Method for making group III nitride articles
Group III (Al, Ga, In)N single crystals, articles and films useful for producing optoelectronic devices (such as light emitting diodes (LEDs), laser diodes (LDs) and photodetectors) and electronic devices (such as high electron mobility transistors (HEMTs)) composed of III-V nitride compounds, and methods for fabricating such crystals, articles and films.
1. A method for forming a bulk crystal structure, the method comprising:
depositing an epitaxial AlN layer on a single-crystal substrate to form an AlN-coated substrate;
growing a GaN nucleation layer on the AlN-coated substrate by HVPE under nucleation layer growth conditions that produce a pitted nucleation layer surface;
growing a GaN transitional layer on the pitted nucleation layer surface by HVPE under transitional layer growth conditions that (i) are different from the nucleation layer growth conditions and (ii) produce a transitional layer surface having a lesser percentage of pits than the pitted nucleation layer surface; and
growing a GaN bulk layer on the transitional layer surface by HVPE.
2. The method of claim 1 , wherein the nucleation layer growth conditions include a first growth temperature, a first ammonia partial pressure, a first V:III ratio, and a first growth rate, the transitional layer growth conditions include a second growth temperature, a second ammonia partial pressure, a second V:III ratio, and a second growth rate, and the transitional layer growth conditions are selected from the group consisting of the first growth temperature being increased to the second growth temperature, the first ammonia partial pressure being reduced to the second ammonia partial pressure, the first growth rate being reduced to the second growth rate, and two or more of the foregoing.
3. The method of claim 2 , wherein the first growth temperature ranges from about 900° C. to about 1000° C., the first V:III ratio ranges from about 10 to about 100, the first growth rate ranges from about 50 μm/hr to about 500 μm/hr, the second growth temperature ranges from about 920° C. to about 1100° C., the second V:III ratio ranges from about 8 to about 80, and the second growth rate ranges from about 50 μm/hr to about 500 μm/hr.
4. The method of claim 1 , wherein a pitting percentage of the pitted nucleation layer surface is greater than about 50%.
5. The method of claim 4 , wherein a diameter of pits of the pitted nucleation layer surface ranges from about 5 to about 50 microns.
6. The method of claim 1 , wherein a pit density within the GaN transitional layer decreases as a function of a thickness of the GaN transitional layer.
7. The method of claim 1 , wherein the transitional layer surface has a smooth surface morphology, and a surface morphology of the GaN bulk layer remains smooth during growth thereof.
8. The method of claim 1 , further comprising introducing an n-type or p-type impurity during at least one of the growth steps.
9. The method of claim 1 , further comprising introducing a deep-level acceptor during at least one of the growth steps.
10. The method of claim 1 , wherein the epitaxial AlN layer is deposited by a technique selected from the group consisting of sputtering, MOVPE, MBE, HVPE, and annealing in ammonia.
11. The method of claim 10 , wherein the epitaxial AlN layer is deposited by sputtering.
12. The method of claim 1 , wherein the GaN nucleation layer is substantially free of cracks and microcracks.