Preparation method for reducing flux method gallium nitride interface redissolution and interface stress

Abstract

The invention discloses a preparation method for reducing flux method gallium nitride interface redissolution and interface stress. The preparation method comprises the following steps: providing a gallium nitride seed crystal; carrying out low-doping growth on the gallium nitride seed crystal in the gallium-sodium melt at a first temperature and a first nitrogen pressure to form a low-doping layer; carrying out secondary growth on the gallium nitride seed crystal in the gallium-sodium melt at a second temperature and a second nitrogen pressure to form a regrowth layer; wherein the first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure. According to the method, in-situ growth is carried out through a two-step method, the low-temperature and low-pressure low-doped layer is constructed at first, then regrowth is carried out based on the low-doped layer, additional processing steps are reduced, the problem of interface redissolution in the growth process of the flux method gallium nitride can be effectively inhibited, interface inclusions and cavities are reduced, and meanwhile, due to introduction of the low-doped layer, the growth efficiency of the gallium nitride is improved. And the interface stress is reduced, so that the quality of the grown crystal is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of gallium nitride semiconductors, and in particular to a preparation method for reducing the interfacial back-dissolution and interfacial stress of gallium nitride by the flux method. Background Art

[0002] Gallium nitride semiconductors are widely used in optoelectronic devices and power devices due to their excellent properties such as large bandgap width, high breakdown field strength, and high stability. At present, the main preparation methods of gallium nitride substrates include hydride vapor phase epitaxy, ammonothermal method, and flux method. The flux method has the advantages of large growth size and high quality of gallium nitride single crystals, and has great industrialization prospects.

[0003] Generally, the flux method is to mix metallic gallium and sodium in a certain proportion. Under the action of the gallium-sodium melt, the nitrogen solubility in the melt increases significantly, and the growth of gallium nitride bulk single crystals is realized at a suitable temperature and nitrogen pressure. However, the gallium nitride substrate is generally placed at the bottom of the crucible. Since the nitrogen source in the melt comes from the dissolution of high-pressure nitrogen, the nitrogen concentration in the melt is too low in the initial stage of growth, and the seed crystal will decompose and back-dissolve in the nitrogen-deficient environment at high temperature, which may cause problems such as voids or inclusions being wrapped in the subsequent growth process, thus affecting the quality of the grown crystal. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a preparation method for reducing the interfacial back-dissolution and interfacial stress of gallium nitride by the flux method.

[0005] To achieve the foregoing invention purpose, the technical solutions adopted by the present invention include: In the first aspect, the present invention provides a preparation method for reducing the interfacial back-dissolution and interfacial stress of gallium nitride by the flux method, which includes: Providing a gallium nitride seed crystal; At a first temperature and a first nitrogen pressure, subjecting the gallium nitride seed crystal to low-doping growth in a gallium-sodium melt to form a low-doping layer; At a second temperature and a second nitrogen pressure, subjecting the gallium nitride seed crystal to secondary growth in a gallium-sodium melt to form a regrowth layer; Wherein, the first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure.

[0006] In the second aspect, the present invention also provides a gallium nitride crystal prepared by the above preparation method, which includes a seed crystal, a low-doping layer, and a regrowth layer stacked in sequence along a selected direction; Wherein, the seed crystal has a stronger fluorescence emission contrast than the low-doping layer, and the interface between the seed crystal and the low-doping layer is a flat surface.

[0007] Based on the above technical solutions, compared with the prior art, the beneficial effects of the present invention at least include: The preparation method provided by the present invention grows in-situ through a two-step method. First, a low-doped layer with low temperature and low pressure is constructed, and then regrowth is carried out based on the low-doped layer, reducing additional processing steps. It can effectively inhibit the interfacial back-dissolution problem during the growth of gallium nitride by the flux method, reduce the appearance of interfacial inclusions and voids. At the same time, the introduction of the low-doped layer reduces the interfacial stress, thereby improving the quality of the grown crystal.

[0008] The above description is only an overview of the technical solution of the present invention. In order to enable those skilled in the art to more clearly understand the technical means of the present application and implement it according to the content of the specification, the following takes the preferred embodiments of the present invention and combines detailed drawings for description as follows. Description of the Drawings

[0009] Figure 1 is a process schematic diagram of the preparation method provided by a typical embodiment of the present invention; Figure 2 is a cross-sectional cathodoluminescence image of a gallium nitride crystal provided by a typical embodiment of the present invention; Figure 3 is a cross-sectional cathodoluminescence image of a gallium nitride crystal provided by a typical comparative case of the present invention. Detailed Embodiments

[0010] As described in the above background art, some technical solutions have proposed technical solutions for growing gallium nitride crystals by a two-step method. For example: The Chinese invention patent with the publication number CN114657640A proposes a high-quality gallium nitride bulk single crystal, its growth method and preparation system, and discloses a method for growing a high-doped gallium nitride layer on a seed crystal, and then growing three-dimensional island-shaped gallium nitride and two-dimensional gallium nitride on the high-doped gallium nitride layer to relieve stress.

[0011] The Chinese invention patent with the publication number CN101410557A proposes a semiconductor substrate, an electronic device, an optical device and their manufacturing methods, and discloses that the GaN layer can be dissolved to a certain extent in the flux according to the time of initiating the growth of the target semiconductor crystal in the flux process.

[0012] The Chinese invention patent with the publication number CN100532658C proposes a production method of a semiconductor crystal, and mentions that in different embodiments, there is an embodiment where a protective film can be formed on the back to adjust the dissolution time in the flux.

[0013] The problem of seed crystal dissolution during the flux growth process has been mentioned in the currently disclosed prior art. In some solutions to solve the problem of seed crystal back-dissolution, AlN or AlGaN etc. are used as buffer layers to resist the dissolution of the seed crystal, but this adds additional processing steps and costs during the growth process. And in some other disclosed solutions, growing a highly doped layer at the interface cannot resist the dissolution of the seed crystal at the initial stage of growth due to the relatively high growth temperature and pressure. In addition, direct high-temperature and high-pressure growth will increase the incorporation level of impurities, further cause changes in the lattice parameters of the growth layer, increase the stress between the growth layer and the seed crystal, and cause problems such as crystal cracking.

[0014] In view of the deficiencies in the prior art, the inventors of this case have, through long-term research and a large number of practices, been able to propose the technical solution of the present invention. The following will further explain the technical solution, its implementation process, principle, etc.

[0015] In the following description, many specific details are set forth in order to provide a thorough understanding of the present invention. However, the present invention may also be implemented in other ways different from those described herein. Therefore, the scope of protection of the present invention is not limited by the specific embodiments disclosed below.

[0016] Moreover, relative terms such as "first" and "second" are only used to distinguish one component or method step with the same name from another, and do not necessarily require or imply any such actual relationship or order between these components or method steps.

[0017] The main technical idea of the present invention is: aiming at the above problems, through low-temperature and low-pressure growth at the initial stage of growth, a low-impurity incorporation layer (referred to as a low-doped layer) is designed. The growth of the low-doped layer belongs to low V / III ratio growth, which ensures a relatively low nitrogen supersaturation in the growth system, greatly inhibits the generation of interface back-dissolution phenomenon, and at the same time the design of the low-doped layer can effectively relieve the interface stress, thereby obtaining high-quality gallium nitride crystals.

[0018] Based on the above technical idea, an embodiment of the present invention provides a preparation method for reducing the interface back-dissolution and interface stress of gallium nitride by the flux method, which includes the following steps: Provide a gallium nitride seed crystal. It should be noted that in the art, the gallium nitride seed crystal is used as a substrate for growth. Therefore, in some prior arts, the gallium nitride seed crystal is often referred to as a substrate according to its function. It can be understood that these two names are interchangeable; At a first temperature and a first nitrogen pressure, carry out low-doped growth of the gallium nitride seed crystal in a gallium-sodium melt to form a low-doped layer; At a second temperature and a second nitrogen pressure, carry out secondary growth of the gallium nitride seed crystal in a gallium-sodium melt to form a regrowth layer; Among them, the first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure.

[0019] The principle of the above technical solution is as follows: The nitrogen saturation in the gallium-sodium melt is related to the system temperature and pressure. Under low temperature and low pressure conditions, the nitrogen saturation of the melt is low. If the growth conditions are in a high temperature and high pressure state at the initial stage of growth, the nitrogen in the melt cannot be dissolved to the saturated state through the external nitrogen source in a short time, then the gallium nitride seed crystal will be decomposed to a certain extent to promote the saturation of nitrogen in the melt to achieve dynamic equilibrium. Therefore, by reducing the temperature and pressure at the initial stage of growth, it is equivalent to reducing the nitrogen saturation in the melt, which can effectively inhibit the dissolution of the interface seed crystal. This situation belongs to low V / III ratio growth, which can reduce the incorporation of impurities during the growth process, and is called the low doping layer here. At the same time, the existence of the low doping layer can reduce the crystal lattice change caused by the incorporation of impurities, improve the lattice matching degree between the seed crystal and the growth layer, thereby reducing the interface stress caused by the lattice matching problem and improving the crystal growth quality.

[0020] Regarding the specific process conditions, in some embodiments, the first temperature is 500-700 °C, and the first nitrogen pressure is 2-3 MPa.

[0021] In some embodiments, the time for low doping growth is 3-5 h.

[0022] In some embodiments, the second temperature is 700-900 °C, and the second nitrogen pressure is 3-5 MPa.

[0023] In some embodiments, the time for secondary growth is 10-100 h.

[0024] In some embodiments, the mass ratio of metallic gallium to metallic sodium in the gallium-sodium melt is (20:40):(60:80).

[0025] In some embodiments, the gallium-sodium melt further includes a nitride additive, and the nitride additive can increase the nitrogen solubility of the gallium-sodium melt compared with the case without addition.

[0026] In some embodiments, the addition amount of the nitride additive in the gallium-sodium melt is 0.05%-0.5% atomic mass percentage.

[0027] In some embodiments, the nitride additive includes any one or a combination of two or more of lithium nitride, calcium nitride, etc.

[0028] As some typical implementation cases of the above technical solution, the growth raw materials of the preparation method are metallic gallium, sodium, and a nitride additive with 0.05%-0.5% atomic mass to increase the N solubility in the melt at the initial stage of growth.

[0029] The growth process flow is as follows: prepare an HVPE gallium nitride seed crystal and adopt two-step in-situ growth: In the first step, for the growth of the low-doped layer, control the growth temperature of the reaction chamber at 500 - 700 °C, the nitrogen pressure at 2 - 3 MPa, and the growth time at 3 - 5 h.

[0030] In the second step, for the growth of the regrowth layer, control the growth temperature of the reaction chamber at 700 - 900 °C, the nitrogen pressure at 3 - 5 MPa, and the growth time at 10 - 100 h.

[0031] Of course, in actual applications, regarding how the seed crystal is prepared and its source, such as self-made or commercially purchased, appropriate selections can be made according to the actual situation, and it is not limited to the HVPE gallium nitride seed crystal in the above example.

[0032] The second aspect of the embodiments of the present invention also provides a gallium nitride crystal prepared by the preparation method provided in any of the above embodiments, which includes a seed crystal, a low-doped layer, and a regrowth layer stacked in sequence along a selected direction; wherein, the seed crystal has a stronger fluorescence emission contrast compared to the low-doped layer, and the interface between the seed crystal and the low-doped layer is a flat surface.

[0033] In the present invention, the interface between the seed crystal and the growth layer can be observed by using cathodoluminescence technology. By finding the vicinity of the interface through a scanning electron microscope, due to the different incorporation of impurities in the growth by the flux method, its cathodoluminescence will be significantly different from that of the seed crystal, thereby the dissolution situation of the growth interface can be judged.

[0034] The technical solutions of the present invention will be further described in detail below through several embodiments in combination with the drawings. However, the selected embodiments are only used to illustrate the present invention and do not limit the scope of the present invention.

[0035] Example 1 This example provides a growth process for a gallium nitride crystal, which is specifically as follows.

[0036] Prepare an HVPE gallium nitride seed crystal and adopt two-step in-situ growth: In the first step, for the growth of the low-doped layer, control the growth temperature of the reaction chamber at 600 °C, the nitrogen pressure at 1.5 MPa, the growth time at 4 h, and use a gallium-sodium melt with an atomic mass ratio of 27:73 and grow with lithium nitride or calcium nitride accounting for 0.1% of the atomic mass of the gallium-sodium melt to form a low-doped layer with a thickness of 3 - 5 μm.

[0037] In the second step, for the growth of the regrowth layer, control the growth temperature of the reaction chamber at 800 °C, the nitrogen pressure at 4 MPa, the growth time at 60 h, and use the same gallium-sodium melt as in the first step for in-situ growth to form a regrowth layer with a thickness of 600 μm.

[0038] The growth results of this embodiment are as follows Figure 2 shown. Due to the significant difference in luminescence caused by the different impurity incorporation levels between the seed crystal and the growth layer, in Figure 2 it can be observed that the luminescence contrast of the seed crystal is stronger, the interface transition between the seed crystal and the growth layer is flat, that is, there is no obvious re-dissolution phenomenon at the growth interface, and a low-doped layer with a lower contrast can be seen above the interface. At the same time, the interface stress during growth is low.

[0039] Example 2 This embodiment is generally the same as Embodiment 1, except that the gallium nitride seed crystal is replaced with a sapphire substrate covered with a gallium nitride material layer with a thickness of 3 - 5 μm.

[0040] For the crystal obtained in this embodiment, its growth interface is also flat without re-dissolution, and the interface stress is small.

[0041] Comparative Example 1 This comparative example is generally the same as Embodiment 1, except that the growth of the low-doped layer in the first step is not carried out, and the growth in the second stage is directly carried out. The growth results, observed through the cathodoluminescence image, show an obvious V-shaped depression contrast distribution between the seed crystal and the growth layer, indicating that there is an obvious seed crystal re-dissolution phenomenon at the growth interface, as Figure 3 shown.

[0042] Comparative Example 2 This comparative example is generally the same as Embodiment 1, except that the low-doped layer in the first step is replaced with a 3 - 5 μm AlN buffer layer grown by MOCVD and then the second step of growth is directly carried out. The growth results show that the AlN layer has re-dissolution, the interface is uneven, showing obvious compressive stress and uneven stress distribution.

[0043] Based on the above embodiments and comparative examples, it can be clearly seen that the key technical means of the technical solution provided by the embodiments of the present invention lies in two-step in-situ growth. Specifically, first, the growth of the low-doped layer is carried out to reduce the nitrogen saturation of the melt and inhibit interface re-dissolution and interface growth stress, and then the growth of the gallium nitride re-growth layer is carried out to grow high-quality gallium nitride crystals, so as to obtain high-quality gallium nitride single crystals with no interface re-dissolution of the seed crystal and low interface stress by using the growth of the low-doped layer.

[0044] It should be understood that the above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those skilled in the art to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A preparation method for reducing the interface dissolution and interface stress of flux-processed gallium nitride, characterized in that: include: Providing gallium nitride seed crystals; At a first temperature and a first nitrogen pressure, the gallium nitride seed crystal is low-doped and grown in a gallium-sodium melt to form a low-doped layer; At a second temperature and a second nitrogen pressure, the gallium nitride seed crystal is allowed to undergo secondary growth in a gallium-sodium melt to form a regrown layer; The first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure.

2. The preparation method according to claim 1, characterized in that: The first temperature is 500-700° C., and the first nitrogen pressure is 2-3 MPa.

3. The preparation method according to claim 1, characterized in that: The low-doping growth time is 3 to 5 hours.

4. The preparation method according to claim 1, characterized in that: The second temperature is 700-900° C., and the second nitrogen pressure is 3-5 MPa.

5. The preparation method according to claim 1, characterized in that: The secondary growth time is 10 to 100 hours.

6. The preparation method according to claim 1, characterized in that: The atomic mass ratio of metallic gallium to metallic sodium in the gallium-sodium melt is (20-40):(60:80).

7. The preparation method according to claim 1 or 6, characterized in that: The gallium-sodium melt also includes a nitride additive, which increases the nitrogen solubility of the gallium-sodium melt compared to when the nitride additive is not added.

8. The preparation method according to claim 7, characterized in that: The amount of the nitride additive added to the gallium-sodium melt is 0.05%-0.5% by atomic mass.

9. The preparation method according to claim 7, characterized in that: The nitride additive includes any one of lithium nitride and calcium nitride or a combination of two or more thereof.

10. The gallium nitride crystal obtained by the preparation method according to any one of claims 1 to 9, characterized in that: It includes a seed crystal, a low-doped layer and a regrown layer stacked in sequence along a selected direction; The seed crystal has a stronger fluorescence contrast than the low-doped layer, and the interface between the seed crystal and the low-doped layer is a flat surface.

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