Microstructures of GaN islands on a stepped sapphire surface

摘要

[[abstract]]We investigated the structural evolution of GaN nucleation layers in the initial growth stages on commercial c-plane sapphires with atomic steps at the surface, using field-emission scanning electron microscopy, synchrotron x-ray scattering, and high-resolution electron microscopy. GaN nucleates into islands preferentially on the atomic steps. The initial small islands of 25 A high have well-ordered cubic sequences and nearly coherent interfacial structures with a large compressive strain of similar to10%. As the islands grow to 50 A high, the strain is drastically reduced, to less than 1%, by generating misfit dislocations at the interface and forming the six-to-seven matched interfacial structure. Interestingly, stacking faults are developed from the GaN/sapphire interface, which induces a cubic-hexagonal transformation. The changes in the stacking order during the initial growth are investigated quantitatively.