Patent Publication Number: US-2019172871-A1

Title: Selector Device Incorporating Conductive Clusters for Memory Applications

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation-in-part of the commonly assigned application bearing Ser. No. 15/157,607 filed on May 18, 2016 by Yang et al. and entitled “Selector Device Incorporating Conductive Clusters for Memory Applications,” the content of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a selector device for memory applications, and more particularly, to embodiments of a two-terminal selector device. 
     A resistance-based memory device normally comprises an array of memory cells, each of which includes a memory element and a selector element coupled in series between two electrodes. The selector element functions like a switch to direct voltage or current through the selected memory element coupled thereto. The selector element may be a three terminal device, such as transistor, or a two-terminal device, such as diode or Ovonic threshold switch (OTS). Upon application of an appropriate voltage or current to the selected memory element, the electrical property of the memory element would change accordingly, thereby switching the stored logic in the respective memory cell. 
       FIG. 1  is a schematic circuit diagram of a memory array  30 , which comprises a plurality of memory cells  32  with each of the memory cells  32  including a two-terminal selector element  34  coupled to a resistance-based memory element  36  in series; a first plurality of parallel wiring lines  38  with each being coupled to a respective row of the memory elements  36  in a first direction; and a second plurality of parallel wiring lines  40  with each being coupled to a respective row of the selector elements  34  in a second direction substantially perpendicular to the first direction. Accordingly, the memory cells  32  are located at the cross points between the first and second plurality of wiring lines  38  and  40 . 
     The resistance-based memory element  36  may be classified into at least one of several known groups based on its resistance switching mechanism. The memory element of Phase Change Random Access Memory (PCRAM) may comprise a phase change chalcogenide compound, which can switch between a resistive phase (amorphous or crystalline) and a conductive crystalline phase. The memory element of Conductive Bridging Random Access Memory (CBRAM) relies on the statistical bridging of metal rich precipitates therein for its switching mechanism. The memory element of CBRAM normally comprises a nominally insulating metal oxide material, which can switch to a lower electrical resistance state as the metal rich precipitates grow and link to form conductive paths upon application of an appropriate voltage. The memory element of Magnetic Random Access Memory (MRAM) typically comprises at least two layers of ferromagnetic materials with an insulating tunnel junction layer interposed therebetween. When a switching current is applied to the memory element of an MRAM device, one of the ferromagnetic layers will switch its magnetization direction with respect to that of the other magnetic layer, thereby changing the electrical resistance of the element. 
     A magnetic memory element normally includes a magnetic reference layer and a magnetic free layer with an electron tunnel junction layer interposed therebetween. The magnetic reference layer, the electron tunnel junction layer, and the magnetic free layer collectively form a magnetic tunnel junction (MTJ). Upon the application of an appropriate current through the MTJ, the magnetization direction of the magnetic free layer can be switched between two directions: parallel and anti-parallel with respect to the magnetization direction of the magnetic reference layer. The electron tunnel junction layer is normally made of an insulating material with a thickness ranging from a few to a few tens of angstroms. When the magnetization directions of the magnetic free and reference layers are substantially parallel or oriented in a same direction, electrons polarized by the magnetic reference layer can tunnel through the insulating tunnel junction layer, thereby decreasing the electrical resistance of the MTJ. Conversely, the electrical resistance of the MTJ is high when the magnetization directions of the magnetic reference and free layers are substantially anti-parallel or oriented in opposite directions. The stored logic in the magnetic memory element can be switched by changing the magnetization direction of the magnetic free layer between parallel and anti-parallel with respect to the magnetization direction of the reference layer. Therefore, the MTJ has two stable resistance states that allow the MTJ to serve as a non-volatile memory element. 
     Based on the relative orientation between the magnetic reference and free layers and the magnetization directions thereof, an MTJ can be classified into one of two types: in-plane MTJ, the magnetization directions of which lie substantially within planes parallel to the same layers, or perpendicular MTJ, the magnetization directions of which are substantially perpendicular to the layer planes. 
     The use of the two-terminal selector element  34  allows the memory cells  32  to attain the minimum cell size of 4F 2 , where F denotes the minimum feature size or one half the minimum feature pitch normally associated with a particular manufacturing process, thereby increasing memory array density. However, conventional bi-directional, two-terminal selector devices, such as Ovonic threshold switch (OTS), have relatively low on/off switching speeds and are prone to current leakage compared with conventional selection transistors. 
     For the foregoing reasons, there is a need for a two-terminal selector device for memory applications that has high on/off switching speeds and low current leakage and that can be inexpensively manufactured. 
     SUMMARY 
     The present invention is directed to a device that satisfies this need. A memory device having features of the present invention comprises an array of memory cells. Each of the memory cells includes a memory element connected to a two-terminal selector element. The two-terminal selector element includes a first electrode and a second electrode with a volatile switching layer interposed therebetween. The second electrode is deposited on top of the volatile switching layer during fabrication. The first electrode has a composition comprising a metal element and the second electrode has a composition comprising the metal element and aluminum element. The metal element may be silver, copper, or nickel. The volatile switching layer may have a composite structure comprising a plurality of conductive particles embedded in an insulating matrix. Alternatively, the volatile switching layer may have a multilayer structure comprising one or more conductive layers interleaved with two or more insulating layers. The memory element may include a magnetic tunnel junction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1  is a schematic circuit diagram of a memory array including a plurality of memory cells with each comprising a memory element and a two-terminal selector element coupled in series between two electrodes; 
         FIG. 2  is a perspective view of a three dimensional memory device in accordance with an embodiment of the present invention; 
         FIGS. 3A and 3B  are cross sectional views of one of memory cells in accordance with different embodiments of the present invention; 
         FIG. 4  is a cross sectional view of a selector element structure in accordance with an embodiment of the present invention; 
         FIGS. 5A-5C  are cross sectional views of three exemplary structures for the volatile switching layer structure in the selector element of  FIG. 4 ; 
         FIGS. 6A-6C  are cross sectional views of exemplary structures for a volatile switching layer structure having two, three, and four switching layers, respectively; 
         FIGS. 7A-7F  are cross sectional views of exemplary structures for a volatile switching layer structure having two switching layers; 
         FIGS. 8A-8F  are cross sectional views of exemplary structures for a volatile switching layer structure having three switching layers; 
         FIGS. 9A-9E  are cross sectional views showing exemplary structures for the first electrode structure of  FIG. 4  having one, two, three, four, and five first electrode layers, respectively; 
         FIGS. 10A-10E  are cross sectional views of exemplary structures for the second electrode structure of  FIG. 4  having one, two, three, four, and five second electrode layers, respectively; 
         FIGS. 11A-11D  are cross sectional views of exemplary structures for a magnetic tunnel junction (MTJ) memory element in accordance with an embodiment of the present invention; 
         FIG. 12A  is an I-V response plot for the two-terminal selector element of  FIG. 4  as an applied voltage cycles from zero to V p  and back; 
         FIG. 12B  is another I-V response plot for the two-terminal selector element of  FIG. 4  as an applied voltage cycles from zero to V p  and back; 
         FIGS. 13A-13D  illustrate various stages in formation of a conductive path in a volatile switching layer by applying a positive voltage to a top electrode; 
         FIG. 14  illustrates formation of a conductive path in a switching layer by applying a positive voltage to a bottom electrode; and 
         FIG. 15  is a cross sectional view of a two-terminal selector element that incorporates therein additional electrode layers in accordance with another embodiment of the present invention. 
     
    
    
     For purposes of clarity and brevity, like elements and components will bear the same designations and numbering throughout the Figures, which are not necessarily drawn to scale. 
     DETAILED DESCRIPTION 
     Where reference is made herein to a material AB composed of element A and element B, the material AB can be an alloy, a compound, or a combination thereof, except where the context excludes that possibility. 
     The term “noncrystalline” means an amorphous state or a state in which fine crystals are dispersed in an amorphous matrix, not a single crystal or polycrystalline state. In case of state in which fine crystals are dispersed in an amorphous matrix, those in which a crystalline peak is substantially not observed by, for example, X-ray diffraction can be designated as “noncrystalline.” 
     The term “at least” followed by a number is used herein to denote the start of a range beginning with that number, which may be a range having an upper limit or no upper limit, depending on the variable being defined. For example, “at least 1” means 1 or more than 1. The term “at most” followed by a number is used herein to denote the end of a range ending with that number, which may be a range having 1 or 0 as its lower limit, or a range having no lower limit, depending upon the variable being defined. For example, “at most 4” means 4 or less than 4, and “at most 40%” means 40% or less than 40%. When, in this specification, a range is given as “a first number to a second number” or “a first number-a second number,” this means a range whose lower limit is the first number and whose upper limit is the second number. For example, “25 to 100 nm” means a range whose lower limit is 25 nm and whose upper limit is 100 nm. 
     An embodiment of the present invention as applied to a memory device having multiple layers of memory cells will now be described with reference to  FIG. 2 . Referring now to  FIG. 2 , the illustrated device comprises two layers of memory cells  102  with each layer of the memory cells  102  formed between a layer of parallel first conductor lines  104  extending along the y-direction and a layer of parallel second conductor lines  106  extending along the x-direction, which is substantially perpendicular to the y-direction. For each layer of the memory cells  102 , each of the first conductor lines  104  couples to one ends (top or bottom) of a respective row of the memory cells  102  along the y-direction, while each of the second conductor lines  106  couples to the other ends (top or bottom) of a respective row of the memory cells  102  along the x-direction. Two adjacent layers of the memory cells  102  share a layer of the second conductor lines  106 . Accordingly, each of the second conductor lines  106  are coupled to two rows of memory cells thereabove and therebeneath, respectively. For reasons of clarity, only two layers of the memory cells  102  are shown in  FIG. 2 . However, the present invention can accommodate as many layers of the memory cells  102  as desired. For example, a third layer of memory cells (not shown) may be formed on top of the top layer of the first conductor lines  104  and another layer of the second conductor lines (not shown) may be formed on top of the third layer of memory cells, and so forth. The first and second conductor lines  104  and  106  may operate as word lines and bit lines, respectively, or vice versa. Each of the memory cells  102  includes a memory element  108  and a two-terminal selector element  122  coupled in series. Each of the memory cells  102  may further include an optional intermediate electrode  112  interposed between the memory element  108  and the two-terminal selector element  122 . 
       FIG. 3A  is a cross sectional view of one of the memory cells  102 , which includes the memory element  108  formed on top of one of the first conductor lines  104  extending along the y-direction, the two-terminal selector element  122  formed on top of the memory element  108 , and the optional intermediate electrode  112  interposed therebetween. One of the second conductor lines  106  forms on top of the two-terminal selector element  122  and extends along the x-direction. In embodiments where the optional intermediate electrode  112  is absent, the two-terminal selector element  122  is directly coupled to the memory element  108 . 
     The stacking order of the two-terminal selector element  122  and the memory element  108  may alternatively be reversed, as illustrated in  FIG. 3B , such that the memory element  108  is formed on top of the two-terminal selector element  122  with the optional intermediate electrode interposed therebetween. Each layer of the memory cells  102  may have the configuration illustrated in  FIG. 3A or 3B . 
     One or more of the first conductor lines  104  and the second conductor lines  106  may be made of any suitable conductor, such as but not limited to copper (Cu), tungsten (W), aluminum (Al), silver (Ag), gold (Au), titanium (Ti), ruthenium (Ru), iridium (Ir), platinum (Pt), palladium (Pd), tantalum (Ta), titanium nitride (TiN x ), tantalum nitride (TaN x ), or any combination thereof. 
     The optional intermediate electrode  112  may be made of any suitable conductor, such as but not limited to copper (Cu), tungsten (W), aluminum (Al), silver (Ag), gold (Au), titanium (Ti), ruthenium (Ru), iridium (Ir), platinum (Pt), palladium (Pd), tantalum (Ta), titanium nitride (TiN x ), tantalum nitride (TaN x ), tungsten silicide (WSi x ), titanium silicide (TiSi x ), cobalt silicide (CoSi x ), nickel silicide (NiSi x ), platinum silicide (PtSix), or any combination thereof. 
     The memory element  108  may change the resistance state thereof by any suitable switching mechanism, such as but not limited to phase change, precipitate bridging, magnetoresistive switching, or any combination thereof. In one embodiment, the memory element  108  comprises a phase change chalcogenide compound, such as but not limited to Ge 2 Sb 2 Te 5  or AgInSbTe, which can switch between a resistive phase and a conductive phase. In another embodiment, the memory element  108  comprises a nominally insulating metal oxide material, such as but not limited to NiO, TiO 2 , or Sr(Zr)TiO 3 , which can switch to a lower electrical resistance state as metal rich precipitates grow and link to form conductive paths upon application of an appropriate voltage. In still another embodiment, the memory element  108  comprises a magnetic free layer and a magnetic reference layer with an insulating electron tunnel junction layer interposed therebetween, collectively forming a magnetic tunnel junction (MTJ). When a switching pulse is applied, the magnetic free layer would switch the magnetization direction thereof, thereby changing the electrical resistance of the MTJ. The magnetic free layer may have a variable magnetization direction substantially perpendicular to a layer plane thereof. The magnetic reference layer may have a fixed magnetization direction substantially perpendicular to a layer plane thereof. Alternatively, the magnetization directions of the magnetic free and reference layers may orientations that are parallel to layer planes thereof. 
     An embodiment of the present invention as applied to the two-terminal selector element  122  will now be described with reference to  FIG. 4 . Referring now to  FIG. 4 , the illustrated selector element  122  includes a first electrode structure  124  and a second electrode structure  126  with a volatile switching layer structure  128  interposed therebetween. The second electrode structure  126  may be deposited onto the volatile switching layer structure  128  during fabrication. 
     The volatile switching layer structure  128 , which may include one or more distinct volatile switching layers, behaves like a volatile device that is nominally insulative in the absence of an applied voltage or current. Upon continuing application of a switching voltage or current, however, the volatile switching layer structure  128  becomes conductive. In an embodiment illustrated in  FIG. 5A , the volatile switching layer structure  128  includes a homogeneous layer  128   a  made of a nominally insulating material or any suitable material that switches its resistance in the presence of an applied field or current, such as but not limited to SiO x , SiN x , AlO x , MgO x , TaO x , VO x , NbO x , TiO x , WO x , HfO x , ZrO x , NiO x , FeO x , YO x , EuO x , SbO x , AsO x , SbO x , SnO x , InO x , SeO x , GaO x , CuGe x S y , CuAg x Ge y S z , GeSb x Te y , AgIn x Sb y Te z , GeTe x , SbTe x , GeSb x , CrO x , SrTi x O y , YZr x O y , LaF x , AgI x , CuI x , RbAg x I y , or any combination thereof. The exemplary compounds may be stoichiometric or non-stoichiometric. The homogeneous layer  128   a  may further include one or more dopant or alloying elements, such as but not limited to Ag, Au, Zn, Sn, Ni, As, and Cu. 
     Alternatively, the volatile switching layer structure  128  may include a composite layer  128   b  comprising a plurality of conductive particles or clusters  130  embedded in a nominally insulating matrix  132  as illustrated in  FIG. 5B . The conductive particles may have metal-rich compositions. The nominally insulating matrix  132  may be made of any suitable material, such as but not limited to SiO x , SiN x , AlO x , MgO x , TaO x , VO x , NbO x , TiO x , WO x , HfO x , ZrO x , NiO x , FeO x , YO x , EuO x , SrO x , AsO x , SbO x , SnO x , InO x , SeO x , GaO x , CeO x , TeO x , CuGe x S y , CuAg x Ge y S z , GeSb x Te y , AgIn x Sb y Te z , GeTe x , SbTe x , GeSb x , CrO x , SrTi x O y , YZr x O y , LaF x , AgI x , CuI x , RbAg x I y , or any combination thereof. The exemplary compounds may be stoichiometric or non-stoichiometric. 
     With continuing reference to  FIG. 5B , the plurality of conductive particles or clusters  130  may be made of a relatively inert metal, or an alloy including one or more inert metals, or a fast electric field enhanced diffuser material, or any combination thereof. Examples of the inert metal include gold (Au), silver (Ag), platinum (Pt), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), osmium (Os), rhenium (Re), and any combinations thereof. Examples of the fast electric field enhanced diffuser material include nickel (Ni), copper (Cu), silver (Ag), gold (Au), zinc (Zn), cobalt (Co), iron (Fe), tungsten (W), molybdenum (Mo), tantalum (Ta), niobium (Nb), aluminum (Al), titanium (Ti), zirconium (Zr), arsenic (As), titanium nitride (TiN x ), zirconium nitride (ZrN x ), tantalum nitride (TaN x ), niobium nitride (NbN x ), tungsten nitride (WN x ), and any combinations thereof. The exemplary nitrides may be stoichiometric or non-stoichiometric. 
     The composite layer structure  128   b  shown in  FIG. 5B  may be fabricated by co-sputtering, whereby the target for the plurality of conductive particles or clusters  130  and the target for the insulating matrix  132  are sputtered at the same time. Alternatively, the composite layer structure  128   b  may be fabricated by alternating sputter deposition of materials corresponding to the conductive particles or clusters  130  and the insulating matrix  132 . The sputter-deposited film by both methods may subsequently subjected to an annealing process to enhance the diffusion or precipitation of the conductive particles or clusters  130 . 
     Still alternatively, the volatile switching layer structure  128  may have a multilayer structure  128   c  comprising one or more conductive layers  134  interleaved with two or more insulating layers  136  as illustrated in  FIG. 5C . The conductive layers  134  may be made of any of the suitable conductive materials described above for the conductive particles or clusters  130 . The thickness of the conductive layers  134  may range from several angstroms to several nanometers. In some cases where the conductive layers  134  are extremely thin, one or more of the conductive layers  134  may be punctured by holes, thereby rendering the layer coverage to be discontinuous in some regions. Similarly, the nominally insulating layers  136  may be made of any of the suitable insulating materials described above for the matrix  132 . In an embodiment, the thicknesses of the conductive layers  134  decrease and/or the thicknesses of the insulating layers  136  increase along the direction of the anti-parallelizing current. 
     The volatile switching layer structure  128  may alternatively include two or more volatile switching layers with each switching layer being a homogenous layer  128   a , a composite layer  128   b , or a multilayer structure  128   c .  FIGS. 6A-6C  illustrate the volatile switching layers structure  128  including two, three, and four switching layers, respectively. 
     Some examples of the volatile switching layer structure  128  having two switching layers are illustrated in  FIGS. 7A-7F .  FIG. 7A  shows an exemplary structure having two homogenous layers  128   a , which may be made of different materials and/or having different dopants if present.  FIG. 7B  shows another exemplary structure including a homogenous layer  128   a  and a composite layer  128   b . In an embodiment, the homogenous layer  128   a  and the matrix  132  of the composite layer  128   b  are made of the same material. In an alternative embodiment, the homogenous layer  128   a  and the matrix  132  of the composite layer  128   b  are made of different materials.  FIG. 7C  shows still another exemplary structure including a homogenous layer  128   a  and a multilayer structure  128   c . In an embodiment, the homogenous layer  128   a  and the insulating layers  136  of the multilayer structure  128   c  are made of the same material. In an alternative embodiment, the homogenous layer  128   a  and the insulating layers  136  of the multilayer structure  128   c  are made of different materials.  FIG. 7D  shows yet another exemplary structure including two composite layer  128   b , which may have different materials for the matrix  132  and/or different materials for the conductive particles or clusters  130 .  FIG. 7E  shows still yet another exemplary structure including a composite layer  128   b  and a multilayer structure  128   c . The matrix  132  of the composite layers  128   b  and the insulating layers  136  of the multilayer structure  128   c  may be made of the same material or different materials. Likewise, the conductive particles or clusters  130  of the composite layer  128   b  and the conductive layers  134  of the multilayer structure  128   c  may be made of the same material or different materials.  FIG. 7F  shows yet still another exemplary structure including two multilayer structures  128   c , which may have different materials for the insulating layers  136  and/or different materials for the conductive layers  134 . Moreover, the stacking order of the volatile switching layers in the exemplary structures illustrated in  FIGS. 7A-7F  may be inverted. 
     Some examples of the volatile switching layer structure  128  having three switching layers are illustrated in  FIGS. 8A-8F .  FIG. 8A  shows an exemplary structure including two homogenous layers  128   a  with a composite layer  128   b  interposed therebetween. The two homogenous layers  128   a  may be made of the same material or different materials. The matrix  132  of the composite layer  128   b  and at least one of the two homogeneous layers  128   a  may be made of the same material. Alternatively, the matrix  132  of the composite layer  128   b  may be made of a different material from the two homogeneous layers  128   a.    
       FIG. 8B  shows another exemplary structure including two composite layers  128   b  with a homogeneous layer  128   a  interposed therebetween. The matrices  132  of the two composite layers  128   b  may be made of the same material or different materials. The conductive particles or clusters  130  of the two composite layers  128   b  may be made of the same material or different materials. The homogeneous layer  128   a  and at least one of the two matrices  132  of the two composite layers  128   b  may be made of the same material. Alternatively, the homogeneous layer  128   a  may be made of a different material from the two matrices  132  of the two composite layers  128   b.    
       FIG. 8C  illustrates still another exemplary structure including two homogenous layers  128   a  with a multilayer structure  128   c  interposed therebetween. The two homogenous layers  128   a  may be made of the same material or different materials. The insulating layers  136  of the multilayer structure  128   c  and at least one of the two homogeneous layers  128   a  may be made of the same material. Alternatively, the insulating layers  136  of the multilayer structure  128   c  may be made of a different material from the two homogeneous layers  128   a.    
       FIG. 8D  illustrates yet another exemplary structure including two multilayer structures  128   c  with a homogeneous layer  128   a  interposed therebetween. The insulating layers  136  of the two multilayer structures  128   c  may be made of the same material or different materials. Likewise, the conductive layers  134  of the two multilayer structures  128   c  may be made of the same material or different materials. The homogeneous layer  128   a  and at least one of the two stacks of insulating layers  136  of the two multilayer structures  128   c  may be made of the same material. Alternatively, the homogeneous layer  128   a  may be made of a different material from the insulating layers  136  of the two multilayer structures  128   c.    
       FIG. 8E  shows still yet another exemplary structure including a composite layer  128   b  and a multilayer structure  128   c  with a homogeneous layer  128   a  interposed therebetween. The matrix  132  of the composite layer  128   b  and the insulating layers  136  of the multilayer structure  128   c  may be made of the same material or different materials. The conductive particles or clusters  130  of the composite layer  128   b  and the conductive layers  134  of the multilayer structure  128   c  may be made of the same material or different materials. The homogeneous layer  128   a  and at least one of the matrix  132  of the composite layer  128   b  and the insulating layers  136  of the multilayer structure  128   c  may be made of the same material. Alternatively, the homogeneous layer  128   a  may be made of a different material from the matrix  132  of the composite layer  128   b  and the insulating layers  136  of the multilayer structure  128   c.    
       FIG. 8F  illustrates yet still another exemplary structure including three homogeneous layers  128   a . The three homogeneous layers  128   a  may be made of different materials and/or have different dopants if present. In an embodiment, the interposing homogenous layer  128   a  is made of a different material from the two peripheral homogeneous layers  128   a , which may be made of the same material and/or have the same dopant if present. 
     The stacking order of the volatile switching layers in the exemplary structures illustrated in  FIGS. 8A-8F  may be inverted. 
     Referring back to  FIG. 4 , the first electrode structure  124  and the second electrode structure  126  of the selector element  122  each may include one or more electrode layers.  FIGS. 9A-9E  show partial views of the selector element  122  including the volatile switching layer structure  128  and various exemplary structures for the first electrode structure  124 . 
       FIG. 9A  illustrates an exemplary structure for the first electrode structure  124  that includes a first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128 . 
       FIG. 9B  illustrates another exemplary structure for the first electrode structure  124  that includes the first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128  and a second electrode layer  124   b  formed adjacent to the first electrode layer  124   a  opposite the volatile switching layer structure  128 . 
       FIG. 9C  illustrates still another exemplary structure for the first electrode structure  124  that includes the first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  124   b  formed adjacent to the first electrode layer  124   a  opposite the volatile switching layer structure  128 , and a third electrode layer  124   c  formed adjacent to the second electrode layer  124   b  opposite the first electrode layer  124   a.    
       FIG. 9D  illustrates yet another exemplary structure for the first electrode structure  124  that includes the first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  124   b  formed adjacent to the first electrode layer  124   a  opposite the volatile switching layer structure  128 , the third electrode layer  124   c  formed adjacent to the second electrode layer  124   b  opposite the first electrode layer  124   a , and a fourth electrode layer  124   d  formed adjacent to the third electrode layer  124   c  opposite the second electrode layer  124   b.    
       FIG. 9E  illustrates still yet another exemplary structure for the first electrode structure  124  that includes the first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  124   b  formed adjacent to the first electrode layer  124   a  opposite the volatile switching layer structure  128 , the third electrode layer  124   c  formed adjacent to the second electrode layer  124   b  opposite the first electrode layer  124   a , the fourth electrode layer  124   d  formed adjacent to the third electrode layer  124   c  opposite the second electrode layer  124   b , and a fifth electrode layer  124   e  formed adjacent to the fourth electrode layer  124   d  opposite the third electrode layer  124   c.    
     The first, second, third, fourth, and fifth electrode layers  124   a - 124   e  of the first electrode structure  124  each may include one or more of the following elements: Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, Fe, Re, Mn, W, Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, Mg, Zn, Cd, In, Ga, Al, B, Pb, Sn, Ge, Si, C, Bi, Sb, As, N, Te, Se, and O to form a suitable conductor material, such as but not limited to Au, Ag, Pt, Pd, Rh, Ir, Ru, Re, Si, Ni, NiSi x , NiCr x , Cu, CuSi x , CuGe x , CuAl x , CuN x , Co, CoSi x , CoCr x , Zn, ZnN x , Fe, FeNi x Cr y , Cr, CrSi x  Al, AlN x , Ti, TiSi x , TiN x , Ta, TaSi x , TaN x , W, WSi x , WN x , Mo, MoSi x , MoN x , Zr, ZrSi x , ZrN x , Hf, HfSi x , HfN x , Nb, NbSi x , NbN x , V, VSi x , VN x , TiAl x , NiAl x , CoAl x , AgO x , CuO x , NiO x , or any combination thereof. For example and without limitation, the first and second electrode layers  124   a  and  124   b  may be made of AgO x  and Ag, respectively. Alternatively, the first and second electrode layers  124   a  and  124   b  may be made of TiN x  and Ag, respectively. Still alternatively, the first and second electrode layers  124   a  and  124   b  may be made of TiN x  and AgAl x , respectively. 
     One or more of the first, second, third, fourth, and fifth electrode layers  124   a - 124   e  of the first electrode structure  124  each may alternatively have a multilayer structure formed by interleaving one or more layers of a first material with one or more layers of a second material. The first and second materials each may include one or more of the following elements: Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, Fe, Re, Mn, W, Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, Mg, Zn, Cd, In, Ga, Al, B, Pb, Sn, Ge, Si, C, Bi, Sb, As, N, Te, Se, and O to form a suitable conductive material, such as but not limited to Au, Ag, Pt, Pd, Rh, Ir, Ru, Re, Si, Ni, NiSi x , NiCr x , Cu, CuSi x , CuGe x , CuAl x , CuN x , Co, CoSi x , CoCr x , Zn, ZnN x , Fe, FeNi x Cr y , Cr, CrSi x  Al, AlN x , Ti, TiSi x , TiN x , Ta, TaSi x , TaN x , W, WSi x , WN x , Mo, MoSi x , MoN x , Zr, ZrSi x , ZrN x , Hf, HfSi x , HfN x , Nb, NbSi x , NbN x , V, VSi x , VN x , TiAl x , NiAl x , CoAl x , AgO x , CuO x , NiO x , or any combination thereof. 
       FIGS. 10A-10E  show partial views of the selector element  122  including the volatile switching layer structure  128  and various exemplary structures for the second electrode structure  126 .  FIG. 10A  illustrates an exemplary structure for the second electrode structure  126  that includes a first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128 . 
       FIG. 10B  illustrates another exemplary structure for the second electrode structure  126  that includes the first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128  and a second electrode layer  126   b  formed adjacent to the first electrode layer  126   a  opposite the switching layer structure  128 . 
       FIG. 10C  illustrates still another exemplary structure for the second electrode structure  126  that includes the first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  126   b  formed adjacent to the first electrode layer  126   a  opposite the volatile switching layer structure  128 , and a third electrode layer  126   c  formed adjacent to the second electrode layer  126   b  opposite the first electrode layer  126   a.    
       FIG. 10D  illustrates yet another exemplary structure for the second electrode structure  126  that includes the first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  126   b  formed adjacent to the first electrode layer  126   a  opposite the volatile switching layer structure  128 , the third electrode layer  126   c  formed adjacent to the second electrode layer  126   b  opposite the first electrode layer  126   a , and a fourth electrode layer  126   d  formed adjacent to the third electrode layer  126   c  opposite the second electrode layer  126   b.    
       FIG. 10E  illustrates still yet another exemplary structure for the second electrode structure  126  that includes the first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128 , the second electrode layer  126   b  formed adjacent to the first electrode layer  126   a  opposite the volatile switching layer structure  128 , the third electrode layer  126   c  formed adjacent to the second electrode layer  126   b  opposite the first electrode layer  126   a , the fourth electrode layer  126   d  formed adjacent to the third electrode layer  126   c  opposite the second electrode layer  126   b , and a fifth electrode layer  126   e  formed adjacent to the fourth electrode layer  126   d  opposite the third electrode layer  126   c.    
     The first, second, third, fourth, and fifth electrode layers  126   a - 126   e  of the second electrode structure  126  each may include one or more of the following elements: Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, Fe, Re, Mn, W, Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, Mg, Zn, Cd, In, Ga, Al, B, Pb, Sn, Ge, Si, C, Bi, Sb, As, N, Te, Se, and O to form a suitable conductive material, such as but not limited to Au, Ag, Pt, Pd, Rh, Ir, Ru, Re, Si, Ni, NiSi x , NiCr x , Cu, CuSi x , CuGe x , CuAl x , CuN x , Co, CoSi x , CoCr x , Zn, ZnN x , Fe, FeNi x Cr y , Cr, CrSi x  Al, AlN x , Ti, TiSi x , TiN x , Ta, TaSi x , TaN x , W, WSi x , WN x , Mo, MoSi x , MoN x , Zr, ZrSi x , ZrN x , Hf, HfSi x , HfN x , Nb, NbSi x , NbN x , V, VSi x , VN x , TiAl x , NiAl x , CoAl x , AgO x , CuO x , NiO x , or any combination thereof. For example and without limitation, the first and second electrode layers  126   a  and  126   b  may be made of AgO x  and Ag, respectively. Alternatively, the first and second electrode layers  126   a  and  126   b  may be made of TiN x  and Ag, respectively. Still alternatively, the first and second electrode layers  126   a  and  126   b  may be made of TiN x  and AgAl x , respectively. 
     One or more of the first, second, third, fourth, and fifth electrode layers  126   a - 126   e  of the second electrode structure  126  each may alternatively have a multilayer structure formed by interleaving one or more layers of a first material with one or more layers of a second material. The first and second materials each may include one or more of the following elements: Au, Ag, Cu, Pt, Pd, Ni, Ir, Rh, Co, Os, Ru, Fe, Re, Mn, W, Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, Mg, Zn, Cd, In, Ga, Al, B, Pb, Sn, Ge, Si, C, Bi, Sb, As, N, Te, Se, and O to form a suitable conductive material, such as but not limited to Au, Ag, Pt, Pd, Rh, Ir, Ru, Re, Si, Ni, NiSi x , NiCr x , Cu, CuSi x , CuGe x , CuAl x , CuN x , Co, CoSi x , CoCr x , Zn, ZnN x , Fe, FeNi x Cr y , Cr, CrSi x  Al, AlN x , Ti, TiSi x , TiN x , Ta, TaSi x , TaN x , W, WSi x , WN x , Mo, MoSi x , MoN x , Zr, ZrSi x , ZrN x , Hf, HfSi x , HfN x , Nb, NbSi x , NbN x , V, VSi x , VN x , TiAl x , NiAl x , CoAl x , AgO x , CuO x , NiO x , or any combination thereof. 
     Referring again to  FIG. 4 , the first electrode structure  124  and the second electrode structure  126  of the selector element  122  may have a “asymmetric” configuration, whereby the two electrode structures  124  and  126  have different numbers of electrode layers and/or different conductive materials for comparable electrode layers (e.g., the first electrode layer  124   a  and the first electrode layer  126   a  are made of different materials). For example and without limitation, an asymmetric selector element  122  may comprise a first electrode structure  124  that includes a first electrode layer  124   a  made of silver, a second electrode structure  126  that includes a first electrode layer  126   a  made of copper, and a volatile switching layer structure  128  including a plurality of silver particles or clusters  130  embedded in a hafnium oxide matrix  132  as illustrated in  FIG. 5B  or at least one layer of silver  134  interleaved with two or more layers of hafnium oxide  136  as illustrated in  FIG. 5C . The second electrode structure  126  of the above exemplary asymmetric selector element may alternatively include a first electrode layer  126   a  made of titanium nitride and a second electrode layer  126   b  made of silver. In an embodiment, the plurality of conductive particles or clusters  130  or the conductor layers  134  in the volatile switching layer structure  128  are made of the same material as at least one electrode layer in at least one of the first and second electrode structures  124  and  126 . For example and without limitation, the plurality of conductive particles or clusters  130  and the second electrode layer  126   b  of the second electrode structure  126  both may be made of Ag, Cu, Co, Ni, or any combination thereof. 
     The first electrode structure  124  and the second electrode structure  126  of the selector element  122  may alternatively have a “symmetric” configuration, whereby the two electrode structures  124  and  126  have the same number of electrode layers and the same conductive material for comparable electrode layers (i.e., the first electrode layer  124   a  and the first electrode layer  126   a  are made of the same material, the second electrode layer  124   b  and the second electrode layer  126   b  are made of the same material, and so on). 
     In an embodiment for the selector element  122  with the symmetric electrode configuration, the volatile switching layer structure  128  includes a plurality of conductive particles or clusters  130  embedded in a matrix  132 . The conductive particles or clusters  130  are made of Ag, Au, Ni, Cu, Co, As, or any combination thereof, while the matrix  132  is made of HfO x , ZrO x , TiO x , NiO x , YO x , AlO x , MgO x , TaO x , SiO x , or any combination thereof. The volatile switching layer structure  128  may have an alternative structure that includes one or more conductive layers  134  interleaved with two or more insulating layers  136 . The conductive layers  134  are made of Ag, Au, Ni, Cu, Co, Ta, As, or any combination thereof, while the insulating layers  136  are made of HfO x , ZrO x , TiO x , NiO x , YO x , AlO x , MgO x , TaO x , SiO x , or any combination thereof. The first and second electrode structures  124  and  126  of the selector element  122  with the symmetric electrode configuration include the first electrode layers  124   a  and  126   a  made of a material that may interact with defects or ions in the volatile switching layer structure  128  in the presence of an electric field, such as but not limited to Ag, Au, Ni, Cu, Co, Ta, Ti, Al, or any combination thereof, thereby acting as “active” electrodes. The first and second electrode structures  124  and  126  may further include the second electrode layers  124   b  and  126   b  that may be relatively inert with respect to the defects or ions in the volatile switching layer structure  128 , such as but not limited to Pt, Pd, Rh, Ir, Ru, Re, Ta, TiN x , ZrN x , HfN x , TaN x , NbN x , TiSi x , CoSi x , NiSi x , or any combination thereof, thereby acting as “inert” electrodes. 
     In another embodiment for the selector element  122  with the symmetric electrode configuration, the volatile switching layer structure  128  includes a plurality of conductive particles or clusters  130  embedded in a matrix  132 . The conductive particles or clusters  130  are made of Ag, Au, Ni, Cu, Co, As, or any combination thereof, while the matrix  132  is made of HfO x , ZrO x , TiO x , NiO x , YO x , AlO x , MgO x , TaO x , SiO x , or any combination thereof. The volatile switching layer structure  128  may have an alternative structure that includes one or more conductive layers  134  interleaved with two or more insulating layers  136 . The conductive layers  134  are made of Ag, Au, Ni, Cu, Co, Ta, As, or any combination thereof, while the insulating layers  136  are made of HfO x , ZrO x , TiO x , NiO x , YO x , AlO x , MgO x , TaO x , SiO x , or any combination thereof. The first and second electrode structures  124  and  126  of the selector element  122  with the symmetric electrode configuration include the first electrode layers  124   a  and  126   a  made of a material that may be relatively inert and may not interact with defects or ions in the volatile switching layer structure  128  in the presence of an electric field, such as but not limited to Pt, Pd, Rh, Ir, Ru, Re, Ta, TiN x , ZrN x , HfN x , TaN x , NbN x , TiSi x , CoSi x , NiSi x , or any combination thereof; and the second electrode layers  124   b  and  126   b  that may act as active electrodes and are made of a material that may interact with defects or ions in the volatile switching layer structure  128  in the presence of an electric field, such as but not limited to Ag, Au, Ni, Cu, Co, Ta, Ti, Al, or any combination thereof. In addition to being relatively inert, the first electrode layers  124   a  and  126   a  may serve as diffusion barrier for the movement of defects or ions between the volatile switching layer structure  128  and the second electrode layers  124   b  and  126   b . The first and second electrode structures  124  and  126  may further include the third electrode layers  124   c  and  126   c  that may be relatively inert and may not interact with defects or ions in the volatile switching layer structure  128 . For example and without limitation, the third electrode layers  124   c  and  126   c  may be made of Pt, Pd, Rh, Ir, Ru, Re, Ta, TiN x , ZrN x , HfN x , CoSi x , NiSi x , or any combination thereof. 
     In still another embodiment for the selector element  122  with the symmetric electrode configuration, the plurality of conductive particles or clusters  130  or the conductive layers  134  in the volatile switching layer structure  128  are made of the same material as at least one electrode layer in the first and second electrode structures  124  and  126 . For example and without limitation, the plurality of conductive particles or clusters  130  and the second electrode layers  124   b  and  126   b  may be made of Ag, Cu, Co, Ni, or any combination thereof. 
       FIG. 11A  shows an exemplary MTJ structure  190  for the memory element  108  that includes a magnetic free layer structure  200  and a magnetic reference layer structure  202  with a tunnel junction layer  204  interposed therebetween. The magnetic free layer structure  200  has a variable magnetization direction  206  substantially perpendicular to the layer plane thereof. The magnetic reference layer structure  202  has a first invariable magnetization direction  208  substantially perpendicular to the layer plane thereof. The magnetic free layer structure  200 , the tunnel junction layer  204 , and the magnetic reference layer structure  202  collectively form a magnetic tunnel junction structure  210 . The exemplary MTJ structure  190  may further include a magnetic fixed layer structure  212  exchange coupled to the magnetic reference layer structure  202  through an anti-ferromagnetic coupling layer  214 . The magnetic fixed layer structure  212  has a second invariable magnetization direction  216  that is substantially perpendicular to the layer plane thereof and is substantially opposite to the first invariable magnetization direction  208  of the magnetic reference layer structure  202 . In an embodiment, the switching voltage of the exemplary structure  190  from the low resistance state to the high resistance state is substantially same as the switching voltage from the high resistance state to the low resistance state by adjusting the offset field, which is the net external magnetic field acting on the magnetic free layer structure  200  along the direction of perpendicular magnetization  208 . In another embodiment, the stray magnetic fields exerted on the magnetic free layer structure  200  by the magnetic reference and fixed layer structures  202  and  212 , respectively, substantially cancel each other, thereby rendering the offset field to be substantially zero or negligible. The stacking order of the layers  212 ,  214 ,  202 ,  204 , and  200  in the exemplary structure  190  may be inverted as shown in  FIG. 11B . 
     Another exemplary MTJ structure  220  for the memory element  108 , as illustrated in  FIG. 11C , includes the magnetic tunnel junction structure  210  and a magnetic compensation layer structure  222  separated from the magnetic free layer structure  200  by a non-magnetic spacer layer  224 . The magnetic compensation layer structure  222  has a third invariable magnetization direction  226  that is substantially perpendicular to the layer plane thereof and is substantially opposite to the first invariable magnetization direction  208  of the magnetic reference layer structure  202 . The magnetic compensation layer structure  222  may be used to generate a magnetic field opposing that exerted by the magnetic fixed layer structure  202  on the magnetic free layer structure  200 . In an embodiment, the switching voltage of the exemplary structure  220  from the low resistance state to the high resistance state is substantially same as the switching voltage from the high resistance state to the low resistance state by adjusting the offset field. In another embodiment, the stray magnetic fields exerted on the magnetic free layer structure  200  by the magnetic reference and compensation layer structures  202  and  222 , respectively, substantially cancel each other, thereby rendering the offset field to be substantially zero or negligible. The stacking order of the layers  202 ,  204 ,  200 ,  224 , and  222  in the exemplary MTJ structure  220  may be inverted as shown in  FIG. 11D . 
     Operation of the two-terminal selector element  110  will now be described with reference to the current-voltage (I-V) response plot illustrated in  FIG. 12A . The I-V plot shows the magnitude of electric current passing through the two-terminal selector element  122  as the voltage applied to the selector element  122  varies. Initially, the current gradually increases with the applied voltage from zero to near a threshold voltage, V th . At or near V th , the current rapidly increases and exhibits a highly non-linear behavior. As the voltage continues to increase beyond V th , the current increase becomes gradual again until reaching I on  and corresponding voltage V p , which are programming current and voltage for the memory element  108 , respectively. The current response behaves like a step function as the applied voltage increases from zero to V p  with the sharp increase occurring at or near V th , which may include a narrow range of voltage values. 
     Without being bound to any theory, it is believed that one or more conductive paths or filaments are formed within the switching layer  128  when the applied voltage, V applied , exceeds V th  as illustrated in  FIG. 13A  for the composite switching layer structure  128   b , resulting in the two-terminal selector element  122  being in a highly conductive state. In response to the applied voltage that is greater than V th , ions and/or ionic particles from at least one of the first and second electrodes  124  and  126  may migrate into the switching layer  128   b  to form conductive bridges between the conductive clusters  130 , thereby forming one or more conductive paths between the first and second electrodes  124  and  126  through the switching layer  128   b . Alternatively, ions and/or ionic particles from the conductive clusters  130  may migrate and form the conductive bridges between the conductive clusters  130  within the switching layer  128   b . Therefore, the ions and/or ionic particles for forming conductive bridges may come from at least one of the first and second electrodes  124  and  126 , or the conductive clusters  130 , or both. It should be noted that there are various possible mechanisms that can cause ions and/or ionic particles to migrate, such as but not limited to electric field, electric current, and joule heating, in the presence of the applied voltage. 
     Referring back to  FIG. 12A , as the voltage applied to the selector element  122  decreases from V p  to near a holding voltage, V hold , that is lower than V th , the current gradually decreases and the selector element  110  remains in the highly conductive state. The conductive paths previously formed in the switching layer  128   b  remain mostly intact as illustrated in  FIG. 13B . 
     At or near V hold , the current rapidly decreases and exhibits a highly non-linear behavior. As the voltage continues to decrease beyond V hold , the current decrease becomes gradual again. When the voltage drops below V hold , the conductive bridges disintegrate and the one or more conductive paths between the electrodes  124  and  126  break down as illustrated in  FIG. 13C , returning the selector element  122  back to a semi-conducting or insulating state. At zero voltage, the conductive bridges disappear and the switching layer  128   b  remains in the original semi-conducting or insulating state as illustrated in  FIG. 13D . 
     With continuing reference to  FIG. 12A , the I-V response of the selector element  122  is characterized by a hysteresis behavior as the applied voltage is increased from zero to V p  and decreased back to zero again. The current response behaves like a step function as the applied voltage increases from zero to V p  with the sharp increase occurring at or near V th . As the voltage decreases from V p  to zero, the current markedly decreases at or near V hold , which is lower than V p . The two-terminal selector element  122  is bi-directional as the polarity of the applied voltage may be reversed as illustrated in the I-V plot of  FIG. 12A . The I-V response corresponding to the opposite polarity is substantially similar to that described above. When V applied  exceeds V th , one or more conductive paths form between the electrodes  124  and  126  as shown in  FIG. 14 , resulting in the two-terminal selector element  122  being in the highly conductive state. 
     Alternatively, the two-terminal selector element  122  may exhibit a different I-V response as illustrated in  FIG. 12B . The I-V plot of  FIG. 12B  differs from that of  FIG. 12A  in that the current remains relatively constant (compliance current, I cc ) even as the applied voltage decreases from V p  to V hold . Therefore, the selector element  122  remains in the highly conductive state and the conductive paths previously formed in the switching layer  128   b  remain mostly intact as illustrated in  FIG. 13B . 
       FIG. 15  shows another embodiment of the present invention as applied to the two-terminal selector element  122 . The first electrode structure  124  of the two-terminal selector element  122  includes a first electrode layer  124   a  formed adjacent to the volatile switching layer structure  128   a  and a second electrode layer  124   b  formed adjacent to the first electrode layer  124   a . The second electrode structure  126  of the two-terminal selector element  122  includes a first electrode layer  126   a  formed adjacent to the volatile switching layer structure  128   a  and a second electrode layer  126   b  formed adjacent to the first electrode layer  126   a . Each of the first and second electrode layers  124   a  and  124   b  of the first electrode structure  124  and the first and second electrode layers  126   a  and  126   b  of the second electrode structure  126  may be made of any material as described above. For example and without limitation, the first electrode layers  124   a  and  126   a  may be made of titanium nitride (TiN x ) and at least one of the second electrode layers  124   b  and  126   b  may be made of silver (Ag) or an alloy of silver and aluminum. 
     While the present invention has been shown and described with reference to certain preferred embodiments, it is to be understood that those skilled in the art will no doubt devise certain alterations and modifications thereto which nevertheless include the true spirit and scope of the present invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by examples given.