Urbach–Martienssen's tails in layered semiconductor GaSe

The dependence of the absorption coefficient on photon energy and temperature near the fundamental absorption edge was measured for layered single crystal, Gallium selenide (GaSe). The exponentially increasing absorption tail was explained as an Urbach–Martienssen's (U–M's) tails in the 10–340 K temperature range. Temperature dependence of the characteristic Urbach's parameters such as steepness parameter σ( T) and Urbach's energy ( E U=k B T/ σ) indicate that absorption is predominantly influenced by the phonon-induced microelectric fields and not compositional or structural potential fluctuations. The evaluated effective phonon energy hν p =17 meV is in good agreement with the A′ 1 (1) homopolar phonon mode obtained from Raman spectra and infrared (IR) measurement for this compound. The absorption process can be considered as an internal Franz–Keldysh effect, caused by the phonon-generated microelectric fields related with charged impurities or defects associated with bulk two-dimensional stacking faults, interstitial atoms, or vacancies in the GaSe lattice and can be described in the framework of the Dow and Redfield (DR) theory.