Method for growing high-quality heteroepitaxial monoclinic gallium oxide crystals

Abstract

The present invention relates to a method for growing high quality heteroepitaxial β-Ga2O3 crystals in the field of chemical vapor deposition, particularly by using the low pressure chemical vapor deposition (LPCVD) method, which comprises the steps of: preparing a substrate with a hexagonal surface cut with different inclinations, with the inclination angle ranging from 2° to 10°; physically transporting the vapor obtained from the heated gallium in the second zone (2) to the sample by means of a pump (4) / argon gas; pumping oxygen into the system through a separate ceramic or refractory metal tube, which is directly and perpendicularly to the surface of the sample on the substrate; transferring oxygen; creating a core layer of β-Ga2O3 on the surface such that the Ga:O surface atomic ratio on the growth surface is in the range of 10:1 to 1:10, thereby ensuring that the Ga and O surface atoms form β-Ga2O3 crystals on the heated substrate; growing the core region of β-Ga2O3 to a thickness of 5 nm to 2000 nm at a growth rate of 10 nm / h to 500 nm / h, and maintaining the growth process on the core layer created in the previous step such that the growth rate of β-Ga2O3 is in the range of 100 nm / h to 10 μm / h.

Description

[Technical Field]

[0001] The present invention relates to semiconductor materials used in solar-blind photodetectors in the high voltage and power electronics field and / or in the defense industry, missile tracking systems in the defense industry, and energy conversion, and in particular to a method for growing β-Ga2O3 crystals.

[0002] More specifically, the present invention relates to a method for growing high quality heteroepitaxial β-Ga2O3 crystals, specifically using low pressure chemical vapor deposition (LPCVD) in the field of chemical vapor deposition. [Background technology]

[0003] Monoclinic gallium oxide (β-Ga2O3) crystals are known to have low thermal conductivity (10-30 W / mK). Therefore, layers with thicknesses of approximately 0.5-1 mm pose significant problems in terms of thermal conduction. On the other hand, the device layer thickness required for electronic devices is approximately 10-20 microns. Therefore, the ability to heteroepitaxially grow a β-Ga2O3 layer of 10-20 microns on a substrate with high thermal conductivity and without compromising crystal quality is of great significance for overcoming this problem at the current state of the art.

[0004] Considering the current situation, β-Ga2O3 is still in the research and development stage at various institutions, and efforts are still ongoing to experimentally clarify its potential as a material. In fact, there is no method for commercially producing devices. Recently, interest has increased due to the availability of high-quality ingots of gallium oxide (β-Ga2O3). In fact, due to the advantage of a wide bandwidth of approximately 4.8 eV, it is expected to have a higher breakdown voltage than competing products. Table 1 shows competing products in the industry and their basic material parameters. JPEG0007796020000001.jpg55102

[0005] All high-quality β-Ga2O3-based electronic devices are fabricated by homoepitaxial growth, i.e., growth processes on superposable substrates. The main growth techniques for β-Ga2O3 thin films in the prior art are dedicated to homoepitaxy on commercially available Ga2O3 substrates, particularly using hydride vapor phase epitaxy (HVPE), molecular beam epitaxy (MBE), and metalorganic vapor phase epitaxy (MOVPE).

[0006] Although some research has been conducted using alternative substrates, the crystalline quality is far from the target. Specifically, the β-Ga2O3 layer is grown by thin-film technology on a β-Ga2O3 substrate cut from an ingot. Therefore, the β-Ga2O3 layer, with a thermal conductivity of 10–30 W / mK, cannot be directly used for high-power applications. The thermal conductivity of β-Ga2O3 is 10–20 times lower than that of GaN and SiC competitors. To compete, the device must have at least the same thermal conductivity. Therefore, these studies have specified a costly and risky process called substrate thinning, in which the substrate is either decomposed down to the device zone or cut with a laser, followed by attachment to a carrier with high thermal conductivity. However, this process is undesirable due to the increased cost.

[0007] Thus, it has been determined that the solutions available in the current state of the art are insufficient to solve the existing problems.

[0008] The inventors' novel crystal growth method has enabled the growth of β-Ga2O3 layers with the highest crystal quality ever achieved on sapphire (Al2O3) substrates. Furthermore, a similar method (0001) has enabled the growth of high-quality β-Ga2O3 layers on 4H- or 6H-SiC, which have higher thermal conductivities (~400 W / mK) than sapphire and similar atomic planes (0001). Instead of using all of these current state-of-the-art processes, it is possible to grow β-Ga2O3 thin film layers for device fabrication directly on SiC, which has high thermal conductivity, by heteroepitaxial growth. In our studies, we have successfully grown high-quality β-Ga2O3 thin films on sapphire crystals, which have a surface structure (0001) very similar to that of SiC crystals, using the present (0001) method. It is expected that similar results will also be achieved on SiC, which has high thermal conductivity, which would be extremely beneficial technologically.

[0009] At the current technological level, in addition to problems related to thermal conductivity, various problems were also observed in terms of product price and size.

[0010] Although β-Ga2O3 crystals can be grown in ingot form, only commercial products with ingot diameters of 4 inches or less have been introduced to the market. This is due to various physical factors, including the limitation of ingot diameter as well as the short physical lifespan of the iridium metal crucibles used in production. For these reasons, the current market price of a 0.5 mm thick, 2 inch β-Ga2O3 substrate, for example, is approximately $2,000. This is comparable in price to competing SiC, weakening the potential competitiveness of β-Ga2O3.

[0011] On the other hand, sapphire is usually in the lower price range, and several semiconductors have been on the market for a long time as 8-inch diameter substrates. For example, the market value of a 2-inch sapphire substrate is about $10. Therefore, growing high-quality β-Ga2O3 on inexpensive, large sapphire substrates has rapidly become an attractive option.

[0012] Semiconductor devices are manufactured commercially, but lithography across the entire substrate allows for as many devices as possible to be simultaneously fabricated on the same semiconductor plate. Therefore, large-diameter plates are preferred for productivity and, therefore, cost reasons. Sapphire substrates, as large as 8 inches in diameter, are commercially available at low cost, making them ideal substrates for the growth of heteroepitaxial β-Ga2O3. Several different research groups have attempted to grow β-Ga2O3 with high crystalline quality using various thin-film growth techniques, but none have been able to achieve growth with the desired levels of dislocation density, growth rate, and smooth surface morphology. X-ray diffraction crystallography (XRC) FWHM (full-width-at-half-maximum) measurement, which is directly related to dislocation density, is the most commonly used nondestructive method for measuring crystalline quality over a wide scan area of ​​approximately 1–2 mm. The FWHM value decreases as crystalline quality increases. β-Ga2O3 (-201) oriented on (0001) sapphire has a similar atomic pattern and can be grown as a single crystal.

[0013] A search of the current state of the art revealed the invention of U.S. Patent Application No. 2016 / 0265137, entitled "Method for growing beta-Ga2O3-based single crystal film, and crystalline layered structure," which is the subject of a U.S. patent application. It was confirmed that this invention discloses a method for growing a β-Ga2O3-based single crystal film using the HVPE method. In this method, the growth process can be performed at a growth temperature of 900°C or higher. The invention disclosed in this application utilizes low-pressure chemical vapor deposition, which is fundamentally different in state of the art from the method disclosed in the literature. Furthermore, the process and reaction parameters are not similar.

[0014] The invention disclosed in the European Patent Registration of European Patent Specification No. 1182697, entitled "Sapphire Substrate, Electronic Component, and Method of Its Manufacture," is also available in the art. It briefly discloses a sapphire substrate having a heteroepitaxial growth surface, the heteroepitaxial growth surface being parallel to a plane obtained by rotating the (0110) plane of the sapphire substrate by 8° to 20° around the c-axis of the sapphire substrate within the crystal lattice of the sapphire substrate. The application further discloses a semiconductor device, an electronic component, and a crystal growth method. The invention does not specifically mention a method for growing high-quality gallium oxide crystals. Furthermore, the invention disclosed in this application achieves surprising technical effects by working with specific values, as can be observed from the process. For example, in the present invention, it is important that the substrate be (0001)-oriented sapphire tilted 6° toward the <11-20> direction.

[0015] In the art, an invention that is the subject of a patent application, U.S. Patent Application Publication No. 2020 / 0161504, entitled "Nanostructure," utilizes plasma-assisted solid-state MBE technology. In MBE, the temperature of the effusion cell can be used to control the growth rate. Reasonable growth rates measured during conventional planar (layer-by-layer) growth are 0.05-2 μm per hour, e.g., 0.1 μm per hour. The technology used there is MBE, which is distinct from the technology of low-pressure chemical vapor deposition (LPCVD), which is the subject of the present invention.

[0016] Another document available in the state of the art is the subject of a U.S. patent application, U.S. Pat. No. 10,593,544, entitled "Method for forming a thin film comprising an ultrawide bandgap oxide semiconductor," which discloses an invention that utilizes a low-pressure chemical vapor deposition (LPCVD) method. An embodiment of the invention utilizes a low-pressure chemical vapor deposition (LPCVD) method, in which the vapor resulting from the evaporation of a material is used as a precursor, and the material has a low vapor pressure at the growth temperature of the thin film. The vapor is carried to a reaction chamber by an inert gas, such as argon, and mixed with a second precursor. The reaction chamber is maintained at an atmospheric pressure such that nucleation of the precursor preferably occurs at the substrate surface rather than in the vapor phase. The low vapor pressure of the material results in a growth rate at the substrate surface that is significantly faster than that achieved using prior art growth methods. Reviewing the description of the invention reveals that the system and reaction parameters used to implement the chemical vapor deposition technique in the process differ from those of the invention disclosed in this application. Furthermore, no guidance was found regarding the technical effect imparted to the present invention by these different technical elements. In particular, the surprising record of an XRC FWHM of 0.049 for heteroepitaxial β-Ga2O3 crystals obtained by the growth method developed by the inventors was not provided for the invention that is the subject of the present application. Summary of the Invention

[0017] The object of the present invention is to develop a novel method for growing high quality heteroepitaxial β-Ga2O3 crystals by overcoming the above-disclosed problems present in the current state of the art.

[0018] Another object of the present invention is to heteroepitaxially grow 10-20 micron thick β-Ga2O3 layers on substrates with high thermal conductivity and without compromising crystalline quality.

[0019] An indirect objective of the present invention is to ensure the direct growth of high quality βGa2O3 thin film layers on SiC with high thermal conductivity by heteroepitaxial growth, which allows the device to be fabricated in a way that is different from the methods used in the current state of the art, which requires the implementation of additional processes.

[0020] Another object of the present invention is to enable cost-effective growth of β-Ga2O3 crystals with optimal quality.

[0021] Another object of the present invention is to grow β-Ga2O3 crystals with improved growth rate and smooth surface morphology.

[0022] The advantage of the present invention is that, as a result of investigations by the present inventors, the XRC FWHM value, which is a value indicating the crystal quality, has been found to be 0.049, the highest value ever obtained. [Brief explanation of the drawings]

[0023] [Figure 1] The system structure of low-pressure chemical vapor deposition (LPCVD) is shown. [Figure 2] FE-SEM surface image of a β-Ga2O3 layer grown on sapphire at a Ga crucible temperature of 795 °C. [Figure 3] The figure shows the XRC measurement results of a Ga2O3 layer grown on sapphire at a Ga crucible temperature of 795°C. [Figure 4] FE-SEM surface image of a β-Ga2O3 layer grown on sapphire at a Ga crucible temperature of 920 °C. [Figure 5] XRC measurements of β-Ga2O3 layers grown on sapphire at a Ga crucible temperature of 920 °C are shown. [Explanation of symbols]

[0024] Part name / Section name 1 Zone 1 2 Zone 2 3 Third Zone 4. Pump 5. First crucible 6. Detailed Description of the Second Crucible Invention

[0025] The present invention discloses a chemical vapor deposition-based method step, more specifically, a method step for growing high quality heteroepitaxial β-Ga2O3 crystals using low-pressure chemical vapor deposition.

[0026] Figure 1 shows the system architecture of the method used in this invention. The system includes a three-zone furnace. Argon gas physically transports vapor from heated gallium (Ga) in the second zone (2) toward the pump (4), i.e., toward the sample. Oxygen is pumped into the system through a separate quartz tube and transferred directly to the vertically oriented sample surface on the substrate. In this way, Ga and O surface atoms form β-Ga2O3 crystals on a substrate heated to an appropriate temperature. The substrate used here is preferably (0001)-oriented sapphire with a 6° tilt to the <11-20> direction. The specific tilt angle (2° to 10°) of the plate has a decisive effect on the monocrystalline nature of the growing β-Ga2O3 layer.

[0027] In the implementation of the system of the present invention, the entire system is maintained within a single furnace with different heating zones at high temperatures. By maintaining different metal crucibles in different zones, desired vapor pressures can be achieved, thereby providing opportunities for doping and alloying processes. As shown in Figure 1, metal vapor is carried by a noble gas, such as Ar, through distribution lines and evenly distributed over the sample, creating pixels at regular intervals. Similarly, O2 (oxygen gas) may also be transported to the substrate via a separate route from the designated pixel locations, either directly or diluted with a carrier gas. In addition to these lines, gases ensuring n-type or p-type doping (e.g., SiC4, H2, etc.) can also be delivered via parallel lines or directly mixed into the O2-carrying line. The sample platform is a rotatable disk capable of holding multiple substrates. In the current state of the art, such a distribution structure is available as a contact showerhead chemical vapor deposition system. In this system, carrier gas or oxygen diluted with carrier gas is delivered into the system from a location close to the center of the rotatable disk platform, thereby providing a radial flow for the gas from the center of the disk to its outer periphery.

[0028] The heteroepitaxial β-Ga2O3 crystal growth method of the present invention using low-pressure chemical vapor deposition (LPCVD) comprises the following steps: a) preparing a substrate having hexagonal surfaces cut at inclinations in different directions, with the inclination angles ranging from 2° to 10°; b) Physically transporting the vapor obtained from the heated gallium in the second zone (2) to the pump (4) / sample by means of a carrier gas (noble gas); c) Oxygen is introduced into the system through a separate ceramic or refractory metal tube and transferred directly onto the substrate at a distance of 0.1-4 cm and an angle of 0°-90°; d) creating a core layer of β-GaO on the surface such that the Ga:O surface atomic ratio on the growth surface is in the range of 10:1 to 1:10, ensuring that the surface atoms of Ga and O form β-GaO crystals on the heated substrate; e) growing a β-GaO core region to a thickness of 5 nm to 2000 nm at a growth rate of 10 nm / h to 500 nm / h; and f) Maintaining the growth process on the core layer prepared in the previous step so that the growth rate of β-Ga2O3 is in the range of 100 nm / h to 10 μm / h.

[0029] The substrate used in this specification is (0001) sapphire or (0001) SiC. (0001) sapphire and (0001) SiC have similar surface atomic packing. In fact, (-201) β-Ga2O3 conforms to these planes, and high-quality β-Ga2O3 structures have been obtained even on sapphire. While cost is not an issue, thermal conductivity is also a major motivation for SiC. As mentioned above, thermal conductivity is an essential limiting factor for high-power electronic devices.

[0030] Preferably, the substrate used in the present invention is (0001) oriented sapphire tilted at 6° to the <11-20> direction, thus ensuring that the grown β-Ga2O3 layer is single crystalline.

[0031] In the present invention, argon (Ar) is preferably used as the carrier gas (rare gas).Furthermore, the ceramic tube or refractory metal tube capable of feeding oxygen into the system is preferably made of quartz.

[0032] The optimal heteroepitaxially obtained β-Ga2O3 layer essentially depends on two conditions. First, nucleation is achieved by maintaining a low growth rate, which ranges between 10 nm and 500 nm. The nucleation stage is described in steps d) and e) above.

[0033] Next, the adatom (surface atom) density ratio on the growth surface in step f) is in the range of 8:1 to 1:4 for Ga and O, respectively. Preferably, this ratio is 2:3. Furthermore, in the present invention, the surface atom ratio of Ga:O on the growth surface in step d) is preferably 2:3.

[0034] The heteroepitaxial β-Ga2O3 crystal growth method of the present invention is characterized in that it includes, in step d), a step of maintaining the sample temperature at 925°C and adjusting the Ga crucible temperature to 795°C, and further includes a step of increasing the Ga crucible temperature to 920°C in step f).

[0035] In the system shown in Figure 1, when the sample temperature was set to 925°C, the Ar flow rate was 300 sccm, the distance between the Ga crucible and the sample was 23 cm, and the Ga crucible temperature was adjusted to 795°C, the scanning electron microscope surface image shown in Figure 2 was obtained, and the vibration curve scan shown in Figure 3 was measured. The deviation of the atomic steps can be observed quite clearly. Furthermore, the FWHM value of the XRC, which indicates the quality of the crystal, was a record high of 0.049. This value proves the technical effect of the present invention through technical data.

[0036] Under the same growth conditions, increasing the Ga crucible temperature to 920 °C increased the growth rate from 100 nm / h to 1000 nm / h, and the surface morphology (Figure 4) evolved into smoother, more perfectly aligned atomic steps. This was primarily due to the 4 sccm O2 flow providing a much higher number of oxygen surface atoms compared to the Ga surface atom count obtained from Ga vapor at 795 °C. Indeed, at 920 °C, the Ga surface atom count increased, while the oxygen and gallium surface atoms were obtained in relatively similar proportions. This confirmed the effectiveness of a value close to the optimal Ga:O surface atomic ratio, the first condition for high-quality growth. Even with the improved surface morphology, the XRC FWHM value increased to 0.158°. Thus, the second condition for achieving optimal heteroepitaxy—a low growth rate—was met. The reason behind the increase in growth rate from 100 nm / h to 1000 nm / h with increasing Ga temperature, as mentioned above, is that surface atoms form bonds by creating defects without finding suitable growth sites.

[0037] Briefly, heteroepitaxial CVD growth of β-Ga2O3 requires a two-stage growth: in the first stage, nucleation is ensured at approximately 100 nm / h with minimal defects, while the growth process continues on the high-quality nucleated β-Ga2O3 layer at a rate of 1000–3000 nm / h.

[0038] The growth process can also be carried out in an undoped state, since different doping elements (Ge, Sn, Si, etc.) and their molecules (N2, H2, SiCl4, etc.) can be used in both growth stages.

[0039] In an embodiment of the present invention, step f) in the growth method of the present invention is followed by step g) of vaporizing a solid source of Ge in the system or introducing SiCl gas mixed with a carrier gas (rare gas) into the system to perform n-type doping of the grown β-GaO.

[0040] Table 2 shows the X-ray diffraction signal width XRC FWHM values, which are indicative of the crystalline quality achieved by various research groups. As can be seen, a FWHM value of 0.0490, much lower than literature values, is obtained with the improved low-pressure deposition system of the present invention. This value was obtained with the growth method disclosed in the present invention. The lowest FWHM values ​​obtained from heteroepitaxial Ga2O3 layers grown by other methods and / or disclosed in other patent documents are about 0.4-0.5. This value demonstrates that the method of the present invention allows for the production of layers with extremely low defect densities. In fact, this value is comparable in quality to some cluster-grown β-Ga2O3 layers and is approximately the FWHM (0.014) of the best cluster-grown β-Ga2O3 substrates with record quality. 0 ) value. This particular difference is a natural result of dislocations formed after the growth process on different substrates and indicates sufficient crystalline quality for the fabricated devices. JPEG0007796020000002.jpg47166

[0041] The products obtained by the method of the present invention can be used in the manufacture of transistors and diodes capable of operating at very high voltages (20-30 kV) and currents (1000-3000 A) in the manufacture of charging stations for electric vehicles (voltage 600-1200 V, current 100 A), in the operation of defense industry products operating at high power, for example, in the manufacture of electromagnetic guns (rail guns).

[0042] Furthermore, it can be used in the manufacture of electronic devices for connecting solar power plants and wind turbines.

[0043] It can also be used to manufacture solar-blind photodetectors, detectors that enable missile tracking, and data transmission for underwater communications.

[0044] Abbreviations used herein LPCVD: Low-Pressure Chemical Vapor Deposition CVD: Chemical Vapor Deposition PECVD: Plasma-Enhanced Chemical Vapor Deposition MOCVD:Metal-Organic Chemical Vapor Deposition HVPE: Halogen Vapor-Phase Epitaxy MBE: Molecular Beam Epitaxy β-Ga2O3: β-gallium oxide

Claims

1. Heteroepitaxial β-Ga was grown by low-pressure chemical vapor deposition (LPCVD). ２ O ３ A method for growing (β-gallium oxide) crystals comprising the steps of: a) preparing a substrate having hexagonal surfaces cut at inclinations in different directions, with the inclination angle ranging from 2° to 10°; b) physically transporting the vapor obtained from the heated gallium in a second zone (2) located at a distance of at least 23 cm from the substrate to a pump (4) / sample by a carrier noble gas; c) introducing oxygen into the system through a separate ceramic or refractory metal tube and transferring the oxygen directly to the sample on the substrate at a distance of 0.1 to 4 cm and an angle of 0° to 90°; d) β-Ga on the surface so that the Ga:O surface atomic ratio on the growth surface is in the range of 10:1 to 1:

10. ２ O ３ By creating a core layer of Ga and O, the surface atoms of the β-Ga ２ O ３ To create crystals, e) β-Ga ２ O ３ growing the core layer to a thickness of 5 nm to 2000 nm at a growth rate of 10 nm / h to 500 nm / h; f) β-Ga ２ O ３ maintaining the growth process on the core layer produced in the previous step so that the growth rate is in the range of 1000 nm / h to 10 μm / h; Heteroepitaxial β-Ga ２ O ３ Crystal growth methods.

2. 2. The heteroepitaxial β-Ga substrate according to claim 1, wherein the Ga:O surface atomic ratio of the growth surface in step f) is in the range of 8:1 to 1:

4. ２ O ３ Crystal growth methods.

3. 3. The heteroepitaxial β-Ga substrate of claim 2, wherein the ratio of Ga:O surface atoms on the growth surface in step f) is 2:

3. ２ O ３ Crystal growth methods.

4. 2. The heteroepitaxial β-Ga substrate according to claim 1, wherein the ratio of Ga:O surface atoms on the growth surface in step d) is 2:3, respectively. ２ O ３ Crystal growth methods.

5. 2. The heteroepitaxial β-GaAs layer according to claim 1, characterized in that n-type or p-type layers are obtained by using different doping elements and / or molecules. ２ O ３ Crystal growth methods.

6. The doping elements used are selected from Ge, Sn, Si and / or the molecules used are N ２ , H ２ , SiCl ４ The heteroepitaxial β-Ga as claimed in claim 5, characterized in that it is selected from ２ O ３ Crystal growth methods.

7. The heteroepitaxial β-GaAs substrate according to claim 1, wherein the substrate is (0001) sapphire or (0001) SiC. ２ O ３ Crystal growth methods.

8. The heteroepitaxial β-Ga substrate according to claim 1 or 7, wherein the substrate is a (0001) oriented sapphire tilted 6° toward the <11-20> direction. ２ O ３ Crystal growth methods.

9. 2. The heteroepitaxial β-GaAs of claim 1, wherein the carrier rare gas is argon (Ar). ２ O ３ Crystal growth methods.

10. 2. The heteroepitaxial β-Ga as claimed in claim 1, wherein the ceramic or refractory metal tube is a quartz tube. ２ O ３ Crystal growth methods.

11. 2. The heteroepitaxial β-Ga crystal according to claim 1, wherein the step d) includes a step of maintaining the sample temperature at 925°C and adjusting the Ga crucible temperature to 795°C. ２ O ３ Crystal growth methods.

12. 2. The heteroepitaxial β-Ga alloy according to claim 1, wherein the step f) includes a step of increasing the Ga crucible temperature to 920° C. ２ O ３ Crystal growth methods.

13. Following the step f), g) a solid source of Ge is vaporized in the system or mixed with a carrier gas (rare gas) to produce SiCl ４ The gas is fed into the system and the grown β-Ga ２ O ３ The heteroepitaxial β-GaAs described in claim 1, 5 or 6, further comprising a step of n-doping the ２ O ３ Crystal growth methods.

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