The defect character of GaN growth on r-plane sapphire

Abstract

We study the influence of the heteroepitaxial interface on the defect content of nonpolar a-plane GaN grown on r-plane sapphire, using transmission electron microscopy techniques and the topological theory of interfacial defects. The structural mismatch is accommodated in different ways along distinct in-plane directions. For the misfit along the [0001] direction of GaN, the I1 basal stacking faults constitute the principal relaxation mechanism, through their delimiting partial dislocations. The misfit along [11¯00] is relaxed by misfit dislocations that have out-of-plane Burgers vectors causing rotational misorientations of the epilayer about [0001]. These misorientations lead to the introduction of low-angle tilt grain boundaries which are defect sources through their associated primary lattice dislocations. Furthermore, semipolar nanocrystals can nucleate on sapphire p-plane nanofacets, causing the introduction of threading dislocations (TDs) and stacking faults, when these nanocrystals are overgrown by the nonpolar epilayer. The semipolar nanocrystals bear a high-symmetry 90° [1¯21¯0] orientation relative to the nonpolar epilayer. The interfacial dislocations at the nonpolar/semipolar boundaries have suitable Burgers vectors to become TDs. These grain boundaries also promote the introduction of stacking faults and pockets of cubic GaN. The coexistence of semipolar orientation variants leads to twin boundaries and associated dislocations.