
                 (Sample C) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Oxygen Plasma Exposure Time (min.) 
                 0 
                 0.5 
                 1 
               
               
                 Initial Resistance Value (Ω) 
                 1.7 
                 650 
                 1850 
               
            
           
           
               
               
               
               
               
               
            
               
                 X-ray Reflectivity 
                 TaO x  Layer 
                 Thickness (nm) 
                 29.0 
                 28.6 
                 28.7 
               
               
                 Measurement 
                 (First Tantalum 
                 δ 
                 29.4 × 10 −6   
                 29.3 × 10 −6   
                 29.3 × 10 −6   
               
               
                 Result 
                 Oxide Layer) 
                 x 
                 1.41 
                 1.43 
                 1.43 
               
               
                   
                 TaO y  Layer 
                 Thickness (nm) 
                 1.0 
                 1.1 
                 1.2 
               
               
                   
                 (Second Tantalum 
                 δ 
                 23.1 × 10 −6   
                 22.3 × 10 −6   
                 24.9 × 10 −6   
               
               
                   
                 Oxide Layer) 
                 y 
                 2.34 
                 2.45 
                 2.07 
               
               
                   
               
            
           
         
       
     
     The following describes the characteristics and so on of the variable resistance elements manufactured in the above manner. 
     (Initial Resistance of Variable Resistance Layer) 
     First, the initial resistance of the variable resistance layer  104  of each of the elements A to C is measured and the result is examined. Here, the initial resistivity of the variable resistance layer  104  of each element was determined by measuring a current flowing upon application of a weak voltage of 50 mV that is lower than a threshold voltage (1 V, for example) between the lower electrode  103  and the upper electrode  105  of each element. 
     Table 1 shows that the element A (oxygen plasma exposure time: zero minute) has resistance of 1.7Ω, the element B (oxygen plasma exposure time: 0.5 minute) has resistance of 650Ω, and the element C (oxygen plasma exposure time: 1 minute) has resistance of 1850Ω. This indicates that the resistance value of the variable resistance layer  104  rises with increase in the oxygen plasma exposure time. 
     Probably, the resistance of 1.72Ω seen in the element A manufactured by zero-minute oxygen plasma exposure is equivalent to contact resistance, and the large resistance values above several hundred ohms seen in the elements B and C are mostly due to the second tantalum oxide layer  104   b  formed by the oxygen plasma processing. 
     (Resistance Change Characteristics) 
     Next, the following describes the characteristics observed when a resistance change occurs upon application of electrical pulses to the elements A to C. 
       FIG. 6  ( a ) to (c) are graphs each showing a relationship between electrical pulses applied and the resistance value of the variable resistance layer of the nonvolatile storage element according to the first experiment. It is to be noted that  FIG. 6  ( a ) to (c) show the results of the elements A to C, respectively. Here, the resistance value of the variable resistance, layer  104  was measured while repeatedly applying two types of electrical pulses alternately between the lower electrode  103  and the upper electrode  105  with a pulse width of 100 nsec. The two types of electrical pulses are a negative voltage of −2.0 V and a positive voltage of 3.0 V which are applied to the upper electrode  105  with reference to the lower electrode  103 . 
     First, referring to  FIG. 6  ( b ) showing the resistance change characteristics of the element B manufactured by 0.5-minute oxygen plasma exposure, it is clear that application of the electrical pulses of the negative voltage −2.0 V to a sample in the initial state immediately after the measurement causes a decrease in the resistance value from 650Ω to about 505Ω. Thereafter, it is possible to see that application of the electrical pulses of the positive voltage 3.0 V causes an increase in the resistance value to 5000Ω, and after that, reversible resistance changes between 50Ω and 5000Ω occur very stably in B mode, which is similar to the characteristics shown in  FIG. 3  ( b ). 
     Further, as is clear from  FIG. 6  ( c ), the element C manufactured by one-minute oxygen plasma exposure also exhibits stable, reversible resistance changes within the measured range. The application of the electrical pulses of −2 V to the element whose initial resistance is 1850Ω causes a decrease in the resistance value to about 200Ω, and then the application of the electrical pulses of +3 V causes an increase in the resistance value to about 2000Ω. In this case, too, the stable resistance changes occur in B mode. 
     Further, it was found that the elements B and C do not exhibit reversible resistance changes in A mode which is similar to the characteristics shown in  FIG. 3  ( a ), but only exhibit the reversible resistance changes in B mode. 
     Referring to  FIG. 6  ( a ) showing the resistance change characteristics of the element A, however, indicates that the resistance does not change even when the two types of electrical pulses −2.0 V and 3.0 V are applied. Since the element A is manufactured by zero-minute oxygen plasma exposure, that is, the element A is manufactured through the deposition of the upper electrode  105  immediately after the deposition of the first tantalum oxide layer  104   a , the second tantalum oxide layer  104   b  probably does not exist, or is very thin even if it does exist. 
     These results appear to indicate a need for the second tantalum oxide layer  104   b  to cause resistance changes. As indicated, with the elements B and C having the second tantalum oxide layer  104   b , the reversible resistance changes in B mode can be observed. 
     The following describes a closer examination of the variable resistance layer  104  in these experiments. 
     (Analysis of Variable Resistance Layer) 
     To analyze the structure of the variable resistance layer  104  in this experiment, samples were prepared by, under the same conditions as that of the elements A to C, depositing a tantalum oxide on a substrate in which an oxide layer having a thickness of 200 nm is formed on a single-crystal silicon substrate, followed by oxygen plasma irradiation. 
     These samples are referred to as samples A, B, and C. Table 1 summarizes the oxygen plasma exposure time and a later-described analysis of each sample. It is to be noted that Pt serving as the upper electrode  105  is not deposited on the samples A to C, and thus the variable resistance layer is exposed. 
       FIG. 7  is a graph showing an X-ray diffraction spectrum of the sample B. This is a measurement of an X-ray diffraction spectrum of a thin film, and therefore, the angle between the X-ray and the sample surface was fixed to 1°, and the diffraction spectrum intensity was measured while varying the angle (2θ) between an extension of the incident X-ray and the detector.  FIG. 7  shows that a peak is reached when 2θ is around 36 degrees, indicating that a tantalum oxide has been formed in the sample B. Since it is a wide peak between 30 and 40 degrees, the crystalline state seems to be amorphous. It is to be noted that another peak which is reached when 2θ is 56 degrees is attributable to the silicon substrate. 
     The samples A and C exhibited the same spectra as that of the sample B, revealing that all of the samples have the variable resistance layer primarily comprising an amorphous tantalum oxide. However, the X-ray diffraction measurement showed very similar spectra among the samples and did not show any dependency on the oxygen plasma exposure time. 
     As described above, the sensitivity of the X-ray diffraction measurement is not so high for the samples used in this experiment. That is to say, the variable resistance layer in the samples A to C is very thin (30 nm thick) and is amorphous as described above, making it difficult to conduct a detail analysis of the tantalum oxides using an ordinary X-ray diffraction spectrum. 
     In view of the above, a more detailed analysis was conducted using a method known as X-ray reflectometry. This is a method of emitting an X-ray entering the sample surface at a shallow angle, and measuring the intensity of the X-ray reflected. 
     Then, fitting is performed presuming an appropriate structural model for the spectrum, so as to evaluate the thickness and the refractive index of the variable resistance layer in the samples A to C. Here, the parameters for the fitting are the layered structure of the variable resistance layer and the thickness and δ (=1−refractive index) of each layer. 
       FIGS. 8  ( a ) and ( b ) each show, as an example, an X-ray reflectivity measurement pattern of the sample B. Here, an angle θ between an incident X-ray and the sample surface and a detector angle (angle θ from the sample surface) were changed in coordination with each other, so as to measure transition in the reflectivity of the X-ray on the sample surface. The angle between an extension of the incident X-ray and the detector is 2θ. In  FIGS. 8  ( a ) and ( b ), the horizontal axis represents the incident angle of the X-ray, and the vertical axis represents the reflectivity of the X-ray. 
       FIG. 8  ( a ) shows a pattern observed at the time of actually measuring the X-ray reflectivity of the sample B (broken line) and a result of fitting which was performed presuming that a single tantalum oxide layer exists on the substrate (solid line), whereas  FIG. 8  ( b ) shows a reflectivity pattern observed at the time of actually measuring the X-ray reflectivity of the sample B (broken line) and a result of fitting which was performed presuming that two tantalum oxide layers exist on the substrate (solid line). 
     Referring to  FIG. 8  ( a ), while the measured values and the fitting result almost match, there are differences at fine levels. On the other hand, referring to  FIG. 8  ( b ), the actually measured reflectivity pattern matches well with the reflectivity pattern obtained from the fitting to the extent that they are hardly distinguishable. The above results appear to indicate that the sample A includes two different tantalum oxide layers, namely, the first and the second tantalum oxide layers. 
     The result of an analysis of the sample B when fitting was performed presuming this double-layered structure showed the following values as shown in Table 1: the thickness of the first tantalum oxide layer was 28.6 nm and δ was 29.3×10 −6 ; and the thickness of the second tantalum oxide layer was about 1.43 nm and δ was 22.3×10 −6 . Generally, δ of metal tantalum is 39×10 −6 , and δ of Ta 2 O 5  is 22×10 −6 . Comparison between these generally known values and the above obtained values appears to indicate that the first tantalum oxide layer is an oxide deficient in oxygen such as TaO 1.43 , which is clearly different from the stoichiometric composition of tantalum. 
     Further, as for the second tantalum oxide layer, calculating its compositional ratio using the δ value gives TaO 2.45 , indicating an oxide similar to Ta 2 O 5  (TaO 2.5 ). However, the second tantalum oxide layer seems to be an oxygen-deficient oxide which is slightly different from the stoichiometric composition. 
     Referring to Table 1, the sample C showed almost the same result. More specifically, when the first tantalum oxide layer is expressed as TaO x , its thickness is approximately 29 nm and x is about 1.4, and when the second tantalum oxide layer is expressed as TaO y , its thickness is approximately 1.2 nm and y is about 2.3. 
     In addition, Table 1 shows that even the sample A manufactured by zero-minute oxygen plasma exposure has the second tantalum oxide layer of about 1 nm in thickness. The sputtering apparatus which deposited the tantalum oxide is kept at high vacuum with a back pressure of 7×10 −4  Pa, and thus it is unlikely that this oxide layer was formed in the sputtering apparatus. 
     Thus, the main part of this layer was probably formed during a time from when the sample A was taken out of the sputtering apparatus after the sputtering and until when the X-ray reflectivity was measured (In fact, the measurement was taken a few days after taking out the sample A from the sputtering apparatus). That is to say, if the second electrode is formed without taking the sample A out of the sputtering apparatus, the second tantalum oxide layer  104   b  probably does not exist, or is as thin as 1 nm or less even if it does exist. 
     The same inference indicates a possibility that even with the samples B and C, the thickness of the second tantalum oxide layer slightly increased due to exposure to the air after being taken out from the sputtering apparatus which deposited the tantalum oxide (before the X-ray reflectivity measurement). However, it is known that the oxidation generally proceeds fast at the beginning and gradually slows down. 
     Therefore, in the case of forming the second tantalum oxide having a higher oxygen content percentage by exposure to oxygen plasma in the sputtering apparatus, it is inferred that the proportion of the second tantalum oxide layer which increased outside the sputtering apparatus is small. 
     The existence of the second tantalum oxide layer is consistent with the fact described above with reference to Table 1 that the initial resistance of the variable resistance layer  104  of the elements B and C is much higher than in the case of providing a single layer of the first tantalum oxide layer. 
     That is to say, the resistance values of the elements B and C are higher by two to three digits than the resistance value of the element A which appears to have no second tantalum oxide layer. This is probably because the elements B and C have, between the first tantalum oxide layer  104   a  and the upper electrode  105 , a highly resistant second tantalum oxide layer  104   b  having a higher oxygen content percentage. 
     Generally, Ta 2 O 5  having a stoichiometric composition is considered to be an insulator. However, as described above, the second tantalum oxide layer is not an insulator because it is deficient in oxygen compared to Ta 2 O 5 . It is to be noted that the present invention follows the general definition of an insulator. To be more specific, a material whose resistivity is 10 8 Ω cm or greater is defined as an insulator (Non Patent Literature: “ Semiconductor Engineering for Integrated Circuits”  (Kougyou Chousakai, 1992) Akira Usami, Shinji Kanefusa, Takao Maekawa, Hajime Tomokage, and Morio Inoue), and a material whose resistivity is smaller than 10 8 Ω cm is defined as a conductor. 
     If the second tantalum oxide layer of the present experiment is an insulator and has a resistivity of 10 8 Ω cm, its resistance value should be approximately 1.4×10 8 Ω cm (resistance value=resistivity×thickness/area) given that it has a circular shape with a diameter of 3 μm (diameter of the element region  107  in this experiment) and has a thickness of 1 nm (roughly estimated thickness of the second tantalum oxide layer). Even when the second tantalum oxide layer has a thickness of 0.1 nm, its resistance value is given by 1.4×10 7 Ω cm. On the other hand, in the elements B and C, the resistance value is approximately 10 3  to 10 4 Ω at most with reference to Table 1, which is lower than in the case of assuming an insulator, by at least three to four digits approximately. 
     This calculation result also indicates that the second tantalum oxide layer formed in the present experiment is not an insulator but a conductive oxide layer. 
     Although this experiment used the X-ray reflectivity measurement for analyzing the second tantalum oxide layer, other instrumental analysis techniques can also be used, including Auger electron spectroscopy analysis (AES), fluorescent X-ray photoelectron spectroscopy analysis (XPS), and electron probe microanalysis (EPMA: also known as WDS, EDS, or EDX depending on the detection method). 
     (Relationship Between Thickness of Second Tantalum Oxide Layer and Resistance Change Phenomenon) 
     The element B and the sample B underwent the sputtering and the oxygen plasma irradiation under the same condition. The element C and the sample C also underwent the sputtering and the oxygen plasma irradiation under the same condition. Thus, in the elements B and C as in the samples B and C, the second tantalum oxide layer  104   b  probably exists between the first tantalum oxide layer  104   a  and the upper electrode  105 . 
     Therefore, it can be said that the second tantalum oxide layer  104   b  having a thickness of 1.1 nm is formed in the element B as in the sample B, and the second tantalum oxide layer  104   b  having a thickness of 1.2 nm is formed in the element C as in the sample C. 
     As previously described, the elements B and C exhibit a stable resistance change phenomenon in B mode. However, the element A which does not have the second tantalum oxide layer having a higher oxygen content percentage does not exhibit the resistance change phenomenon. This appears to indicate that the second tantalum oxide layer is essential for a resistance change. In the present experiment, the composition of this second tantalum oxide layer, when expressed as TaO y , is sufficient if it has a thickness of about 1.1 nm and y of about 2.1. 
     (Relationship Between Thickness of First Tantalum Oxide Layer and Resistance Change Phenomenon) 
     Next, a variable resistance element (referred to as an element B′) was manufactured which includes a first tantalum oxide layer having a thickness different from that in the above element B, and the resistance change characteristics of this variable resistance element were examined. 
     The element B′ is different from the element B only in thickness of the first tantalum oxide layer  104   a . While the thickness of the first tantalum oxide layer  104   a  of the element B was 30 nm, the thickness of the first tantalum oxide layer  104   a  of the element B′ was 90 nm. In manufacturing the element B′, the oxygen plasma exposure time was 0.5 minutes as in the case of the element B. Therefore, with the element B′ too, the thickness of the second tantalum oxide layer  104   b  is probably 1 to 2 nm approximately. 
     When electrical pulses of a negative voltage −2.0 V and a positive voltage 3.0 V with a pulse width of 100 nsec were alternately and repeatedly applied between the upper electrode  105  and the lower electrode  103  of the element B, the element B exhibited such resistance change characteristics that the resistance value changed from about 500Ω to 20Ω when −2.0 V was applied, and thereafter, reversible resistance changes stably occurred in B mode between about 20Ω and about 200 Ω. 
     The above results show that the thickness of the first tantalum oxide layer does not have a significant impact on the resistance change phenomenon of the nonvolatile storage element according to the present experiment. 
     As described above, the first experiment involved deposition of the first tantalum oxide layer in the sputtering apparatus, followed by oxidation using oxygen plasma to form the second tantalum oxide layer. However, this method could not thickly form the second tantalum oxide layer due to the apparatus used. Thus, as the second experiment, the following describes the operation of the variable resistance element in the case where a second tantalum oxide layer is thickly formed. 
     (Manufacturing Method of Variable Resistance Element) 
     The structure and the manufacturing method of the variable resistance element used in the second experiment are basically the same as in the first experiment. However, for convenience of the oxidation process, the second experiment is different from the first experiment in condition under which tantalum oxide is deposited and size of the nonvolatile storage element formed. Hereinafter, a process for manufacturing a nonvolatile element is described with reference to  FIG. 5  ( a ) to (c). 
     First, as shown in  FIG. 5  ( a ), thermal oxidation produces the oxide layer  102  having a thickness of 200 nm on the substrate  101  which is a single-crystal silicon. Then, by sputtering, a Pt thin film having a thickness of 100 nm is deposited on the oxide layer  102  as the lower electrode  103 . After that, the first tantalum oxide layer  104   a  is formed on the lower electrode  103  by reactive sputtering, with tantalum used as the target. 
     Here, the first tantalum oxide layer  104   a  was deposited under the condition described below. That is, the substrate was placed in the sputtering apparatus and then the sputtering apparatus was vacuumed to about 8×10 −6  Pa. Then, the sputtering was performed for 20 seconds with tantalum used as the target, power set to 1.6 kW, a flow of an argon gas at 34 sccm, a flow of an oxygen gas at 21 sccm, and the pressure in the sputtering apparatus kept at 0.17 Pa. With this, the first tantalum oxide layer was deposited with a thickness of 30 nm, a resistivity of 6 mΩcm, and an oxygen content percentage of about 61 at % (TaO 1.6 ). 
     Next, as shown in  FIG. 5  ( b ), the top surface of the first tantalum oxide layer  104   a  was oxidized to modify the surface property. Here, as shown in Table 2, elements D and E were manufactured with different oxidation methods. More specifically, with the element D, after the sputtering was performed, the substrate was taken out of the sputtering apparatus and placed in an oxygen plasma generation apparatus in which oxidation was performed by exposing the substrate to oxygen plasma while heating the substrate to keep at a temperature of 250° C. As for the element E, the substrate was placed in a lamp annealing apparatus in which the oxidation was performed using a flow of an oxygen gas while heating the substrate to keep at a temperature of 300° C. Such oxidation processes produced the second tantalum oxide layer  104   b  having an oxygen content percentage higher than that of the first tantalum oxide layer  104   a  (An analysis of film composition of the second tantalum oxide layer is described later). 
     Subsequently, by sputtering, a Pt thin film having a thickness of 150 nm was formed on the second tantalum oxide layer  104   b  as the upper electrode  105 . To prevent oxidization of the second tantalum oxide layer  104   b  in the air, the upper electrode  105  was formed immediately after the deposition of the second tantalum oxide layer  104   b . Lastly, photolithography processing created the pattern  106  using a photoresist, and dry etching produced the element region  107 . The element region  107  here is assumed to be a rectangle with each side being 0.5 μm in length. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Element D 
                 Element E 
               
               
                   
                 (Sample D) 
                 (Sample E) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 Oxidation Method 
                 Oxygen Plasma Oxidation 
                 Lamp Annealing Oxidation 
               
               
                   
                 (250° C.) 
                 (300° C.) 
               
               
                 Initial Resistance Value (Ω) 
                 426 
                 610 
               
            
           
           
               
               
               
               
               
            
               
                 X-ray 
                 TaO x  Layer 
                 Thickness (nm) 
                 26.6 
                 27.9 
               
               
                 Reflectivity 
                 (First Tantalum 
                 δ 
                 28.5 × 10 −6   
                 28.2 × 10 −6   
               
               
                 Measurement 
                 Oxide Layer) 
                 x 
                 1.54 
                 1.59 
               
               
                 Result 
                 TaO y  Layer 
                 Thickness (nm) 
                 8.1 
                 7.3 
               
               
                   
                 (Second Tantalum 
                 δ 
                 22.2 × 10 −6   
                 22.8 × 10 −6   
               
               
                   
                 Oxide Layer) 
                 y 
                 2.47 
                 2.38 
               
               
                   
               
            
           
         
       
     
     (Resistance Change Characteristics of Elements D and E) 
     Next, the following describes the characteristics observed when a resistance change occurs upon application of electrical pulses to the elements D and E which were actually manufactured in the second experiment. 
       FIGS. 9  ( a ) and ( b ) are graphs each showing a relationship between electrical pulses applied and the resistance value of the variable resistance layer included in the variable resistance element of the second experiment. Here,  FIGS. 9  ( a ) and ( b ) show the results of the elements D and E, respectively. 
     First, the following describes the result of the element D in which the second tantalum oxide layer was formed through the oxidation using oxygen plasma. As is clear from the result shown in  FIG. 9  ( a ), application of a negative voltage of −1.3 V to the upper electrode of the variable resistance element immediately after being manufactured causes a decrease in the resistance value, which was initially about 400Ω, to about 200Ω, and application of a positive voltage of 1.5 V increases the resistance value to about 2000Ω. After that, upon applying the electrical pulses of the positive voltage 1.5 V and the negative voltage −1.3 V alternately, stable resistance changes occur in B mode, with the resistance value varying between 200 Ω to 3000 Ω approximately. 
     Next, referring to  FIG. 9  ( b ) showing the result of the element E which underwent the oxidation through lamp annealing, it is clear that stable resistance changes in B mode occur also in the element E. More specifically, the resistance, which was initially about 600Ω, decreases to about 300 Ω upon application of the negative voltage of −1.3 V and increases to about 5000Ω upon application of the positive voltage of 1.5 V. After that, upon applying the electrical pulses of the positive voltage 1.5 V and the negative voltage −1.3 V alternately, stable resistance changes occur in B mode with the resistance value varying between 200Ω to 5000Ω approximately. 
       FIG. 10  illustrates hysteretic current-voltage characteristics showing how the resistance of the element D changes. The horizontal axis represents the voltage applied to the upper electrode  105  with reference to the lower electrode  103 , and the vertical axis represents the value of a current flowing in the element D. 
     As  FIG. 10  shows, application of a positive voltage to the upper electrode  105  side with reference to the lower electrode  103  increases the current almost in proportion to the voltage, and the current rapidly decreases when the positive voltage exceeds the voltage at the point A. This shows a change in the resistance from a low resistance state to a high resistance state. 
     On the other hand, in gradual application of a negative voltage to the upper electrode  105  side with reference to the lower electrode  103  (equivalent to application of a positive voltage to the lower electrode  103  side with reference to the upper electrode  105 ) in the high resistance state, the current rapidly increases when the negative voltage exceeds the voltage at the point B. This shows a change in the resistance from the high resistance state to the low resistance state. 
       FIG. 10  further shows that the change from the low resistance state to the high resistance state does not occur before the point A and the change from the high resistance state to the low resistance state does not occur before the point B. 
     Thus, it is clear that the element D exhibits the resistance changes in B mode and requires a larger driving current for the resistance change from the low resistance state to the high resistance state than for the resistance change from the high resistance state to the low resistance state. 
     (Analysis of Variable Resistance Layer) 
     To analyze the structure of the variable resistance layer  104  used in this experiment, samples were prepared by, under the same conditions as that of the elements D and E, depositing a tantalum oxide on a substrate in which an oxide layer having a thickness of 200 nm is formed on a single-crystal silicon substrate, followed by oxidation. These samples are referred to as samples D and E. The results of the X-ray reflectivity measurement on these samples are shown in Table 2. It is to be noted that in the samples D and E, the second tantalum oxide layer is exposed as in the case of the samples A to C. 
     Referring to Table 2, the sample D which was oxidized using oxygen plasma has the second tantalum oxide layer TaO y  whose thickness is 8.1 nm, which is, as originally intended, thicker than that of the samples A to C. Further, y is 2.47, indicating that oxygen is more deficient in TaO y  than in Ta 2 O 5  having a stoichiometric composition. As for the sample E which was oxidized using the lamp annealing apparatus, the thickness of the second tantalum oxide layer TaO y  was 7.3 nm and y was 2.38. 
     (Cross-Section Observation of Nonvolatile Storage Element) 
     As described above, the second tantalum oxide layer formed in the variable resistance element in this experiment has a thickness of 7 to 8 nm approximately. Such a thickness is sufficient for observing the existence of the second tantalum oxide layer through cross-section observation of the nonvolatile element using a transmission electron microscope. Thus, cross-section observation was actually performed on the variable resistance element in which the second tantalum oxide layer was formed through the oxidation using oxygen plasma. 
       FIGS. 11  ( a ) and ( b ) show the observation result. These diagrams clearly show the first electrode comprising Pt, the first tantalum oxide layer, the second tantalum oxide layer, and the second electrode comprising Pt. In addition, the diagrams show that, though there is some unevenness, the thickness of the first tantalum oxide layer is about 28 nm, and the thickness of the second tantalum oxide layer is about 8 nm. 
     These values almost match the result of the X-ray reflectivity measurement on the sample D which was manufactured under the same oxidation condition (as Table 2 shows, the thickness of the first tantalum oxide layer is 26.6 nm and the thickness of the second tantalum oxide layer is 8.1 nm). 
     The above result made it clear that the second tantalum oxide layer does actually exist in the nonvolatile storage element used in the second experiment. In addition, it proves the validity of the analysis using the X-ray reflectivity measurement. 
     The third experiment is now described. 
     In the case of the elements A to C used in the above-described first experiment, the oxygen content percentage of the first tantalum oxide layer  104   a  was 58 at % (TaO 1.4 ). The first tantalum oxide layer  104   a  of the elements D and E used in the second experiment had a similar oxygen content percentage of 61 at % (TaO 1.6 ). 
     In contrast, the variable resistance element used in the third experiment includes the first tantalum oxide layer whose oxygen content percentage is changed to be a little greater. The structure of the variable resistance element used in the third experiment is the same as in the first and the second experiments, and is thus not shown. 
     With reference to  FIG. 5 , the following describes a method of manufacturing the variable resistance element used in the third experiment, which was manufactured using the first tantalum oxide layer having a different oxygen content percentage, and also describes the resistance change characteristics and so on of that variable resistance element. 
     (Relationship Between Oxygen Flow Ratio in Sputtering and Composition) 
     First, the following describes the condition under which the tantalum oxide layer of the third experiment was manufactured and an analysis result of the oxygen content percentage in the third experiment. The tantalum oxide was manufactured using the same method as in the first experiment. However, the oxygen content percentage of the tantalum oxide is controlled by adjusting the oxygen flow ratio employed in sputtering. 
     Describing in accordance with the specific processes of sputtering, first, a substrate is placed in a sputtering apparatus, and the sputtering apparatus is vacuumed to about 7×10 −4  Pa. Then, the sputtering is performed with tantalum used as the target, power set to 250 W, the total gas pressure of the argon gas and the oxygen gas set to 3.3 Pa, and the preset temperature of the substrate set to 30° C. Here, the flow ratio of the oxygen gas is varied from 0.8% to 6.7%. 
     Since the object of the third experiment is to examine the composition, SiO 2  deposited 200 nm in thickness on Si (Silicon) was used as the substrate, and the sputtering time was adjusted to make the thickness of the tantalum oxide layer about 100 nm. Unlike in the first experiment, the oxygen plasma exposure was not performed in the third experiment. 
       FIG. 12  shows a result of analyzing, using the Rutherford backscattering spectrometry (RBS) and the Auger electron spectroscopy (AES), the composition of the tantalum oxide layer manufactured in the above manner.  FIG. 12  shows that the oxygen content percentage of the tantalum oxide layer changes from approximately 40 at % (TaO 0.66 ) to approximately 70 at % (TaO 2.3 ) when an oxygen partial pressure ratio is changed from 0.8% to 6.7%. More specifically, it is clear that the oxygen content percentage of the tantalum oxide layer can be controlled by varying the oxygen flow ratio. 
     It appears that the sample prepared for the composition measurement was oxidized with oxygen in the air during a time between the deposition on the substrate and the measurement, thereby forming a high oxygen content percentage layer on the sample surface. However, because the surface was etched before the measurement using the RBS and the AES, it is possible to ignore the impact that the high oxygen content percentage layer formed on the surface has on the measurement of the oxygen content percentage. 
     Although the present experiment used the Rutherford backscattering spectrometry (RBS) and the Auger electron spectroscopy (AES) for analyzing the tantalum oxide layer, other instrumental analysis techniques can also be used, including fluorescent X-ray photoelectron spectroscopy analysis (XPS) and electron probe microanalysis (EPMA). 
     (Composition of First Tantalum Oxide Layer and Resistance Change Characteristics) 
     Next, the following describes the resistance change characteristics observed when the variable resistance element  100  is manufactured by forming the variable resistance layer  104  using, as the first tantalum oxide layer  104   a  and the second tantalum oxide layer  104   b , each of tantalum oxide layers having different oxygen content percentages. 
     The variable resistance element  100  was manufactured using the same method as in the first experiment. To be more specific, thermal oxidation produced the oxide layer  102  having a thickness of 200 nm on the substrate  101  which is a single-crystal silicon, and then, by sputtering, a Pt thin film having a thickness of 100 nm was deposited on the oxide layer  102  as the lower electrode  103 . Then, the sputtering was performed to form the first tantalum oxide layer  104   a , with tantalum used as the target, power set to 250 W, the total gas pressure of the argon gas and the oxygen gas set to 3.3 Pa, and the preset temperature of the substrate set to 30° C. 
     In this experiment, each element was manufactured with the flow ratio of the oxygen gas varied from 0.8% to 6.7%. 
     The sputtering time was adjusted such that the thickness of the first tantalum oxide layer  104   a  becomes 30 nm. Thereafter, the top surface of the first tantalum oxide layer  104   a  was exposed to oxygen plasma for 30 seconds, so as to form the second tantalum oxide layer  104   b . Lastly, by sputtering, a Pt thin film having a thickness of 150 nm was formed on the second tantalum oxide layer  104   b  as the upper electrode  105 , so as to form the variable resistance element  100 . 
     The resistance change phenomenon of the variable resistance element formed in the above manner was measured. The result of the measurement showed a satisfactory resistance change phenomenon. More specifically, with the nonvolatile storage element using a tantalum oxide film represented by a range in  FIG. 12  from point a (an oxygen flow ratio of approximately 1.7% and an oxygen content percentage of approximately 45 at %) to point  13  (an oxygen flow ratio of approximately 5% and an oxygen content percentage of approximately 65 at %), the high resistance value was equal to or more than five times larger than the low resistance value. 
       FIGS. 13  ( a ) and ( b ) each show a result of measuring the resistance change characteristics, which vary according to the number of pulses applied, of a sample having the oxygen content percentage at the point a or the point β.  FIGS. 13  ( a ) and ( b ) show that with both the oxygen content percentages at the points α and β, each high resistance value is equal to or more than five times larger than the low resistance value. Thus, a compositional range in which the oxygen content percentage is 45 to 65 at %, that is, a range of 0.8≦x≦1.9 when the variable resistance layer is expressed as TaO x  is a more appropriate range of the variable resistance layer (the oxygen content percentage of 45 at % and the oxygen content percentage of 65 at % correspond to x=0.8 and x=1.9, respectively). 
     As described so far in the first to third experiments, it was found that with the variable resistance element shown in  FIG. 4 , the variable resistance layer  104  having a layered structure including (i) the first oxygen-deficient tantalum oxide layer having a composition TaO x  (0.8≧x≧1.9) and provided in contact with the lower electrode and (ii) the second oxygen-deficient tantalum oxide layer having a composition TaO y  (2.1≦y&lt;2.5) and provided in contact with the upper electrode exhibits stable resistance changes in B mode in which the following resistance changes repeatedly take place: the resistance state changes into the low resistance state when negative voltage pulses are applied to the upper electrode side with reference to the lower electrode; and the resistance state changes into the high resistance state when positive voltage pulses are applied to the upper electrode side with reference to the lower electrode. 
     The variable resistance element having such a structure did not exhibit the resistance changes in A mode which are resistance changes of the reversed polarity. Further, with this structure, the thickness of the second oxygen-deficient tantalum oxide layer between 1 nm and 8 nm inclusive was favorable in exhibiting the stable resistance changes in B mode. 
     Next, the following describes the fourth experiment related to a variable resistance element which exhibits stable resistance changes in A mode, which are opposite to the resistance changes described so far. 
     (Structure of Variable Resistance Element) 
       FIG. 14  is a cross-sectional view showing a structural example of a variable resistance element according to the fourth experiment. As shown in  FIG. 14 , the variable resistance element  100  used in the fourth experiment includes: a substrate  101 ; an oxide layer  102  formed on the substrate  101 ; a lower electrode  103  formed on the oxide layer  102 ; an upper electrode  105 ; and a variable resistance layer  104  located between the lower electrode  103  and the upper electrode  105 . 
     Here, the variable resistance layer  104  includes: a first tantalum containing layer (hereinafter referred to as “first tantalum oxide layer”)  104   a  having a lower oxygen content percentage; and a second tantalum containing layer (hereinafter referred to as “second tantalum oxide layer”)  104   b  formed on the first tantalum oxide layer  104   a  and having a higher oxygen content percentage. 
     The variable resistance element used in the fourth experiment is different from that used in the first to third experiments in that the second tantalum oxide layer  104   b  is provided in contact with the lower electrode  103 , and the first tantalum oxide layer  104   a  is provided in contact with the upper electrode  105 . 
     (Manufacturing Method of Variable Resistance Element) 
     Next, a manufacturing method of the variable resistance element  100  used in the present experiment is described with reference to  FIG. 15  ( a ) to (c). 
     First, as shown in  FIG. 15  ( a ), thermal oxidation produces the oxide layer  102  having a thickness of 200 nm on the substrate  101  which is a single-crystal silicon. Then, by sputtering, a Pt thin film having a thickness of 100 nm is deposited on the oxide layer  102  as the lower electrode  103 . After that, by sputtering, the second tantalum oxide layer  104   b  is formed about 3 nm in thickness on the lower electrode  103 , with Ta 2 O 5  used as the target. 
     Next, as shown in  FIG. 15  ( b ), the first tantalum oxide layer  104   a  is formed on the second tantalum oxide layer  104   b  by reactive sputtering, with tantalum used as the target. For forming the first tantalum oxide layer  104   a , the sputtering is performed for 18 seconds with tantalum used as the target, power set to 1.6 kW, a flow of an argon gas at 34 sccm, a flow of an oxygen gas at 21 sccm, and the pressure in the sputtering apparatus kept at 0.17 Pa. With this, the first tantalum oxide layer was deposited 27 nm in thickness, with the resistivity of 6 mΩcm and the oxygen content percentage of about 61 at % (TaO 1.6 ). 
     This produces, on the surface of the second tantalum oxide layer  104   b , the first tantalum oxide layer  104   a  having an oxygen content percentage lower than that of the second tantalum oxide layer  104   b . Such a layered structure of the second tantalum oxide layer  104   b  and the first tantalum oxide layer  104   a  is the structure of the variable resistance layer  104 . 
     Subsequently, by sputtering, a Pt thin film having a thickness of 150 nm is formed on the first tantalum oxide layer  104   a  as the upper electrode  105 . Lastly, photolithography processing creates a pattern  106  using a photoresist, and dry etching produces an element region  107  as shown in  FIG. 15  ( c ). 
     With the above-described manufacturing method, an element F was manufactured. The element region  107  here is assumed to be a rectangle with each side being 0.5 μm in length. 
     (Resistance Change Characteristics of Element F) 
     Next, the following describes the resistance change characteristics observed when a resistance change occurs upon application of electrical pulses to the element F which was actually manufactured in the fourth experiment. 
       FIG. 16  is a graph showing the resistance value of the variable resistance layer of the element F, in the case of continuously applying pulses of a positive voltage 1.5 V and a negative voltage −1.8 V alternately on the upper electrode with reference to the lower electrode. The pulse width was set to 100 nsec. 
     First, upon application of a positive voltage of 1.5 V to the upper electrode, the resistance value decreases to about 200Ω, and upon application of a negative voltage of −1.8 V, the resistance value increases to about 20000Ω. After that, upon applying the electrical pulses of the positive voltage 1.5 V and the negative voltage −1.8 V alternately, stable resistance changes occur in A mode, with the resistance value varying between 100 Ω to 8000Ω approximately. 
       FIG. 17  shows hysteretic current-voltage characteristics indicating how the resistance of the element F changes. The horizontal axis represents the voltage applied to the upper electrode  105  with reference to the lower electrode  103 , and the vertical axis represents the value of a current flowing in the element F. 
     As  FIG. 17  shows, application of a negative voltage to the upper electrode  105  side with reference to the lower electrode  103  increases the current almost in proportion to the voltage, and the current rapidly decreases when the negative voltage exceeds the voltage at the point A. In other words, it shows a change in the resistance from a low resistance state to a high resistance state. 
     On the other hand, in gradual application of a positive voltage to the upper electrode  105  side with reference to the lower electrode  103  (equivalent to application of a negative voltage to the lower electrode  103  side with reference to the upper electrode  105 ) in the high resistance state, the current rapidly increases when the negative voltage exceeds the voltage at the point B. In other words, it shows a change in the resistance from the high resistance state to the low resistance state. 
       FIG. 17  further shows that the change from the low resistance state to the high resistance state does not occur before the point A and the change from the high resistance state to the low resistance state does not occur before the point B. 
     Thus, it indicates that the element F exhibits the resistance changes in A mode and requires a larger driving current for the resistance change from the low resistance state to the high resistance state than for the resistance change from the high resistance state to the low resistance state. 
     (Inference of Variable Resistance Layer) 
     The following examines the structure of the variable resistance layer  104  included in the variable resistance element used in the fourth experiment, and especially examines the composition of the second tantalum oxide layer formed in the present experiment by sputtering, with Ta 2 O 5  used as the target. 
     Due to the plasma in the sputtering process, the composition of the second tantalum oxide layer is not exactly Ta 2 O 5  but is a composition which slightly lacks oxygen. Thus, it is inferred that a tantalum oxide TaO y  (y=2.3 to 2.4) was formed which contains less oxygen than in the composition of the target. 
     Thus, with the variable resistance element shown in  FIG. 17 , the structure of the variable resistance layer  104  in this experiment can be identified as follows: the second tantalum oxide layer  104   b  has a composition TaO y  (y=2.3 to 2.4) with a thickness of 3 nm; and the first tantalum oxide layer  104   a  has a composition TaO x  (x=1.6) with a thickness of 27 nm. 
     As described so far regarding the fourth experiment, it was found that with the variable resistance element shown in  FIG. 14 , the variable resistance layer  104  having a layered structure including (i) the second oxygen-deficient tantalum oxide layer having the composition TaO y  (y=2.3 to 2.4) and provided in contact with the lower electrode and (ii) the first oxygen-deficient tantalum oxide layer having the composition TaO x  (x=1.6) and provided in contact with the upper electrode exhibits the stable resistance changes in A mode in which the following resistance changes repeatedly take place: the resistance state changes into the low resistance state when positive voltage pulses are applied to the upper electrode with reference to the lower electrode; and the resistance state changes into the high resistance state when negative voltage pulses are applied to the upper electrode with reference to the lower electrode. 
     The variable resistance element having such a structure did not exhibit the resistance changes in B mode which are resistance changes of the reversed polarity. Further, with this structure, the thickness of the second oxygen-deficient tantalum oxide layer was 3 nm. 
     By combining the structure of the variable resistance element in the fourth experiment and the compositions of the variable resistance layers in the first to third experiments, it is possible to adequately infer that with the structure of the variable resistance element shown in  FIG. 14 , the variable resistance element using the variable resistance layer  104  having a layered structure including (i) the first oxygen-deficient tantalum oxide layer  104   a  having a composition TaO x  (0.8≦x≦1.9) and (ii) the second oxygen-deficient tantalum oxide layer  104   b  having a composition TaO y  (2.1≦y&lt;2.5) exhibits the stable resistance changes in A mode in which the following resistance changes repeatedly take place: the resistance state changes into the low resistance state when positive voltage pulses are applied to the upper electrode side with reference to the lower electrode; and the resistance state changes into the high resistance state when negative voltage pulses are applied to the upper electrode side with reference to the lower electrode. 
     In addition, it can be inferred that the variable resistance element having such a structure does not exhibit the resistance changes in B mode which are the resistance changes of the reversed polarity. Furthermore, it is possible to infer that, with this structure too, the thickness of the second oxygen-deficient tantalum oxide layer between 1 nm and 8 nm inclusive is favorable in exhibiting the stable resistance changes in A mode. 
     (Variable Resistance Nonvolatile Storage Device According to Embodiment of Present Invention) 
     Next, the following describes, as an embodiment of the present invention, a 1T1R nonvolatile storage device for which the above-mentioned variable resistance element is used. 
     (1T1R Nonvolatile Storage Device having NMOS Structure) 
       FIG. 18  is a block diagram showing a structure of a nonvolatile storage device according to an embodiment of the present invention. 
     As shown in  FIG. 18 , a nonvolatile storage device  200  according to this embodiment includes a memory main portion  201  on a semiconductor substrate, and the memory main portion  201  includes: a memory array  202 ; a row selection circuit  208 ; a row driver  207  including word line drivers WLD and source line drivers SLD; a column selection circuit  203 ; a write circuit  206  for writing data; a sense amplifier  204  which detects an amount of a current flowing in a selected bit line and determines whether stored data indicates “1” or “0”; and a data input and output circuit  205  which performs an input and output process of input and output data via a terminal DQ. 
     The nonvolatile storage device  200  further includes, as a power source for writing  211 , a power source for low resistance (LR) writing  212  and a power source for high resistance (HR) writing  213 . An output V 2  of the power source for low resistance (LR) writing  212  is supplied to the row driver  207 , whereas an output V 1  of the power source for high resistance (HR) writing  213  is supplied to the write circuit  206 . 
     Moreover, the nonvolatile storage device  200  includes: an address input circuit  209  which receives address signals inputted from the outside; and a control circuit  210  which controls operations of the memory main portion  201  based on a control signal inputted from the outside. 
     The memory array  202  includes: word lines WL 0 , WL 1 , WL 2  . . . ; bit lines BL 0 , BL 1 , BL 2  . . . ; NMOS transistors N 11 , N 12 , N 13 , N 21 , N 22 , N 23 , N 31 , N 32 , N 33  . . . (hereinafter, referred to as “transistors N 11 , N 12  . . . ”); and variable resistance elements R 11 , R 12 , R 13 , R 21 , R 22 , R 23 , R 31 , R 32 , R 33  . . . (hereinafter, referred to as “variable resistance elements R 11 , R 12  . . . ”). These NMOS transistors, and variable resistance elements form memory cells M 11 , M 12 , M 13 , M 21 , M 22 , M 23 , M 31 , M 32 , M 33  . . . (hereinafter, referred to as “memory cells M 11 , M 12  . . . ”). Here, the word lines and the bit lines are formed above the semiconductor substrate and arranged to intersect with each other. The NMOS transistors are provided at corresponding points where the word lines WL 0 , WL 1 , WL 2  . . . and the bit lines BL 0 , BL 1 , BL 2  . . . intersect with each other. The variable resistance elements are connected in series with the transistors N 11 , N 12  . . . on a one-on-one basis. It is to be noted that each of the variable resistance elements R 11 , R 12  . . . is the variable resistance element described above as the basic data of the present invention. 
     As shown in  FIG. 18 , the gates of the transistors N 11 , N 21 , N 31  . . . are connected to the word line WL 0 , the gates of the transistors N 12 , N 22 , N 32  . . . are connected to the word line WL 1 , the gates of the transistors N 13 , N 23 , N 33  . . . are connected to the word line WL 2 , and the gates of the transistors N 14 , N 24 , N 34  . . . are connected to the word line WL 3 . 
     Furthermore, the transistors N 11 , N 21 , N 31  . . . and the transistors N 12 , N 22 , N 32  . . . are commonly connected to the source line SL 0 , and the transistors N 13 , N 23 , N 33  . . . and the transistors N 14 , N 24 , N 34  . . . are commonly connected to the source line SL 2 . 
     Moreover, the variable resistance elements R 11 , R 12 , R 13 , R 14  . . . are connected to the bit line BL 0 , the variable resistance elements R 21 , R 22 , R 23 , R 24  . . . are connected to the bit line BL 1 , and the variable resistance elements R 31 , R 32 , R 33 , R 34  . . . are connected to the bit line BL 2 . 
     The address input circuit  209  receives address signals from an external circuit (not shown), and outputs row address signals to the row selection circuit  208  and column address signals to the column selection circuit  203  based on the address signals. Here, the address signals are signals indicating an address of a specific memory cell selected from among the memory cells M 11 , M 12  . . . . 
     In a data write cycle, the control circuit  210  outputs to the write circuit  206  a write command signal instructing application of a voltage for writing, according to input data Din inputted into the data input and output circuit  205 . On the other hand, in a data read cycle, the control circuit  210  outputs to the sense amplifier  204  a read command signal instructing a read operation. 
     The row selection circuit  208  receives the row address signals outputted from the address input circuit  209 , and according to the row address signals, the row driver  207  causes a word line driver WLD corresponding to one of the word lines WL 0 , WL 1 , WL 2  . . . to apply a predetermined voltage to the selected word line. 
     Similarly, the row selection circuit  208  receives the row address signals outputted from the address input circuit  209 , and according to the row address signals, the row driver  207  causes a source line driver SLD corresponding to one of the source lines SL 0 , SL 2  . . . to apply a predetermined voltage to the selected source line. 
     In addition, the column selection circuit  203  receives the column address signals outputted from the address input circuit  209 , selects one of the bit lines BL 0 , BL 1 , BL 2  . . . according to the column address signals, and applies the voltage for writing or the voltage for reading to the selected bit line. 
     In the case of receiving the write command signal from the control circuit  210 , the write circuit  206  outputs to the column selection circuit  203  a signal instructing an application of the voltage for writing to the selected bit line. 
     Moreover, in the data read cycle, the sense amplifier  204  detects an amount of a current flowing in the selected bit line to be read, and determines whether stored data indicates “1” or “0”. The resulting output data DO is outputted to an external circuit via the data input and output circuit  205 . 
     The power source for writing  211  includes the power source for low resistance (LR) writing  212  and the power source for high resistance (HR) writing  213 , and the output of the power source for low resistance (LR) writing  212  and the output of the power source for high resistance (HR) writing  213  are inputted to the row driver  207  and the write circuit  206 , respectively. 
     In the case of using the element D, which has been described as the basic data of the present invention, as the variable resistance elements R 11 , R 12  . . . , for example, and given that a voltage at the point A of  FIG. 10  is referred to as a threshold voltage for high resistance writing V HR  and a voltage at the point B of  FIG. 10  is referred to as a threshold voltage for low resistance writing V LR , the power source for HR writing  213  serves as a power circuit capable of applying, to the variable resistance elements R 11 , R 12  . . . , a positive voltage higher than the threshold voltage for high resistance writing V HR , whereas the power source for LR writing  212  serves as a power circuit capable of applying, to the variable resistance elements R 11 , R 12  . . . , a negative voltage higher than an absolute value of the threshold voltage for low resistance writing V LR . 
       FIG. 19  is a cross-sectional view showing a structure (two-bit structure) of memory cells  300  corresponding to the part C in  FIG. 18  and an enlarged view of one of variable resistance elements  309 . 
     Transistors  317  and the variable resistance elements  309  correspond to the transistors N 11  and N 12  and the variable resistance elements R 11  and R 12  shown in  FIG. 18 , respectively. 
     The memory cells  300  are formed by sequentially forming, above a semiconductor substrate  301 , a second N-type diffusion layer region  302   a , first N-type diffusion layer regions  302   b , gate insulator films  303   a , gate electrodes  303   b , first vias  304 , first wiring layers  305 , second vias  306 , second wiring layers  307 , third vias  308 , the variable resistance elements  309 , fourth vias  310 , and a third wiring layer  311 . 
     The third wiring layer  311  connected to the fourth vias  310  corresponds to the bit line BL 0 , and the first wiring layers  305  and the second wiring layers  307  which are connected to the second N-type diffusion layer region  302   a  of the transistors  317  correspond to the source line SL 0  extending perpendicularly to the diagram. 
     The voltage at the semiconductor substrate  301  is 0 V, and is supplied by a commonly-known structure via a 0-V power source line (not shown). 
     As shown in the enlarged view in  FIG. 19 , each of the variable resistance elements  309  is formed by providing, on one of the third vias  308 , the variable resistance layer  309   b  between the lower electrode  309   a  and the upper electrode  309   c , and the variable resistance element  309  is connected to one of the fourth vias  310  connected to the third wiring layer  311 . 
     Here, the variable resistance layer  309   b  comprises an oxygen-deficient transition metal oxide and, in the present example, has a layered structure in which the following layers are stacked: a conductive, first oxygen-deficient tantalum oxide layer  309   b - 1  having a composition TaO x  (where x=1.54); and a conductive, second oxygen-deficient tantalum oxide layer  309   b - 2  having a composition TaO y  (where y=2.47). Here, the thickness of TaO x  (where x=1.54) is 26.6 nm, and the thickness of TaO y  (where y=2.47) is 2.47 nm. Further, the lower electrode  309   a  and the upper electrode  309   c  comprise the same material, which is Pt (platinum) in this example. The lower electrode  309   a  is connected to the second N-type diffusion layer region  302   b  of the transistor through the vias, whereas the upper electrode  309   c  is connected, through the via, to the bit line BL 0  formed by the third wiring layer  311 . 
     (Operations of Variable Resistance Nonvolatile Storage Device) 
     With reference to timing charts shown in  FIG. 20  ( a ) to (c), the following describes example operations of the variable resistance nonvolatile storage device formed in the above manner, performed in a write cycle to write data and in a read cycle to read data. 
       FIG. 20  ( a ) to (c) are timing charts each showing example operations of the nonvolatile storage device according to an embodiment of the present invention. It is to be noted that the example operations are shown with an assumption that data “1” corresponds to the case where the variable resistance layer is in the high resistance state and data “0” corresponds to the case where the variable resistance layer is in the low resistance state. Further, described here is only the case where data is written and read to and from the memory cell M 11 . 
     In  FIG. 20  ( a ), the voltage V 2  generated by the power source for LR writing  212  is determined to have a value of a voltage which causes a voltage larger than the threshold voltage for low resistance writing V AR  to be applied to the variable resistance elements R 11 , R 12  . . . . 
     In  FIG. 20  ( b ), the voltage V 1  generated by the power source for HR writing  213  is determined to have a value of a voltage which causes a voltage larger than the threshold voltage for high resistance writing V HR  to be applied to the variable resistance elements R 11 , R 12  . . . . 
     The method for determining V 1  and V 2  is described later. 
     In  FIG. 20  ( c ), Vread refers to a voltage for reading, which is generated by the sense amplifier  204  and has a value of a voltage which causes a voltage equal to or less than the threshold voltage for high resistance writing V HR  to be applied to the variable resistance elements R 11 , R 12  . . . . 
     Furthermore, in  FIG. 20  ( a ) to (c), VDD corresponds to a power voltage supplied to the nonvolatile storage device  200 . 
     In the write cycle in which the data “0” is written into the memory cell M 11  as shown in  FIG. 20  ( a ), firstly, the selected bit line BL 0  and the selected source line SL 0  are set to the voltage V 2 . Next, the selected word line WL 0  is set to the voltage VDD to turn on the NMOS transistor N 11  of the selected memory cell M 11 . Current does not flow at this stage because the voltage V 2  is applied to both the second N-type diffusion layer region  302   a  and the first N-type diffusion layer region  302   b  of the transistor  317 . 
     Then, the selected bit line BL 0  is set to the voltage 0 V for a predetermined period, and after the predetermined period, a pulse waveform corresponding to the voltage V 2  is applied again to the bit line BL 0 . At this stage, in the variable resistance element  309 , a negative voltage having an absolute value larger than the threshold voltage for low resistance writing V AR  is applied to the upper electrode  309   c  with reference to the lower electrode  309   a , so that a writing is performed to change from a high resistance value to a low resistance value. Then, the word line WL 0  is set to the voltage 0 V to turn off the transistor  317 , thereby completing the writing of the data “0”. 
     In the write cycle in which the data “1” is written into the memory cell M 11  as shown in  FIG. 20  ( b ), firstly, the selected bit line BL 0  and the selected source line SL 0  are set to the voltage 0 V. Next, the selected word line WL 0  is set to the voltage VDD to turn on the NMOS transistor N 11  of the selected memory cell M 11 . 
     Then, the selected bit line BL 0  is set to the voltage V 1  for a predetermined period, and after the predetermined period, a pulse waveform corresponding to the voltage 0 V is applied again to the bit line BL 0 . At this stage, in the variable resistance element  309 , a positive voltage higher than the threshold voltage for high resistance writing V HR  is applied to the upper electrode  309   c  with reference to the lower electrode  309   a , so that a writing is performed to change from a low resistance value to a high resistance value. Then, the word line WL 0  is set to the voltage 0 V, thereby completing the writing of the data “1”. 
     In the read cycle in which data is read from the memory cell M 11  as shown in  FIG. 20  ( c ), firstly, the selected bit line BL 0  and the selected source line SL 0  are first set to the voltage 0 V. Next, the selected word line WL 0  is set to the voltage VDD to turn on the NMOS transistor N 11  of the selected memory cell M 11 . 
     Then, the selected bit line BL 0  is set to the read voltage Vread for a predetermined period, and the sense amplifier  204  detects the value of the current flowing in the selected memory cell M 11  to determine whether the stored data is the data “0” or the data “1”. Then, the word line WL 0  is set to the voltage 0 V, thereby completing the data reading operation. 
     (Characteristics of 1T1R Memory Cells) 
     The following describes the 1T1R memory cells M 11 , M 12  . . . according to an embodiment, and especially describes the structures of the NMOS transistors N 11 , N 12  . . . . 
     According to the present embodiment, as shown in the enlarged view in  FIG. 19 , the variable resistance layer  309  has a structure which includes, on the upper electrode  309   c  side, the conductive, second oxygen-deficient tantalum oxide layer  309   b - 2  having a composition TaO y  (where y=2.47) and a higher oxygen content percentage. Application of a positive voltage to the upper electrode  309   c  with reference to the lower electrode  309   a  appears to cause a progress in the oxidation phenomenon near the interface between the upper electrode  309   c  and the second oxygen-deficient tantalum oxide layer  309   b - 2  and thus the resistance state changes into the high resistance state, whereas application of a voltage in the opposite direction appears to cause a progress in a redox phenomenon and thus the resistance state changes into the low resistance state. This means that it is possible to fix, to one pattern, the resistance change state with respect to the voltage application direction. 
       FIG. 21  shows a relationship between a voltage applied to a variable resistance element and the resistance value of the variable resistance element when 2.2 V is applied to both ends of a memory cell. 
     A voltage application direction  1  shows the characteristics observed when a predetermined positive voltage is applied to the bit lines BL 0 , BL 1  . . . and 0 V is applied to the source lines SL 0 , SL 1  . . . in  FIG. 18 ; that is, the characteristics observed when a positive voltage is applied to the upper electrode  309   c  with reference to the lower electrode  309   a.    
     In addition, a voltage application direction  2  shows the characteristics observed when 0 V is applied to the bit lines BL 0 , BL 1  . . . and a predetermined positive voltage is applied to the source lines SL 0 , SL 1  . . . in  FIG. 18 ; that is, the characteristics observed when a negative voltage is applied to the upper electrode  309   c  with reference to the lower electrode  309   a.    
     For example, when the element resistance value is 1000Ω, the voltage application direction  1  indicates that approximately 2.1 V can be applied to the variable resistance element and that a current of 2.1 mA can be driven (2.1 V/1000Ω). In the case of the voltage application direction  2 , it indicates that approximately 1.25 V can be applied to the variable resistance element and that a current of 1.25 mA can be driven (1.25 V/1000Ω). 
     This shows that in the voltage application direction  1  in which the influence of the substrate bias effect in the NMOS transistor is small, a current 1.7 times larger than in the case of the voltage application direction  2  can be driven. 
     Further, the value of the voltage V 2  generated by the power source for LR writing  212  shown in  FIG. 20  ( a ) can be determined using the characteristics of the voltage application direction  2 . 
     For instance, when the resistance value of the variable resistance element  309  is 10 kΩ in the high resistance state, it is clear that with application of 2.2 V to the both ends of the memory cell, a voltage up to approximately 1.5 V can be applied to the variable resistance element  309  (point C in  FIG. 21 ). The current value at this time is 0.15 mA (1.5 V/10 kΩ). 
     When the threshold voltage for low resistance writing V LR , which is applied for the resistance change to low resistance state of the variable resistance element  309 , is −1.1 V (point B in  FIG. 10 ), for example, a voltage larger than the threshold voltage for low resistance writing V AR  can be applied to the variable resistance element  309  given that the voltage V 2  is set to 2.2 V and there is a current drive capability of 0.15 mA or greater in the power source for LR writing  212 . 
     Likewise, the value of the voltage V 1  generated by the power source for HR writing  213  shown in  FIG. 20  ( b ) can be determined using the characteristics of the voltage application direction  1 . 
     For instance, when the resistance value of the variable resistance element  309  is 1000Ω in the low resistance state, it is clear that with application of 2.2 V to the both ends of the memory cell, a voltage up to approximately 2.1 V can be applied to the variable resistance element  309  (point D in  FIG. 21 ). The current value at this time is 2.1 mA (2.1 V/1000Ω). 
     When the threshold voltage for high resistance writing V HR , which is applied for the resistance change to high resistance state of the variable resistance element  309 , is 1.2 V (point A in  FIG. 10 ), for example, the voltage V 1  is set to 2.2 V by the power source for HR writing  213 , and when there is a current drive capability of 2.1 mA or greater, a voltage larger than the threshold voltage for high resistance writing V HR  can be applied to the variable resistance element  309 . More preferably, the voltage V 1  may be determined to be a lower voltage (for instance, 1.8 V) with a voltage value having a margin above a certain level. 
     It is also possible, at the designing stage, to determine an approximate voltage using the above-mentioned method, and at the product inspection stage, to also use a commonly-known conventional method of making, while checking the operation, fine adjustments so as to determine optimum voltages of the voltages V 1  and V 2  at which changes in the resistance occur stably. 
     As described above, the variable resistance nonvolatile storage device of the present embodiment includes the variable resistance element in which: the second oxygen-deficient tantalum oxide layer having a higher oxygen content percentage is provided on the upper electrode side; and the first oxygen-deficient tantalum oxide layer having a lower oxygen content percentage is provided on the lower electrode side. Thus, the voltage application direction (driving polarity) which stably causes a resistance change in one direction (resistance change to low resistance state or resistance change to high resistance state) is uniquely determined for each of the memory cells. 
     Further, the memory cell is formed by connecting the lower electrode and one of the N-type diffusion layer regions of the NMOS transistor. Therefore, voltage application for the resistance change from the low resistance to the high resistance, which requires a larger current, can be reliably performed in the voltage application direction  1 , thereby eliminating the need of taking into account the case of the voltage application direction  2 . As a result, the memory cell can be designed with an optimum transistor size. 
     This means that especially the voltage V 1  of the power source for HR writing can be further reduced when there is a margin in the current drive capability, thereby leading to reduction in voltage and reduction in power consumption. 
     In addition, because the driving polarity is uniquely determined, there is no need to manage information identifying the modes of the resistance change characteristics, thereby allowing for a simple and inexpensive circuit configuration. 
     (Other Structural Examples of 1T1R Memory Cell) 
       FIG. 22  ( a ) to (f) are circuit diagrams each showing a circuit configuration of a 1T1R memory cell used for a commonly-known variable resistance element, including the 1T1R memory cell described in the embodiment. 
       FIG. 22  ( a ) shows a configuration in which the NMOS transistor described in the embodiment is used. 
       FIG. 22  ( b ) shows a configuration which differs from the configuration shown in  FIG. 22  ( a ) in that the bit line and the source line are interchanged. 
       FIG. 22  ( c ) shows a configuration which differs from the configuration shown in  FIG. 22  ( b ) in that the source line is connected to a reference power source which supplies a fixed reference voltage. In this case, the writing state is controlled by raising or lowering the bit line voltage with reference to the reference voltage. 
       FIG. 22  ( d ) shows a configuration which differs from the configuration shown in  FIG. 22  ( a ) in that a PMOS transistor is used instead of an NMOS transistor. In this case, a high electrical potential such as the power supply voltage VDD is supplied as the substrate voltage for the PMOS transistor. In addition, although  FIG. 22  ( d ) differs from  FIG. 22  ( a ) in that the memory cell is selected by setting the word line to a low level, other control methods are the same as in the configuration shown in  FIG. 22  ( a ) in which the NMOS transistor is used. 
       FIG. 22  ( e ) shows a configuration which differs from the configuration shown in  FIG. 22  ( d ) in that the bit line and the source line are interchanged. 
       FIG. 22  ( f ) shows a configuration which differs from the configuration shown in  FIG. 22  ( e ) in that the source line is connected to a reference power source which supplies a fixed reference voltage. In this case, the writing state is controlled by raising or lowering the bit line voltage with reference to the reference voltage. 
       FIG. 23  ( a ) to (f) are diagrams each showing how a variable resistance element and a transistor according to the present invention are connected for realizing the circuits shown in  FIG. 22  ( a ) to (f). 
     Here, a variable resistance layer  309   e  comprises an oxygen-deficient tantalum oxide as is the variable resistance layer  309   b , and has a structure in which: a second oxygen-deficient tantalum oxide layer  309   e - 2  having a higher oxygen content percentage is provided in contact with the lower electrode; and a first oxygen-deficient tantalum oxide layer  309   e - 1  having a lower oxygen content percentage is provided in contact with the upper electrode. 
     The description of  FIG. 23  ( a ) is omitted since the configuration shown therein is identical to the configuration shown in  FIG. 19 . 
       FIG. 23  ( b ) shows a configuration which differs from the configuration shown in  FIG. 23  ( a ) in that the bit line and the source line are interchanged. In the configuration of  FIG. 23  ( b ), the second oxygen-deficient tantalum oxide layer  309   b - 2  having a higher oxygen content percentage is provided in contact with the upper electrode, and the first oxygen-deficient tantalum oxide layer  309   b - 1  having a lower oxygen content percentage is provided in contact with the lower electrode. The upper electrode  309   c  that is in contact with the interface which easily causes a resistance change (that is, the second oxygen-deficient tantalum oxide layer  309   b - 2 ) is connected to the source line, whereas the lower electrode  309   a  that is in contact with the interface which does not easily cause a resistance change (that is, the first oxygen-deficient tantalum oxide layer  309   b - 1 ) is connected to the bit line via an NMOS transistor. 
     In this case, as in the case of  FIG. 23  ( a ), the source line and the word line are wired in the same direction, and the bit line is wired vertically to the source line and the word line. 
     In the configuration shown in  FIG. 23  ( c ), the upper electrode  309   c  that is in contact with the interface which easily causes a resistance change is connected to a reference power source, whereas the lower electrode  309   a  that is in contact with the interface which does not easily cause a resistance change is connected to the bit line via an NMOS transistor. 
     Unlike  FIG. 23  ( a ),  FIG. 23  ( d ) shows a configuration in which: the second oxygen-deficient tantalum oxide layer  309   e - 2  having a higher oxygen content percentage is provided on the lower electrode side; and the first oxygen-deficient tantalum oxide layer  309   e - 1  having a lower oxygen content percentage is provided on the upper electrode side. An upper electrode  309   f  that is in contact with the interface which does not easily cause a resistance change (that is, the first oxygen-deficient tantalum oxide layer  309   e - 1 ) is connected to the bit line, whereas a lower electrode  309   d  that is in contact with the interface which easily causes a resistance change (that is, the second oxygen-deficient tantalum oxide layer  309   e - 2 ) is connected to the source line via a PMOS transistor. In this case, as in  FIG. 23  ( a ), the source line and the word line are wired in the same direction, and the bit line is wired vertically to the source line and the word line. 
       FIG. 23  ( e ) shows a configuration which differs from the configuration shown in  FIG. 23  ( d ) in that the bit line and the source line are interchanged, and that the upper electrode  309   f  that is in contact with the interface which does not easily cause a resistance change is connected to the source line, whereas the lower electrode  309   d  that is in contact with the interface which easily causes a resistance change is connected to the bit line via a PMOS transistor. 
     In this case, as in  FIG. 23  ( d ), the source line and the word line are wired in the same direction, and the bit line is wired vertically to the source line and the word line. 
       FIG. 23  ( f ) shows a configuration in which the upper electrode  309   f  that is in contact with the interface which does not easily cause a resistance change is connected to a reference power source, whereas the lower electrode  309   d  that is in contact with the interface which easily causes a resistance change is connected to the bit line via a PMOS transistor. 
       FIG. 24  is a cross-sectional view corresponding to the part C (two bits) shown in  FIG. 18  and an enlarged view of one of variable resistance elements  409  in the case where 1T1R memory cells  400  according to the configuration shown in  FIG. 23  ( d ) which include PMOS transistors are used in a nonvolatile storage device. It is to be noted that the same numeral references are given to the structural elements common to the memory cells  300  shown in  FIG. 19  and descriptions thereof will not be repeated. 
     The memory cells  400  are formed by sequentially forming, above a semiconductor substrate  301 , an N well  418 , a second P-type diffusion layer region  402   a , first P-type diffusion layer regions  402   b , gate insulator films  303   a , gate electrodes  303   b , first vias  304 , first wiring layers  305 , second vias  306 , second wiring layers  307 , third vias  308 , the variable resistance elements  409 , fourth vias  310 , and a third wiring layer  311 . 
     The third wiring layer  311  connected to the fourth vias  310  corresponds to the bit line BL 0 , and the first wiring layers  305  and the second wiring layers  307  which are connected to the second P-type diffusion layer region  402   a  of transistors  417  correspond to the source line SL 0  extending perpendicularly to the diagram. The N well is supplied with a power supply voltage VDD of a nonvolatile storage device  200  through a commonly-known structure via a VDD power source line (not shown). 
     As shown in the enlarged view in  FIG. 24 , each of the variable resistance elements  409  is formed by providing, on the third via  308 , the variable resistance layer  309   e  between the lower electrode  309   d  and the upper electrode  309   f , and each variable resistance element  409  is connected to one of the fourth vias  310  connected to the third wiring layer  311 . 
     Here, unlike in the case of the 1T1R memory cell ( FIG. 23  ( a ) to (c)) including the NMOS transistor, the 1T1R memory cell ( FIG. 23  ( d ) to (f)) including the PMOS transistor has a structure in which: the second oxygen-deficient tantalum oxide layer  309   e - 2  which easily causes a resistance change and has a higher oxygen content percentage is provided on the side of the lower electrode  309   d  which is connected to one of the first P-type diffusion layer regions  402   b  of the transistors  417 ; and the first oxygen-deficient tantalum oxide layer  309   e - 1  which does not easily cause a resistance change and has a lower oxygen content percentage is provided on the upper electrode  309   f  side. 
     The driving direction of the transistors  417  in which the influence of the substrate bias effect is small and the current drive capability is large is the direction in which the second P-type diffusion layer region  402   a  serves as a source and a voltage at the second P-type diffusion layer region  402   a  approaches the voltage (VDD) at the N well  418  serving as the substrate voltage for the PMOS transistor; that is, it is the direction in which the lower electrode  309   d  is set to a high level and the upper electrode  309   f  is set to a low level. 
     In order to match the resistance change direction from the low resistance state to the high resistance state, which requires a larger current, with the above-mentioned voltage application direction, the second oxygen-deficient tantalum oxide layer  309   e - 2  having a higher oxygen content percentage is provided on the lower electrode  309   d  side, and the first oxygen-deficient tantalum oxide layer  309   e - 1  having a lower oxygen content percentage is provided on the upper electrode  309   f  side. When a positive voltage is applied to the lower electrode  309   d  with reference to the upper electrode  309   f , the oxidation phenomenon progresses near the interface of the lower electrode  309   d , thereby causing a change into the high resistance state. 
     Although the NMOS transistor is often used in the 1T1R memory cell in general, there could be cases where the PMOS transistor is used in the memory cell, including the following cases: 
     For instance, there is a case where in a selected memory cell, only a threshold voltage of a transistor of the memory cell is set low in order to obtain a larger current for driving the transistor. In this case, there is an increase in leak current to non-selected memory cells connected to a bit line to which the selected memory cell belongs. This will probably result in deterioration in the read characteristics. 
     As a method for avoiding the increase in the leak current while maintaining the driving current of the selected memory cell, there is a method of electrically dividing a region of the semiconductor substrate  301  into some blocks and adjusting substrate voltage of the blocks so as to increase the threshold voltage of transistors in blocks other than a block to which the selected memory cell belongs, thereby reducing the leak current. 
     Generally, many CMOS semiconductor devices use a P-type silicon semiconductor for the semiconductor substrate  301 . Thus, to implement such a structure, in the case of forming the transistor of the memory cell with an NMOS transistor, it is necessary to adopt a well structure known as a triple-well structure, for instance, and to electrically divide the substrate region into some blocks. In such a case, a new manufacturing step needs to be added, which leads to an increase in cost. 
     On the other hand, in the case of forming the transistor of the memory cell with a PMOS transistor, it is only necessary to design the layout of the N well  418  in a desired unit and electrically divide the N well  418  into some blocks. Thus, there seems to be an advantage that the N well  418  can be divided into the blocks without an additional manufacturing step. 
     The cross-sectional views shown in  FIGS. 19 and 24  correspond to  FIGS. 23  ( a ) and  23  ( d ), respectively. 
     The cross-sectional views corresponding to  FIGS. 23  ( b ) and  23  ( c ) showing the configurations using the NMOS transistor are the same as the cross-sectional view in  FIG. 19  except for the wiring layer to which the source line, the bit line, and the reference power source are connected. Thus, the descriptions of the cross-sectional views corresponding to  FIGS. 23  ( b ) and  23  ( c ) are omitted. 
     Further, the cross-sectional views corresponding to  FIGS. 23  ( e ) and  23  ( f ) showing the configurations using the PMOS transistor are the same as the cross-sectional view in  FIG. 24  except for the wiring layer to which the source line, the bit line, and the reference power source are connected. Thus, the descriptions of the cross-sectional views corresponding to  FIGS. 23  ( e ) and  23  ( f ) are omitted. 
     Table 3 shows, for each of the memory cell structures corresponding to  FIG. 23  ( a ) to (f), a method for controlling the bit line and the source line in the case where low resistance writing is performed on the resistance element and in the case where high resistance writing is performed on the resistance element. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 3 
               
             
            
               
                   
                   
               
               
                   
                 High Resistance 
                 Low Resistance 
               
               
                   
                 Writing 
                 Writing 
               
            
           
           
               
               
               
               
               
               
            
               
                 Memory 
                 Corresponding 
                   
                 Source 
                   
                 Source 
               
               
                 Cell Type 
                 Diagram 
                 Bit Line 
                 Line 
                 Bit Line 
                 Line 
               
               
                   
               
               
                 a 
                 FIG. 23(a) 
                 H 
                 L 
                 L 
                 H 
               
               
                 b 
                 FIG. 23(b) 
                 L 
                 H 
                 H 
                 L 
               
               
                 c 
                 FIG. 23(c) 
                 L 
                   
                 H 
               
               
                 d 
                 FIG. 23(d) 
                 L 
                 H 
                 H 
                 L 
               
               
                 e   
                 FIG. 23(e) 
                 H 
                 L 
                 L 
                 H 
               
               
                 f 
                 FIG. 23(f) 
                 H 
                   
                 L 
               
               
                   
               
               
                     indicates data missing or illegible when filed 
               
            
           
         
       
     
     For each memory cell, the voltage application direction (driving polarity) which stably causes a resistance change in one direction (resistance change to low resistance state or resistance change to high resistance state) is uniquely determined based on Table 3. Thus, there is no need to manage the information identifying the modes of the resistance change characteristics, thereby allowing for a simple circuit configuration. 
     It is to be noted that although Pt is used as the electrode material in the present embodiment, Ir, Pd, Ag or Cu may be used instead. 
     Although the present embodiment has shown the example of the variable resistance element in which the tantalum oxide is used as the variable resistance layer, the material of the variable resistance layer is not limited to the tantalum oxide. To manufacture an element having the same structure as in the present embodiment, it is possible to use any material as long as it stably shows the resistance change characteristics in A mode or B mode. 
     The inventors of the present application have acquired from an experiment basic data which allows inference that the resistance change characteristics in A mode or B mode stably occur in, for example, a variable resistance element in which a hafnium oxide is used as the variable resistance layer and in a variable resistance element in which a zirconium oxide is used as the variable resistance layer. 
     The following describes that experiment. 
     (Variable Resistance Element Using Oxygen-Deficient Hafnium Oxide as Variable Resistance Layer) 
     The following describes the fifth experiment regarding a variable resistance nonvolatile storage element which uses an oxygen-deficient hafnium oxide as the variable resistance layer. 
     (Structure of Variable Resistance Element) 
       FIG. 25  is a cross-sectional view showing a structural example of the variable resistance element used in the fifth experiment. 
     As shown in  FIG. 25 , a variable resistance element  1100  used in the fifth experiment includes: a substrate  1101 ; an oxide layer  1102  formed on the substrate  1101 ; a lower electrode  1103  formed on the oxide layer  1102 ; an upper electrode  1108 ; and a variable resistance layer  1107  located between the lower electrode  1103  and the upper electrode  1108 . 
     Here, the variable resistance layer  1107  includes: a second hafnium containing layer (hereinafter referred to as “second hafnium oxide layer”)  1105  having a low oxygen content percentage; a first hafnium containing layer (hereinafter referred to as “first hafnium oxide layer”)  1104  formed under the second hafnium oxide layer  1105  and having an oxygen content percentage higher that that of the second hafnium oxide layer  1105 ; and a third hafnium containing layer (hereinafter referred to as “third hafnium oxide layer”)  1106  formed on the second hafnium oxide layer  1105  and having an oxygen content percentage higher that that of the second hafnium oxide layer  1105 . 
     The materials of the lower electrode  1103  and the upper electrode  1108  are, for example, Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), Ni (nickel), W (tungsten), Cu (copper), Al (aluminum), Ta (tantalum), Ti (titanium), TiN (titanium nitride), TaN (tantalum nitride), and TiAIN (titanium aluminum nitride). 
     Although the substrate  1101  may be a single-crystal silicon substrate or a semiconductor substrate, it is not limited to these substrates. The variable resistance layer  1107  can be formed at a relatively low substrate temperature and thus can be formed on a resin material and the like. 
     (Manufacturing Method of Variable Resistance Element) 
     Next, a manufacturing method of the variable resistance element  1100  used in the fifth experiment is described with reference to  FIG. 25 . 
     First, as shown in  FIG. 25 , thermal oxidation produces the oxide layer  1102  having a thickness of 200 nm on the substrate  1101  which is a single-crystal silicon. Then, by sputtering, a Pt thin film having a thickness of 100 nm is deposited on the oxide layer  1102  as the lower electrode  1103 . After that, the second hafnium oxide layer  1105  is formed on the lower electrode  1103  by reactive-sputtering a hafnium target in an atmosphere of an Ar gas and an O 2  gas. 
     The first hafnium oxide layer  1104  whose oxygen content percentage is higher than that of the second hafnium oxide layer  1105  is formed by exposure of the surface of the lower electrode  1103  to the air at the time of forming the second hafnium oxide layer  1105 . Further, after forming the second hafnium oxide layer  1105 , the third hafnium oxide layer  1106  whose oxygen content percentage is higher than that of the second hafnium oxide layer  1105  is formed by exposure to plasma of the Ar gas and the O 2  gas used in the sputtering. 
     This layered structure in which the first hafnium oxide layer  1104 , the second hafnium oxide layer  1105 , and the third hafnium oxide layer  1106  are stacked is the structure of the variable resistance layer  1107 . 
     Subsequently, by sputtering, a Pt thin film having a thickness of 150 nm is deposited on the third hafnium oxide layer  1106  as the upper electrode  1108 . 
     Lastly, photolithography processing and dry etching create an element region  1109 . The element region  1109  has a circular shape with a diameter of 3 μm. 
     With the above-described manufacturing method, plural variable resistance elements were manufactured under different manufacturing conditions. The following describes the details. 
     (Composition of Hafnium Oxide Layer) 
     First, the following describes the conditions under which the oxygen-deficient hafnium oxide layer of the fifth experiment was manufactured and an analysis of the oxygen content percentage in the fifth experiment. The oxygen-deficient hafnium oxide layer was manufactured by reactive sputtering, that is, a process of sputtering the hafnium target in an atmosphere of an Ar (argon) gas and an O 2  gas. 
     First, a substrate was placed in a sputtering apparatus and then the sputtering apparatus was vacuumed to about 3×10 −5  Pa. Then, sputtering was performed with hafnium used as the target, power set to 300 W, the total gas pressure of the argon gas and the oxygen gas set to 0.9 Pa, and the preset temperature of the substrate set to 30° C. In this experiment, five types of hafnium oxide layers were formed with the flow ratio of the O 2  gas to the Ar gas varied from 2% to 4%. 
     First, with an object of this experiment being an examination of the composition, SiO 2  deposited 200 nm in thickness on Si was used as the substrate, and the sputtering time was adjusted to make the thickness of the five types of the hafnium oxide layers about 50 nm. 
       FIG. 26  shows a result of analyzing, using the Rutherford backscattering spectrometry (RBS), the compositions of the hafnium oxide layers manufactured in the above manner. In  FIG. 26 , the points G, H, I, J, and K respectively show the compositions of the hafnium oxide layers of samples G, H, I, J, and K manufactured at five different O 2  gas flow ratios.  FIG. 26  shows that the oxygen content percentage of the hafnium oxide layer changes from approximately 37.7 at % (HfO 0.6 ) to approximately 69.4 at % (HfO 2.3 ) when the oxygen flow ratio is changed from 2% to 4%. 
     Although this experiment used the Rutherford backscattering spectrometry (RBS) for analyzing the hafnium oxide layers, other instrumental analysis techniques can also be used, including Auger electron spectroscopy (AES), fluorescent X-ray photoelectron spectroscopy analysis (XPS), and electron probe microanalysis (EPMA). 
     The above result made it clear that: the oxygen content percentage of the hafnium oxide layer can be controlled using the oxygen flow ratio; and a hafnium oxide layer (sample K) which seems to excessively contain oxygen was formed as well as oxygen-deficient hafnium oxide layers (samples G to 3) which have an oxygen content percentage lower than 66.7 at %, which is the oxygen content percentage of HfO 2  that is a stoichiometric oxide of hafnium. 
     (Oxygen Content Percentage and Resistance Change Characteristics of Hafnium Oxide Layer) 
     Fives types of variable resistance elements were manufactured by depositing a hafnium oxide as the variable resistance layer  1107  under the same conditions as for the samples G to K, and then forming the upper electrode  1108  on the variable resistance layer  1107 . The thickness of all the variable resistance layers  1107  was set to 30 nm. These elements are referred to as elements G, H, I, J, and K. 
     The resistance change characteristics of the nonvolatile storage elements manufactured in the above manner were measured. 
     First, the resistance change characteristics observed upon application of electrical pulses to the element I (the oxygen flow ratio of about 3.0% and the oxygen content percentage of the hafnium oxide layer of about 56.8 at %) were closely examined. 
       FIGS. 27  ( a ) and ( b ) each show how the resistance of the element I changes upon repeated application of electrical pulses. In  FIGS. 27  ( a ) and ( b ), the horizontal axis represents the number of electrical pulses applied between the lower electrode  1103  and the upper electrode  1108 , and the vertical axis represents the resistance value. 
     First,  FIG. 27  ( a ) shows a result of measuring the resistance value upon applying, between the upper electrode  1108  and the lower electrode  1103 , such electrical pulses that voltages at the upper electrode  1108  are +1.5 V and −1.2 V alternately with reference to the lower electrode  1103 , with the pulse width of 100 nsec. In this case, the application of the electrical pulses of the voltage +1.5 V changed the resistance value to be between 500 and 700Ω, whereas the application of the electrical pulses of the voltage −1.2 V changed the resistance value to be about 140Ω. In other words, the element I exhibited the resistance changes in B mode in which the resistance change to high resistance state is performed upon application of electrical pulses of a higher voltage to the upper electrode  1108  than to the lower electrode  1103 . 
     Although the details are omitted, an additional experiment showed a result indicating that the resistance changes in B mode occur near the upper electrode  1108 . 
     Next,  FIG. 27  ( b ) shows a result of measuring the resistance value when the applied voltages are changed such that a larger negative voltage is applied. In this case, electrical pulses having voltages of −1.5 V and +1.2 V were applied alternately to the upper electrode  1108  with reference to the lower electrode  1103 . The application of the electrical pulses of the voltage −1.5 V caused the resistance change to high resistance state with the resistance value between 900 and 1200Ω, whereas the application of the electrical pulses of the voltage +1.2 V caused the resistance change to low resistance state with the resistance value of about 150Ω. In other words, the element I exhibited the resistance changes in A mode in which the resistance change to low resistance state is performed upon application of electrical pulses of a higher voltage to the upper electrode  1108  than to the lower electrode  1103 . 
     Although the details are omitted, an additional experiment showed a result indicating that the resistance changes in A mode occur near the lower electrode  1103 . 
     Further, an experiment was conducted with the other elements in the same manner as the element I, so as to find out with what percentage of oxygen content in an oxygen-deficient hafnium oxide used for the variable resistance layer, the elements exhibit resistance changes. 
     The result of the experiment showed that aside from the element I, the element H (the oxygen flow ratio of about 2.7% and the oxygen content percentage of the hafnium oxide layer of about 46.6 at %) and the element  3  (the oxygen flow ratio of about 3.3% and the oxygen content percentage of the hafnium oxide layer of about 62 at %) exhibit favorable resistance changes in which the high resistance value is equal to or greater than four times the low resistance value. 
     From the above findings, it can be said that the bipolar variable resistance element exhibits high-speed resistance changes when its variable resistance layer is manufactured at an oxygen flow ratio of 2.6% to 3.3% and has an oxygen content percentage of 46.6 to 62 at %. In other words, when the variable resistance layer is expressed as HfO x , a more appropriate compositional range of the variable resistance layer is 0.9≦x≦1.6 (the oxygen content percentage of 46.6 at % and the oxygen content percentage of 62 at % correspond to x=0.9 and x=1.6, respectively). 
     The element G, that is, the variable resistance element which was manufactured at the oxygen flow ratio of 2.0% and includes the hafnium oxide layer having an oxygen content percentage of about 37.7 at % (HfO 0.6 ), has a small initial resistance. Thus, the forming was not possible, and the resistance did not change. Also, the element K, that is, the non-volatile element which was manufactured at the oxygen flow ratio of 4.2% and includes the hafnium oxide layer having an oxygen content percentage of about 69.4 at % (HfO 2.3 ), has a great initial resistance. Thus, soft breakdown was not possible even upon application of a direct current voltage of 5 V, and the resistance did not change. 
     (Analysis of Variable Resistance Layer) 
     Since the resistance changes in A mode and B mode appear to occur near the upper electrode or near the lower electrode, a closer analysis of the structure of the oxygen-deficient hafnium oxide layer near the upper electrode and the lower electrode was further conducted. 
     To analyze the structure of the variable resistance layer  1107  included in the variable resistance element  1100 , a hafnium oxide layer of the sample I was formed to have an oxygen content percentage of 56.8% and a thickness of 50 nm. Since the analysis requires a relatively large region in which an oxygen-deficient hafnium oxide is deposited, a sample specifically for the analysis was separately prepared by depositing an oxygen-deficient hafnium oxide on a substrate with no element pattern, in which an oxide layer having a thickness of 200 nm is formed on a single-crystal silicon substrate. 
     In this experiment, too, using the above-described X-ray reflectometry, an X-ray was emitted to enter the sample surface at a shallow angle, and the intensity of the X-ray reflected was measured. Then, fitting was performed presuming an appropriate structural model for an obtained spectrum, so as to evaluate the thickness and the refractive index of the variable resistance layer in the analysis sample which was specifically prepared for the analysis. Here, the parameters for the fitting are the layered structure of the oxygen-deficient hafnium oxide and the thickness and δ (=1−refractive index) of each layer. 
       FIGS. 28  ( a ) and ( b ) show the measurement results. In these graphs, the horizontal axis represents the incident angle of the X-ray, and the vertical axis represents the reflectivity of the X-ray. Here, an angle θ between the X-ray and the sample surface and a detector angle (angle θ from the sample surface) were changed in coordination with each other, so as to measure transition in the reflectivity of the X-ray on the sample surface. The angle between an extension of the incident X-ray and the detector is 2θ. 
       FIG. 28  ( a ) shows a pattern observed at the time of actually measuring the X-ray reflectivity of the analysis sample (broken line) and a result of fitting which was performed presuming that a single oxygen-deficient hafnium oxide layer exists on the substrate (solid line), whereas  FIG. 28  ( b ) shows a reflectivity pattern observed at the time of actually measuring the X-ray reflectivity of the analysis sample (broken line) and a result of fitting which was performed presuming that three oxygen-deficient hafnium oxide layers exist on the substrate (solid line). 
     Referring to  FIG. 28  ( a ), while the measured values and the fitting result almost match, there are differences at fine levels. On the other hand, referring to  FIG. 28  ( b ), the actually measured reflectivity pattern matches well with the reflectivity pattern obtained from the fitting to the extent that they are hardly distinguishable. 
     The above results appear to indicate that the oxygen-deficient hafnium oxide layer includes: the first oxygen-deficient hafnium oxide layer  1104  on the lower electrode side; the second oxygen-deficient hafnium oxide layer  1105  in the middle; and the third oxygen-deficient hafnium oxide layer  1106  on the upper electrode side. 
     The analysis result obtained when fitting was performed presuming this triple-layered structure showed the following values: the thickness of the first oxygen-deficient hafnium oxide layer was 3.9 nm and δ was 24.2×10 −6 ; the thickness of the second oxygen-deficient hafnium oxide layer was 45.5 nm and δ was 26.0×10 −6 ; and the thickness of the third oxygen-deficient hafnium oxide layer was 3.3 nm and δ was 24.2×10 −6 . 
     Although it is difficult to derive a correct composition from these δ values, a rough estimation can be made from the facts that δ of hafnium metals is 31.2×10 −6  and δ of HfO 2  having a stoichiometric composition is 24.2×10 −6 , and so on. That is to say, since δ of the second oxygen-deficient hafnium oxide layer is intermediate between δ of hafnium metals and δ of HfO 2 , it appears to be a hafnium oxide having a non-stoichiometric composition as originally intended. 
     In addition, based on the δ values, the first and the third oxygen-deficient hafnium oxide layers are predicted to be HfO 1.94  approximately, and are thus estimated to be hafnium oxides very close to HfO 2  (oxygen content percentage of 66.7%) having a stoichiometric composition. 
     The following describes the analysis using the X-ray reflectometry. 
     First, for the purposes of simplifying later calculations, fitting is performed presuming that the variable resistance layer has a double-layered structure. More specifically, the fitting is performed to calculate δ and the thickness of a high oxygen containing layer and a low oxygen containing layer, assuming that the high oxygen containing layer exists near the upper electrode and that the layer other than the high oxygen containing layer is the low oxygen containing layer (calculation 1). It is to be noted that the fitting is performed using a least squares method. 
     The calculation 1 roughly determines the values of the thickness and δ of the high oxygen containing layer which exists near the upper electrode. 
     The following calculation is performed assuming that the variable resistance layer has a triple-layered structure. 
     Fitting is performed to calculate a new δ and thicknesses of the first, second, and third variable resistance films under such conditions that δ and the thickness of the high oxygen containing layer are the initial values of δ and the thickness of the first variable resistance film and that δ of the first variable resistance film is the same as δ of the third variable resistance film (calculation 2). With this calculation, δ and the thicknesses of the first, second, and third variable resistance films were determined through the fitting. 
     Generally, in the case of using the X-ray reflectometry, the layer farthest from the surface in the triple-layered structure cannot be easily measured. The reason why this calculation of the layered structure of the variable resistance films was performed using highly precise data on the third variable resistance layer as the data on the first layer is because the element I exhibited a resistance change phenomenon both in B mode and A mode, which led to an inference that a high oxygen containing layer similar to the third layer was formed also near the lower electrode. 
     Table 4 shows the result of measuring the samples G to K using the X-ray reflectometry in the same manner as above. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Element G 
                 Element H 
                 Element I 
                 Element J 
                 Element K 
               
               
                   
                 (Sample G) 
                 (Sample H) 
                 (Sample I) 
                 (Sample J) 
                 (Sample K) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Oxygen Flow Ratio (%) 
                 2.0 
                 2.7 
                 3.0 
                 3.3 
                 4.0 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 X-ray 
                 Third 
                 Thickness (nm) 
                 3.6 
                 3.4 
                 3.3 
                 3.6 
                 — 
               
               
                 Reflectivity 
                 Layer 
                 δ 
                 24.5 × 10 −6   
                 24.4 × 10 −6   
                 24.2 × 10 −6   
                 24.1 × 10 −6   
                 — 
               
               
                 Measurement 
                   
                 y 
                  1.86 
                  1.89 
                  1.94 
                  1.97 
                 — 
               
               
                 Result 
                 Second 
                 Thickness (nm) 
                 38.1  
                 42.7  
                 45.5  
                 47.7  
                 60.3  
               
               
                   
                 Layer 
                 δ 
                 28.9 × 10 −6   
                 27.2 × 10 −6   
                 26.0 × 10 −6   
                 25.3 × 10 −6   
                 22.7 × 10 −6   
               
               
                   
                   
                 x 
                  0.65 
                  1.12 
                  1.43 
                  1.65 
                  2.35 
               
               
                   
                 First 
                 Thickness (nm) 
                 2.7 
                 3.5 
                 3.9 
                 3.9 
                 — 
               
               
                   
                 Layer 
                 δ 
                 24.5 × 10 −6   
                 24.4 × 10 −6   
                 24.2 × 10 −6   
                 24.1 × 10 −6   
                 — 
               
               
                   
                   
                 y 
                  1.86 
                  1.89 
                  1.94 
                  1.97 
                 — 
               
               
                   
               
            
           
         
       
     
     From the result of measuring the samples H, I and J which were made under the same conditions as the well-operated elements H, I and J, respectively, it can be said that a preferable thickness of the m first or third layer is between 3 nm and 4 nm inclusive. It can also be said that a preferable y value of the first or third layer is 1.8&lt;y&lt;2. 
     It is to be noted that the first and third hafnium oxide layers can also be formed through deposition using sputtering or chemical vapor deposition. In the case of sputtering, for example, first, sputtering is performed under a condition of a high oxygen gas flow ratio at the time of deposition, so that a hafnium oxide having a high oxygen content percentage and high resistance can be formed. For example, in the case of the sputtering condition described above in relation to the manufacturing method of the variable resistance element  1100 , the hafnium oxide layers can be formed by setting the oxygen gas flow ratio to 4% approximately. 
     (Mechanism of Resistance Change Phenomenon) 
     The following considers the mechanism of the resistance change phenomenon observed in the fifth experiment, and examines based on the mechanism, the structure of the variable resistance element which can uniquely exhibit the resistance change phenomenon either in A mode or B mode. 
     First, it is to be noted that the roles of the first hafnium oxide layer  1104  and the third hafnium oxide layer  1106  are not clearly known because the mechanism of the resistance change phenomenon is not understood well at present. 
     In this experiment, however, considering that the resistance changes of the variable resistance nonvolatile element occur due to movements of oxygen ions in the interface between an electrode and a hafnium oxide layer, the first hafnium oxide layer  1104  and the third hafnium oxide layer  1106  may have a role of effectively applying a voltage to a region near the interface. That is, the resistance change phenomenon seems to occur as oxygen ions gather and diffuse near the interface between the lower electrode  1103  and the first hafnium oxide layer  1104  and near the interface between the upper electrode  1108  and the third hafnium oxide layer  1106  due to an electric field. 
     To be more specific, applying a positive voltage to the upper electrode  1108  causes negatively-charged oxygen ions to gather on the upper electrode  1108  side, forming a high resistance layer, which means that the element performs the resistance change to thigh resistance state. On the other hand, applying a negative voltage causes diffusion of the oxygen ions in the hafnium oxide layer, resulting in the resistance change to low resistance state of the element. 
     Here, since the third hafnium oxide layer  1106 , which is a high resistance layer, exists in contact with the upper electrode  1108 , a large voltage is applied to the third hafnium oxide layer  1106 , causing injection of oxygen into the hafnium oxide layer  1106 . As a result, the oxygen content percentage increases and the third hafnium oxide layer  1106  approaches HfO 2  having a stoichiometric composition known as an insulator. That is to say, the third hafnium oxide layer  1106  plays a part in the resistance changes. 
     The gathering and diffusion of oxygen ions can also take place on the lower electrode  1103  side. In that case, the first hafnium oxide layer  1104 , which is a high resistance layer provided in contact with the lower electrode  1103 , plays a part in the resistance changes. 
     With such a mechanism as described above, in the case where the third hafnium oxide layer  1106  plays a larger part in the resistance changes, it appears that the resistance changes in B mode occur; that is, the resistance change to high resistance state is performed upon application, to the upper electrode  1108  provided in contact with the third hafnium oxide layer  1106 , of electrical pulses having a voltage higher than a voltage applied to the lower electrode  1103 , whereas the resistance change to low resistance state is performed upon application of a negative voltage. 
     On the other hand, in the case where the first hafnium oxide layer  1104  plays a larger part in the resistance changes, it appears that the resistance changes in A mode occur; that is, the resistance change to high resistance state is performed upon application, to the lower electrode  1103  provided in contact with the first hafnium oxide layer  1104 , of electrical pulses having a voltage higher than a voltage applied to the upper electrode  1108 , whereas the resistance change to low resistance state is performed upon application of a negative voltage. 
     The experimental result that the element G, in which the first hafnium oxide layer  1104  and the third hafnium oxide layer  1106  have a low oxygen content percentage, did not exhibit the resistance changes indicates that the voltage is evenly applied to the hafnium oxide layer  1105  if the first hafnium oxide layer  1104  and the third hafnium oxide layer  1106 , which are high resistance layers, do not exist, and thus high resistance layers which are almost insulators are not easily formed near the electrodes. As a result, it appears that the resistance change phenomenon does not occur easily. 
     Further, taking into account that the hafnium oxide layers near the electrodes have a role of locally applying a large voltage to a region near the electrodes in the variable resistance layer, it seems that the resistance of the hafnium oxide layers near the electrodes need to be larger than the resistance of the second hafnium oxide layer, which is an oxygen supplying layer. For this reason, it is sufficient if the first or third hafnium oxide layer expressed as HfO y  satisfies x&lt;y&lt;2. As also for the thickness of the first or third hafnium oxide layer, it is sufficient as along as the thickness falls within a range which allows the first or third hafnium oxide layer to locally apply a large voltage. 
     From the viewpoint of simple manufacturing, the first and third hafnium oxide layers having a thickness of 1 nm or greater are suitable for implementation. Moreover, from the viewpoint of increase in the element resistance which is to be brought about by future miniaturization, a thickness of 5 nm or smaller is probably suitable for implementation. 
     (Preferable Structure of Variable Resistance Element using Hafnium Oxide as Variable Resistance Layer) 
     Such a mechanism as described above seems to indicate that the variable resistance element which can uniquely exhibit the resistance changes either in A mode or B mode can be obtained by providing only one of the first hafnium oxide layer  1104  and the third hafnium oxide layer  1106  of the variable resistance layer. 
     This idea is also supported by the results of the first to fourth experiments related to the variable resistance element in which a variable resistance layer containing a tantalum oxide is used; that is, the results that the variable resistance element in which a tantalum oxide layer having a higher oxygen content percentage is provided in contact with only one of the upper electrode and the lower electrode uniquely exhibited the resistance changes in A mode or B mode. 
       FIGS. 29  ( a ) and ( b ) are cross-sectional views each showing a structure of such a variable resistance element.  FIGS. 29  ( a ) and ( b ) omit the substrate and the oxide layer for convenience. 
     With a variable resistance element  1100 A shown in  FIG. 29  ( a ), the hafnium oxide layer  1104  having a higher oxygen content percentage is deposited only on the lower electrode  1103 . Sequentially depositing the hafnium oxide layer  1104  and the hafnium oxide layer  1105  makes the structure of a variable resistance layer  1107 A. 
     With the variable resistance element  1100 A having such a structure, the hafnium oxide layer  1104  provided in contact with the lower electrode  1103  appears to play a part in the resistance changes and to cause the resistance changes in A mode. 
     As for a variable resistance element  1100 B shown in  FIG. 29  ( b ), the hafnium oxide layer  1105 , the hafnium oxide layer  1106  having a higher oxygen content percentage, and the upper electrode  1108  are sequentially provided. Sequentially depositing the hafnium oxide layer  1105  and the hafnium oxide layer  1106  makes the structure of a variable resistance layer  1107 B. 
     With the variable resistance element  1100 B having such a structure, the hafnium oxide layer  1106  provided in contact with the upper electrode  1108  appears to play a part in the resistance changes and to cause the resistance changes in B mode. 
     The present invention also includes a variable resistance nonvolatile storage device having the variable resistance element  1100 A and a variable resistance nonvolatile storage device having the variable resistance element  1100 B. Such variable resistance nonvolatile storage devices also achieve the same advantageous effect as that of the variable resistance nonvolatile storage device described in the embodiment. 
     (Variable Resistance Element Using Oxygen-Deficient Zirconium Oxide as Variable Resistance Layer) 
     Next, the following describes the sixth experiment regarding a variable resistance nonvolatile storage element which uses an oxygen-deficient zirconium oxide as the variable resistance layer. 
     (Structure of Variable Resistance Element) 
       FIG. 30  ( c ) is a cross-sectional view showing a structural example of the variable resistance element used in the sixth experiment. 
     As shown in  FIG. 30  ( c ), a variable resistance element  2100  used in the sixth experiment includes: a substrate  2101 ; an oxide layer  2102  formed on the substrate  2101 ; a lower electrode  2103  formed on the oxide layer  2102 ; an upper electrode  2107 ; and a variable resistance layer  2106  located between the lower electrode  2103  and the upper electrode  2107 . Here, the variable resistance layer  2106  includes: a first zirconium oxide layer  2104  having a lower oxygen content percentage (hereinafter referred to as “first zirconium oxide layer”); and a second zirconium oxide layer  2105  formed on the first zirconium oxide layer  2104  and having a higher oxygen content percentage (hereinafter referred to as “second zirconium oxide layer”). 
     The materials of the lower electrode  2103  and the upper electrode  2107  are, for example, Pt (platinum), Ir (iridium), Pd (palladium), Ag (silver), Ni (nickel), W (tungsten), Cu (copper), Al (aluminum), Ta (tantalum), Ti (titanium), TiN (titanium nitride), TaN (tantalum nitride), and TiAIN (titanium aluminum nitride). 
     Although the substrate  2101  may be a single-crystal silicon substrate or a semiconductor substrate, it is not limited to these substrates. The variable resistance layer  2106  can be formed at a relatively low substrate temperature and thus can be formed on a resin material and the like. 
     (Manufacturing Method of Variable Resistance Element) 
     Next, a manufacturing method of the variable resistance element  2100  used in the sixth experiment is described with reference to  FIG. 30  ( a ) to (c). 
     First, as shown in  FIG. 30  ( a ), thermal oxidation produces the oxide layer  2102  having a thickness of 200 nm on the substrate  2101  which is a single-crystal silicon. Then, by sputtering, a Pt thin film having a thickness of 100 nm is deposited on the oxide layer  2102  as the lower electrode  2103 . After that, the first zirconium oxide layer  2104  is formed on the lower electrode  2103  by reactive sputtering, with zirconium used as the target. 
     Next, as shown in  FIG. 30  ( b ), the top surface of the first zirconium oxide layer  2104  is oxidized to modify the surface property. This forms, on the surface of the first zirconium oxide layer  2104 , the second zirconium oxide layer  2105  whose oxygen content percentage is higher than that of the first zirconium oxide layer  2104 . This layered structure in which the first zirconium oxide layer  2104  and the second zirconium oxide layer  2105  are stacked is the structure of the variable resistance layer  2106 . 
     Subsequently, by sputtering, a Pt thin film having a thickness of 150 nm is formed on the second zirconium oxide layer  2105  as the upper electrode  2107 . Lastly, photolithography processing creates a pattern  2108  using a photoresist, and dry etching produces an element region  2109 . 
     With the above-described manufacturing method, three types of variable resistance elements were manufactured under different manufacturing conditions. The following describes the details. 
     First, the layered structure of the substrate  2101 , the oxide layer  2102 , and the lower electrode  2103  comprising Pt was formed in the above-described manner. After that, the first zirconium oxide layer  2104  was formed on the lower electrode  2103  by reactive sputtering, that is, a process of sputtering a zirconium target in an argon gas and an oxygen gas ( FIG. 30  ( a )). 
     The film forming condition here was as follows: vacuum (back pressure) in the sputtering apparatus was approximately 2×10 −5  Pa before sputtering; power was 300 W when sputtering; the total gas pressure of the argon gas and the oxygen gas was 0.93 Pa; the flow ratios of the oxygen gas were 2.0%, 2.7% and 3.3%; the preset temperature of the substrate was 25° C.; and the time period for film formation was approximately four minutes. 
     With this, the first zirconium oxide layer  2104  was deposited with a thickness of about 30 nm to 40 nm and an oxygen content percentage of about 48%, about 59%, or about 67% corresponding to one of the above mentioned oxygen gas flow ratios; in other words, the first zirconium oxide layer  2104  was deposited which can be expressed as ZrO x , where x is 0.9, 1.4 or 2.0. 
     In manufacturing the variable resistance elements, the formation of the first zirconium oxide layer  2104  and the second zirconium oxide layer  2105  was followed by the formation of the upper electrode  2107 , both performed in the sputtering apparatus. More specifically, after depositing the first zirconium oxide layer  2104 , a shutter was inserted between the zirconium target and the oppositely placed substrate  2101  with no change to the sputtering condition, including the condition of the gas pressure and the power, and it was kept in this state for about 10 to 30 seconds. 
     This caused oxidation of the top surface of the first zirconium oxide layer  2104  due to oxygen plasma. As a result, the second zirconium oxide layer  2105  whose oxygen content percentage is higher than that of the first zirconium oxide layer  2104  was formed on the surface of the first zirconium oxide layer  2104 . 
     Thereafter, the upper electrode  2107  comprising Pt was formed on the second zirconium oxide layer  2105  in the above-described manner ( FIG. 30  ( b )). 
     Then, the element region  2109  was formed through the photolithography processing ( FIG. 30  ( c )). 
     With the above processes, three types of variable resistance elements were manufactured under different manufacturing conditions. These variable resistance elements are referred to as elements L, M, and N. It is to be noted that the element region  2109  of the elements L, M, and N has a circular pattern with a diameter of 3 μm. 
     (Analysis of Variable Resistance Layer) 
     To analyze the structure of the variable resistance layer  2106  in the sixth experiment, samples were prepared by, under the same conditions as that of the elements L, M, and N, depositing a zirconium oxide on a substrate in which an oxide layer having a thickness of 200 nm is formed on a single-crystal silicon substrate, followed by oxygen plasma irradiation. These samples are referred to as samples L, M, and N. 
     Table 5 summarizes the oxygen gas flow ratio and a later-described analysis of each sample. It is to be noted that Pt equivalent to the upper electrode  2107  is not deposited on the samples L to N, and thus the variable resistance layer  2106  is exposed. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                 Element L 
                 Element N 
                 Element M 
               
               
                   
                 (Sample L) 
                 (Sample N) 
                 (Sample M) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Oxygen Flow Ratio (%) 
                 2.0 
                 2.7 
                 3.3 
               
            
           
           
               
               
               
               
               
               
            
               
                 X-ray Reflectivity 
                 ZrO x  Layer 
                 Thickness (nm) 
                 33.5 
                 38.5 
                 41.9 
               
               
                 Measurement 
                 (First Zirconium 
                 δ 
                 17.8 × 10 −6   
                 17.2 × 10 −6   
                 16.4 × 10 −6   
               
               
                 Result 
                 Oxide Layer) 
                 x 
                 0.93 
                 1.42 
                 2.03 
               
               
                   
                 ZrO y  Layer 
                 Thickness (nm) 
                 5.0 
                 3.9 
                 — 
               
               
                   
                 (Second Zirconium 
                 δ 
                 16.7 × 10 −6   
                 16.5 × 10 −6   
                 — 
               
               
                   
                 Oxide Layer) 
                 y 
                 1.79 
                 1.97 
                 — 
               
               
                   
               
            
           
         
       
     
     In the sixth experiment, too, using the above-described X-ray reflectometry, an X-ray was emitted to enter the surface of each sample at a shallow angle, and the intensity of the X-ray reflected was measured. Then, fitting was performed presuming an appropriate structural model for an obtained spectrum, so as to evaluate the thickness and the refractive index of the variable resistance layer in the samples L to N. Here, the parameters for the fitting are the layered structure of the variable resistance layer and the thickness and δ (=1−refractive index) of each layer. 
       FIGS. 31  ( a ) and ( b ) and  FIG. 32  each show an example of the X-ray reflectivity measurement pattern of the samples M and N. Here, an angle θ between the X-ray and the sample surface and a detector angle (angle θ from the sample surface) were changed in coordination with each other, so as to measure transition in the reflectivity of the X-ray on the sample surface. The angle between an extension of the incident X-ray and the detector is 2θ. In  FIGS. 31  ( a ) and ( b ) and  FIG. 32 , the horizontal axis represents the incident angle θ of the X-ray, and the vertical axis represents the reflectivity of the X-ray. 
       FIG. 31  ( a ) shows a pattern observed at the time of actually measuring the X-ray reflectivity of the sample M (broken line) and a result of fitting which was performed presuming that a single zirconium oxide layer exists on the substrate (solid line).  FIG. 31  ( b ) shows a reflectivity pattern observed at the time of actually measuring the X-ray reflectivity of the sample M (broken line) and a result of fitting which was performed presuming that two zirconium oxide layers exist on the substrate (solid line).  FIG. 32  shows a pattern observed at the time of actually measuring the X-ray reflectivity of the sample N (broken line) and a result of fitting which was performed presuming that a single zirconium oxide layer exists on the substrate (solid line). 
     Referring to  FIG. 31  ( a ), while the measured values and the fitting result almost match, there are differences at fine levels. On the other hand, referring to  FIG. 31  ( b ), the actually measured reflectivity pattern matches well with the reflectivity pattern obtained from the fitting to the extent that they are hardly distinguishable. The above results appear to indicate that the sample M includes two different zirconium oxide layers, namely, the first and the second zirconium oxide layers. 
     The result of an analysis of the sample M when fitting was performed presuming this double-layered structure showed the following values as shown in Table 5: the thickness of the first zirconium oxide layer was 38.5 nm and δ was 17.2×10 −6 ; and the thickness of the second zirconium oxide layer was about 3.9 nm and δ was 16.5×10 −6 . 
     Generally, δ can be theoretically calculated based on the density of the film; δ of metal zirconium having a density of 6.798 g/cm 3  is 19.0×10 −6 , and δ of ZrO 2  having a density of 5.817 g/cm 3  is 16.9×10 −6 . Comparison between these values and the above obtained values appears to indicate that the first zirconium oxide layer is a zirconium oxide such as ZrO 1.42  which is an oxide deficient in oxygen and is clearly different from the stoichiometric composition of zirconium oxide. Further, as for the second zirconium oxide layer, calculating its compositional ratio using the δ value gives ZrO 1.97 . This indicates that the second zirconium oxide layer is an oxide similar to ZrO 2  yet seems to be an oxygen-deficient oxide which is different from the stoichiometric composition. 
     As shown in Table 5, the sample L exhibited almost the same results. More specifically, when the first zirconium oxide layer is expressed as ZrO x , its thickness is approximately 33.5 nm and x is about 0.93, and when the second zirconium oxide layer is expressed as ZrO y , its thickness is approximately 5.0 nm and y is about 1.79. 
     In comparison, referring to the analysis result of the sample N shown in  FIG. 32 , the result of fitting which was performed presuming that a single zirconium oxide layer exists on the substrate matches well with the actually measured reflectivity pattern. That means, in the sample N, it appears that the structure is not a double-layered structure and the second zirconium oxide layer does not exist. 
     Here, as shown in Table 5, when the single zirconium oxide layer is expressed as ZrO x , its thickness is approximately 41.9 nm and the x is about 2.0 which indicates a stoichiometric compositional ratio. 
     The element L and the sample L underwent the sputtering and the oxygen plasma irradiation under the same condition. The element M and the sample M also underwent the sputtering and the oxygen plasma irradiation under the same condition. Thus, in the elements L and M as in the samples L and M, the second zirconium oxide layer  2105  probably exists between the first zirconium oxide layer  2104  and the upper electrode  2107 . 
     Thus, it can be said that the second zirconium oxide layer  2105  having a thickness of 3.9 nm and a compositional ratio in which y is 1.97 is formed in the element M as in the sample M. 
     Although the sixth experiment used the X-ray reflectivity measurement for analyzing the second zirconium oxide layer, other instrumental analysis techniques can also be used, including Auger electron spectroscopy analysis (AES), fluorescent X-ray photoelectron spectroscopy analysis (XPS), and electron probe microanalysis (EPMA: also known as WDS, EDS, or EDX depending on the detection method). 
     (Variable Resistance Characteristics) 
     Next, the following describes: the characteristics observed when electrical pulses are applied to the element M to cause the resistance change in the element M, which is a representative example of the double-layered structure including the first zirconium oxide layer  2104  and the second zirconium oxide layer  2105  which were actually manufactured in the sixth experiment; and the characteristics observed when electrical pulses are applied to the element N in which the second zirconium oxide layer  2105  does not seem to exist. 
       FIGS. 33  ( a ) and ( b ) are graphs each showing a relationship between electrical pulses applied and the resistance value of the variable resistance layer included in the variable resistance element of the sixth experiment. It is to be noted that  FIGS. 33  ( a ) and ( b ) show the results of the elements M and N, respectively. Here, the resistance value of the variable resistance layer  2106  was measured while repeatedly applying two types of electrical pulses (positive voltage and negative voltage) alternately with a pulse width of 100 nsec between the lower electrode  2103  and the upper electrode  2107 . 
     First, referring to  FIG. 33  ( a ) showing the resistance change characteristics of the element M which was manufactured at the oxygen gas flow ratio of 2.7%, it is clear that application of electrical pulses having a positive voltage of +2.3 V to the sample immediately after measuring its initial resistance causes a decrease in the resistance value from about 500 kΩ to about 3 kΩ. This is known as a forming process, and since the initial resistance of about 500 kΩ is very high, the resistance value needs adjustment to be close to a resistance change range of 1105 to 3 kΩ. Further, the forming process only involves a single application of pluses having a positive voltage of +2.3 V, and thus as compared to the conventional technique, it is simpler and requires a lower voltage. 
     After that, the element M exhibited the above-described resistance changes in B mode in which application of electrical pulses having a negative voltage of −1.0 V causes a decrease in the resistance value to about 110Ω and application of electrical pulses having a positive voltage of +1.7 V causes an increase in the resistance value to about 3 kit. Then, reversible resistance changes occurred stably between 110Ω and 3 kΩ. 
     Referring to  FIG. 33  ( b ) showing the resistance change characteristics of the element N which was manufactured at the oxygen gas flow ratio of 3.3%, the initial resistance was very high at 6.8 MΩ, and the element N exhibited no resistance change even when electrical pulses were applied while gradually changing the positive voltage from +0.1 V to +10.0 V or the negative voltage from −0.1 V to −10.0 V, with a pulse width of 100 nsec. The resistance value decreased to about 30Ω when a positive voltage of +10.0 V and a negative voltage of −10.0 V were alternately and repeatedly applied several times. However, no resistance change was exhibited thereafter. 
     These results clarified that reversible resistance changes can be observed only when the second zirconium oxide layer  2105  exists. However, when the second zirconium oxide layer  2105  having a high oxygen content percentage does not exist, a pulse having a width of 100 ns, for example, cannot bring about the resistance change phenomenon. This indicates that the second zirconium oxide layer is essential to bring about high-speed resistance changes. The composition of this second zirconium oxide layer, when expressed as ZrO y , is sufficient as long as it satisfies x&lt;y&lt;2. A further preferable value of y is 1.9&lt;y&lt;2. 
     From the viewpoint of simple manufacturing, the second zirconium oxide layer having a thickness of 1 nm or greater is suitable for implementation. Moreover, from the viewpoint of increase in the element resistance which is to be brought about by future miniaturization, a thickness of 5 nm or smaller is probably suitable for implementation. 
     (Preferable Structure of Variable Resistance Element Using Hafnium Oxide as Variable Resistance Layer) 
     The above results show a tendency identical to that shown in the result of the fifth experiment in relation to the variable resistance element in which the variable resistance layer comprising the hafnium oxide is used. The variable resistance element in which the variable resistance layer comprising the zirconium oxide is used also appears to exhibit resistance changes with the same mechanism as that examined in the fifth experiment. 
     This idea is supported also by the fact that the element M exhibited the stable resistance changes in B mode after a few and low voltage forming process. That means, the element M exhibited the resistance changes in B mode probably because the zirconium oxide layer  2105  provided in contact with the upper electrode  2107  and having a high oxygen content percentage played a part in the resistance changes. 
     This suggests that by providing the zirconium oxide layer  2105 , which has a high oxygen content percentage, in contact with the lower electrode  2103 , a variable resistance element can be obtained which exhibits the resistance changes in A mode. 
       FIG. 34  is a cross-sectional view showing a structure of such a variable resistance element as a variation.  FIG. 34  omits the substrate and the oxide layer for convenience. 
     With a variable resistance element  2100 A shown in  FIG. 34 , the zirconium oxide layer  2105  having a high oxygen content percentage, the zirconium oxide layer  2104 , and the upper electrode  2107  are sequentially provided on the lower electrode  2103 . A variable resistance layer  2106 A is formed by sequentially depositing the zirconium oxide layer  2105  and the zirconium oxide layer  2104 . 
     With the variable resistance element  2100 A having such a structure, the zirconium oxide layer  2104  provided in contact with the lower electrode  2103  appears to play a part in the resistance changes and to cause the resistance changes in A mode. 
     The present invention also includes a variable resistance nonvolatile storage device having the above-described variable resistance element  2100  and a variable resistance nonvolatile storage device having the above-described variable resistance element  2100 A. Such variable resistance nonvolatile storage devices also achieve the same advantageous effect as that of the variable resistance nonvolatile storage device described in the embodiment. 
     Although only an exemplary embodiment of the variable resistance nonvolatile storage device according to the present invention has been described above, the present invention is not limited to this embodiment. Various modifications which can be conceived by those skilled in the art without materially departing from the novel teachings and advantages of the present invention are intended to be included within the scope of the present invention. 
     (Variation of Variable Resistance Element Used in Nonvolatile Storage Device) 
     For example, although the embodiment has described, as an example, the nonvolatile storage device that uses a variable resistance element in which the upper electrode and the lower electrode both comprise Pt, it is also possible to use a variable resistance element in which the upper electrode and the lower electrode comprise different materials. 
     From another experiment conducted by the inventors of the present application, it was found that the mode of the resistance change characteristics is also unique in a variable resistance element formed by providing a variable resistance layer comprising a transition metal oxide having a uniform oxygen content percentage, between a second electrode comprising a material having a standard electrode potential which is higher than that of the transition metal of the transition metal oxide and a first electrode comprising a material having a standard electrode potential which is lower than that of the second electrode. 
     Although the details are omitted, the experiment showed a result that the above-described resistance changes in B mode occurred in, for example, a variable resistance element in which the upper electrode comprises Pt that is an example of the second electrode material and the lower electrode comprises W (tungsten) that is an example of the first electrode material. Here, as compared to W, Pt has a high standard electrode potential and is not easily oxidized. 
     In this case, when voltages having different polarities are alternately applied between the upper electrode and the lower electrode, the oxygen ions are not absorbed into the upper electrode comprising Pt that is not easily oxidized, but are absorbed into the lower electrode comprising W that is easily oxidized. Thus, the gathering and diffusion of the oxygen ions occur more actively near the interface between the variable resistance layer and the upper electrode, thereby causing the resistance changes in B mode. 
     In view of the above, the electrode provided in contact with a transition metal oxide layer having a higher oxygen content percentage may comprise Pt or the like having a higher standard electrode potential (not easily oxidized), whereas the electrode provided in contact with a transition metal oxide layer having a lower oxygen content percentage may comprise W or the like having a lower standard electrode potential (easily oxidized). 
     This way, the oxygen ions are not absorbed into the Pt electrode from the transition metal oxide layer having a higher oxygen content percentage, but are absorbed into the W electrode from the transition metal oxide layer having a lower oxygen content percentage, and thus there is a greater tendency in which the oxygen ions gather and diffuse in the transition metal oxide layer having a higher oxygen content percentage. As a result, the mode of the resistance change characteristics is strongly fixed. 
     This is more preferable in implementing a characteristic structure of the present invention in which a memory cell is formed by connecting a variable resistance element and a transistor in a direction in which a substrate bias effect in the transistor is smaller, depending on the mode of the variable resistance element. 
     The inventors of the present application have verified the aforementioned concept holds true by conducting the seventh experiment using the actual variable resistance element having a layered structure in which variable resistance layers comprising a transition metal oxide are stacked. 
     In the seventh experiment, an element O and an element P were manufactured according to the method of manufacturing a variable resistance element described in Embodiments. The element O includes (i) a variable resistance layer comprising a tantalum oxide as an example of one of oxygen-deficient oxides, (ii) an upper electrode comprising Pt as an example of a material having a higher standard electrode potential than Ta, and (iii) a lower electrode comprising TaN as an example of a material having a lower standard electrode potential than that in the upper electrode. The element P includes (i) a variable resistance layer comprising the tantalum oxide as an example of one of the oxygen-deficient oxides, and (ii) the upper electrode and the lower electrode both comprising Pt as an example of a material having a higher standard electrode potential than Ta. 
     According to the measurement by the inventors, the standard electrode potential of TaN is +0.48 eV. Furthermore, according to Non Patent Literature 2 (referred to as NPL 2 hereinafter), “CRC HANDBOOK of CHEMISTRY and PHYSICS”, DAVID R. LIDE, Editor-in-chief, 84th Edition 2003-2004, CRC PRESS, the standard electrode potential of Pt is +1.18 eV, and the standard electrode potential of Ta is −0.6 eV. 
     The element O includes the lower electrode comprising TaN and having a thickness of 50 nm, the first tantalum oxide layer comprising TaO x  (x=1.57) and having a thickness of 43 nm, the second tantalum oxide layer comprising TaO y  (y=2.47) and having a thickness of 7 nm, and the upper electrode comprising Pt and having a thickness of 50 nm. 
     The element P includes the lower electrode comprising Pt and having a thickness of 50 nm, the first tantalum oxide layer comprising TaO x  (x=1.59) and having a thickness of 44 nm, the second tantalum oxide layer comprising TaO y  (y=2.38) and having a thickness of 7 nm, and the upper electrode comprising Pt and having a thickness of 50 nm. 
     Each of the elements O and P was a square having a side of 0.5 μm. The second tantalum oxide layer of each of the elements O and P was formed by oxidizing the top surface of the first tantalum oxide layer through the plasma oxidation process. Each of the values of the thicknesses and compositions is an actual measured value obtained by measuring an X-ray reflectivity. 
     Since in each of the elements O and P, the second tantalum oxide layer that is in contact with the upper electrode was set to an oxygen content percentage higher than that of the first tantalum oxide layer that is in contact with the lower electrode, the element underwent a resistance change in the B mode. 
     Furthermore, intending that a mode of the resistance change in the element O was stably fixed to the B mode, the upper electrode comprised Pt having the standard electrode potential higher than that of Ta (i.e. Pt is hardly oxidized), and the lower electrode comprised TaN having the standard electrode potential lower than that of Pt (i.e. TaN is easily oxidized). 
     The hysteretic resistance-voltage characteristics in the resistance change of these elements O and P were measured. In this measurement, the electrical pulses were applied to each of the elements without passing though a load resistor or a transistor, and the resistance value of the element was measured. 
       FIGS. 35  ( a ) and ( b ) are graphs showing the hysteretic resistance-voltage characteristics measured for the elements O and P. In these graphs, the horizontal axis represents the voltage applied to the upper electrode with reference to the lower electrode, and the vertical axis represents the resistance value of each of the elements calculated using a value of a current flowing through the element. 
     As apparent from the comparison between  FIGS. 35  ( a ) and ( b ), while the element O comprising different materials in the upper and lower electrodes exhibited the stable resistance values in the low resistance state, the element P comprising the same material in the upper and lower electrodes did not exhibit the stable resistance values in the low resistance state. Furthermore, although the elements O and P have almost the same dimensions, the element O had the variation range of resistance values between the high and low resistance states larger than that of the element P (almost 10 times). 
     As a result, a mode of the resistance change in the element O was more stably fixed to the B mode than that of the element P as intended. Consequently, it has been verified that the element O stably exhibited the clear resistance change with the larger variation range of resistance values. 
     Next, the eighth experiment has been conducted to measure the resistance change characteristics of a variable resistance element with application of electrical pulses. 
     In the eighth experiment, elements Q and R each comprising different materials in the upper and lower electrodes were further manufactured. 
     The element Q includes the lower electrode comprising TaN and having a thickness of 50 nm, the first tantalum oxide layer comprising TaO x  (x=1.57) and having a thickness of 44 nm, the second tantalum oxide layer comprising TaO y  (y=2.47) and having a thickness of 6 nm, and the upper electrode comprising Ir and having a thickness of 50 nm. 
     The element R includes the lower electrode comprising TaN and having a thickness of 50 nm, the first tantalum oxide layer comprising TaO x  (x=1.57) and having a thickness of 44 nm, the second tantalum oxide layer comprising TaO y  (y=2.47) and having a thickness of 6 nm, and the upper electrode comprising Pd and having a thickness of 50 nm. 
     According to NPL 2, the standard electrode potential of Ir is +1.156 eV, and the standard electrode potential of Pd is +0.951 eV. 
     Each of the elements Q and R was a square having a side of 0.5 p.m. The second tantalum oxide layer of each of the elements Q and R was formed by oxidizing the top surface of the first tantalum oxide layer through the plasma oxidation process. Each of the values of the thicknesses and compositions is an actual measured value obtained by measuring an X-ray reflectivity. 
     Furthermore, intending that a mode of the resistance change in each of the elements Q and R was stably fixed to the B mode as the element O, the upper electrode comprised Ir and Pd having the standard electrode potential higher than that of Ta (i.e. Ir and Pd are hardly oxidized), and the lower electrode comprised TaN having the standard electrode potential lower than those of Ir and Pd (i.e. TaN is easily oxidized). 
     The resistance values of each of the elements Q, R, O, and P were measured with each application of alternate electrical pulses of a negative voltage of −1.5 V and a positive voltage of +2.0 V, through a load resistor of 1 kΩ. The resistance change characteristics of each of the elements O, P, Q, and R underwent the resistance change in the B mode, where the element is changed to the low resistance state with application of the negative voltage, and the element is changed to the high resistance state with application of the positive voltage. 
       FIG. 36  ( a ) to (d) are graphs showing respective measurement results of the elements O, P, Q, and R. 
     As apparent from the comparison between  FIG. 36  ( a ) to (d), while the elements O, Q, and R each comprising different materials in the upper and lower electrodes had the resistance values that are evenly distributed (exhibited the stable resistance values) in the low resistance state, the element P comprising the same material in the upper and lower electrodes did not have partly sufficiently small resistance values in the low resistance state. Furthermore, each of the elements O and R had the variation range of resistance values when having changes in the resistance states, larger than that of the element P. 
     As a result, it has been verified that the variable resistance element comprising different materials in the upper and lower electrodes such that the resistance change characteristics were stably fixed to one of modes is superior to the variable resistance element comprising the same material in the upper and lower electrodes, because the variable resistance element comprising the different materials stably exhibited the clear resistance change with the larger variation range of resistance values. 
     Next, the ninth experiment has been conducted to study the characteristics of a memory cell with a transistor for practical use, using a 1T1R circuit in which a variable resistance element is connected in series with a transistor. Such a 1T1R circuit is an example of an equivalent circuit of a memory cell for practical use. 
     In the ninth experiment, 1T1R circuits were manufactured which include: 1T1R circuits in each of which a transistor is connected in series with a variable resistance element including a lower electrode comprising TaN and an upper electrode comprising Pt as the electrode materials comprised in the element O; and 1T1R circuits in each of which a transistor is connected in series with a variable resistance element including a lower electrode and an upper electrode both comprising Pt as the electrode material comprised in the element P. 
     Each of the transistors in the 1T1R circuits was turned on with application of a gate voltage of +2.4 V, and a resistance change occurred in each of the variable resistance elements with application of alternate electrical pulses of −1.8 V and +1.8 V between the upper electrode of the variable resistance element and a source of the transistor that are both ends of the 1T1R circuit, with reference to the source. In each resistance change, a current value was measured with application of a reading voltage of 0.4 V to each of the 1T1R circuits. 
       FIG. 37  is a distribution chart illustrating the distribution of the measured current values, using a set of two top and bottom vertical bars for each of the 1T1R circuits. The top vertical bar represents distribution of values of currents that flow after the variable resistance element is changed to the low resistance state, and the bottom vertical bar represents distribution of values of currents that flow after the variable resistance element is changed to the high resistance state. 
     As apparent from  FIG. 37 , the 1T1R circuit using the variable resistance element including the lower electrode comprising TaN and the upper electrode comprising Pt has a larger difference in the average current values between the high resistance state and the low resistance state than the 1T1R circuit using the variable resistance element including the lower electrode and the upper electrode both comprising Pt. 
     As a result, it has been verified that the variable resistance element including the upper and lower electrodes comprising different materials such that the resistance change characteristics were stably fixed to one of modes is superior to the variable resistance element including the upper and lower electrodes comprising the same material, in that even the 1T1R circuit including the variable resistance element and the transistor stably exhibited the clear resistance change with the larger difference in the average current values between the high resistance state and the low resistance state. 
     As described above, the variable resistance element includes (i) a variable resistance layer having a layered structure in which transition metal oxide layers with different oxygen content percentages are stacked, and (ii) the upper and lower electrodes comprising different materials such that the resistance change characteristics are stably fixed to one of modes. Such a variable resistance element has an advantage of the clear resistance change with the larger variation range of resistance values than that of the variable resistance element including the upper and lower electrodes comprising the same material. 
     The advantage is probably based on the next mechanism. 
     Since in each of the elements O, P, Q, and R, the second tantalum oxide layer that is in contact with the upper electrode was set to an oxygen content percentage higher than that of the first tantalum oxide layer that is in contact with the lower electrode, the gathering and diffusion of oxygen ions occurred more actively near the interface with the upper electrode of the second tantalum oxide layer, thereby causing the resistance change in the B mode. 
     Furthermore, intending that the resistance change characteristics of the elements O, Ω, and R were stably fixed to one of modes, the upper electrode comprised Pt, Ir, and Pd having the standard electrode potential higher than that of Ta (hardly oxidized), and the lower electrode comprised TaN having the standard electrode potential lower than those of Pt, Ir, and Pd (easily oxidized). 
     In this case, when voltages having different polarities were alternately applied between the upper electrode and the lower electrode, the mode of the resistance change was stably fixed to the B mode. This is because oxygen ions were not absorbed into the upper electrode comprising Pt, Ir, and Pd that are not easily oxidized, but were absorbed into the lower electrode comprising TaN that is easily oxidized. Thus, gathering and diffusion of the oxygen ions probably occurred more actively near the interface with the upper electrode of the variable resistance layer. 
     Probably, such a mechanism generally works when the aforementioned magnitude relationship in standard electrode potential between a transition metal comprised in a variable resistance layer, an upper electrode, and a lower electrode is satisfied. 
     Thus, it can be expected that a variable resistance element including a variable resistance layer having a layered structure in which hafnium oxide layers or zirconium oxide layers are stacked stably exhibits the clear resistance change with the larger variation range of resistance values, based on the mechanism using different and suitable materials in the upper and lower electrodes, as the variable resistance element including a variable resistance layer having a layered structure in which tantalum oxide layers are stacked. 
     Further, one may consider forming the first transition metal oxide layer and the second transition metal oxide layer included in the variable resistance layer of the variable resistance element, using oxide layers comprising different types of transition metals. In this case, according to the types of the transition metals used, the oxygen content percentage of each oxide layer is determined such that the resistance value of the first transition metal oxide layer is greater than that of the second transition metal oxide layer. 
     As described earlier, in the variable resistance layer, the second transition metal oxide layer whose resistance value is higher than that of the first transition metal oxide layer has a role of locally applying a large voltage. As described above, it appears that with this voltage, the gathering and diffusion of the oxygen ions playing a part in the resistance changes occur more actively in the second transition metal oxide layer, and as a result, the mode of the resistance change characteristics is fixed. 
     With this idea, even when different types of transition metals are used, it is probably possible, as in the embodiment, to obtain a variable resistance element which can fix the mode of the resistance change characteristics by making the resistance value of the second transition metal oxide layer larger than the resistance value of the first transition metal oxide layer. 
     That is to say, the present invention also includes the variable resistance element in which the first transition metal oxide layer and the second transition metal oxide layer of the variable resistance layer are oxide layers comprising different types of transition metals. 
     (Impurities in Variable Resistance Layer) 
     Although not described above, it is a known technique to implant predetermined impurities, such as an additive for adjusting the resistance value, into the variable resistance layer of the variable resistance element. This technique may be applied to the variable resistance element used in the variable resistance nonvolatile storage device of the present invention. For example, adding nitrogen to the variable resistance layer leads to an increase in the resistance value of the variable resistance layer, thereby improving the reactivity of the resistance changes. 
     More specifically, regarding the variable resistance element in which an oxygen-deficient transition metal oxide is used for the variable resistance layer, the limitation in the claims that the variable resistance layer includes: a first region containing a first oxygen-deficient transition metal oxide having a composition MO x ; and a second region containing a second oxygen-deficient transition metal oxide having a composition MO y  (where x&lt;y) is not intended to exclude the first region and the second region containing predetermined impurities (e.g. an additive for adjusting the resistance value) other than the corresponding compositional transition metal oxides. 
     INDUSTRIAL APPLICABILITY 
     As described above, the present invention makes it possible to provide, in a small layout area, a variable resistance nonvolatile storage device formed with 1T1R memory cells each using a variable resistance element. Thus, the present invention is useful in providing a memory which is high in integration and small in area, for example. 
     REFERENCE SIGNS LIST 
     
         
           100  Variable resistance element 
           101  Substrate 
           102  Oxide layer 
           103  Lower electrode 
           104  Variable resistance layer 
           104   a ,  104   b  Tantalum oxide layer 
           105  Upper electrode 
           106  Pattern 
           107  Element region 
           200  Nonvolatile storage device 
           201  Memory main portion 
           202  Memory array 
           203  Column selection circuit 
           204  Sense amplifier 
           205  Data input and output circuit 
           206  Write circuit 
           207  Row driver 
           208  Row selection circuit 
           209  Address input circuit 
           210  Control circuit 
           211  Power source for writing 
           212  Power source for low resistance (LR) writing 
           213  Power source for high resistance (HR) writing 
           300  Memory cells 
           301  Semiconductor substrate 
           302   a ,  302   b  N-type diffusion layer region 
           303   a  Gate insulator film 
           303   b  Gate electrode 
           304 ,  306 ,  308 ,  310  Via 
           305 ,  307 ,  311  Wiring layer 
           309  Variable resistance element 
           309   a ,  309   d  Lower electrode 
           309   b ,  309   e  Variable resistance layer 
           309   b - 1 ,  309   e - 1  Tantalum oxide layer having a lower oxygen content percentage 
           309   b - 2 ,  309   e - 2  Tantalum oxide layer having a higher oxygen content percentage 
           309   c ,  309   f  Upper electrode 
           317  Transistors 
           400  Memory cells 
           402   a ,  402   b  P-type diffusion layer region 
           409  Variable resistance element 
           417  Transistors 
           418  N well 
           1001  Semiconductor substrate 
           1002  Source region 
           1003  Drain region 
           1004  Gate oxide film 
           1005 ,  1031  Gate electrode 
           1006  Transistor 
           1007  Lower electrode 
           1008  Variable resistance layer 
           1009  Upper electrode 
           1010  Variable resistance element 
           1011 ,  1021  Memory cell 
           1012  Bit line 
           1013  Source line 
           1029 ,  1030  N-type diffusion layer region 
           1100 ,  1100 A,  1100 B Variable resistance element 
           1101  Substrate 
           1102  Oxide layer 
           1103  Lower electrode 
           1104 ,  1105 ,  1106  Hafnium oxide layer 
           1107 ,  1107 A,  1107 B Variable resistance layer 
           1108  Upper electrode 
           1109  Element region 
           2100 ,  2100 A Variable resistance element 
           2101  Substrate 
           2102  Oxide layer 
           2103  Lower electrode 
           2104 ,  2105  Zirconium oxide layer 
           2106 ,  2106 A Variable resistance layer 
           2107  Upper electrode 
           2108  Pattern 
           2109  Element region 
           3301  Lower electrode 
           3302  Variable resistance layer 
           3303  Upper electrode