Positive temperature variation of the bandgap energy in the single-crystalline chalcopyrite semiconductor AgInS2

The complex dielectric-function spectra, ε(E ) = ε 1 (E ) + iε 2 (E ), of the AgGaSe 2 chalcopyrite semiconductor have been measured by spectroscopic ellipsometry (SE) for light polarizations perpendicular (E ? c) and parallel to the c-axis (E k c) in the 1.6-5.3 eV photon-energy range at room temperature. The measured ε(E ) spectra reveal distinct structures of critical points in the Brillouin zone. Analysis of the numerically derived ε(E ) spectra facilitates the precise determination of critical-point energies. By performing the band-structure and dielectric-function calculations, these critical points are successfully assigned to specific points in the Brillouin zone. The dielectric-related optical constants of AgGaSe 2 , such as the complex refractive index n* = n + ik, the absorption coefficient α, and the normal-incidence reflectivity R, are presented.