Strain-rate insensitive photoindentation pop-in behavior in ZnS single crystals at room temperature

The structural and mechanical reliability of inorganic semiconductors for practical applications is determined by the rate at which they can deform and sustain externally applied strain. In this research, nanoindentation experiments under three strain rates and two light conditions were performed on single-crystal ZnS with an 80 nm radius Berkovich tip at a peak load of 60 µN. Significant pop-in events were observed in all indentation tests. The calculated maximum resolved shear stress at the first pop-in approximated the theoretical strength of ZnS, indicating a homogeneous dislocation nucleation process. The cumulative spreads of the maximum shear stress were found to be insensitive to the strain rate, and the distribution at a small strain rate was slightly broader than that of the other two strain rates because of thermal noise. Calculated activation energy ΔG required for the dislocation nucleation indicates that dislocation nucleation in ZnS could occur with external stress and without much assistance of thermal energy, leading to weak dependence of the first pop-in on the strain rate. At three strain rates, light consistently showed little influence on the pop-in behavior and dislocation nucleation process.