An XPS study of amorphous thin films of mixed oxides In2O3-SnO2 system deposited by co-evaporation

XPS core level measurements are used to observe the surface changes in amorphous thin films of mixed oxides In2O3-SnO2 system deposited by co-evaporation. The effects of changes in composition (in mol%), film thickness, substrate temperature, post-deposition annealing, and etching with Ar+ ions on the binding energies of In(3d) and Sn(3d) doublets in mixed oxides In2O3-SnO2 system are presented. XPS core level In(3d) and Sn(3d) spectra at various compositions exhibit the characteristic 3d(5/2) and 3d(3/2) doublets. The positions of the In(3d) and Sn(3d) lines are those as expected for In3+ ions in In2O3 and Sn4+ ions in SnO2. The initial decrease in binding energy with an increase in Sn content in In2O3 lattice is caused by the Sn atom substitution of In atom, giving out one extra electron. The increase in binding energy above the critical Sn content (10 mol% SnO2) is caused by the defects formed by Sn atoms, which act as carrier traps rather than electron donors. The decrease in binding energy with film thickness is caused by the increase in free-carriers density, which is generated by oxygen vacancy acting as two electrons donor. The decrease in binding energy with a substrate and annealing temperatures is due either to the severe deficiency of oxygen, which deteriorates the film properties and reduces the mobility of the carriers or to the diffusion of Sn atoms from interstitial locations into the In cation sites and the formation of indium and tin species of lower valence states so that the In3+ and the Sn4+ oxidation states may be changed to the In2+ and the Sn2+ oxidation states respectively. The new oxidation states, In2+ and Sn2+, formed due to ion etching and annealing the samples can be attributed to the internal electron transfer from oxygen 2p to the In 5s and Sn 5s levels both in In2O3 and SnO2.