Abstract
Float zone silicon (FZ-Si) is typically assumed to be an extremely high quality material, with high minority carrier lifetimes and low concentrations of recombination active defects. However, minority carrier lifetime in FZ-Si has previously been shown to be unstable following thermal treatments between 450 and 700 °C, with a range of unidentified deep level states being linked to reduced carrier lifetime. There are suspicions that nitrogen doping, which occurs from the growth atmosphere, and intrinsic point defects play a role in the degradation. This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type FZ-Si samples. We find that nitrogen-doped samples experience increased degradation due to higher concentrations of deep level defects during thermal treatments compared to nitrogen-lean samples. In an attempt to explain this difference, in-diffusion of nickel has been used as a marker to demonstrate the existence of higher vacancy concentrations in the nitrogen-doped samples. The origin of the recombination active defects responsible for the thermally induced lifetime degradation in FZ-Si crystals is discussed.
Highlights
Float zone (FZ) growth of silicon allows the production of high purity wafers, with significantly lower concentrations of impurities than in Si grown by directional solidification methods, and crucially, lower concentrations of light impurities, such as oxygen and carbon, than those found in silicon grown using the Czochralski (Cz) technique.1 Low oxygen concentrations are achieved by the FZ process because the silicon melt is not in contact with a quartz crucible, which in the case of Cz-Si results in oxygen concentrations of the order 1018 cmÀ3.2 Silicon wafers grown by the float zone method often have high minority carrier lifetime and better doping uniformity than the Cz material
This study aims to address this by using deep level transient spectroscopy (DLTS), minority carrier transient spectroscopy, Laplace DLTS, and photoluminescence lifetime measurements to study recombination active defects in nitrogen-doped and nitrogen-lean n-type Float zone silicon (FZ-Si) samples
We have investigated the role of nitrogen doping in the formation of thermally activated deep level defects in FZ-Si using photoluminescence (PL) carrier lifetime measurements as well as deep level transient spectroscopy (DLTS), high resolution Laplace DLTS (L-DLTS),20 and minority carrier transient spectroscopy (MCTS)
Summary
Float zone (FZ) growth of silicon allows the production of high purity wafers, with significantly lower concentrations of impurities than in Si grown by directional solidification methods, and crucially, lower concentrations of light impurities, such as oxygen and carbon, than those found in silicon grown using the Czochralski (Cz) technique. Low oxygen concentrations (typically below 1016 cmÀ3) are achieved by the FZ process because the silicon melt is not in contact with a quartz crucible, which in the case of Cz-Si results in oxygen concentrations of the order 1018 cmÀ3.2 Silicon wafers grown by the float zone method often have high minority carrier lifetime and better doping uniformity than the Cz material. Low oxygen concentrations (typically below 1016 cmÀ3) are achieved by the FZ process because the silicon melt is not in contact with a quartz crucible, which in the case of Cz-Si results in oxygen concentrations of the order 1018 cmÀ3.2 Silicon wafers grown by the float zone method often have high minority carrier lifetime and better doping uniformity than the Cz material. They are used in power devices where a closely controlled lifetime is required and for the production of the highest efficiency solar cells where very high lifetime is a prerequisite. There is evidence for the formation of electrically active nitrogen-oxygen complexes in Si, these are seen to occur in nitrogen doped
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