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Method for growing group III-nitride crystals in supercritical ammonia using an autoclave 実績あり

外国特許コード F120006301
整理番号 E06710WO
掲載日 2012年3月12日
出願国 アメリカ合衆国
出願番号 92139605
公報番号 20100158785
公報番号 8709371
出願日 平成17年7月8日(2005.7.8)
公報発行日 平成22年6月24日(2010.6.24)
公報発行日 平成26年4月29日(2014.4.29)
国際出願番号 US2005024239
国際公開番号 WO2007008198
国際出願日 平成17年7月8日(2005.7.8)
国際公開日 平成19年1月18日(2007.1.18)
優先権データ
  • 2005US024239 (2005.7.8) WO
発明の名称 (英語) Method for growing group III-nitride crystals in supercritical ammonia using an autoclave 実績あり
発明の概要(英語) A method of growing high-quality, group-III nitride, bulk single crystals.
The group III-nitride bulk crystal is grown in an autoclave in supercritical ammonia using a source material or nutrient that is a group III-nitride polycrystals or group-III metal having a grain size of at least 10 microns or more and a seed crystal that is a group-III nitride single crystal.
The group III-nitride polycrystals may be recycled from previous ammonothermal process after annealing in reducing gas at more then 600° C.
The autoclave may include an internal chamber that is filled with ammonia, wherein the ammonia is released from the internal chamber into the autoclave when the ammonia attains a supercritical state after the heating of the autoclave, such that convection of the supercritical ammonia transfers source materials and deposits the transferred source materials onto seed crystals, but undissolved particles of the source materials are prevented from being transferred and deposited on the seed crystals.
従来技術、競合技術の概要(英語) BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention is related to the growth of Group-III nitride crystals, and more particularly, to the growth of Group-III nitride crystals in supercritical ammonia using an autoclave.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x].
A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled "References." Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN) and its ternary and quaternary alloys incorporating aluminum and indium (AlGaN, InGaN, AlInGaN) has been well established for fabrication of visible and ultraviolet optoelectronic devices and high-power electronic devices.
These devices are typically grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide since GaN wafers are not yet available.
The heteroepitaxial growth of group III-nitride causes highly defected or even cracked films, which deteriorate the performance and reliability of these devices.
In order to eliminate the problems arising from the heteroepitaxial growth, group III-nitride wafers sliced from bulk crystals must be used.
However, it is very difficult to grow a bulk crystal of group III-nitride, such as GaN, AlN, and InN, since group III-nitride has a high melting point and high nitrogen vapor pressure at high temperature.
Up to now, a couple of methods, such as high-pressure high-temperature synthesis [1,2] and sodium flux [3,4], have been used to obtain bulk group III-nitride crystals.
However, the crystal shape obtained by these methods is a thin platelet because these methods are based on a melt of group III metal, in which nitrogen has very low solubility and a low diffusion coefficient.
A new technique called ammonothermal growth has the potential for growing large bulk group III-nitride crystals, because supercritical ammonia used as a fluid has high solubility of source materials, such as group III-nitride polycrystals or group III metal, and has high transport speed of dissolved precursors.
This ammonothermal method [5-9] has a potential of growing large bulk group III-nitride crystals.
However, in the previously disclosed technique, there was no quantitative assessment for the grain size of the source material.
If GaN or AlN is chosen as a source material, the only commercially available form is a powder of a size less than 10 microns, and usually 0.1.about.1 microns.
This small powder is easily blown by the convective flow of supercritical ammonia and transported onto the seed crystals, resulting in polycrystalline growth.
The main idea of the ammonothermal growth is taken from a successful mass production of artificial quartz by hydrothermal growth.
In the hydrothermal growth of artificial quartz, an autoclave is divided into two regions: a top region and a bottom region.
Source material, known as the nutrient, such as polycrystalline SiO2, is placed in the bottom region and seed crystals, such as single crystalline SiO2, are placed in the top region.
The autoclave is filled with water and a small amount of chemicals known as mineralizers are added to the water to increase the solubility of SiO2.
Sodium hydroxide or sodium carbonate is a typical mineralizer.
In addition, the temperature in the bottom region is kept higher than that in the top region.
In the case of ammonothermal growth, ammonia is used as a fluid.
It is challenging to fill the autoclave with liquid ammonia safely, without contamination.
In particular, oxygen is a detrimental impurity source in ammonothermal growth.
Both the ammonia and mineralizers favor oxygen and moisture.
Therefore, it is very important to load all solid sources and ammonia in an air-free environment.
Another important issue is the boiling point of ammonia.
In the case of hydrothermal growth, water is in a liquid phase at room temperature.
However, ammonia is in a gas phase at room temperature and the vapor pressure at room temperature is about 150 psi.
It is necessary to cool the autoclave and condense gaseous ammonia to fill liquid ammonia into an autoclave or an internal chamber.
When the size of the autoclave is small (e.g., small enough to fit in a glove-box), all solid sources (i.e., nutrient, mineralizers, seed crystals, etc.) can be loaded into the autoclave in a glovebox, and ammonia can be condensed in the autoclave by cooling the entire autoclave.
However, when the autoclave is large (e.g., too large to fit in a glove-box), it is practically very difficult to cool the entire autoclave to condense the ammonia.
These difficulties can be solved by using an internal chamber within the autoclave.
However, use of an internal chamber creates another problem, which is to balance pressure inside and outside of the internal chamber.
Notwithstanding the above, what is needed in the art are new methods for the growth of group-III nitride structures, as well as new apparatus for performing such methods.
The present invention satisfies these needs.

特許請求の範囲(英語) [claim1]
1. A method for growing group III-nitride crystals, comprising: (a) loading source materials and seed crystals into a reaction vessel, wherein part or all of the source materials are prepared by a recycling process for a nutrient used in a previous ammonothermal process or fragments of the group III-nitride crystals grown in the previous ammonothermal process and the recycling process includes annealing the nutrients or the fragments at more than 600 deg. C. in a reducing environment;
(b) filling the reaction vessel with ammonia; and
(c) raising the reaction vessel's temperature to attain a supercritical state for the ammonia wherein convection of the supercritical ammonia transfers the source materials and deposits the transferred source materials onto the seed crystals.
[claim2]
2. The method of claim 1, wherein the source materials have a grain size of at least 10 microns.
[claim3]
3. The method of claim 1, wherein the source materials are a group-III nitride polycrystals.
[claim4]
4. The method of claim 3, wherein the group III-nitride polycrystals are synthesized from group III halides.
[claim5]
5. The method of claim 4, wherein the group III-nitride is GaN.
[claim6]
6. The method of claim 1, wherein the source materials are a group-III metal.
[claim7]
7. The method of claim 1, wherein the source materials are a mixture of group-III metal and group-III nitride polycrystals.
[claim8]
8. The method of claim 7, wherein the group III-nitride is GaN.
[claim9]
9. The method of claim 1, wherein the seed crystals are group-III nitride crystals.
[claim10]
10. The method of claim 1, wherein the reducing environment contains hydrogen or ammonia.
[claim11]
11. The method of claim 1, wherein the reaction vessel has longer dimension along the vertical direction, the reaction vessel is divided into a top region and a bottom region with a baffle plate therebetween, the source materials and seed crystals are placed in separate ones of the top and bottom regions, and the top region is kept at a different temperature than the bottom region.
[claim12]
12. The method of claim 1, wherein the source materials are held in a mesh basket and the mesh basket is made of Ni or Ni-based alloy that contains at least 30% of Ni.
[claim13]
13. The method of claim 1, wherein undissolved particles of the source materials are prevented from being transferred and deposited on the seed crystals during the convection of the supercritical ammonia.
[claim14]
14. A method for growing group III-nitride crystals, comprising: (a) loading source materials and seed crystals into a reaction vessel;
(b) filling an internal chamber of the reaction vessel with ammonia;
(c) raising the reaction vessel's temperature to attain a supercritical state for the ammonia, wherein convection of the supercritical ammonia transfers the source materials and deposits the transferred source materials onto the seed crystals; and
(d) releasing the ammonia from the internal chamber into the reaction vessel, when the ammonia attains the supercritical state, such that the released ammonia fills a space between the internal chamber's outer walls and the reaction vessel's inner walls, to balance the pressure between inside and outside of the internal chamber.
[claim15]
15. The method of claim 14, wherein the source materials are held in a mesh basket and the mesh basket is made of Ni or Ni-based alloy that contains at least 30% of Ni.
  • 発明者/出願人(英語)
  • FUJITO KENJI
  • HASHIMOTO TADAO
  • NAKAMURA SHUJI
  • JAPAN SCIENCE AND TECHNOLOGY AGENCY
国際特許分類(IPC)
米国特許分類/主・副
  • 423/409
  • 117/84
参考情報 (研究プロジェクト等) ERATO NAKAMURA Inhomogeneous Crystal AREA
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