Optical properties of single-crystalline chalcopyrite semiconductor AgInSe2

The optical properties of single-crystalline AgInSe2 have been studied by optical absorption, spectroscopic ellipsometry (SE), and photoreflectance (PR) spectroscopy. Optical absorption measurements suggest that AgInSe2 is a direct-gap semiconductor having an optical band gap of E-0 similar to 1.2 eV at T=15-300 K. The pseudodielectric-function spectra, epsilon(E)=epsilon(1)(E)+i epsilon(2)(E), of AgInSe2 are determined by SE for light polarization perpendicular (E perpendicular to c) and parallel to the c axis (E parallel to c) in the E=1.2-5.3 eV spectral region at room temperature. The measured SE spectra reveal distinct structures at energies of the critical points (E-n with n=0-8) in the Brillouin zone. The temperature-dependent PR spectra are obtained at T=15-300 K in the 1.2-1.7 eV spectral ranges. The lowest band-gap energy E-0 of AgInSe2 shows unusual temperature dependence at T <= 120 K. The resultant temperature coefficients dE(0)/dT are positive at T <= 120 K and negative above 120 K, and are explained by considering the effects of thermal expansion and electron-phonon interaction. The spin-orbit and crystal-field splitting parameters are also determined to be Delta(so)=321 meV and Delta(cr)=-172 meV, respectively, and are discussed from an aspect of the electronic energy-band structure consequences. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3309953]