Correlation of indentation-induced phase transformations with the degree of relaxation of ion-implanted amorphous silico
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The probability for amorphous silicon (a-Si) to phase transform under indentation testing is statistically determined as a function of annealing temperature from the probability of a pop-out event occurring on the unloading curve. Raman microspectroscopy is used to confirm that the presence of a pop-out event during indentation is a clear signature that a-Si undergoes phase transformation. The probability for such a phase transformation increases with annealing temperature and reaches 100% at a temperature of 340 °C, a temperature well before the temperature where the average bond-angle distortion is fully minimized. This suggests that multiple processes are occurring during full relaxation.
I. INTRODUCTION
Over recent years, amorphous silicon (a-Si) has found widespread use in large area electronics. Two particular examples are thin-film transistors1 and second generation photovoltaic cells.2 Despite its use in such mature application areas, much research continues into its structure and properties as a result of a lack of detailed understanding of these areas. Studies of a-Si indicate that many of its properties are dependent on the preparation technique and thermal history.3 For example, when a-Si obtained by Si ion implantation into crystalline silicon (c-Si) is thermally annealed, the Si-Si network undergoes extensive restructuring or structural relaxation toward a state of lowest free energy. This relaxation leads to changes in its vibrational,4–6 structural,7 mechanical,8 and electrical9 properties. Property variations are also observed to be influenced by its microstructure and impurity content. It is also known that hydrogen interacts with defects in the a-Si matrix, thus reducing the number of band gap states,10 which results in improvement in the electrical (transport) properties of a-Si. In addition, a-Si prepared by vacuum evaporation is known to produce a-Si layers that contain voids, and such layers may take up impurities such as hydrogen, oxygen, and carbon upon exposure to air.11 Especially, atomic hydrogen is known to be a successful passivator of dangling bonds, can thus also reduce the Si-Si network strain12 and hence also contributes to changes in the network as observed by Raman microspectroscopy.13 However, a-Si prepared by ion implantation is widely reported to be pure and voidless.14 Thus, ion-implanted a-Si, where variation in microstructure a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2013.32 1056
J. Mater. Res., Vol. 28, No. 8, Apr 28, 2013
http://journals.cambridge.org
Downloaded: 21 Mar 2015
and impurities can be eliminated, is an ideal amorphous material for studying the effect of thermal annealing on its structure. As ion-implanted a-Si is formed via highly nonequilibrium processes, it is to be expected that its structure contains a high concentration of defects, e.g., dangling bonds, floating bonds, and vacancy type defects. Despite the lack of voids, upon annealing, hydrogen diffuses into a-Si from the environment, and thus coul
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