Vickers indentation tests were performed, on the (001) faces of single crystals, in darkness or under laser light having a wavelength which was close to the band absorption edge. When loads of 0.196N or less were applied to the indenter, illumination resulted in a decrease in the Vickers hardness. This confirmed the previously reported softening effect of photonic excitation during compression experiments. However, the effect of illumination was less marked for microhardness than for plastic flow data. The spectral dependence of the negative photoplastic effect was investigated, and it was shown that the mechanism which was responsible for the illumination-induced softening operated on both sides of the band absorption edge. The defect structure around micro-indents was studied by means of transmission electron microscopy (200kV or 1MV); with particular regard to indentation rosettes. Rosette arms (which expanded along perpendicular <110> directions) contained perfect dislocations with Burgers vectors that were parallel to the surface, but micro-twins were formed only on {111} planes in a zone with [110]. Perfect dislocations nucleated, in the bulk, as elongated half-loops. On the other hand, twinning dislocations nucleated on the indented surface. In the dark, perpendicular rosette arms had approximately the same length and no asymmetry was observed under the present experimental conditions of low applied load and ambient temperature. Under infra-red illumination, the rosette pattern had a well-defined 2-fold symmetry. That is, the movement of α dislocations was enhanced by photonic excitation whereas the movement of β dislocations appeared to be rather insensitive to illumination. The results were explained in terms of radiation-enhanced dislocation glide mechanisms.

Vickers Indentation on the {001} Faces of GaAs under Infrared Illumination and in Darkness S.Koubaïti, C.Levade, G.Vanderschaeve, J.J.Couderc: Philosophical Magazine A, 2000, 80[1], 83-104