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Title: Optical spectroscopy of group III-nitride quantum well structures
Author: Hylton, Nicholas
ISNI:       0000 0004 2688 238X
Awarding Body: The University of Manchester
Current Institution: University of Manchester
Date of Award: 2009
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Presented in this thesis is a detailed investigation of the optical properties of sets of InGaN/GaN and InGaN/AIGaN quantum well structures, supported by the results of microstructural experiments from the University of Cambridge. The effect of the depth of a single InGaN/GaN quantum well below the sample surface on its optical properties was investigated. Oscillations in the photoluminescence excitation spectra of the samples were observed at temperatures below 50 K and at photon energies above the GaN band gap. These features were attributed to the excitation of electrons at multiples of the LO-phonon energy above the GaN conduction band minimum. The rapid relaxation of the electrons and their capture into the quantum well led to a shift in the photoluminescence peak energy, and therefore the oscillations in the excitation spectra. The shifts in the photoluminescence spectra were attributed to a modification to the carrier distribution amongst localised states in the quantum well. A comparison of the optical properties of a set of semi-polar InGaN/GaN quantum well structures with those of nominally identical polar control samples was made. Evidence for a reduction in the electric field strength across the quantum wells was observed in the form of blue-shifts of the photoluminescence peak energy, increased rates of radiative recombination and sharpening of the quantum well excitation spectra, with respect to the controls. However, the reduction in the electric field strength across the wells does not result in an improvement in the room temperature internal quantum efficiency; an observation which was attributed to an increased density of non-radiative recombination centres due to an order of magnitude increase in the threading dislocation density in the semi-polar samples. The optical properties of a set of InGaN/AIGaN multiple quantum well samples with no capping layer, a GaN capping layer and a magnesium doped capping layer were also compared. It was found that the intensity of the quantum well luminescence in the capped samples was reduced at T = 6 K compared to that of the uncapped quantum wells, due to carriers generated in the cap layer not being captured into the wells. At room temperature the difference in quantum well luminescence intensity between the capped and uncapped samples increased due to the lower carrier density in the wells in the capped samples. In the case of the uncapped sample the higher carrier density resulted in the saturation of the impurity luminescence bands meaning that more carriers could contribute to the quantum well luminescence. However, in the capped samples the impurity luminescence was not saturated and so it is possible that carriers could be thermalised out of the wells and be captured at impurity sites, hence not contributing to the quantum well luminescence. Finally, the optical properties of InGaN/GaN multiple quantum well structures grown on 2 and 6 inch silicon substrates were studied. It was discovered that the reflectivity of silicon varies at low temperatures, which inevitably affects the outcome of measurements of internal quantum efficiency. After taking into account the effects of the changes to the reflectivity of the substrate, the room temperature photoluminescence internal quantum efficiencies of the samples were estimated to be 33 %,57 % (2 inch substrates) and 38 % (6 inch substrate).
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available