Phase change material and phase change type memory device

Abstract

This phase change material undergoes a phase change between a first crystal phase and a second crystal phase, which has a crystal structure having a lattice constant different from that of the first crystal phase, in response to heating; and this phase change material is mainly composed of a composition that is expressed by a composition formula SmxTe100-x, wherein x is 45.0 (at%) or more.

Description

Technical Field

[0001] The present invention relates to a phase change material and a phase change memory device.

Background Art

[0002] In recent years, with the rapid market expansion of mobile electronic devices such as mobile phones, the development of next-generation non-volatile memories to replace Flash memories has been actively carried out. As candidates for next-generation non-volatile memories, magnetoresistive random access memories (MRAM), ferroelectric random access memories (FeRAM), phase change random access memories (PCRAM), resistive random access memories (ReRAM), etc. have been actively researched and developed. Among them, PCRAM has a simple memory cell structure, and thus is excellent in terms of manufacturing cost and integration density compared to other memories.

[0003] Generally, a phase change material is used for the information recording layer of PCRAM, and information is recorded by utilizing the change in electrical resistance accompanying the phase change between an amorphous phase with a relatively high electrical resistance of the phase change material and a crystalline phase with a lower electrical resistance than the amorphous phase.

[0004] The phase change between the amorphous phase and the crystalline phase of the phase change material is realized by utilizing the Joule heat generated by applying an electric pulse. For example, the phase change material is Joule heated to a temperature equal to or higher than the melting point Tm and changes to an amorphous phase with a high electrical resistance, thereby recording information indicating a reset state [0]. Also, for example, the phase change material is Joule heated to a temperature equal to or higher than the crystallization temperature Tc and lower than the melting point Tm and changes to a crystalline phase with a low electrical resistance, thereby recording information indicating a set state [1].

[0005] Currently, as the phase change material used in PCRAM, Ge-Sb-Te-based compounds (GST) used in DVD-RAM etc. have been widely studied (see, for example, Non-Patent Documents 1, 2, and 3).

[0006] In addition to this, various phase change materials that can be used in PCRAM are being studied. For example, Patent Document 1 and Non-Patent Document 4 disclose a superlattice-type phase change material in which Ge-Te compounds and Sb-Te compounds are laminated. Patent Document 2 discloses a phase change material containing SnTe and Sb 2 Te 3 In Patent Document 2, a phase change material containing Te is disclosed. Non-Patent Document 5 discloses a phase change material composed of MnTe. Non-Patent Document 6 discloses a phase change material containing In 2 Se 3 In Non-Patent Document 6, a phase change material containing Se is disclosed.

[0007] Among these, the Ge-Te compound disclosed in Patent Document 1 and Non-Patent Document 4, when used as PCRAM, the position of Ge (germanium) atoms changes slightly, and a phase change occurs between crystal states (transition operation). The Sb-Te compound assists the transition operation in the Ge-Te compound. In this phase change, it is not necessary to change to the amorphous phase, and it is disclosed in Patent Document 1 and Non-Patent Document 4 that the electrical resistance changes greatly and the power required at that time is small.

[0008] Non-Patent Document 5 discloses that a phase change material composed of MnTe undergoes a crystal / crystal phase change between the α phase and the β' phase. That is, the phase change material composed of MnTe, like the phase change materials disclosed in Patent Document 1 and Non-Patent Document 4, undergoes a phase change between crystal states, and the electrical resistance changes greatly.

[0009] The phase change material composed of In 2 Se 3 disclosed in Non-Patent Document 6 is a two-dimensional layer material. In 2 Se 3It is disclosed that a crystal / crystal phase change occurs due to Joule heating generated by applying an electric pulse. In 2 Se 3 it is disclosed that due to Joule heating, the van der Waals gap between two-dimensional layers changes, and with the phase change, the electrical resistance changes significantly.

[0010] Among these, the phase change materials disclosed in Patent Document 1, Non-Patent Documents 4, 5, and 6 exhibit a phase change between crystal phases upon heating. In the present embodiment, this phase change is referred to as a crystal / crystal phase change.

Prior Art Documents

Patent Documents

[0011]

Patent Document 1

Patent Document 2

Non-Patent Documents

[0012]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Summary of the Invention

Problems to be Solved by the Invention

[0013] With the demand for further high performance of memory devices, problems of existing phase change materials such as GST as disclosed in Non-Patent Document 1 and Non-Patent Document 2 have begun to be pointed out. As the first problem, the Joule energy applied to change the phase change material from the low-resistance crystalline phase to the high-resistance amorphous phase is high, and the data rewrite power consumed when rewriting data is large. When changing the phase change material from the crystalline phase to the amorphous phase, since the phase change material is Joule-heated to a temperature above the melting point of the phase change material, the data rewrite power becomes large. As the second problem, the amorphous phase of the phase change material is thermally unstable and inferior in heat resistance. PCRAM is expected to be applied in fields where high temperatures are used such as the automotive field in the future, and it is expected to be used at 150°C for 10 years (see Non-Patent Document 3). The phase change material may change to the crystalline phase when left at a high temperature in the amorphous phase for a long time, and improvement of data retention characteristics is desired. Further, with the further miniaturization of PCRAM memory cells, there is a possibility that the problem of thermal disturbance between adjacent cells in which Joule heat is transmitted from adjacent cells and the data of the PCRAM memory cells is rewritten unintentionally becomes apparent. In order to solve such problems, the phase change material is expected to have further improved thermal stability in both the amorphous phase and the crystalline phase.

[0014] In Patent Document 1 and Non-Patent Document 4, since both the state with high electrical resistance and the state with low electrical resistance are crystal phases, it is not necessary to change the crystal phase to an amorphous phase when rewriting data. Therefore, the power consumption for rewriting data from the low-resistance crystal phase to the high-resistance crystal phase is significantly reduced. However, since the superlattice-type phase change material described in Patent Document 1 and Non-Patent Document 4 has a structure in which different compounds are laminated, a diffusion reaction may occur between different compound layers at high temperatures, leading to deterioration of characteristics, and there are concerns about its use in a high-temperature environment, especially long-term use in a high-temperature environment.

[0015] The superlattice-type phase change material described in Patent Document 2, similar to the superlattice-type phase change material described in Patent Document 1 etc., may cause a diffusion reaction between different compound layers at high temperatures, leading to deterioration of characteristics. That is, there are concerns about its use in a high-temperature environment, especially long-term use in a high-temperature environment.

[0016] In Non-Patent Document 5, in the phase change material composed of MnTe, since a rapid resistance change occurs in the β' phase of MnTe at about 220 °C, there are concerns about its use in a high-temperature environment, especially long-term use in a high-temperature environment.

[0017] In Non-Patent Document 6, 2 Se 3 a phase change material formed by 2 Se 3 is disclosed. 2 Se 3 is a single material, and since the electrical resistance changes greatly due to the change of the van der Waals gap existing in the crystal, similar to the MnTe phase change material described in Non-Patent Document 5, there is no possibility of a diffusion reaction between compound layers, which is a concern in superlattice-type phase change materials. However, 2 Se 3 the temperature at which the electrical resistance changes rapidly is about 200 °C, and it is not easy to apply it in fields where high temperatures are used, such as the automotive field. Also,

[0018] As described above, a low-power PCRAM using a phase change material that greatly changes its electrical resistance due to a crystal / crystalline phase change has been proposed. However, an invention of a new phase change material with excellent practicality that has data retention ability in a high-temperature environment and is resistant to thermal disturbance between adjacent cells is expected.

[0019] The present invention has been made in view of the problems of the conventional phase change materials described above, and aims to provide a phase change material having a novel composition suitable for obtaining a phase change memory element with excellent practicality, and a phase change memory element applying the phase change material.

Means for Solving the Problems

[0020] The phase change material according to one aspect of the present invention is a phase change material that undergoes a phase change between a first crystal phase and a second crystal phase having a crystal structure different from that of the first crystal phase in response to heating, comprising a composition represented by the compositional formula Sm x Te 100-x as a main component, in the compositional formula, x is 45.0 (at.%) or more.

[0021] In the phase change material according to the above aspect, in the compositional formula, x may be 45.0 (at.%) or more and 60.0 (at.%).

[0022] In the phase change material according to the above aspect, in the compositional formula, x may be 48.0 (at.%) or more and 53.0 (at.%).

[0023] In the phase change material according to the above aspect, the first crystal phase and the second crystal phase may have an NaCl-type crystal structure.

[0024] In the phase change material according to the above aspect, the phase change temperature from the first crystal phase to the second crystal phase may be 350°C or higher.

[0025] In the phase change material according to the above aspect, the electrical resistance of the second crystal phase may be at least 50 times or more the electrical resistance of the first crystal phase.

[0026] The phase change memory element according to the second aspect of the present invention is a first electrode, a memory layer electrically connected to the first electrode, and a second electrode electrically connected to the memory layer. The memory layer is composed of a phase change material according to the first aspect.

Advantages of the Invention

[0027] The phase change material of the present invention has a novel composition and can provide a phase change memory element with excellent practicality.

Brief Description of the Drawings

[0028]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Embodiments for Carrying Out the Invention

[0029] Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. Note that the drawings used in the following description may show the characteristic parts enlarged for the sake of clarity of the features of the present invention. For this reason, the dimensional ratios of the respective components may be different from the actual ones.

[0030] The inventors of the present invention conducted various experiments to search for a novel phase change material in which the electrical resistance changes due to a crystal / crystalline phase change without being amorphized. The novel phase change material to be searched preferably has a large change in electrical resistance due to a crystal / crystalline phase change, and a high phase change temperature when changing from a crystal phase with a low electrical resistance to a crystal phase with a high electrical resistance. As a result of various experiments, the inventors of the present invention found that the object of the present invention can be achieved in a material having the following characteristics. Some of the experimental results obtained by the inventors of the present invention will be described later with reference to FIGS. 2 to 6. In the following embodiments, a material that undergoes a phase change between a first crystal phase and a second crystal phase having different electrical resistances is defined as the phase change material according to the present embodiment.

[0031] [Phase change material] The phase change material according to the present embodiment is a material that undergoes a phase change between a first crystal phase and a second crystal phase having a lattice constant different from that of the first crystal phase in response to heating, contains Sm and Te, and contains a composition represented by the following composition formula (1) as a main component. Sm x Te 100-x ···(1)

[0032] In the formula, x is the atomic concentration of Sm (samarium) atoms (hereinafter, also referred to as at.%). The Sm atomic concentration x is 45.0 or more.

[0033] In particular, the phase change material represented by the composition formula (1) exhibits a crystal / crystalline phase change between the first crystal and a second crystal phase having a different electrical resistance from the first crystal, and has a high phase change temperature when changing from the first crystal phase to the second crystal phase. Therefore, the phase change material according to the present embodiment has high thermal stability. The first crystal phase exhibits a crystal structure having a larger lattice constant than the second crystal phase, for example.

[0034] The reason for setting the atomic concentration of Sm atoms to 45.0 (at.%) or more is that at this atomic concentration, the phase change material represented by the composition formula (1) undergoes a phase change from the first crystal phase to the second crystal phase, and the electrical resistance differs between the first crystal phase and the second crystal phase. In the present embodiment, a novel phase change material that undergoes a phase change from the first crystal phase to the second crystal phase can be provided. Also, when the atomic concentration of Sm atoms is 45.0 (at.%) or more, the phase change temperature T t from the first crystal phase to the second crystal phase is, for example, 350°C or more and 520°C or less. Therefore, the phase change material according to the present embodiment has a higher phase change temperature and higher data retention ability in a high-temperature environment compared to phase change materials that undergo a phase change between crystal phases such as MnTe and In 2 Se 3 etc. Also, the phase change temperature is lower than the phase change temperature of phase change materials that undergo a phase change from a crystal phase such as GST to an amorphous phase, and the data writing power can be reduced. This is because when Sm is 45.0 (at.%) or more, the as-deposited state in which the phase change material is in a deposited state does not become an amorphous phase.

[0035] The atomic concentration of Sm atoms is preferably 45.0 (at.%) or more and 60.0 (at.%) or less, more preferably 48.0 (at.%) or more and 53.0 (at.%) or less, and even more preferably 48.0 (at.%) or more and 52.0 (at.%) or less. When the atomic concentration of Sm atoms is 45.0 (at.%) or more and 60.0 (at.%) or less, the difference in electrical resistance between the first crystal phase and the second crystal phase of the phase change material becomes clearer.

[0036] The reason why it is preferable that the atomic concentration of Sm atoms is 48.0 (at.%) or more and 53.0 (at.%) or less is as follows. The first reason is that when the Sm content is 48.0 (at.%) or more, the change in electrical resistance between the first crystal phase and the second crystal phase is particularly large. The electrical resistance of the second crystal phase relative to that of the first crystal phase is preferably 50 times or more, more preferably 100 times or more. The second reason is that when the Sm content is 48.0 (at.%) or more, the resistance to phase separation can be enhanced. That is, even when the phase change repeatedly occurs from the first crystal phase to the second crystal phase and from the second crystal phase to the first crystal phase, phase separation of the phase change material into two phases can be sufficiently suppressed. Also, it has been confirmed in the examples described later that by setting the atomic concentration of Sm atoms to 53.0 or less, a clear change in resistance value is shown between the first crystal phase and the second crystal phase.

[0037] Incidentally, the composition of the composition represented by the composition formula (1) of the phase change material is measured by scanning electron microscopy - energy dispersive X-ray spectroscopy (SEM-EDS).

[0038] In the present embodiment, the main component indicates that it is 50 mass% or more in the composition. The composition change material preferably consists essentially of the composition represented by the composition formula (1), more preferably consists of the composition represented by the composition formula (1). When the phase change material in the present embodiment consists essentially of the composition represented by the composition formula (1), it means that the composition represented by the composition formula (1) is 99.5 mass% or more in the phase change material. The phase change material may contain a third metal element, a fourth metal element, or inevitable impurities different from the Sm element and the Te element constituting the composition represented by the composition formula (1). Thus, even if the phase change material according to the present embodiment contains elements other than the Sm element and the Te element within the above range, the effects of the present embodiment can be obtained.

[0039] The phase change materials in the first crystal phase and the second crystal phase have, for example, an NaCl-type crystal structure. The phase change from the first crystal phase to the second crystal phase of the phase change material according to this embodiment and the fact that the first crystal phase and the second crystal phase have a crystal structure can be confirmed by X-ray diffraction or an electron microscope image (FFT image).

[0040] The phase change material according to this embodiment is manufactured by forming a film on various substrates by a physical vapor deposition method such as sputtering using various targets containing Sm and Te within a predetermined composition range. For the target used in the manufacture of the phase change material according to this embodiment, the film formation output may be changed by adjusting the concentration by multi-component sputtering using pure Sm and pure Te, and then forming a film, or a film may be formed using a Sm-Te binary alloy target with the components adjusted in advance. Here, the temperature of the substrate on which the phase change material according to the embodiment is formed is appropriately selected within the temperature range from room temperature to 800°C as required.

[0041] It has been confirmed in the examples described later that the phase change material according to this embodiment has a high phase change temperature of 350°C or higher at which it changes phase between the first crystal phase and the second crystal phase. Since the phase change material according to the present invention changes phase between crystal phases, it is not necessary to melt the phase change material with Joule heat to amorphize it, and regarding the phase change from a low-resistance crystal phase to a high-resistance crystal phase, the data rewrite power can be reduced. Further, in the phase change material according to the present invention, the phase change temperature when changing phase from the first crystal phase with low electrical resistance to the second crystal phase with high electrical resistance is, for example, as high as 350°C or higher, and the crystal phase with low electrical resistance is thermally stable even in a high-temperature environment. Since the phase change material according to the present invention has low data rewrite power for the phase change from a low-resistance crystal phase to a high-resistance crystal phase and the crystal phase with low electrical resistance is thermally stable even in a high-temperature environment, it is possible to configure a more practical phase change type memory element using the phase change material according to the present invention.

[0042] [Phase Change Type Memory Element] The phase change type memory element according to this embodiment includes a memory layer composed of the phase change material according to the above embodiment. FIG. 1(a) is a schematic cross-sectional view of a phase change type memory element 10 according to an embodiment of the present invention, and FIG. 1(b) is a plan view of the phase change type memory element 10 shown in FIG. 1(a).

[0043] The phase change type memory element 10 has, for example, a substrate 1, a first electrode 2, an insulating layer 3, a memory layer 4, and a second electrode 5. The substrate 1 is, for example, a SiO 2 / Si substrate that spreads in the in-plane direction. The first electrode 2 is disposed, for example, on the surface of the substrate 1. The first electrode 2 is, for example, a metal film having a rectangular planar shape and having a through hole H formed therein. The hole provided in the first electrode 2 is, for example, a circular hole provided in the central portion of the substrate. The first electrode 2 is formed of a metal such as W, TiN, TiW, Al, or Cu. Further, the first electrode 2 may be formed of a lower electrode formed of a metal electrode such as W and TiW and a nitride electrode such as TiN, and a heating electrode formed of a nitride electrode such as TiN and laminated on the surface of the lower electrode opposite to the substrate 1.

[0044] The insulating layer 3 is, for example, an insulating film disposed on the surface of the first electrode 2 and having the same planar shape as the first electrode 2. The insulating layer 3 is formed of an insulator such as SiN.

[0045] The memory layer 4 is electrically connected to the first electrode 2. The memory layer 4 has, for example, a columnar portion 4A that penetrates the central portions of the first electrode 2 and the insulating layer 3 and contacts the substrate 1, and a flat plate portion 4B that is disposed on the surfaces of the insulating layer 3 and the columnar portion 4A and has the same planar shape as the first electrode 2 and the insulating layer 3 in plan view. The columnar portion 4A is, for example, a cylindrical member. The memory layer 4 is electrically connected to the first electrode 2 via the outer wall of the columnar portion 4A.

[0046] The second electrode 5 is electrically connected to the memory layer 4 and is insulated from the first electrode 2, for example. The second electrode 5 is, for example, a metal film disposed on the surface of the flat portion of the memory layer 4 and having the same planar shape as the first electrode 2 and the insulating layer 3. The second electrode 5 is formed of a metal such as W, TiN, TiW, Al, Cu, etc., similar to the first electrode 2. The second electrode 5 is electrically connected to the memory layer 4 by the back surface thereof coming into contact with the surface of the memory layer 4. Further, since the insulating layer 3 and the memory layer 4 are disposed between the second electrode 5 and the first electrode 2 and the second electrode 5 does not have a portion in contact with the first electrode 2, the second electrode 5 is insulated from the first electrode 2.

[0047] Note that the phase change type memory element according to this embodiment is not limited to the structure shown in the above example. For example, in the above example, a through hole H is provided in the central portion in the in-plane direction of the first electrode 2 and the insulating layer 3, and a pillar portion 4A is provided penetrating the through hole H. However, the positions of the through hole H and the pillar portion 4A penetrating the through hole H may be positions other than the central portions of the first electrode 2 and the insulating layer 3. Further, the phase change type memory element according to this embodiment is not limited to the structure having only one through hole H in the first electrode 2 and the insulating layer 3 as shown in FIG. 1, and may be a configuration in which no through hole H is provided in the first electrode 2 and the insulating layer 3, or a configuration in which two or more through holes H and pillar portions 4A are provided in the first electrode 2 and the insulating layer 3.

[0048] The phase change type memory element 10 is manufactured by sequentially laminating the first electrode 2, the insulating layer 3, the memory layer 4, and the second electrode 5 on the surface of the substrate 1 by utilizing techniques well known in semiconductor manufacturing processes such as photolithography and sputtering. In order to form the pillar portion 4A of the memory layer 4, first, as a preparation step, a through hole is formed by irradiating the central portions of the first electrode 2 and the insulating layer 3 with a focused ion beam in advance. Next, it is formed by embedding the phase change material according to the above embodiment in the through hole.

Example

[0049] Hereinafter, examples of the present invention will be described. The present invention is not limited to only the following examples.

[0050] [Table 1] shows the measured values of the compositions and physical properties of the phase change materials according to the examples described below and the phase change materials according to the comparative examples.

[0051]

Table 1

[0052] Hereinafter, with reference to FIG. 1, the phase change material according to the embodiment and the phase change memory element using the phase change material will be described in more detail. Each of the phase change materials according to Examples 1 to 4 and the phase change material according to Comparative Example 1 has, as a main component, a composition represented by the composition formula Sm x Te 100-x shown.

[0053] [Example 1] As Example 1, by the method shown below, a phase change material composed essentially of the composition represented by the composition formula Sm 2 / Si substrate, a pure Sm target, and a pure Te target were installed in an RF sputtering apparatus (manufactured by ULVAC, model number: MUE-201C-HC1). Next, using the RF sputtering apparatus, the input power of Sm was set to 23 W and the input power of Te was set to 10 W in an Ar atmosphere, and sputtering was performed under the conditions of a gas pressure of 3.8×10 49.2 Te 50.8 to form a phase change material thin film with a film thickness of 100 nm on the SiO

[0054] / Si substrate. The input power of the Te target was fixed at 10 W, and the composition was controlled by changing the input power of the Sm target between 15 and 28 W. 2 / Si substrate, a pure Sm target, and a pure Te target were installed in an RF sputtering apparatus (manufactured by ULVAC, model number: MUE-201C-HC1). Next, using the RF sputtering apparatus, the input power of Sm was set to 23 W and the input power of Te was set to 10 W in an Ar atmosphere, and sputtering was performed under the conditions of a gas pressure of 3.8×10 -7 Torr and room temperature to form a phase change material thin film with a film thickness of 100 nm on the SiO 2 / Si substrate. The input power of the Te target was fixed at 10 W, and the composition was controlled by changing the input power of the Sm target between 15 and 28 W.

[0055] [Example 2] The sputtering conditions were changed such that the input power of Sm was 25 W and the input power of Te was 10 W, and essentially the composition formula Sm 50.2 Te 49.8A sample was prepared in the same manner as in Example 1, except that a thin film of the phase change material composed of the composition shown by

[0056] [Example 3] The sputtering conditions were changed such that the input power of Sm was 28 W and the input power of Te was 10 W, and a thin film of the phase change material composed of the composition essentially represented by the formula Sm 52.8 Te 47.2 was formed. A sample was prepared in the same manner as in Example 1, except for this point.

[0057] [Example 4] The sputtering conditions were changed such that the input power of Sm was 20 W and the input power of Te was 10 W, and a thin film of the phase change material composed of the composition essentially represented by the formula Sm 46.5 Te 53.5 was formed. A sample was prepared in the same manner as in Example 1, except for this point.

[0058] [Comparative Example 1] The sputtering conditions were changed such that the input power of Sm was 15 W and the input power of Te was 10 W, and a thin film of the phase change material composed of the composition essentially represented by the formula Sm 42.6 Te 57.4 was formed. A sample was prepared in the same manner as in Example 1, except for this point.

[0059] For the samples of Examples 1 to 4 and Comparative Example 1, the composition of the phase change material was measured by SEM-EDS using a scanning electron microscope equipped with EDS (manufactured by JEOL Ltd., model number: JSM-7100F). As a result, the phase change materials of Example 1, Example 2, Example 3, Example 4, and Comparative Example 1 were essentially Sm 49.2 Te 50.8 , Sm 50.2 Te 49.8 , Sm 52.8 Te 47.2 , Sm 46.5 Te 53.5 and Sm 42.6 Te 57.4It was confirmed that it is a composition consisting of. That is, in the phase change materials according to Examples 1 to 4 and Comparative Example 1, the Sm atom concentration x in the composition essentially constituting the phase change material is a numerical value within the range of 45.0 (at.%) ≤ x. On the other hand, in the phase change material according to Comparative Example 1, the Sm atom concentration x in the composition is 42.6 (at.%), which is a numerical value outside the range of 45.0 (at.%) ≤ x.

[0060] [Table 1] shows the atomic concentrations of Sm and Te, which include impurities inevitably contained in the film-forming raw materials. Usually, such inevitable impurities are on the order of several ppm to several tens of ppm, and thus do not have a great influence on the physical properties of the phase change material after film formation.

[0061] Next, the electrical resistance and phase change temperature were determined for the samples of Examples 1 to 4 and Comparative Example 1. The physical properties shown in [Table 1] are the phase change temperature T from the first crystal phase with low electrical resistance to the second crystal phase with high electrical resistance t (°C), the electrical resistance R of the first crystal phase 1 (Ω), the electrical resistance R of the second crystal phase 2 (Ω), and the electrical resistance ratio ΔR (= R 2 / R 1 ). Here, the first crystal phase and the second crystal phase are defined by the magnitude of the lattice constant determined by electron beam diffraction described later, and the lattice constant of the first crystal phase is larger than that of the second crystal phase. The phase change temperature T t and the electrical resistance R 2 and R 1 were measured by measuring the electrical resistance during the heating process using the two-terminal method. Each of the phase change materials according to Examples 1 to 4 and the phase change material according to Comparative Example 1 was heated by a "Heat Treatment and Analysis Apparatus UHV-P4 type" manufactured by Universal Systems Co., Ltd. The heating rate during the measurement of the phase change temperature T t and the electrical resistance R 2 and R 1 was 9.2 °C / min, and the heating was stopped when reaching one of the temperatures of 440 °C, 490 °C, 500 °C, and 580 °C.

[0062] Figure 2 shows the relationship between the temperature and the electrical resistance during the measurement of the phase change materials according to Examples 1, 2, 3, and 4 and the phase change material according to Comparative Example 1. In Figure 2, the horizontal axis represents the temperature (°C), and the vertical axis represents the electrical resistance (Ω). In Figure 2, each curve represents the trajectory of the phase change materials according to Example 1, Example 2, Example 3, and Example 4. t and the electrical resistance R 2 and R 1 The phase change temperature T

[0063] is defined as the temperature at which the differential value of the curve is maximum. t is defined as the temperature at which the differential value of the curve is maximum.

[0064] The electrical resistance R 1 of the first crystal phase is defined as the electrical resistance when each of the phase change materials according to Examples 1 to 4 reaches 50°C in the temperature rising process after starting the temperature rising. Also, the electrical resistance R 2 of the second crystal phase is defined as the electrical resistance when each of the phase change materials according to Examples 1 to 4 drops to 50°C in the temperature dropping process after stopping the temperature rising after the phase change. The electrical resistance ratio ΔR, that is, the magnification of the electrical resistance of the second crystal phase with respect to the electrical resistance of the first crystal phase, is the electrical resistance R 2 of the second crystal phase divided by the electrical resistance R 1 of the first crystal phase, and is defined as the value obtained by rounding the quotient to the second decimal place. Note that the upper limit of the resistance measuring instrument (Agilent's "Data Acquisition / Switch Unit 34970A (Plug-in Module: 34901A)") used in this example was 1.2×10 8 (Ω). Therefore, when the electrical resistance R 2 has an electrical resistance of 1.2×10 8 (Ω) or more, the electrical resistance R 2 is described as ">1.2×10 8 (Ω)".

[0065] As shown in Figure 2, the phase change materials of Examples 1 to 3 all have a phase change temperature T tIt has, and in Examples 1 to 3, all of them undergo a phase change from the first crystal phase to the second crystal phase, and the electrical resistance increases. In Examples 1 to 3, the electrical resistance ratio ΔR between the first crystal phase and the second crystal phase is as large as 50 or more. That is, in Examples 1 to 3, the electrical resistance of the second crystal phase is 50 times or more that of the first crystal phase.

[0066] On the other hand, Example 4 shows a phase change from the second crystal phase having a high electrical resistance to the first crystal phase having a low electrical resistance, and the electrical resistance ratio ΔR between the first crystal phase and the second crystal phase was 5.6 times or more. Also, in Comparative Example 1, no change in electrical resistance due to the phase change was observed.

[0067] More specifically, as shown in FIG. 2, Examples 1, 2, and 3 are in a state where a phase change material is formed on a substrate. In the as-deposited state, it shows a low electrical resistance of 2.2×10 4 (Ω), 3.2×10 4 (Ω), and 2.1×10 3 (Ω). The electrical resistance during the cooling process of Examples 1, 2, and 3 shows a change of only a gentle decrease. On the other hand, the electrical resistance in the as-deposited state of Example 4 was 1.2×10 8 (Ω) or more.

[0068] Also, the respective phase change temperatures T t of Examples 1, 2, 3, and 4 are all 350 (°C) or more, namely 352 (°C), 420 (°C), 520 (°C), and 489 °C. Also, the electrical resistance during the cooling process of Examples 1, 2, and 3 shows a change of only a gentle increase.

[0069] FIG. 3 is an X-ray diffraction pattern corresponding to Example 1. It was confirmed that the first crystal phase shows a NaCl-type crystal structure, and the second crystal phase shows a NaCl-type crystal structure with a smaller lattice constant than the first crystal phase.

[0070] FIG. 4 is a diagram showing the switching characteristics of a phase change memory element using the phase change material corresponding to Example 1 as a memory layer.

[0071] The measurement of the switching characteristics of the phase change memory element was performed using a phase change memory element having a configuration as shown in FIG. 1. The substrate 1 employed a SiO 2 / Si substrate. The materials of the first electrode 2 and the second electrode 5 were TiN (titanium nitride). The insulating layer 3 was a SiN insulating layer, and the film thickness of the insulating layer 3 was 100 nm. After forming a φ500 nm hole in the first electrode 2 and the insulating layer 3 using a focused ion beam, a phase change material was deposited on the surface of the SiN insulating layer by 200 nm by photolithography and sputtering to form the memory layer 4. The second electrode 5 was laminated on the memory layer 4.

[0072] In the initial state, the phase change memory element was in a state where the phase change material was deposited on the substrate, and it was the first crystal phase with low electrical resistance. The electrical resistance value of the first crystal phase was 5.91×10 2 Ω. The phase change memory element was applied with a pulse current having a pulse width of 30 ns while gradually increasing the height from 8.7×10 -6 (A). When the height of the pulse was about 2.9×10 -5 (A), the phase change memory element changed from the first crystal phase to the second crystal phase with high electrical resistance. That is, the phase change memory element changed from the set state to the reset state. The electrical resistance of the second crystal phase was 9.15×10 5 (Ω). Further, when the height of the pulse was 2.3×10 -4 (A), the phase change memory element changed from the second crystal phase to the first crystal phase. That is, the phase change memory element changed from the reset state to the set state. The electrical resistance of the first crystal phase after the phase change was 1.42×10 2 (Ω), which was about the same value as the electrical resistance in the initial state. From this, it was confirmed that the phase change material according to the embodiment including Example 1 can write and erase information by Joule heating using an electric pulse.

[0073] Next, the phase change material of Example 1 was observed using a high-resolution transmission electron microscope, and a fast Fourier transform (FFT) image was obtained from the bright-field image. FIG. 5 is an FFT image of the memory layer when the phase change memory device using the phase change material of Example 1 as the memory layer performed a memory switching operation. FIG. 5(a) is an FFT image of the first crystal phase as-deposited, and FIG. 5(b) is an FFT image of the second crystal phase obtained by heating to 440°C. As a result of these analyses, it was confirmed that both the first crystal phase and the second crystal phase have an NaCl-type crystal structure, which is consistent with the X-ray diffraction results.

[0074] FIG. 6 is a graph showing the interatomic distances estimated from the inverse Fourier transform (IFFT) image of the FFT image of FIG. 5. In FIG. 6, the vertical axis represents the IFFT image contrast intensity, and the horizontal axis represents the interatomic distance. From FIG. 6, the lattice constant of the first crystal phase was 6.44 Å, and the lattice constant of the second crystal phase was 6.18 Å. From this result, it was confirmed that the lattice constant of the first crystal phase is larger than that of the second crystal phase.

Industrial Applicability

[0075] The phase change material of the present invention exhibits a phase change between the first crystal phase and the second crystal phase, and has a very high phase change temperature. Therefore, it can be used in non-volatile semiconductor memories using the phase change material. In addition, it can be used not only in semiconductor memories but also in optical recording media such as DVD-RAM that utilize the reflectivity of laser light in different crystal phases, similar to GST. The present invention is not limited by the above-described examples. That is, other examples, aspects, etc. within the scope of the technical idea of the present invention are naturally included.

Explanation of Signs

[0076] 1 Substrate 2 First electrode 3 Insulating layer 4 Memory layer 5 Second electrode 10 Phase change memory device

Claims

1. A phase change material that undergoes a phase change between a first crystal phase and a second crystal phase having a crystal structure different from that of the first crystal phase in response to heating, Composition formula Sm ｘ Te １００－ｘ The composition contains as a main component a composition represented by the formula: wherein in the composition formula, x is 46.5 (at.%) or more and 52.8 (at.%) or less, the phase change material.

2. The phase change material according to claim 1, wherein in the composition formula, x is 48.0 (at.%) or more.

3. The phase change material according to claim 1 or 2, wherein the first crystal phase and the second crystal phase have a NaCl-type crystal structure.

4. The phase change material according to any one of claims 1 to 3, wherein the phase change temperature from the first crystal phase to the second crystal phase is 350°C or higher.

5. The phase change material according to any one of claims 1 to 4, wherein the electrical resistance of the second crystal phase is at least 50 times or more the electrical resistance of the first crystal phase.

6. A first electrode, a memory layer electrically connected to the first electrode, and a second electrode electrically connected to the memory layer, and the memory layer is composed of the phase change material according to any one of claims 1 to 5, a phase change type memory element.

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