The precipitation rate of intentionally introduced iron during low-temperature heating is studied among a variety of single-crystal and polycrystalline silicon solar cell materials. A correlation exists between the iron precipitation rate and the carrier recombination rate in dislocation-free as-grown material, suggesting that diffusion-length-limiting defects in as-grown material are structural defects which accelerate iron precipitation. Phosphorous diffusion gettering was found to be particularly ineffective at improving diffusion length after intentional iron contamination in materials with high iron precipitation rates. We propose that intragranular structural defects in solar cell silicon greatly enhance transition metal precipitation during cooling from the melt and become highly recombination-active when decorated with these impurities. The defects then greatly impair diffusion length improvement during phosphorus gettering and limit carrier lifetimes in as-grown material.

Gate oxide integrity (GOI) tests, surface photovoltage and deep level transient spectroscopy of Czochralski silicon wafers reveal oxide degradation at heavy precipitation, defect-controlled recombination lifetime and defect-induced deep levels. Electron beam induced current measurements on those wafers before and after intentional metal decoration reveal relatively shallow levels in the non-decorated state and deep levels in the decorated state. It is shown that the actual contamination level determines the usefulness of GOI tests for predicting material performance in device processing.

Edge-defined film-fed growth (EFG) Si grown with carbon monoxide (CO) added to the Ar ambient during crystal growth yields solar cells with higher efficiencies than when grown without CO. This increase in cell efficiency is not fully understood. Surface photovoltage, deep-level transient spectroscopy, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy were used to determine the minority carrier diffusion lengths, impurity distributions, and defect structures in uncontaminated, Cr contaminated, and V contaminated EFG material grown with and without CO added to the Ar ambient. We conclude that ‘‘SiC-like’’ complexes in the near-surface region of the CO ambient material act as gettering sites during crystal growth, and that this gettering action results in lower bulk impurity levels and higher solar cell efficiencies.

Electron microscopy techniques were applied to the study of intragranular microdefects in as-grown and intentionally Fe contaminated polycrystalline silicon solar cell material. Electron-energy-loss spectroscopy (EELS) imaging revealed bright contrasts in metal diffused samples and with smaller concentration in as-grown material, probably due to metal precipitates. High-resolution and analytical electron microscopy identified Cu- and Fe-silicide particles in the contaminated specimen. These results demonstrate that intragranular microdefects exist in polycrystalline Si which act as nucleation sites for metal contaminants and lend support to the model that metal-decorated microdefects are decisive lifetime killers in as-grown material

Efficiency-limiting defects in polycrystalline silicon were studied using intentional metal contamination and various thermal and gettering treatments. Recently we have determined that regions of polysilicon material with the highest as-grown diffusion length are found to contain dissolved iron after growth, indicating retarded iron precipitation during post-growth cooling in these regions. Intragranular structural defects are responsible for low diffusion length previous results. In order to better understand the interaction of metallic impurities with these structural defects, phosphorus and aluminum gettering and P/Al co-gettering, have been performed on a variety of silicon solar cell materials after identical transition metal contamination. Our results show that polysilicon is more resistant to diffusion length increase than single crystal material. A model is proposed in which the predominant diffusion-length limiting factors in low diffusion length material are structural defects, whose recombination activity depends upon their density and degree of transition metal decoration.

A systematic study of iron gettering has been performed with Edge‐defined Film‐fed Growth (EFG) and Bayer cast polycrystalline solar cell material, in comparison with Czochralski and float‐zone silicon, to identify minority carrier diffusion length limiting mechanisms in these materials and illustrate the usefulness of iron precipitation kinetics as a characterization tool. Iron was intentionally diffused into a variety of samples of both materials, selected to represent the wide range of diffusion lengths found in EFG and Bayer material. After quenching, the diffusion length was found to have decreased drastically in all materials, due to the formation of electrically‐active iron‐related precipitates. Isochronal anneals of EFG material at temperatures up to 950 °C partially recovered the diffusion lengths, to an extent proportional to the original values. A clear correlation between iron precipitation rate at 235 °C and as‐grown diffusion length exists for EFG material, indicating that structural defects in EFG material that serve as nucleation sites for heterogeneous iron precipitation dominate the carrier diffusion length. Bayer and Czochralski material have more moderate minority carrier diffusion lengths and iron precipitation rates than EFG, and float zone silicon has by far the lowest precipitation and recombination rates of all materials. Among all materials, a direct relationship between iron precipitation and carrier recombination rates is found, illustrating the strong dependence of carrier diffusion length on the concentration of structural defects that increase the rate of interstitial iron precipitation.

A systematic study is performed of the precipitation of interstitial iron in polycrystalline silicon for photovoltaic applications. The material is grown by the edge-defined, film-fed ribbon growth (EFG) technique and intentionally contaminated with iron at high temperature. Comparing different grains of the same ribbon, the iron precipitation rate is found to correlate directly with the carrier diffusion length. The precipitation rate is much greater than can be explained by the observed dislocation density. In high-oxygen EFG silicon, iron precipitation kinetics show that an order of magnitude increase in dislocation concentration, from 5 x 10(6) to 5 x 10(7) cm-2, does not significantly increase the rate of iron precipitation over that observed in dislocation-free standard EFG material. These results indicate that a high concentration of hitherto unidentified bulk defects exists in EFG material, which act as nucleation sites for iron precipitation, and might limit the carrier lifetime in this material.

Modulated optical reflectance (MOR) images of plastically-deformed silicon show slip bands created by the deformation, which are also visible in electron beam induced conductivity images. Annealing the material decreases the electrical activity of the dislocations, but has no effect on the MOR image. MOR measurements of implanted and annealed silicon wafers show that a high MOR response is caused by decreased diffusivity in the surface layer. We conclude that the MOR contrast of defects in silicon is due primarily to scattering by dislocations and ionized impurities, and that carrier recombination is not significant.

The investigation of defects in silicon by modulated optical reflectance measurements has proven to be a powerful and easy-to-use method of nondestructive materials characterization. This technique has been used to monitor ion implant dose [1] and measure polishing damage [2] in silicon wafers, and to map O2 swirl precipitates in Czochralski-grown silicon [3]. Laser-induced modulated reflectance offers advantages over some related thermal-wave techniques: it is contactless, and because it can be performed at modulation frequencies of several MHz, it offers micron-scale resolution. Its noncontact and nondestructive nature makes this technique attractive for production-line use in the semiconductor industry.

An increase in the incidence of solar ultraviolet radiation upon oceans and lakes, as a consequence of anthropogenic diminishing of the ozone in the stratosphere, might well have a significant effect upon primary producers and other aquatic organisms in these waters. Existing data were inadequate for accurately estimating the penetration of biologically effective dosages of present and potential future levels of UV irradiance into various natural waters. An underwater spectroradiometer, designed specifically to measure spectral irradiance in the middle ultraviolet (MUV, 280-340 nm), has been designed, constructed and used to obtain ecologically important data.