Abstract
It is known that the interstitial iron concentration in silicon is reduced after annealing silicon wafers coated with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films. The underlying mechanism for the significant iron reduction has remained unclear and is investigated in this work. Secondary ion mass spectrometry (SIMS) depth profiling of iron is performed on annealed iron-contaminated single-crystalline silicon wafers passivated with PECVD silicon nitride films. SIMS measurements reveal a high concentration of iron uniformly distributed in the annealed silicon nitride films. This accumulation of iron in the silicon nitride film matches the interstitial iron loss in the silicon bulk. This finding conclusively shows that the interstitial iron is gettered by the silicon nitride films during annealing over a wide temperature range from 250 °C to 900 °C, via a segregation gettering effect. Further experimental evidence is presented to support this finding. Deep-level transient spectroscopy analysis shows that no new electrically active defects are formed in the silicon bulk after annealing iron-containing silicon with silicon nitride films, confirming that the interstitial iron loss is not due to a change in the chemical structure of iron related defects in the silicon bulk. In addition, once the annealed silicon nitride films are removed, subsequent high temperature processes do not result in any reappearance of iron. Finally, the experimentally measured iron decay kinetics are shown to agree with a model of iron diffusion to the surface gettering sites, indicating a diffusion-limited iron gettering process for temperatures below 700 °C. The gettering process is found to become reaction-limited at higher temperatures.
Highlights
This hypothesis was based on the reports that showed that the recombination activity and the concentration of interstitial iron in silicon were reduced after hydrogen incorporation, via exposure to hydrogen plasma,18–20 hydrogen ion implantation,21 wet chemical etching,22 or deposition of plasma-enhanced chemical vapour deposited (PECVD) silicon nitride films
The initial interstitial iron concentrations in the FZ-Si wafers resulted from Fe ion implantation and the subsequent distribution anneal were found to be in the range of 2 Â 1012–1013 cmÀ3, from quasi-steady-state photoconductance (QSSPC) lifetime measurements
The disappearance of interstitial iron in silicon after annealing silicon wafers with PECVD silicon nitride films is found to be due to the segregation gettering of Fe by the SiNx, as evidenced by Secondary ion mass spectrometry (SIMS) depth profiling of the Fe concentration in the annealed SiNx layer and the Si substrate
Summary
Iron is a harmful and common metallic impurity in the silicon materials used for solar cells and microelectronics. In the solar cell fabrication process, the detrimental impact of iron is commonly mitigated by the gettering of iron to the heavily phosphorus doped regions near the wafer surfaces, during the high temperature phosphorus diffusion process used for the p-n junction formation. Heavy boron diffusion or aluminium alloying processes could achieve similar gettering effects, if well integrated into the solar cell fabrication processes. In Czochralski silicon for microelectronics, the gettering of dissolved iron is commonly achieved by driving iron precipitation at intentionally introduced oxide precipitates in the silicon bulk, via low temperature annealing. in multicrystalline silicon materials for solar cells, the overall impact of iron can be alleviated by driving iron precipitation at the crystallographic defects, the process is much less effective than the high temperature phosphorus diffusion gettering approach.Numerous studies have shown that annealing silicon wafers with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiNx) films at high firing temperatures causes the concentration of dissolved iron (i.e., interstitial iron) in silicon to be reduced significantly. The mechanism for this iron reduction has remained unresolved, and it has been hypothesised that it might be caused by the hydrogenation of iron in silicon. This hypothesis was based on the reports that showed that the recombination activity and the concentration of interstitial iron in silicon were reduced after hydrogen incorporation, via exposure to hydrogen plasma, hydrogen ion implantation, wet chemical etching, or deposition of PECVD silicon nitride films. While there have been reports of the detection of new defect levels assigned to possible Fe-H complexes, from theoretical calculations the binding energy of Fe-H pairs was found to be weak, indicating the unlikelihood of the Fe-H pairs withstanding the relatively high temperatures used for firing. In multicrystalline silicon materials for solar cells, the overall impact of iron can be alleviated by driving iron precipitation at the crystallographic defects, the process is much less effective than the high temperature phosphorus diffusion gettering approach. Numerous studies have shown that annealing silicon wafers with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiNx) films at high firing temperatures causes the concentration of dissolved iron (i.e., interstitial iron) in silicon to be reduced significantly.. Numerous studies have shown that annealing silicon wafers with plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiNx) films at high firing temperatures causes the concentration of dissolved iron (i.e., interstitial iron) in silicon to be reduced significantly.13–17 The mechanism for this iron reduction has remained unresolved, and it has been hypothesised that it might be caused by the hydrogenation of iron in silicon.. Others have suggested that the reduction was due to the accelerated precipitation of iron driven by hydrogenenhanced iron diffusivity. this hypothesis cannot explain the observed iron loss at higher annealing temperatures, at which the iron solubility limits were well above the dissolved iron concentrations in silicon, meaning that such an iron relaxation precipitation process should not proceed
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