Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Reexamination Certificate
2002-03-26
2004-11-30
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S094000, C117S106000, C117S915000, C117S952000
Reexamination Certificate
active
06824610
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for producing a gallium nitride crystal substrate, and a gallium nitride crystal substrate.
2. Prior Art
GaN compound semiconductors, such as gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (GaAlN), have drawn attention as materials for blue light-emitting diodes (LEDs) and laser diodes (LDs). Further, by virtue of good heat resistance and environmental resistance, GaN compound semiconductors have been used in the optical devices, as well as in the development of elements for electronic devices utilizing these features.
In the GaN compound semiconductors, however, it is difficult to grow bulk crystal, and, for this reason, GaN substrates, which can be put to practical use, have not been produced yet. Sapphire is a substrate for the growth of GaN which is presently extensively put to practical use, and it is common practice to epitaxially grow GaN on a single crystal sapphire substrate, for example, by metal-organic vapor phase epitaxy (MOVPE).
The sapphire substrate is different from GaN in lattice constant, and, thus, when GaN is grown directly on the sapphire this problem, Japanese Patent Laid-Open No. 188983/1988 discloses a method wherein a buffer layer of AlN or GaN is once grown at a low temperature on the sapphire substrate, for reducing strain of lattice and GaN is then grown on the buffer layer.
Even in the case of the growth of GaN using the low-temperature grown buffer layer, however, a difference in lattice constant between the substrate and the crystal occurs, and, consequently, GaN has numerous defects which are expected to be an obstacle to the production of GaN-base LDs. Further, due to the difference in a coefficient of linear expansion between the sapphire substrate and GaN, warpage occurs in the substrate after epitaxy, and, in the worst case, the substrate is disadvantageously cracked.
For this reason, the development of GaN bulk substrates has been eagerly desired.
Although the growth of a large bulk GaN crystal is very difficult, a method has recently been proposed wherein a thick film of GaN is heteroepitaxially grown on a substrate, for example, by HVPE and the substrate is then removed to provide a free standing substrate of GaN.
By the way, at the present time, any technique for separating GaN grown on the sapphire substrate by etching has not been developed. Although an attempt to mechanically remove the sapphire substrate by polishing has also been made, the warpage of the substrate is increased in the process of polishing and the probability of cracking of the substrate is high. For this reason, this method has not been put to practical use.
Here Jpn. J. Appl. Phys. Vol. 38 (1999) Pt. 2, No. 3A reports a method wherein, after the growth of a thick GaN film on a sapphire substrate by HVPE, laser pulses are applied to separate only the GaN film. However, this method also is likely to cause cracking of the substrate.
Further, Japanese Patent Laid-Open No. 12900/2000 discloses a method using an easily removable substrate. In this method, a thick GaN film is grown on a GaAs substrate by HVPE and the GaAs substrate is then removed by etching. According to this method, a large GaN substrate can be prepared in a relatively high yield. This method, however, has a problem that the GaAs substrate is disadvantageously decomposed during the growth of the GaN crystal and arsenic (As) is included as an impurity into the GaN.
Selective growth using a patterned mask is effective for reducing the defect density of epitaxially grown GaN, and, for example, Japanese Patent Laid-Open No, 312971/1998 discloses a technique for this. Since, however, there is no method for easily separating the substrate, the above technique cannot be effectively utilized in the production of a free standing substrate of GaN.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to solve the problems of the prior art and to provide a process for producing, in a simple manner, a gallium nitride crystal substrate, which has low defect density and has not been significantly contaminated with impurities, and a gallium nitride crystal substrate produced by the production process.
According to the first feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing a gallium nitride film on the metal film to form a laminate substrate, and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the second feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the metal film;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the third feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
depositing a metal film on a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing a gallium nitride film on the metal film;
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material on the gallium nitride film;
again depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
According to the fourth feature of the invention, a process for producing a gallium nitride crystal substrate, comprises the steps of:
forming a mask region and a gallium nitride selective growth region formed of a patterned mask material an a starting substrate, which is any one of a single crystal sapphire substrate, a substrate comprising a single crystal gallium nitride film grown on a sapphire substrate, and a single crystal semiconductor substrate;
depositing, using the selective growth region as an origin, a gallium nitride film on the selective growth region and the mask region;
forming a metal film on the gallium nitride film;
again depositing a gallium nitride film on the metal film to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film again deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production processes according to the first to fourth features of the invention having the above respective constructions may further comprise the steps of;
depositing a metal film on a starting substrate, which is the free standing gallium nitride crystal substrate;
depositing a gallium nitride film on the metal film to form a laminate substrate; and
removing the starting substrate from the laminate substrate with the gallium nitride film deposited thereon to prepare a free standing gallium nitride crystal substrate.
The production process
Kuroda Naotaka
Shibata Masatomo
Barnes & Thornburg LLP
Conte James B.
Kunemund Robert
NEC Corporation
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