Structures of boundaries and corners of fully-closed hexagonal domains in HVPE-AlN film

Prismatic stacking fault (PSF) is a common and important lattice defect in wurtzite nitrides. This work reports unusual nano-sized hexagonal-shaped domains in AlN films grown on (0001) SiC substrate by hydride vapor phase epitaxy (HVPE), based on atomic-resolution scanning transmission electron microscopy and first-principles calculations. The domain boundaries are all PSFs lying parallel to {112¯0} planes, and comprise alternating 4-membered-rings (tetra-rings) and 8-membered-rings (octa-rings). The six corners of a single domain, each defined by the intersection of two neighbouring PSFs, are observed for the first time to have well-assembled structures containing one, two or three octa-rings in different configurations. Distinct features include that each domain corner carries a 1/6<112¯0> corner partial dislocation (CPD) arranged tangentially there, and that a net zero of the Burgers vectors is obtained summing up all six CPDs of a domain. In addition, all six side PSF boundaries are found to simultaneously intersect with I1 basal stacking fault (BSF), and a mechanism is proposed to account for the formation of an edge-type CPD from two intersected screw-type stair-rod dislocations (SRDs). Hence, the fully-closed PSF domain is a unique self-assembled system not only as a textbook example to demonstrate a solution of the argued disconnection problem at the PSF-PSF intersection, but also as a defective system which can help relax the stress remained in the film growth process. Based on these facts, a hexagonal island growth mechanism is discussed for the formation of the observed domain.