Effect of oxygen on nanoscale indentation-induced phase transformations in amorphous silicon
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1185-II02-07
Effect of oxygen on nanoscale indentation-induced phase transformations in amorphous silicon S. Ruffell1 and J. S. Williams1 1 Department of Electronic Materials Engineering, Research School of Physics and Engineering, Australian National University, Canberra, ACT, 0200, Australia ABSTRACT 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. INTRODUCTION Nanoindentation-induced phase transformations in Si have attracted significant interest over the last few decades with more recent studies reported in nanoindentation of ion-implanted amorphous Si (a-Si) [2-6]. During loading, a transformation to the β-Sn phase (Si-II) occurs at a critical pressure of ~12 GPa. On unloading, the Si-II further transforms to either amorphous silicon (a-Si) or a mixture of high pressure polycrystalline phases (Si-III and Si-XII); the latter being favoured for slow unloading and is usually accompanied by a pop-out event [2, 3]. These pressure-induced transformations are well characterized [2-6], but the exact mechanisms behind the phenomena are still not well understood. In particular the proposed formation of the crystalline phases by a nucleation and growth mechanism has not been studied in detail. Recent studies have investigated these phase transformations during indentation of ionimplanted a-Si [5, 7]. It was found that the Si-III/Si-XII phases form more readily in an a-Si matrix compared to c-Si e.g. volumes of Si-III/Si-XII formed in a-Si with unloading rates over 3 orders of magnitude greater than the unload rates required to form the phases in c-Si. However, recent work by the current authors on plasma enhanced chemical vapour deposited (PECVD) aSi films found that the films do not undergo these phase transformations. The reasons for this are not understood but one possibility is that the high impurity content in the deposited films compared to a “pure” film created by ion-implantation prevents the formation of the high pressure phases. In particular, O and H are found in high concentrations (1019 to 1021 cm-3) in PECVD deposited films compared to ≤1018 cm-3 in a-Si formed by Si ion-implantation [8]. The aim of this study is to study the effect of O on the indentation-ind
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