Abstract
A decade ago Gilman proposed that the hardness of semi-conducting materials was controlled by a semi-conducting to metallic phase transformation at the very high hydrostatic compressive stresses developed under the indenter. In this work we report on a systematic investigation of the hardness and elastic modulus properties of a number of elemental semi-conductors (Si and Ge), as well as compound semi-conductors (GaAs, InP, GaN, ZnO) using nano-indentation and associated cross-sectional TEM. In all instances cross-section were made of impressions at various indentation loads using focused ion beam milling (FIB). Indentations were primarily made with a small spherical tipped indenter, which enabled the transition from elastic to elastic-plastic behaviour to be quantified. Complimentary observations of the residual impressions with AFM and Raman micro-probe spectroscopy were made as well as some electrical conductivity measurements during the indentation cycle. It was observed that only in the case of silicon was definitive evidence found for a pressure induced phase transformation beneath the indenter. In all other materials deformation occurred by classic plastic deformation, namely dislocation motion and twinning. In the case of silicon it was found that the resultant phase transformation observed in TEM sections from beneath the indenter depended strongly on the unloading rate. This presentation is based upon the PhD thesis of Ms J. Bradby.
