Anisotropy of optical absorption in GaSe studied by midinfrared spectroscopy

Slab samples of epsilon-GaSe have been prepared with the optical axis (the c axis) in the plane of the face. Previously, most optical studies were made on slabs cleaved from crystals, which have the c axis perpendicular to the face. The infrared absorption spectra of epsilon-GaSe were measured at room temperature in the spectral range of 4000-450 cm(-1) for the e-ray (polarized parallel to the optical axis) and the o-ray (polarized perpendicular to the optical axis). The maximum value of the optical absorption coefficient, alpha, for both geometries, does not exceed 2.5 cm-1 (Eparallel toc). We find anisotropy in the transmission in the spectral range below similar to2250 cm(-1), due to the free carrier absorption, only in the e ray. At T=300 K, acoustic lattice vibrations are the dominant scattering mechanism for carriers moving parallel to the c axis. Absorption bands were observed in e-ray spectra at 891, 945, 1093, and 1270 cm(-1). These bands may be due to multiphonon absorption, or to localized vibrational modes stemming from unintentional impurities. It is suggested that the sharp absorption at 891 cm(-1) may be due to hole transitions from the lowest-lying valence band in the center of the Brillouin zone to the top of the valence band (Gamma(6)(+)-->Gamma(4)(-) transition). (C) 2003 American Institute of Physics.