MANUFACTURING METHOD OF Mg3Sb2 THIN FILM AND Mg3Sb2 THIN FILM

Abstract

To grow a Mg3Sb2 thin film with an extremely high orientation on a substrate.SOLUTION: A substrate plane of a substrate used here is a c-plane {0001} plane, of sapphire. Since an orientation of a Mg3Sb2 thin film is thought to depend on a surface condition of a sapphire substrate, heat treatment (a heat treatment process) is performed for the substrate before the crystal growth, and a change in an orientation condition is investigated through the heat treatment. In a case of performing the heat treatment process at 900°C or more (an upper case), compared to a case of not performing the heat treatment process (a lower case), no peak of a (10-11) plane or a (01-11) plane is observed, but only peaks of (a substrate plane) and (a (0001) plane of Mg3Sb2) are observed. Namely, it has been confirmed that the Mg3Sb2 thin film has a (0001) orientation.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for producing a thin film containing Mg3Sb2 as a main component, and to an Mg3Sb2 thin film. [Background technology]

[0002] Mg3Sb2, a Zintl phase material, is known as a semiconductor and a high-performance thermoelectric conversion material, as described in, for example, Patent Document 1. Its manufacturing method includes a sintering method, as described in, for example, Patent Document 2. In the sintering method, bulk Mg3Sb2 is obtained as a sintered body (polycrystal).

[0003] Mg3Sb2 can be doped to produce n-type or p-type Mg3Sb2. To achieve high thermoelectric conversion efficiency, the semiconductor material Mg3Sb2 must have high (electron) mobility. However, Mg3Sb2 obtained by the firing method is polycrystalline and has low mobility. However, it was expected that highly oriented or even single-crystalline Mg3Sb2 could be obtained by heteroepitaxially growing it on a single-crystalline substrate of another material. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-190953 [Patent Document 2] Japanese Patent Publication No. 2022-114404 Summary of the Invention [Problem to be solved by the invention]

[0005] As with GaN and the like, sapphire (α-Al2O3) is particularly preferably used as the substrate material for Mg3Sb2 growth, and the c-plane {0001} is particularly preferably used as the plane orientation.

[0006] We were able to grow an Mg3Sb2 thin film on the c-plane of sapphire by molecular beam epitaxy. However, X-ray diffraction results showed that the thin film had two different orientations, and its mobility (electrical conductivity) was insufficient. Therefore, it was hoped that by improving the orientation, we could obtain an Mg3Sb2 thin film with higher mobility.

[0007] The present invention has been made in view of the above problems, and an object of the present invention is to provide an invention that solves the above problems. [Means for solving the problem]

[0008] In order to solve the above problems, the present invention has the following configurations. The method for producing an Mg3Sb2 thin film of the present invention is characterized by comprising a heat treatment step of performing a heat treatment on a c-plane sapphire (α-Al2O3) substrate at 900 to 1400°C in an oxygen atmosphere, and a growth step of growing the Mg3Sb2 thin film by epitaxial growth on the substrate after the heat treatment step. The method for producing an Mg3Sb2 thin film of the present invention is characterized in that in the growth step, the substrate after the heat treatment step is heated in a vacuum at a temperature in the range of 500°C to 550°C, and Mg and Sb are supplied as molecular beams to the main surface of the substrate to grow the Mg3Sb2 thin film. The method for producing an Mg3Sb2 thin film of the present invention is characterized in that the heat treatment step is carried out in the atmosphere. The Mg3Sb2 thin film of the present invention is characterized by being formed with a (0001) orientation on a substrate made of c-plane sapphire (α-Al2O3). [Effects of the Invention]

[0009] Since the present invention is configured as described above, it is possible to grow a highly oriented Mg3Sb2 thin film on a substrate. [Brief explanation of the drawings]

[0010] [Figure 1]FIG. 2 is a diagram schematically illustrating an apparatus configuration in a growth step in the manufacturing method according to the embodiment of the present invention. [Figure 2] FIG. 1 is a diagram showing two types of plane orientations that can be epitaxially grown on a c-plane sapphire substrate in Mg3Sb2. [Figure 3] 1 shows X-ray diffraction patterns of Mg3Sb2 thin films with and without heat treatment. [Figure 4] 1 shows the results of measuring electrical conductivity with and without a heat treatment process. DETAILED DESCRIPTION OF THE INVENTION

[0011] A method for producing Mg3Sb2 according to an embodiment of the present invention will be described. Figure 1 is a diagram showing a schematic diagram of an apparatus configuration in a growth step for growing an Mg3Sb2 thin film on a substrate in this manufacturing method. This manufacturing method uses the well-known molecular beam epitaxy (MBE) method. The substrate surface of the substrate 10 used here is the c-plane {0001} of sapphire (α-Al2O3 with a hexagonal crystal structure).

[0012] The substrate 10 is fixed to a substrate holder 20 in a vacuum chamber 100, and an Mg source 21 as a supply source of Mg (magnesium) and an Sb source 22 as a supply source of Sb (antimony) are installed facing the substrate surface. Although not shown in the figure, the vacuum chamber 100 is evacuated by a vacuum pump, and the degree of vacuum is, for example, 10 -7 Pa~10 -8 The temperature is set to a range of 500°C to 550°C. In addition, a resistance heater and a temperature sensor (not shown) are installed inside the substrate holder 20, so that the temperature of the substrate 10 can be controlled before or during growth. When growing Mg3Sb2, the temperature is preferably in the range of 500°C to 550°C, and within this range, an Mg3Sb2 thin film with particularly high crystallinity can be obtained.

[0013] The Mg source 21 and Sb source 22 are filled with high-purity Mg and Sb, respectively, and their temperatures are controlled by resistance heating. The temperature of the Mg source 21 is set, for example, in the range of 250°C to 350°C, and the temperature of the Sb source 22 is set, for example, in the range of 400°C to 500°C. Molecular beams of these materials are irradiated onto the substrate surface, growing a thin film of Mg3Sb2 on the substrate 10. Here, since the vapor pressure of Mg is higher than that of Sb, an Mg3Sb2 thin film can be obtained by making the Mg content in excess (e.g., Mg:Sb=4:1) rather than the stoichiometric composition of Mg3Sb2 (Mg:Sb=3:2). The above configuration is similar to that of a well-known molecular beam epitaxy apparatus.

[0014] This manufacturing method is characterized by the processing direction before the substrate 10 is made into the form shown in Fig. 1. This point will be explained below.

[0015] Due to the difference in lattice constants, the crystal plane of trigonal Mg3Sb2 epitaxially grown on the c-plane of sapphire is limited to either the (0001), (10-11), or (01-11) plane. Figure 2 shows the crystal structure of Mg3Sb2, where the hatched plane in Figure 2(a) represents the (0001) plane and the hatched plane in Figure 2(b) represents the (10-11) plane.

[0016] The bottom panel of Figure 3 shows the X-ray diffraction pattern after Mg3Sb2 growth when a commercially available sapphire substrate with the above-mentioned plane orientation was used as the substrate 10. In Figure 3, ▼ indicates the peak of the c-plane of the sapphire substrate, and ● indicates the peak of the (0001) plane of Mg3Sb2 shown in Figure 2(a). Note that a peak other than the one mentioned above is also observed near the leftmost ● peak. This peak is either the (10-11) or (01-11) plane of Mg3Sb2 mentioned above. It is difficult to distinguish which of these it is, but in any case, it is a peak other than the (0001) plane of Mg3Sb2. In other words, the Mg3Sb2 thin film formed in this case contains a mixture of a (0001)-oriented component and either a (10-11) or (01-11)-oriented component.

[0017] In this case, the electron mobility in the Mg3Sb2 thin film is reduced due to scattering at the grain boundaries. Therefore, higher electron mobility is expected by decreasing at least the (10-11) or (01-11) orientation component and increasing the (0001) orientation component.

[0018] Since the orientation of the Mg3Sb2 thin film is thought to depend on the surface condition of the sapphire substrate (c-plane), the substrate 10 was subjected to a heat treatment (heat treatment step) before the crystal growth described above, and the resulting change in the orientation state was investigated. Here, heating was performed in an electric furnace at a predetermined temperature in an air atmosphere for 10 minutes. The X-ray diffraction results without this heat treatment are shown in the bottom row of Figure 3.

[0019] Here, in both the substrate 10 that had undergone the pretreatment process and the substrate 10 that had not, in the configuration of Figure 1, a heat treatment was performed in a vacuum at 600°C for 6 hours or more before growth (a state in which Mg and Sb were supplied to the substrate 10 side) in order to remove adhering gas components from the surface of the substrate 10, and the temperature of the substrate 10 during growth was set to 550°C. The film thickness of the grown Mg3Sb2 is determined by the growth time, but here it was set to 100 nm.

[0020] The X-ray diffraction results for this case are shown in the upper part of Figure 3. The temperatures used in the heat treatment were 800°C (second from the bottom), 900°C (third from the bottom), and 1000°C (top). At 800°C, the diffraction pattern was unchanged from that without heat treatment, with peaks from the (10-11) or (01-11) planes. However, when heat treatment was performed at 900°C or higher, these peaks disappeared, and only peaks from the ▼ (substrate plane) and ● (Mg3Sb2 (0001) plane) were observed. This confirmed that the Mg3Sb2 thin film had a (0001) orientation.

[0021] The influence of the heat treatment process on the orientation state of Mg3Sb2 as described above is due to changes in the surface state of the sapphire substrate 10 (c-plane). M. Yoshimoto, T. Maeda, T. Ohnishi, H. Koinuma, O. Ishiyama, M. Shinohara, M. Kubo, R. Miura, and A. Miyamoto, "Atomic-scale formation of ultrasmooth surfaces on sapphire substrates for high-quality thin-film fabrication," Applied Physics Letters, vol. 67(18), p. 2615 (1995), describes such changes in the state of the sapphire c-plane due to heat treatment. According to this literature, when viewed with an AFM, the c-plane of sapphire that has not undergone heat treatment has many minute irregularities due to differences between the actual surface and the exact c-plane, and from a microscopic perspective, there are few areas that are strictly composed of the c-plane. In contrast, after the heat treatment process, many terrace-like regions formed with precise c-planes were formed. This change in the state of the sapphire c-plane surface is thought to have made it easier for the (0001) plane of Mg3Sb2 to grow.

[0022] We also measured the electrical conductivity of the Mg3Sb2 thin film after growth. Figure 4 shows the results of a comparison of electrical conductivity with and without a heat treatment process. Here, the temperature of the heat treatment process was 1000°C. The heat treatment process more than doubled the electrical conductivity. This is thought to be due to the fact that the heat treatment process increased the orientation and reduced the number of crystal grain boundaries, resulting in increased (electron) mobility.

[0023] As mentioned above, the temperature of the heat treatment step is preferably 900°C or higher. If the temperature is lower than 900°C, the above effect is insufficient, and the effect of eliminating the (10-11) or (01-11) orientation becomes insufficient. Although the upper limit of the heat treatment temperature is not clear from the above experimental results, a temperature of, for example, 1400°C or lower is preferable as a temperature at which the crystal structure of the sapphire surface is reliably maintained. However, since a low temperature is preferable to facilitate the manufacturing process, the temperature of the heat treatment step is particularly preferably in the range of 900°C to 1000°C shown in Figure 3. The heat treatment time is preferably 10 to 60 minutes to cause the above-mentioned changes in the surface state.

[0024] Although the heat treatment is performed in the air in the above example, it is preferable that oxygen is included since oxygen, a constituent element of sapphire, is involved in the above-mentioned change in the surface structure. Therefore, the atmosphere does not have to be air.

[0025] In the above example, MBE was used in the growth process, but it is clear that the above heat treatment process is also effective when other growth methods are used, as long as they can similarly grow Mg3Sb2 epitaxially on a c-plane sapphire substrate.

[0026] Such a high-mobility Mg3Sb2 thin film is particularly suitable as a thermoelectric conversion material. Since the Mg3Sb2 thin film is formed on a sapphire substrate with high mechanical strength, it is easy to fabricate a thermoelectric conversion element by forming electrodes on the thin film. Heat conduction to the Mg3Sb2 thin film can also be achieved via the sapphire substrate.

[0027] Furthermore, Mg3Sb2 thin films can also be used as semiconductor materials. As described in Patent Documents 1 and 2, Mg3Sb2 thin films have conventionally been manufactured as sintered bodies, but a form in which a high-quality Mg3Sb2 thin film is formed on an insulating sapphire substrate and an electrode is formed on the Mg3Sb2 thin film is preferable for use in various general semiconductor elements. In this case, the conductivity type (p-type or n-type) of Mg3Sb2 can be easily controlled by adding impurities.

[0028] The present invention has been described above based on an embodiment. This embodiment is merely an example, and it will be understood by those skilled in the art that various modifications are possible in the combination of the respective components, and that such modifications are also within the scope of the present invention. [Explanation of symbols]

[0029] 10 Substrate 20 Substrate holder 21 Mg Sauce 22 Sb sauce 100 Vacuum Chamber

Claims

1. Mg ３ Sb ２ A method for producing a thin film, comprising: c-plane sapphire (α-Al ２ O ３ a heat treatment step of subjecting the substrate to a heat treatment at 900°C to 1400°C in an oxygen atmosphere; The Mg is epitaxially grown on the substrate after the heat treatment step. ３ Sb ２ a growing step of growing a thin film; Mg ３ Sb ２ Thin film manufacturing method.

2. In the growing step, In a vacuum, the substrate after the heat treatment step is heated to a temperature in the range of 500°C to 550°C, and Mg and Sb are supplied as molecular beams to the main surface of the substrate, thereby forming the Mg ３ Sb ２ 2. The method of claim 1, wherein a thin film is grown. ３ Sb ２ Thin film manufacturing method.

3. 3. The Mg alloy according to claim 1, wherein the heat treatment step is carried out in air. ３ Sb ２ Thin film manufacturing method.

4. c-plane sapphire (α-Al ２ O ３ ) on a substrate, characterized in that it is formed with a (0001) orientation. ３ Sb ２ Thin film.

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