Photocarrier diffusion lengths of high-growth-rate microcrystalline silicon

Abstract We report on photocarrier transport of high-growth-rate microcrystalline Si (μc-Si) in conjunction with the lateral size, σ L , of crystallites’ conglomerate (grain) determined from the atomic force microscope (AFM) topographic images on the basis of fractal concepts. μc-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. μc-Si thicknesses, d , were varied from 0.53 μm to 5.6 μm. With an increase in d , σ 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 σ 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 σ L . Moreover, their ratio, L amb / σ L , was below unity, implying the intra-grain carrier diffusion. From these results, the role of the grain (column) boundaries for photocarrier diffusion in μc-Si is discussed.