Optical properties of single-crystalline chalcopyrite semiconductor AgInSe2

Abstract

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 E0∼1.2 eV at T=15–300 K. The pseudodielectric-function spectra, ε(E)=ε1(E)+iε2(E), of AgInSe2 are determined by SE for light polarization perpendicular (E⊥c) and parallel to the c axis (E∥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 (En 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 E0 of AgInSe2 shows unusual temperature dependence at T≤120 K. The resultant temperature coefficients dE0/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 Δso=321 meV and Δcr=−172 meV, respectively, and are discussed from an aspect of the electronic energy-band structure consequences.