The composition dependence of the optical band gap in Ge-Se-In thin films

Bulk glasses with the chemical composition Ge-y Se94-yIn6 (8 <= y <= 30 at%) have been prepared from high purity constituent elements. Fragments of the bulk glasses are used to deposit thin films by vacuum thermal evaporation. The optical band gaps (E-g s) of the as-deposited films have been measured. The allowed optical transition is found to be indirect. The relation of Eg to the covalent coordination number, Z, is demonstrated by varying the composition parameter y of the thin films. A maximum in the compositional dependence of E-g is attained at Z = 2.63. The cohesive energies (CE) of the investigated samples have also been calculated using the method suggested by the chemical bond approach. It is found that the composition dependence of the CE also possesses a peak at Z = 2.63. A plausible explanation based on the bonding considerations between the constituents has been provided for the understanding of the E-g-Z and CE-Z dependences for these Ge-Se-In thin films. (C) 2006 Elsevier B.V. All rights reserved.