Temperature dependence of the optical absorption edge in amorphous Cd-As and Zn-P thin films

Temperature dependencies of the optical absorption edge in the amorphous Cd29As71 and Zn32P68 films have been obtained from transmission measurements in the 5–300 K temperature range. The absorption edges for these films were found to follow the Tauc power law with the optical pseudogap EG significant increase with decreasing temperature. A nearly linear dependence of EG on temperature was found in the range from 100 K to about 300 K, while below 100 K the energy gap is almost independent of temperature. For the Cd29As71 film EG was found to be equal to 0.76 eV at 5 K and 0.63 eV at 300 K, while for Zn32P68 film, the energy gap changes from 1.46 to 1.27 eV in the same temperature range. In contrast to EG, the Urbach energy decreases rather slightly with decreasing temperature. The observed change of the optical gap may be attributed to an increase of the mobility gap due to contraction of interatomic distances with decreasing temperature.