Method for producing nanowire

Abstract

A first raw material (121) of In and a second raw material (122) of Se are supplied onto a substrate (101) so that the surface of the substrate (101) reaches an In excess of the stoichiometric composition of In4Se3, thereby growing a nanowire (102) of In4Se3 on the substrate (101). After forming the nanowire (102) in this manner, the nanowire (102) can be selectively peeled from the substrate (101), and the peeled nanowire (102) can be transferred to another substrate (not depicted in the drawings).

Description

How to make nanowires

[0001] The present invention relates to a method for producing nanowires.

[0002] In-Se compounds have multiple crystalline phases, among which the crystalline phase with the highest In composition ratio is InSe (Non-Patent Document 1). InSe is an indirect transition semiconductor with a band gap of approximately 0.8 eV (Non-Patent Document 2). Se deficiency causes n-type conductivity, and the carrier density at room temperature is 0.89 x 10 17 -33.27 x 10 17 cm -3 , carrier mobility is 83.4 to 242.7 cm 2 V -1 s -1 It has been reported that the change occurs in the range of In4Se, where Se is missing from In4Se3. 3-δ At δ = 0.65, a high thermoelectric figure of merit (ZT = 1.48) has been reported, and it is attracting attention as a next-generation thermoelectric material (Non-Patent Document 4). In addition, due to its band gap corresponding to infrared wavelengths and high carrier mobility, it is expected to be used in photoelectric devices that respond to light in the infrared region.

[0003] As a method for synthesizing In4Se3, bulk synthesis techniques such as the Czochralski method (Non-Patent Document 5) and the Bridgman method (Non-Patent Document 4), and a thin film preparation technique such as molecular beam epitaxy (Non-Patent Document 2) have been reported.

[0004] As the information society advances, the demand for miniaturization of semiconductors is increasing, and nanoscale structures are attracting attention from both fundamental and applied perspectives. Nanowires, in particular, are practical for nanodevice applications because their structure allows for easy extraction of signals in the form of current or voltage along the axial direction. To date, nanowire growth techniques have been established for various semiconductor materials, including Si (Non-Patent Document 6). Furthermore, nanowires have been applied to gate-all-around FETs (Non-Patent Document 7), gas sensors (Non-Patent Document 8), and thermoelectric devices (Non-Patent Document 9), taking advantage of their high surface-to-volume ratio. However, no method for growing nanowire structures has been established for In4Se3.

[0005] T. Godecke et al., "Stable and metastable phase equilibria of the In-Se system", Journal of Phase Equilibria , vol. 19, pp. 572-576, 1998.L. de Brucker et al., "Determination of the direct bandgap value in In4Se3 thin films", Journal of Physics: Condensed Matter, vol. 34, no. 42, 2022.Jong-Soo Rhyee et al., "Thermoelectric properties and anisotropic electronic band structure on the In4Se3-x compounds", Applied Physics Letters, vol. 95, no. 21, 212106, 2009.Jong-Soo Rhyee et al., "Peierls distortion as a route to high thermoelectric performance in In4Se3-δ crystals", Nature, vol. 459, pp. 965-968, 2009.K. Fukutani et al., "High-Resolution Angle-Resolved Photoemission Study of Quasi-One-Dimensional Semiconductor In4Se3", Journal of the Physical Society of Japan, vol. 84, 074710, 2015.N. Wang et al., "Growth of nanowires", Materials Science and Engineering R, vol. 60, Issues 1-6,pp. 1-51, 2008.B. Yang et al., "Vertical Silicon-Nanowire Formation and Gate-All-Around MOSFET", IEEE Electron Device Letters, vol. 29, no. 7, pp. 791-794, 2008.X. Chen et al., "Nanowire-based gas sensors", Sensors and Actuators B: Chemical, vol. 177, pp. 178-195, 2013. al., "Thermoelectrics of Nanowires", Chemical Reviews, vol. 119, pp. 9260-9302, 2019.

[0006] In order to apply In4Se3 to nanodevices, it is necessary to establish a technology for fabricating In4Se3 nanowires.

[0007] The present invention has been made to solve the above problems, and aims to make it possible to produce In4Se3 nanowires.

[0008] The method for producing nanowires according to the present invention grows InSe nanowires by supplying a first source of In and a second source of Se onto a substrate so that the surface of the substrate has an In excess relative to the stoichiometric composition of InSe.

[0009] As described above, according to the present invention, a first source of In and a second source of Se are supplied onto a substrate so that the surface of the substrate has an excess of In compared to the stoichiometric composition of InSe, thereby enabling the production of InSe nanowires.

[0010] FIG. 1 is a cross-sectional view illustrating a nanowire fabrication method according to an embodiment of the present invention. FIG. 2 is a perspective view illustrating the state of nanowire 102. FIG. 3 is a photograph showing the state of InSe nanowires grown on the surface of a substrate, an InSe thin film, and bulk InSe grown by autocatalytic VLS growth from In particles. FIG. 4 is an optical microscope image showing the state of InSe nanowires grown on a sapphire substrate and transferred to a SiO / Si substrate using a thermoplastic sacrificial layer method with a resin film. FIG. 5 is a length histogram of multiple InSe nanowires grown with different Se source supply rates. FIG. 6A is a photograph showing a cross section of an InSe nanowire cut perpendicular to its longitudinal axis. FIG. 6B is a photograph showing the results of observation of a cross section of an InSe nanowire cut perpendicular to its longitudinal axis using a high-angle annular dark-field scanning transmission electron microscope. FIG. 6C is an electron diffraction image of a cross section of an InSe nanowire cut perpendicular to its longitudinal axis.

[0011] A method for fabricating nanowires according to an embodiment of the present invention will now be described with reference to Figure 1. In this fabrication method, a first source material 121 of In and a second source material 122 of Se are supplied onto a substrate 101, thereby growing InSe nanowires 102 on the substrate 101. Nanowires 102 can be grown on the substrate 101 by well-known vapor phase growth methods such as chemical vapor deposition, physical vapor deposition, metalorganic vapor phase deposition, and molecular beam epitaxy.

[0012] 1, a first source material 121 and a second source material 122 are supplied to the surface of a substrate 101 heated to a predetermined temperature so that the surface has an In excess relative to the stoichiometric composition of InSe, thereby growing single-crystal InSe nanowires 102 on the substrate 101. After the nanowires 102 are formed in this manner, the nanowires 102 can be selectively peeled off from the substrate 101 and transferred to another substrate (not shown) (transfer step).

[0013] A typical size of the nanowire 102 is a width w of ∼10 3 nm, length L is 1-10 2μm, thickness t ~ 10-10 2 nm (Figure 2).

[0014] A more detailed explanation follows. In this example, metal-organic chemical vapor deposition is used. A sapphire substrate is used, and trimethylindium (TMIn) is used as the In source (first source), and diethylselenium (DESe) is used as the Se source (second source). The substrate heating temperature can be in the range of 450 to 550°C.

[0015] In4Se3 nanowires can be grown on the substrate by supplying TMIn at 0.374 μmol / min and DESe in the range of 11.1 to 44.3 μmol / min using Ar as a carrier gas.

[0016] When the supply of DESe is less than the above range, agglomeration of In occurs dominantly, resulting in the formation of numerous In particles on the substrate surface, which act as catalyst particles from which bulk InSe grows by self-catalytic vapor-liquid-solid (VLS) growth.

[0017] On the other hand, if the supply amount of DESe exceeds the above range, only the InSe phase grows on the substrate surface. In the In-Se crystal phase diagram (Non-Patent Document 1), there is no crystal phase with a higher In composition than InSe, so InSe is obtained on an In-excess growth surface (substrate surface). The supply amounts of In and Se raw materials that result in an In excess can vary depending on the growth method and the structure of the growth apparatus, but if In aggregates on the substrate surface to form In particles, it can be determined that the conditions are In-excessive.

[0018] On the growth surface where InSe nanowires are obtained, the self-catalyzed VLS growth occurs from the In particles mentioned above, and bulk InSe and bulk InSe grow. InSe, which has a composition similar to InSe, grows directly onto the substrate as a thin film (Figure 3; InSe film).

[0019] Therefore, the VLS-grown bulk InSe and bulk InSe, as well as the thin film grown InSe, coexist with the InSe nanowires. The InSe nanowires can be peeled off using a resin film or the like because the area of ​​contact with the substrate is small relative to their surface area.

[0020] On the other hand, growth species other than the nanowires (bulk InSe, bulk InSe, and thin InSe films) are not exfoliated because they adhere strongly to the substrate. As a result, it is possible to exfoliate only the InSe nanowires, which can then be transferred onto any other substrate.

[0021] Figure 4 shows an optical microscope image of InSe nanowires grown on a sapphire substrate, which were then transferred onto a SiO / Si substrate (another substrate; Au markers attached) using a thermoplastic sacrificial layer method (reference literature) using a resin film.

[0022] In the fabrication method according to the embodiment, the growth rate of In4Se3 nanowires can be controlled by the amount of Se source material supplied. Figure 5 shows a histogram of the lengths of multiple In4Se3 nanowires grown with different amounts of Se source material supplied. Here, the growth conditions (In source material supplied amount, growth temperature, growth time, etc.) other than the amount of Se source material supplied are constant. The average length (L average ) increases with increasing Se source supply rate. This is because the Se source supply rate determines the growth rate of InSe nanowires under In-excess growth conditions.

[0023] The In4Se3 nanowires are single crystals. A cross section of an In4Se3 nanowire cut perpendicular to its longitudinal axis (Figure 6A) was observed using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) (Figure 6B). Only the atomic arrangement corresponding to the ab plane is clearly observed. Figure 6C is an electron diffraction image of the cross section shown in Figure 6A, and only diffraction spots (400 spot, 060 spot, etc.) attributed to the ab plane are observed. This indicates that no domains with different crystal orientations exist, making the nanowires single crystals.

[0024] From the viewpoint of device applications, being a single crystal is an important requirement for realizing high yields. Furthermore, while the grain boundaries present in polycrystals cause carrier scattering, which reduces carrier mobility, single crystals do not have this effect, which improves the response of electronic and photoelectric devices.

[0025] In the above example, a sapphire substrate is used, but the substrate is not limited to this, and other substrates such as a Si substrate, a SiO2 substrate, a GaAs substrate, or a mica substrate can also be used.

[0026] In the above example, DESe and TMIn are used as raw materials, but the present invention is not limited to these. For the In raw material, other raw materials such as indium (In) and triethylindium (TEIn) can be used, and for the Se raw material, selenium (Se), dimethylselenium (DMSe), and hydrogen selenide (HSe) can be used.

[0027] If different raw materials and substrates are used, the substrate heating temperature range suitable for growth may vary from the above 450 to 550° C. However, since the In4Se3 crystal phase becomes thermally unstable above 550° C., the substrate heating temperature range suitable for growth is below 550° C.

[0028] As described above, according to embodiments of the present invention, a first source of In and a second source of Se are supplied onto a substrate so that the substrate surface has an In excess relative to the stoichiometric composition of InSe, thereby enabling the production of InSe nanowires. The embodiments of the present invention establish a crystal growth technique capable of mass-producing InSe nanowires, enabling the use of InSe nanowires in a wide range of industrial applications. Until now, the synthesis of the InSe phase has been difficult due to the complexity of the In-Se crystalline phase diagram. However, by utilizing the aggregation of In particles on the growth surface, it is now possible to synthesize InSe nanowires using general thin film growth techniques.

[0029] It should be noted that the present invention is not limited to the above-described embodiments, and it is clear that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention.

[0030] [Reference] S. Fan et al., "Transfer assembly for two-dimensional van der Waals heterostructures", 2D Materials, vol. 7, no. 2, 022005, 2020.

[0031] 101...substrate, 102...nanowire, 121...first raw material, 122...second raw material.

Claims

1. A method for producing nanowires, which comprises growing In4Se3 nanowires by supplying a first source of In and a second source of Se onto a substrate so that the surface of the substrate has an excess of In compared to the stoichiometric composition of In4Se3.

2. The method for producing nanowires according to claim 1, wherein single-crystal In4Se3 nanowires are grown.

3. A method for producing nanowires according to claim 1 or 2, comprising the steps of selectively peeling the nanowires from the substrate and transferring the peeled nanowires to another substrate.

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