Electrical and optical properties of doped a-SiGe:H deposited by RF-sputtering

The effects of doping on the electrical and optical properties of RF-sputtered a-Si_xGe_1-x:H (x=0.70) are investigated. The films were deposited at a constant substrate temperature of 200°C, and n and p-doped samples were obtained by introducing controlled amounts of phosphine and diborane, respectively, in the sputtering atmosphere. The conductivity increased by about two orders of magnitude for the n-type samples and by one order of magnitude for the p-type samples. Thermoelectric power measurements confirmed that doping was achieved. The optical gap did not seem to vary upon doping. The presence of germanium and oxygen is responsible for an increase of the localized states in the gap and of the width of the conduction band tail states. Therefore, these films show less sensitivity to doping than those of a-Si:H deposited under similar conditions. A model for the density of states in the gap, which includes the effects of impurities (oxygen), , is presented in order to explain the observed results