Controlling Indentation-induced Phases of Silicon

Abstract

Silicon (Si) is the backbone of the semiconductor industry. The widespread use of Si is largely due to the useful electrical and optical properties of the material in its standard, diamond cubic (dc) crystal structure. However, in recent years, there has been an increasing interest in the properties of Si with different crystal structures (phases). In particular, the metastable phases formed through pressure application have been the topic of much study due to their promising properties. For example, the body-centred cubic phase (bc8-Si) has been reported to have an ultra-narrow band-gap whereas the rhombohedral phase (r8-Si) has been predicted to have an improved absorption coefficient across the solar spectrum. A mixture of these two exotic phases can be formed directly from a standard dc-Si semiconductor wafer using the application of pressure through point loading via indentation. This thesis addresses several challenges regarding the formation, stability, and properties of this bc8/r8 structure.

The process involving the nucleation of the bc8/r8 phase via indentation was investigated in detail. Specifically, the interplay between the nucleation of the bc8/r8 phase with other plastic deformation processes within the surrounding crystalline lattice (labelled collectively as ``crystalline defects'' within this work) that occur is studied. It was shown that both phase transformation and the formation of crystalline defects are nucleation limited. Thus, holding a volume of Si at high pressure for a duration increases the likelyhood that th material will plastically deform. It is also shown that these two forms of plastic deformation act as competing mechanisms, with the mode of incipient plasticity playing a dominant role in the shape and volume of the final phase transformed region. Indentations in which phase transformation occur before crystalline defects occur result in larger, more uniform regions of phase transformed material.

The phase fraction of r8-Si within the mixed structure is determined using Rietveld analysis of x-ray diffraction (XRD) data. The mixed structure was found to be predominantly r8-Si, with this phase comprising 60 to 80 percent of the structure. These results also show that there is residual stress within the bc8/r8 structure that causes an elongation of the unit cell along the axis of indentation. The optical absorption from a thin film of the bc8/r8 structure is measured using spectrophotometry. There is a clear increase in absorption due to the presence of the bc8/r8 structure. As the optical properties of bc8-Si are known from the literature, an estimate for the r8-Si absorption coefficient is calculated from this absorption increase.

The stability of the bc8/r8 mixed structure under thermal annealing is explored. A transformation from r8-Si to a novel phase with an, as yet, unknown crystal structure (Si-XIII) is reported after annealing to 100 degrees C. Si-XIII further transforms to a hexagonal structure (hd-Si) at 240 degrees C, and then transforms back to dc-Si (in a nanocrystalline form) at 750 degrees C. A transformation from bc8-Si to hd-Si is also proposed at a temperature below 240 degrees C. Therefore, there is a temperature range where hd-Si is both stable and the sole crystalline phase present within the transformed region. Laser-induced annealing results were also presented, and a similar transformation pathway was reported.

Thus, the formation, stability, and technologically interesting properties of the bc8/r8 structure is presented. This forms a framework for future studies into the scalability of this structure to technologically relevant sizes.