Fluorine-Induced Microvoids in Amorphous Silicon

The effects of fluorine incorporation on the microstructural and electronic properties of a-Si:H:F with 1–7 at.% F have been systematically studied. Films were prepared by direct photo-CVD of disilane with xenon difluoride as the fluorine source. The fluorine content was measured by infrared spectroscopy, xray photoelectron spectroscopy (XPS) and electron microprobe. Infrared spectra show that as the fluorine content increases, silicon dihydride bonding increases. Density measurements confirm that this is associated with an increase in microvoid content. With the increase in F and SiH2, the photoconductivity decreases over two orders of magnitude. A review of the literature shows that the appearance of SiH2 is a universal result of >1% F incorporation and is not limited to the present study. This suggests that fluorination of amorphous silicon is not beneficial for photovoltaic application.