Photoplastic effects in nanoindentation experiments

The impact of light on hardness of II-VI semiconductor ZnSe was studied by nanoindentation. A positive photoplastic effect (PPE), that is a hardness increase by 20% from 2.05 GPa in darkness to 2.45 GPa under illumination (16 mN load), was found. The PPE amplitude (hardness change) exhibited saturation for moderate light intensities of about 15 mW cm(-2). It increased with decreasing loading speed and decreasing effective straining. The spectral sensitivity of the nanoindentation PPE is similar to that found in uniaxial deformation experiments, but displaying somewhat higher effects at shorter wavelengths. The PPE proved to be a dynamic effect; no alteration of penetration depth was found when changing the illumination state in the case of constant load. When varying the illumination state during loading, the resulting penetration depth was governed by the ratio of the volume that was displaced under light to that displaced in darkness.