On the physical nature of a photomechanical effect

A photomechanical effect was studied in single-crystal Si by a unified method. The dependences of the photomechanical effect on the spectrum and intensity of light, the residual photomechanical effect (the persistence of crystal softening for some time after the termination of indention), the effect of illumination on microhardness anisotropy, and the temperature dependence of the photomechanical effect were studied. Based on the experimental results and on the analysis of relevant published data, the correlation between the photomechanical effect and the corresponding density of photogenerated nonequilibrium carriers (so-called antibonding quasiparticles) is determined. This correlation suggests a mechanism according to which, in covalent crystals, microhardness decreases mostly owing to chemical bond weakening and isotropization caused by the photogenerated antibonding quasiparticles.