| Nanoheteroepitaxy (NHE) is a novel approach to the dislocation reduction in lattice mismatched heterostructures. In this thesis patterned Si3 N4 NHE structure has been used for the defect reduction in the GaN film grown by MOCVD. This approach allows NHE to be performed on refractory substrates such as SiC, sapphire and previously-grown GaN on SiC.; For GaN grown on the patterned Si3N4 NHE structure, new mechanisms for dislocation reduction have been observed. These additional defect reduction mechanisms that are operative during the NHE process even when the NHE pattern size is larger than 40 nm include: (1) blocking of the substrate defects with the Si3N4 mask, (2) the bending of defects at local free-surfaces, and (3) the annealing phenomenon inside the nanoscale holes.; These mechanisms contributed to the dislocation reduction in the NHE GaN films. Coalesced films on NHE structure have been achieved by using low pressure (20 Torr) and low V/III ratios.; We also investigated GaN on dense silicon nanopillar arrays formed on (111) silicon substrate using an anodic-aluminum-oxide (AAO) membrane, etch-mask process. Silicon nanopillars are believed to be among the smallest patterned objects that have been used as substrates for GaN epitaxial growth by MOCVD. New defect reduction mechanisms have been observed in GaN epilayer on the highly compliant pillar arrays. While defects have not been eliminated in these samples the TD concentration is well below <108 cm-2.; In summary, the work described in this thesis has shown new mechanisms for defect reduction on patterned Si3N4 NHE structure. This allows a further defect reduction beyond what is expected from the basic strain argument of NHE method. Coalesced GaN film on IL patterned samples has been achieved. Future work is to build some device structures on top of the coalesced film. |
