Crystal, phase change memory, method for manufacturing crystal, and method for manufacturing phase change memory

Abstract

Provided is a crystal body in which phase change occurs between at least two phases selected from among: an ordered phase which is constituted from the elements Sr, Bi, Ni and O and in which a first layer, which comprises a first composition having a perovskite structure constituted from Sr element, Ni element and O element, and a second layer, which comprises a second composition having a halite structure constituted from Sr element, Bi element and O element, are alternately layered, and the arrangement of Sr element and Bi element in the second composition in the second layer is ordered; an unordered phase in which the first layer and a second layer comprising the second composition are alternately layered and the arrangement of Sr element and Bi element in the second composition in the second layer is unordered; and a double perovskite phase having a double perovskite structure represented by the general formula SrBi0.5Ni0.5O3-x.

Description

[Technical Field]

[0001] The present invention relates to a crystal, a phase change memory, a method for manufacturing a crystal, and a method for manufacturing a phase change memory. This application claims priority based on Japanese Patent Application No. 2022-075155, filed on April 28, 2022, the contents of which are incorporated herein by reference. [Background technology]

[0002] With the aim of achieving the Sustainable Development Goals (SDGs), there are growing expectations for the realization of a new society (Society 5.0) in which all people and things are connected through the Internet of Things (IoT). Further development of electronic devices such as computers is essential for the realization of Society 5.0, and in recent years, attention has been focused on a hierarchical level of non-volatile memory called storage-class memory, which bridges the performance gap between main memory and storage.

[0003] In recent years, phase-change random-access memory (PRAM) has been viewed as a promising storage-class memory (e.g., Non-Patent Document 1). Phase-change memory has a phase-change material that can take two states: an ordered phase with low electrical resistance and a disordered phase with high electrical resistance, and records 0 / 1 based on the difference in electrical resistance of the phase-change material between the two states. For example, the phase-change memory in Non-Patent Document 1 mainly uses chalcogenide materials such as GeSbTe alloys as the phase-change material, changes the phase-change material between a crystalline state with low electrical resistance and an amorphous state with high electrical resistance, and records 0 / 1 based on the difference in electrical resistance between the crystalline and amorphous states.

[0004] Since Tellurium is designated as a toxic substance, in recent years, there has been a demand for chalcogen-free phase-change materials made from raw materials other than chalcogenides for practical use. Furthermore, chalcogenide materials such as GeSbTe alloys were originally developed 40 years ago as optical disc materials that utilize changes in reflectivity. The emergence of an entirely new group of phase-change materials based on electrical resistance changes is expected.

[0005] Also, Sr 2.5 Bi 0.5 A material having NiO5 crystals has been reported (Non-Patent Document 2). 2.5 Bi 0.5 The NiO5 crystal consists of a layer of SrNiO3 perovskite structure and (Sr 1.5 Bi 0.5 )O2 rock salt structure has a structure in which layers are stacked alternately. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] M. Wuttig et al., Nat. Mater. 6, 824 (2007) [Non-patent document 2] J. Solid State Chem. 184, 2011, 3262. Summary of the Invention [Problem to be solved by the invention]

[0007] In the phase-change memory disclosed in Non-Patent Document 1, chalcogenide materials such as GeSbTe alloys used as phase-change materials contain Te, which is designated as a toxic substance. Furthermore, the phase-change materials described in Non-Patent Document 1 and Patent Document 1 are based on the principle of phase control between crystalline and amorphous phases. In practice, this type of phase control requires phase-change materials to have low melting points. No phase-change materials other than chalcogenide materials have been reported that can control the phase change between crystalline and amorphous phases at low melting points. Perovskite oxides, such as those described in Patent Document 1, have higher melting points than chalcogenide materials, and other phase-change materials are needed. Therefore, in order to develop a completely new group of phase-change materials, it is desirable to reversibly control the phase of a substance using a different principle than conventional methods. If a completely new material whose phase can be reversibly controlled using a different principle can be developed, it will be possible to explore phase-change materials for all substances, going beyond the scope of chalcogenide materials.

[0008] In Non-Patent Document 2, Sr 2.5 Bi 0.5 NiO5 crystal (Sr 1.5 Bi 0.5 It is unclear whether the 02 rock salt layer may have a different crystal structure, such as a disordered phase, that differs from the ordered phase. If it does have a different crystal structure, it is unclear whether the difference in crystal structure will result in a difference in electrical resistance sufficient for a phase-change material. Therefore, it is not possible to provide a new phase-change material based on the report in this literature.

[0009] The present invention has been made in consideration of the above circumstances, and aims to provide a crystalline body of a new phase-change material that exhibits a phase change other than a phase change between a crystalline phase and an amorphous phase based on a different principle than has been previously known, a phase-change memory that utilizes the crystalline body, a method for manufacturing the crystalline body, and a method for manufacturing the phase-change memory. [Means for solving the problem]

[0010] [1] A crystalline body according to one embodiment of the present invention comprises: It is composed of Sr, Bi, Ni and O elements. an ordered phase in which a first layer made of a first composition and a second layer made of a second composition different from the first composition are alternately laminated, the first composition having a perovskite structure made of Sr, Ni, and O elements, the second composition having a rock salt structure made of Sr, Bi, and O elements, and the second layer having an ordered phase in which the arrangement of Sr and Bi elements of the second composition is ordered; a first layer made of a first composition and a second layer made of a second composition different from the first composition, the first composition having a perovskite structure made of Sr, Ni, and O elements, the second composition having a rock salt structure made of Sr, Bi, and O elements, the second layer having a disordered phase in which the arrangement of Sr and Bi elements of the second composition is disordered; General formula SrBi 0.5 Ni 0.5 O 3-x a double perovskite phase having a composition represented by (0.3≦x≦1.3) (1) and a double perovskite structure; The phase changes between at least two of the phases.

[0011] [2] In the crystal of [1] above, in the ordered phase and the disordered phase, the first composition is represented by the general formula SrNiO3 ... (2), and the second composition is represented by the general formula Sr 1.5 Bi 0.5 It may be represented by O2...(3).

[0012] [3] The crystalline body of [1] or [2] above may undergo a phase change between the ordered phase and the double perovskite phase.

[0013] [4] Any of the crystals described in [1] to [3] above may undergo a phase change between the disordered phase and the double perovskite phase.

[0014] [5] Any of the crystals described in [1] to [4] above may undergo a phase change between the ordered phase, the ordered phase, and the disordered phase.

[0015] [6] In any one of the crystals [1] to [5] above, the electrical resistivity in the disordered phase is 1.0 × 10 3 ~1.0×10 5 It may be [mΩ·cm].

[0016] [7] In any of the crystals of [1] to [5] above, the electrical resistivity when in the double perovskite phase may be 1.0×10 8 [mΩ·cm] or more.

[0017] [8] The phase change memory according to one aspect of the present invention includes a memory layer made of any of the crystals of [1] to [7] above, and a plurality of electrodes electrically connected to the memory layer.

[0018] [9] The method for manufacturing a crystal according to one aspect of the present invention is as follows: A mixed powder of a strontium source material, a bismuth source material, and a nickel source material mixed based on the stoichiometric ratio of the general formula Sr 2.5 Bi 0.5 NiO 5-x (where 0 < x < 2.5) ··· (4) is compacted to form a pellet in a compacting step, the pellet is heated at 950°C or higher in an air atmosphere in a first heating step to produce a crystal precursor, the crystal precursor is cooled in a cooling step, and the crystal precursor is heated at a temperature of 350°C or higher and lower than 950°C in a second heating step.

[0019]

[10] In the method for manufacturing a crystal of [9] above, in the second heating step, the crystal precursor may be heated at a temperature of 350°C or higher and lower than 650°C.

[0020]

[11] In the method for manufacturing a crystal of [9] above, in the second heating step, the crystal precursor may be heated at a temperature of 650°C or higher and lower than 950°C in an air atmosphere.

[0021]

[12] The method for manufacturing a crystal of

[10] above further includes a third heating step of cooling the crystal precursor after the second heating step and then heating it again. In the third heating step, the crystal precursor may be heated at 500 to 1000°C, and in the third heating step, the crystal precursor may be heated at a temperature of 650°C or higher and lower than 950°C in an air atmosphere.

[0022]

[13] A method for manufacturing a phase-change memory according to one aspect of the present invention includes the step of forming a memory layer using any one of the methods for manufacturing a crystalline body described above in [9] to

[12] . [Effects of the Invention]

[0023] According to the present invention, it is possible to provide a crystalline body of a new phase-change material that exhibits a phase change other than a phase change between a crystalline phase and an amorphous phase based on a different principle than has been previously known, a phase-change memory that utilizes the crystalline body, a method for manufacturing the crystalline body, and a method for manufacturing the phase-change memory. [Brief explanation of the drawings]

[0024] [Figure 1] FIG. 1 is a diagram showing the crystal structure of a crystalline body according to one embodiment of the present invention, which shows a crystal structure in an ordered phase. [Figure 2] FIG. 1 is a diagram showing the crystal structure of a crystalline body according to one embodiment of the present invention, showing the crystal structure in a disordered phase. [Figure 3] FIG. 1 is a diagram showing the crystal structure of a crystalline body according to an embodiment of the present invention, showing a double perovskite phase. [Figure 4] 1A and 1B are diagrams for explaining a phase change of a crystal according to one embodiment of the present invention. [Figure 5] 1 is a flowchart of a method for producing a crystal according to one embodiment of the present invention. [Figure 6] 6 is a flowchart of a method for producing a crystal according to a modified example of FIG. 5. [Figure 7] 1 is a cross-sectional view of a phase change memory according to an embodiment of the present invention; [Figure 8] 1 shows X-ray diffraction patterns of the crystals of Examples 1-1 to 1-6 and Reference Examples 1-1 to 1-5. [Figure 9] 1 shows X-ray diffraction patterns of the crystals of Examples 2-1 to 2-2 and Reference Examples 2-1 to 2-2. [Figure 10] 1 shows X-ray diffraction patterns of the crystals of Example 3-1, Reference Example 3-1, and Reference Example 3-2. [Figure 11]1 is a graph showing the electrical resistivity of Examples 1-1 and 1-2 and Reference Examples 1-4 and 1-6. DETAILED DESCRIPTION OF THE INVENTION

[0025] An example of an embodiment of the present invention will be described in detail below with reference to the drawings. Note that the drawings used in the following description may show characteristic parts enlarged for the sake of convenience in order to make the features of the present invention easier to understand. Therefore, the dimensional ratios of each component may differ from the actual ones.

[0026] [Crystal] A crystalline body according to one embodiment of the present invention is composed of Sr, Bi, Ni, and O elements, and is a phase change material that undergoes a phase change between at least two of an ordered phase, a disordered phase, and a double perovskite phase, which will be described in detail later. The crystalline body according to one embodiment of the present invention exhibits a phase change at least between an ordered phase and a disordered phase, between an ordered phase and a double perovskite phase, or between a disordered phase and a double perovskite phase, and may also undergo a phase change between a disordered phase, an ordered phase, and a double perovskite phase. The ordered phase has an alternating laminate structure in which first layers made of a first composition and second layers made of a second composition different from the first composition are alternately laminated, the first composition has a perovskite structure made of Sr, Ni, and O elements, the second composition has a rock salt structure made of Sr, Bi, and O elements, and in the second layer, the Sr and Bi elements of the second composition are arranged in an orderly manner. The disordered phase has an alternating laminate structure in which first layers made of a first composition and second layers made of a second composition different from the first composition are alternately laminated, the first composition has a perovskite structure made of Sr, Ni, and O elements, the second composition has a rock salt structure made of Sr, Bi, and O elements, and in the second layer, the arrangement of the Sr and Bi elements of the second composition is disordered. The double perovskite phase has the general formula SrBi 0.5 Ni 0.5 O 3-xIt has a double perovskite structure expressed by (0.3≦x≦1.3) (1).

[0027] Fig. 1 is a diagram showing the crystal structure of a crystalline body according to one embodiment of the present invention, showing a crystal structure of an ordered phase in which the arrangement of Sr and Bi elements in the second composition is ordered. The crystalline body 1a shown in Fig. 1 has an alternating laminate structure in which first layers L1a made of the first composition and second layers L2a made of the second composition are alternately laminated.

[0028] The first composition constituting the first layer L1a has a perovskite structure represented by the elements Sr, Ni, and O. The first composition is represented by the general formula SrNiO3 (2).

[0029] The second composition constituting the second layer L2a has a rock salt structure composed of Sr, Bi and O elements. 1.5 Bi 0.5 O2···(3). In the second layer L2a, the arrangement of Sr and Bi elements is orderly. Specifically, within the plane having the same c-axis direction component, the atom nearest to the Sr atom is a Bi atom, and the atom nearest to the Bi atom is also an Sr atom. With this arrangement, Sr elements are surrounded by Bi elements, and Bi elements are surrounded by Sr elements. Furthermore, in the crystal 1a, within the plane having the same c-axis direction component, Sr elements and Bi elements are alternately and periodically arranged.

[0030] The crystal 1a has a layered structure in which first layers L1a and second layers L2a are alternately stacked, and is represented by the general formula Sr 2.5 Bi 0.5 NiO 5-x (wherein 0≦x≦0.8) (5). In the crystalline body 1a, at least one of the first composition represented by the above formula (2) constituting the first layer L1a and the second composition represented by the above formula (3) constituting the second layer L2a may have an oxygen deficiency. In such a case, x in the above formula (5) has a positive value. When the crystalline body 1a does not have an oxygen deficiency, x=0 in the above formula (5).

[0031] The electrical resistivity of the crystalline body 1a is, for example, 11 to 18 mΩ·cm at room temperature. The electrical resistivity of the crystalline body 1a is measured by a four-terminal method. The measurement by the four-terminal method conforms to the method described in "Experimental Chemistry Lectures 7 - Electrical and Magnetic Properties -" (5th edition), edited by the Chemical Society of Japan, Maruzen, 2004.

[0032] FIG. 2 is a diagram showing the crystal structure of a crystalline body according to one embodiment of the present invention, showing a crystalline structure in which the arrangement of Sr and Bi elements in the second composition is disordered. In the crystalline body 1b shown in FIG. 2, the same components as those in the crystalline body 1a shown in FIG. 1 are denoted by the same reference numerals, and a description thereof will be omitted. The crystalline body 1b shown in FIG. 2 has an alternating laminate structure in which first layers L1a and second layers L2b are alternately laminated. The crystalline body 1b has a structure in which the local atomic arrangement is changed from that of the crystalline body 1a.

[0033] The second composition constituting the second layer L2b has a rock salt structure composed of Sr, Bi and O elements. 1.5 Bi 0.5 O2···(3). The second layer L2b has a different arrangement of Sr and Bi atoms than the second layer L2a. In the second layer L2b, the arrangement of Sr and Bi atoms is disordered. Specifically, within a plane having the same c-axis direction component, the Sr and Bi elements are arranged differently from those in the second layer L2a. That is, within a plane having the same c-axis direction component, the second layer L2b of the crystal 1b includes a location where the atom nearest to the Sr atom is an Sr atom or a location where the atom nearest to the Bi atom is a Bi atom. Due to this arrangement, the crystal 1b includes a location where the atom nearest to the Sr atom is an Sr atom or a location where the atom nearest to the Bi atom is a Bi atom. Therefore, the crystal 1b includes a location where one Sr atom is not surrounded by its four nearest Bi atoms and a location where one Bi atom is not surrounded by its four nearest Sr atoms. Furthermore, in the crystal 1b, the periodic structure of Sr elements and Bi elements is disordered within the plane having the same c-axis direction component.

[0034] The crystal 1b has a layered structure in which first layers L1a and second layers L2b are alternately stacked, and is represented by the general formula Sr 2.5 Bi 0.5 NiO 5-x (wherein 0≦x≦0.8) (5). Like crystalline body 1a, crystalline body 1b may have an oxygen deficiency in at least one of the first composition and the second composition. When at least one of the first composition and the second composition has an oxygen deficiency, x is a positive value. When crystalline body 1b does not have an oxygen deficiency, x=0.

[0035] Crystal 1b exhibits a higher electrical resistivity than crystal 1a. For example, the electrical resistivity of crystal 1b at room temperature is 3.5×10 3 ~1.2×10 4 The electrical resistivity of the crystal 1b is, for example, 1.0×10 at room temperature. 3 ~1.3×10 5 [mΩ·cm], 6.3×10 4 ~1.0×10 5 It is preferable that the resistance is [mΩ·cm].

[0036] FIG. 3 is a diagram showing the crystal structure of a crystal according to one embodiment of the present invention, and is represented by the general formula SrBi 0.5 Ni 0.5 O 3-x (0.3≦x≦1.3) (1) shows a double perovskite phase having a double perovskite structure. In the double perovskite phase shown by the above formula (1), x may satisfy 0.3≦x≦0.9, or 0.7≦x≦1.3. Most preferably, x satisfies 0.7≦x≦0.9. Crystal 1c has oxygen deficiency, so x is a positive value.

[0037] The crystal 1c shown in Fig. 3 has an alternating laminate structure in which, for example, first layers La and second layers Lb are alternately laminated with Sr elements interposed therebetween. The first layer La and the second layer Lb have, for example, symmetrical arrangements of Bi and Ni elements. In such a symmetrical arrangement, Bi atoms in the first layer La overlap with Ni atoms in the second layer Lb, and Ni atoms in the first layer La overlap with Bi atoms in the second layer Lb. In the crystal structure shown in Figure 3, the general formula of the double perovskite phase is AB' 0.5 B´´ 0.5 The A site in O3 is occupied by a Sr atom, and the B' and B'' sites are selectively occupied by a Bi atom and a Ni atom.

[0038] The double perovskite structure of crystalline body 1c can be confirmed by the diffraction pattern obtained by X-ray diffraction measurement, which shows peaks at 2θ=18.4°, 21.3°, 30.4°, and 35.7°.

[0039] In the crystalline body 1c, O atoms are arranged in the ±a-axis direction, ±b-axis direction, and ±c-axis direction of Bi atoms and Ni atoms.

[0040] The crystalline body 1c, in which the first layer L1c and the second layer L2c are in a perovskite phase and which is cubic as a whole, exhibits a higher electrical resistivity than the crystalline body 1a, in which the second layer L2a is in an ordered phase, and the crystalline body 1b, in which the second layer L2b is in a disordered phase. The electrical resistivity of the crystalline body 1c at room temperature is, for example, 1.1 × 10 9 ~1.8×10 9 is 1.5 × 10 times the electrical resistivity of crystalline 1b at room temperature. 5 ~3.2×10 5 The electrical resistivity of the crystal 1c is, for example, 1.0 × 10 at room temperature. 8 [mΩ·cm] or more, and 1.0×10 10 [mΩ·cm] or more is preferable, and 2.0×10 10 It is more preferable that the resistivity is [mΩ·cm] or more.

[0041] The crystal structures of the crystals 1a, 1b, and 1c change when heated. That is, the crystals 1a, 1b, and 1c can undergo phase changes between crystal phases when heated. FIG. 4 is a diagram illustrating the phase changes of a crystal according to one embodiment of the present invention. The crystals 1a, 1b, and 1c can be heated by applying a predetermined pulse voltage, and can be phase-changed between their respective states at room temperature when cooled.

[0042] When crystalline body 1a is heated to temperature Tab and then cooled, its crystal structure changes to that of crystalline body 1b. When crystalline body 1a is heated to temperature Tac and then cooled, its crystal structure at room temperature changes to that of crystalline body 1c.

[0043] The temperature Tab is, for example, 350°C or higher and lower than 650°C. The temperature Tac is, for example, 650°C or higher and lower than 950°C. The heating time depends on the heating temperature, but is, for example, 10 minutes or longer. The heating time is, for example, 48 hours or shorter. When the crystalline body 1a is heated at a low temperature of, for example, lower than 350°C or a high temperature of, for example, 950°C or higher, the crystalline structure thereof at room temperature is maintained.

[0044] When crystalline body 1b is heated to a temperature Tbc and then cooled, its crystal structure changes to that of crystalline body 1c. When crystalline body 1b is heated to a temperature Tba and then cooled, its crystal structure at room temperature changes to that of crystalline body 1a.

[0045] The temperature Tbc is, for example, 650 to 900°C. The temperature Tba is, for example, a temperature of 900°C or higher. The heating time depends on the heating temperature, but is, for example, 10 minutes or longer. The heating time is, for example, 48 hours or shorter. When the crystalline body 1c is heated and cooled at a low temperature, it retains its crystalline structure at room temperature.

[0046] When crystalline body 1c is heated to a temperature Tca and then cooled, its crystalline structure at room temperature changes to that of crystalline body 1a. Temperature Tca is, for example, 950°C or higher. The heating time depends on the heating temperature, but is, for example, 10 minutes or longer. The heating time is, for example, 48 hours or shorter. When crystalline body 1c is heated and cooled at a low temperature, its crystalline structure at room temperature is maintained.

[0047] The crystal phase change between the above crystals 1a, 1b, and 1c can also be carried out in a short time shorter than the above time by applying a pulse voltage, etc. Using the above phase change, the above crystal may undergo a phase change between an ordered phase and a double perovskite phase, a phase change between a disordered phase and a double perovskite phase, or a phase change between an ordered phase and a disordered phase. When a crystal in an ordered or disordered phase is heated to undergo a phase change into a crystal in a double perovskite phase, a crystal and an amorphous body substantially composed of Sr, Ni, and O may be generated separately from the crystal in the double perovskite phase. The amounts of Sr, Bi, and Ni in these crystals and amorphous bodies can be expressed by the general formula Sr 2.5 Bi 0.5 NiO 5-x (wherein 0≦x≦0.8) (5) The general formula SrBi 0.5 Ni 0.5 O 3-x This is the amount of substance obtained by subtracting the amount of substance of the corresponding element in the double perovskite phase crystal expressed by (0.3≦x≦1.3) (1).

[0048] The phase change occurs, for example, in the air.

[0049] [Method of manufacturing crystals] 5 is a flowchart of a method for producing a crystal according to one embodiment of the present invention. 2.5 Bi 0.5 NiO 5-xA mixed powder of a strontium source material, a bismuth source material, and a nickel source material mixed based on the stoichiometric ratio of (where 0 < x < 2.5) ··· (4) is compacted, and a pellet is formed in a compacting step; the pellet is heated in the air to produce a crystal precursor in a first heating step; a cooling step of cooling the crystal precursor; and a second heating step of heating the crystal precursor.

[0050] <Compacting step> First, powders of a strontium source material, a bismuth source material, and a nickel source material are mixed based on the stoichiometric ratio of the above formula (4) to obtain a mixed powder. Here, the amounts of the strontium source material, the bismuth source material, and the nickel source material are adjusted so that the composition ratios of the Sr element, the Bi element, and the Ni element in the above formula (4) are obtained, and the amount of the O element may not be considered.

[0051] The mixing of the above powders can be performed, for example, in a glove box. As the strontium source material, for example, strontium oxide can be used. As the bismuth source material, for example, bismuth oxide can be used. As the nickel source material, for example, nickel(II) oxide can be used. For the mixing of these powders, for example, a ball mill, a pestle, and a mortar are used. Then, the mixed powder is compacted by a tableting machine or the like to form a pellet. The above metal oxides may be appropriately mixed with a metal single body and used.

[0052] <First heating step> Next, the pellets are heated to 950°C or higher in an air atmosphere. An electric furnace or the like can be used to heat the pellets. The pellets are heated, for example, under conditions including a heating temperature of 950°C or higher, preferably 750 to 1100°C. The pellets are heated, for example, at a heating rate of 100°C / h to the above heating temperature and maintained at the above heating temperature for 5 to 48 hours. The pellets are preferably heated by gradually increasing the heating temperature in several stages. For example, the pellets are heated at a low temperature (750°C or higher but lower than 900°C) for about 5 hours, then at a medium temperature (900°C or higher but lower than 950°C) for 15 hours, and then at a high temperature (950°C or higher but lower than 1100°C) for about 48 hours while maintaining the temperature. By including a high-temperature heating step in the first heating step, the impurity concentration of the resulting crystal can be reduced.

[0053] By carrying out the first heating step in an air atmosphere, oxygen is taken into the mixed powder of the raw material powders. The first heating step produces the crystalline body 1a as a crystalline precursor for producing the crystalline bodies 1b and 1c.

[0054] <First cooling process> Next, the pellets heated to the above heating temperature are cooled (first cooling step) For example, in the same atmosphere as in the first heating step, they are cooled from the above heating temperature to room temperature at a cooling rate of 100°C / h.

[0055] <Second heating process> Next, the crystal precursor is heated. For example, the crystal precursor (pellet) is heated. The crystal precursor can be heated using an electric furnace or the like, and it is also possible to utilize the application of a pulse voltage. The crystal precursor can be heated, for example, in an air atmosphere. The heating temperature of the crystal precursor in the second heating step is 350°C or higher and lower than 950°C. When producing a crystal 1b in which the second layer L2b has a disordered arrangement and is a disordered phase, the crystal precursor is heated at a temperature of 350 to 650°C. When producing a crystal 1c having a double perovskite structure, the crystal precursor is heated at a temperature of 650°C or higher and lower than 950°C. The crystal precursor is heated, for example, at a rate of 200°C / h to the above heating temperature and maintained at the above heating temperature for 24 hours.

[0056] <Second cooling process> Next, for example, the pellets heated to the above heating temperature are cooled (second cooling step) from the above heating temperature to room temperature at a cooling rate of 200°C / h in the same atmosphere as in the second heating step.

[0057] Crystals 1b and 1c can be produced by the above-described procedure. Note that crystal 1c can also be produced by other methods. FIG. 6 is a flowchart of a method for producing a crystal according to a modification of FIG. 5, showing a method for producing crystal 1c. Specifically, crystal 1b can be produced as a crystal precursor in the second heating step, and after the second heating step, the crystal precursor is cooled (second cooling step) and then heated again in a third heating step.

[0058] <Third heating step> After cooling the crystal precursor in the second cooling step, the crystal precursor (pellet) is heated at a heating temperature of 500 to 1000°C. The heating temperature in the third heating step may be 650°C or higher and lower than 950°C. The crystal precursor can be heated in an electric furnace or the like, and application of a pulse voltage can also be utilized. The temperature rise rate and the holding time at the heating temperature can be the same as in the second heating step.

[0059] <Third cooling process> Next, for example, the pellets heated at the above heating temperature are cooled (third cooling step). For example, the pellets can be cooled in the same atmosphere and at the same temperature decreasing rate as in the second heating step.

[0060] Furthermore, as shown in Figure 4, considering that a phase change from crystalline body 1c to crystalline body 1a is also possible, crystalline body 1c may be formed as a crystalline precursor in the second heating step, and then the crystalline precursor may be cooled and then heated again at temperature Tca to produce crystalline body 1a.

[0061] Furthermore, when crystalline body 1a or crystalline body 1b is heated and undergoes a phase change to crystalline body 1c, as described above, a crystalline body essentially composed of Sr, Ni, and O elements, and an amorphous body may be generated as impurities separate from crystalline body 1c. However, by subsequently heating at 900°C or higher, these can be integrated to produce crystalline body 1a.

[0062] [Phase change memory] Fig. 7 is a cross-sectional view of a phase-change memory according to one embodiment of the present invention. The phase-change memory 100 shown in Fig. 7 includes a memory layer 10 made of crystalline bodies 1a, 1b, and 1c according to the above embodiment, and a plurality of electrodes 11 and 12 electrically connected to the memory layer 10. The phase-change memory 100 includes, for example, a first electrode 11, a memory layer 10, and a second electrode 12.

[0063] The memory layer 10 is made up of the crystals 1a, 1b, and 1c according to the above embodiment. The memory layer 10 is electrically connected to the first electrode 11 and the second electrode 12 described below, and is disposed between the first electrode 11 and the second electrode 12 in the stacking direction, for example.

[0064] The first electrode 11 is, for example, a member for electrically writing and reading data to and from the memory layer 10. The first electrode 11 is, for example, a plug-shaped material. FIG. 7 shows the first electrode 11 having the same surface area of ​​the plane intersecting the stacking direction as the second electrode 12, but the surface area of ​​the plane intersecting the stacking direction of the first electrode 11 is arbitrary. The first electrode 11 is, for example, a heater made of a material that can be heated by passing electricity through it. The second electrode 12 is electrically connected to the memory layer 10 and is provided, for example, spaced apart from the first electrode 11. The second electrode 12 is disposed, for example, on the surface of the memory layer 10 in the stacking direction. The second electrode 12 is formed, for example, from a conductive material such as a metal.

[0065] The phase-change memory 100 is manufactured by a manufacturing method including a step of forming a memory layer using the above-described crystalline manufacturing method. In the phase-change memory 100, the first electrode 11 and the second electrode 12 are stacked using well-known techniques in semiconductor manufacturing processes, such as photolithography and sputtering. The memory layer 10 can be made of, for example, the crystalline material according to the above-described embodiment. Note that while FIG. 7 shows an example in which the electrically heatable first electrode 11 is provided, a member made of an electrically heatable heater material may be further provided between the first electrode 11 and the memory layer 10, and a material with low resistance may be used for the first electrode 11.

[0066] According to the phase change memory 100, for example, by placing a probe in contact with the first electrode 11 and the second electrode 12 and applying a predetermined pulse voltage between the first electrode 11 and the second electrode 12, the crystalline body constituting the memory layer 10 can be heated and undergo a phase change. By applying a pulse voltage, it is possible to change the crystalline structure in a short time. The crystalline bodies 1a, 1b, and 1c have significantly different electrical resistivities, and therefore can be used as a phase change memory.

[0067] The crystal according to this embodiment can record information in a multi-value manner (ternary recording) depending on its crystal structure. It is also possible to select two crystal structures from the crystal structures of crystal 1a, crystal 1b, and crystal 1c, and record information in binary between these crystal phases.

[0068] Furthermore, according to this embodiment, a chalcogen-free phase change memory can be realized. Furthermore, the memory layer of the phase change memory 100 is made of oxide, and has high chemical stability in the atmosphere.

[0069] The present invention is not limited to the above configuration, and various modifications can be made within the scope of the gist described in the claims. [Example]

[0070] Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0071] [Example 1] First, strontium (II) oxide, bismuth (III) oxide, and nickel (II) oxide were prepared as strontium, bismuth, and nickel sources in a mass ratio of 2.5:0.25:1, and mixed in a glove box using a pestle and mortar to prepare a mixed powder. 2.5 Bi 0.5 NiO 4.25 The ratio of the amounts of strontium, bismuth, and nickel is the same as the ratio of the amounts of strontium, bismuth, and nickel in the above composition formula.

[0072] Next, a pressure of 20 MPa was applied to about 0.1 g of the mixed powder using a hydraulic press (manufactured by Riken Seiki, model number: CDM-5PA) to mold it into a pellet with a diameter of about 5 mm.

[0073] The pellets were then heated and cooled in the air using an electric furnace (Yamada Electric Co., Ltd., model number: MSTF-1520) to produce a crystal precursor. The heating conditions were as follows: increase in temperature to 750°C at a rate of 100°C / h and hold for 5 hours, increase in temperature to 900°C at a rate of 100°C / h and hold for 15 hours, increase in temperature to 1100°C and hold for 48 hours, and then cool to room temperature at a rate of 100°C / h.

[0074] Here, the crystal precursor was subjected to XRD measurement using an X-ray diffractometer (manufactured by Bruker, model number: D8 DISCOVER) to identify the composition of the crystal precursor. The measurement conditions were as follows: (ray source: CuK α The tube voltage was 40 kV, the tube current was 40 mA, and the incidence angle was 10° to 100°.

[0075] In addition, the occupancy rate of each element site was determined by Rietveld analysis of the diffraction pattern obtained by X-ray diffraction measurement, and the composition of the crystal was determined by adding up these data. XRD measurement revealed that the crystal precursor consisted of a first layer consisting of a first composition with a perovskite structure represented by the general formula SrNiO3, as shown in Figure 1, and a second layer consisting of a first composition with the general formula (Sr 1.5 Bi 0.5 )O2, and the second layer made of the second composition has an alternating laminate structure in which the second layer made of the second composition has a rock salt structure represented by the general formula Sr 2.5 Bi 0.5 NiO 5-x It was confirmed that the crystal was represented by the formula:

[0076] [Example 1-1] The crystal precursor prepared in Example 1 was placed in the electric furnace used in Example 1, and the temperature was increased to 400°C at a rate of 200°C / h. The crystal precursor was heated at 400°C for 24 hours in an air atmosphere and then cooled to room temperature.

[0077] [Examples 1-2 to 1-6] In Examples 1-2 to 1-6, the same treatment as in Example 1-1 was carried out except that the heating temperature was changed. The heating temperatures in Examples 1-2 to 1-6 are summarized in Table 1.

[0078] [Reference example 1-1~Reference example 1-5] For Reference Examples 1-1 to 1-5, the same treatment as in Example 1-1 was carried out except that the heating temperature was changed. The heating temperatures for Reference Examples 1-1 to 1-5 are summarized in Table 1.

[0079] XRD measurements were performed on Examples 1-1 to 1-6 and Reference Examples 1-1 to 1-5 in the same manner as in Example 1, and the crystal structures and compositions were identified by Rietveld analysis. Figure 8 shows the X-ray diffraction patterns of the crystals of Examples 1-1 to 1-6 and Reference Examples 1-1 to 1-5. The results of the crystalline phase at room temperature for each Example and Reference Example are summarized in Table 1. Table 2 also shows the results of composition analysis of the crystals of Examples 1-1 to 1-6 and Reference Examples 1-1 to 1-5 at room temperature.

[0080] In Examples 1-4 to 1-6, heating yielded three separate structures: two crystalline phase materials with different crystal compositions and an amorphous material. Figure 8 and Table 1 show the measurement results for one of the three separate structures obtained in Examples 1-4 to 1-6. From the XRD diffraction patterns, the composition of one of the crystalline structures in Examples 1-4 to 1-6 is shown in Table 2 as above, and the composition of the other crystalline structure is shown in Table 2 as the general formula Sr6Ni5O 15 It was confirmed that the crystals were those shown in Table 1. In Table 1, the "Crystalline" column indicates the crystalline phase of the observed crystals. Crystals shown as disordered phase exhibit a crystalline structure similar to Crystalline Form 1b. Similarly, crystals shown as double perovskite phase exhibit a crystalline structure similar to Crystalline Form 1c, and crystals shown as ordered phase exhibit a crystalline structure similar to Crystalline Form 1a.

[0081] [Table 1]

[0082] [Table 2]

[0083] In the crystals of Examples 1-4 to 1-6, the composition ratio of the O element was 2.1 to 2.3 times the composition ratio of the Sr element.

[0084] From the results of Examples 1-1 to 1-6 and Reference Examples 1-1 to 1-5, it can be seen that the crystal precursor of Example 1, in which the crystal phase of the second layer has an ordered phase in which the arrangement of Sr and Bi elements is ordered, undergoes a phase change to a disordered phase in which the arrangement of Sr and Bi elements in the second layer is disordered when heated at 400 to 600°C, and when heated at 700 to 900°C, the composition of the crystal changes to the general formula SrBi 0.5 Ni 0.5 O 3-x It was confirmed that the phase change occurred to a crystalline phase with a double perovskite crystal structure shown in (1), and that the phase change did not occur when heated below 300°C or above 1000°C.

[0085] [Example 2] First, a crystal precursor (crystal body) of a disordered phase in which the arrangement of Sr elements and Bi elements is disordered was produced in the second layer by the same method as in Example 1-2.

[0086] [Example 2-1] The crystal precursor prepared in Example 2 was placed in the electric furnace used in Example 1, and the temperature was increased to 800°C at a rate of 200°C / h. The crystal precursor was heated at 800°C for 24 hours in an air atmosphere and then cooled to room temperature.

[0087] [Example 2-2] The same treatment as in Example 2-1 was carried out, except that the heating temperature was changed to 1000°C.

[0088] [Reference example 2-1] The same treatment as in Example 2-1 was carried out, except that the heating temperature was changed to 200°C.

[0089] [Reference example 2-2] The same treatment as in Example 2-1 was carried out, except that the heating temperature was changed to 800°C.

[0090] XRD measurements were performed on Examples 2-1 and 2-2 and Reference Examples 2-1 and 2-2 in the same manner as in Example 1 to identify the crystal structure and composition. Figure 9 shows the X-ray diffraction patterns of the crystals of Examples 2-1 and 2-2 and Reference Examples 2-1 and 2-2. The results of the crystal phase at room temperature for each Example and Reference Example are summarized in Table 3. In Example 2-1, three separate structures were obtained by heating: two crystalline phase materials with different crystal compositions and an amorphous material. Figure 9 and Table 3 show the measurement results of one structure (crystalline substance) of the three separate structures obtained in Example 2-1.

[0091] [Table 3]

[0092] In Example 2-1, a crystal having an alternate layer structure of an ordered phase and a rock salt structure was heated and cooled, and then a crystal having an alternate layer structure of a disordered phase and a rock salt structure was obtained. By heating and cooling the crystal again at a predetermined temperature, a crystal having a general formula of SrBi 0.5 Ni 0.5 O 3-x (where x is 0.3≦x≦1.3) 0.5 Ni 0.5 O 2.3 The other crystal of Example 2-1 was found to have the general formula Sr6Ni5O from the XRD diffraction pattern. 15 It was confirmed that the crystal of Example 2 was a crystal represented by the formula: In addition, it was confirmed that by heating the crystal of Example 2 at a high temperature, an ordered phase in which the arrangement of Sr and Bi elements in the second layer was orderly could be obtained without changing the composition. It was also confirmed that the crystal structure did not change even when the crystal of Example 2 was heated at a low temperature.

[0093] [Example 3] First, a crystalline precursor (crystalline body) having a double perovskite phase was prepared in the same manner as in Examples 1-5. In Example 3, three separate structures were obtained: two crystalline phase materials with different crystal compositions and an amorphous material. In the following Example 3-1 and Reference Examples 3-1 and 3-2, these structures were collectively referred to as the crystalline precursor.

[0094] [Example 3-1] The crystal precursor prepared in Example 3 was placed in the electric furnace used in Example 1, and the temperature was increased to 1000°C at a rate of 200°C / h. The crystal precursor was heated at 1000°C for 24 hours in an air atmosphere and then cooled to room temperature.

[0095] [Reference example 3-1] The same treatment as in Example 3-1 was carried out, except that the heating temperature was changed to 500°C.

[0096] [Reference example 3-2] The same treatment as in Example 3-1 was carried out, except that the heating temperature was changed to 800°C.

[0097] XRD measurements were performed on Example 3-1 and Reference Examples 3-1 and 3-2 in the same manner as in Example 1 to identify the crystal structure and composition. Figure 10 shows the X-ray diffraction patterns of the crystals of Example 3-1 and Reference Examples 3-1 and 3-2. The results of the crystal phase at room temperature for each Example and Reference Example are summarized in Table 4. In Example 3-1, three separate structures (crystal precursors), consisting of two crystalline phase materials with different crystal compositions and an amorphous material, were converted into a single crystal by heating, whereas in Reference Examples 3-1 and 3-2, the three separate structures remained. Figure 10 and Table 4 show the measurement results for one of the three separate structures obtained in Reference Examples 3-1 and 3-2.

[0098] [Table 4]

[0099] From Example 3-1, it was confirmed that by heating a crystal precursor consisting of three structures including a crystal exhibiting a double perovskite crystal structure at high temperature, a single ordered crystal phase in which the arrangement of Sr and Bi elements is orderly can be obtained in the second layer. It was also confirmed that the crystal structure does not change even when this crystal is heated at low temperature.

[0100] [Reference example 1-6] A crystal precursor (crystal material) of an ordered phase in which the arrangement of Sr and Bi elements is orderly was produced in the second layer in the same manner as in Reference Example 1-4, except that the heating temperature was changed to 1050°C.

[0101] <Electrical resistivity measurement> The electrical resistivity of the crystals (ordered phase) prepared in Reference Examples 1-4 and 1-6 and the crystals (disordered phase) prepared in Examples 1-1 and 1-2 was measured by the four-terminal method using a physical property evaluation device (manufactured by Quantum Design Co., Ltd.) The electrical resistivity of the crystals was measured while changing the temperature of the crystals using a liquid helium cryostat.

[0102] 11 is a graph showing the electrical resistivity of Examples 1-1 and 1-2, and Reference Examples 1-4 and 1-6. The electrical resistivity of the ordered phase crystals of Examples 1-4 and 1-6 was about 10 [mΩ·cm] at room temperature. The electrical resistivity of the disordered phase crystals of Examples 1-1 and 1-2 was about 1.0×10 4 [mΩ·cm]. When the crystal (double perovskite phase) prepared in Example 3 was also measured using the physical property evaluation device, it was at the measurement limit. The measurement limit of the physical property evaluation device was 1.0 × 10 8 [mΩ cm], 1.0×10 8 The electrical resistivity of the crystal of Example 3, which is a double perovskite phase, is 1.0 × 10 8Furthermore, when the electrical resistivity of the crystal of Example 3 was measured by the four-terminal method using a Graphical Source Meter 2461 (manufactured by Keithley Instruments) instead of the physical property evaluation device described above, the electrical resistivity of the crystal of Example 3, which is a double perovskite phase, was confirmed to be approximately 1.0 × 10 at room temperature. 10 It was confirmed that the resistance was [mΩ·cm].

[0103] Therefore, by comparing the electrical resistivities of the crystals of Examples 1-1 and 1-2, the crystals of Reference Examples 1-4 and 1-6, and the crystal of Example 3, it was found that the electrical resistivities of the crystals of the disordered phase and the double perovskite phase were approximately 1.0 × 10 compared to the electrical resistivity of the crystals of the ordered phase. 3 times, approximately 1.0×10 7 The electrical resistivity of the double perovskite phase crystal is approximately 10,000 times that of the disordered phase crystal, and it was confirmed that the electrical resistivity changes significantly depending on the crystalline phase.The above differences in electrical resistivity are sufficient for use as phase-change memory, and it was confirmed that these crystalline phase changes can be utilized for application in phase-change memory. [Explanation of symbols]

[0104] 1a, 1b, 1c: crystal body, 10: memory layer, 11: first electrode, 12: second electrode, 100: phase change memory, sub: substrate

Claims

1. It is composed of Sr element, Bi element, Ni element and O element, an alternating laminate structure in which a first layer made of a first composition and a second layer made of a second composition different from the first composition are alternately laminated, the first composition having a perovskite structure made of Sr, Ni, and O elements, the second composition having a rock salt structure made of Sr, Bi, and O elements, and an ordered phase in which the arrangement of Sr and Bi elements of the second composition is ordered in the second layer; a first layer made of a first composition and a second layer made of a second composition different from the first composition, the first composition having a perovskite structure made of Sr, Ni, and O elements, the second composition having a rock salt structure made of Sr, Bi, and O elements, the second layer having a disordered phase in which the Sr and Bi elements of the second composition are arranged in a disordered manner; General formula SrBi ０．５ Ni ０．５ O ３－ｘ (0.3≦x≦1.3) ... (1) A double perovskite phase having a double perovskite structure, A crystalline substance that undergoes a phase change between at least two of the above phases.

2. In the ordered phase and the disordered phase, The first composition has the general formula SrNiO ３ ...(2) The second composition has the general formula Sr １．５ Bi ０．５ O ２ The crystalline body according to claim 1, represented by the formula (3).

3. The crystalline body according to claim 1 , which undergoes a phase change between the ordered phase and the double perovskite phase.

4. The crystalline body according to claim 1 , which undergoes a phase change between the disordered phase and the double perovskite phase.

5. The crystalline body of claim 1 , which undergoes a phase change between the ordered phase and the disordered phase.

6. The electrical resistivity of the disordered phase is 1.0×10 ３ ~1.0 x 10 ５ The crystal according to claim 1, wherein the viscosity is [mΩ·cm].

7. When the double perovskite phase is formed, the electrical resistivity is 1.0×10 ８ The crystal according to claim 1, wherein the tensile strength is [mΩ·cm] or more.

8. A phase-change memory comprising: a memory layer made of the crystalline body according to any one of claims 1 to 7; and a plurality of electrodes electrically connected to the memory layer.

9. General formula Sr ２．５ Bi ０．５ NiO ５－ｘ a compacting step of compacting a mixed powder of a strontium source material, a bismuth source material, and a nickel source material, which are mixed based on the stoichiometric ratio of (wherein 0<x<2.5) (4), to form a pellet; a first heating step of heating the pellet at 950°C or higher in an air atmosphere to prepare a crystal precursor; a cooling step of cooling the crystal precursor; a second heating step of heating the crystal precursor in an air atmosphere at a temperature of 350°C or higher but lower than 950°C.

10. The method for producing a crystalline body according to claim 9 , wherein the crystalline precursor is heated at a temperature of 350° C. or higher and lower than 650° C. in the second heating step.

11. The method for producing a crystalline body according to claim 9 , wherein the crystalline precursor is heated in the second heating step at a temperature of 650° C. or higher and lower than 950° C. in an air atmosphere.

12. The method further includes, after the second heating step, a second cooling step of cooling the crystal precursor and a third heating step of heating it again, The method for producing a crystalline body according to claim 10 , wherein the crystalline precursor is heated in the air atmosphere at a temperature of 650° C. or higher and lower than 950° C. in the third heating step.

13. A method for manufacturing a phase-change memory, comprising the step of forming a memory layer using the method for manufacturing a crystal according to any one of claims 9 to 12.

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