A strategy to reduce the density of threading dislocations in AlN epilayers grown onto sapphire substrates was reported. The threading dislocations experienced a re-direction of their line orientation which was found to coincide with imposed increased in both the V/III ratio and the overall flux rate; leading to the formation of an internal sub-interface delineated by the changes in dislocation orientation. Threading dislocations either experienced large kinks and then re-directed into threading orientation or formed dipole half-loops via the annihilation of re-directed threading segments of opposite sign, with the latter leading to a significant dislocation density reduction. These phenomena could be accounted for by a transition in growth mode from atomic step flow to 2-dimensional layer-by-layer growth which accompanied the imposed changes in V/III ratio and flux. As this occurred, macrosteps (several atomic layers thick) laterally overgrew pre-existing dislocation outcrops. Image forces initiated the re-direction processes and created trailing segments parallel to the interface between the advancing macrostep and the surface outcrop. This horizontal segment could be forced to re-direct into threading orientation if another macrostep traveling in the opposite direction was encountered. Image forces again nucleated the re-directed segment, which was then replicated as the crystal grew. A dipole half-loop formed if 2 dislocations of opposite sign were redirected so as to meet one another.

Reduction of Threading Dislocation Densities in AlN/Sapphire Epilayers Driven by Growth Mode Modification. J.Bai, M.Dudley, W.H.Sun, H.M.Wang, M.A.Khan: Applied Physics Letters , 2006, 88[5], 051903 (3pp)