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Title: Atom probe tomography analysis of near surface, low concentration impurities in single crystal silicon
Author: Douglas, James
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2018
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Advances in semiconductor device manufacture have led to modern nanoelectronic devices incorporating features with dimensions less than ten nanometres, fabricated within tens of nanometres of silicon substrate surfaces. To refine materials processing and optimise the correlation between fabricated nanostructures and device performance, high resolution characterisation techniques, such as atom probe tomography (APT), are required. In this thesis, surface sensitive specimen preparation methodologies for APT, utilising focused ion beam techniques using metal capping layers and low energy milling, were developed. This resulted in a reproducible method of precisely placing a metal/silicon interface at the apex of an APT nanoscale tip. This approach was utilised to characterise the surface regions of two silicon material systems with technological applications. Low concentration, low energy phosphorus ion implantation is currently being investigated for fabrication of qubit arrays for large scale quantum computers. APT was used to investigate phosphorus implantation depth accuracy. 14 kV phosphorus ions with predicted peak concentrations between 0.1 and 0.2 at.% and peak depths of ~ 22 nm were implanted into planar substrates and pre-fabricated silicon posts. Measured profiles matched well with simulation but showed a reduced peak concentration and an extended tail region, likely from ion channelling. Phosphorus diffusion is used to getter transition metals, particularly iron, to the surfaces of silicon solar cell material, however, the exact physical mechanism underpinning this is not well understood. Secondary ion mass spectrometry depth profiles were previously used to propose the interaction of oxide particles formed near the surface. In this study, single crystal silicon substrates were contaminated with pure iron and subjected to an industrially-relevant phosphorus diffusion process. APT analyses showed oxygen-rich regions in direct contact with silicon phosphide precipitates, confirming a heterogeneous distribution of oxygen in the surface regions. This can be used to inform processing conditions for modern silicon photovoltaics.
Supervisor: Moody, Michael ; Bagot, Paul Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Atom Probe Microscopy ; Semiconductors ; Physical Sciences ; Atom Probe Tomography ; Material Sciences