Final PhD Seminar

Indentation-induced Phases of Silicon

Date & time

Fri 30 Jun 2017, 2–3pm

Location

Room:

Seminar Room (414)

Audience

Members of RSPE welcome

Contact

(02)61251598
Mr Sherman Wong
ANU

Silicon (Si) is the backbone of the semiconductor industry, with its huge impact largely due to its useful electrical and optical properties in its standard, diamond cubic 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, a body-centred cubic phase (bc8-Si) has been reported to have an ultra-narrow band-gap whereas a 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 onto a sample surface using point-loading of pressure via indentation. This thesis addresses several challenges regarding the formation, stability, and properties of this bc8/r8 mixed structure.

Firstly, 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 mixed structure that causes an elongation of the unit cell along the axis of indentation. Further, the 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 mixed structure. As the optical properties of bc8-Si have been reported, an estimate for the r8-Si absorption coefficient is calculated from this absorption increase.

Secondly, the nucleation of phase transformation under indentation-induced pressure was investigated. Such transformation is commonly observed alongside other plastic deformations within the surrounding crystalline lattice (labelled collectively as ``crystalline defects'' within this work). It was shown that both phase transformation and the formation of crystalline defects are nucleation limited. Thus, holding the sample at maximum load for a duration increased the number of these plastic deformations that were observed. Further, it is 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 within which phase transformation is the first form of plastic deformation result in larger, more uniform regions of phase transformed material. 

Finally, 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 Celsius. Si-XIII further transforms to a hexagonal structure (hd-Si) at 240 degrees Celsius, and then transforms back to dc-Si (in a nanocrystalline form) at 750 degrees Celsius. A transformation from bc8-Si to hd-Si is also proposed at a temperature below 240 degrees Celsius. 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.

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