Optical characterisation of group III-nitride semiconductors

• Main Author: Othick, Catherine Ann
• Other Authors: Dawson, Philip
• Published: University of Manchester 2011
• Subjects: 621.38152
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.553292

Research presented in this thesis focuses on the optical characterisation of InGaN/GaN quantum well (QW) and quantum dot (QD) structures and ScGaNepi layers, supported by microscopy results from the University of Cambridge. Reported in the first part of this thesis are the optical properties of sets of high In fraction (~25%) multiple QW structures designed to emit in the green part of the spectrum. Sets of InGaN/GaN QW structures were investigated which were grown using traditional methods but by varying QW growth temperature. These samples were found to have very broad photoluminescence linewidths and 1-10% room temperature IQE. Changes to the growth procedures had little effect on the improvement of the samples' luminescence properties. It was found that the first two wells of these samples were thicker and/or contained more indium than subsequent wells grown in the stack. The added thickness and/or indium content of these InGaN/GaN QWs resulted in lower energy emission than the rest of the QW stack, broadening the photoluminescence linewidth and decreasing the IQE. A modification of the growth procedures was developed to ensure that the individual QWs have as similar properties as possible so that the contribution to the PL linewidth due to well-to-well variations was significantly reduced. These modified procedures were used to produce new sets of InGaN/GaN 10 QW structures. These new structures showed a marked increase (~30%) in IQE and a significant decrease in PL linewidth. Furthermore, a set of 1, 3, 5 and 10 QW structures grown under modified growth procedures were investigated to determine the optimum number of QWs needed. It was found that 3 QWs provided a significant improvement to the IQE over the 1 QW samples; however, no additional improvements were realised by growing additional QWs. The second part of this thesis explores the use of macroscopic optical spectroscopy methods to study the properties of QDs. InGaN QDs are typically studied using spatially resolved techniques which allow for the study of individual dots in a structure. A sample which was known to contain InGaN QDs was investigated; however it was determined that macroscopic spectroscopic techniques were unable to determine the existence of QDs in the structure. Difficulties creating highly efficient green or near-UV light emitting InGaN semiconductors have lead to an interest in alternative material structures. One suggested alternative is to replace indium with the Group IIIB transition metal, scandium. The final part of this thesis explores a series of ~ 260 μm thick MBE grown ScGaN layers on 500 μm thick MOCVD-grown GaN templates. It was found that these materials, believed to contain up to 8% scandium, emit a broad spectrum violet luminescence. This broad luminescence spectrum resolves into multiple, narrower features with either increased substrate temperature during growth or by decreasing the scandium effusion cell temperature. The absorption spectrum, on the other hand, only shows evidence of GaN and samples grown at the highest substrate temperature revealed an exciton absorption feature near the band edge of GaN, a sign of increased crystal quality. These results have lead to questions as to whether the violet luminescence is due to the ternary alloy ScGaN or rather from shallow, radiative defects in the material.