A Phase Diagram of Low Temperature Epitaxial Silicon Grown by Hot-wire Chemical Vapor Deposition for Photovoltaic Devices

We have investigated the low-temperature epitaxial growth of thin silicon films by hotwire chemical vapor deposition (HWCVD). Using reflection high energy electron diffraction (RHEED) and transmission electron microscopy (TEM), we have found conditions for epitaxial growth at low temperatures achieving twinned epitaxial growth up to 6.8 μm on Si(100) substrates at a substrate temperature of 230°C. This opens the possibility of growing high quality films on low cost substrates. The H2:SiH4 dilution ratio was set to 50:1 for all growths. Consistent with previous results, the epitaxial thickness is found to decrease with an increase in the substrate temperature. INTRODUCTION HWCVD epitaxial growth on large-grained templates is one strategy for the fast, lowtemperature growth of large-grained films with hydrogen-passivated low-angle grain boundaries. We propose a structure in which a polycrystalline silicon template with grain sizes on the order of ten microns is fabricated on glass coated with a transparent, conductive oxide (TCO), by a solid-phase crystallization process called selective nucleation solid phase epitaxy (SNSPE) [1]. The template layer is designed to serve as the n+ layer of the device, and as the epitaxial template for n and p+ layers grown by HWCVD, using phosphine and tri-methyl boron as dopants (Fig. 1). Figure 1: Schematic of proposed photovoltaic device incorporating epitaxial Si growth on a large-grained polycrystalline template fabricated by SNSPE. SNSPE Poly-Si n+template HWCVD epitaxy 1-10 μm n-type layer Glass Grain Boundaries