Spectral bandwidth enhancement in gallium nitride superluminescent light emitting diodes

Goldberg, Graham Robert (2021) Spectral bandwidth enhancement in gallium nitride superluminescent light emitting diodes. PhD thesis, University of Glasgow.

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This thesis is concerned with the design and development of short wavelength, ~400nm, superluminescent light emitting diodes (SLEDs). The enhancement of spectral bandwidth through the use of multiple passive and active contacts allows the spectral bandwidth to be increased beyond that possible with a single active contact. Such devices may find application in biomedical imaging applications such as optical coherence tomography (OCT). Thus far limited research has been conducted into short wavelength devices for OCT due to immaturity of development of suitable materials and their associated narrow bandwidths. Prior to this work gallium nitride (GaN) SLEDs have been reported with bandwidths of ~5nm, which at 400nm offers an axial resolution in the region of 10µm. This thesis describes and explains the research conducted towards the objective of achieving broader bandwidth.

Initially, an experimental setup capable of characterising shorter wavelength devices was constructed, and a comprehensive review of the literature and current state of the art is presented. An OCT setup was then constructed, allowing for direct measurement of device resolution and comparison to values obtained using both the point spread function and from theory. Non-contact metrology using a GaN SLED was then carried out for the first time. For a two contact SLED the role of bias conditions and their effect on the absorber section and device emission bandwidth was then examined, after which three section GaN SLEDs were developed, allowing for different current injection levels to be applied to the active regions. Optimal drive conditions for the two active contacts, along with a short-circuited absorber section, allowed a spectral bandwidth of ~20nm to be obtained. Device resolution was again analysed under the new conditions, yielding 3.3μm, and compared to the Rayleigh criterion. This offered a system resolution limit value. Once successfully developed, short wavelength GaN SLEDs with a broad bandwidth could sufficiently increase axial resolution to allow detailed imaging of the cornea and its respective layers, early detection of skin cancers, and imaging teeth in non-ionising dental applications. Future routes to developing even broader bandwidth devices, and hence higher resolution sources, are discussed.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: This work has been published as an IEEE journal paper and an Applied Physics Letter
Keywords: Gallium nitride, Superluminescent light emitting diodes, optical coherence tomography, bandwidth, resolution, multi-section
Subjects: Q Science > QC Physics
R Medicine > RE Ophthalmology
T Technology > TK Electrical engineering. Electronics Nuclear engineering
Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
Funder's Name: Engineering and Physical Sciences Research Council (EPSRC)
Supervisor's Name: Hogg, Professor Richard A and Childs, Doctor David T D
Date of Award: 2021
Depositing User: Dr Graham R Goldberg
Unique ID: glathesis:2021-82076
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 24 Mar 2021 16:41
Last Modified: 26 Mar 2021 09:42
Thesis DOI: 10.5525/gla.thesis.82076
URI: http://theses.gla.ac.uk/id/eprint/82076
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