Reduction of threading dislocation densities in AlN/sapphire epilayers driven by growth mode modification

A strategy to reduce the density of threading dislocations (TDs) in AlN epilayers grown on sapphire substrates is reported. The TDs experience a redirection of their line orientation which is found to coincide with imposed increases in both of V/III ratio and overall flux rate leading to the formation of an internal subinterface delineated by the changes in dislocation orientation. Threading dislocations either experience large kinks and then redirect into threading orientation or form dipole half loops via annihilation of redirected threading segments of opposite sign with the latter leading to a significant dislocation density reduction. These phenomena can be accounted for by a transition of growth mode from atomic step flow to two-dimensional layer-by-layer growth which accompanies the imposed changes in V/III ratio and flux. As this occurs, macrosteps (several atomic layers thick) laterally overgrow pre-existing dislocation outcrops. Image forces initiate the redirection processes and create trailing segments parallel to the interface between the advancing macrostep and the surface outcrop. This horizontal segment can be forced to redirect into threading orientation should another macrostep traveling in the opposite direction be encountered. Image forces again nucleate the redirected segment which is then replicated as the crystal grows. A dipole half loop will form if two dislocations with opposite sign are redirected so as to meet each other.