Disorder and the optical properties of amorphous silicon grown by molecular beam epitaxy

We have produced a novel form of amorphous silicon (a-Si) using ultra-high-vacuum molecular beam epitaxy. By depositing silicon atoms onto a fused quartz substrate at ∼100°C, we have obtained a silicon based material that lacks the characteristic periodicity of crystalline silicon but nevertheless has 98% of its density. Infrared and secondary ion mass spectroscopic measurements demonstrate that there are only small amounts of hydrogen (0.1 at.%) in this clean form of a-Si, which contrasts dramatically with the case of conventional a-Si. The optical properties of this new form of a-Si are compared with those of conventional a-Si, and conclusions are drawn regarding the amount of disorder.