Low density of threading dislocations in AlN grown on sapphire

Abstract

We report on high resolution x-ray diffraction studies of the crystalline perfection and the relaxation of elastic strain in AlN grown by MBE on sapphire(0001). Thin (200–300 nm thick) AlN layers were grown with a very low density of threading screw dislocations. A density of 1.75−8.5×105 cm−2, the lowest value ever reported for III-Nitride epitaxial layers, was observed in a surface layer formed over a defective nucleation layer. Residual elastic strain was found in investigated AlN layers. Stress was found to be close to that expected from thermal expansion mismatch between the AlN and sapphire(0001). A model for the structural transformation of crystalline defects accounts for these observations.