The aging of tungsten filaments and its effect on wire surface kinetics in hot-wire chemical vapor deposition

Wire-desorbed radicals present during hot-wire chemical vapor deposition growth have been measured by quadrupole mass spectrometry. New wires produce Si as the predominant radical for temperatures above 1500 K, with a minor contribution from SiH3, consistent with previous measurements; the activation energy for the SiH3 signal suggests its formation is catalyzed. Aged wires also produce Si as the predominant radical (above 2100 K), but show profoundly different radical desorption kinetics. In particular, the Si signal exhibits a high temperature activation energy consistent with evaporation from liquid silicon. The relative abundance of the other SiHx species suggests that heterogeneous pyrolysis of SiH4 on the wire may be occurring to some extent. Chemical analysis of aged wires by Auger electron spectroscopy suggests that the aging process is related to the formation of a silicide at the surface, with silicon surface concentrations as high as 15 at. %. A limited amount (2 at. %) of silicon is observed in the interior as well, suggesting that diffusion into the wire occurs. Calculation of the relative rates for the various wire kinetic processes, coupled with experimental observations, reveals that silicon diffusion through the silicide is the slowest process, followed by Si evaporation, with SiH4 decomposition being the fastest

Synthesis of Large-Grained Polycrystalline Silicon by Hot-Wire Chemical Vapor Deposition for Thin Film Photovoltaic Applications

In this study, we investigate the fabrication of large-grained polycrystalline silicon by hot-wire chemical vapor deposition (HWCVD) and its suitability for thin-film photovoltaic applications. We have devised two strategies for the fast, low-temperature growth of thin polycrystalline silicon films on glass substrates. The first is the direct growth of polycrystalline silicon on SiO₂ by HWCVD. We use atomic force microscopy (AFM) to characterize fully continuous polycrystalline silicon films grown by HWCVD on SiO₂, as well as the nucleation density of silicon islands formed in the early stages of HWCVD growth, as a function of temperature and hydrogen dilution (H₂:SiH₄). Our observations of the nucleation kinetics of Si on SiO₂ can be explained by a rate-equation pair-binding model, from which we derive an estimate for the prefactor and activation energy for surface diffusion of Si on SiO₂ during HWCVD growth and assess the viability of this method for the rapid growth of large-grained polycrystalline silicon on SiO₂. The second strategy uses large-grained (~100 µm) polycrystalline silicon layers fabricated by selective nucleation and solid-phase epitaxy (SNSPE) on SiO₂ substrates as templates for epitaxial growth by HWCVD. Using reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM), we have derived a phase diagram for Si on Si(100) consisting of epitaxial, twinned epitaxial, mixed epitaxial/polycrystalline, and polycrystalline phases of growth on Si(100) in the 50 nm-2 µm thickness regime. Evidence is also presented for epitaxial growth on SNSPE templates, which use nickel nanoparticles as nucleation sites for the solid-phase crystallization of phosphorus-doped amorphous silicon on SiO₂. Minority carrier lifetimes for films on Si(100), as measured by resonant-coupled photoconductive decay experiments, range from 5.7 to 14.8 microseconds while those for films on SNSPE templates range from 5.9 to 19.3 microseconds. Residual nickel present in the SNSPE templates does not significantly affect the lifetime of films grown on SNSPE templates, making the growth of epitaxial layers by HWCVD on SNSPE templates a possible strategy for the fabrication of thin-film photovoltaics.</p