Accurate experimental determination of the Poisson’s ratio of GaN using high-resolution x-ray diffraction

Abstract

An accurate Poisson’s ratio value of 0.183±0.003 for a typical c-axis-oriented GaN film grown by metal-organic vapor-phase epitaxy deposition has been determined using a wafer bending apparatus combined with high-resolution x-ray diffraction lattice parameter measurements. The precision of this method has improved ten fold over typical methods used for thin film samples, enabling future study of the effects of doping, compositional changes, or structural defects on the Poisson’s ratio of GaN. The obtained Poisson’s ratio value is lower than most calculated values, which is attributed to the presence of strain-relieving edge dislocations in the GaN sample. Unstrained film lattice parameters can also be found using this method, and are shown to be a=3.1884±0.0002Å and c=5.1850±0.0002Å (assuming an unstrained c∕a ratio of 1.6262). A brief review of Poisson’s ratio and unstrained lattice parameter values for GaN in the literature is also given.