Patent Publication Number: US-11043533-B2

Title: Switch and method for fabricating the same, and resistive memory cell and electronic device, including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation of U.S. application Ser. No. 15/432,806 filed on Feb. 14, 2017, which claims priority under 35 U.S.C. § 119(a) to Korean patent application number 10-2016-0074641 filed on Jun. 15, 2016, which is incorporated herein in its entirety by reference. 
    
    
     BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to a memory circuit or device, and an application thereof in an electronic device. 
     2. Description of the Related Art 
     In recent years, semiconductor devices capable of storing information in various electronic devices such as portable communication devices become desirable according to miniaturization, low power, high performance, and diversification of the electronic devices, and studies on the semiconductor devices have been actively conducted. 
     Such semiconductor devices include semiconductor devices which can store data using a characteristic that switches between different resistant states according to a voltage or a current applied thereto, for example, a resistive random access memory (RRAM), a phase-change random access memory (PRAM), a ferroelectric random access memory (FRAM), a magnetic random access memory (MRAM), an E-fuse, etc. 
     SUMMARY 
     Embodiments provide a switch having improved characteristics and a method for fabricating the same, and a resistive memory and an electronic device, including the same. 
     According to an embodiment of the present disclosure, there is provided a switch including: a first electrode layer; a second electrode layer disposed over the first electrode layer; and a selecting element layer interposed between the first electrode layer and the second electrode layer, the selecting element layer including a gas region with a plurality of gas ions in which a current flows or is does not flow according a value of a voltage applied to the switch, wherein the current flows across the gas region in an on-state, and the current does not flow across the gas region in an off-state. 
     According to an embodiment of the present disclosure, there is provided a resistive memory cell including: a first electrode layer; a second electrode layer disposed over the first electrode layer; a third electrode layer disposed over the second electrode layer; a variable resistance layer interposed between the first electrode layer and the second electrode layer; and a selecting element layer interposed between the second electrode layer and the third electrode layer, the selecting element layer including a gas region with a plurality of gas ions in which a current flows or does not flow according to a voltage applied to the second and third electrode layers, wherein, if the voltage is less than a threshold value, current does not flow through the selecting element layer into the variable resistance layer, and, if the voltage is equal to or greater than the threshold value, the current flows through the selecting element layer into the variable resistance layer. 
     According to an embodiment of the present disclosure, there is provided an electronic device including: a memory element storing data; and a selecting element electrically connected to the memory element, the selecting element including a gas region in which a current flows or does not flow according to a value of a signal applied to the electronic device, the selecting element preventing the current from flowing to the memory element when the value is less than a threshold value, and allowing the current to flow to the memory element when the value is equal to or greater than the threshold value, thereby controlling access to the memory element. 
     According to an embodiment of the present disclosure, there is provided a method for fabricating a switch, the method including: forming a first electrode layer; forming a second electrode layer over the first electrode layer; and implanting ions between the first electrode layer and the second electrode layer to form a gas region in which a current flows or does not flow according to a voltage applied to the switch. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present disclosure will now be described more fully hereinafter with reference to the accompanying drawings; however, they may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete. 
       In the drawing figures, dimensions may be exaggerated for clarity of illustration. It will be understood that when an element is referred to as being “between” two elements, it can be the only element between the two elements, or one or more intervening elements may also be present. Like reference numerals refer to like elements throughout. 
         FIG. 1A  is a cross-sectional view illustrating a switch according to an embodiment of the present disclosure. 
         FIG. 1B  is a cross-sectional view illustrating a switch according to another embodiment. 
         FIGS. 2A and 2B  are a perspective view and a cross-sectional view, respectively, illustrating a resistive memory cell according to an embodiment of the present disclosure. 
         FIGS. 3A and 3B  are a perspective view and a cross-sectional view, respectively, illustrating a resistive memory cell according to another embodiment of the present disclosure. 
         FIGS. 4A and 4B  are perspective views illustrating an electronic device including cell arrays having a cross-point structure according to embodiments of the present disclosure. 
         FIGS. 5A to 5C  are cross-sectional views illustrating a method for fabricating the switch of  FIG. 1A  and the resistive memory cell of  FIGS. 2A and 2B  according to an embodiment of the present disclosure. 
         FIGS. 6A to 6C  are cross-sectional views illustrating a method for fabricating the switch of  FIG. 1B  and the resistive memory cell of  FIGS. 3A and 3B  according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, various embodiments will be described in detail with reference to the accompanying drawings. 
     The drawings are not necessarily drawn to scale, and, in some embodiments, at least some of structures shown in the drawings may be exaggerated to clearly describe features of the embodiments. When a multi-layered structure having two or more layers is disclosed in a drawing or detailed description, the relative positional relationship or arrangement order of the layers merely reflects a specific embodiment, and thus, embodiments of the present disclosure are not limited thereto. The relative positional relationship or arrangement order of the layers may be changed. Also, the drawing or detailed description of the multi-layered structure may not reflect all layers existing in a specific multi-layered structure (e.g., one or more additional layers may exist between two layers). For example, when a first layer exists on a second layer or a substrate in the multi-layered structure in the drawing or detailed description, the first layer can be directly formed on the second layer or the substrate. In addition, one or more other layers may exist between the first layer and the second layer or between the first layer and the substrate. 
       FIG. 1A  is a cross-sectional view illustrating a switch SW according to an embodiment of the present disclosure. 
     Referring to  FIG. 1A , the switch SW is a device that has an on-state or off-state, based on the value of a voltage or a current applied thereto. For example, when a voltage is applied to the switch SW and the value of the applied voltage is less than a threshold voltage, the switch SW is in the off-state. When the value of the applied voltage is equal to or greater than the threshold voltage, the switch SW is in the on-state. The switch SW includes a gas region (or a gas layer) with a plurality of gas ions. If a conductive path is generated in the gas region, the switch SW is in the on-state. If the conductive path disappears, the switch SW is in the off-state. 
     The switch SW includes a first electrode layer  11 , a selecting element layer  12 , and a second electrode layer  13 . The first electrode layer  11  and the second electrode layer  13  are spaced apart from each other, and the selecting element layer  12  is interposed between the first electrode layer  11  and the second electrode layer  13 . Here, the first electrode layer  11 , the selecting element layer  12 , and the second electrode layer  13  may be stacked in a first direction (e.g., a third direction III of  FIG. 2A ), which is a vertical direction with respect to a substrate. However, embodiments of the present disclosure are not limited thereto, and the first electrode layer  11 , the selecting element layer  12 , and the second electrode layer  13  may be stacked in a second direction (e.g., a first direction I or a second direction II of  FIG. 2A ), which is a horizontal direction with respect to the substrate. 
     The first and second electrode layers  11  and  13  are used to provide the selecting element layer  12  with a voltage or a current applied to the switch SW, and may be formed of a conductive material. The first and second electrode layers  11  and  13  may include metal, metal nitride, precious metal, etc. For example, the first and second electrode layers  11  and  13  may include any one of titanium (Ti), titanium nitride (TiN), tungsten (W), tungsten nitride (WNx), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), copper (Cu), silver (Ag), platinum (Pt) and iridium (Ir), or a combination thereof. 
     The first and second electrode layers  11  and  13  may include the same metal, or may include different kinds of metals, respectively. As an example, if the first electrode layer  11  includes a precious metal, the second electrode layer  13  includes a general metal other than the precious metal of the first electrode layer  11 . The precious metal may be gold (Au), silver (Ag), platinum (Pt), iridium (Ir), or palladium (Pd), and the general metal may be a metal element other than the precious metals. As another example, if the second electrode layer  13  includes a precious metal, the first electrode layer  11  includes a general metal instead of the precious metal of the second electrode layer  13 . 
     The selecting element layer  12  has a switching characteristic, such that, when the value of the applied voltage or current is less than a threshold value, the selecting element layer  12  substantially cuts off a current, and, when the value of the applied voltage or current is equal to or greater than the threshold value, the selecting element layer  12  allows current to flow through the selecting element layer  12 . Specifically, when the value of the applied voltage is less than a threshold value, the selecting element layer  12  substantially prevents the current from flowing through the selecting element layer  12 . The selecting element layer  12  includes a gas region having the threshold switching characteristic, and the gas region may be located at an interface between the first electrode layer  11  and the second electrode layer  13 . 
     When the value of a voltage applied to the switch SW is equal to or greater than the threshold value, a current flows through the gas region. For example, electrons are moved by direct tunneling in the gas region, or a conductive path is formed in the gas region. As a result, the current flows between the first electrode layer  11  and the second electrode layer  13 . Thus, the switch SW is in the on-state, and the current flowing in the on-state is referred to as an on-current (I on ). In contrast, when the value of a voltage or a current applied to the switch SW is less than the threshold value or when no voltage or current is applied to the switch SW, the direct tunneling does not occur in the gas region, and the conductive path is not formed. That is, the gas region substantially prevents a current from flowing through the gas region, and the gas region has a characteristic of a nonconductor. Therefore, the switch SW is in the off-state, and the current in the off-state is referred to as an off-current (T off ). 
     Here, the conductive path may be one or more conductive filaments. The conductive filaments may include conductive ions, which are moved into the gas region from the first electrode layer  11 , from the second electrode layer  13 , or from the first and second electrode layers  11  and  13 . For example, if the first electrode layer  11  includes a precious metal such as platinum (Pt) or iridium (Ir) and the second electrode layer  13  includes a general metal such as titanium (Ti) or tungsten (W), ions of the general metal are moved into the gas region from the second electrode layer  13 , thereby forming the conductive filaments. 
     Meanwhile, the gas region has a predetermined thickness T. The thickness T is determined, for example, by considering a size of the switch SW, material characteristics (e.g., melting points, diffusion concentrations, and diffusion coefficients into the gas region) of the first and second electrode layers  11  and  13 , or the like. Specifically, the thickness T of the gas region may be increased as a value of the diffusion coefficient of the first electrode layer  11  or the second electrode layer  13  into the gas region increases. For example, the gas region may have a thickness T of 10 to 200 Å. When a current flows due to the direct tunneling, the thickness T of the gas region may be smaller compared with when the current flows due to the conductive filament. 
     The gas region may substantially entirely or partially exist at the interface between the first electrode layer  11  and the second electrode layer  13 . As an example, the gas region may exist as a gas layer corresponding to the selecting element layer  12  at the interface between the first electrode layer  11  and the second electrode layer  13 . In this case, the first electrode layer  11  and the second electrode layer  13  are completely separated from each other by the gas layer, and thus a current does not flow in the off-state. As another example, the gas region may be partially formed at the interface between the first electrode layer  11  and the second electrode layer  13 . In other words, the gas region may be formed at a portion of the interface. In this case, an area of contact between the first electrode layer  11  and the second electrode layer  13  corresponds to the remaining portion at the interface. Also, a low off-current (I off ) having a small magnitude can flow through the gas region due to gas characteristics, and hence a total magnitude of a current flowing between the first electrode layer  11  and the second electrode layer  13  in the off-state can be limited. 
     According to the above-described embodiments, if the value of the applied voltage or current is equal to or greater than the threshold value, the current flows through the gas region. For example, when electrons are moved by direct tunneling in the gas region, or when a conductive path is formed in the gas region, current flows through the gas region. Thus, the switch SW is in the on-state. In an embodiment, a conductive filament is formed in the gas region, thereby forming a conductive path between the first electrode layer  11  and the second electrode layer  13 . Conductive ions may be moved into the gas region from the first electrode layer  11 , from the second electrode layer  13 , or from the first and second electrode layers  11  and  13 , thereby forming the conductive filament. 
     Referring to  FIG. 1B , in a switch SW according to another embodiment of the present disclosure, a selecting element layer  12  includes a gas region (or a gas layer)  12 A and an insulating layer  12 B. The insulating layer  12 B is interposed between the first electrode layer  11  and the second electrode layer  13 , and the gas region  12 A is located at an interface between the insulating layer  12 B and the second electrode layer  13 . For example, the insulating layer  12 B may include an oxide, a nitride, or a combination thereof. 
     When the selecting element layer  12  includes the insulating layer  12 B, a parasitic resistance of the switch SW may be adjusted. When a voltage applied to the switch SW gradually increases before reaching a threshold voltage, a magnitude of a current does not significantly increase. After the applied voltage reaches the threshold voltage, the current increases rapidly. In this case, a current-voltage curve of the switch SW may have a desirable shape according to an application of the switch SW, a type of a device in which the switch SW is disposed, etc. Such a desirable shape of the current-voltage curve may be obtained by adjusting a thickness of the insulating layer  12 B. For example, a resistance value of the switch SW may be increased by increasing the thickness of the insulating layer  12 B, or the resistance value of the switch SW may be decreased by decreasing the thickness of the insulating layer  12 B. When the resistance value of the switch SW varies, a slope of the current-voltage curve may be adjusted to match the desirable shape. 
     When the gas region  12 A partially exists in a portion of the interface between the first electrode layer  11  and the second electrode layer  13 , due to the presence of the insulating layer  12 B, it is possible to prevent the first electrode layer  11  and the second electrode layer  13  from directly contacting each other by the insulating layer  12 B. That is, the first electrode layer  11  and the second electrode layer  13  can be insulated from each other by the insulating layer  12 B. 
     According to the above-described embodiments, if the value of the applied voltage or current is equal to or greater than the threshold value, a conductive filament is formed in the gas region  12 A, and a vacancy chain is formed in the insulating layer  12 B, so that a conductive path is generated between the first electrode layer  11  and the second electrode layer  13 . Thus, the switch SW is in the on-state. For example, conductive ions may be moved into the gas region  12 A from the first electrode layer  11 , from the second electrode layer  13 , or from the first electrode layer  11  and the second electrode layer  13 , thereby forming the conductive filament. Also, atoms may be moved in the insulating layer  12 B, thereby forming vacancies, and the formed vacancies may be connected to each other, thereby forming the vacancy chain. 
       FIGS. 2A and 2B  illustrate a resistive memory cell MC according to an embodiment of the present disclosure.  FIG. 2A  illustrates a perspective view and  FIG. 2B  illustrates a cross-sectional view taken along a line A-A′ of  FIG. 2A . Hereinafter, descriptions of a selecting element SE of the resistive memory cell MC similar to those of the switch of  FIG. 1A  will be omitted for the interest of brevity. 
     Referring to  FIGS. 2A and 2B , the resistive memory cell MC includes a memory element ME storing data therein, and a selecting element SE electrically connected to the memory element ME and controlling access to the memory element ME. Here, the selecting element SE may be a switch. 
     The memory element ME may include a first electrode layer  21 , a variable resistance layer  22 , and a second electrode layer  23 , and the selecting element SE may include the second electrode layer  23 , a selecting element layer  24 , and a third electrode layer  25 . In this case, the first electrode layer  21  may be a lower electrode, the second electrode layer  23  may be a middle electrode, and the third electrode layer  25  may be an upper electrode. In addition, the memory element ME and the selecting element SE share the second electrode layer  23 , e.g., the middle electrode. Here, the middle electrode may be a conductive path for electrically connecting the lower electrode to the upper electrode. Accordingly, it is possible to implement the resistive memory cell MC having a 1S1R structure. 
     For reference, in  FIGS. 2A and 2B , the selecting element SE is located at an upper portion of the resistive memory cell MC and the memory element ME is located at a lower portion of the resistive memory cell MC, but embodiments of the present disclosure are not limited thereto. In another embodiment (not shown), the memory element ME may be located at the upper portion of the resistive memory cell MC and the selecting element SE may be located at the lower portion of the resistive memory cell MC. In addition, the selecting element SE and the memory element ME may be stacked in a first direction I, in a second direction II, or in a third direction III. 
     The first to third electrode layers  21 ,  23 , and  25  are configured to provide the variable resistance layer  22  with a voltage or a current applied to the resistive memory cell MC, and may be formed of a conductive material. The first to third electrode layers  21 ,  23 , and  25  may include metal, metal nitride, precious metal, etc. For example, the first to third electrode layers  21 ,  23 , and  25  may include any one of titanium (Ti), titanium nitride (TiN), tungsten (W), tungsten nitride (WNx), tantalum (Ta), tantalum nitride (TaN), aluminum (Al), copper (Cu), silver (Ag), platinum (Pt) and iridium (Ir), or a combination thereof. 
     The second electrode layer  23  and the third electrode layer  25  may be formed of the same material, or may be formed of different materials. For example, the second electrode layer  23  and the third electrode layer  25  may be formed of different kinds of metals. 
     Here, the first electrode layer  21  may extend in the first direction I, and the third electrode layer  25  may extend in the second direction II intersecting the first direction I. The second electrode layer  23  may extend in the first direction I. In another embodiment, the second electrode layer  23  is patterned and the patterned second electrode (not shown) may be located at an intersection of the first electrode layer  21  and the third electrode layer  25 . Also, the first to third electrode layers  21 ,  23 , and  25  may be sequentially stacked in the third direction III intersecting the first and second directions I and II. 
     The variable resistance layer  22  may operate as a variable resistor switching between different resistance states based on the applied voltage or current. As an example, if a conductive path penetrating the variable resistance layer  22  is generated in the variable resistance layer  22 , the variable resistance layer  22  is in a low-resistance state. Also, if the conductive path in the variable resistance layer  22  disappears, the variable resistance layer  22  is in a high-resistance state. As another example, when the variable resistance layer  22  includes a metal oxide containing oxygen vacancies, a conductive path according to behavior of the oxygen vacancies is generated or disappears in the variable resistance layer  22 , and therefore, the resistance state of the variable resistance layer  22  may be changed. Accordingly, the variable resistance layer  22  can store data depending on a resistance state thereof. For example, if the variable resistance layer  22  has the high-resistance state, data indicative of a logic low value ‘0’ is stored in the memory element ME. If the variable resistance layer  22  has the low-resistance state, data indicative of a logic high value ‘1’ is stored in the memory element ME. In other embodiments, the conductive path may be formed in various manners according to the kind of the variable resistance layer  22 , the structure of a layer, and operational characteristics. 
     Here, the variable resistance layer  22  may include various material used in a resistive random access memory (RRAM), a phase-change random access memory (PRAM), a ferroelectric random access memory (FRAM), a magnetic random access memory (MRAM), and the like. For example, the variable resistance layer  22  may include a transition metal oxide, a metal oxide such as a perovskite-based material, a phase change material such as a chalcogenide-based material, a ferroelectric material, a ferromagnetic material, and the like. Also, the variable resistance layer  22  may have a single-layered structure or a multi-layered structure. 
     The selecting element layer  24  may have a switching characteristic that, if the value of the applied voltage or current is less than a threshold value, the selecting element layer  24  substantially prevents current flowing through the selecting element layer  24 , and, if the value of the applied voltage or current is equal to or greater than the threshold value, the selecting element layer  24  allows current to flow through the selecting element layer  24 . When the selecting element layer  24  allows the current to flow, a magnitude of the flowing current rapidly increases with an increase in the applied voltage or current. Also, the selecting element layer  24  includes a gas region located at an interface between the second electrode layer  23  and the third electrode layer  25 . 
     According to the above-described embodiment, when the selecting element SE is in an on-state to allow the current to flow, the current smoothly flows into the memory element ME. Thus, a predetermined write voltage or a predetermined read voltage is applied to the resistive memory cell MC, to store data in the variable resistance layer  22  or to read the stored data in the variable resistance layer  22 . Here, the write voltage or the read voltage may have substantially the same level as or a higher level than a threshold voltage having a level sufficiently high to cause the selecting element SE to have the on-state. 
       FIGS. 3A and 3B  illustrate a resistive memory cell MC according to another embodiment of the present disclosure.  FIG. 3A  illustrates a perspective view and  FIG. 3B  illustrates a sectional view taken along a line B-B′ of  FIG. 3A . Hereinafter, descriptions of a selecting element SE of the resistive memory cell MC similar to those of the switch of  FIG. 1B  will be omitted for the interest of brevity. 
     Referring to  FIGS. 3A and 3B , the resistive memory cell MC includes a memory element ME storing data therein, and the selecting element SE electrically connected to the memory element ME and controlling access to the memory element ME. Here, the selecting element SE includes a selecting element layer  24 , the selecting element layer  24  includes a gas region (or a gas layer)  24 A and an insulating layer  24 B, and the gas region  24 A is located at a boundary between the insulating layer  24 B and a third electrode layer  25 . 
       FIGS. 4A and 4B  are perspective views illustrating an electronic device including cell arrays having a cross-point structure according to embodiments of the present disclosure. 
     Referring to  FIG. 4A , a cell array according to an embodiment of the present disclosure includes first lines  31  extending in parallel with each other and in a first direction I and second lines  32  extending in parallel with each other and in a second direction II. Here, the first lines  31  may be located at a first level, and the second lines  32  may be located at a second level different from the first level. For example, the first lines  31  and the second lines  32  may be stacked in a third direction III, and the second lines  32  may be located over the first lines  31 . 
     In addition, switches SW or resistive memory cells MC may be located at intersections of the first lines  31  and the second lines  32 . The switches SW or the resistive memory cells MC are located between the first lines  31  and the second lines  32 , and may be electrically connected to the first and second lines  31  and  32 . 
     Here, the switches SW may have the structures described above with reference to  FIGS. 1A and 1B , and the resistive memory cells MC may have the structures described above with reference to  FIGS. 2A to 3B . For example, lower electrode layers (e.g., first electrode layers  21  of  FIGS. 2B and 3B ) of the resistive memory cells MC may be electrically connected to the first lines  31 , and upper electrode layers (e.g., third electrode layers  25  of  FIGS. 2B and 3B ) of the resistive memory cells MC may be electrically connected to the second lines  32 . 
     Referring to  FIG. 4B , a cell array according to an embodiment of the present disclosure includes first lines  31  extending in parallel to each other and in first direction I, second lines  32  extending in parallel to each other and in second direction II, and third lines  33  extending in parallel to each other and in the first direction I. Here, the first lines  31  may be located at a first level, the second lines  32  may be located at a second level, and the third lines  33  may be located at a third level different from the first and second levels. For example, the first to third lines  31  to  33  may be sequentially stacked in the third direction III. 
     First switches SW 1  or first resistive memory cells MC 1  may be located at intersections of the first lines  31  and the second lines  32 , and second switches SW 2  or second resistive memory cells MC 2  may be located at intersections of the second lines  32  and the third lines  33 . The first switches SW 1  or the first resistive memory cells MC 1  may be located between the first lines  31  and the second lines  32 , and the second switches SW 2  or the second resistive memory cells MC 2  may be located between the second lines  32  and the third lines  33 . The first switches SW 1  or the first resistive memory cells MC 1  may be electrically connected to the first and second lines  31  and  32 , and the second switches SW 2  or the second resistive memory cells MC 2  may be electrically connected to the second and third lines  32  and  33 . 
     In addition, the first switches SW 1  and the second switches SW 2  may have the same structure, or may have structures symmetrical with respect to the second lines  32 . Similarly, the first resistive memory cells MC 1  and the second resistive memory cells MC 2  may have the same structure, or may have structures symmetrical with respect to the second lines  32 . 
     According to the above-described embodiments, resistive memory cells MC are stacked as a cell array having a cross-point structure, so that the degree of integration of a memory can be improved. Also, switches SW or selecting elements SE impede a current, so that it is possible to substantially prevent the leakage of a current of the resistive memory cells MC included in the cell array. 
     For example, when a first voltage having a first level V 1  is applied to a resistive memory cell MC selected in the cell array at an intersection between a selected one of the first lines  31  and a selected one of the second lines  32 , a second voltage having a second level V 2  substantially equal to half of the first level V 1  is applied to a plurality of unselected resistive memory cells sharing the selected one of the first lines  31  or the selected one of the second lines  32  with the selected resistive memory cell MC. Therefore, in order to prevent the leakage of a current through one or more of the unselected resistive memory cells, the switch SW or the selecting element SE substantially prevents current from flowing into a memory element ME when a voltage having a level equal to or smaller than the half of the first level V 1  is applied to the unselected resistive memory cells. In other words, a material that has a low off-current (T off ) characteristic and a high on-current (I on ) characteristic is used for a selecting element layer of the switch SW or the selecting element SE. According to the embodiments of the present disclosure, the selecting element layer including a gas region is used, thereby preventing the leakage of current through the unselected resistive memory cells MC. 
       FIGS. 5A to 5C  are cross-sectional views illustrating a method for fabricating the switch of  FIG. 1A  and the resistive memory cell of  FIGS. 2A and 2B  according to an embodiment of the present disclosure. Hereinafter, descriptions similar to those described above will be omitted for the interest of brevity. 
     Referring to  FIG. 5A , a first electrode layer  51 , a variable resistance layer  52 , a second electrode layer  53 , and a third electrode layer  54  are formed. For example, after a first electrode material layer (not shown), a variable resistance material layer (not shown), and a second electrode material layer (not shown) are formed, the first electrode material layer, the variable resistance material layer, and the second electrode material layer are patterned to form first electrode layer  51 , variable resistance layer  52 , and second electrode layer  53 . The first electrode layer  51 , the variable resistance layer  52  and the second electrode layer  53  may have the shape of a line extending in a first direction (e.g., the first direction I of  FIG. 2A ). Subsequently, after a third electrode material layer (not shown) is formed, at least the third electrode layer material layer is patterned to form third electrode layer  54  having the shape of a line extending in a second direction (e.g., the second direction II of  FIG. 2A ). Here, the first direction and the second direction intersect each other, and may perpendicularly intersect each other. 
     The shapes of the variable resistance layer  52  and the second electrode layer  53  may be changed depending on an etching depth in the patterning process to form the third electrode layer  54 . As an example, when only the third electrode material layer (not shown) is etched in the patterning process, the first electrode layer  51 , the variable resistance layer  52 , and the second electrode layer  53  may respectively have the shape of a line extending in the first direction, and the third electrode layer  54  may have the shape of a line extending in the second direction. As another example, when the third electrode material layer (not shown), the second electrode layer  53 , and the variable resistance layer  52  are etched in the patterning process to form the third electrode layer  54 , a second electrode pattern (not shown), and a variable resistance pattern (not shown), respectively. In this example, the first electrode layer  51  may have the shape of a line extending in the first direction, the third electrode layer  54  may have the shape of a line extending in the second direction, and the variable resistance pattern (not shown) and the second electrode pattern (not shown) may have the shape of an island located at an intersection of the first electrode layer  51  and the third electrode layer  54 . 
     The second electrode layer  53  and the third electrode layer  54  may be formed of the same material, or may be formed of different materials. As an example, the second electrode layer  53  and the third electrode layer  54  may be formed of the same metal layer. In this case, a deposition process may be divided into sub-processes such that an interface is formed between the second electrode layer  53  and the third electrode layer  54 . As another example, the second electrode layer  53  and the third electrode layer  54  may be formed of different kinds of metal layers. 
     Referring to  FIG. 5B , in order to form a gas region (or a gas layer), ions of an element that is a gas at room temperature are implanted into the interface between the second electrode layer  53  and the third electrode layer  54 . For example, hydrogen ions, helium ions, or a combination thereof are implanted at a high concentration into the interface between the second electrode layer  53  and the third electrode layer  54 , using an ion implantation process. At this time, a projection range (Rp) is adjusted such that a concentration of the implanted ions has a maximum value at the interface between the second electrode layer  53  and the third electrode layer  54 . For example, the Rp may be adjusted by changing implant energy in the ion implantation process. Accordingly, implanted ions  55  accumulate at the interface between the second electrode layer  53  and the third electrode layer  54 . 
     As an example, when the second electrode layer  53  and the third electrode layer  54  are formed of the same metal material, one or more defects exist at the interface between the second electrode layer  53  and the third electrode layer  54 . Therefore, in order to reduce Gibbs free energy at the interface, the implanted ions are gathered at a high concentration proximate to the defects at the interface, which is a thermodynamically unstable interface. As another example, when the second electrode layer  53  and the third electrode layer  54  are formed of different kinds of metal materials, a defect exists at the interface between the second electrode layer  53  and the third electrode layer  54 . In addition, a lattice mismatch due to the different kinds of metals exists at the interface between the second electrode layer  53  and the third electrode layer  54 . Here, the term “lattice mismatch” in the present disclosure may refer not only to a phenomenon where two layers having different lattice constants are brought together but also to a defect induced by the phenomenon. Therefore, the implanted ions  55  are disposed in a high concentration proximate to the defect and the lattice mismatch at the interface between the second electrode layer  53  and the third electrode layer  54 . 
     Referring to  FIG. 5C , the ions  55  accumulated at the interface between the second electrode layer  53  and the third electrode layer  54  are bonded to each other, thereby forming a gas region. Accordingly, a gas region that may include hydrogen gas, helium gas, or a mixture gas thereof is formed. 
     In this case, an annealing process may be performed so as to promote bonding between the ions  55 . For example, a post deposition annealing (PDA) process may be performed at a temperature of 500° C. or less. Accordingly, a selecting element layer  56  including the gas region can be formed. 
       FIGS. 6A to 6C  are cross-sectional views illustrating a method for fabricating the switch of  FIG. 1B  and the resistive memory cell of  FIGS. 3A and 3B  according to an embodiment of the present disclosure. Hereinafter, descriptions similar to those described above will be omitted for the interest of brevity. 
     Referring to  FIG. 6A , a first electrode layer  61 , a variable resistance layer  62 , a second electrode layer  63 , an insulating layer  64 , and a third electrode layer  65  are formed. Here, the first electrode layer  61  may have the shape of a line extending in a first direction (e.g., the first direction I of  FIG. 3A ), and the third electrode layer  65  may have the shape of a line extending in a second direction (e.g., the second direction II of  FIG. 3A ). Also, the variable resistance layer  62 , the second electrode layer  63 , and the insulating layer  64  may respectively have the shape of a line extending in the first direction, or may respectively have the shape of an island located at an intersection of the first electrode layer  61  and the third electrode layer  65 . 
     Referring to  FIG. 6B , ions  66  are implanted into an interface between the insulating layer  64  and the third electrode layer  65 . A defect and a lattice mismatch exist at the interface between the insulating layer  64  and the third electrode layer  65 , and therefore, a high concentration of ions  66  are proximate to the defect and the lattice mismatch at the interface. 
     Referring to  FIG. 6C , the ions  66  concentrated at the interface between the insulating layer  64  and the third electrode layer  65  are bonded to each other, thereby forming a gas region (or a gas layer)  67 . In this case, after the ions  66  are implanted, an annealing process may be performed so as to promote bonding between the ions  66 . Accordingly, a selecting element layer including the insulating layer  64  and the gas region  67  can be formed. 
     According to an embodiment of the present disclosure, a selecting element layer includes a gas region. Current flows into the gas region, or the current flowing into the gas region is substantially cut off according to a value of a voltage or a current applied to a switching device including the selecting element layer. Thus, it is possible to provide a switching device exhibiting nonlinear current-voltage behavior. Also, an off-current (T off ) can be minimized, thereby ensuring an excellent on/off ratio. 
     Embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a descriptive sense and not for purpose of limitation. In some instances, features, characteristics, and/or elements described in connection with a particular embodiment may be used alone or in combination with features, characteristics, and/or elements described in connection with other embodiments, unless otherwise specifically indicated. Accordingly, it will be understood by those of skill in the art that various changes in form and details may be possible.