Photocarrier diffusion lengths of high-growth-rate microcrystalline silicon

We report on photocarrier transport of high-growth-rate microcrystalline Si (μc-Si) in conjunction with the lateral size, σL, 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, σL increased from 70 nm to 590 nm. At the same time, the ambipolar diffusion lengths, Lamb, of photocarriers, observed using the steady-state photocarrier grating (SSPG) technique, increased from 50 nm to 420 nm. Log–log plots of Lamb versus d and σL versus d were both expressed as a power law with an exponent of 0.9, yielding a simple linear relation between Lamb and σL. Moreover, their ratio, Lamb/σL, 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.