Variable resistance element and memory device

According to one embodiment, a variable resistance element includes first and conductive layers and first and second layers. The first conductive layer includes a first element including at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The second conductive layer includes at least one selected from the group consisting of platinum, gold, iridium, tungsten, palladium, rhodium, titanium nitride, and silicon. A first layer contacts the first conductive layer, and is provided between the first and second conductive layers. The first layer includes a first material. The first material is insulative. The second layer includes a second element and a second material and is provided between the first layer and the second conductive layer. The second element includes at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The second material is different from the first material.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2017-048602, filed on Mar. 14, 2017; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a variable resistance element and a memory device.

BACKGROUND

A memory device that uses a variable resistance element has been proposed. Stable operations of the variable resistance element are desirable.

DETAILED DESCRIPTION

According to one embodiment, a variable resistance element includes a first conductive layer, a second conductive layer, a first layer, and a second layer. The first conductive layer includes a first element including at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The second conductive layer includes at least one selected from the group consisting of platinum, gold, iridium, tungsten, palladium, rhodium, titanium nitride, and silicon. A first layer contacts the first conductive layer, and is provided between the first conductive layer and the second conductive layer. The first layer includes a first material. The first material is insulative. The second layer includes a second element and a second material and is provided between the first layer and the second conductive layer. The second element includes at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The second material is different from the first material.

According to another embodiment of the invention, a memory device includes the variable resistance element recited above, a first interconnect, a second interconnect, and a controller. The first interconnect is electrically connected to the first conductive layer. The second interconnect is electrically connected to the second conductive layer. The controller is electrically connected to the first interconnect and the second interconnect. The controller is configured to implement a first operation of applying a first voltage to the first interconnect, the first voltage being positive when referenced to a potential of the second interconnect, a second operation of applying a second voltage to the first interconnect, the second voltage being negative when referenced to the potential of the second interconnect, and a third operation of applying a third voltage to the first interconnect, the third voltage being smaller than the first voltage and being positive when referenced to the potential of the second interconnect. The absolute value of a first current flowing in a stacked body via the first interconnect and the second interconnect in the third operation implemented after the first operation is greater than the absolute value of a second current flowing in the stacked body in the third operation implemented after the second operation, the stacked body including the first conductive layer, the second conductive layer, the first layer, and the second layer. The absolute value of a fourth current flowing in the stacked body via the first interconnect and the second interconnect when a fourth voltage is applied to the first interconnect is greater than the absolute value of a fifth current flowing in the stacked body via the first interconnect and the second interconnect when a fifth voltage is applied to the first interconnect, the fourth voltage being positive when referenced to the potential of the second interconnect, the fifth voltage being negative when referenced to the potential of the second interconnect, the fourth voltage being ⅓ of the third voltage, the absolute value of the fifth voltage being ⅓ of the third voltage.

The drawings are schematic and conceptual; and the relationships between the thickness and width of portions, the proportions of sizes among portions, etc., are not necessarily the same as the actual values thereof. Further, the dimensions and proportions may be illustrated differently among drawings, even for identical portions.

In the specification and drawings, components similar to those described or illustrated in a drawing thereinabove are marked with like reference numerals, and a detailed description is omitted as appropriate.

First Embodiment

FIG. 1is a schematic cross-sectional view illustrating a variable resistance element according to a first embodiment.

As shown inFIG. 1, the variable resistance element110according to the embodiment includes a first conductive layer11, a second conductive layer12, a first layer21, and a second layer22.

The first conductive layer11includes a first element. The first element includes at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The first conductive layer11is, for example, silver.

The second conductive layer12includes at least one selected from the group consisting of platinum, gold, iridium, tungsten, palladium, rhodium, titanium nitride, and silicon. The second conductive layer12includes, for example, Si including an impurity. In such a case, the second conductive layer12includes, for example, Si doped with a high concentration of phosphorus (P). The resistivity of the second conductive layer12is, for example, 0.005 Ωcm or less.

For example, the work function of the first conductive layer11is smaller than the work function of the second conductive layer12. Such a relationship of the work functions is obtained by using the materials recited above.

A direction from the second conductive layer12toward the first conductive layer11is taken as a Z-axis direction. The Z-axis direction corresponds to the stacking direction.

The first layer21is provided between the first conductive layer11and the second conductive layer12. The first layer21contacts the first conductive layer11. The first layer21includes a first material. The first material is insulative. The first layer21is, for example, aluminum oxide.

The second layer22is provided between the first layer21and the second conductive layer12. The second layer22includes a second element22aand a second material22b. The second element22aincludes at least one selected from the group consisting of silver, copper, aluminum, nickel, and titanium. The second material22bis different from the first material recited above (the material of the first layer21). For example, the second element22ais silver. For example, the second material is hafnium oxide. In the second layer22, the silver is doped into the hafnium oxide. The concentration of silver in the second layer22is, for example, 1×1020atoms/cm3or more. For example, the second layer22contacts the first layer21and the second conductive layer12.

The thickness (the length along the Z-axis direction) of the first layer21is, for example, not less than 0.2 nanometers (nm) and not more than 1.5 nm. The first layer21functions as, for example, a tunneling insulating film. The thickness of the first layer21is thinner than the thickness of the second layer22. The thickness of the second layer22is greater than 1.5 nm and not more than 10 nanometers. The second layer22functions as a functional portion in which the resistance changes.

The first conductive layer11, the second conductive layer12, the first layer21, and the second layer22are included in a stacked body25. The stacked body25functions as one memory cell.

The electrical resistance of the stacked body25has multiple mutually-different states. One of the multiple states is a high resistance state STH. Another one of the multiple states is a low resistance state STL. The electrical resistance of the stacked body25in the low resistance state STL is lower than the electrical resistance in the high resistance state STH.

FIG. 2is a graph illustrating a characteristic of the variable resistance element according to the first embodiment.

FIG. 2illustrates a characteristic of the variable resistance element110recited above. In the example, the first conductive layer11is a silver layer. The second conductive layer12is Si including phosphorus (P). The first layer21(the first material) is aluminum oxide. The second layer22is hafnium oxide doped with silver.

A current Ic that flows in the stacked body25when an applied voltage Vap is applied between the first conductive layer11and the second conductive layer12is measured.FIG. 2shows the relationship between the applied voltage Vap and the current Ic. The applied voltage Vap is the potential of the first conductive layer11when referenced to the potential of the second conductive layer12. When the applied voltage Vap is positive, the potential of the first conductive layer11is higher than the potential of the second conductive layer12. When the applied voltage Vap is negative, the potential of the first conductive layer11is lower than the potential of the second conductive layer12. The current Ic is a logarithmic display.

As shown inFIG. 2, when the applied voltage Vap is positive, as the absolute value of the applied voltage Vap is increased, the current Ic becomes large; and the low resistance state STL is obtained. When the absolute value of the applied voltage Vap is reduced from the low resistance state STL, the current Ic becomes small. Further, the current Ic is small also when the applied voltage Vap is set to be negative and the absolute value of the negative applied voltage Vap is increased.

As the positive applied voltage Vap increases and decreases, the characteristic of the current Ic has hysteresis; and multiple states (the high resistance state STH and the low resistance state STL) are obtained. These multiple states are used to store information.

In the low resistance state STL as shown inFIG. 2, the characteristic of the current Ic has a large asymmetry between when the applied voltage Vap is positive and when the applied voltage Vap is negative. The asymmetry is based on the special structure of the stacked body25according to the embodiment. As described below, due to the asymmetry, the stacked body25(the memory cell) is not easily affected by disturbances when an unintended voltage is applied to the stacked body25(the memory cell).

FIG. 3AandFIG. 3Bare graphs illustrating characteristics of the variable resistance element.

Characteristics of the variable resistance element110according to the embodiment and a variable resistance element119of a reference example are illustrated in these figures. In the variable resistance element119of the reference example, a titanium oxide layer is provided as a variable resistance layer between a first electrode and a second electrode. The variable resistance element119is a generally-known oxide-based variable resistance element.FIG. 3Ashows one example of the characteristic of the variable resistance element110according to the embodiment.FIG. 3Bshows one other example of the characteristic of the variable resistance element110according to the embodiment.

In the variable resistance element119of the reference example as shown inFIG. 3AandFIG. 3B, when the polarity of the applied voltage Vap is positive, the characteristic is equal to the characteristic of the variable resistance element110(the solid line); when negative, the characteristic is illustrated by the broken line; and the characteristic of the current Ic is substantially symmetric.

Conversely, in the one example of the variable resistance element110as shown by the solid line ofFIG. 3A, the current Ic when the applied voltage Vap is negative is extremely small compared to the current Ic when the applied voltage Vap is positive. In other words, the variable resistance element110has a rectifying property in the negative-side bias direction.

For example, the stacked body25transitions to the low resistance state STL (the set state or the on-state) when a positive set voltage Vs is applied to the stacked body25. Then, the stacked body25transitions to the high resistance state STH when a negative reset voltage Vrs is applied to the stacked body25. The current Ic that flows when the set voltage Vs is applied is a set current Is. The current Ic that flows when the reset voltage Vrs is applied is a reset current Irs. The applied voltage Vap at which the magnitude of the current Ic becomes a prescribed value (the set current Is) or more corresponds to the set voltage Vs. The applied voltage Vap at which the magnitude of the current Ic becomes a prescribed value (the reset current Irs) or less corresponds to the reset voltage Vrs.

In the case where the multiple memory cells (the stacked body25) are provided as described below, a negative voltage (an intermediate voltage) is applied to the unselected cells or the half-selected cells. The absolute value of the negative intermediate voltage is less than the absolute value of the negative reset voltage Vrs. The stacked body25undesirably transitions to the high resistance state STH if the current Ic that flows is large when the negative intermediate voltage is applied to the memory cells. In other words, a reset disturbance occurs.

In the characteristic ofFIG. 2, the reset disturbance is suppressed when the current Ic is small when the applied voltage Vap is negative. In the variable resistance element119of the reference example as recited above, the symmetry of the positive and negative characteristics of the current Ic is high; and the current Ic is large when the applied voltage Vap is negative. Therefore, the effect of the reset disturbance is large. Stable operations are difficult.

Conversely, in the variable resistance element110according to the embodiment as shown inFIG. 2, the positive and negative characteristics of the current Ic are asymmetric. The current Ic is extremely small when the applied voltage Vap is negative. Therefore, the effect of the reset disturbance can be suppressed. Stable operations are obtained.

In the one other example of the variable resistance element110as shown inFIG. 3Bas well, the current Ic is extremely small when the applied voltage Vap is negative. Further, the applied voltage Vap (the reset voltage Vrs) at which the current Ic becomes a prescribed magnitude or less is extremely low (negative and having a large absolute value). The absolute value of the reset voltage Vrs is greater than the absolute value of the set voltage Vs. In such a case, the effect of the reset disturbance can be suppressed further. Stable operations are obtained.

Thus, in the variable resistance element110according to the embodiment, the voltage-current characteristic is asymmetric with respect to the positive and negative polarities. An example of the operations of the variable resistance element110in which such a characteristic is obtained will now be described.

FIG. 4AtoFIG. 4Dare schematic views illustrating characteristics of the variable resistance element according to the first embodiment.

FIG. 4Acorresponds to the off-state (the high resistance state STH) of the variable resistance element110(the stacked body25).FIG. 4Bcorresponds to the state in which the applied voltage Vap is applied to the stacked body25in this state. In the example, the applied voltage Vap is positive.FIG. 4Ccorresponds to the state in which the positive applied voltage Vap is applied continuously.FIG. 4Dcorresponds to the state in which the applied voltage Vap is removed after the state shown inFIG. 4C.

As shown inFIG. 4B, a filament22cis formed of the second element22a(e.g., silver) when the positive applied voltage Vap is applied to the stacked body25. The filament22cis formed in the portion of the second layer22on the second conductive layer12side.

As shown inFIG. 4C, the filament22cextends toward the first conductive layer11side from the second conductive layer12. There are also cases where a portion (the second element22a) of the filament22centers the first layer21. At this time, a tunneling current in the first layer21is dominant prior to the filament22creaching the first conductive layer11.

Therefore, the growth of the filament22cstops as shown inFIG. 4D. The tunneling current can flow through the first layer21in this state. This state corresponds to the on-state (the low resistance state STL).

FIG. 5AtoFIG. 5Fare schematic views illustrating characteristics of the variable resistance element according to the first embodiment.

FIG. 5AtoFIG. 5Ccorrespond to the off-state (the high resistance state STH).FIG. 5DtoFIG. 5Fcorrespond to the on-state (the low resistance state STL).FIG. 5AandFIG. 5Dcorrespond to when the applied voltage Vap is negative.FIG. 5BandFIG. 5Ecorrespond to when the applied voltage Vap is 0.FIG. 5CandFIG. 5Fcorrespond to when the applied voltage Vap is positive. A vacuum level VL, the level of a conduction band CB, the level of a valence band VB, and a Fermi level EF are shown in these figures.

As shown inFIG. 5A, etc., for example, an electron affinity EA of the first layer21(the first material) is smaller than the electron affinity EA of the second layer22(the second material22b).

In the low resistance state STL, the first layer21functions as a tunneling barrier of the electron current. On the other hand, the filament22cis formed and the Fermi level EF is formed in the second layer22. The electrons flow through the first layer21due to tunneling conduction. The barrier thickness of the first layer21is larger when a negative voltage is applied than when a positive voltage is applied.

For example, in the low resistance state STL in which the filament22cis formed inside the first layer21, the resistance is higher when the negative voltage is applied to the first layer21than when the positive voltage is applied. Thereby, it is considered that a shape that is asymmetric around the zero voltage is obtained for the current-voltage characteristic.

As shown inFIG. 5AtoFIG. 5F, the work function of the first conductive layer11is smaller than the work function of the second conductive layer12. By such a combination of materials, for example, a combination of barriers such as that recited above is obtained; and an operation that utilizes the asymmetry with respect to the positive and negative voltages is obtained.

In the embodiment, it is favorable for the relative dielectric constant of the first layer21(the first material) to be higher than the relative dielectric constant of the second material22bof the second layer22. Thereby, the filament22cis formed stably in the portion on the second conductive layer12side.

In one example, the first material includes an oxide including at least one selected from the group consisting of Si, Mg, Ca, Hf, Zr, Sc, Ga, Al, Y, La, and Ce. In such a case, the second material22bincludes an oxide including at least one selected from the group consisting of Ta, Nb, Mo, V, Cu, Fe, Mn, Ge, Zn, Gd, and W.

In one example, the first material includes an oxide including at least one selected from the group consisting of Si, Mg, Ca, Al, Y, La, and Ce. In such a case, the second material22bincludes Hf, Zr, Sc, and Ga.

In one example, the first material includes an oxide including at least one selected from the group consisting of Si, Mg, and Ca. In such a case, the second material22bincludes an oxide including at least one selected from the group consisting of Al, Y, La, and Ce.

In one example, the first material includes an oxide including at least one selected from the group consisting of Si, Mg, Ca, Hf, Zr, Sc, Ga, Al, Y, La, and Ce. In such a case, the second material22bincludes a chalcogenide compound.

In one example, the first material includes aluminum oxide. In such a case, the second material22bincludes hafnium oxide.

In one example, the first layer21(the first material) includes, for example, at least one selected from the group consisting of SiON, HfO2, ZrO2, Al2O3, MgO, CaO, Y2O3, La2O3, and CeO2. The first layer21(the first material) includes, for example, Al2O3. For example, the first layer21can be formed by PVD.

In one example, the second material22bof the second layer22includes an oxide including at least one element selected from the group consisting of Hf, Zr, Sc, Ga, Al, Ta, Nb, Mo, V, Cu, Ge, Ce, La, Gd, Y, and W. The second material of the second layer22includes, for example, HfO2. In such a case, the second element22ais, for example, silver.

The concentration of the second element22ain the second layer22is, for example, not less than 1.1×1020atoms/cm3and not more than 2×1022atoms/cm3. When the concentration is a value in this range, for example, the filament22cis formed stably from the second conductive layer12side. In the case where the concentration is less than 1.1×1020atoms/cm3, for example, the concentration of the second element22a(Ag) does not increase easily in the region on the second conductive layer12side. In such a case, for example, ions of the first element (Ag) may ionize from the first conductive layer11, enter the first layer21, and be reduced; and a filament may be formed undesirably from the first conductive layer11side. Thereby, there are cases where it is difficult to obtain the desired rectifying property. On the other hand, in the case where the concentration exceeds 2×1022atoms/cm3, the leakage current is large when a positive voltage that is smaller than the set voltage Vs is applied in the high resistance state STH. Therefore, a stable switching operation is not obtained.

In the embodiment, for example, the first layer21substantially does not include the second element22a. Or, in the case where the first layer21includes the second element22a, the concentration of the second element22ain the second layer22is higher than the concentration of the second element22ain the first layer21.

For example, the first layer21can be formed by PVD (physical vapor deposition). For example, the second layer22can be formed by PVD. For example, the first conductive layer11and the second conductive layer12can be formed by CVD (chemical vapor deposition), etc.

An example of a memory device using the variable resistance element110(the stacked body25) will now be described.

FIG. 6AandFIG. 6Bare schematic perspective views illustrating the memory device according to the first embodiment.

As shown inFIG. 6A, the memory device210includes the variable resistance element110(the stacked body25), a first interconnect51, and a second interconnect52.

The first interconnect51is electrically connected to the first conductive layer11. The second interconnect52is electrically connected to the second conductive layer12. For example, the first interconnect51extends in a Y-axis direction. For example, the second interconnect52extends in an X-axis direction. The Y-axis direction is perpendicular to the Z-axis direction. The X-axis direction is perpendicular to the Z-axis direction and the Y-axis direction.

The stacked body25is provided in a region where a portion of the first interconnect51and a portion of the second interconnect52overlap in the Z-axis direction.

As shown inFIG. 6B, multiple first interconnects51(e.g., bit lines BL) and multiple second interconnects52(e.g., word lines WL) may be provided. One stacked body25(the variable resistance element110) is provided at each crossing portion between the multiple first interconnects51and the multiple second interconnects52.

In the example, the interconnects and the variable resistance elements110(the stacked bodies25) are provided on a substrate10s. The upper surface (e.g., the major surface) of the substrate10sis aligned with the X-Y plane. In the embodiment, the direction of the major surface of the substrate10sis arbitrary. For example, the Z-axis direction may be aligned with the major surface of the substrate10s.

The multiple stacked bodies25(the variable resistance elements110) correspond to multiple memory cells. Information is stored in the multiple memory cells.

FIG. 7AtoFIG. 7Care schematic views illustrating the memory device according to the first embodiment.

FIG. 7Ais a schematic circuit diagram.FIG. 7Bshows one example of the voltages of the memory device.FIG. 7Cshows one other example of the voltages of the memory device.

As shown inFIG. 7A, the multiple bit lines BL include, for example, an unselected bit line BL1(NS), a selected bit line BL2(S), and an unselected bit line BL3(NS). These bit lines BL correspond to the first interconnects51. The multiple word lines WL include, for example, an unselected word line WL1(NS), a selected word line WL2(S), and an unselected word line WL3(NS). These word lines WL correspond to the second interconnects52.

The multiple bit lines BL are connected to a first circuit71. The multiple word lines WL are connected to a second circuit72. The first circuit71and the second circuit72are included in a controller70. The potentials of the multiple bit lines BL and the potentials of the multiple word lines WL are controlled by the controller70(the first circuit71and the second circuit72). Thus, the memory device210may further include the controller70.

The stacked body25that is positioned at the crossing portion between the selected bit line BL2(S) and the selected word line WL2(S) corresponds to a selected cell CS. The stacked bodies25that are positioned at the crossing portion between the selected bit line BL2(S) and the unselected word line WL1(NS) and the crossing portion between the selected bit line BL2(S) and the unselected word line WL3(NS) correspond to half-selected cells CH1. The stacked bodies25that are positioned at the crossing portion between the unselected bit line BL1(NS) and the selected word line WL2(S) and the crossing portion between the unselected bit line BL3(NS) and the selected word line WL2(S) correspond to half-selected cells CH2. The stacked bodies that are positioned at the crossing portions between the unselected bit lines and the unselected word lines correspond to unselected cells CN.

FIG. 7Bshows one example of the applied voltage Vap (volts (V)) applied to each of the cells in a read operation (Read), a set operation (Set), and a reset operation (Reset). In the example ofFIG. 7A, the set voltage Vs is applied to the selected cell CS in the set operation. In such a case, a voltage of “Vs/2” is applied to the half-selected cells CH1and CH2. In such a case, the applied voltage Vap of the unselected cells CN is 0 V. On the other hand, in the reset operation, the reset voltage Vrs is applied to the selected cell CS. In such a case, a voltage of “−Vrs” is applied to the half-selected cells CH1and CH2. In such a case, the applied voltage Vap of the unselected cells CN is 0 V. A negative voltage being applied to the half-selected cells corresponds to a reset disturbance. The example corresponds to a “V/2 method.” In the “V/2 method,” the absolute value of the negative voltage applied to the half-selected cells is ½ of the absolute value of the reset voltage Vrs.

FIG. 7Cshows one other example of the applied voltage Vap (volts (V)) applied to the cells in the read operation (Read), the set operation (Set), and the reset operation (Reset). In the example ofFIG. 7B, the set voltage Vs is applied to the selected cell CS in the set operation. In such a case, a voltage of “Vs/3” is applied to the half-selected cells CH1and CH2. In such a case, the applied voltage Vap of the unselected cells CN is 0 V. On the other hand, in the reset operation, the reset voltage Vrs is applied to the selected cell CS. At this time, a voltage of “Vrs/3” is applied to the half-selected cells CH1and CH2. In such a case, the applied voltage Vap of the unselected cells CN is 0 V. A negative voltage being applied to the half-selected cells corresponds to a reset disturbance. The example corresponds to a “V/3 method.” In the “V/3 method,” the absolute value of the negative voltage applied to the half-selected cells is ⅓ of the absolute value of the reset voltage Vrs in the example.

For example, the example ofFIG. 7Bcorresponds to an example of a binary memory device. For example, the example ofFIG. 7Ccorresponds to an example of a multi-bit memory device that is greater than binary.

In a voltage setting such as that illustrated inFIG. 7Bas well, it is desirable to obtain stable characteristics (a stable difference between the characteristics in the low resistance state STL and the high resistance state STH).

According to the embodiment, stable characteristics are obtained using the “V/3 method” illustrated inFIG. 7Cas well.

An example of a characteristic and an example of the setting of the voltages according to the embodiment will now be described.

FIG. 8is a schematic view illustrating a characteristic of the memory device according to the first embodiment.

The horizontal axis ofFIG. 8is the applied voltage Vap. The vertical axis is the current Ic (logarithmic display).

The applied voltage Vap corresponds to the potential difference between the first conductive layer11and the second conductive layer12. The first conductive layer11is electrically connected to the first interconnect51. The second conductive layer12is electrically connected to the second interconnect52. Practically, the applied voltage Vap may be set to be the potential difference between the first interconnect51and the second interconnect52.

The applied voltage Vap is supplied by the controller70. The controller70is electrically connected to the first interconnects51(e.g., the bit lines BL) and the second interconnects52(e.g., the word lines WL).

The controller70is configured to implement the first operation (the set operation), the second operation (the reset operation), and the third operation (the read operation).

In the first operation (the set operation) as shown inFIG. 8, the controller70applies a first voltage V1(the set voltage Vs) to the first interconnect51(the first conductive layer11). The first voltage V1is a voltage that is positive when referenced to the potential of the second interconnect52(the second conductive layer12).

In the second operation (the reset operation), the controller70applies a second voltage V2(the reset voltage Vrs) to the first interconnect51(the first conductive layer11). The second voltage V2is a voltage that is negative when referenced to the potential of the second interconnect52(the second conductive layer12).

In the third operation (the read operation), the controller70applies a third voltage V3(a read voltage Vrd) to the first interconnect51(the first conductive layer11). The third voltage V3is a voltage that is positive when referenced to the potential of the second interconnect52(the second conductive layer12). The third voltage V3is smaller than the first voltage V1.

In the low resistance state STL, the current Ic for the third voltage V3(the read voltage Vrd) corresponds to a first current I1. In the high resistance state STH, the current Ic for the third voltage V3(the read voltage Vrd) corresponds to a second current I2. Thus, the absolute value of the current Ic (the first current I1) flowing in the stacked body25via the first interconnect51and the second interconnect52in the third operation (the read operation) implemented after the first operation (the set operation) is greater than the absolute value of the current Ic (the second current I2) flowing in the stacked body25in the third operation (the read operation) implemented after the second operation (the reset operation). Thus, the current Ic that flows when the same read voltage Vrd is applied is different between the low resistance state STL and the high resistance state STH.

The voltage (a positive voltage) of ⅓ of the third voltage V3(the read voltage Vrd) is taken as a fourth voltage V4(Vrd/3). In the low resistance state STL, the current Ic for the fourth voltage V4(Vrd/3) corresponds to a fourth current I4. On the other hand, a fifth voltage V5(−Vrd/3) is a negative voltage; and the absolute value of the fifth voltage V5is ⅓ of the read voltage Vrd. In the low resistance state STL, the current Ic for the fifth voltage V5(−Vrd/3) corresponds to a fifth current I5.

The absolute value of the current Ic (the fourth current I4) flowing in the stacked body25via the first interconnect51and the second interconnect52when the fourth voltage V4(a voltage that is positive when referenced to the potential of the second interconnect52) is applied to the first interconnect51is greater than the absolute value of the fifth current I5flowing in the stacked body25via the first interconnect51and the second interconnect52when the fifth voltage V5(a voltage that is negative when referenced to the potential of the second interconnect52) is applied to the first interconnect51. As described above, the fourth voltage V4is ⅓ of the third voltage V3. The absolute value of the fifth voltage V5is ⅓ of the third voltage V3.

Because the fourth current I4when applying the fourth voltage V4that is positive and has the absolute value of ⅓ of the read voltage Vrd (the third voltage V3) is larger than the fifth current I5when applying the fifth voltage V5that is negative and has the absolute value of ⅓ of the read voltage Vrd (the third voltage V3), in the case where the operation of the “V/3 method” recited above is performed as well, a stable difference of the currents (difference of the electrical resistances) between the low resistance state STL and the high resistance state STH is obtained when the read voltage Vrd is applied in the third operation. For example, multi-bit storing is obtained stably.

The variable resistance element119of the reference example (referring toFIG. 3AandFIG. 3B) is reset easily because similar currents flow on the negative voltage side and on the positive voltage side. Conversely, in the variable resistance element110according to the embodiment, the error of being erroneously reset is suppressed because the current on the negative voltage side is small. Thus, according to the embodiment, a stable array operation that is resistant to the reset disturbance is obtained.

In the embodiment, the absolute value of the first voltage V1(the set voltage Vs) may be less than the absolute value of the second voltage V2(the reset voltage Vrs) (referring toFIG. 3B). Thereby, more stable memory operations can be implemented. For example, the absolute value of the fifth voltage V5recited above is less than ⅓ of the absolute value of the second voltage V2.

The first current I1(the absolute value) recited above is, for example, not less than 100 times the fourth current I4(the absolute value). Stable storing and read operations can be implemented using such a large difference.

Second Embodiment

FIG. 9is a schematic cross-sectional view illustrating a variable resistance element according to a second embodiment.

As shown inFIG. 2, the variable resistance element120according to the embodiment also includes the first conductive layer11, the second conductive layer12, the first layer21, and the second layer22. In the variable resistance element120, the characteristics are different between two regions (a first region r1and a second region r2) in the second layer22. The first conductive layer11, the second conductive layer12, and the first layer21are similar to those of the variable resistance element110; and a description is therefore omitted.

In the second layer22, the second region r2is provided between the first region r1and the second conductive layer12. The concentration of the second element22a(e.g., silver) included in the first region r1is lower than the concentration of the second element22aincluded in the second region r2. There may be a case where the first region r1substantially does not include the second element22a. In the variable resistance element120, more stable operations are obtained as described below.

FIG. 10AtoFIG. 10Care schematic views illustrating characteristics of the variable resistance element according to the second embodiment.

FIG. 10Acorresponds to the off-state (the high resistance state STH) of the variable resistance element110(the stacked body25).FIG. 10Bcorresponds to the state in which a positive applied voltage Vap is applied to the stacked body25in this state.FIG. 10Ccorresponds to the state in which the applied voltage is 0 V after the positive applied voltage Vap is applied.

FIG. 10Acorresponds to the high resistance state STH.

As shown inFIG. 10B, a positive applied voltage Vap that is higher than the set voltage Vs is applied to the first conductive layer11in the high resistance state STH. Thereby, the filament22cis formed from the second conductive layer12toward the first conductive layer11. Inside the second layer22, a thick and stable filament is formed in the region (the second region r2) where the concentration of the second element22a(e.g., silver) is high. On the other hand, a fine unstable filament22cis formed in the region (the first region r1) where the concentration of the second element22ais low.

Therefore, when the applied voltage Vap is returned to 0 as shown inFIG. 10C, the second element22aundesirably diffuses from the filament22cinside the first region r1; and the filament22cthat is in the first region r1disappears.

The filament22cis re-formed in the first region r1when reading the low resistance state by applying the read voltage Vrd (the positive voltage) to the first conductive layer11. As a result, the current Ic increases abruptly for a voltage that is higher than some threshold voltage. The high nonlinearity (the rectifying property) of the current Ic around the threshold voltage is obtained.

For example, when applying the read voltage Vrd to the selected cell CS in a memory device in which multiple memory cells are provided, for example, a voltage of Vrd/3 is applied according to the operation method to the cells (the half-selected cells) that are connected to the same bit line and word line as the selected cell CS. In the embodiment, the ratio of the current Ic for the read voltage Vrd to the current Ic for the voltage of Vrd/3 is sufficiently greater than 100. Thus, a large nonlinearity is obtained. Thereby, read errors in the read operation can be suppressed. Thereby, stable operations are obtained. Even in the case where the range of the applied voltage Vap is low and small, a stable memory operation is obtained. Thereby, the power consumption can be reduced.

According to the embodiment, a variable resistance element and a memory device can be provided that are resistant to the reset disturbance in which a negative voltage is applied.

According to the embodiment, a variable resistance element and a memory device can be provided in which stable operations are possible.

In this specification, the state of being electrically connected includes the state in which two conductors are in direct contact. The state of being electrically connected includes the state in which two conductors are connected by another conductor (e.g., an interconnect, etc.). The state of being electrically connected includes the state in which a switching element (a transistor or the like) is provided on a path between two conductors, and a state is formable in which a current flows in the path between the two conductors.

Hereinabove, exemplary embodiments of the invention are described with reference to specific examples. However, the embodiments of the invention are not limited to these specific examples. For example, one skilled in the art may similarly practice the invention by appropriately selecting specific configurations of components included in variable resistance elements and memory devices such as conductive layers, layers, interconnects, and controller etc., from known art. Such practice is included in the scope of the invention to the extent that similar effects thereto are obtained.

Moreover, all semiconductor devices practicable by an appropriate design modification by one skilled in the art based on the semiconductor devices described above as embodiments of the invention also are within the scope of the invention to the extent that the spirit of the invention is included.