The Effects of High Pressure and Shear on the Structure of Silicon

Silicon is a cornerstone of modern technology, predominantly utilised in its conventional diamond cubic (dc-Si) phase for a variety of semiconductor devices. However, there are intrinsic limitations of dc-Si which have spurred significant interest in alternative semiconductors, including other phases of silicon. These novel phases possess a range of advantageous properties that could be used for various applications. However, there is currently a lack of knowledge of how to reliably synthesise these phases, which is limiting their use. The overarching aim of this thesis is to determine the mechanisms behind how exotic phases of silicon form. A particular focus will be the use of non-hydrostatic compression conditions, where high pressures and shear stresses can be used to promote phase transformations from different types of silicon precursors. The effects of non-hydrostatic conditions on the structure of powdered dc-Si precursors were investigated both in situ [during high pressure treatment in a diamond anvil cell (DAC)] and ex situ (following treatment by analysing the recovered sample). A central focus was to determine how the application of shear-stress influences the phase transformation pathway. Notably, the results revealed that non-hydrostatic conditions lowered the transition threshold of dc-Si to the metallic white tin silicon (β-Sn-Si) phase to ~9 GPa, lower than ~11-12 GPa usually needed to begin this transformation under hydrostatic conditions. Furthermore, a distinct preferred crystallographic orientation was found to occur in the recovered body-centred cubic silicon (bc8-Si) phase in which the crystals were found to be aligned with their <110> directions normal to the compression axis. The origin of this orientation was found to be caused by the extremely anisotropic simple hexagonal silicon (sh-Si) parent phase’s tendency to orientate under the influence of the anisotropic stress field, which was then passed on to subsequent daughter-phases via displacive phase transformations. The speed of decompression was also found to be important, with a rapid release of the pressure leading to the formation of bulk amorphous silicon (a-Si) containing small clusters of bc8-Si crystals ~10-50 nm in size. This thesis investigated the impact of actively applying shear-stress on the phase transformations of powdered dc-Si utilising a novel rotational DAC (rDAC) in which one anvil could be rotated relative to the other. This work revealed that even small rotations at low pressures could induce significant changes in microstructure. It was shown that the metallic β-Sn-Si phase could be formed by applying shear at pressures as low as ~4 GPa (more than 50% lower than in the absence of shear). However, the resulting bc8/r8-Si (rhombohedral silicon) mixture formed upon pressure release, which is commonly considered stable, reverted to dc-Si after a relaxation period of ~24 hours. This indicated that the phases produced by applying shear may not be as stable as those under pressure alone. Significant shear was found to produce a mixture of stable dc/bc8-Si phases in the recovered sample at pressures of ~5.6 GPa. This work showed that using an rDAC to increase the level of shear is a promising method of promoting phase transformations in silicon with the potential of increasing the yield of novel phases. Investigations into the impact of non-hydrostatic pressure on different types of silicon precursors, including single crystal dc-Si and bc8-Si, were conducted. It was found that compressing single crystal dc-Si along the <100> direction results primarily in deformation along {111} planes, producing stacking faults which accumulate, forming β-Sn-Si, which subsequently transforms into r8-Si and bc8-Si upon decompression. This result provided new insights into the mechanism of silicon phase transformations by showing that they are both shear mediated and growth limited. In contrast, compression along the <111> direction in single crystal silicon resulted in the formation of bands of a-Si throughout the dc-Si crystal, separated by defects along the {111} planes. This result was explained by the transformation of β-Sn-Si into a-Si as the stress released too quickly for other transformations to occur. Overall, this work advances the understanding of silicon’s phase transformation behaviour under extreme conditions by highlighting the vital roles of non-hydrostatic compression and shear. The findings offer a useful avenue for facilitating the formation of novel silicon materials that have potentially useful electrical and optical properties by lowering the pressure transformation threshold required for their formation.<p></p>