Vickers indentation on the {001} faces of GaAs under infrared illumination and in darkness

Vickers indentation tests have been performed on the (001) faces of GaAs single crystals, in darkness and under laser light illumination with a wavelength close to the band absorption edge. When low loads (0.196 N or less) are applied to the indenter, illumination results in a decrease in the Vickers hardness. This confirms the softening effect of photonic excitation (negative photoplastic effect), as previously reported by Mdivanyan and Shikhsaidov (1988, Phys. Stat. sol., (a), 107, 131) from compression experiments. However, the effect of illumination is less marked on microhardness than on plastic flow. The spectral dependence of the negative photoplastic effect has been investigated; it is shown that the mechanism responsible for the illumination-induced softening is operative on both sides of the band absorption edge. The defect structure around the microindents has been studied by transmission electron microscopy (200 kV and 1 MV), with particular attention to indentation rosettes. Rosette arms (which expand along perpendicular (110) directions) contain perfect dislocations with Burgers vector parallel to the surface, but microtwins are formed only in (111) planes in zone with [110]. Perfect dislocations nucleate in the bulk as elongated half-loops; in contrast, twinning dislocations nucleate on the indented surface. In darkness, perpendicular rosette arms have approximately the same length; the alpha-beta asymmetry is not observed in the experimental conditions (light applied load, room temperature). Under infrared illumination the rosette pattern presents a well marked twofold symmetry; the movement of alpha dislocations is enhanced under photonic excitation whereas the movement of beta dislocations appears rather insensitive to illumination. These results are discussed in connection with the radiation-enhanced dislocation glide mechanisms.