New insight into pressure-induced phase transitions of amorphous silicon: the role of impurities

The pressure-induced phase transformations of a form of amorphous silicon (a-Si) with well characterized impurity levels and structure are examined at pressures up to 40 GPa using in situ synchrotron X-ray radiation. At ∼12 GPa crystallization commences, but it is not completed until ∼16 GPa. At higher pressures, not all the crystalline phases observed for crystalline silicon (c-Si) appear. On pressure release, none of the metastable crystalline phases observed for c-Si nucleate. Instead an amorphous phase is re-formed. This is in contrast to all previous diamond-anvil studies on a-Si. If full pressure-induced crystallization occurred, the material remained crystalline on unloading. The formation of a-Si upon unloading was only observed when a high-density amorphous phase was reported on loading. The fully characterized nature of the a-Si used in this current study allows for the interpretation of this significant diversity in terms of impurity content of the a-Si used. Namely, this suggests that `ideal' (pure, voidless, structurally relaxed) a-Si will follow the same transition pathway as observed for c-Si, while crystallization of a-Si forms with a high impurity content is retarded or even inhibited. The a-Si used here straddles both regimes and thus, although full crystallization occurs, the more complex crystalline structures fail to nucleate.