Photocarrier diffusion lengths of high-growth-rate microcrystalline silicon

We report on photocarrier transport of high-growth-rate microcrystalline Si (mu c-Si) in conjunction with the lateral size, sigma(L), of crystallites' conglomerate (grain) determined from the atomic force microscope (AFM) topographic images on the basis of fractal concepts. pc-Si films were prepared using very-high-frequency plasma-enhanced chemical vapor deposition at a high deposition rate of 6.8 +/- 0.5 nm/s. mu c-Si thicknesses, d, were varied from 0.53 mu m to 5.6 mu m. With an increase in d, sigma(L) increased from 70 nm to 590 nm. At the same time, the ambipolar diffusion lengths, L-amb, of photocarriers, observed using the steady-state photocarrier grating (SSPG) technique, increased from 50 nm to 420 nm. Log-log plots of L-amb versus d and sigma(L) versus d were both expressed as a power law with an exponent of 0.9, yielding a simple linear relation between L-amb and sigma(L). Moreover, their ratio, L-amb/sigma(L), was below unity, implying the intra-grain carrier diffusion. From these results, the role of the grain (column) boundaries for photocarrier diffusion in mu c-Si is discussed. (c) 2007 Elsevier B.V. All rights reserved.