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An AlN annealing cap has been developed for GaN films grown on sapphire substrates that enables them to be annealed at temperatures as high as 1300 °C for times as long as 30 min or times as long as 120 min at a temperature of 1150 °C without creating a significant number of electrically active N vacancies or new thermal etch pits due to the preferential evaporation of N. However, the pits in the as-grown sample became larger and their sidewalls became steeper, and the flat areas of the film became a little rougher when the annealing time and/or temperature was larger. The films became more relaxed as indicated by a >10% reduction in the X-ray rocking curve peak widths and the creation of a more uniform convex bow. Both the net carrier concentration and electron mobility were reduced by the anneal suggesting that compensating point defects had been created. One possible explanation is that more C in these metal organic chemical vapor deposition (MOCVD) grown films occupied N sites where it is believed to be a deep acceptor.

Hydride vapor phase epitaxy (HVPE) grown GaN was evaluated for high power Schottky diodes (SDs) because it contains much less carbon and grows much more rapidly than other typical growth methods. The results are encouraging for applications

GaN films were grown by metal organic chemical vapor deposition on TaC substrates that were created by pulsed laser deposition of TaC onto (0 0 0 1) SiC substrates at ∼1000 °C. This was done to determine if good quality TaC films could be grown, and if good quality GaN films could be grown on this closely lattice matched to GaN, conductive material. This was done by depositing the TaC on on-axis and 3° or 8° off-axis (0 0 0 1) SiC at temperatures ranging from 950 to 1200 °C, and examining them using X-ray diffraction, scanning electron microscopy, atomic force microscopy, and transmission electron microscopy. The GaN films were grown on as-deposited TaC films, and films annealed at 1200, 1400, or 1600 °C, and examined using the same techniques. The TaC films were polycrystalline with a slight (1 1 1) texture, and the grains were ∼200 nm in diameter. Films grown on-axis were found to be of higher quality than those grown on off-axis substrates, but the latter could be improved to a comparable quality by annealing them at 1200–1600 °C for 30 min. TaC films deposited at temperatures above 1000 °C were found to react with the SiC. GaN films could be deposited onto the TaC when the surface was nitrided with NH3 for 3 min at 1100 °C and the low temperature buffer layer was AlN. However, the GaN did not nucleate easily on the TaC film, and the crystallites did not have the desired (0 0 0 1) preferred orientation. They were ∼10 times larger than those typically seen in films grown on SiC or sapphire. Also the etch pit concentration in the GaN films grown on the TaC was more than 2 orders of magnitude less than it was for growth on the SiC.

AlN deposited on 4H–SiC by pulsed laser deposition (PLD) did not noticeably deteriorate when it was annealed in an Ar atmosphere at 1500 °C for 30 min, but it contained numerous thermal etch pits when it was annealed at 1600 °C as determined by SEM and AFM, and it completely evaporated at the higher annealing temperatures thereby allowing the silicon to evaporate preferentially that produced a pitted SiC surface. When the AlN film was annealed in nitrogen for 30 min, the film formed a protective cover up to 1650 °C, and when a sapphire cover was employed, it protected the SiC surface up to 1700 °C. The AlN surface was altered when it was annealed in nitrogen by both the evaporation of N from the surface and reactions with the N in the atmosphere. This produced a rougher surface as determined by SEM and AFM that was composed of more randomly oriented crystallites as determined by X-ray and TEM analyses, but it still protected the SiC surface below. Thermodynamic calculations are in qualitative agreement with these analyses.