Optical absorption and photoref lectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe_2

摘要

Optical absorption and photorenectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe_2 for light polarization perpendicular (E ⊥ c) and parallel to the c-axis (E ‖ c) at T= 15-300 K. Optical absorption measurements suggest that AgGaSe_2 is a direct-gap semiconductor having an optical band gap of E_0 ～ 1.8 eV at T= 15-300 K. The temperature-dependent PR spectra are obtained at T = 20-300 K in the 1.8-2.5 eV spectral ranges. The lowest band-gap energy E_0 of AgGaSe_2 shows unusual temperature dependence at T ≤ 80 K. The resultant temperature coefficients dE_0/dT are positive at T ≤ 70 K and negative above 70 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 Δ_(so) = 327meV and Δ_(cr)= -288 meV at T= 20 K, respectively.