Band gap energies of semiconducting sulphides and selenides

Thin films of some semiconducting sulphides (ZnS, CdS and Bi2S3) and selenides (ZnSe, CdSe and Bi2Se3) have been prepared using the chemical deposition method. The band gap energy of the films was determined by optical absorption measurements while the activation energy was determined from resistance measurements. The band gap energies obtained from the absorption measurements ranged from 1.42 eV for Bi2Se3 to 3.72 eV for ZnS while the activation energies obtained from the resistance measurements ranged from 0.31 eV for Bi2Se3 to 0.59 eV for ZnSe. The high optical band gap obtained for the films could be attributed to very small size in chemically deposited films which lead to electrical isolation of individual grains, or quantum well structure. The small values of the activation energies are due to the presence of impurity levels in the energy band gap of the films.