We apply n- and p-type polycrystalline silicon (poly-Si) films on tunneling SiOx to form passivated contacts to n-type Si wafers. The resulting induced emitter and n+/n back surface field junctions of high carrier selectivity and low contact resistivity enable high efficiency Si solar cells. This work addresses the materials science of their performance governed by the properties of the individual layers (poly-Si, tunneling oxide) and more importantly, by the process history of the cell as a whole. Tunneling SiOx layers (<2 nm) are grown thermally or chemically, followed by a plasma enhanced chemical vapor deposition growth of p+ or n+ doped a-Si:H. The latter is thermally crystallized into poly-Si, resulting in grain nucleation and growth as well as dopant diffusion within the poly-Si and penetration through the tunneling oxide into the Si base wafer. The cell process is designed to improve the passivation of both oxide interfaces and tunneling transport through the oxide. A novel passivation technique involves coating of the passivated contact and whole cell with atomic layer deposited Al2O3 and activating them at 400 °C. The resulting excellent passivation persists after subsequent chemical removal of the Al2O3. The preceding cell process steps must be carefully tailored to avoid structural and morphological defects, as well as to maintain or improve passivation, and carrier selective transport. Furthermore, passivated contact metallization presents significant challenges, often resulting in passivation loss. Suggested remedies include improved Si cell wafer surface morphology (without micropyramids) and postdeposited a-Si:H capping layers over the poly-Si.

In this paper, we will describe the nature of defects and impurities in thick epitaxial-Si layers and their influence on the cell efficiency. These wafers have very low average dislocation density. Stacking faults (SFs) are the main defect in epi layers. They can occur in many configurations—be isolated, intersecting, and nested. When nested, they can be accompanied by formation of coherent twins resulting in dendritic growth, with pyramids protruding out of the wafer surface. Such pyramids create large local stresses and punch out dislocations. The main mechanism of dislocation formation is through pyramids. Stacking faults degrade solar cell performance. Analyses of the solar cells have revealed that the nested SFs have a controlling effect on the solar cell performance. A well-controlled growth can minimize defect generation and produce wafers that can yield cell efficiencies close to 20%.

This paper identifies some mechanisms that lead to problems in back Al contact formation. Major issues are related to a basic problem that the Al melt has a large surface tension and tries to ball up during the firing step. Other issues arise from dissolution of the Si-Al interface and entrapment of glass within the Si-Al alloy. Si diffusion into Al can be applied to control the melt, while cooling rate can help improve the structure of various regions of the back contact for a favorable series resistance. We also discuss a modified time-temperature profile that can lead to a deep and uniform back-surface field.

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.

Solid-phase crystallization (SPC) rates are compared in amorphous silicon films prepared by three different methods: hot-wire chemical vapor deposition (HWCVD), plasma-enhanced chemical vapor deposition (PECVD), and electron-beam physical vapor deposition (e-beam). Random SPC proceeds approximately 5 and 13 times slower in PECVD and e-beam films, respectively, as compared to HWCVD films. Doping accelerates random SPC in e-beam films but has little effect on the SPC rate of HWCVD films. In contrast, the crystalline growth front in solid-phase epitaxy experiments propagates at similar speed in HWCVD, PECVD, and e-beam amorphous Si films. This strongly suggests that the observed large differences in random SPC rates originate from different nucleation rates in these materials while the grain growth rates are relatively similar. The larger grain sizes observed for films that exhibit slower random SPC support this suggestion.

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 incorporate narrow-gap amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (HWCVD) into single-junction n-i-p solar cells, and improve both fill factor (FF) and open-circuit voltage (Voc) by bandgap grading. The Tauc bandgap (ET) of the a-SiGe:H is as low as about 1.25 eV. Previously [1], we obtained a short-circuit current density (Jsc) up to 20 mA/cm2 in an n-i-p device incorporating an ungraded 120-nm i-layer of 1.25-eV a-SiGe:H. However, without buffer layers or bandgap profiling, the fill factor was only 38%, likely due to an abrupt bandgap transition and poor hole collection. To overcome these problems, we have used composition bandgap profiling throughout the i-layer and improved both Voc and FF significantly without any Jsc loss. The solar cell efficiency is improved from 3.55% to 5.85% and Voc rises from 0.475 to 0.550 eV. This improved single-junction a-SiGe:H solar cell has a quantum efficiency of about 48% at l=800 nm and about 15% at l=900 nm. We present details of the bandgap profiling and its effect on device performance.

We have found that narrow-bandgap—1.25 < Tauc Gap < 1.50 eV—amorphous silicon germanium (a-SiGe:H) alloys grown by hot-wire chemical vapor deposition (hot-wire CVD) can be improved by lowering both substrate and filament temperatures. We systematically study films deposited using a one-tungsten filament, decreasing filament temperature (Tf) from our standard temperature of 2150° down to 1750°C, and fixing all other deposition parameters. By decreasing Tf at the fixed substrate temperature (Ts) of 180°C, the Ge-H bonding increases, whereas the Si-H2 bonding is eliminated. Films with higher Ge-H bonding and less Si-H2 have improved photoconductivity. For the series of films deposited using the same germane gas fraction at 35%, the energy where the optical absorption is 1x104 (E04) drops from 1.54 to 1.41 eV with decreasing Tf. This is mainly due to the combination of an increasing Ge solid fraction (x) in the film, and an improved homogeneity and compactness due to significant reduction of microvoids, which was confirmed by small angle X-ray scattering (SAXS). We also studied a series of films grown by decreasing the Ts from our previous standard temperature of 350°C down to 125°C, fixing all other deposition parameters including Tf at 1800°C. By decreasing Ts, both the total hydrogen content (CH) and the Ge-H bonding increased, but the Si-H2 bonding is not measurable in the Ts range of 180°-300°C. The E04 increases from 1.40 to 1.51 eV as Ts decreased from 350° to 125°C, mainly due to the increased total hydrogen content (CH). At the same time, the photo-to-dark conductivity ratio increases almost three orders of magnitude over this range of Ts.

We find that hydrogen diffuses as H+, H0, or H- in hydrogenated amorphous silicon depending on its location within the i-layer of a p-i-n device. We annealed a set of five p-i-n devices, each with a thin deuterium-doped layer at a different location in the i-layer, and observed the D-diffusion using secondary ionmass spectrometry (SIMS). When H-diffuses in a charged state, electric fields in the device strongly influence the direction and distance of diffusion. When D is incorporated into a device near the p-layer, almost all of the D-diffusion occurs as D+, and when the D is incorporated near the n-layer, most of the D-diffusion occurs as D-. We correlate the preferential direction of D-motion at given depth within the i-layer, with the local Fermi level (as calculated by solar cell simulations), to empirically determine an effective correlation energy for mobile-H electronic transitions of 0.39 ± 0.1 eV. Using this procedure, the best fit to the data produces a disorder broadening of the transition levels of ∼0.25 eV. The midpoint between the H0/+ and the H0/- transition levels is ∼0.20 ± 0.05 eV above midgap.

Optically assisted, metal induced crystallization (MIC) was used to convert amorphous Si films, deposited on Al coated glass substrates, into polycrystalline Si (pc-Si). The study investigated the effects of deposition temperature, process temperature, and film thickness on the grain orientation, grain size, and crystallization front of the processed films. Furthermore, we have attempted to examine the role of Al in MIC – in particular, whether the metal can be confined to the interface while grain enhancement occurs.

We successfully grow high-quality hydrogenated amorphous-silicon-germanium alloys (a-SiGe:H) by the hot-wire chemical-vapor deposition (HWCVD) technique using silane and germane gas mixtures. These alloys display electronic properties as good as those grown by the plasma-enhanced chemical-vapor deposition (PECVD) technique, when comparing materials with the same optical bandgaps. However, we grow materials with good electrical properties at high deposition rates—up to 40 Å/s, compared to 1–4 Å/s for PECVD materials. Our alloys exhibit similar trends with increasing Ge content to alloys grown by PECVD. The defect density, the dark conductivity, and the degree of nanostructural heterogeneity (as measured by small-angle X-ray scattering) all increase with increasing germanium content in the alloy. The nanostructural heterogeneity displays a sharp transition between 9 at.% and 14 at.% germanium. PECVD- grown a-SiGe:H alloys exhibit a similar transition at 20 at.% Ge. The photoconductivity and the ambipolar diffusion length of the alloys decrease with increasing germanium content. For a fixed silane-to-germane gas ratio, all material properties improve substantially when increasing substrate temperature (Tsub) from 220°C to 375°C. Increasing Tsub also narrows the optical bandgap and lowers the hydrogen content in the alloys for the same germane-to-silane gas ratio.

Measurements of cosmogenic nuclides made in situ in the Earth's surface are being used to help resolve a wide range of geologic and chronologic questions. Cosmogenic nuclides (3He, 10Be, 14C, 21Ne, 26Al 36C1 are presently used) can reveal rock exposure history information leading to estimates of timing of surface forming events, rates and styles of erosion, and timing and durations of episodes of burial. Depending on the problems being tackled, a significant source of error (±10–25%) for any cosmogenic nuclide method is the present uncertainty in the spatial and temporal variability of the rates of production of these in-situ nuclides.