Abstracts Category : Other

Multi-Microscopy Characterisation of III-Nitride Devices and Materials

by Christopher Xiang Ren

III-nitride optoelectronic devices have become ubiquitous due to their ability to emit lightefficiently in the blue and green spectral ranges. Specifically, III-nitride light emitting diodes(LEDs) have become widespread due to their high brightness and efficiency. However,III-nitride devices such as single photon sources are also the subject of research and arepromising for various applications. In order to improve design efficient devices and improvecurrent ones, the relationship between the structure of the constituent materials and theiroptical properties must be studied. The optical properties of materials are often examined byphotoluminescence or cathodoluminescence, whilst traditional microscopy techniques such atransmission electron microscopy and scanning electron microscopy are used to elucidatetheir structure and composition. This thesis describes the use of a dual-beam focussed ionbeam/scanning electron microscope (FIB/SEM) in bridging the gap between these two typesof techniques and providing a platform on which to perform correlative studies between theoptical and structural properties of III-nitride materials.The heteroepitaxial growth of III-nitrides has been known to produce high defect densities,which can harm device performance. We used this correlative approach to identify hexagonaldefects as the source of inhomogeneous electroluminescence (EL) in LEDs. HyperspectralEL mapping was used to show the local changes in the emission induced by the defects.Following this the FIB/SEM was used to prepare TEM samples from the apex of the defects,revealing the presence of p-doped material in the active region caused by the defect. APSYSsimulations confirmed that the presence of p-doped material can enhance local EL.The deleterious effects of defects on the photoelectrochemical etching of cavities werealso studied. We performed TEM analysis of an edge-defect contained in unetched material onthe underside of a microdisk using FIB/SEM sample preparation methods. The roughness andmorphology of microdisk and nanobeam cavities was studied using FIB-tomography (FIBT),demonstrating how the dual-beam instrument may be used to access the 3D morphology ofcavities down to the resolution of the SEM and the slicing thickness of the FIB.This tomography approach was further extended with electron tomography studies ofthe nanobeam cavities, a technique which provided fewer issues in terms of image series alignment but also the presence of reconstruction artefacts which must be taken into accountwhen quantitatively analysing the data.The use of correlative techniques was also used to establish the link between high Sicontent in an interlayer running along the length of microrods with changes in the opticalemission of these rods.The combination of CL, FIB/SEM and TEM-based techniques has made it possible togain a thorough understanding of the link between the structural and optical properties in awide variety of III-nitride materials and devices.