Effects of in-situ Heat-Treatment on Residual Stress Dependent Optical Properties of ZnI2 Thin Films

Abstract

ZnI2, a hygroscopic chemical compound in thin film form has been investigated for residual stress dependent optical properties in in-situ heat-treated conditions up to 80oC. The tetragonal structured ZnI2 thin films were grown by thermal evaporation on glass substrates with deposition rate optimized at 1-2 nm/s and vacuum maintained at 10−6 Torr. Quartz crystal thickness monitor was used to monitor the film thickness and deposition rate during the film growth. Measurements like UV spectroscopy, X-ray diffraction and scanning electron microscopy were performed under low relativity humidity conditions (~40%). For in situ heat-treatment, films were heated inside the film growth glass chamber with inbuilt thermocouple to do temperature measurements. The in-situ heat-treatment of above-mentioned ZnI2 thin films shows a linear decrease in film thickness with increase of in-situ treatment temperature. The x-ray diffraction studies revealed a decrease in residual stress parameter with decreasing film thickness. Morphological studies support the internal stress behavior based on density effect or packing density of the films. The direct type-band gap for ZnI2 films found to increase almost linearly with increasing heat treatment temperature (and corresponding decreasing film thickness) from 3.25 eV to 4.06 eV. In-situ experiments estimate a much larger value of 8.39 eV of the band gap for a stress free perfect crystal of ZnI2 and give an indication of a large value of its deformation potential.