A non-melting superlattice phase change thin film material

Abstract

The present invention belongs to the field of microelectronic devices and storage technology, and more specifically, relates to a non-melting superlattice phase change film material, which includes a phase change layer and a pinning layer, wherein the phase change layer and the pinning layer are alternately stacked to form a periodic structure, wherein the phase change layer is made of GeTe material, and the pinning layer is made of TiTe2 material; during operation, when the phase change layer begins to undergo a non-melting phase change, the pinning layer will not undergo a phase change, so as to improve the stability of the pinning layer during the phase change process. Compared with the existing (Sb2Te3)(GeTe)2 non-melting phase change material, the non-melting superlattice phase change film material of the present invention significantly improves the stability of the pinning layer of the non-phase change part in the superlattice during the phase change process through the optimization of the pinning layer material and the superlattice structure, so that the non-melting superlattice phase change film material has better stability and a higher number of cycles.

Description

Technical Field

[0001] The present invention belongs to the field of microelectronic devices and storage technology, and more specifically, relates to a non-melting superlattice phase change thin film material. Background Art

[0002] Phase change memory (PCRAM) is considered to be one of the next generation storage technologies with the greatest potential to replace flash memory. In the write operation (RESET) process of traditional phase change memory devices, the phase change material needs to reach the melting temperature to achieve the transition from crystalline to amorphous state, resulting in excessive device power consumption, serious thermal crosstalk problems, and difficulty in improving integration and data storage reliability. In order to further reduce the power consumption of phase change memory devices and improve storage reliability, researchers have proposed the concept of non-melting phase change materials. Compared with traditional melting phase change materials, non-melting phase change materials only need to change between different crystal structures at low temperature (low energy drive) to complete the change of device resistance and store information. The transition process between different crystal structures of non-melting phase change materials is achieved through the interlayer migration of atoms, and there is no need to raise the material temperature above the melting point. The energy required for non-melting phase change only needs to be greater than the energy barrier required to overcome the interlayer migration of atoms. Therefore, the operating power consumption of non-melting phase change materials is much lower than that of traditional melting phase change materials.

[0003] Superlattice (Sb2Te3)(GeTe)2 is the most studied non-melting phase change material. Its phase change process is concentrated in the lattice of (GeTe)2 layer. Based on the different atomic arrangement structures in (GeTe)2 layer, up to three different superlattice structures can be formed. These three structures have adjacent Te-Te atomic layers, which cannot bond with each other and form a large van der Waals gap (about ), which provides a large space for atomic layer migration. At present, atomic migration and crystalline structure transformation in the non-melting phase transition process occur near the van der Waals gap. Ideally, in the process of realizing low-energy driven non-melting phase transition, only the (GeTe)2 layer undergoes non-melting phase transition, while the (Sb2Te3) layer does not undergo phase transition.

[0004] However, the (Sb2Te3) layer in the (Sb2Te3)(GeTe)2 non-melting superlattice phase change material is also a sulfur compound. Simulation calculations have found that the (Sb2Te3) layer can also undergo a structural change from crystalline order to amorphous disorder under certain energy excitation. Therefore, during the non-melting phase change process, the stability of the (Sb2Te3) layer is relatively poor. Summary of the invention

[0005] In view of the defects of the prior art, the purpose of the present invention is to provide a non-melting superlattice phase change thin film material, which aims to solve the problem of poor stability of the existing (Sb2Te3)(GeTe)2 non-melting superlattice phase change material by optimizing the pinning layer material and the superlattice structure.

[0006] To achieve the above-mentioned purpose, the present invention provides a non-melting superlattice phase change thin film material, which includes a phase change layer and a pinning layer; the phase change layer and the pinning layer are alternately stacked to form a periodic structure, the phase change layer is made of GeTe material, and the pinning layer is made of TiTe2 material; during operation, when the phase change layer begins to undergo a non-melting phase change, the pinning layer will not undergo a phase change, so as to improve the stability of the pinning layer during the phase change process.

[0007] Preferably, the crystal structure of the single-layer pinning layer is at least two layers of TiTe2.

[0008] Preferably, the crystal structure of the single-layer phase change layer is a three-layer GeTe lattice.

[0009] Further preferably, the three-layer GeTe lattice has four lattice arrangements from top to bottom or from bottom to top, namely Ge-Te-Ge-Te-Ge-Te, Ge-Te-Ge-Te-Te-Ge, Ge-Te-Te-Ge-Ge-Te and Te-Ge-Ge-Te-Ge-Te.

[0010] Further preferably, during the non-melting phase change process of the phase change layer, the three-layer GeTe lattice transforms between the four lattice arrangements, so that the non-melting superlattice phase change thin film material forms four different crystal structures.

[0011] Further preferably, the crystal constants a and b of the four different crystal structures are respectively Its crystal constant c is or

[0012] Further preferably, during the non-melting phase change process of the phase change layer, the three-layer GeTe lattice changes between two lattice arrangements of Ge-Te-Ge-Te-Ge-Te and Te-Ge-Ge-Te-Ge-Te.

[0013] In general, the above technical solution conceived by the present invention has the following beneficial effects compared with the prior art:

[0014] (1) In the non-melting superlattice phase change thin film material provided by the present invention, the pinning layer is made of TiTe2 material and the phase change layer is made of GeTe material. When the phase change layer begins to undergo a non-melting phase change, the pinning layer will not undergo a phase change. Compared with the traditional (Sb2Te3)(GeTe)2 non-melting superlattice phase change material in which the pinning layer (Sb2Te3) can undergo an amorphous transformation under a certain energy excitation, the pinning layer of the present invention has better stability during the non-melting phase change process, thereby improving the non-melting phase change stability of the entire phase change thin film material.

[0015] (2) When the crystal structure of the single-layer phase change layer of the present invention is a three-layer GeTe lattice, TiTe2 is more stable in the superlattice structure, and no atomic migration occurs during the non-melting phase change process. This not only improves the operating temperature window of the non-melting phase change, but also helps to improve the reliability of the non-melting phase change.

[0016] (3) As the number of TiTe2 layers in the pinned layer increases, the window of the non-melting phase transition operating temperature increases, and the stability of the TiTe2 layer during the non-melting phase transition of the thin film material also increases.

[0017] (4) When the three-layer GeTe lattice in the phase change layer transforms between the two lattice arrangements of Ge-Te-Ge-Te-Ge-Te and Te-Ge-Ge-Te-Ge-Te, the energy difference of the system is smaller, and the two lattice structures are easy to transform into each other; and the migration barrier between the two structures is equivalent to the migration barrier of traditional (Sb2Te3)(GeTe)2 non-melting phase change material, indicating that the superlattice phase change thin film material of the present invention can also complete a non-melting phase change process similar to that of the (Sb2Te3)(GeTe)2 non-melting phase change material. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 Schematic diagram of four superlattice structures A, B, C, and D of the non-melting superlattice phase change thin film material (TiTe2)(GeTe)3 of Example 1 of the present invention;

[0019] Figure 2 1 is an energy barrier diagram for converting structure A into structure D in the non-melting superlattice phase change thin film material (TiTe2)(GeTe)3 of Example 1 of the present invention;

[0020] Figure 3 It is a detection diagram of atomic mean square displacement of the prior art non-melting superlattice phase change thin film material (Sb2Te3)(GeTe)2 at different temperatures;

[0021] Figure 4 This is a detection diagram of atomic mean square displacement of non-melting superlattice phase change thin film material (TiTe2)(GeTe)3 at different temperatures in Example 1 of the present invention;

[0022] Figure 5 This is a detection diagram of atomic mean square displacement of the non-melting superlattice phase change thin film material (TiTe2)2(GeTe)3 at different temperatures in Example 2 of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0024] The present invention provides a non-melting superlattice phase change thin film material, which includes a phase change layer and a pinning layer; the phase change layer and the pinning layer are alternately stacked to form a periodic structure. During operation, when the phase change layer begins to undergo a non-melting phase change, the pinning layer will not undergo a phase change.

[0025] The non-melting superlattice phase change film material does not require a high-energy electric pulse to raise the material temperature to the melting point, so there is no amorphization process and amorphous state. It only needs low-energy drive to achieve a non-melting phase change process by transforming from one lattice structure to another lattice structure, thereby reducing the power consumption of the phase change. The present invention optimizes the superlattice structure of the pinning layer material and the entire phase change material so that the pinning layer in the non-melting superlattice phase change film material will not undergo a phase change during the non-melting phase change process. Compared with the (Sb2Te3) layer in the traditional (Sb2Te3)(GeTe)2 non-melting superlattice phase change film material, the stability is better.

[0026] In some embodiments, the phase change layer is made of GeTe material and the pinning layer is made of TiTe2 material. The non-melting superlattice phase change film material expression can be written as ((TiTe2) m (GeTe) n ) x , where m and n represent the number of TiTe2 layers in the single-layer pinning layer and the number of GeTe layers in the single-layer phase change layer, respectively, and x represents the single-layer pinning layer (TiTe2) m and a single phase change layer (GeTe) n The number of periods of , m, n and x are all positive integers.

[0027] In some embodiments, after multiple simulation verifications, the crystal structure of a single-layer phase change layer is preferably a three-layer GeTe lattice, i.e., n=3. Compared with the traditional two-layer GeTe lattice (n=2), the pinned layer in the superlattice phase change film material has better stability during the non-melting phase change process.

[0028] In some embodiments, VASP is used to treat (TiTe2) mThe (GeTe)3 superlattice was subjected to molecular dynamics calculations. Compared with the difference in the starting temperature of the violent motion of the atomic layers between Sb2Te3 and GeTe in the existing (Sb2Te3)(GeTe)2 non-melting phase change material, (TiTe2) m The difference in the starting temperature of the violent movement of atomic layers between TiTe2 and GeTe in (GeTe)3 (i.e., the operating window) is larger, so the stability is significantly improved. As the number of TiTe2 layers m increases, the operating window increases, and the stability of the TiTe2 layer during the non-melting phase change of the material also increases.

[0029] In some embodiments, due to the superlattice (TiTe2) m There are three layers of GeTe binary structure in (GeTe)3. If one layer of GeTe binary structure has two positive and negative situations, then usually three layers of GeTe should have 2 3 = 8 structures, and because of the symmetry of TiTe2, four different lattice structures can be obtained by repeating two by two. During the non-melting phase transition, the transition between different lattice structures is achieved through the migration of Ge atoms, thereby realizing a low-power non-melting phase transition from one lattice structure to another.

[0030] The above technical solution is described in detail below in conjunction with specific embodiments.

[0031] Example 1

[0032] In this example, materials studio software was used to build four different structures of (TiTe2)(GeTe)3 superlattices, and VASP was used for atomic relaxation to optimize the crystal structure. The specific parameters are as follows:

[0033] The crystal constants a, b, and c of structure A are

[0034] The crystal constants a, b, and c of the B structure are

[0035] The crystal constants a, b, and c of the C structure are

[0036] The crystal constants a, b, and c of the D structure are

[0037] The structures of A, B, C, and D are all hexagonal crystals.

[0038] like Figure 1As shown, the three-layer GeTe lattice arrangement in structure A is Ge-Te-Ge-Te-Ge-Te, the three-layer GeTe lattice arrangement in structure B is Ge-Te-Ge-Te-Te-Ge, the three-layer GeTe lattice arrangement in structure C is Ge-Te-Te-Ge-Ge-Te, and the three-layer GeTe lattice arrangement in structure D is Te-Ge-Ge-Te-Ge-Te.

[0039] In this example, VASP was used to perform energy system calculations and molecular dynamics calculations.

[0040] The general parameters for the (TiTe2)(GeTe)3 superlattice calculation are:

[0041] PAW_PBE Te_GW 22Mar2012, PAW_PBE Ti_sv_GW05Dec2013, and PAW_PBE Ge_d_GW 19Mar2013 pseudopotentials were used for Te, Ti, and Ge, respectively. The convergence accuracy of the ion step was 1.0e-07ev, the cutoff energy was 350ev, and the k-point was 8×8×2 Monkhorst-Pack.

[0042] In this example, VASP was used to calculate the lattice energy of the four structures of the (TiTe2)(GeTe)3 superlattice. The system energies of structure A, structure B, structure C, and structure D were -41.605eV, -41.667eV, -59.042eV, and -41.581eV, respectively. It can be seen that the difference in system energy between the two structures A and D is small, which means that the two lattice structures A and D are easy to transform into each other.

[0043] This example uses the neb algorithm of VASP-VTST to calculate the migration barrier from structure A to structure D, and divides the straight line path from the initial state to the final state into 8 segments for force convergence calculation. Figure 2 As shown, the migration barrier between the A and D structures of the (TiTe2)(GeTe)3 superlattice is 3.42eV. Compared with the migration barrier of the (Sb2Te3)(GeTe)2 non-melting phase change material of 2.56eV, the difference is not much in the same order of magnitude. That is, the (TiTe2)(GeTe)3 superlattice thin film material can also complete the non-melting phase change process similar to the (Sb2Te3)(GeTe)2 non-melting phase change material.

[0044] Example 2

[0045] This embodiment provides a (TiTe2)2(GeTe)3 non-melting superlattice phase change thin film material. Similar to Embodiment 1, this embodiment uses materials studio software to build a (TiTe2)2(GeTe)3 crystal structure model, and uses VASP for atomic relaxation to optimize the crystal structure.

[0046] The present invention uses the molecular dynamics algorithm of VASP to calculate the mean square displacement and standard deviation of each layer of atoms in the c-axis direction of three different superlattice structures, (Sb2Te3)(GeTe)2 superlattice, (TiTe2)(GeTe)3 superlattice and (TiTe2)2(GeTe)3 superlattice, at different temperatures. The specific parameters are: temperature from 800K to 1600K, time for each step is 5fs, and the total number of steps is 8000.

[0047] The calculation results are as follows Figure 3 , Figure 4 and Figure 5 As shown in the figure, the mean square displacement on the vertical axis represents the stability of each layer of atoms of each material at different temperatures. The increase in mean square displacement during the heating process indicates that the lattice stability has deteriorated. We call the difference in the starting temperature of the violent motion of the non-phase change part and the phase change part in the superlattice the operating window. It can be seen that there is almost no difference in the violent motion temperature of the atomic layer between Sb2Te3 and GeTe in the (Sb2Te3)(GeTe)2 superlattice (see Figure 3 ), while the temperature difference between the atomic layers of TiTe2 and GeTe in the (TiTe2)(GeTe)3 superlattice is 24K (see Figure 4 ), the temperature difference between the atomic layers of TiTe2 and GeTe in the (TiTe2)2(GeTe)3 superlattice is 264K (see Figure 5 ). Compared with the existing (Sb2Te3)(GeTe)2 non-melting phase change materials, (TiTe2) m The operating window of (GeTe)3 is significantly increased, so the stability of the TiTe2 layer is significantly improved compared to the Sb2Te3 layer; and with the increase of the number of TiTe2 layers m in the pinned layer, the operating window also increases, indicating that the stability of the TiTe2 layer in the non-melting phase change process of the material is also improved.

[0048] Compared with the traditional non-melting phase change memory material (Sb2Te3)(GeTe)2, the (TiTe2) m During the non-melting phase transition of the (GeTe)3 superlattice material, the stability of the part of the TiTe2 layer where atomic migration does not occur is significantly improved, which not only improves the operating temperature window of the non-melting phase transition, but also helps to improve the reliability of the non-melting phase transition.

[0049] It will be easily understood by those skilled in the art that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A non-melting superlattice phase change thin film material, characterized in that: It comprises a phase change layer and a pinning layer, wherein the phase change layer and the pinning layer are alternately stacked to form a periodic structure, the phase change layer is made of GeTe material, and the pinning layer is made of TiTe2 material, wherein the crystal structure of the phase change layer is a three-layer GeTe lattice; During operation, when the phase change layer begins to undergo a non-melting phase change, the pinned layer will not undergo a phase change, so as to improve the stability of the pinned layer during the phase change process.

2. The non-melting superlattice phase change thin film material according to claim 1, characterized in that: The pinning layer has a crystal structure of at least two layers of TiTe2.

3. The non-melting superlattice phase change thin film material according to claim 1, characterized in that: The three-layer GeTe lattice has four lattice arrangement modes from top to bottom or from bottom to top, namely Ge-Te-Ge-Te-Ge-Te, Ge-Te-Ge-Te-Te-Ge, Ge-Te-Te-Ge-Ge-Te and Te-Ge-Ge-Te-Ge-Te.

4. The non-melting superlattice phase change thin film material according to claim 3, characterized in that: During the non-melting phase change process of the phase change layer, the three-layer GeTe lattice changes between the four lattice arrangements, so that the non-melting superlattice phase change thin film material forms four different crystal structures.

5. The non-melting superlattice phase change thin film material according to claim 4, characterized in that: The crystal constants a and b of the four different crystal structures are 4Å and 4Å respectively, and the crystal constant c is 18.5363Å or 19.5371Å.

6. The non-melting superlattice phase change thin film material according to claim 3, characterized in that: During the non-melting phase change of the phase change layer, the three-layer GeTe lattice changes between two lattice arrangements of Ge-Te-Ge-Te-Ge-Te and Te-Ge-Ge-Te-Ge-Te.

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