N-type Czochralski silicon solidification: Oxygen- and copper-related defects formation
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This PhD work was focused on the formation of oxygen- and copper-related defects in ntype Czochralski silicon for solar cells. These defects are a very important source of material and solar cell performance degradation, and a better understanding of the mechanism behind their formation is still needed. The first scope of this thesis was to understand the impact of process parameters on the quality of n-type silicon ingots. We have seen that the oxygen incorporation into the crystal is very sensitive to the evaporation rate at the free melt surface and a proper control of the transition from neck to body (crown plus shoulder) is of extreme importance to the material, as large diameter changes are needed. In industry, the crown pull speed is set to a specific value; the crown shape may anyway change from ingot to ingot. This shape can be indirectly correlated to the average melt temperature after necking, where lower average temperature leads to a higher growth rate of the crown. Longer time to grow the crown results in higher oxygen evaporation at the free melt surface and consequently a lower oxygen concentration is measured at the top of the ingot body. Thermal donor annealing heat treatment results in an improvement of the average lifetime and reduction of its inhomogeneities. However, only small improvements are found on the early body region with higher oxygen concentration due to the formation of larger oxygen-related defects. The following work on the optimization of the process was based on changes of the crown pulling speed (from 0.6 to 0.8 mm/min). It was seen that higher pull speed results in a more elongated crown and consequently lower oxygen incorporation into the crystal body. The formation of oxygen precipitates is very high at regions below the P-band and the lifetime degradation is correlated to the high precipitate density. The precipitation at the first part of the body is reduced for ingots grown with higher crown pull speeds, although a strict control of the process parameters (e.g. heater power) is still needed. Lower solid-liquid interface deflections are found for the top of the ingot, grown with higher crown pull speed. As the interface is normally concave towards the melt during further body growth, small interface shape changes take place in this ingot, with lower probability of structures loss. After optimization of the growth of the Cz silicon ingots, the focus has been set on the impact of impurities on the quality of n-type Cz silicon ingots. In order to study the impact of metallic impurities, phosphorus and oxygen on the defects generation, four ingots were grown with different impurity contents. The main difference between top and middle of the ingots is the oxygen concentration due to its higher concentration compared to other impurities. Metallic impurities are still a limiting factor for lifetime at the ingot middle, and can have a different impact depending on feedstock quality. The reduction of the oxygen incorporation is effective on the overall improvement of the electrical properties, mainly after high temperature processes. In this work, ring-pattern defects were only visualized after thermal oxidation, but still without significant fluctuations of the interstitial oxygen distribution. Striations are correlated to the oxygen precipitate density, except for ingots grown with lower oxygen incorporation. However, the precipitates density is not enough to correlate with the lifetime fluctuations, as their nature may differ from one striation to another. After heat treatment, a large reduction of the average lifetime is obtained at the top of the ingots due to the oxygen precipitation and formation of stacking faults. The degradation of the solar cells performance, made from these materials, is mainly attributed to the precipitation of oxygen during their fabrication. The formation of ring-pattern defects in as-grown samples is correlated to the interaction between vacancies and oxygen during crystal cooling. The local differences can be minor through the as-grown samples radial direction, but may be enhanced during high temperature processes, as the ones during solar cells fabrication. At last, a study of lifetime degradation close to the ingot end was carried out. We have initially measured a large lifetime degradation of the material quality on a block, obtained from the end of an n-type ingot at room temperature. The source of its degradation was unknown until we measured a high average Cu concentration in this block with GDMS. Cu depth profiles were measured and a very high concentration was found close to the sample surface, with a decrease towards the bulk. Lifetime degradation in regions of moderate Cu contamination can thus be associated to the continuous Cu precipitation close to the samples surface. It is a result of the high diffusivity and low solubility of Cu in silicon, even at room temperature. The ingot end may thus be a source of contamination for the rest of the ingot and need to be quickly removed. The Cu concentration and precipitation close to the surface depends on the surface area, where a larger area enhances the precipitation close to the surface and reduction of the bulk contamination.