Effect of fluorine on the structural and electronic properties ofa-Si:H:F

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. 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, suggesting that fluorine induces the formation of voids which are lined with ${\mathrm{SiH}}_2$. With the increase in F and ${\mathrm{SiH}}_2$, the photoconductivity of the material decreases over 4 orders of magnitude. A review of the literature shows that the appearance of ${\mathrm{SiH}}_2$ is a universal result of >1 at. % F incorporation by many techniques and is not limited to the present study. Mechanisms by which fluorine can induce structural changes are evaluated. These results are contrasted with the use of fluorinated process gases to deposit microcrystalline films and Si-Ge alloys. This has implications for the incorporation of fluorine in photovoltaic devices.