N-K-edge EXAFS study of epitaxial GaN films

X-ray absorption measurements at the N and O-K-edges are used to study the local microstructure in cubic and hexagonal GaN films grown by ECR-MBE and HVPE. A distortion in the local microstructure is identified in the 1st nearest neighbor (nn) shell, consisting of Ga atoms, in both the cubic and hexagonal samples. Two N-Ga distances are identified, R(1) and R(2), where R(1) is the expected distance of 1.95 Angstrom while R(2)=R(1)+0.25 Angstrom. The same distortion is detected in the next nn shell containing Ga atoms, where the two distances are 3.7 Angstrom and 4.1 Angstrom. All the reported distance variations are larger than the error-bar. The nitrogen 2nd nn neighbor is found at the expected distance of 3.12 Angstrom while N deficiency is not detected. Finally, the O-Ga distance is found equal to 1.60 Angstrom and therefore it can be proposed that the oxygen atom occupies interstitial positions.