Modification of Nanocrystalline ZnS Films by Au Irradiation: Role of Energy Loss Processes

The changes in structural, optical, and surface morphological properties of nanocrystalline ZnS films induced by 35 keV, 2 MeV, and 100 MeV Au irradiations have been investigated. The as-grown films have been found to be composed of wurtzite crystal structure with a grain size and an optical band gap of about 17 nm and 3.56 eV, respectively. MeV Au irradiation has been found to result in an increase in grain size and a decrease in band gap. There is also a change over from compressive stress in the starting film to tensile stress after MeV Au irradiation. The effects of irradiation are seen to be stronger at 100 MeV than that at 2 MeV. However, no changes in grain size and band gap have been observed in following irradiation at the keV energy. The 35 keV and 2 MeV Au irradiations are found to produce rough surfaces the characteristics of which have been studied through the analysis of the power spectral density. Interestingly enough, 100 MeV Au irradiation is found to produce smoother surface as compared to that produced at lower irradiation energies. In the large spatial frequency, q, the Fourier exponent, has been found to have values close to 3 for the as-grown, 35 keV, and 100 MeV Au irradiated films. However, a value close to 4 has been found in case of 2 MeV Au irradiation. The lower values could be understood in a model where volume diffusion dominates, the higher value corresponding to a surface diffusion dominated process.