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Superacid-based passivation for accurate measurements of bulk carrier lifetime in silicon wafers
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Pointon, Alex Ivan (2022) Superacid-based passivation for accurate measurements of bulk carrier lifetime in silicon wafers. EngD thesis, University of Warwick.
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Official URL: http://webcat.warwick.ac.uk/record=b3965261~S15
Abstract
This thesis investigates the treatment of silicon surfaces by the superacid bis(trifluoromethane)sulfonimide (TFSI). The aim is to develop high quality surface passivation with sufficient longevity for reliable use in the determination of accurate bulk carrier lifetimes in silicon wafers, particularly those used for solar cells.
The role of the solvent was first investigated and it was found that TFSI in hexane provides improved temporal stability, better passivation and improved solution longevity compared to solvents longevity compared to solvents used previously. Sample storage conditions, particularly humidity, were found to strongly influence the passivation stability. The optimised TFSI-hexane passivation scheme was applied to a set of 3 Ωcm n-type wafers, and, at 1015 cm-3 injection, the best-case effective surface recombination velocity is 0.69+0.04 cm s-1, with bulk lifetimes measured up to the intrinsic lifetime limit at high injection and > 43 ms at lower injection.
To elucidate the mechanism of passivation, others solutions based on molecules containing a triflyl group, like TFSI, were used to passivate silicon. The study shows that the presence of CF3SO2 groups and not the solution’s superacidity is critical for passivation, opening up the possibility of using the new solutions on materials more sensitive to acidic environments. Kelvin probe measurements established that the mechanism of passivation comprises both chemical and field effect passivation (from a negatively charged thin film).
Finally, the role of the silicon surface preparation was investigated. Lifetime measurements showed clean silicon surfaces can be temporarily passivated by a short treatment in both HF(2%):HCI(2%) and HF(50%) solutions. XPS and AFM showed treatment with strong HF solutions result in a roughened fluorine-terminated surface. Subsequent superacid-derived surface passivation on the different chemically-treated surfaces show considerably better passivation on surfaces treated with HF(2%) : HCI(2%) compared with HF alone, highlight the importance of surface pre-treatment in the creation of high quality functionalised surfaces.
Item Type: | Thesis (EngD) | ||||
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Subjects: | Q Science > QD Chemistry T Technology > TK Electrical engineering. Electronics Nuclear engineering |
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Library of Congress Subject Headings (LCSH): | Silicon -- Surfaces, Semiconductor wafers, Superacids | ||||
Official Date: | 2022 | ||||
Dates: |
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Institution: | University of Warwick | ||||
Theses Department: | School of Engineering | ||||
Thesis Type: | EngD | ||||
Publication Status: | Unpublished | ||||
Supervisor(s)/Advisor: | Murphy, John D. (John Douglas) ; Grant, Nicholas E. | ||||
Format of File: | |||||
Extent: | xiv, 142 pages : illustrations, charts | ||||
Language: | eng |
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