Formation of ZnO by Annealing of Thermally Evaporated Zinc in Oxygen Ambient for Solar Cell Application

Zinc oxide films have been formed from thermally evaporated zinc films on polished and textured silicon wafers after heat treatment at temperatures between 400-900°C for 10min in oxygen ambient. These ZnO films show crystalline structure and the values of their refractive index are found to increase slightly from 1.85 for films annealed at 400°C to 2.05 for the films annealed at 900°C. The films formed at a heat-treatment of 450°C have a refractive index value of 1.9. These films have a direct band gap of 3.3eV that corresponds to the absorption edge at 376nm. Consequently these films are highly transparent in the entire 400-1200 nm range and are useful for silicon solar cell application. ZnO films of 80nm thickness on textured silicon exhibit an average reflectivity of ~4 % over the 400-1100nm range; however, the lowest reflectivity is found to be 2.3%. Surface morphological studies have been carried out using SEM for the ZnO thin films deposited on polished silicon wafers. The grain size of ZnO film was found to be ~20nm corresponding to annealing at 450C. XRD analysis has been done for investigating the crystallographic structure and grain size of the ZnO thin films. The grain size of the ZnO thin films increases with heating temperature from 20nm corresponding to an annealing temperature of 400C to 42nm corresponding to annealing at 900C.