The high pressure phase transformations of silicon and germanium at the nanoscale

Abstract

Semiconducting materials are critical for the electronics industry with the two most important semiconductors being Si and Ge. Most Si and Ge has a diamond cubic (dc) structure. However, many other phases of Si and Ge that are accessible via the application of high pressure. Compression to 10-11 GPa leads to both Si and Ge phase transforming to a metallic (b-Sn) structure. On decompression, the b-Sn phase transforms into one of several metastable phases; bc8-Si, r8-Si, hd Si and Ge, and st12-Ge depending on a number of factors including temperature, decompression rate and shear.

The difference between nanoscale and bulk behaviour is a recurring theme of materials science over the past decades. However, this effect has not been investigated with respect to high pressure phase transformations of Si and Ge. To determine the effect of size on the phase transformations of Si and Ge, nanowires (NWs) were compressed and low load nanoindentation was performed. By using these two methods, other effects such as large pressure gradients in a sample and interaction with an underlying substrate could be probed. To add further understanding, the effect on temperature and decompression rate on these small volumes of Si and Ge is also investigated. To study the effects of size SiNWs of two sizes (80-150 nm and 200-250 nm) and GeNWs (40-60 nm) in diameter were compressed using a DAC, and low load nanoindentation of Si and Ge was performed at various temperatures and decompression rates. The materials were analysed using x-ray diffraction, Raman spectroscopy, and transmission electron microscopy.

At ambient temperature that both sets of SiNWs experienced a suppressed dc-Si to b-Sn-Si phase transformation, with some of the smaller diameter SiNWs observed to phase transform directly to sh-Si. On decompression b-Sn-Si was found to persist until lower pressures than in bulk-Si, and a-Si was the dominant end phase. These suppressed phase transformations were attributed to the small size of the SiNWs making nucleation of new crystalline phases difficult.

The effect of temperature on the high pressure phase transformation of the SiNWs was also investigated. Temperature was found to have a significant impact on the end phases formed. At low temperatures, a-Si was the dominant end phase, at moderate temperatures bc8-Si and dc-Si were present, and at the high temperatures dc-Si was the dominant end phase. This behaviour differed to bulk Si.

Nanoindentation at ambient temperature and 105C was performed, noting that the phase transformed material volume is also small. At low temperatures, a-Si was the dominant end phase. At the 105C, a-Si was the dominant phase for fast unloading, however the portion of r8/bc8-Si increased with the next lowest unloading rate. For indents in dc-Si at the slowest unloading rate, the only end phase observed was dc-Si. This phase may have formed via nucleation and growth from the underlying crystalline substrate.

A similar study was performed on Ge. Like in Si, the dc-Ge to b-Sn-Ge phase transformation was suppressed on compression and b-Sn-Ge persisted until lower pressures than bulk. The end phases of GeNWs were found to be a-Ge, hd-Ge, and dc-Ge. For nanoindentation of Ge, it was found that lowering the temperature of nanoindentation promotes the formation of r8-Ge and a-Si end phases instead of defective dc-Ge at ambient temperature.

These results further the understanding on how size, temperature and decompression affect the pressure induced phase transformation pathways of Si and Ge are formed. These results are of technological significance as the synthesis of near phase pure nanowires should allow for the testing of their properties.