Nanoindentation of Ion Implanted and Deposited Amorphous Silicon
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R10.3.1/T6.3.1
Nanoindentation of Ion Implanted and Deposited Amorphous Silicon J. S. Williams, B. Haberl, J. E. Bradby Department of Electronic Materials Engineering, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, ACT 0200, Australia ABSTRACT The deformation behavior of both ion-implanted and deposited amorphous Si (a-Si) films has been studied using spherical nanoindentation, followed by analysis using Raman spectroscopy and cross-sectional transmission electron microscopy (XTEM). Indentation was carried out on both unannealed a-Si films (the so-called unrelaxed state) and in ion implanted films that were annealed to 450oC to fully relax the amorphous film. The dominant mode of deformation in unrelaxed films was via plastic flow of the amorphous phase rather than phase transformation, with measured hardness being typically 75-85% of that of crystalline Si. In contrast, deformation via phase transformation was clearly observed in the relaxed state of ion implanted a-Si, with the load–unload curves displaying characteristic discontinuities and Raman and XTEM indicating the presence of high-pressure crystalline phases Si-III and Si-XII following pressure release. In such cases the measured hardness was within 5% of that of the crystalline phase. INTRODUCTION It is well known that diamond cubic Si-I undergoes a series of pressure-induced phase transformations during mechanical loading using both diamond anvil and indentation apparatus. It has been shown that Si-I first transforms to a metallic β-Sn (Si-II) phase during loading at a pressure of ~11 GPa [1-5]. During pressure release, the material undergoes further transformation to crystalline phases (Si-III and Si-XII) and, during fast unloading in indentation experiments, can transform to an a-Si phase [3]. For a-Si, early indentation measurements on ion-implanted amorphous films showed that the mechanical deformation appeared to depend on the preparation conditions or the “state” of the amorphous phase [6]. The so-called relaxed a-Si state [7] (in which the implanted amorphous layer is annealed to ~450 °C prior to indentation) exhibited very similar load–unload indentation curves to those of crystalline Si, including a similar hardness. However, unannealed (or unrelaxed) a-Si appeared to deform plastically at lower loads and exhibit a somewhat lower hardness. The more recent indentation measurements of Follstaedt et al. [8] confirmed that the hardness of relaxed ion implanted a-Si approached that of Si-I but that unrelaxed a-Si was slightly softer. Our very recent measurements [9] on ion implanted a-Si have shown that the relaxed state undergoes phase transformations under indentation whereas the unrelaxed state appears to flow plastically. In this paper, we extend such measurements to compare the indentation behaviour of both ion implanted and CVD-deposited amorphous Si. EXPERIMENTAL DETAILS In this current study continuous layers of a-Si were prepared by one of two methods: by ionimplantation of (100) Si-I wafe
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