Linear Al-composition dependence of strain-free bandgap in AlxGa1-xN (0 ≤ x < 0.2) via optical spectroscopy and theoretical calculation

This study presents the strain-free bandgap energy derived from the pseudomorphic Al_xGa_1-xN layer, obtained through theoretical calculations and experiments. The Al_xGa_1-xN films with various Al compositions (0 ≤ x < 0.2) were coherently grown on GaN/sapphire templates by metal-organic chemical vapor deposition, keeping their thicknesses smaller than their critical thicknesses. The c-lattice constants of GaN layers onto which the Al_xGa_1-xN layers were coherently grown were calculated considering the strain effects both from sapphire substrate and Al_xGa_1-xN epilayer, and confirmed by measuring absolute c-lattice constants using two different diffraction planes (0002) and (0004) by XRD. The strain-induced shift in the Al_xGa_1-xN bandgap energy due to the difference in thermal expansion coefficients between sapphire and nitrides and in-plane biaxial strain caused by a lattice-mismatch with GaN was calculated based on the band parameters given in the literature. The excitonic transition energy for fully-strained Al_xGa_1-xN films was obtained based on the temperature-dependent photoluminescence, and confirmed also by observing exciton-related features in reflectance measurements at room temperature. By accounting for the bandgap shift induced by strain and the exciton binding energies, the Al-composition dependence of the strain-free bandgap energy was determined, yielding no bowing parameter.