 This density is 1.8±0.1% less dense than crystalline Si at 300 K. Silicon is one of the few elements that expands upon cooling and has a lower density as a solid than as a liquid.Unhydrogenated a-Si has a very high defect density which leads to undesirable semiconductor properties such as poor photoconductivity and prevents doping which is critical to engineering semiconductor properties.By introducing hydrogen during the fabrication of amorphous silicon, photoconductivity is significantly improved and doping is made possible.