Strain accommodation and interfacial structure of AlN interlayers in GaN

Abstract

The strain accommodation mechanisms at AlN interlayers in GaN, grown by radio-frequency plasma assisted molecular beam epitaxy, are studied using transmission electron microscopy techniques and atomistic modelling. Interlayers of various thicknesses grown within GaN epilayers deposited on both sapphire and silicon substrates have been employed. Interlayers of thickness below 6 nm do not exhibit line defects although local roughness of the upper interlayer interface is observed as a result of the Al adatom kinetics and higher interfacial energy compared to the lower interface. Above 6 nm, introduction of a-type misfit and threading dislocations constitutes the principal relaxation mechanism. Due to strain partitioning between AlN and GaN, threading dislocations adopt inclined zig-zag lines thus contributing to the relief of alternating compressive-tensile elastic strain across the AlN/GaN heterostructure. The observed dislocation configurations are consistent with a model of independent motion by climb or ancillary glide in response to their localized three-dimensional strain environment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)