Effect of thermal annealing on the structural and optical properties of ZnO thin films deposited by the reactive e-beam evaporation technique

Crystalline zinc oxide (ZnO) thin films with highly preferential c-axis-oriented crystals were prepared using the reactive e-beam evaporation technique. Prior to deposition, ZnO targets were prepared from ZnO (99.999%) powder. Post-deposition thermal annealing was performed at various temperatures ranging from 200 to 700 degrees C for 2 h in air to investigate the effect of annealing on the structural and optical properties. Structural characterization including that of the crystal structure, crystal orientation, phase, stress, strain, grain size and surface morphology was carried out using x-ray diffraction (XRD) and atomic force microscopy (AFM). Optical characterization including transmission, absorption coefficient and band gap estimation was carried out using a spectrophotometer. The XRD results showed that the films were highly c-axis oriented before and after annealing. Crystallinity and grain size improved with annealing temperature. AFM results showed that the surface morphology improved with annealing temperature. Optical transmittance increases slightly and the band gap decreases with increasing annealing temperature. The effect of the stress formation during thin film deposition and its variation with post-deposition heat treatment and the effect of this stress on optical properties of the thin films were also studied. The residual compressive stress in as-deposited thin films relaxes with heat treatment and becomes tensile with further increase in annealing temperature. The optical band gap decreases with increasing grain size and decreases with increasing tensile stress.