Determination of the local microstructure of epitaxial AlN by x-ray absorption

Hexagonal AlN, epitaxially grown on GaAs, is studied using x-ray absorption measurements at the N-, O-, and AI-K edges. The measured distances of the nitrogen nearest neighbors (nn) are found shorter than expected by 0.04-0.13 Angstrom for the first to third nn shells, respectively. Nitrogen atoms are fourfold coordinated with Al as first nearest neighbors (nn), while Al atoms are fourfold coordinated with N and O atoms. The Al-N distance is 1.85 Angstrom, i.e., it is smaller by 0.04 Angstrom than expected, A reduction in the nn distances is also observed in the second and third nn shells where the N-N and the N-Al distances are found 0.07 and 0.13 Angstrom shorter than expected, respectively. The reduction in the nn distances is attributed to the presence of oxygen contamination. The Al-O distance is only 0.18 Angstrom smaller than the Al-N distance, i.e., O is a substitutional impurity in the N sublattice. The characteristic angular dependence of the N-K edge NEXAFS spectra verifies the hexagonal structure of the AIN film. Moreover, from the dependence of the areas under the NEXAFS resonances versus the angle of incidence theta the directions of maximum electron charge densities are determined. Finally, the p-partial density of states in the conduction band is discussed. (C) 1997 Americam Institute of Physics.