Stock material or miscellaneous articles – Composite – Of inorganic material
Reexamination Certificate
1999-07-27
2001-10-02
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
Composite
Of inorganic material
C428S336000, C428S337000, C428S697000, C423S412000, C117S928000
Reexamination Certificate
active
06296956
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the growth of semiconductor materials. More particularly, the invention relates to the growth of bulk single crystals of aluminum nitride.
BACKGROUND OF THE INVENTION
The physical and electronic properties of aluminum nitride (AlN) give it great potential for a wide variety of semiconductor applications. AlN has a wide energy bandgap (6.2 electron volts), high breakdown electric field and extremely high thermal conductivity. In fact, in Chow et. al
Wide Bandgap Compound Semiconductors for Superior High Voltage Unipolar Power Devices
(IEEE Transactions on Electron Devices, Vol. 41, No. 8, 1994) ranking all semiconductors materials, AlN is reported to have, excluding diamond, the highest figure of merit for unipolar power device performance.
In addition, the high thermal conductivity and high optical transmissivity (i.e., low optical density) of AlN make AlN an excellent candidate substrate material. Also, AlN is likely to be the optimum substrate for the growth of pseudo-binary inter metallic compounds such as Al
0.8
In
0.2
N which have extremely high figures of merit for semiconductor performance (up to 4,413,000 times silicon). Although AlN has extraordinary properties for a semiconductor material and has tremendous commercial potential, AlN based semiconductor devices have been limited by the unavailability of large, low defect AlN single crystals. In the most successful prior work, Slack and McNelly demonstrated a method for growing AlN single crystals via sublimation in
AlN Single Crystals
(Journal of Crystal Growth 42, 1977). However, the time required to grow a 12 mm by 4 mm crystal was approximately 150 hours. This growth rate is far too low to ever allow the commercial production of AlN single crystals.
SUMMARY OF THE INVENTION
The present invention enables one to grow bulk, low impurity AlN single crystals at growth rates greater than 0.5 mm per hour. The invention in its most basic form is a method for allowing a high flux of vaporized AlN to deposit in an isotropic manner on a growing crystal interface without interference caused by significant impurity atoms, impurity molecules or accumulated non-stoichiometric vapor constituents being present at the growing crystal interface.
One feature of the invention is a unique effusion system that is utilized to sweep impurity atoms/molecules and non-stoichiometric vapor constituents in the vapor away from the growing crystal interface. As shown by Abernathy in
Congruent Vapor Transport
(Journal of Crystal Growth 47, 1979) impurity atom or impurity molecule build up at a growing crystal interface dramatically reduces the crystal growth rate. Also, it has been found that inconsistencies between the stoichiometry of the vapor at the source and the stoichiometry of the vapor required for optimum growth conditions at the growing crystal interface can result in the accumulation of non-stoichiometric vapor constituents at the growing crystal interface. In one embodiment of the invention, a novel effusion system utilizes a series of effusion holes in fluid communication with a master effusion outlet to provide effusion directly at the growing crystal interface as the crystal grows. Preferably, the total effusion rate is maintained at or near a predetermined percentage (e.g. 30%) of the total vapor flow rate (N
e
=30% of N
t
). Other embodiments of the effusion system are disclosed. These systems are designed to provide optimum impurity effusion at the growing crystal interface, preferably at a constant rate throughout the crystal growth run. This constant effusion system significantly increases the AlN crystal growth rate by sweeping away matter that blocks deposition of the desired vapor species and substantially decreases impurity levels in the grown crystal.
Another feature of the invention is a growth system that provides a significant decrease in the ratio of crucible height to diameter (H:D aspect ratio) while maintaining a uniform thermal profile across the growing crystal interface. In this regard, Slack and McNelly, supra, used a radio frequency heating apparatus which provided a relatively compact system, but would not allow the use of a crucible having a low ratio of height to diameter without creating a highly non uniform thermal gradient. By decreasing the crucible height to diameter ratio, several important advantages may be achieved. First, the viscous interaction of the source vapor with the walls of the crucible is dramatically reduced, resulting in improved fluid dynamics characteristics within the crucible, i.e., vastly improved mass transport. Second, the low H:D aspect ratio enables the growth system to operate with a significantly lower absolute temperature difference between the source and the growing crystal interface, while maintaining an optimum thermal gradient between the source and the growing crystal interface. This lower temperature difference means that the stoichiometry of the vapor at the growing crystal interface (i.e., the stoichiometry required for optimum crystal growth) may be relatively close to the stoichiometry of the vapor at the source, resulting in less accumulated non-stoichiometric vapor constituents at the growing crystal interface. As mentioned above, this desirable condition permits vastly improved growth rates, while also permitting the growth of exceptionally large diameter AlN single crystals (e.g. 2 inch diameter).
In certain preferred embodiments of the invention a flat heated plate or parallel flat heated plates are used instead of a typical cylindrical heating arrangement. These flat plate designs, which preferably are carried out by resistance heating, provide a highly uniform thermal profile at the growing crystal interface and a steep thermal gradient to allow the growth of very large diameter AlN and AlN alloy single crystals at relatively high growth rates, and are adapted to be used in conjunction with effusion systems as discussed above.
The constant flux of source vapor may be provided in a number of ways, for example, from an AlN crystal or crystals, sintered AlN, hydrostatically pressed AlN, AlN powder or other solid form containing AlN, Al or N. Other embodiments of the invention utilize gas injection to provide a constant flux of source vapor. The source gas may take the form of AlCl
3
, NH
3
, vaporized Al, N
2
, atomic nitrogen, nitrogen ions, N
2
in combination with nitrogen ions, N
2
that has been excited via microwave, laser or other energy source, or other gases that contain nitrogen or aluminum, alone or in combination. Furthermore, source vapor may be provided in part from solid or liquid source material and in part from a gas source material, for example, the use of source vapor from molten Al combined with N
2
.
The gas injector system can also be used to increase the effective effusion by providing a flow of effusion-assist gas such as N
2
or argon. The use of an effusion-assist gas increases the flow rate of gas past the growing crystal interface by adding additional flow to the source vapor flow.
The growth system may also utilize an injected gas that serves as both a source gas and an effusion-assist gas, for example, by injection of N
2
or ammonia (NH
3
). In this regard, the cracking of ammonia provides nitrogen to react with Al atoms located on the surface of the growing AlN crystal to form AlN.
The bulk crystals of the present invention may be grown with sufficient size for commercial application, particularly for the electronics industry. While any useful size crystal may be grown according to the invention, in most applications the crystals will have a diameter of two to three inches and above.
The bulk single crystals of the invention are typically grown as intrinsic AlN, doped AlN, or AlN alloys and compounds containing significant amounts of AlN, for example, greater than 50% AlN.
Because the resistance heated, constant effusion crystal growth systems of preferred embodiments of the invention allow very high growth rates and shorter run times, slightly lower growth temperature
Cree Inc.
Faust Richard S.
Jones Deborah
Stein Stephen
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