Terahertz and Photoluminescence Characterisation of GaN-based Quantum Well Structures

Abstract

In this thesis, the terahertz emission properties of bare sapphire, wurtzite GaN epilayers grown on sapphire and wurtzite InGaN/GaN quantum wells (QWs) are presented. Also presented are the optical properties of wurtzite GaN/AlGaN QWs structures. GaN/AlGaN QWs grown on c-plane sapphire are shown to exhibit a similar temperaturedependence of the photoluminescence (PL) peak energy to that of InGaN/GaN QWs often reported in literature. This temperature dependent behaviour, termed “S-shape” and commonly accepted as a spectral signature of carrier localisation in InGaN/GaN QWs, is shown to be exhibited by the GaN/AlGaN QWs studied. Other spectral signatures of carrier localisation commonly reported in the literature on InGaN/GaN are also observed in GaN/AlGaN QWs. This includes a non single-exponential PL decay and a spectral dependence of the τ1/e times, i.e the time taken for the time resolved PL intensities to fall by 1/e, prompting the question as to what could be the localisation mechanism in a GaN/AlGaN QW. The question arises because of the fundamentally different natures of InGaN/GaN and GaN/AlGaN QWs, the former having a well consisting of a binary material and the latter having a well consisting of a ternary alloy. A combined experimental and theoretical approach is used to show that the localisation mechanism in GaN/AlGaN QWs is excitonic, at least for narrow wells, in contrast to what is seen in InGaN/GaN QWs. The anomalous temperature dependence of the PL peak corresponding to the zero-phonon line is shown to be more pronounced in GaN/AlGaN QWs with wider wells, and it is also shown that the change in the τ1/e decay times across the low temperature PL spectrum increases as the well width increases. It is further shown that the holes are localised in the quantum well as they sample the interface roughness at the well/barrier interface introduced by the random alloy fluctuations in the barriers. The terahertz emission of c-plane InGaN/GaN QWs is also investigated, and it is shown that the emitted field amplitudes increase with indium alloy content and also increase as the number of wells is increased. Using test samples consisting of bare sapphire substrates and GaN-on-sapphire epilayers, it is shown that the terahertz emission is non-existent in the sapphire sample and 40 times weaker in the GaN epilayer compared to the InGaN/GaN MQWs. It is also established that the macroscopic polarisation of the QW sample plays a role in the terahertz emission by the use of a series of InGaN/GaN QWs band engineered to have a desired electric field across the QW heterostructures, with the conclusion that terahertz emission spectroscopy (TES) can indeed be used to sample the internal electrostatic field of the QWs studied. And finally a formalism for the terahertz generation in ZnTe crystals is developed and described, with the mechanism detailed analytically and a family of temporal profiles that are exact solutions to Maxwell’s equations used to model the generation and propagation of terahertz pulses in the time domain. The characteristics of the propagation of the terahertz waveforms through the filters and mirrors of an optical table are described, including the Fabry-Perot effects due to the generated pulses undergoing multiple reflections at the various interfaces along the propagation path. The physics of electro-optical sampling of the terahertz pulses in nonlinear EO crystals via frequency mixing is also described and used to simulate the sampling of the pulses in a real experimental setup. Potential issues with such simulations are described, with the use of an optics simulation software yielding results of propagated Gaussian beams through setups modelled to replicate the experimental apparatus used for the TES experiments.

Date of Award	1 Aug 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Darren Graham (Supervisor) & Peter Mitchell (Supervisor)