Epitaxy and strain in the growth of GaN on AlN: A polarized x-ray absorption spectroscopy study

We present a study of the local structure of 7-8 nm thick GaN epilayers deposited on AlN by molecular beam epitaxy at temperatures ranging between 620 and 790 degreesC by x-ray absorption spectroscopy, exploiting the polarization dependence of the technique. Both the near-edge and extended spectra suggest that atomic intermixing between GaN and AlN takes place at most for one monolayer. An interpretation of the near-edge spectra of wurtzite GaN and of the variation with the sample orientation in the framework of multiple scattering theory is provided. Values of the in-plane and out-of-plane strain due to heteroepitaxial growth are obtained from the interatomic distances in the second coordination shell around Ga; we compare the obtained values to literature values of the elastic constants, finding good agreement with most of them.