Use of radio frequency power, silicon tetrafluoride and methane as parameters to tune structural properties of hydrogenated microcrystalline silicon carbon alloys

In the search for a material with electrical properties similar to those of amorphous silicon or amorphous silicon germanium, but stable under light soaking, hydrogenated microcrystalline silicon–carbon alloy (µc-Si1 − xCx:H) thin films are a promising candidate. The interest in these materials lies in the possibility of varying the effective band gap by changing the amount of carbon in the alloy composition, while keeping a high crystalline fraction to maintain stability under light-soaking. In this study, µc-Si1 − xCx:H thin films were deposited by radio frequency (RF) plasma enhanced chemical vapour deposition using a silane and methane gas mixture highly diluted in hydrogen. Three deposition parameters were investigated as a means to control the film crystallinity: the RF power density, the methane flow rate and the presence of a small amount of silicon tetrafluoride in the gas mixture. Although all three parameters can result in materials with a Raman crystalline volume fraction above 50%, it is shown that they result in very different microstructures, as evidenced by x-ray diffraction and scanning transmission electron microscopy analyses. A growth model is proposed to explain the influence of each of these parameters on the final film structure.