Substrate temperature dependence of electrical conduction in nanocrystalline CdTe:TiO2 sputtered films

Abstract

TiO2 thin films with high volume fraction (∼50­70 %) of CdTe nanoparticles were prepared by radio frequency (rf) magnetron sputtering from a composite TiO2:CdTe target. With increase in substrate temperature Ts from room temperature (RT ∼300 K) to 373 K, a transition from an ordered structure exhibiting metallic-type conduction to a disordered structure exhibiting nonmetallic-type conduction was observed for annealed nanocrystalline CdTe:TiO2 films. The annealed RT-deposited films showed a large coalescence of distinct islands (size ∼0.3­0.7 µm) mainly of Cd and CdTe, and as result, a 3D network was realized. For metallic regime films, electrical conduction is essentially due to electrical percolation through Cd/CdTe crystallites embedded in an amorphous TiO2 matrix. However, the annealed high Ts films consisted of noncoalescent, small islands (size ∼0.15­0.3 µm) of Cd and CdTe embedded in amorphous TiO2 matrix. Here, the conduction is essentially by hopping mechanism via thermally activated tunneling.