NK-edge x-ray-absorption study of heteroepitaxial GaN films

The microstructure of GaN films grown by electron cyclotron resonance (ECR) molecular-beam epitaxy (MBE) and hydride vapor-phase epitaxy (HVPE) is studied using x-ray-absorption measurements at the N $K$ edge. The local microstructure around the N atom is found to be distorted. The nearest neighbors of the N atom are four Ga atoms, $N_1$ of which are located at a distance $R_1=1.95\mathrm{}\AA ,$ which is expected, and $N_2$ at a distance $R_2{=R}_1+0.25\mathrm{}\AA .$ This distortion is more pronounced in the cubic ECR-MBE and the HVPE hexagonal samples where the ratio $N_2{/N}_1$ is equal to 1. In the rest of the samples the distortion is smaller and the ratio $N_2{/N}_1$ takes the value 0.33. Nitrogen deficiency is not detected in the second nearest-neighbor shell and the N atoms are found at the expected distance of 3.11 Å.