Preparation method for reducing interfacial back-dissolution and interfacial stress of gallium nitride by flux method

Abstract

The present invention discloses a preparation method for reducing interface back-dissolution and interface stress of gallium nitride by the flux method. The preparation method includes: providing a gallium nitride seed crystal; performing low-doping growth of the gallium nitride seed crystal in a gallium-sodium melt at a first temperature and a first nitrogen pressure to form a low-doping layer; performing secondary growth of 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. By means of in-situ growth in two steps, the present invention first constructs a low-doping layer at low temperature and low pressure, and then performs regrowth based on the low-doping layer, reducing additional processing steps, effectively suppressing the problem of interface back-dissolution during the growth of gallium nitride by the flux method, reducing the occurrence of interface inclusions and voids, and at the same time, the introduction of the low-doping layer reduces the interface stress, thereby improving the quality of the grown crystal.

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 interface re-dissolution and interface stress of gallium nitride produced by a flux process. Background Art

[0002] Gallium nitride semiconductors are widely used in optoelectronics and power devices due to their excellent properties, including a wide bandgap, high breakdown field strength, and high stability. Currently, the main methods for preparing gallium nitride substrates include hydride vapor phase epitaxy, ammonothermal method, and flux method. The flux method offers the advantages of growing large gallium nitride single crystals with high quality, and holds great promise for industrialization.

[0003] Typically, the flux method involves mixing metallic gallium and sodium in a specific ratio. The presence of a gallium-sodium melt significantly increases the nitrogen solubility of the melt, enabling the growth of bulk GaN single crystals at appropriate temperatures and nitrogen pressures. However, the GaN substrate is typically placed at the bottom of a crucible. Because the nitrogen source in the melt comes from the dissolution of high-pressure nitrogen, the nitrogen concentration in the melt is too low during the initial growth phase. This can cause the seed crystal to decompose and dissolve back in a high-temperature, nitrogen-deficient environment. This can lead to problems such as voids or inclusions during subsequent growth, compromising the quality of the resulting crystal. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention aims to provide a preparation method for flux-processed gallium nitride that reduces interface dissolution and interface stress.

[0005] To achieve the aforementioned object of the invention, the technical solutions adopted by the present invention include:

[0006] In a first aspect, the present invention provides a method for reducing interfacial dissolution and interfacial stress of flux-processed gallium nitride, comprising:

[0007] Provide gallium nitride seed crystals;

[0008] At a first temperature and a first nitrogen pressure, the gallium nitride seed crystal is allowed to perform low-doping growth in a gallium-sodium melt to form a low-doping layer;

[0009] 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;

[0010] The first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure.

[0011] In a second aspect, the present invention further provides a gallium nitride crystal obtained by the above-mentioned preparation method, which comprises a seed crystal, a low-doped layer and a regrown layer stacked in sequence along a selected direction;

[0012] Among them, the seed crystal has a stronger fluorescence emission contrast than the low-doped layer, and the interface between the seed crystal and the low-doped layer is a flat surface.

[0013] Based on the above technical solutions, compared with the prior art, the beneficial effects of the present invention at least include:

[0014] 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 suppress the problem of interfacial back-dissolution 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.

[0015] The above description is only an overview of the technical solutions 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 in accordance with the content of the specification, the following describes the preferred embodiments of the present invention in conjunction with detailed drawings as follows. Description of the Drawings

[0016] Figure 1 is a process schematic diagram of the preparation method provided by a typical embodiment of the present invention;

[0017] Figure 2 is a cross-sectional cathodoluminescence image of a gallium nitride crystal provided by a typical embodiment of the present invention;

[0018] Figure 3 is a cross-sectional cathodoluminescence image of a gallium nitride crystal provided by a typical comparative example of the present invention. Detailed Embodiments

[0019] 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:

[0020] 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.

[0021] 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 when the growth of the target semiconductor crystal is initiated in the flux process.

[0022] Chinese invention patent publication number CN100532658C proposes a method for producing semiconductor crystals, mentioning that among different embodiments, one embodiment can form a protective film on the back to adjust the time it takes to dissolve in a flux.

[0023] The currently published prior art mentions the problem of seed crystal dissolution during the flux growth process. In order to solve the seed crystal dissolution problem, some solutions use AlN or AlGaN as a buffer layer to resist the dissolution of the seed crystal, but this adds additional processing steps and costs during the growth process. Other solutions have announced the growth of a highly doped layer at the interface, which cannot resist the dissolution of the seed crystal in the early stages of growth due to the high growth temperature and pressure. In addition, direct high temperature and high pressure growth will increase the level of impurity incorporation, further causing changes in the lattice parameters of the growth layer, increasing the stress between the growth layer and the seed crystal, and causing problems such as crystal cracking.

[0024] In view of the shortcomings of the prior art, the inventors of this case, after long-term research and extensive practice, have proposed the technical solution of the present invention. The following will further explain this technical solution, its implementation process and principles.

[0025] In the following description, many specific details are set forth to facilitate a full 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 to the specific embodiments disclosed below.

[0026] Moreover, relational terms such as “first” and “second” are merely used to distinguish one component or method step from another with the same name, but do not necessarily require or imply any actual relationship or order between these components or method steps.

[0027] The main technical idea of this invention is: to address the above problems, a low-impurity incorporation layer (called a low-doping layer) is designed through low-temperature and low-pressure growth in the early stage of growth. The growth of the low-doping layer belongs to low V / III ratio growth, which ensures low nitrogen supersaturation in the growth system and greatly suppresses the occurrence of interface dissolution. At the same time, the design of the low-doping layer can effectively relieve interface stress, thereby obtaining high-quality gallium nitride crystals.

[0028] Based on the above technical ideas, an embodiment of the present invention provides a preparation method for reducing interfacial dissolution and interfacial stress of flux-processed gallium nitride, which includes the following steps:

[0029] Providing a gallium nitride seed crystal. It should be noted that in the art, a gallium nitride seed crystal is used as a growth substrate. Therefore, in some prior arts, a gallium nitride seed crystal is often referred to as a substrate according to its function. It is understandable that the two terms are interchangeable.

[0030] At the first temperature and the first nitrogen pressure, the gallium nitride seed crystal is subjected to low-doping growth in a gallium-sodium melt to form a low-doping layer;

[0031] At the second temperature and the second nitrogen pressure, the gallium nitride seed crystal is subjected to secondary growth in a gallium-sodium melt to form a regrowth layer;

[0032] Wherein, the first temperature is lower than the second temperature, and the first nitrogen pressure is lower than the second nitrogen pressure.

[0033] 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.

[0034] Regarding the specific process conditions, in some embodiments, the first temperature is 500-700 °C, and the first nitrogen pressure is 2-3 MPa.

[0035] In some embodiments, the time for the low-doping growth is 3-5 h.

[0036] In some embodiments, the second temperature is 700-900 °C, and the second nitrogen pressure is 3-5 MPa.

[0037] In some embodiments, the time for the secondary growth is 10-100 h.

[0038] In some embodiments, the mass ratio of metallic gallium to metallic sodium in the gallium-sodium melt is (20:40):(60:80).

[0039] In some embodiments, the gallium-sodium melt further includes a nitride additive, and the nitride additive increases the nitrogen solubility of the gallium-sodium melt compared to when not added.

[0040] In some embodiments, the addition amount of the nitride additive in the gallium-sodium melt is 0.05%-0.5% atomic mass percentage.

[0041] In some embodiments, the nitride additive includes any one of lithium nitride, calcium nitride, etc., or a combination of two or more thereof.

[0042] As some typical implementation cases of the above technical solution, the growth raw materials of the preparation method are metallic gallium and sodium, as well as 0.05%-0.5% atomic mass of nitride additives to improve the N solubility in the melt in the early stage of growth.

[0043] The growth process is to prepare HVPE GaN seed crystals and adopt a two-step in-situ growth process:

[0044] In the first step, the low-doped layer is grown. The growth temperature of the reaction chamber is controlled to be 500~700℃, the nitrogen pressure is 2~3MPa, and the growth time is 3~5h.

[0045] In the second step, the regrowth layer is grown, and the growth temperature of the reaction chamber is controlled to be 700~900℃, the nitrogen pressure is 3~5MPa, and the growth time is 10~100h.

[0046] Of course, in actual applications, the process by which the seed crystal is prepared and its source, such as homemade or commercially purchased, can be appropriately selected according to the actual situation and is not limited to the HVPE gallium nitride seed crystal in the above example.

[0047] A second aspect of an embodiment of the present invention further provides a gallium nitride crystal prepared by the preparation method provided in any of the above-mentioned embodiments, which includes a seed crystal, a low-doped layer and a regrown layer stacked in sequence along a selected direction; wherein the seed crystal has a stronger fluorescence luminescence contrast than the low-doped layer, and the interface between the seed crystal and the low-doped layer is a flat surface.

[0048] In the present invention, the interface between the seed crystal and the growth layer can be observed using cathode fluorescence technology. The vicinity of the interface is found by a scanning electron microscope. Due to the different impurities incorporated in the flux growth method, its cathode fluorescence luminescence will be significantly different from that of the seed crystal, thereby judging the dissolution condition of the growth interface.

[0049] The technical solution of the present invention is further described in detail below through several embodiments and in conjunction with the accompanying drawings. However, the selected embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention.

[0050] Example 1

[0051] This embodiment provides a gallium nitride crystal growth process, which is specifically described as follows.

[0052] Prepare HVPE GaN seed crystals using two-step in-situ growth:

[0053] First step: growth of the low-doped layer. Control the growth temperature of the reaction chamber at 600 °C, the nitrogen pressure at 1.5 MPa, and the growth time at 4 h. Use lithium nitride or calcium nitride with an atomic mass ratio of gallium to sodium in the melt of 27:73 and accounting for 0.1% of the atomic mass of the gallium-sodium melt for growth to form a low-doped layer with a thickness of 3 - 5 μm.

[0054] Second step: growth of the regrown layer. Control the growth temperature of the reaction chamber at 800 °C, the nitrogen pressure at 4 MPa, and the growth time at 60 h. Use the same gallium-sodium melt in situ as in the first step for growth to form a regrown layer with a thickness of 600 μm.

[0055] The growth result of this embodiment is as 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, a stronger luminescence contrast of the seed crystal can be observed in Figure 2 . The interface transition between the seed crystal and the growth layer is flat, that is, there is no obvious back-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.

[0056] Example 2

[0057] This embodiment is generally the same as Example 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.

[0058] For the crystal obtained in this embodiment, its growth interface is also flat without back-dissolution, and the interface stress is small.

[0059] Comparative Example 1

[0060] This comparative example is generally the same as Example 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 result, observed through the cathodoluminescence image, shows an obvious V-shaped depression contrast distribution between the seed crystal and the growth layer, indicating an obvious seed crystal back-dissolution phenomenon at the growth interface, as Figure 3 shown.

[0061] Comparative Example 2

[0062] This comparative example is generally the same as Example 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 result shows that the AlN layer undergoes back-dissolution, the interface is uneven, showing obvious compressive stress and uneven stress distribution.

[0063] 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 the two-step in-situ growth, specifically, first growing a low-doped layer to reduce the nitrogen saturation of the melt to inhibit interface dissolution and interface growth stress, and then growing a high-quality gallium nitride crystal by growing a gallium nitride regrowth layer, thereby utilizing the low-doped layer growth to obtain a high-quality gallium nitride single crystal with no interface dissolution of the seed crystal and low interface stress.

[0064] It should be understood that the above embodiments are merely illustrative of the technical concepts and features of the present invention. Their purpose is to enable those skilled in the art to understand the contents of the present invention and implement them accordingly. They are not intended to limit the scope of protection of the present invention. Any equivalent changes or modifications made in accordance with the spirit and substance of the present invention are intended to be encompassed within the scope of protection of the present invention.

Claims

1. A preparation method for reducing interfacial back-dissolution and interfacial stress of gallium nitride by the flux method, characterized in that, Comprising: Providing a gallium nitride seed crystal; Under 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; Under 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; the first temperature is 500-700 °C, the first nitrogen pressure is 2-3 MPa; the time for the low-doping growth is 3-5 h; the second temperature is 700-900 °C, the second nitrogen pressure is 3-5 MPa; the time for the secondary growth is 10-100 h; the mass ratio of metallic gallium to metallic sodium in the gallium-sodium melt is (20-40):(60-80).

2. The preparation method according to claim 1, wherein The gallium-sodium melt further comprises a nitride additive, and the nitride additive increases the nitrogen solubility of the gallium-sodium melt compared to when not added.

3. The preparation method according to claim 2, wherein The addition amount of the nitride additive in the gallium-sodium melt is 0.05%-0.5% atomic mass percentage.

4. The preparation method according to claim 3, characterized in that, The nitride additive comprises any one or a combination of two or more of lithium nitride and calcium nitride. The gallium nitride crystal prepared by the preparation method according to any one of claims 1-4, characterized in that, Comprising 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 compared to the low-doping layer, and the interface between the seed crystal and the low-doping layer is a flat surface.

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