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Title: Strain balancing of self-assembled InAs/GaAs quantum dots grown by metal-organic vapour phase epitaxy
Author: Lewis-Roberts, Timothy S.
ISNI:       0000 0004 7428 2805
Awarding Body: University of Sheffield
Current Institution: University of Sheffield
Date of Award: 2018
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In this thesis, the strain balancing of self-assembled InAs/GaAs quantum dots (QDs), grown by metal-organic vapour phase epitaxy (MOVPE) is investigated. Initially, the optimisation of the important QD growth parameters, InAs coverage, growth rate, V/III ratio during the QD growth and the V/III ratio during the capping layer growth is explored. Incorporation of a strain-balancing layer will allow the close vertical stacking of QD layers, giving the potential for increased volumetric QD density and hence increased optical gain. Strain balancing can only be achieved practically using a GaAszP1-z layer. Therefore, modelling is performed of different phosphorus concentrations to understand the effects of placing a large potential barrier between QD layers on the electrical characteristics and the lasing threshold current density. In addition, X-ray diffraction (XRD) spectra are modelled to estimate the thickness of GaAs0.8P0.2 required to strain balance a QD layer. Based on this modelling, the implementation of a strain-balancing layer, which does not adversely affect the electrical characteristics and threshold current density, was attempted for three different QD layer separations: 50, 30 and 20 nm. Incorporation of this layer is shown to improve the performance of a device with a 30 nm spacing between QD layers, although performance is still inferior to that of a 50 nm device without strain balancing. Successful laser fabrication of the 50 and 30 nm structures with strain balancing gave laser operation up to 240 and 200 K, respectively. Further optimisation of the QD growth resulted in a room temperature laser with a layer separation of 50 nm, however, this was without the strain-balancing layer. Modelling of a relatively new type of semiconductor laser structure, a photonic crystal surface emitting laser (PCSEL) is performed to develop the incorporation of QDs. Three material systems for the photonic crystal are considered: void/GaAs, InGaP/GaAs and AlAs/GaAs, where the potential performance of each material system is discussed and compared. Preliminary investigations of the effect of the overgrowth temperature on the QD emission and the effect of incorporating an AlAs layer on the electrical characteristics were performed.
Supervisor: Mowbray, David ; Clarke, Edmund ; Hogg, Richard ; Stevens, Benjamin Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available