Optical Characterisation of Group III - Nitride Semiconductors

Abstract

AbstractOptical Characterisation of Group III - Nitride SemiconductorsCatherine Ann Othick, Ph.D., The University of Manchester, 2011Research presented in this thesis focuses on the optical characterisation ofInGaN/GaN quantum well (QW) and quantum dot (QD) structures and ScGaNepilayers, supported by microscopy results from the University of Cambridge.Reported in the first part of this thesis are the optical properties of sets ofhigh In fraction (~25%) multiple QW structures designed to emit in the green partof the spectrum. Sets of InGaN/GaN QW structures were investigated which weregrown using traditional methods but by varying QW growth temperature. Thesesamples were found to have very broad photoluminescence linewidths and 1 - 10 %room temperature IQE. Changes to the growth procedures had little effect on theimprovement of the samples' luminescence properties. It was found that the firsttwo wells of these samples were thicker and/or contained more indium thansubsequent wells grown in the stack. The added thickness and/or indium contentof these InGaN/GaN QWs resulted in lower energy emission than the rest of theQW stack, broadening the photoluminescence linewidth and decreasing the IQE.A modification of the growth procedures was developed to ensure that theindividual QWs have as similar properties as possible so that the contribution to thePL linewidth due to well-to-well variations was significantly reduced. Thesemodified procedures were used to produce new sets of InGaN/GaN 10 QWstructures. These new structures showed a marked increase (~30%) in IQE and asignificant decrease in PL linewidth. Furthermore, a set of 1, 3, 5 and 10 QWstructures grown under modified growth procedures were investigated to determinethe optimum number of QWs needed. It was found that 3 QWs provided asignificant improvement to the IQE over the 1 QW samples; however, noadditional improvements were realised by growing additional QWs.The second part of this thesis explores the use macroscopic opticalspectroscopy methods to study the properties of QDs. InGaN QDs are typicallystudied using spatially resolved techniques which allow for the study of individualdots in a structure. A sample which was known to contain InGaN QDs wasinvestigated; however it was determined that macroscopic spectroscopic techniqueswere unable to determine the existence of QDs in the structure.Difficulties creating highly efficient green or near-UV light emitting InGaNsemiconductors have lead to an interest in alternative material structures. Onesuggested alternative is to replace indium with the Group IIIB transition metal,scandium. The final part of this thesis explores a series of ~ 260 micro metre thick MBEgrownScGaN layers on 500 micro metre thick MOCVD-grown GaN templates. It wasfound that these materials, believed to contain up to 8% scandium, emit a broadspectrum violet luminescence. This broad luminescence spectrum resolves intomultiple, narrower features with either increased substrate temperature duringgrowth or by decreasing the scandium effusion cell temperature. The absorptionspectrum, on the other hand, only shows evidence of GaN and samples grown atthe highest substrate temperature revealed an exciton absorption feature near theband edge of GaN, a sign of increased crystal quality. These results have lead toquestions as to whether the violet luminescence is due to the ternary alloy ScGaN orrather from shallow, radiative defects in the material.

Date of Award	31 Dec 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Dawson (Supervisor)