Giant & Controllable Photoplasticity and Photoelasticity in Compound Semiconductors

We show that the wide-band gap compound semiconductors ZnO, CdS, and ZnS feature large photo-plastic and photo-elastic effects that are mediated by point defects. We measure the mechanical properties of single crystals (ZnO and CdS) and ceramics (ZnS) using nanoindentation, and we find that elasticity and plasticity vary strongly with moderate illumination. For instance, the elastic stiffness of CdS can increase by 20% due to blue illumination of intensity 1.4 mW/cm². Above-band-gap illumination (<i>e.g.</i> UV light) has the strongest effect, and the relative effect of sub-band gap illumination varies between samples – a clear sign of defect-mediated processes. We show giant optomechanical effects can be tuned by materials processing and varying point defect concentration. The photo-plastic effect can be understood by a long-established theory of charged dislocation motion. The photo-elastic effect requires a new theoretical framework. Using density functional theory (DFT), we find that the point defect ionization is accompanied by large lattice distortions and large changes in elastic constants in both CdS and ZnS. DFT predicts a large photoelastic effect, on the order of 5% for realistic point defect concentration. Our results update the longstanding but lesser-studied field of semiconductor optomechanics, and suggest interesting applications.