Urbach-Martienssen's tails in layered semiconductor GaSe

The dependence of the absorption coefficient on photon energy and temperatu re near the fundamental absorption edge was measured for layered single cry stal, Gallium selenide (GaSe). The exponentially increasing absorption tail was explained as an Urbach-Martienssen's (U-M's) tails in the 10-340 K tem perature range. Temperature dependence of the characteristic Urbach's param eters such as steepness parameter sigma(T) and Urbach's energy (E-U = k(B)T /sigma) indicate that absorption is predominantly influenced by the phonon- induced microelectric fields and not compositional or structural potential fluctuations. The evaluated effective phonon energy hv(p) = 17 meV is in go od agreement with the A(I)(')(1) homopolar phonon mode obtained from Raman spectra and infrared (IR) measurement for this compound. The absorption pro cess can be considered as an internal Franz-Keldysh effect, caused by the p honon-generated microelectric fields related with charged impurities or def ects associated with bulk two-dimensional stacking faults, interstitial ato ms, or vacancies in the GaSe lattice and can be described in the framework of the Dow and Redfield (DR) theory. (C) 1999 Elsevier Science Ltd. All rig hts reserved.