Microcrystalline silicon-germanium solar cells fabricated using VHF PECVD

We report on the properties of microcrystalline silicon-germanium (muc-Si_1-xGe_x:H) alloy and solar cells fabricated using very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD). p+nn+ structures are deposited at 275-300degC using mixtures of silane, 10%germane (rest is hydrogen) and hydrogen. Raman and XRD techniques were used to determine the crystalline nature of the films. From quantum efficiency data we observe enhanced infrared absorption with incorporation of germanium as compared to microcrystalline silicon (muc-Si:H) devices. The transport properties measured include minority carrier lifetime, diffusion length and mobility. The minority carrier lifetime values, measured using reverse recovery technique, in muc-Si_1-xGe_x:H are found to be in the range of 150-250 ns. The diffusion length of holes measured in device structures was 0.5-2 mum, comparable to the typical values found in muc-Si:H. Hence, assuming a diffusion controlled transport, the estimated mobility, from diffusion length and lifetime measurements in muc-Si_1-xGe_x:H is higher than that of muc-Si:H.