Characterization and control of crystal nucleation in amorphous electron beam evaporated silicon for thin film solar cells

The kinetics of crystal nucleation in high-rate electron beam evaporated amorphous Si for polycrystalline thin film solar cells was systematically studied on SiN and selected ZnO:Al-coated glass substrates with dissimilar surface topographies by employing Raman spectroscopy, transmission electron microscopy, and optical microscopy. The influence of the surface topography of the substrate and the disorder of the deposited amorphous Si could be correlated to the respective characteristics of the transient and steady state regime of the nucleation rate. The steady state nucleation rate I-ss, its corresponding activation energy E-Iss, and consequently the size of the grains in the crystallized Si were found to be governed by the interplay between the surface roughness and the deposition temperature. The steady state nucleation rate I-ss increased gradually upon increasing the substrate roughness, while lowering the deposition temperature of the amorphous Si on rough textures resulted in a decline of I-ss. The time-lag tau, which represents a distinctive parameter for the transient regime, was only slightly affected by the substrate topography. The deposition temperature, however, had a significant influence on tau, with tau increasing by a factor of 8 upon lowering the deposition temperature from 300 to 200 degrees C for all substrate topographies. These characteristics could be correlated with the increasing structural disorder of the deposited a-Si upon decreasing the deposition temperature. Based on this analysis, we could determine design rules for the controlled preparation of large-grained poly-Si in minimized processing time on any of the used substrate types by individually adjusting the deposition temperature and implementing nucleation layers. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3627373]