Optical absorption in tin selenide thin films

Tin selenide (SnSe) thin films have been prepared by encapsulated selenization technique. The Sn/Se stacked films were deposited by vacuum evaporation and annealed at 200 degrees Celsius for 3 hours to form a stoichiometric SnSe compound. Optical absorption measurements were made on the as- prepared sample using spectrophotometer in the range from UV to visible region (200 - 900 nm). The absorption coefficient, (alpha) was found to be greater than 105 cm-1 that suggested the occurrence of either indirect allowed or direct forbidden optical transition. A further investigations on the (ahv)1/2 and (ahv)2/3 plots against photon energy have been carried out and the resulting optical bandgap obtained from indirect allowed transitions were 0.95 eV for 40.5 nm sample and reduced to 0.79 eV for 125 nm sample thickness. In the case of direct forbidden transition, the energy gaps were between 1.20 - 1.08 nm. The results also showed that the band gap decreased with increasing sample thickness.