Spectroscopic ellipsometry measurements of Al xGa 1−xN in the energy range 3–25 eV

Abstract

The dielectric functions of the wide-bandgap semiconductors GaN and Al x Ga 1− x N (hexagonal phase on c-sapphire) are determined directly for the first time in the spectral range 3–25 eV using an ellipsometry set-up operating with synchrotron radiation at the Berlin electron storage ring BESSY I. The compositional dependence of the transition energies of interband-critical points located in the vacuum ultraviolet spectral region, which are not accessible to ellipsometers using conventional light sources, was determined for 0≤ x≤1. Additional measurements with a visible-UV ellipsometer have been performed to determine precisely the fundamental gap E 0. A systematic parabolic shift of E 0 towards higher energies was found with increasing Al content while higher interband transitions show a linear shift. Differences in morphological properties of a MBE- and a MOCVD-grown GaN sample and their influence on the measured dielectric function are discussed.