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Title: Atom probe tomography of crystallographic defects in silicon
Author: Tweddle, David
ISNI:       0000 0004 7971 6983
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2019
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High performance multicrystalline silicon (HPMC-Si) is the dominant material used in photovoltaic (PV) devices. This material however contains a large number of defects, such as grain boundaries and dislocations. The decoration of such defects by transition metals and other impurities causes deep levels in the band gap, significantly affecting the performance and efficiency of PV devices. In the photovoltaic industry, processing techniques, such as hydrogen passivation and gettering, are used to increase the minority carrier lifetime, reducing the recombination activity of crystallographic defects. However, these techniques are not always effective and the reasons for this are currently not always well understood. In this thesis, a correlative microscopy approach is developed, incorporating electron beam induced current (EBIC), electron back scatter diffraction (EBSD) and atom probe tomography (APT). This approach combines the characterisation of recombination activity, crystallographic information, and chemical composition of individual defects after various stages of processing. These methods were employed to determine the cause of contamination from room temperature colloidal silica polishing and also investigate the evolution of grain boundaries at various stages of gettering and hydrogenation. In addition, this approach was also applied to alternative PV materials of Red Zone HPMC-Si and n-type HMPC-Si, demonstrating wide-scale applicability. Nitrogen was observed to correlate with an increase in recombination activity at certain grain boundaries. In addition, the light elements carbon and oxygen were also observed to decorate grain boundaries. No transition metals, except copper, were detected. This may be due to the detection limit of APT, which was calculated to be as high as 1015 cm-3 for Fe. Furthermore, applying this new multiscale correlative microscopical approach to the analysis of deuterium (2H) passivated wafers, hydrogen has been directly observed to decorate crystallographic defects in silicon and the extent of segregation quantified. These new insights have the potential to directly address the question as to why some grain boundaries respond positively to hydrogenation, whereas others do not.
Supervisor: Moody, Michael ; Wilshaw, Peter ; Hamer, Phillip Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Materials