Spectroscopic characterization of processing-induced property changes in doped ZnO films

The optical response and electronic conductivity of neat and trivalent cation doped ZnO films are dependent upon film deposition parameters and subsequent post-deposition processing. As-deposited films which show high resistivity can be made conducting through gas phase reduction with hydrogen at elevated temperatures or by electrochemical reduction at room temperature using both aqueous and nonaqueous solvents. Resistive sputter and solution deposited films which exhibit c-axis or random crystallite orientation respectively, become conducting after these treatments and show increased infrared reflectivity owing to free carrier absorption. Raman measurements are used to confirm the wurtzite crystalline phase and dopant incorporation into the lattice. Optical properties of selected films are determined from transmission and spectroscopic ellipsometry measurements using models parameterized with previously determined microstructural information. Based upon the electrochemical results, a mechanism is proposed to describe the reduction process which requires the generation of atomic hydrogen as a reducing species. (C) 1997 Elsevier Science S.A.