We present a systematic study on the correlation of hydrogen dilution profiles to structural properties materials and solar cell performance in nc-Si:H solar cells. We deposited nc-Si:H single-junction solar cells using a modified very high frequency (VHF) glow discharge technique on stainless steel substrates with various profiles of hydrogen dilution in the gas mixture during deposition. The material properties were characterized using Raman spectroscopy, X-TEM, AFM, and C-AFM. The solar cell performance correlates well with the material structures. Three major conclusions are made based on the characterization results. First, the optimized nc-Si:H material does not show an incubation layer, indicating that the seeding layer is well optimized and works as per design. Second, the nanocrystalline evolution is well controlled by hydrogen dilution profiling in which the hydrogen dilution ratio is dynamically reduced during the intrinsic layer deposition. Third, the best nc-Si:H single-junction solar cell was made using a proper hydrogen dilution profile, which caused a nanocrystalline distribution close to uniform throughout the thickness, but with a slightly inverse nanocrystalline evolution. We have used the optimized hydrogen dilution profiling and improved the nc-Si:H solar cell performance significantly. As a result, we have achieved an initial active-area cell efficiency of 9.2% with a nc-Si:H single-junction structure, and 15.4% with an a-Si:H/a-SiGe:H/nc-Si:H triple-junction solar cell structure.

The ‘cone kinetics’ model, which explains development of cone-shaped inclusions during nanocrystalline silicon film growth and during low-temperature silicon epitaxy breakdown, is extended to protocrystalline (‘edge’) amorphous silicon, Si heterojunctions and other Si film morpholologies. We generalize the physics underlying cone formation and present diagrams that delineate the deposition regimes giving rise to the different film morphologies; these regimes are determined by the nucleation rate of the second phase and the relative growth rate of the phases present. The model predicts cone growth during thin-film deposition by plasma-enhanced and other chemical vapor deposition techniques when isotropic growth is coupled with the isolated nucleation of a second phase with a higher growth rate. Protocrystalline amorphous silicon and other embedded crystallite phases are formed when a second phase successfully nucleates but has a smaller growth rate than the surrounding amorphous film. The cone kinetics phase diagram provides a simple explanation of the various nanocrystalline film morphologies observed when silane precursors are diluted with different concentrations of hydrogen gas.

Fast epitaxial growth of several microns thick Si at glass-compatible temperatures by the hot-wire CVD technique is investigated, for film Si photovoltaic and other applications. Growth temperature determines the growth phase (epitaxial or disordered) and affects the growth rate, possibly due to the different hydrogen coverage. Stable epitaxy proceeds robustly in several different growth chemistry regimes at substrate temperatures above 600°C. The resulting films exhibit low defect concentrations and high carrier mobilities.

Previously, we reported improved silicon epitaxy by hot-wire chemical vapor deposition (HWCVD) between about 600 and 650°C. Such temperatures are compatible with the thickening of large-grained Si seed layers on borosilicate glasses or other inexpensive substrates. Here, we provide detailed real-time spectroscopic ellipsometry (RTSE) and x-ray diffraction (XRD) analysis of two films grown near 600°C. A film grown at 594°C shows breakdown to a polycrystalline phase, while a film grown at 627°C is entirely epitaxial. Transmission electron microscopy (TEM) of this epitaxial film shows dislocation defects that originate at the substrate/film interface, suggesting that an optimized surface preparation could yield lower defect densities.

During the last few years, hot-wire chemical vapor deposition (HWCVD) has been explored as a low-temperature process for epitaxially thickening c-Si seeds layers on low cost substrates. Here, we demonstrate HWCVD epitaxy on thin polycrystalline silicon seed layers on borosilicate glass substrates. The crystal Si seeds are large-grained (~10 µm) polycrystalline silicon that were fabricated by Al-induced crystallization of a-Si. We report the growth of 0.5 µm of epitaxy at ~670°C.

We grow epitaxial silicon films onto (100) silicon wafers from pure silane by hot-wire chemical vapor deposition (HWCVD). The films grow epitaxially for a thickness hepi before a Si:H cones nucleate and expand. We study the dependence of hepi on growth rate and the differences between Ta and W filaments. The surface morphology of thin but completely epitaxial films are studied in order to correlate the surface roughness during growth with the eventual epitaxial breakdown thickness. Surface roughness, strain and H at the wafer/film interface are not likely to cause the observed breakdown.

Solid state crystallization of hydrogenated amorphous silicon (a-Si:H) prepared by hot-wire CVD is studied in solid phase epitaxy mode. By using a novel optical method combined with cross-sectional TEM and SIMS, a reduction of epitaxial growth speed is observed with increase in a-Si:H film thickness. Namely, in films thinner that 0.5 micron, solid phase epitaxy velocity depends linearly on film thickness. As the film thickness increases beyond 1 micron, the average velocity of solid phase epitaxy decreases considerably with respect to that in thinner films. In this regime, its velocity becomes also time-dependent: initial slow propagation of crystallization front gets considerably accelerated after the front has traveled above 400nm. SIMS thickness profiles of hydrogen shows considerably more residual hydrogen in thicker films after the start of solid phase epitaxy. In addition, prolonged pre-dehydrogenation at lower temperatures results in the increase in the average epitaxy speed in thicker films. These phenomena are likely related to delayed hydrogen outdiffusion in thicker films, which also leads to time-dependent speed of the solid-phase epitaxy front. We conclude that excess residual hydrogen reduces the rate of solid-phase crystalline growth.

We have studied silicon films grown epitaxially on silicon wafers using hot-wire chemical vapor deposition (HWCVD) with a tantalum filament. Silicon films were grown on (100)-oriented hydrogen terminated silicon wafers at temperatures from 175°C to 480°C, using a Ta filament 5 cm from the substrate to decompose pure SiH4 gas. The progression of epitaxy was monitored using real-time spectroscopic ellipsometry (RTSE). Analysis using RTSE, transmission electron microscopy (TEM), and scanning electron microscopy shows that at a characteristic thickness, hepi all of the films break down into a-Si:H cones. Below 380°C, both hepi and the thickness of the transition to pure a-Si:H increase with increasing temperature. Above 380°C, hepi was not observed to increase further but TEM images show fewer defects in the epitaxial regions. Secondary ion-mass spectrometry shows that the oxygen concentration remains nearly constant during growth (<1018 cm-3). The hydrogen concentration is found to increase substantially with film thickness from 5·1018 to 5·1019 cm-3, likely due to the incorporation of hydrogen into the a-Si:H cones that grow after the breakdown of epitaxy.

We use in-situ real-time spectroscopic ellipsometry to observe the breakdown of silicon epitaxy during growth by hot-wire chemical vapor deposition (HWCVD) on Si (100) substrates. Representative data is presented for the two types of epitaxy breakdown that we have observed: 1) an immediate transition to hydrogenated amorphous silicon (a-Si:H), and 2) a slower transition where a-Si:H cones nucleate and grow until they eclipse further epitaxial growth. Simple models, consistent with transmission-electron and atomic-force micrographs, describe the evolution of both types of breakdown showing that real-time spectroscopic ellipsometry is a useful tool for monitoring the growth of epitaxial silicon.