Initial stages of microcrystalline silicon film growth

Abstract

μc-Si:H films with thicknesses increasing from 5 to 350 nm, which are deposited by the new closed-chamber cyclic chemical vapor deposition method, have been studied. The structure of the samples versus film thickness has been investigated by scanning tunneling microscopy (STM), scanning electron microscopy (SEM), Raman scattering, and ultra-violet (UV)-reflection spectroscopy. The carrier transport is characterised by dark and photoconductivities, as well as by the ambipolar diffusion length obtained from the steady-state photocarrier grating method (SSPG). Optical absorption is measured by transmission spectroscopy and by the constant photocurrent method (CPM). A gradual change of all properties has been found with increasing film thickness. While the transition from amorphous to highly crystalline structure is completed within some 50–80 nm, the lateral carrier transport properties saturate at about 200 nm. The transition layer thickness can be reduced by soft deposition conditions during the nucleation phase. A growth model of initially cone-shaped crystallites merging into a compact crystalline layer at higher thicknesses is proposed. Transport could then proceed on percolation paths along the crystallites.