Mid-infrared ellipsometry, Raman and X-ray diffraction studies of AlxGa1−xN/AlN/Si structures

We report an investigation of the optical and structural properties of wurtzite phase AlxGa1−xN/AlN structure grown on Si(111) within the compositional range of 0 ≤ x ≤ 1. The study focuses on providing essential physical quantities for the fabrication process control, namely the composition dependence of phonon mode energy and refractive index. Three complementary techniques, infrared ellipsometry, Raman spectroscopy and X-ray diffraction, have been used to minimize uncertainties in our analysis. Based on the high quality and nearly strain-free AlxGa1−xN/AlN double layer samples, we determined the calibration curve for the A1(LO) phonon mode. We have also constructed the ellipsometry model which uses a-priori knowledge of experimentally measured A1(TO) phonon mode frequencies. From the best model fit to the collected ellipsometry spectra of the entire sample series, we obtained the anisotropic refractive indices of the AlxGa1−xN alloys with a very satisfactory accuracy.