Dependence of Optical Band Gap on Residual Stress in Group IIB Iodide (ZnI2, CdI2, HgI2) Films

Group IIB iodides are layered structured semiconductors and are chosen particularly for studying the residual stress dependence of band gap because of their anisotropy along and across the layers. The development of residual stress with film thickness, substrate temperature, annealing temperature etc. was studied using X-ray diffraction in these films grown by vacuum evaporation. The three compounds show completely different growth and residual stress behaviors. The room temperature optical band gaps of these films were determined by optical absorption measurements. Only ZnI2 exhibits excitonic absorption at room temperature. The optical band gaps of ZnI2 and HgI2 correlate linearly with residual stress while CdI2 shows nonlinear behavior. The magnitude of band gap variation with residual stress can be interpreted as due to the affected cation-cation, cation-anion and anion-anion bond lengths. The ionicity of these three compounds decreases linearly with their cation-anion radii ratio and the band gap increases with ionicity.