Hot-wire chemical vapor deposition of silicon and silicon nitride for photovoltaics: Experiments, simulations, and applications.

摘要

Hot-wire chemical vapor deposition is a promising technique for deposition of thin film silicon for photovoltaics. Fundamental questions remain, however, about the gas-phase and surface-kinetic processes involved. To this end, the nature of the decomposition process has been studied in detail by use of mass spectrometry. Catalysis was evident for SiH₃ production with the use of a new wire, while aged wires appear to produce radicals by a non-catalyzed route. Large quantities of silicon were present at the surface, consistent with a silicide layer.; Threshold ionization mass spectrometry revealed large quantities of the SiH2 radical, attributed to heterogeneous pyrolysis on the walls of the reactor. At dilute (1% in He) silane pressures of up to 2 Torr, a negligible amount of ions and silicon agglomerates (Si₂, Si₂H, Si ₂H₆) were detected. Density functional theory calculations reveal an energetically favorable route for the reaction of Si and SiH ₄, producing Si₂H₂ and H₂. Two-dimensional Monte Carlo simulations were used to model a hot-wire reactor, showing that filament arrays can be used to improve film growth uniformity. Continuum simulations predict a maximum growth rate of 10 nm/s for dilute (1%) silane conditions and a rate of 50 nm/s for pure silane.; Hot-wire chemical vapor deposition was used to deposit silicon nitride films with indices of refraction from 1.8–2.5 and hydrogen content from 9–18 atomic %. By tuning the SiH₄/NH₃ flow ratio, films in which the hydrogen was predominantly bound to N or Si could be produced. Platinum-diffused silicon samples, capped by a hydrogenated silicon nitride layer revealed, upon annealing at 700°C, platinum-hydrogen complexes with a bulk concentration of 1014 cm−3. Photovoltaic cells employing a hot-wire nitride layer were found to have comparable electrical properties to those using plasma nitride layers.; Finally, a method for in situ generation of SiH ₄ by atomic hydrogen etching was evaluated. Using a cooled crystalline silicon target in an H/H₂ ambient produced negligible etching, while a cooled amorphous silicon film target was etched at a rate of up to 14 nm/min. In the latter case, net deposition at 0.6 nm/min onto a heated Ge(100) substrate resulted.