Transport anisotropy in microcrystalline silicon studied by measurement ofambipolar diffusion length

We have studied charge transport anisotropy in microcrystalline silicon (mu c-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by su rface photovoltage method (SPV). We have developed a SPV evaluation procedu re which allowed us to exclude the effect of light scattering at the natura lly rough surface of the muc-Si:H. The procedure allows us to deduce not on ly the diffusion length, but also the depth of the depletion layer at the s urface and recombination coefficients at both top and bottom interfaces of the film. With growing muc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increa se of the diffusion length and of the surface depletion layer depth. For al l samples the diffusion length perpendicular to the substrate was several t imes higher than the diffusion length parallel to it, clearly confirming pr evious indication of the transport anisotropy resulting from the measuremen ts of coplanar and sandwich conductivity. (C) 2001 American Institute of Ph ysics.