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In silicon crystals manufactured by the Czochralski method, oxygen is incorporated as a contaminant originating primarily from dissolution of the quartz crucible used to contain the molten silicon feedstock. The oxygen can be found either as interstitials, agglomerates, or as oxide particles. Particular kinds of agglomerates are known to lead to the formation of thermal donors—electronic states in the bandgap of the silicon base material. These can act as sites for recombination of excited charge carriers and are called thermal donors due to their ability to inject two electrons into the conduction band of silicon by thermal excitation, leading to enhanced electrical conductivity. Neither of these features are desirable in Cz‐Si manufacture. Herein, the behavior of thermal donors as a function of the pull speed, and the position in the ingot, is investigated. Thermal treatment is provided, first for the formation of thermal donors and then for their removal. The aim is to investigate methods for minimalizing their incorporation in the crystal in the first place and then how they can be removed. Hyperspectral imaging and spectroscopy combined with Fourier‐transform infrared spectroscopy are used. [ABSTRACT FROM AUTHOR]

We report results from a national project about impurities in high performance multicrystalline silicon: Contamination sources, transport routes, interaction with crystal defects and impact on solar cell efficiency parameters. Several ingots were produced in a lab scale furnace. Growth parameters and crucible types were varied, and high purity quartz crucibles were compared to novel silicon nitride crucibles. The material was characterized by a range of methods including FTIR, GDMS, NAA, EBSD, dislocation etching, lifetime analysis. Wafers were processed, and lifetime was measured during the cell processing to analyse the effect of separate processing steps. Models were constructed to simulate the impurity transport during the crystallization. The diffusivity in quartz and silicon nitride crucibles required in the modelling is not generally known, and experiments were performed to evaluate them. Variations in dislocation density have the strongest impact on lifetime, even for this high quality material. The quartz crucible has the highest contamination potential, but the uncertainty in diffusion parameters makes it difficult to conclude if this potential is reached, and for which elements. We therefore make an overview showing which value of the diffusion coefficient shift the contamination dominance between feedstock/crucible/coating. The use of high purity crucibles enables comparable or even higher lifetime material to be made in lab scale compared to industrial settings. Our results indicate that a thin layer of higher purity, lower porosity quartz on the inside of the crucible can be an effective method to transfer this improvement to industrial conditions. The cell processing indicates that the diffusion/gettering sequence profoundly decreases the lifetime by rendering the random high angle grain boundaries electrically active. However, this effect is reversed after firing of the anti-reflective coating, when the remaining active defects are dislocation clusters and CSL boundaries. Silicon nitride crucibles appear to be a promising low oxygen alternative to traditional quartz crucibles in terms of purity. [ABSTRACT FROM AUTHOR]

Four industrial-scale n-type Czochralski silicon crystals were grown with different impurity contents, i.e. metallics, phosphorus and oxygen. Horizontal slices were obtained from the top and middle of the crystals and were characterized in terms of lifetime and both defect and impurity distribution. The slabs characterization was performed both under as-grown conditions and after 2-step oxidation. Solar cells were fabricated from neighboring wafers with n-PASHA process and the overall efficiency determined. The crystal grown under low oxygen incorporation conditions has shown higher average as-grown lifetimes compared to the other crystals. The addition of lower phosphorus concentration to the melt resulted in an improvement of the lifetime of the crystal while the use of low grade feedstock has mainly affected the quality at the middle of the corresponding crystal. However, the major difference between top and middle heights was still the interstitial oxygen concentration for all crystals under investigation. The distribution of the interstitial oxygen concentration has not shown significant fluctuations through the samples radial direction. In addition, significant void fluctuations were found through the slabs radial direction, and demonstrate possible variations in the vacancy incorporation at the solidification interface during crystal growth. After heat treatment, large fluctuations of the lifetime were found through the samples radial direction, and are mainly associated to the local fluctuations of the oxygen precipitate density. However, these variations were suppressed in the crystal of lower oxygen incorporation. Solar cells performance degradation was mainly correlated with oxygen precipitation during their fabrication. It was also demonstrated that their efficiency is largely affected by the formation of detrimental striations of high periodicity. Their formation was thus associated to fluctuations of the vacancy concentration due to transient growth and their interaction with oxygen during crystal cooling. These oxygen nuclei may grow during solar cells processing and become even more detrimental to their performance. [ABSTRACT FROM AUTHOR]

Abstract: Oxygen induced stacking faults (OSFs) are mainly seen in oxygen rich wafers from the seed end of Cz-silicon crystals. In wafers this ring shaped OSF-region delineates a border between two defect regions; usually silicon self-interstitials dominate outside and vacancies inside this ring. High temperature treatment (>800°C) leads to oxygen precipitation in the border region. These precipitates act as nucleation sites for stacking faults. The standard procedure for characterizing the OSF-rings is to expose an oxidized sample to a preferential etchant, e.g. the highly toxic Wright solution. In this work photoluminescence-imaging is compared to preferential etching and visual inspection. Vertical samples from an n-type Cz-crystal containing an OSF-boundary are studied before and after wet oxidation. High resolution PL-images, which allow for a close inspection of the OSF-area, reveal a complex band-structure of this border region. [Copyright &y& Elsevier]