Raman and photoreflectance study of CuIn 1 − x Ga xSe 2 epitaxial layers

Abstract

The structural and optical properties of CuIn 1 − x Ga xSe 2 epitaxial layers were studied in dependence of composition by Photoreflectance (PR) spectroscopy at room and low temperatures and by Raman spectroscopy at room temperature. For compositions with [Ga] / ([In] + [Ga]) varying between 0.08 and 0.82, the band gap, determined by fitting the PR-spectra with a third derivative functional form, shows a quadratic dependence on composition. The Raman spectra, recorded under excitation with a vertically polarized Kr +-laser beam operated at the 647.1 nm line in the region 50–700 cm − 1 , consist mainly of peaks assigned to the A 1- and B 2-phonon modes. The A 1-mode dominates the spectra and shifts linearly, with the increase of the [Ga] / ([In] + [Ga]) content, from 172 cm − 1 (A 1-mode frequency of CuInSe 2) to 181 cm − 1 (A 1-mode frequency of CuGaSe 2). Combining the Raman and PR-data, an analytical expression has been derived which correlates the band gap energy with the A 1 Raman mode frequency for a given composition of the quaternary compound.