Resistance change element, storage device, and neural network device

Abstract

To provide a resistance change element improved in repetition characteristics and linearity.SOLUTION: A resistance change element 10 includes: a first electrode 21; a second electrode 22; a first transition metal compound layer 23 which is provided between the first electrode and the second electrode and is a metal compound containing lithium ion at lattice positions; a second transition metal compound layer 24 which is provided between the first transition metal compound layer and the second electrode and is a metal compound containing lithium ions at lattice positions; and a lithium ion conductor layer 25 which is provided between the first transition metal compound layer and the second transition metal compound layer and is a solid material which passes lithium ions and hardly passes electrons.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] FIELD Embodiments of the present invention relate to a resistive change element, a memory device, and a neural network device. [Background technology]

[0002] In recent years, neural network devices implemented in hardware have been researched. Neuromorphic neural networks that mimic the human brain, i.e., brain-type neural networks, are also known. Brain-type neural networks are neural networks that mimic the human brain, operate with low energy consumption, and have high error tolerance.

[0003] In the field of brain-type neural networks, the development of new hardware is desired in addition to algorithms. In the development of new hardware, the development of new resistive random access memories is desired. Various resistive random access memories (ReRAMs) have been proposed as elements for neuron or synapse circuits. However, all types of resistive random access memories have large variations in the characteristics of each element. For this reason, large-scale brain-type neural networks using resistive random access memories have not been developed.

[0004] For example, one of the conventional ReRAMs is TiO 2There is known a device having a structure in which metal electrodes are provided on both ends of a transition metal oxide such as . In a ReRAM of this structure, the amount or distribution of oxygen vacancies present in the transition metal oxide is changed by applying a voltage or current pulse between the metal electrodes. In a ReRAM of this structure, if the oxygen vacancies in the transition metal oxide increase, electrons are introduced near the oxygen vacancies to maintain electrical neutrality, so that the device enters a low resistive state (LRS). Conversely, in a ReRAM of this structure, if the oxygen vacancies in the transition metal oxide decrease, the device enters a high resistive state (HRS). However, the ReRAM of this structure has a large variation in characteristics. For this reason, it is difficult to apply a ReRAM of this structure to a large brain-type neural network. In recent years, a thin-film battery-type ReRAM using lithium ions instead of oxygen vacancies has been proposed, but further improvement in the linearity and repeatability of the device is desired. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2023-43142 A [Non-patent literature]

[0006] [Non-Patent Document 1] T. Marukame et. al., “Lithium-ion-based resistive devices of LiCoO2 / LiPON / Cu with ultrathin interlayers of titanium oxide for neuromorphic computing”, IEEE J. El. Dev. Soc. Special Issue, 2023, p.1-9 [Non-Patent Document 2] MG Verde et. al., “Elucidating the Phase Transformation of Li4Ti5O12 Lithiation at the Nanoscale”, ACS Nano 2016, 10, p.4312-4321. [Non-Patent Document 3] Juan Carlos Gonzalez-Rosillo1 et. al., “Lithium-Battery Anode Gains Additional Functionality for Neuromorphic Computing through Metal-Insulator Phase Separation”, Adv. Mater. 2020, doi: 10.1002 / adma.201907465, p.1-27 [Non-Patent Document 4] K. Mukai et.al., “Understanding the Zero-Strain Lithium Insertion Scheme of Li[Li1 / 3Ti5 / 3]O4: Structural Changes at Atomic Scale Clarified by Raman Spectroscopy”, J. Phys. Chem. C 2014, 118, p.2992-2999 Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a variable resistance element, a memory device, and a neural network device that are excellent in repeatability and linearity. [Means for solving the problem]

[0008] The resistance change element according to the embodiment includes a first electrode, a second electrode, a first transition metal compound layer provided between the first electrode and the second electrode and which is a metal compound containing lithium ions at lattice sites, a second transition metal compound layer provided between the first transition metal compound layer and the second electrode and which is a metal compound containing lithium ions at lattice sites, and a lithium ion conductor layer provided between the first transition metal compound layer and the second transition metal compound layer and which is a solid substance which allows lithium ions to pass through but does not easily allow electrons to pass through. [Brief description of the drawings]

[0009] [Figure 1] FIG. 2 is a diagram showing an example of a cross-sectional structure of the resistance change element according to the first embodiment. [Diagram 2] FIG. 4 is a diagram showing an example of a variable resistance element according to the first embodiment in a low resistance state. [Diagram 3] FIG. 4 is a diagram showing an example of a case where the resistance change element according to the first embodiment is in a high resistance state. [Figure 4] A graph showing the electromotive force of the transition metal compound Li4+εTi5O12 (0<ε<3) against metallic lithium during discharge. [Diagram 5] 5A and 5B are diagrams showing state changes of a second transition metal compound layer when a set pulse is applied to the resistance change element according to the first embodiment. [Figure 6] 5A and 5B are diagrams showing state changes of a second transition metal compound layer when a set pulse is applied to the resistance change element according to the first embodiment. [Figure 7] 5A and 5B are diagrams showing state changes of a second transition metal compound layer when a set pulse is applied to the resistance change element according to the first embodiment. [Figure 8] 4 is a diagram showing a state of the variable resistance element according to the first embodiment when an input pulse is applied thereto, in which the variable resistance element is in a low resistance state. FIG. [Figure 9] FIG. 11 is a diagram showing an example of the configuration of a storage device according to a second embodiment. [Figure 10] FIG. 11 is a diagram showing an example of the configuration of a storage device according to a third embodiment. [Figure 11] FIG. 13 is a diagram showing an example of the configuration of a storage device according to a fourth embodiment. [Figure 12] FIG. 13 is a diagram showing an example of the configuration of a storage device according to a fifth embodiment. [Figure 13] FIG. 13 is a diagram showing an example of the configuration of a neural network device according to a sixth embodiment. [Figure 14] FIG. 13 is a diagram for explaining one layer of a neural network in the neural network device according to the sixth embodiment. [Figure 15] FIG. 13 is a diagram for explaining a product-sum operation performed by a product-sum operation circuit included in the neural network device according to the sixth embodiment. [Figure 16] FIG. 13 is a diagram showing an example of the configuration of a neural network device according to the seventh embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, an embodiment will be described with reference to the drawings. In the following description, components having the same functions and configurations are denoted by the same reference numerals.

[0011] 1. First embodiment (1.1 Structure of the resistance change element) First, a description will be given of the configuration of the variable resistance element 10 according to the first embodiment. Fig. 1 is a diagram showing an example of the cross-sectional structure of the variable resistance element 10 according to the first embodiment.

[0012] The resistance change element 10 is an element whose electrical resistance changes. More specifically, the resistance change element 10 changes into a low resistance state (LRS) in which a current can flow from one electrode to the other electrode, or into a high resistance state (HRS) in which almost no current flows, by the movement of lithium ions inside the resistance change element 10. For example, the resistance change element 10 can store data by assigning data "0" and data "1" according to the state of the resistance change element 10. Hereinafter, an operation of changing the state of the resistance change element 10 from a high resistance state to a low resistance state, or from a low resistance state to a high resistance state, is referred to as a write operation. Also, an operation of reading out the state of the resistance change element 10 is referred to as a read operation. The resistance change element 10 is used, for example, as a memory element for a neuron circuit or a synapse circuit in a brain-type neural network.

[0013] As shown in FIG. 1, the resistance change element 10 includes a first electrode 21, a second electrode 22, a first transition metal compound layer 23, a second transition metal compound layer 24, and a lithium ion conductor layer 25. The resistance change element 10 has a laminated structure in which the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are laminated in this order from the first electrode 21 side. The resistance change element 10 may further include a layer (not shown) between the first electrode 21 and the second electrode 22. Hereinafter, the direction along the bonding surface of each layer is defined as the X direction. The direction intersecting the X direction and along the bonding surface is defined as the Y direction. The direction intersecting the X direction and the Y direction and perpendicular to the bonding surface is defined as the Z direction. The Z direction can also be expressed as the "stacking direction" in which each layer is stacked.

[0014] In the resistance change element 10, lithium ions can move between the first transition metal compound layer 23 and the second transition metal compound layer 24 via the lithium ion conductor layer 25. The direction in which the lithium ions move is the stacking direction (i.e., the Z direction) in which the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are stacked.

[0015] For example, the bonding surfaces of the first electrode 21, the first transition metal compound layer 23, the lithium ion conductor layer 25, the second transition metal compound layer 24, and the second electrode 22 are generally the same in shape and size. More specifically, for example, the bonding surface between the first electrode 21 and the first transition metal compound layer 23, the bonding surface between the first transition metal compound layer 23 and the lithium ion conductor layer 25, the bonding surface between the lithium ion conductor layer 25 and the second transition metal compound layer 24, and the bonding surface between the second transition metal compound layer 24 and the second electrode 22 are generally the same in shape and size. Note that the expression "generally the same" may include an error due to manufacturing variations.

[0016] The thickness of the first transition metal compound layer 23 in the Z direction (stacking direction) is taken as thickness T1, and the thickness of the second transition metal compound layer 24 in the Z direction (stacking direction) is taken as thickness T2. For example, in this embodiment, the thickness T2 of the second transition metal compound layer 24 is less than the thickness T1 of the first transition metal compound layer 23 (T1>T2). Note that the thickness T2 of the second transition metal compound layer 24 may be the same as the thickness T1 of the first transition metal compound layer 23, or may be thicker than the thickness T1 of the first transition metal compound layer 23. That is, the thicknesses T1 and T2 may have a relationship of T1≦T2.

[0017] The first electrode 21 and the second electrode 22 are connected to, for example, an external circuit (not shown). A voltage is applied between the first electrode 21 and the second electrode 22 from the external circuit. The first electrode 21 and the second electrode 22 include a conductive material. The first electrode 21 and the second electrode 22 may have the same configuration or different configurations.

[0018] The first transition metal compound layer 23 is connected to the first electrode 21 (in contact with the first electrode 21). The second transition metal compound layer 24 is connected to the second electrode 22 (in contact with the second electrode 22).

[0019] Each of the first transition metal compound layer 23 and the second transition metal compound layer 24 contains lithium ions (Li 1+) at the lattice site. For example, it can be confirmed that the lithium ions are arranged at the lattice site by observation using AFM (Atomic Force Microscopy). The lithium ions contained in the first transition metal compound layer 23 and the second transition metal compound layer 24 move between the first transition metal compound layer 23 and the second transition metal compound layer 24 by applying a voltage between the first electrode 21 and the second electrode 22. Furthermore, in each of the first transition metal compound layer 23 and the second transition metal compound layer 24, electrons may move inside by applying a voltage between the first electrode 21 and the second electrode 22. In this case, electrons may move between the first electrode 21 and the first transition metal compound layer 23. Furthermore, electrons may move between the second electrode 22 and the second transition metal compound layer 24.

[0020] Each of the first transition metal compound layer 23 and the second transition metal compound layer 24 may be, for example, a metal oxide containing lithium ions at lattice positions. In the following, in this embodiment, a case where the first transition metal compound layer 23 and the second transition metal compound layer 24 are oxides containing lithium and titanium will be described. The first transition metal compound layer 23 is Li 4+ε1 Ti 5 O 12 The second transition metal compound layer 24 is represented by Li 4+ε2 Ti 5 O 12 Here, the variable ε 1 and ε 2 That is, 0≦ε 2 <ε 1 The relationship is ≦3. Composition 4+ε 1 represents the composition (concentration) of lithium ions in the first transition metal compound layer 23. Composition 4+ε 2 represents the composition of lithium ions in the second transition metal compound layer 24. 4+ε1 Ti 5 O 12 and Li 4+ε2 Ti 5 O 12 None of them are solid solutions, and Li 4 Ti 5 O 12 and Li 7 Ti5 O 12 For example, Li 4 Ti 5 O 12 and Li 7 Ti 5 O 12 This can be confirmed by analysis such as X-ray diffraction (XRD).

[0021] Li 4 Ti 5 O 12 Li is a spinel-type transition metal oxide that has insulating properties. 7 Ti 5 O 12 is a rock-salt transition metal oxide that exhibits metallic conductivity. Spinel transition metal oxide Li 4 Ti 5 O 12 and rock-salt transition metal oxide Li 7 Ti 5 O 12 In both cases, lithium ions are arranged at the lattice sites. 1 and composition 4+ε 2 represents the amount of Li present in the first transition metal compound layer 23 and the second transition metal compound layer 24. 4 Ti 5 O 12 and Li 7 Ti 5 O 12 For example, the ratio of Li 7 Ti 5 O 12 As the proportion of increases, the composition 4+ε 1 Similarly, the Li 7 Ti 5 O 12 As the proportion of increases, the composition 4+ε 2 The first transition metal compound layer 23 and the second transition metal compound layer 24 have different compositions of lithium ions arranged at lattice sites. 1 and ε 2 That is, ε 2 <ε 1That is, the composition is 4+ε 1 and composition 4+ε 2 That is, 4+ε 2 <4+ε 1 Therefore, the first transition metal compound layer 23 has a higher Li content than the second transition metal compound layer 24. 7 Ti 5 O 12 In other words, the first transition metal compound layer 23 has a higher composition of lithium ions arranged at lattice sites than the second transition metal compound layer 24.

[0022] For example, the composition of the first transition metal compound layer 23 is 4+ε 1 is at most 7. That is, ε 1 In other words, the first transition metal compound layer 23 may have a Li 7 Ti 5 O 12 Including Li 4 Ti 5 O 12 In addition, the second transition metal compound layer 24 may have a composition of 4+ε 2 is at least 4. That is, ε 2 = 0. In other words, the second transition metal compound layer 24 may be Li 4 Ti 5 O 12 Including Li 7 Ti 5 O 12 It may be a state that does not include

[0023] The first transition metal compound layer 23 may be realized by the same base material as the second transition metal compound layer 24 as described above, but may be realized by using a spinel-type transition metal oxide containing lithium ions at lattice sites, such as LiTi 2 O 4 , Li 4 Ti 5 O 12 , Li[CrTi]O 4 , (Li 1 / 2 Fe 1 / 2 )[Li 1 / 2 Fe 1 / 2 Ti]O 4 , or Li 1 / 2Mn 2 O 4 The first transition metal compound layer 23 may be LiTiO, which is a rock-salt transition metal oxide containing lithium ions at lattice sites. 2 , Li 7 Ti 5 O 12 , Li 2 [CrTi]O 4 , or (Li 3 / 2 Fe 1 / 2 )[Li 1 / 2 Fe 1 / 2 Ti]O 4 Furthermore, the first transition metal compound layer 23 may be a mixture of the above-mentioned spinel-type transition metal oxide and rock salt-type transition metal oxide.

[0024] The second transition metal compound layer 24 is a mixture of a spinel-type transition metal oxide containing lithium ions at the lattice sites and a rock salt-type transition metal oxide. For example, the spinel-type transition metal oxide Li 0.5 MnO 2 and λ-type cubic metal oxide λMnO 2 The oxide may be a mixture of oxides having different crystal structures, such as:

[0025] The lithium ion conductor layer 25 is formed between the first transition metal compound layer 23 and the second transition metal compound layer 24. For example, two surfaces of the lithium ion conductor layer 25 facing the Z direction (stacking direction) are in contact with the first transition metal compound layer 23 and the second transition metal compound layer 24, respectively. The lithium ion conductor layer 25 is a solid material that allows lithium ions to pass through and does not allow electrons to pass through easily. In other words, the lithium ion conductor layer 25 is a solid material that does not allow electrons to pass through easily compared to the ease with which lithium ions pass through. That is, the lithium ion conductor layer 25 is a solid material through which electrons pass through more difficultly than lithium ions. For example, the lithium ion conductor layer 25 can be realized by LiPON. Note that the lithium ion conductor layer 25 is not limited to LiPON and may be other materials as long as it has the above-mentioned characteristics. The lithium ion conductor layer 25 is also called a solid electrolyte.

[0026] (1.2 State of the resistance change element) Next, a description will be given of the state of the resistance change element 10. First, a description will be given of the resistance change element 10 in the low resistance state (LRS). Fig. 2 is a diagram showing an example of the resistance change element 10 in the low resistance state.

[0027] 2, when the variable resistance element 10 is in a low resistance state, the second transition metal compound layer 24 contains a predetermined amount or more of lithium ions. The variable resistance element 10 changes state based on the composition of lithium ions in the second transition metal compound layer 24. When the variable resistance element 10 is in a low resistance state, the second transition metal compound layer 24 contains a spinel-type transition metal oxide Li 4 Ti 5 O 12 and Li, a rock-salt transition metal oxide exhibiting metallic conductivity. 7 Ti 5 O 12 For example, in the second transition metal compound layer 24 in the low resistance state, there are Li ions extending in the Z direction, one end of which is in contact with the lithium ion conductor layer 25 and the other end of which is in contact with the second electrode 22. 7 Ti 5 O 12 One or more filaments are formed.

[0028] The second transition metal compound layer 24 in the low resistance state contains a predetermined amount or more of lithium ions, which are positive ions. 7 Ti 5 O 12 The filament of the first electrode 21 is in contact with the second electrode 22. In this state, a voltage is applied to the resistance change element 10 such that the voltage on the first electrode 21 side is higher than the voltage on the second electrode 22 side. For example, a positive voltage pulse is applied to the first electrode 21 such that the voltage on the first electrode 21 side is higher than the voltage on the second electrode 22 side. This causes lithium ions to move from the first transition metal compound layer 23 to the second transition metal compound layer 24. Electrons are supplied to the second electrode 22 from an external circuit. The electrons supplied from the second electrode 22 are converted into Li 7 Ti 5 O12 The metal is taken into the second transition metal compound layer 24 through the filament. Therefore, when a voltage higher on the first electrode 21 side than on the second electrode 22 side is applied to the variable resistance element 10 in the low resistance state (for example, when a positive voltage pulse is applied to the first electrode 21), the variable resistance element 10 can pass a current in a direction from the first electrode 21 to the second electrode 22.

[0029] In this manner, the resistance change element 10 is in a low resistance state when the second transition metal compound layer 24 contains a predetermined amount or more of lithium ions.

[0030] The resistance value of the variable resistance element 10 in the low resistance state is determined by the combined resistance value of the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25. In many cases, the resistance value of the second transition metal compound layer 24 is sufficiently larger than the resistance values ​​of the first transition metal compound layer 23 and the lithium ion conductor layer 25. Therefore, the resistance value of the variable resistance element 10 in the low resistance state is substantially determined by the resistance value of the second transition metal compound layer 24.

[0031] For example, the resistance value of the second transition metal compound layer 24 is 100% when the second transition metal compound layer 24 is made of an insulating spinel-type transition metal oxide Li 4 Ti 5 O 12 and Li, a rock-salt transition metal oxide exhibiting metallic conductivity. 7 Ti 5 O 12 Therefore, the resistance value of the variable resistance element 10 in the low resistance state changes linearly with the composition of lithium ions in the second transition metal compound layer 24.

[0032] Next, the variable resistance element 10 in the high resistance state (HRS) will be described. Fig. 3 is a diagram showing an example of the variable resistance element 10 in the high resistance state.

[0033] 3, when the resistance change element 10 is in a high resistance state, the second transition metal compound layer 24 contains less than a predetermined amount of lithium ions.2 The second transition metal compound layer 24 is made of an insulating spinel-type transition metal oxide Li 4 Ti 5 O 12 The majority of the transition metal oxides are Li 7 Ti 5 O 12 In this state, when a voltage is applied to the variable resistance element 10 such that the voltage on the first electrode 21 side is higher than the voltage on the second electrode 22 side (for example, when a positive voltage pulse is applied to the first electrode 21), an electric field is applied to the second electrode 22 such that electrons move in a direction toward the second transition metal compound layer 24. However, since the second transition metal compound layer 24 is an insulating layer, electrons supplied from the second electrode 22 side are not taken into the second transition metal compound layer 24. Therefore, when the variable resistance element 10 is in the high resistance state, the variable resistance element 10 cannot pass a current in a direction from the first electrode 21 to the second electrode 22 even when a voltage is applied to the first electrode 21 side such that the voltage is higher than the voltage on the second electrode 22 side.

[0034] In this manner, the resistance change element 10 is in a high resistance state in which no current flows in the direction from the first electrode 21 to the second electrode 22 when the amount of lithium ions contained in the second transition metal compound layer 24 is less than a predetermined amount.

[0035] The resistance value of the variable resistance element 10 in the high resistance state is substantially determined by the resistance value of the second transition metal compound layer 24. The resistance value of the variable resistance element 10 in the high resistance state is preferably higher so that the difference between the resistance value in the high resistance state and the resistance value in the low resistance state becomes large.

[0036] For example, when the thickness T2 of the second transition metal compound layer 24 is thinner than the thickness T1 of the first transition metal compound layer 23, the resistance change element 10 can have the lithium ions contained in the second transition metal compound layer 24 at a certain value or less in the high resistance state, and can stably achieve a high resistance value. Furthermore, the resistance change element 10 having such a configuration can have a relatively small amount of lithium ions contained in the second transition metal compound layer 24. Therefore, when the resistance change element 10 is changed from a low resistance state to a high resistance state, the lithium ions can be moved from the second transition metal compound layer 24 to the first transition metal compound layer 23 in a relatively short time. Therefore, the resistance change element 10 having such a configuration is preferable in that the write operation can be made faster. Furthermore, the resistance change element 10 having such a configuration can have a relatively large amount of lithium ions contained in the first transition metal compound layer 23 in the low resistance state, and therefore the amount of current that the resistance change element 10 can pass can be increased. Therefore, when the resistance change element 10 having such a configuration is used as a memory element for a neuron circuit or a synapse circuit in a brain-type neural network, a relatively large current can be flowed from the resistance change element 10 in the low resistance state to the neuron circuit or the synapse circuit at high speed. This is preferable in that it is possible to make the operation speed of the resistance change element 10 faster because it is only necessary to move a small amount of lithium ions under a large amount of current. In addition, the resistance change element 10 having such a configuration (T1>T2) is preferable in that it can suppress the operation delay of the neuron circuit or the synapse circuit. In other words, the resistance change element 10 having such a configuration (T1>T2) is preferable in that it can improve the processing capacity of the neuron circuit or the synapse circuit.

[0037] The thickness T2 of the second transition metal compound layer 24 may be the same as the thickness T1 of the first transition metal compound layer 23 or may be thicker than the thickness T1 of the first transition metal compound layer 23. The thicker the thickness of the second transition metal compound layer 24 in the direction of lithium ion movement, the higher the resistance value can be achieved in the high resistance state. In this way, the difference between the resistance value in the high resistance state and the resistance value in the low resistance state of the resistance variable element 10 can be increased. Therefore, when the resistance variable element 10 having such a configuration is used as a memory element for a neuron circuit or a synapse circuit in a brain-type neural network, for example, the leakage current from the resistance variable element 10 in the high resistance state to the neuron circuit or the synapse circuit can be blocked with higher accuracy. Therefore, the resistance variable element 10 having such a configuration (T1≦T2) is preferable in that it can suppress malfunction of the neuron circuit or the synapse circuit.

[0038] In the variable resistance element 10, the lithium ion concentration in the first transition metal compound layer 23 and the second transition metal compound layer 24 changes every time the resistance changes. Therefore, each layer expands and contracts repeatedly every time the resistance changes. As a result, the life of the variable resistance element 10 is shortened. In this embodiment, by selecting materials with little expansion and contraction for the first transition metal compound layer 23 and the second transition metal compound layer 24 and reducing the film thickness of each layer, it is possible to realize a variable resistance element 10 that is resistant to expansion and contraction and has high performance. For example, the film thickness of each of the first transition metal compound layer 23 and the second transition metal compound layer 24 can be set to 100 nm or less.

[0039] The conductivity of the lithium ion conductor layer 25 is 10 -2 Scm -1 When the thickness of the lithium ion conductor layer 25 is increased, the resistance value of the resistance change element 10 in the low resistance state is determined by the resistance value of the lithium ion conductor layer 25. Therefore, in order to utilize the resistance value of the second transition metal compound layer 24 up to the low resistance state, the thinner the lithium ion conductor layer 25, the better. For example, in this embodiment, the thickness of the lithium ion conductor layer 25 can be set to 100 nm or less.

[0040] (1.3 Relationship between lithium ion composition and electromotive force) Next, the relationship between the composition of lithium ions and electromotive force will be explained. 4+ε Ti 5 O 12 FIG. 13 is a graph showing the electromotive force against metallic lithium during discharge (0<ε<3).

[0041] As shown in Figure 4, Li 4+ε Ti 5 O 12 The electromotive force associated with metallic lithium is constant when 0<ε<3. Therefore, when the resistance change element 10 is in a low resistance state, no electromotive force is generated between the first electrode 21 and the second electrode 22, regardless of the difference in the composition of lithium ions contained in the first transition metal compound layer 23 and the second transition metal compound layer 24.

[0042] (1.4 Write Operation) Next, the write operation of the resistance change element 10 will be described.

[0043] First, a case where the resistance change element 10 is changed from a high resistance state to a low resistance state in a write operation will be described. Figures 5 to 7 are diagrams showing state changes of the second transition metal compound layer 24 when a set pulse is applied to the resistance change element 10.

[0044] In the following description, a voltage pulse applied to the resistance change element 10 to change the resistance change element 10 from a high resistance state to a low resistance state is referred to as a "set pulse." Also, a voltage pulse applied to the resistance change element 10 to change the resistance change element 10 from a low resistance state to a high resistance state is referred to as a "reset pulse." In a write operation, either a set pulse or a reset pulse is applied to the resistance change element 10.

[0045] When changing the resistance change element 10 from a high resistance state to a low resistance state, the control circuit that controls the resistance change element 10 applies a set pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21. Note that applying a set pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21 is the same as applying a set pulse of a negative voltage lower than the voltage of the first electrode 21 to the second electrode 22.

[0046] Insulating spinel-type transition metal oxide Li 4 Ti 5 O 12 Metallic Conduction in Rock-Salt Transition Metal Oxides Formed in Li 7 Ti 5 O 12 Although no detailed study has been done on the growth process of , it is assumed that the growth is a phase-separated type under an external electric field.

[0047] As shown in state (a) of FIG. 5, first, a set pulse is applied to the resistance change element 10 such that the voltage of the first electrode 21 becomes V+ΔV and the voltage of the second electrode 22 becomes V. That is, a voltage higher than the voltage of the second electrode 22 is applied to the resistance change element 10. This causes the Li 4 Ti 5 O 12 In the lithium ion conductor layer 25, Li 7 Ti 5 O 12 The filaments penetrate (grow) into the second transition metal compound layer 24. The penetration location depends on the microstructure of the second transition metal compound layer 24 and is a portion where the electric field is relatively strong.

[0048] As shown in Figure 5(b), Li 7 Ti 5 O 12 The growth of the filament stops when it reaches the second electrode 22. 7 Ti 5 O 12 The filament is saturated with lithium ions. 7 Ti 5 O 12No more lithium ions flow into the filament. This means that the lithium ion conductor layer 25 and the Li 7 Ti 5 O 12 This means that an effective high resistance interface occurs between the filament and the

[0049] As shown in state (c) of FIG. 6, a high electric field region is generated in the interface region of the high resistance interface.

[0050] As shown in state (d) of Figure 6, in the high electric field region, i.e., in the area where the electric field is strong, new Li 7 Ti 5 O 12 filament growth begins.

[0051] As shown in state (e) of Figure 7, Li 7 Ti 5 O 12 The conductivity of the variable resistance element 10 is increased by increasing the area of ​​the spinel-type transition metal oxide Li 4 Ti 5 O 12 and Li, a rock-salt transition metal oxide exhibiting metallic conductivity. 7 Ti 5 O 12 Therefore, the conductivity of the resistance change element 10 depends linearly on the composition of lithium ions in the second transition metal compound layer 24.

[0052] In addition, Li 7 Ti 5 O 12 There may be multiple locations where the filament enters. For this reason, the Li filament intrusion described using the states (a) to (e) in Figs. 5 to 7 may be 7 Ti 5 O 12 The growth of the filament can occur at multiple locations in the second transition metal compound layer 24.

[0053] Next, a case where the resistance change element 10 is changed from a low resistance state to a high resistance state in a write operation will be described. When changing the resistance change element 10 from a low resistance state to a high resistance state, a control circuit that controls the resistance change element 10 applies a reset pulse of a negative voltage lower than the voltage of the second electrode 22 to the first electrode 21. Note that applying a reset pulse of a negative voltage lower than the voltage of the second electrode 22 to the first electrode 21 is the same as applying a reset pulse of a positive voltage higher than the voltage of the first electrode 21 to the second electrode 22.

[0054] The lithium ions contained in the second transition metal compound layer 24 pass through the lithium ion conductor layer 25 and move to the first transition metal compound layer 23 due to the electric field created by the reset pulse. This causes the second transition metal compound layer 24 to change from a state containing lithium ions to a state not containing lithium ions. That is, the second transition metal compound layer 24 is composed of a rock salt type transition metal oxide Li 7 Ti 5 O 12 Therefore, the second transition metal compound layer 24 is in a state in which it cannot take in electrons from the second electrode 22. This causes the second transition metal compound layer 24 to change from a conductor to an insulator. As a result, the resistance change element 10 changes to a high resistance state.

[0055] (1.5 Specific example of input operation of input pulse to resistive change element) Next, a description will be given of a pulse input to the resistance change element 10. Fig. 8 is a diagram showing the state of the resistance change element 10 to which an input pulse is applied when the resistance change element 10 is in a low resistance state.

[0056] In the following description, when data is read from the resistance change element 10 or when the resistance change element 10 is used as a resistive element, a voltage pulse applied to the resistance change element 10 is referred to as an “input pulse.” For example, the input pulse is a voltage pulse having a lower voltage than the set pulse.

[0057] When reading data from the resistance change element 10 or when the resistance change element 10 is used as a resistive element, a circuit (memory device) incorporating the resistance change element 10 applies an input pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21. Note that applying an input pulse of a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21 is the same as applying an input pulse of a negative voltage lower than the voltage of the first electrode 21 to the second electrode 22.

[0058] The lithium ions contained in the first transition metal compound layer 23 pass through the lithium ion conductor layer 25 and move to the second transition metal compound layer 24 due to the electric field created by the input pulse. When the lithium ions reach the second transition metal compound layer 24, the second transition metal compound layer 24 captures electrons from the second electrode 22. As a result, the resistance change element 10 in the low resistance state can pass a current when an input pulse is applied.

[0059] The charge amount of the lithium ions contained in the first transition metal compound layer 23 has an upper limit. In other words, the amount of lithium ions that can move from the first transition metal compound layer 23 to the second transition metal compound layer 24 has an upper limit. 3 The size of Li 7 Ti 5 O 2 The charge of the lithium ion in is 1.5×10 -11 In this case, the size of Li 7 Ti 5 O 2 The variable resistance element 10 including the first transition metal compound layer 23 having a base material of 1.5×10 4 The control circuit that controls the variable resistance element 10 may execute control for reading the state of the variable resistance element 10 so as not to exceed the upper limit of the current (number of current pulses) that can be passed through the variable resistance element 10.

[0060] In contrast, when the resistance change element 10 is in a high resistance state, even if lithium ions reach the second transition metal compound layer 24 due to the electric field created by the input pulse, the second transition metal compound layer 24 does not contain a sufficient amount of lithium ions at lattice positions, and therefore cannot take in electrons from the second electrode 22. As a result, the resistance change element 10 in the high resistance state cannot pass a current even when an input pulse is applied.

[0061] (1.6 Effects of this embodiment) With the configuration according to this embodiment, the resistance change element 10 can achieve a low resistance state and a high resistance state by changing the composition of lithium ions contained in the lattice sites of the first transition metal compound layer 23 and the second transition metal compound layer 24. This can improve the repeatability and linearity of the resistance change element.

[0062] For example, a conventional thin-film battery-type resistance change element using lithium ions changes the composition of the lithium ions contained in the lattice gaps of a transition metal compound layer to realize a low resistance state and a high resistance state. In this case, the volume of the transition metal compound layer changes depending on the composition of the lithium ions contained in the lattice gaps. For example, the amount of volume change reaches several percent. If the volume expands / contracts repeatedly with the change in the resistance state of the resistance change element, the resistance change element deteriorates. For this reason, sufficient repeatability cannot be obtained. That is, the life of the resistance change element is relatively short. In addition, the state of the resistance change element depends on the composition of the lithium ions in the transition metal compound layer. However, the resistance value of the resistance change element does not change linearly with the composition of the lithium ions. That is, the conductivity of the resistance change element does not change linearly with the number of voltage pulses applied to the resistance change element.

[0063] In contrast, the resistance change element 10 according to this embodiment includes a first electrode 21, a first transition metal compound layer 23, a lithium ion conductor layer 25, a second transition metal compound layer 24, and a second electrode 22. The first transition metal compound layer 23 and the second transition metal compound layer 24 include lithium ions at lattice positions. The resistance change element 10 can change the composition of the lithium ions included at the lattice positions of the first transition metal compound layer 23 and the second transition metal compound layer 24 based on the voltage applied to the resistance change element 10. This allows the resistance change element 10 to achieve a low resistance state and a high resistance state. The resistance change element 10 changes the composition of the lithium ions at the lattice positions. Therefore, the resistance change element 10 can suppress the volume change of the first transition metal compound layer 23 and the second transition metal compound layer 24 due to the change in state to 1% or less. The resistance change element 10 can suppress the deterioration of the resistance change element 10 due to the volume change, and can improve the repeatability of the resistance change element 10. Therefore, the life of the resistance change element 10 can be improved.

[0064] The second transition metal compound layer 24 of the resistance change element 10 according to this embodiment is made of, for example, a spinel-type transition metal oxide Li 4 Ti 5 O 12 and Li, a rock-salt transition metal oxide exhibiting metallic conductivity. 7 Ti 5 O 12 The resistance value of the second transition metal compound layer 24 is determined by the ratio of the spinel-type transition metal oxide Li 4 Ti 5 O 12 and rock-salt transition metal oxide Li 7 Ti 5 O 12 It depends on the volume ratio of the second transition metal compound layer 24 to the second transition metal compound layer 24. Therefore, the resistance value of the variable resistance element 10 in the low resistance state changes linearly with the composition of the lithium ions contained in the second transition metal compound layer 24. Therefore, the resistance value of the variable resistance element 10 in the low resistance state can be controlled by the composition of the second transition metal compound layer 24. Therefore, the conductivity of the variable resistance element 10 can change linearly with respect to the number of voltage pulses applied to the variable resistance element 10, that is, the linearity can be improved.

[0065] 2. Second Embodiment Next, a memory device 30 according to a second embodiment will be described. The memory device 30 according to the second embodiment uses the resistance change element 10 described in the first embodiment as a memory element.

[0066] (2.1 Storage Device Configuration) First, an example of the configuration of a memory device 30 according to this embodiment will be described. Fig. 9 is a diagram showing an example of the configuration of a memory device 30 according to a second embodiment. In the example of Fig. 9, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity.

[0067] As shown in FIG. 9, the memory device 30 includes a resistance change element 10, a switch 31, a control circuit 32, and an output circuit 33.

[0068] The switch 31 is a circuit that shorts or opens the second electrode 22 of the resistance change element 10 and the ground. One end of the transmission path of the switch 31 is connected to the second electrode 22 of the resistance change element 10 and the output circuit 33, and the other end of the transmission path is connected (grounded) to the ground. The switch 31 receives a switching signal from the control circuit 32. The switching signal is a signal that switches the switch 31 to a short-circuited state (on state) or an open state (off state). The switch 31 is controlled to a short-circuited state or an open state based on the switching signal. For example, the switch 31 is short-circuited during a write operation and is open during a read operation. The switch 31 is realized by, for example, a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor).

[0069] The control circuit 32 controls the write operation and read operation in the resistance change element 10. The control circuit 32 is connected to, for example, the first electrode 21 and the switch 31 of the resistance change element 10. The control circuit 32 is also connected to a circuit (not shown) that manages the state of the resistance change element 10. For example, the control circuit 32 receives a state setting signal from the circuit that manages the state of the resistance change element 10 during a write operation. The state setting signal is a signal that instructs the resistance change element 10 to be in a high resistance state or a low resistance state. The control circuit 32 applies a set pulse or a reset pulse to the first electrode 21 based on the state setting signal.

[0070] During a write operation, the control circuit 32 shorts the switch 31. The second electrode 22 of the resistance change element 10 is connected to ground. This puts the resistance change element 10 in a state in which a set pulse or a reset pulse can be applied. Then, the control circuit 32 applies the set pulse or the reset pulse to the first electrode 21 based on the state setting signal. This puts the resistance change element 10 in a low resistance state or a high resistance state.

[0071] More specifically, when the control circuit 32 receives a state setting signal corresponding to a low resistance state during a write operation, the control circuit 32 applies a set pulse to the resistance change element 10. That is, the control circuit 32 applies a positive voltage higher than the voltage of the second electrode 22 to the first electrode 21, and moves the lithium ions contained in the first transition metal compound layer 23 to the second transition metal compound layer 24. This brings the resistance change element 10 into a low resistance state.

[0072] Furthermore, during a write operation, when the control circuit 32 receives a state setting signal corresponding to a high resistance state, for example, the control circuit 32 applies a reset pulse to the resistance change element 10. That is, the control circuit 32 applies a negative voltage lower than the voltage of the second electrode 22 to the first electrode 21, and moves the lithium ions contained in the second transition metal compound layer 24 to the first transition metal compound layer 23. This brings the resistance change element 10 into a high resistance state.

[0073] In a read operation, the control circuit 32 opens the switch 31. The second electrode 22 of the resistance change element 10 is disconnected from the ground. This allows the resistance change element 10 to be readable by an external circuit (not shown).

[0074] In this state, for example, an input pulse is applied to the first electrode 21 of the resistance change element 10 from an external circuit. More specifically, for example, an input pulse having a voltage higher than the voltage of the second electrode 22 is applied to the first electrode 21 of the resistance change element 10 from the external circuit. Note that the memory device 30 may further include a pulse generating circuit that generates an input pulse when a read instruction is received from the external circuit. For example, the pulse generating circuit is connected to the first electrode 21.

[0075] The output circuit 33 outputs an output signal indicating whether or not a current flows through the resistance change element 10 at the timing when an input pulse is applied to the resistance change element 10. The output circuit 33 is connected to the second electrode 22 of the resistance change element 10 and the switch 31. For example, the output circuit 33 has a capacitor 41. One electrode of the capacitor 41 is connected to the second electrode 22 of the resistance change element 10, and the other electrode is connected to ground. The output circuit 33 outputs the voltage of the connection point between the capacitor 41 and the second electrode 22 of the resistance change element 10 as an output signal. For example, data can be read out by comparing the voltage of the output signal with a predetermined value (reference voltage).

[0076] During a read operation, when an input pulse is applied to the resistance change element 10 in the low resistance state, a current flows through the resistance change element 10. As a result, the output circuit 33 outputs an output signal having a voltage higher than a predetermined value.

[0077] Furthermore, when an input pulse is applied to the resistance change element 10 in the high resistance state during a read operation, no current flows through the resistance change element 10. As a result, the output circuit 33 outputs an output signal with a voltage equal to or lower than a predetermined value.

[0078] (2.2 Effects of this embodiment) With the configuration according to this embodiment, the same effects as those of the first embodiment can be obtained.

[0079] Furthermore, the memory device 30 according to this embodiment includes a resistance change element 10, a switch 31, a control circuit 32, and an output circuit 33. When the control circuit 32 receives an instruction to change the resistance change element 10 to a low resistance state (a state setting signal corresponding to a low resistance state) during a write operation, the control circuit 32 can change the resistance change element 10 to a low resistance state. When the control circuit 32 receives an instruction to change the resistance change element 10 to a high resistance state (a state setting signal corresponding to a high resistance state) during a write operation, the control circuit 32 can change the resistance change element 10 to a high resistance state.

[0080] Furthermore, in the memory device 30 according to this embodiment, an input pulse is applied to the resistance change element 10 in a read operation. At this time, if the resistance change element 10 is in a low resistance state, the output circuit 33 can output an output signal with a voltage higher than a predetermined value. Also, if the resistance change element 10 is in a high resistance state, the output circuit 33 can output an output signal with a voltage equal to or lower than a predetermined value. The state of the resistance change element 10 can be determined based on the voltage of the output signal. That is, data can be read from the resistance change element 10.

[0081] The memory device 30 according to this embodiment can change the state of the resistance change element 10 in response to an instruction from an external circuit, and can output the state of the resistance change element 10 to the external circuit. Therefore, the memory device 30 can function as a storage device that stores binary information.

[0082] 3. Third Embodiment Next, a storage device 30 according to a third embodiment will be described. In the third embodiment, a configuration of the storage device 30 different from that of the second embodiment will be described. The following description will focus on the differences from the second embodiment.

[0083] (3.1 Storage Device Configuration) First, an example of the configuration of a memory device 30 according to the present embodiment will be described. Fig. 10 is a diagram showing an example of the configuration of a memory device 30 according to a third embodiment. In the example of Fig. 10, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity.

[0084] As shown in FIG. 10, the memory device 30 includes a resistance change element 10, a switch 31, a control circuit 32, an output circuit 33, and a positive / negative inversion circuit 51.

[0085] The positive / negative inversion circuit 51 is a circuit that switches the connection between the resistance change element 10 and the external circuit, the switch 31, and the control circuit 32 based on a control signal received from the control circuit 32. In other words, the positive / negative inversion circuit 51 switches the connection relationship between the first electrode 21 and the second electrode 22 and other circuits.

[0086] The positive / negative inversion circuit 51 includes terminals TN1 to TN6. The terminal TN1 is connected to an external circuit and an output terminal of a set pulse and a reset pulse of the control circuit 32. The terminal TN1 is configured to be connectable to either a terminal TN2 or a terminal TN3. The terminal TN2 is connected to a terminal TN6 and a first electrode 21 of the resistance change element 10. The terminal TN3 is connected to a terminal TN5. The terminal TN4 is connected to the switch 31 and the output circuit 33. The terminal TN3 is configured to be connectable to either a terminal TN5 or a terminal TN6. The terminal TN5 is connected to the terminal TN3 and a second electrode 22 of the resistance change element 10.

[0087] The positive / negative inversion circuit 51 is set to either a combination of connecting the terminal TN1 and the terminal TN2 and connecting the terminal TN4 and the terminal TN5, or a combination of connecting the terminal TN1 and the terminal TN3 and connecting the terminal TN4 and the terminal TN6, based on a control signal received from the control circuit 32. For example, when the combination of connecting the terminal TN1 and the terminal TN2 and connecting the terminal TN4 and the terminal TN5 is set, any of an input pulse, a set pulse, or a reset pulse can be applied to the first electrode 21. Then, the second electrode 22 is connected to the switch 31 and the output circuit 33. Also, when the combination of connecting the terminal TN1 and the terminal TN3 and connecting the terminal TN4 and the terminal TN6 is set, any of an input pulse, a set pulse, or a reset pulse can be applied to the second electrode 22. Then, the first electrode 21 is connected to the switch 31 and the output circuit 33.

[0088] For example, in a read operation, the positive / negative inversion circuit 51 switches between applying an input pulse applied from an external circuit to the first electrode 21 as a first input pulse having a voltage higher than that of the second electrode 22, and applying the input pulse to the second electrode 22 as a second input pulse having a voltage higher than that of the first electrode 21. As a result, the first input pulse and the second input pulse are applied alternately to the resistance change element 10 in the read operation.

[0089] The switch 31 of this embodiment is a circuit that shorts or opens between the terminal TN4 of the positive / negative inversion circuit 51 and the ground. One end of the transmission path of the switch 31 is connected to the terminal TN4 of the positive / negative inversion circuit 51 and the output circuit 33, and the other end of the transmission path is connected to the ground. The other configurations are the same as those described in the second embodiment.

[0090] The control circuit 32 of this embodiment is further connected to a positive / negative inversion circuit 51. Other connections are the same as those described in the second embodiment. The control circuit 32 transmits a control signal to the positive / negative inversion circuit 51. The control signal is a signal that controls the connection of the positive / negative inversion circuit 51.

[0091] Furthermore, during a read operation, the control circuit 32 can switch between the first input pulse and the second input pulse every time the first input pulse or the second input pulse is applied a predetermined number of times, i.e., switches the connection of the positive / negative inverter circuit 51.

[0092] For example, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that the first input pulse is applied to the resistance change element 10 after changing the state of the resistance change element 10 (after the write operation). Then, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that the second input pulse is applied to the resistance change element 10 after the first input pulse is applied to the resistance change element 10 a predetermined number of times in the read operation. Then, the control circuit 32 switches the connection of the positive / negative inversion circuit 51 so that the first input pulse is applied to the resistance change element 10 after the second input pulse is applied to the resistance change element 10 a predetermined number of times in the read operation. The control circuit 32 repeats such a switching process in the read operation. For example, when the first input pulse is applied to the first electrode 21, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. On the other hand, when the second input pulse is applied to the second electrode 22, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. In a read operation, when a first input pulse and a second input pulse are alternately applied to the resistance change element 10, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 by the first input pulse move (back) to the first transition metal compound layer 23 by the second input pulse.

[0093] The output circuit 33 outputs an output signal indicating whether or not a current flows through the resistance change element 10 at the timing when the first input pulse or the second input pulse is applied from an external circuit to the resistance change element 10. The configuration of the output circuit 33 is the same as that described in the second embodiment. For example, when the first input pulse is applied to the resistance change element 10, the output circuit 33 outputs the voltage of the second electrode 22 of the resistance change element 10 as an output signal. Also, for example, when the second input pulse is applied to the resistance change element 10, the output circuit 33 outputs the voltage of the first electrode 21 of the resistance change element 10 as an output signal.

[0094] (3.2 Effects of this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0095] Further, the memory device 30 according to the present embodiment includes a resistance change element 10, a switch 31, a control circuit 32, an output circuit 33, and a positive / negative inversion circuit 51. The positive / negative inversion circuit 51 can switch the connection of the resistance change element 10. The positive / negative inversion circuit 51 can switch between a case where a first input pulse is applied to the first electrode 21 and a case where a second input pulse is applied to the second electrode 22 in a read operation. When the first input pulse is applied to the first electrode 21, the lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. When the second input pulse is applied to the second electrode 22, the lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, in a read operation, by switching the connection of the resistance change element 10, the lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 can be returned to the first transition metal compound layer 23. As a result, when the resistance change element 10 is in a low resistance state, the memory device 30 can pass a current through the resistance change element 10 without exceeding the upper limit of the current that can be passed through the resistance change element 10 (the upper limit of movable lithium ions). Therefore, the memory device 30 can execute a read operation of the resistance change element 10 by an external circuit without imposing a limit on the number of times an input pulse is applied.

[0096] 4. Fourth Embodiment Next, a memory device 30 according to a fourth embodiment will be described. In the fourth embodiment, a configuration of the memory device 30 different from those of the second and third embodiments will be described. The memory device 30 of this embodiment is effective in the case where an electromotive force is generated between the first transition metal compound layer 23 and the second transition metal compound layer 24 of the resistance change element 10. The following description will focus on the differences from the second and third embodiments.

[0097] (4.1 Storage Device Configuration) First, an example of the configuration of a memory device 30 according to this embodiment will be described. Fig. 11 is a diagram showing an example of the configuration of a memory device 30 according to a fourth embodiment. In the example of Fig. 11, the first transition metal compound layer 23, the second transition metal compound layer 24, and the lithium ion conductor layer 25 are omitted for the sake of simplicity.

[0098] As shown in FIG. 11, the memory device 30 includes a resistance change element 10, a switch 31, a control circuit 32, an output circuit 33, and a short-circuit switch 52.

[0099] The short-circuit switch 52 is a circuit that shorts or opens between the first electrode 21 and the second electrode 22 of the resistance change element 10 based on a control signal received from the control circuit 32. One end of a transmission path of the short-circuit switch 52 is connected to the first electrode 21, and the other end of the transmission path is connected to the second electrode 22. The short-circuit switch 52 is realized by, for example, a MOSFET.

[0100] One end of the transmission path of the switch 31 of this embodiment is connected to the second electrode 22, the short switch 52, and the output circuit 33, and the other end of the transmission path is connected to the ground. The other configurations are the same as those described in the second embodiment.

[0101] The control circuit 32 of this embodiment is further connected to the short-circuit switch 52. The other connections are the same as those described in the second embodiment. The control circuit 32 transmits a control signal to the short-circuit switch 52. The control signal of this embodiment is a signal for switching the short-circuit switch 52 to a short-circuit state (on state) or an open state (off state). In a read operation, for example, the control circuit 32 shorts the first electrode 21 and the second electrode 22 by putting the short-circuit switch 52 in a short-circuit state for a predetermined time every time an input pulse is applied a predetermined number of times. For example, an electromotive force may be generated in the resistance change element 10 by short-circuiting between the first electrode 21 and the second electrode 22. In this case, lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 by the input pulse move (return) to the first transition metal compound layer 23 by the electromotive force generated in the resistance change element 10.

[0102] The output circuit 33 of this embodiment is connected to the second electrode 22, the short switch 52, and the switch 31. The other configurations are the same as those described in the second embodiment.

[0103] (4.2 Effects of this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0104] Further, the memory device 30 according to the present embodiment includes a resistance change element 10, a switch 31, a control circuit 32, an output circuit 33, and a short-circuit switch 52. The short-circuit switch 52 can short-circuit the first electrode 21 and the second electrode 22 of the resistance change element 10. In a read operation, the control circuit 32 can electrically connect the first electrode 21 and the second electrode 22 by shorting the short-circuit switch 52 for a predetermined time every time an input pulse is applied a predetermined number of times. When the first electrode 21 and the second electrode 22 are short-circuited, an electromotive force is generated in the resistance change element 10. The electromotive force causes lithium ions to move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by short-circuiting the first electrode 21 and the second electrode 22 using the short-circuit switch 52, the lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 by the input pulse can be returned to the first transition metal compound layer 23. As a result, when the resistance change element 10 is in a low resistance state, the memory device 30 can pass a current through the resistance change element 10 without exceeding the upper limit of the current that can be passed through the resistance change element 10 (the upper limit of movable lithium ions). Therefore, the memory device 30 can execute a read operation of the resistance change element 10 by an external circuit without imposing a limit on the number of times an input pulse is applied.

[0105] 5. Fifth Embodiment Next, a storage device 30 according to a fifth embodiment will be described. In the fifth embodiment, a configuration of the storage device 30 different from that of the second to fourth embodiments will be described. The following description will focus on the differences from the second to fourth embodiments.

[0106] (5.1 Storage Device Configuration) First, an example of the configuration of the storage device 30 according to this embodiment will be described. Fig. 12 is a diagram showing an example of the configuration of the storage device 30 according to the fifth embodiment.

[0107] As shown in FIG. 12, the memory device 30 includes a resistance change element 10, a switch 31, a control circuit 32, and an output circuit 33.

[0108] In this embodiment, in a read operation, an input pulse (hereinafter referred to as a "forward pulse") having a voltage higher than the voltage of the second electrode 22 and an input pulse (hereinafter referred to as a "reverse pulse") having a voltage lower than the voltage of the second electrode 22 are alternately and repeatedly applied to the first electrode 21 of the resistance change element 10 from an external circuit. That is, the input pulse includes a forward pulse and a reverse pulse. While the forward pulse is applied to the resistance change element 10, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. Also, while the reverse pulse is applied to the resistance change element 10, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by alternately applying the forward pulse and the reverse pulse, the lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 by the forward pulse move (return) to the first transition metal compound layer 23 by the reverse pulse. In addition, a normal pulse may be applied to the resistance change element 10, followed by an inverted pulse, or a normal pulse may be applied to the resistance change element 10 after an inverted pulse is applied. The storage device 30 may further include a pulse generating circuit that receives a read instruction from an external circuit and generates a normal pulse and an inverted pulse in succession when the read instruction is received. For example, the pulse generating circuit is connected to the first electrode 21.

[0109] The configurations of the switch 31 and the control circuit 32 are similar to those described in the second embodiment.

[0110] The output circuit 33 of this embodiment outputs an output signal indicating whether or not a current flows through the resistance change element 10 at the timing when a forward pulse is applied to the resistance change element 10. The output circuit 33 outputs an output signal indicating that a current does not flow at the timing when an inverted pulse is applied to the resistance change element 10.

[0111] For example, the output circuit 33 includes an N-type MOSFET 60, a first resistor element 61, a second resistor element 62, a third resistor element 63, and a capacitor 41.

[0112] The gate of the N-type MOSFET 60 is connected to the second electrode 22 of the resistance change element 10 and the switch 31 .

[0113] One terminal of the first resistor element 61 is connected to the gate of the N-type MOSFET 60. The other terminal of the first resistor element 61 is connected to ground.

[0114] The power supply voltage Vdd is applied to one terminal of the second resistor element 62. The other terminal of the second resistor element 62 is connected to the drain of the N-type MOSFET 60.

[0115] One terminal of the third resistor element 63 is connected to the source of the N-type MOSFET 60. The other terminal of the third resistor element 63 is connected to ground.

[0116] One electrode of the capacitor 41 is connected to the source of the N-type MOSFET 60. The other electrode of the capacitor 41 is connected to ground.

[0117] The output circuit 33 outputs the voltage at the connection point between the capacitor 41 and the source of the N-type MOSFET 60 as an output signal.

[0118] The output signal in the read operation will be briefly described below. For example, while a forward pulse is applied to the resistance change element 10 in the low resistance state, a current flows through the resistance change element 10. During this time, a voltage higher than the threshold voltage is applied to the gate of the N-type MOSFET 60. Therefore, the N-type MOSFET 60 is turned on, and the drain-source is in a conductive state. As a result, the output circuit 33 outputs an output signal at the power supply voltage level during the period when a forward pulse is applied to the resistance change element 10 in the low resistance state.

[0119] Furthermore, when a forward pulse is applied to the resistance change element 10 in the high resistance state, no current flows through the resistance change element 10. Therefore, a voltage lower than the threshold voltage is applied to the gate of the N-type MOSFET 60. As a result, the N-type MOSFET 60 is turned off, and the drain-source is put into a non-conductive state. As a result, when a forward pulse is applied to the resistance change element 10 in the high resistance state, the output circuit 33 outputs an output signal of the ground level.

[0120] When an inversion pulse is applied to the resistance change element 10, no current flows through the resistance change element 10, as in the case where a normal pulse is applied to the resistance change element 10 in a high resistance state. Therefore, a voltage lower than the threshold voltage is applied to the gate of the N-type MOSFET 60. As a result, the N-type MOSFET 60 is turned off, and the drain-source is brought into a non-conductive state. As a result, when an inversion pulse is applied to the resistance change element 10, the output circuit 33 outputs an output signal of the ground level.

[0121] (5.2 Effects of this embodiment) With the configuration according to this embodiment, the same effects as those of the first and second embodiments can be obtained.

[0122] Further, the memory device 30 according to the present embodiment includes a resistance change element 10, a switch 31, a control circuit 32, and an output circuit 33. In a read operation, the memory device 30 can alternately apply a forward pulse and a reverse pulse to the resistance change element 10. When a forward pulse is applied to the resistance change element 10, lithium ions move from the first transition metal compound layer 23 to the second transition metal compound layer 24. On the other hand, when a reverse pulse is applied to the resistance change element 10, lithium ions move from the second transition metal compound layer 24 to the first transition metal compound layer 23. Therefore, by alternately applying a forward pulse and a reverse pulse to the resistance change element 10, the lithium ions that have moved from the first transition metal compound layer 23 to the second transition metal compound layer 24 can be returned to the first transition metal compound layer 23. As a result, when the resistance change element 10 is in a low resistance state, the memory device 30 can pass a current through the resistance change element 10 so as not to exceed the upper limit of the current that can be passed through the resistance change element 10 (the upper limit of the movable lithium ions). Therefore, the memory device 30 can enable the state of the resistance change element 10 to be read by an external circuit without imposing a limit on the number of times the data can be read.

[0123] (6. Sixth Embodiment) Next, a neural network device 70 according to a sixth embodiment will be described. The neural network device 70 according to the sixth embodiment uses the storage device 30 described in the second to fifth embodiments as a storage circuit.

[0124] (6.1 Configuration of Neural Network Device) First, a description will be given of an example of the configuration of the neural network device 70. Fig. 13 is a diagram showing an example of the configuration of the neural network device 70 according to the sixth embodiment.

[0125] As shown in FIG. 13, the neural network device 70 includes an arithmetic circuit 71, an inference weight storage circuit 72, a learning weight storage circuit 73, and a learning control circuit 74.

[0126] The arithmetic circuit 71 executes arithmetic processing according to a neural network. The arithmetic circuit 71 is realized by, for example, an electric circuit including an analog circuit. For example, the arithmetic circuit 71 receives M (M is an integer equal to or greater than 2) input signals (x 1 ,…,x M ) and outputs an output signal (z). The arithmetic circuit 71 may output a plurality of output signals.

[0127] The inference weight storage circuit 72 stores, for example, a plurality of inference weights used in the arithmetic processing according to the neural network by the arithmetic circuit 71. The inference weight storage circuit 72 includes a plurality of memory devices 30 corresponding to the plurality of inference weights. That is, the inference weight storage circuit 72 includes a plurality of resistance change elements 10 that store a plurality of inference weights. The inference weight storage circuit 72 stores, for example, L (L is an integer equal to or greater than 2) inference weights (w d1 ,…,w dL ) is stored in the inference weight memory circuit 72. Each of the multiple inference weights has an N value (N is an integer equal to or greater than 2). In other words, each of the multiple inference weights is a digital value. The inference weight memory circuit 72 converts the output voltage of the memory device 30 into an N-value inference weight and transmits it to the arithmetic circuit 71. For example, the inference weight memory circuit 72 converts the output voltage of the memory device 30 into an inference weight w di (i is an integer between 1 and L), a preset weight conversion threshold (T 0 ,T 1 ,…,T N-2 ) to set the weight (S 0 ,S 1 ,S 2 ,…,S N-1 For example, the inference weight storage circuit 72 converts the voltage value V i But, T k ≦V i <T k+1 (k is an integer between 0 and N-3), then the inference weight w di =S kFor example, if N=2, the inference weight storage circuit 72 can be used as a binary weight circuit. This allows the arithmetic circuit 71 to use a plurality of inference weights, each represented by N values, to execute arithmetic processing according to a neural network at high speed using an analog circuit. The inference weight storage circuit 72 may be incorporated inside the arithmetic circuit 71.

[0128] The learning weight storage circuit 73 stores a plurality of weights corresponding to a plurality of inference weights in the learning process of the neural network. For example, the learning weight storage circuit 73 stores L weights (w 1 ,…,w L Each of the weights is a continuous value (e.g., an analog quantity or a digital value of a predetermined number of bits). The learning weight storage circuit 73 stores the weights (w 1 ,…,w L ) to send.

[0129] In the learning process of the neural network, the learning control circuit 74 stores the initial values ​​of the multiple weights in the learning weight storage circuit 73. The learning control circuit 74 repeats the update process multiple times. In the update process, the learning control circuit 74 calculates the update amount (Δw 1 ,…,Δw L ) and transmits it to the learning weight storage circuit 73. The learning weight storage circuit 73 updates the stored weights based on the update amount. The learning control circuit 74 may execute the update process only once. After the learning process, the learning control circuit 74 stores, in the inference weight storage circuit 72, multiple values ​​corresponding to the weights stored in the learning weight storage circuit 73 as multiple inference weights.

[0130] In this way, the learning control circuit 74 executes the learning process of the neural network using multiple weights expressed as continuous values. This allows the learning control circuit 74 to increase or decrease each of the multiple weights by minute amounts in the learning process, thereby enabling the neural network to learn with high accuracy.

[0131] (6.2 Layer structure of neural network) Next, an example of the configuration of layers of a neural network will be described. Fig. 14 is a diagram for explaining one layer of a neural network in neural network device 70.

[0132] 14, the neural network includes, for example, one or more layers. The arithmetic circuit 71 includes a circuit that executes an arithmetic operation corresponding to the layer as shown in FIG.

[0133] In order to perform layer operations, the arithmetic circuit 71 outputs N (N is an integer equal to or greater than 2) intermediate signals (y 1 ~y N The j-th (j is an integer not less than 1 and not more than N) product-sum operation circuit 80_j of the N product-sum operation circuits 80 outputs the j-th intermediate signal (y j ) Each of the N multiply-add circuits 80 receives M input signals (x 1 ~x M )

[0134] (6.3 Multiply-and-accumulate operation using a multiply-and-accumulate circuit) Next, a description will be given of the product-sum calculation performed by the product-sum calculation circuit 80. FIG.

[0135] As shown in FIG. 15, each of the N product-sum calculation circuits 80 receives M inference weights (w 1j ,w 2j ,…,w ij ,…,w Mj) is given.

[0136] Each of the N product-sum calculation circuits 80 outputs an intermediate signal obtained by binarizing a product-sum calculation value of the M input signals and the M inference weights. For example, the product-sum calculation circuit 80_j corresponding to the j-th intermediate signal executes the calculation of the formula (1) in an analog manner.

[0137]

number

[0138] In formula (1), y j represents the j-th intermediate signal. i w represents the i-th (i is an integer between 1 and M) input signal. ij represents an inference weight to be multiplied to the i-th input signal among the M inference weights. In formula (1), f(X) represents a function that binarizes the value X in the parentheses with a predetermined threshold value. Furthermore, y j A bias that is a constant may be added to

[0139] (6.4 Effects of this embodiment) The neural network device 70 according to this embodiment stores inference weights used in calculations in the neural network by the storage device 30 using the resistance change elements 10 described in the first to fifth embodiments. This allows the neural network device 70 to store inference weights by the resistance change elements 10 with small variation in characteristics, and therefore allows calculations to be performed with high accuracy.

[0140] (7. Seventh embodiment) Next, a neural network device 70 according to a seventh embodiment will be described. In the seventh embodiment, the configuration of the neural network device 70 that differs from that of the sixth embodiment will be described. The following description will focus on the differences from the sixth embodiment.

[0141] (7.1 Configuration of Neural Network Device) First, a description will be given of an example of the configuration of the neural network device 70. Fig. 16 is a diagram showing an example of the configuration of the neural network device 70 according to the seventh embodiment.

[0142] As shown in FIG. 16, the neural network device 70 includes an arithmetic circuit 71, a learning weight storage circuit 73, and a learning control circuit 74.

[0143] The arithmetic circuit 71 executes arithmetic processing according to a neural network. The arithmetic circuit 71 is realized by, for example, an electric circuit including an analog circuit. For example, the arithmetic circuit 71 receives M (M is an integer equal to or greater than 2) input signals (x 1 ,…,x M ) and outputs an output signal (z). The arithmetic circuit 71 may output a plurality of output signals.

[0144] The learning weight storage circuit 73 stores a plurality of weights corresponding to a plurality of inference weights in the learning process of the neural network. The learning weight storage circuit 73 includes a plurality of storage devices 30 corresponding to a plurality of weights. That is, the learning weight storage circuit 73 includes a plurality of resistance change elements 10 that store a plurality of inference weights. The learning weight storage circuit 73 stores, for example, L weights (w 1 ,…,w L ) Each of the multiple weights is a continuous value (for example, an analog amount or a digital value of a predetermined number of bits). The learning weight storage circuit 73 transmits the multiple weights to the arithmetic circuit 71. The arithmetic circuit 71 uses the output values ​​(weights) of the learning weight storage circuit 73 as they are.

[0145] In the learning process of the neural network, the learning control circuit 74 stores the initial values ​​of the weights in the learning weight storage circuit 73. Then, the learning control circuit 74 repeats the update process a number of times. In the update process, the learning control circuit 74 calculates the update amount (Δw 1 ,…,Δw L) and transmits it to the learning weight storage circuit 73. The learning weight storage circuit 73 updates each of the stored weights based on the update amount. Note that the learning control circuit 74 may execute the update process only once.

[0146] (7.2 Effects of this embodiment) The neural network device 70 according to this embodiment stores weights used in calculations in the neural network by a storage device 30 using the resistance change elements 10 described in the first to fifth embodiments. A learning control circuit 74 executes a learning process for the neural network using a plurality of weights expressed as continuous values. This allows the learning control circuit 74 to increase or decrease each of the plurality of weights by minute amounts in the learning process, thereby enabling the neural network to learn with high accuracy.

[0147] (8. Modifications, etc.) Although some embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their modifications are included in the scope and spirit of the invention, and are included in the scope of the invention and its equivalents described in the claims. [Explanation of symbols]

[0148] 10...Resistance change element 21...1st electrode 22…Second electrode 23...First transition metal compound layer 24...Second transition metal compound layer 25...Lithium ion conductor layer 30…Storage device 31…Switch 32...Control circuit 33...Output circuit 41…Capacitor 51...Positive / negative inversion circuit 52...Short circuit switch 60…N-type MOSFET 61...First resistor element 62...Second resistor element 63...Third resistor element 70...Neural network device 71...Arithmetic circuit 72...Inference weight memory circuit 73...Learning weight memory circuit 74...Learning control circuit 80...Multiply-and-accumulate circuit

Claims

1. A first electrode; A second electrode; a first transition metal compound layer provided between the first electrode and the second electrode and being a metal compound containing lithium ions at lattice sites; a second transition metal compound layer provided between the first transition metal compound layer and the second electrode, the second transition metal compound layer being the metal compound containing the lithium ions at the lattice sites; a lithium ion conductor layer provided between the first transition metal compound layer and the second transition metal compound layer, the lithium ion conductor layer being a solid material that allows the lithium ions to pass through but does not allow electrons to pass through easily; Equipped with Resistance change element.

2. the first transition metal compound layer is either a rock-salt type transition metal oxide or a spinel type transition metal oxide, or a mixture of the rock-salt type transition metal oxide and the spinel type transition metal oxide; The variable resistance element according to claim 1 .

3. The rock salt transition metal oxide is LiTiO ２ , Li ７ Ti ５ O １２ , Li ２ [CrTi]O ４ , (Li ３／２ Fe １／２ ) [Li １／２ Fe １／２ Ti]O ４ Consisting of any of the following, or a mixture of these: The variable resistance element according to claim 2 .

4. The spinel-type transition metal oxide is LiTi ２ O ４ , Li ４ Ti ５ O １２ , Li[CrTi]O ４ , (Li １／２ Fe １／２ ) [Li １／２ Fe １／２ Ti]O ４ Consists of any of the following, or a mixture of these: The variable resistance element according to claim 2 .

5. The second transition metal compound layer is a rock salt type transition metal oxide Li ７ Ti ５ O １２ or spinel-type transition metal oxide Li ４ Ti ５ O １２ or a mixture thereof, The variable resistance element according to claim 1 .

6. The second transition metal compound layer is a λ-type cubic metal oxide λMnO ２ and spinel-type transition metal oxide Li １／２ MnO ２ It is a mixture of The variable resistance element according to claim 1 .

7. a composition of the lithium ions in the second transition metal compound layer is lower than a composition of the lithium ions in the first transition metal compound layer; The variable resistance element according to claim 1 .

8. The second transition metal compound layer has a thickness smaller than that of the first transition metal compound layer. The variable resistance element according to claim 1 .

9. the second transition metal compound layer has a thickness equal to or greater than the thickness of the first transition metal compound layer; The variable resistance element according to claim 1 .

10. The second transition metal compound layer changes to a low resistance state or a high resistance state based on the composition of the lithium ions contained in the second transition metal compound layer. The variable resistance element according to claim 1 .

11. the first transition metal compound layer is in contact with a first surface of the lithium ion conductor layer, the second transition metal compound layer is in contact with a second surface of the lithium ion conductor layer opposite to the first surface; The variable resistance element according to claim 1 .

12. The shape of the first surface is the same as the shape of the second surface. The variable resistance element according to claim 11 .

13. A first electrode; A second electrode; a first transition metal compound layer provided between the first electrode and the second electrode, containing lithium ions and made of either a rock-salt type transition metal oxide or a spinel type transition metal oxide, or a mixture of the rock-salt type transition metal oxide and the spinel type transition metal oxide; A Li ７ Ti ５ O １２ Or Li ４ Ti ５ O １２ a second transition metal compound layer composed of any one of the above or a mixture thereof; a lithium ion conductor layer provided between the first transition metal compound layer and the second transition metal compound layer, the lithium ion conductor layer being a solid material that allows the lithium ions to pass through but does not allow electrons to pass through easily; Resistance change element.

14. The variable resistance element according to claim 1 , A control circuit connected to the resistance change element and configured to control a write operation and a read operation using the resistance change element; an output circuit connected to the resistance change element and configured to output an output voltage based on the read operation; Equipped with storage device.

15. When the control circuit is to put the resistance change element into a low resistance state in the write operation, the control circuit applies a pulse of a positive voltage higher than a voltage of the second electrode to the first electrode. The storage device of claim 14.

16. a first circuit configured to connect the output circuit to either the first electrode or the second electrode of the resistance change element; In the read operation, the first circuit switches from a state in which an input pulse is applied to the first electrode and the second electrode is connected to the output circuit to a state in which the input pulse is applied to the second electrode and the first electrode is connected to the output circuit. The storage device of claim 14.

17. a second circuit configured to be able to short-circuit the first electrode and the second electrode, In the read operation, the second circuit shorts the first electrode and the second electrode after application of the input pulse to the resistance change element is completed. The storage device of claim 14.

18. the output circuit includes a transistor having a gate connected to the second electrode of the resistance change element; In the read operation, a normal pulse and an inversion pulse obtained by inverting the normal pulse are alternately applied to the first electrode of the resistance change element, the output circuit outputs a first voltage when the resistance change element is in a low resistance state when the forward pulse is applied to the first electrode, and outputs a second voltage lower than the first voltage when the resistance change element is in a high resistance state. The storage device of claim 14.

19. A computation circuit configured to perform computation according to a neural network; A memory circuit including the variable resistance element according to claim 1 , the memory circuit being configured to cause the variable resistance element to store a weight used in the arithmetic processing; Equipped with Neural network device.

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