Optical absorption and photoreflectance spectroscopy of the single-crystalline chalcopyrite semiconductor AgGaSe2

Optical absorption and photoreflectance (PR) spectra have been measured on the single-crystalline chalcopyrite semiconductor AgGaSe2 for light polarization perpendicular (E ⊥ c) and parallel to the c-axis (E ‖ c) at T = 15–300 K. Optical absorption measurements suggest that AgGaSe2 is a direct-gap semiconductor having an optical band gap of E0 ∼ 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 E0 of AgGaSe2 shows unusual temperature dependence at T ≤ 80 K. The resultant temperature coefficients dE0/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 Δso = 327 meV and Δcr = −288 meV at T = 20 K, respectively.