First formed dislocations in microcompressed c-oriented GaN micropillars and their subsequent interactions

c-oriented GaN micropillars created from single crystals containing ∼103 or ∼106 dislocations/cm2 and a thick heteroepitaxially grown film containing ∼109 were compressed to study methods to accommodate strain during heteroepitaxial growth. The yield stress in the 103 samples was found to be the highest, and it was the lowest in the 109 samples. The 103 and 106 pillars often failed catastrophically but the 109 pillars almost never did. This was linked to the high stresses required to generate sufficient pyramidal dislocations to accommodate plastic strain and dislocation interactions, which precipitated axial fracture. Transmission electron microscopy analysis shows categorically that the first formed dislocations are ⅓⟨1123_⟩{1122} dislocations, and that a few ⅓⟨1123_⟩{0111} dislocations found were formed by a cross slip in the vicinity of where the former dislocations interacted. When compared with the similar stress patterns created in the heteroepitaxial growth of AlGaN films on GaN substrates, the analysis suggests that there is no pathway for creating basal plane dislocations during growth from the pyramidal dislocations, which require high applied stresses; the basal plane dislocations would provide relief for the mismatch strain while not penetrating the region where active devices are fabricated in the film. Rather, it will be necessary to find a method for creating shear stress in the basal plane during growth to form them directly.