Reduction of dislocation density of aluminium nitride buffer layer grown on sapphire substrate

An aluminium nitride (AlN) buffer layer with 200 nm thickness was grown on (0001) sapphire substrate using the metal-organic vapour phase epitaxy (MOVPE) method in a low-pressure furnace, followed by a clean-up treatment of sapphire substrate at 1100 degrees C. Thereafter, the AlN buffer layer was annealed at a high temperature in the range of 1500 degrees C to 1700 degrees C for 2 hours under the atmosphere of N-2+ CO. The objective of this research is to determine the microstructure changes with different annealing temperatures. Cross-sectional TEM has revealed that, after annealing at 1500 degrees C, two types of defects remained in the AlN buffer layer: inverted cone shape domains and threading dislocations. The former domains were observed in an image taken with diffraction of g=0002, but not in an image with g= 10 (1) over bar0. The morphology and the diffraction condition for the image contrast strongly, suggesting that the domains are inversion domains. The threading dislocations were invisible in the image taken with the diffraction of g= 0002, revealing that they were a-type dislocations. However, after annealing at 1600 degrees C, the inversion domains coalesced with each other to give a two-layer structure divided by a single inversion domain boundary at the centre of the AlN buffer layer. The density of threading dislocation was roughly estimated to be 5x109 cm(-2) after annealing at 1500 degrees C, and to be reduced to 5x10(8) cm(-2) after annealing at 1600 degrees C. These experimental results validate the fact that the annealing temperature around 1600 degrees C is high enough to remove the defects by the diffusion process. Therefore, it was discovered that high temperature annealing is an effective treatment to alter the microstructure of AlN thin films and remove defects by the diffusion process. Annealing at high temperature is recommended to increase the emission efficiency for fabrication of optoelectronic devices.