Hot-Wire Chemical Vapor Deposition of Silicon and Silicon Nitride for Photovoltaics: Experiments, Simulations, and Applications

Citation

Holt, Jason Knowles (2003) Hot-Wire Chemical Vapor Deposition of Silicon and Silicon Nitride for Photovoltaics: Experiments, Simulations, and Applications. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9S6-8J66. https://resolver.caltech.edu/CaltechETD:etd-11262002-143938

Abstract

Hot-wire chemical vapor deposition is a promising technique for deposition of thin amorphous, polycrystalline, and epitaxial silicon films for photovoltaic applications. Fundamental questions remain, however, about the gas-phase and surface-kinetic processes involved. To this end, the nature of the wire decomposition process has been studied in detail by use of mass spectrometry. Atomic silicon was the predominant radical formed for wire temperatures above 1500 K, and catalysis was evident for SiH₃ production with the use of a new wire. Aged wires appear to produce radicals by a non-catalyzed route and chemical analysis of these wires reveal large quantities of silicon at the surface, consistent with the presence of a silicide layer. This study is the first of its kind to correlate radical desorption kinetics with filament aging for the hot-wire chemical vapor deposition technique.

Threshold ionization mass spectrometry revealed large quantities of the SiH₂ radical, attributed to heterogeneous pyrolysis on the walls of the reactor. At dilute (1%) silane pressures of up to 2 Torr, a negligible amount of ions and silicon agglomerates (Si₂, Si₂H, Si₂H₆) were detected. Density functional theory calculations reveal an energetically favorable route for the reaction of Si and SiH₄, producing Si₂H₂ and H₂. The trace amounts of Si₂H₂ observed experimentally, however, may suggest that an intermediate spin state transition involved in this reaction is slow under the hot-wire conditions used. Monte Carlo simulations of the hot-wire reactor suggest SiH₃ is the predominant growth species under conditions leading to amorphous and polycrystalline growth. The flux of atomic hydrogen, rather than the identity of the precursor, appears to be the more important factor in governing the amorphous-to-microcrystalline transition that occurs upon hydrogen-dilution. Two-dimensional Monte Carlo simulations were used to model a hot-wire reactor for the first time, showing that filament arrays can be used to improve film growth uniformity. Under conditions where agglomerate formation does not occur, 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, each of which reveal different hydrogen release kinetics. Platinum-diffused silicon samples, capped by a hydrogenated silicon nitride layer revealed, upon annealing at 700^oC, platinum-hydrogen complexes with a bulk concentration of 10¹⁴ cm^-3. This constitutes the first direct evidence for bulk silicon passivation by hydrogen release from a silicon nitride layer and hydrogen complex formation. 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 SiH₄ 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. A method for more efficient etching of crystalline silicon materials was proposed.

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