Nanobelt production method

Abstract

In this production method, a first raw material (121) of a group III element and a second raw material (122) of a group VI element are supplied onto a substrate (101) to form a catalyst (102) of a group III element on the substrate (101), a nanobelt (103) of a group III-VI semiconductor is grown from the catalyst (102) in a direction parallel to the surface of the substrate, and the nanobelt (103) is formed on the substrate (101) by a self-catalyzed VLS (vapor-liquid-solid) method.

Description

How to make nanobelts

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

[0002] Among III-VI semiconductors, which are composed of the group 13 (group III) elements Ga and In and the group 16 (group VI) elements S and Se, GaS, GaSe, and InSe, which have a 1:1 composition ratio, form atomic layer structures. These two-dimensional layered materials have interlayer wave functions that depend on the number of stacked layers, making it possible to control a wide range of physical properties by utilizing this. For example, increasing the number of layers from a single layer changes the band gap due to the emergence of subbands.

[0003] In particular, III-VI atomic layer semiconductors exhibit a large bandgap dependency on the number of layers. For example, the bandgap of GaS varies from blue light (2.5 eV, bulk) to ultraviolet light (3.4 eV, single layer), while that of InSe varies from infrared light (1.3 eV, bulk) to green light (2.2 eV, single layer). For this reason, two-dimensional layered materials based on the above-mentioned atomic layer semiconductors are expected to be applied to photoelectric devices capable of responding to light over a wide wavelength range from infrared to ultraviolet light.

[0004] When two-dimensional layered materials are reduced in size in the in-plane direction, they become nanobelt structures with minute widths. In recent years, with the advancement of microfabrication technology and the application of photonic crystals, optical elements have been miniaturized down to the nanoscale, and nanobelts are expected to be applied to optoelectronic integrated devices.

[0005] G. Shen et al., "Vapor-Solid Growth of One-Dimensional Layer-Structured Gallium Sulfide Nanostructures", American Chemical Society Nano, vol. 3, no. 5, pp. 1115-1120, 2009.E. Sutter et al., "Vapor-Liquid-Solid Growth and Optoelectronics of Gallium Sulfide van der Waals 21, pp. 4335-4342, 2021.

[0006] However, currently, the crystal growth technology for III-VI atomic layer semiconductor nanobelts has not been established, and there are very few reported examples (Non-Patent Documents 1, 2, and 3). From the viewpoint of industrial application, a technology for fabricating nanobelts by crystal growth that can be mass-produced on a wafer scale is required.

[0007] The present invention has been made to solve the above problems, and aims to enable mass production of nanobelts on a wafer scale by crystal growth.

[0008] The method for producing a nanobelt according to the present invention comprises supplying a first source of a Group III element and a second source of a Group VI element onto a substrate, forming a catalyst for the Group III element on the substrate, and growing a nanobelt of a Group III-VI semiconductor from the catalyst in a direction parallel to the surface of the substrate.

[0009] As described above, according to the present invention, nanobelts are formed on a substrate by the autocatalytic VLS method by supplying a first source of a group III element and a second source of a group VI element onto the substrate, and thus nanobelts can be mass-produced on a wafer scale by crystal growth.

[0010] FIG. 1A is a cross-sectional view illustrating a state during an intermediate process for explaining a method for fabricating a nanobelt according to an embodiment of the present invention. FIG. 1B is a cross-sectional view illustrating a state during an intermediate process for explaining a method for fabricating a nanobelt according to an embodiment of the present invention. FIG. 2A is a photograph showing a cross-sectional transmission electron microscope image of a Ga catalyst and a GaS nanobelt formed on a sapphire substrate. FIG. 2B is a characteristic diagram showing an energy dispersive X-ray analysis spectrum measured at the GaS nanobelt. FIG. 2C is a characteristic diagram showing an energy dispersive X-ray analysis spectrum measured at the Ga catalyst. FIG. 2D is a photograph showing the results of high-angle annular dark-field scanning transmission electron microscopy observation of the growth interface between the Ga catalyst and the GaS nanobelt within a rectangular frame near the Ga catalyst in FIG. 2A. FIG. 2E is an explanatory diagram showing the growth of a GaS nanobelt from clustered Ga catalyst particles in a direction parallel to the armchair edge. Figure 3 shows an atomic force microscope image (a) of GaS grown at a source supply ratio of [DES] / [TEGa] = 2.40, and an atomic force microscope image (b) of a GaS sample grown at a source supply ratio of [DES] / [TEGa] = 8.65. Figure 4 shows a characteristic diagram illustrating changes in crystal phase and crystal shape due to changes in growth conditions. Figure 5A shows a cross-sectional transmission electron microscope image of an In catalyst and InSe nanobelts formed on a sapphire substrate. Figure 5B shows a characteristic diagram illustrating an energy dispersive X-ray analysis spectrum measured at the InSe nanobelt. Figure 5C shows a characteristic diagram illustrating an energy dispersive X-ray analysis spectrum measured at the In catalyst. Figure 5D shows a photograph of the growth interface between the In catalyst and the InSe nanobelts within the rectangular frame near the In catalyst in Figure 5A, observed using a high-angle annular dark-field scanning transmission electron microscope. Fig. 5E is an explanatory diagram showing the growth of InSe nanobelts from clustered particulate In catalysts in a direction parallel to the armchair edge. Fig. 6A is a plan view showing the configuration of a photodetector using a GaS nanobelt as a photodetector. Fig. 6B is a cross-sectional view showing the configuration of a photodetector using a GaS nanobelt as a photodetector.

[0011] 1A and 1B, a method for fabricating a nanobelt according to an embodiment of the present invention will be described. This fabrication method involves supplying a first source material 121 of a group III element and a second source material 122 of a group VI element onto a substrate 101, thereby forming a catalyst 102 of the group III element on the substrate 101, and then growing a nanobelt 103 of a group III-VI semiconductor from the catalyst 102 in a direction parallel to the surface of the substrate 101. The nanobelt 103 is formed on the substrate 101 by an autocatalytic vapor-liquid-solid (VLS) method.

[0012] For example, as shown in FIG. 1A , a first source material 121 of a group III element (group 13 element) is supplied onto a substrate 101, thereby forming a catalyst 102 of the group III element on the substrate 101 (first step). The group III element can be Ga or In. The catalyst 102 is formed in the form of particles in which the group III element forms aggregates (clusters). The catalyst 102 can be considered to be droplets of the group III element.

[0013] Next, as shown in FIG. 1B , a first source material 121 and a second source material 122 of a group VI element (group 16 element) are supplied, and nanobelts 103 of a group III-VI semiconductor are grown from the catalyst 102 (second step). The nanobelts 103 are grown from the catalyst 102 in a direction parallel to the surface of the substrate 101. The group VI element can be S or Se. The grown nanobelts 103 can be composed of any one of the group III-VI semiconductors GaS, GaSe, and InSe. Here, the growth direction of the nanobelts 103 is parallel to the armchair edge of the nanobelts 103.

[0014] When a Group VI element is incorporated into the catalyst 102 made of a Group III element and the concentration exceeds the saturation concentration, nanobelts 103 made of a Group III-VI semiconductor are precipitated from the catalyst 102 and grow.

[0015] The present invention will be described in more detail below using examples.

[0016] Example 1 In Example 1, a method for producing a nanobelt made of GaS (GaS nanobelt) will be described. The typical size of a GaS nanobelt is a width w in a plan view of approximately 102 ~10 3 nm, and the length L is approximately 10 4 The typical thickness t of the GaS nanobelt is approximately 10-10 2 nm.

[0017] In the fabrication of GaS nanobelts, sapphire can be used as the substrate material. Furthermore, triethylgallium (TEGa) can be used as the first source material, and diethylsulfide (DES) can be used as the second source material. Using these sources, GaS nanobelts can be grown by metalorganic chemical vapor deposition (MOCVD). Alternatively, molecular beam epitaxy (MBE) can be used. The GaS crystalline phase of the fabricated GaS nanobelts was confirmed using Raman spectroscopy and X-ray diffraction.

[0018] For example, in the first step, only TEGa is supplied as a raw material onto a sapphire substrate at a growth temperature (substrate temperature) of 740°C to form Ga clusters and use them as a Ga catalyst. Then, in the second step, DES and TEGa are supplied onto the sapphire substrate at the same growth temperature (substrate temperature) as above, under the condition of [DES supply amount] / [TEGa supply amount]=0.961. This allows GaS nanobelts to grow from the Ga catalyst in a direction parallel to the surface of the sapphire substrate.

[0019] Furthermore, by maintaining the supply rates of DES (second source) and TEGa (first source) constant at a source supply ratio where TEGa (first source) is in excess, Ga catalysts can be formed and GaS nanobelts can be grown. The source supply ratio where TEGa is in excess in this case depends on the growth temperature. For example, at a growth temperature of 700°C, [DES] / [TEGa]≦3.36.

[0020] Figure 2A shows a cross-sectional transmission electron microscope (TEM) image of a Ga catalyst and GaS nanobelts grown on a sapphire substrate. Because they were grown by self-catalyzed VLS growth, the Ga catalyst is present on one side of the GaS nanobelts.

[0021] Next, the energy dispersive X-ray (EDX) spectrum measured at the fabricated GaS nanobelt is shown in Figure 2B. The EDX spectrum measured at the fabricated Ga catalyst is shown in Figure 2C. Characteristic X-ray peaks due to Ga and S were observed from the GaS nanobelt. Furthermore, characteristic X-ray peaks due to Ga were observed from the Ga catalyst. Other characteristic X-ray peaks (C, O, and Al) are due to the carbon coating formed during the fabrication of the sapphire substrate and cross-sectional TEM sample.

[0022] The interface between the Ga catalyst and the GaS nanobelt growth within the rectangular frame near the Ga catalyst in Figure 2A was observed using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM image) and is shown in Figure 2D. As shown in Figure 2D, the atomic arrangement of GaS was clearly confirmed in the GaS nanobelt.

[0023] As shown in the schematic diagram of Figure 2E, GaS nanobelts grow from clustered Ga catalyst particles in the in-plane direction of the GaS atomic layers (parallel to the armchair edges). Figure 2E (a) is a plan view seen from the normal direction of the substrate plane, which is perpendicular to the c-axis. Figure 2E (b) is a side view seen from the direction perpendicular to the c-axis.

[0024] Figure 3(a) shows an atomic force microscope (AFM) image of GaS grown at a source supply ratio of [DES] / [TEGa] = 2.40. In this case, the grown GaS has a nanobelt shape. Figure 3(b) shows an AFM image of a GaS sample grown at a source supply ratio of [DES] / [TEGa] = 8.65. In this case, the grown GaS has a triangular shape (island shape). This change in shape due to the difference in source supply cost indicates that the growth mechanism changes from autocatalytic VLS growth to two-dimensional island growth as the Ga source supply ratio decreases.

[0025] Next, changes in the crystal phase and crystal shape due to changes in growth conditions (growth temperature and raw material supply ratio) will be explained with reference to Fig. 4. In Fig. 4, open squares indicate conditions under which GaS nanobelts were formed, open circles indicate conditions under which GaS was formed in islands (triangular islands), and filled circles indicate conditions under which GaS was formed. GaS nanobelts were formed under conditions where the growth temperature was lower and the TEGa supply ratio was higher (in the range where [DES] / [TEGa] was smaller) within the growth range.

[0026] Example 2 Next, Example 2 will be described. In Example 2, a method for fabricating a nanobelt made of InSe (InSe nanobelt) will be described. Fabrication of the InSe nanobelt is similar to that described above, and sapphire can be used as the substrate material. Furthermore, trimethylindium (TMIn) can be used as the first source material, and diethylselenium (DESe) can be used as the second source material. Using these sources, InSe nanobelts can be grown by metalorganic chemical vapor deposition (MOCVD). Alternatively, molecular beam epitaxy (MBE) can be used.

[0027] For example, DESe and TMIn are supplied onto a sapphire substrate at a growth temperature (substrate temperature) of 450° C. and under the condition of [DESe supply rate] / [TMIn supply rate]=59.1, which forms an In catalyst and allows InSe nanobelts to grow from the In catalyst in a direction parallel to the surface of the sapphire substrate.

[0028] Figure 5A shows a cross-sectional TEM image of an InSe nanobelt and an In catalyst grown on a sapphire substrate. Because the nanobelt was grown by self-catalytic VLS growth, the In catalyst is present on one side of the InSe nanobelt.

[0029] Next, the EDX spectrum measured at the fabricated InSe nanobelt is shown in Figure 5B. The EDX spectrum measured at the fabricated In catalyst is shown in Figure 5C. Characteristic X-ray peaks due to In and S were observed from the InSe nanobelt. Furthermore, characteristic X-ray peaks due to In were observed from the In catalyst.

[0030] The interface between the In catalyst and the InSe nanobelt growth within the rectangular frame near the In catalyst in Fig. 5A was observed using a high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM image) and is shown in Fig. 5D. As shown in Fig. 5D, the atomic arrangement of InSe was clearly confirmed in the InSe nanobelt.

[0031] As shown in the schematic diagram of Figure 5E, InSe nanobelts grow from clustered particulate In catalysts in the in-plane direction of the InSe atomic layer (direction parallel to the armchair edge). Figure 5E (a) is a plan view seen from the normal direction of the substrate plane perpendicular to the c-axis. Figure 5E (b) is a side view seen from the direction perpendicular to the c-axis.

[0032] Next, a device using a nanobelt fabricated by the nanobelt fabrication method according to the embodiment will be described. For example, a photodetector can be constructed using a GaS nanobelt as a photodetector. As shown in FIGS. 6A and 6B, a GsS nanobelt 202 is formed on a substrate 201, and a first electrode 203a is connected to one end of the GsS nanobelt 202, and a second electrode 203b is connected to the other end of the GsS nanobelt 202. Note that FIG. 6B shows a cross section taken along line bb' in FIG. 6A.

[0033] The first electrode 203a and the second electrode 203b can be composed of a lower 150-nm-thick Cr layer 211 that contacts the GsS nanobelt 202, and a 100-nm-thick Au layer 212 formed on the Cr layer 211. To facilitate extraction of photocurrent from the edges (side surfaces) of the GsS nanobelt 202, the Cr layer 211 is made thicker than the GsS nanobelt 202 (approximately 100 nm). The Cr layer 211 can also be a Ti layer or an In layer.

[0034] By contacting a portion of the first electrode 203a and the second electrode 203b with the side or end face of the GsS nanobelt 202, photocurrent extraction is efficient, improving response. When the GsS nanobelt 202 is irradiated with light (excitation light) with a wavelength shorter than 490 nm, which corresponds to the band gap of GaS (2.5 eV), a photocurrent flows between the first electrode 203a and the second electrode 203b. Furthermore, the dark current is below the detection limit (0.15 pA). The photodetector's specifications, under conditions of an excitation light wavelength of 300 nm and an applied voltage of 5 V, were an ON / OFF ratio of >1840 and a response of 220 mA / W.

[0035] As described above, according to the present invention, nanobelts are formed on a substrate by the autocatalytic VLS method by supplying a first source of a group III element and a second source of a group VI element onto the substrate, thereby enabling mass production of nanobelts on a wafer scale by crystal growth.

[0036] According to the present invention, a method for fabricating III-VI atomic layer semiconductor nanobelts by self-catalytic vapor-phase lithography (VLS) growth has been established, enabling mass production on a wafer scale. The width of the fabricated nanobelts is comparable to the size of photonic crystals, making them a new material suitable for next-generation integrated optoelectronic devices. Furthermore, photodetectors using the fabricated nanobelts according to the present invention exhibit extremely low dark current, resulting in a high ON / OFF ratio in the blue light region, enabling photodetection in microscopic areas.

[0037] It should be noted that the present invention is not limited to the embodiments described above, 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.

[0038] 101...substrate, 102...catalyst, 103...nanobelt, 121...first raw material, 122...second raw material.

Claims

1. A method for producing nanobelts, comprising supplying a first raw material of group III elements and a second raw material of group VI elements onto a substrate to form a catalyst of group III elements on the substrate, and growing III-VI group semiconductor nanobelts from the catalyst in a direction parallel to the surface of the substrate.

2. The method for producing nanobelts according to claim 1, comprising: a first step of forming the catalyst on the substrate by supplying the first raw material onto the substrate; and a second step of growing the nanobelts from the catalyst by supplying the first raw material and the second raw material, wherein the nanobelts are formed on the substrate by a self-catalytic VLS method.

3. The method for producing nanobelts according to claim 1 or 2, wherein the group III element is Ga or In, the group VI element is S or Se, and the III-VI group semiconductor is any one of GaS, GaSe, and InSe.

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