Structural characterization of pressure-induced amorphous silicon

Abstract

We investigate the structure and mechanical properties of pressure-induced (PI) amorphous silicon $(a\text{-Si})$ and compare this to the more extensively characterized case of $a\text{-Si}$ created by ion implantation. To study the effect of thermal history we also examine the structure of both PI and ion-implanted $a\text{-Si}$ after a low-temperature ``relaxation'' anneal $(450\text{ }\ifmmode^\circ\else\textdegree\fi{}\text{C})$. Indentation testing suggests that structural changes are induced by thermal annealing. As-prepared forms of $a\text{-Si}$ deform via plastic flow, while relaxed forms of $a\text{-Si}$ transform to high-pressure crystalline phases. These structural changes are confirmed by more explicit measurements. Raman microspectroscopy shows that the short-range order as expressed by the average bond-angle distortion of the as-prepared amorphous phases is the same and reduced by the same amount following the low-temperature anneal. Fluctuation electron microscopy demonstrates that the as-prepared PI $a\text{-Si}$ displays a much lower variance of the diffracted intensity, a feature directly correlated with the medium-range order, than the as-prepared ion-implanted $a\text{-Si}$. However, relaxation brings this variance of the two networks to the same intermediate level. The mechanical tests and structural probes indicate that annealing the amorphous silicon network can bring it to a common state with the same structure and properties regardless of the initial state. This final state might be the closest attainable to the continuous random network model.