Effect of oxygen on nanoscale indentation-induced phase transformations in amorphous silicon

Ion-implantation has been used to introduce oxygen concentration-depth profiles into nominally oxygen-free amorphous silicon (a-Si). The effect of O concentrations in excess of 10¹⁸ cm^-3 on the formation of high pressure crystalline phases (Si-III and Si-XII) during indentation unloading has been studied. By examination of unloading curves and post-indent Raman microspectroscopy O is found to inhibit the so-called pop-out event during unloading and, therefore, the formation of the crystalline phases. Furthermore, at high O concentrations (> 10²¹ cm^-3) the formation of these phases is reduced significantly such that under indentation conditions used here the probability of forming the phases is reduced to almost zero. We suggest that the bonding of O with Si reduces the formation of Si-III/XII during unloading through a similar mechanism to that of oxygen-retarded solid phase crystallization of a-Si.