Single-crystal – oriented-crystal – and epitaxy growth processes; – Forming from vapor or gaseous state – With decomposition of a precursor
Utility Patent
1998-04-09
2001-01-02
Kunemund, Robert (Department: 1765)
Single-crystal, oriented-crystal, and epitaxy growth processes;
Forming from vapor or gaseous state
With decomposition of a precursor
C117S090000, C117S096000, C117S097000, C117S952000
Utility Patent
active
06168659
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for producing gallium nitride crystals that can be used in short wavelength semiconductor lasers, high temperature and high speed transistors and the like, for example.
Semiconductor lasers are widely used in reading and writing information from and into optical discs, respectively. Since the amount of information that can be stored in an optical disc per unit area is inversely Proportional to the square of wavelength of a semiconductor laser used, it is imperative to make the wavelength of laser as short as possible in order to realize a high density recording. Gallium nitride has a large forbidden band gap of 3.4 eV and also belongs to a direct transition semiconductor, thereby enabling the fabrication of a mixed crystal of aluminum nitride and indium nitride. Therefore, gallium nitride makes it possible to produce readily a semiconductor junction double heterostructure involving different forbidden band gaps, which is needed to realize semiconductor lasers and is expected to be useful as a material for short wavelength lasers having the wavelength of around 400 nm.
In addition, the gallium nitride has the features of a large forbidden band gap, an insulation break down electric field of as large as 5×10
6
V/cm and a saturation drift velocity of as high as 1.5×10
7
cm/s, showing great promise for a high temperature and high speed transistor material.
For the crystal growth of gallium nitride base materials, a heteroepitaxial growth method using sapphire as a substrate has been generally employed since an excellent gallium nitride substrate is not available. In addition to the organometallic vapor deposition and molecular beam epitaxy that are used in general, thick film growth according to a fast film forming method of halide VPE with a growth speed of 100 microns/hr or higher, wherein ammonia is used as a raw material of nitrogen and hydrogen chloride supplied via a heated gallium surface is used as a raw material of gallium, has been receiving an attention for a further improvement in crystallization and R&D in this area have been carried out actively.
The method for producing gallium nitride crystals according to the prior art halide VPE method with sapphire used as the substrate has been utilized to form gallium nitride crystals to a thickness of 100 micron or larger.
Next, an explanation will be made on the prior art method for producing gallium nitride crystals. The prior art method for producing gallium nitride crystals is shown in FIGS.
33
(
a
)-
33
(
b
), wherein the reference numeral
11
shows a sapphire substrate and the reference numeral
12
shows gallium nitride crystals.
The sapphire substrate
11
with a film thickness of 600 microns is heated to 1000° C., for example, and gallium nitride is formed thereon to 100 micron thick by having an ammonia gas, for example, and gallium chloride formed by supplying a hydrogen chloride gas via the surface of metal gallium that is heated at 850° C. reacted with each other. However, with the foregoing method for producing gallium nitride crystals, crystal strain energy due to a difference in lattice constant between sapphire and gallium nitride is stored in the sides of sapphire and gallium nitride when the gallium nitride is formed on the sappier substrate. Since the film thickness of gallium nitride is smaller than that of sapphire, the crystal strain energy per one lattice of gallium nitride is larger than that of sapphire, resulting in causing crystal defects such as crystal dislocation and the like at the side of gallium nitride and the difficulty in obtaining thick film gallium nitride crystals that are excellent in crystallization. Further, since the substrate is formed of sapphire that has no electric conductivity, a selective etching process has to be applied to nitride base semiconductors in order to form electrodes in such applications as semiconductor laser, light emitting diode and the like with a resulting complexity of production process. Furthermore, when compared with the case wherein electrodes are formed on both upper and lower surfaces of a substrate, series resistance becomes large with two electrodes formed only on the upper surface of the foregoing substrate, thereby causing a problem of increasing the operating voltage for the foregoing semiconductor laser and light emitting diode. Also, when the prior art method for producing gallium nitride is used in applying to transistors, a limit has been imposed on the handling output power of these transistors due to the limit in sapphire's thermal conductivity that is as small as 0.11 W/cm K.
SUMMARY OF THE INVENTION
The present invention deals with the problems involved with the prior art method for producing gallium nitride crystals and aims at supplying gallium nitride crystals that are excellent in crystallization, gallium nitride base semiconductor lasers and light emitting diodes that require a simple production process and have a low operating voltage, and gallium nitride base transistors that are capable of handling high output power.
A method for producing gallium nitride crystals of the present invention is characterized by forming gallium nitride thick film crystals after having a single crystal silicon thin film formed on an amorphous silicon dioxide thin film that was formed on a silicon substrate, whereby crystal defects such as crystal dislocation and the like caused at the time of heteroepitaxial growth made to be Produced in t side of single crystal silicon thin film formed on the foregoing amorphous silicon dioxide, not in the side of gallium nitride, thus resulting in gaining a knowledge of enabling the large reduction in the crystal defects of gallium nitride and the production of gallium nitride thick film crystals with excellent crystallization and so implemented according to the foregoing knowledge. Further, a knowledge has been gained to the effect that after having formed the foregoing gallium nitride thick film crystals the foregoing silicon substrate, silicon dioxide and silicon thin film are eliminated, thereby enabling the production of gallium nitride crystals that are electro-conductive and excellent in heat dissipation, and so implemented according to the foregoing knowledge.
A first type of embodiments of the invention have a structure, wherein after having an amorphous, silicon dioxide thin film formed on a silicon substrate and a single crystal silicon thin film formed on the foregoing silicon dioxide thin film, gallium nitride is formed on the foregoing silicon thin film. Accordingly, by forming the gallium nitride thick film on the single crystal silicon thin film that has been formed on the amorphous silicon dioxide thin film, such crystal defects as crystal dislocation and the like caused at the time of heteroepitaxial growth are produced not in the side of gallium nitride but in the side of the single crystal silicon thin film formed on the foregoing amorphous silicon dioxide, thereby enabling the large reduction of crystal defects in gallium nitride and the forming of gallium nitride thick film crystals that are excellent in crystallization.
A second type of embodiments of the invention have a structure, wherein after having an amorphous silicon dioxide formed on a silicon substrate and a single crystal thin film formed on the foregoing silicon dioxide thin film, gallium nitride is formed on the foregoing silicon thin film and then, in succession, the foregoing silicon substrate and silicon dioxide and silicon films are eliminated. Accordingly, in the same way as in the invention of the first type of embodiments, the crystal defect are allowed to decline substantially, thereby enabling the forming of gallium nitride thick film crystals that are excellent in crystallization. In having a pn junction structure of gallium nitride base semiconductor laser and light emitting diode formed on this gallium nitride film, each respective electrode can be formed on both surfaces of the gallium nitride-since the foregoing silicon icon subs
Baba Takaaki
Ueda Tetsuzo
Yuri Masaaki
Kunemund Robert
Matsushita Electronics Corporation
McDermott & Will & Emery
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