Semiconductor device manufacturing: process – Making device or circuit emissive of nonelectrical signal – Compound semiconductor
Reexamination Certificate
1999-10-08
2001-10-23
Pham, Long (Department: 2823)
Semiconductor device manufacturing: process
Making device or circuit emissive of nonelectrical signal
Compound semiconductor
C438S689000
Reexamination Certificate
active
06306675
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to heteroepitaxial layer formation in the fabrication of semiconductor devices and, more particularly, to such formation on a silicon carbide substrate surface.
Increasingly, laser diodes (LD) and light emitting diodes (LED) based on gallium nitride (GaN) thin films are being used because of their range of electrical, thermal, and optical properties. For a GaN-based laser diode to have a long life, i.e. several thousand hours of continuous operation, it is important to minimize the density of defects (usually in the form of dislocations) in the GaN film. This requires a defect density of less than about 1×10
7
cm
−2
(i.e. less than 10 million defects per square centimeter).
One attempt at achieving such a low defect density involves complex schemes of growing the GaN film epitaxially on a sapphire substrate. These growth schemes are time consuming, and their use impedes commercialization of devices based on GaN.
Another attempt is to grow the GaN thin film using a silicon carbide (SiC) substrate with a buffer layer of aluminum nitride (AlN) between the substrate and the GaN film. Such an attempt has potential because the GaN/AlN/SiC system has close matching of lattice parameters, namely 3.19/3.11/3.08 Å. Prior heteroepitaxial structures of GaN and AlN on a SiC substrate have been fabricated as described in Torres et al., “Growth of AlN and GaN on 6H-SiC(0001) Using a Helium Supersonic Beam Seeded with Ammonia,”
Appl. Phys. Lett
. 71 (10) (Sep. 8, 1997). In that process, the SiC substrate was first degreased, deoxidized, and degassed up to 500° C. Then, the SiC substrate was annealed to 900° C. for 20 minutes. Next, a buffer layer of AlN was grown on the SiC substrate at a temperature of between 560 and 1000° C. Finally, a GaN layer approximately 0.1 &mgr;m thick was grown on the AlN layer at a temperature exceeding 700° C. In a typical structure where the AlN layer was grown at 900° C. and the GaN layer was grown at 800° C., defect densities of 2×10
11
cm
−2
and 2×10
10
cm
−2
were measured for the AlN and GaN layers, respectively. Notwithstanding the reasonably good lattice matching of the GaN/AlN/SiC system, this process still produced relatively high defect densities.
Hallin et al., in “In situ preparation for Eligh-Quality SiC Chemical Vapour Deposition,”
J. Crystal Growth
181, pp. 241-53 (1997), have shown that etching a SiC substrate in hydrogen at 1500 to 1600° C. improves the quality, with respect to defect formation, of an epitaxial layer grown on the substrate. However, a GaN thin film heteroepitaxial structure has still not been fabricated with low defect density.
SUMMARY OF THE INVENTION
The object of the present invention is to provide such a low-defect density gallium nitride heteroepitaxial structure, and a method for making such a structure.
On a silicon carbide surface, a gallium nitride film or a quantum-well heterostructure using alloys of indium nitride (InN), gallium nitride, and aluminum nitride is formed on the silicon carbide substrate. Prior to gallium nitride film formation, the substrate surface is held at an elevated temperature and exposed to hydrogen for etching to remove mechanical damage and defects and so as to reveal atomic steps and terraces for heteroepitaxial deposition of gallium nitride. The resulting interface between the silicon carbide and the gallium nitride is sharp and clean, and low defect density is achieved on the surface of the gallium nitride film or quantum-well heterostructure formed on the silicon carbide substrate.
Alternatively, on a SiC surface, an aluminum nitride film may be formed as a buffer layer, and a gallium nitride film or a quantum-well heterostructure using alloys of indium nitride, gallium nitride, and aluminum nitride is formed on the aluminum nitride film. Prior to aluminum nitride layer formation, the substrate surface is held at an elevated temperature and exposed to hydrogen for etching to remove mechanical damage and defects and so as to reveal atomic steps and terraces for heteroepitaxial deposition of aluminum nitride. With a resulting interface between the silicon carbide and the aluminum nitride which is sharp and clean, low defect density is achieved in the gallium nitride film or quantum-well heterostructure formed on the aluminum nitride layer.
Preferably, in both types of structures the etching of the substrate is performed at a temperature from about 1550 to 1700° C. Preferably, the substrate is also exposed to a mixture of hydrogen and at least one inert gas, such as helium or argon. In preferred embodiments, the aluminum nitride layer and the gallium nitride film or quantum-well heterostructure are formed using selected energy epitaxy (SEE) deposition. Alternatively, these layers can be formed using molecular beam epitaxy deposition.
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Hallin et al, “In situ substrate preparation for high-quality SiC chemical vapor deposition,”Journal of Crystal Growth181, pp. 241-253 (1997).
Torres et al., “Growth of AlN and GaN on 6H-SiC(0001) Using a Helium Supersonic Beam Seeded with Ammonia,”Appl. Phys. Lett. 71 (10), pp. 1365-1367 (Sep. 8, 1997).
Owman et al., “Removal of polishing-induced damage from 6H-SiC(0001) substrates by hydrogen etching,”Journal of Crystal Growth167, pp. 391-395 (1996).
Doak R. Bruce
Edwards, Jr. John L.
Smith David J.
Torres Victor M.
Tsong Ignatius S. T.
Arizona Board of Regents Acting on behalf of Arizona State Unive
Baker & Botts L.L.P.
Pham Long
Shukla Ravindra B.
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