N K-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 Angstrom, which is expected, and N-2 at a distance R-2=R-1+0.25 Angstrom. This distortion is mon 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 Angstrom.