Multi-Microscopy Characterisation of III-Nitride Devices and Materials

III-nitride optoelectronic devices have become ubiquitous due to their ability to emit light efficiently 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 are promising for various applications. In order to improve design efficient devices and improve current ones, the relationship between the structure of the constituent materials and their optical properties must be studied. The optical properties of materials are often examined by photoluminescence or cathodoluminescence, whilst traditional microscopy techniques such a transmission electron microscopy and scanning electron microscopy are used to elucidate their structure and composition. This thesis describes the use of a dual-beam focussed ion beam/scanning electron microscope (FIB/SEM) in bridging the gap between these two types of techniques and providing a platform on which to perform correlative studies between the optical 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 hexagonal defects as the source of inhomogeneous electroluminescence (EL) in LEDs. Hyperspectral EL 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. APSYS simulations confirmed that the presence of p-doped material can enhance local EL. The deleterious effects of defects on the photoelectrochemical etching of cavities were also studied. We performed TEM analysis of an edge-defect contained in unetched material on the underside of a microdisk using FIB/SEM sample preparation methods. The roughness and morphology 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 of cavities down to the resolution of the SEM and the slicing thickness of the FIB. This tomography approach was further extended with electron tomography studies of the 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 account when quantitatively analysing the data. The use of correlative techniques was also used to establish the link between high Si content in an interlayer running along the length of microrods with changes in the optical emission of these rods. The combination of CL, FIB/SEM and TEM-based techniques has made it possible to gain a thorough understanding of the link between the structural and optical properties in a wide variety of III-nitride materials and devices.EPSR

Epitaxial growth of iii-nitride nanostructures and their optoelectronic applications

Light-emitting diodes (LEDs) using III-nitride nanowire heterostructures have been intensively studied as promising candidates for future phosphor-free solid-state lighting and full-color displays. Compared to conventional GaN-based planar LEDs, III-nitride nanowire LEDs exhibit numerous advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect due to the effective lateral stress relaxation, promising high efficiency full-color LEDs. Beside these advantages, however, several factors have been identified as the limiting factors for further enhancing the nanowire LED quantum efficiency and light output power. Some of the most probable causes have been identified as due to the lack of carrier confinement in the active region, non-uniform carrier distribution, and electron overflow. Moreover, the presence of large surface states and defects contribute significantly to the carrier loss in nanowire LEDs. In this dissertation, a unique core-shell nanowire heterostructure is reported, that could overcome some of the aforementioned-problems of nanowire LEDs. The device performance of such core-shell nanowire LEDs is significantly enhanced by employing several effective approaches. For instance, electron overflow and surface states/defects issues can be significantly improved by the usage of electron blocking layer and by passivating the nanowire surface with either dielectric material / large bandgap energy semiconductors, respectively. Such core-shell nanowire structures exhibit significantly increased carrier lifetime and massively enhanced photoluminescence intensity compared to conventional InGaN/GaN nanowire LEDs. Furthermore, AlGaN based ultraviolet LEDs are studied and demonstrated in this dissertation. The simulation studies using Finite-Difference Time-Domain method (FDTD) substantiate the design modifications such as flip-chip nanowire LED introduced in this work. High performance nanowire LEDs on metal substrates (copper) were fabricated via substrate-transfer process. These LEDs display higher output power in comparison to typical nanowire LEDs grown on Si substrates. By engineering the device active region, high brightness phosphor-free LEDs on Cu with highly stable white light emission and high color rendering index of \u3e 95 are realized. High performance nickel?zinc oxide (Ni-ZnO) and zinc oxide-graphene (ZnO-G) particles have been fabricated through a modified polyol route at 250?C. Such materials exhibit great potential for dye-sensitized solar cell (DSSC) applications on account of the enhanced short-circuit current density values and improved efficiency that stems from the enhanced absorption and large surface area of the composite. The enhanced absorption of Ni-ZnO composites can be explained by the reduction in grain boundaries of the composite structure as well as to scattering at the grain boundaries. The impregnation of graphene into ZnO structures results in a significant increase in photocurrent consequently due to graphene\u27s unique attributes including high surface area and ultra-high electron mobility. Future research directions will involve the development of such wide-bandgap devices such as solar cells, full color LEDs, phosphor free white-LEDs, UV LEDs and laser diodes for several applications including general lighting, wearable flexible electronics, water purification, as well as high speed LEDs for visible light communications

Design, growth and fabrication and characterisation of InGaN Micro Light Emitting Diodes using a Direct Epitaxial Approach

Free standing micro disks have been the focus of a significant amount of research in recent years. This is due to the ease in creating low threshold micro lasers and micro array devices. Unfortunately, there are issues with the fabrication process that limit the overall efficiency and quality of such devices. The top-down approach used with these micro disk leads to severe sidewall damage from the etching process. Therefore, we present a novel approach using a direct epitaxial method to selectively grow micro disks in a patterned SiO2 template. This thesis presents the design process in which we made these devices and the use of characterisation to optimise the method to create highly efficient micro-LEDs. We also take these devices further and created micro laser cavities using a hybrid epitaxial/ dielectric cavity. Using lattice matched nanoporous GaN/undoped GaN Distributed Bragg Reflectors (DBR) and a dielectric SiO2/SiN based DBR, we can create optically pumped micro disks arrays with stimulated emission with a wavelength of 510nm. Finally, we investigate a new limiting factor in the growth of ultra-small micro disks (<3.5μm) in the form the circularity of the micro disks themselves rather than the roughness of the sidewall

Novel III-Nitride Semiconductors for Solar Hydrogen Production

III-nitride materials are crucially becoming the most important and promising class of semiconductors for future optoelectronic devices including solid state lighting and solar energy applications. Presently, there are still many challenges in regards to the wide scale uptake of these devices, including low efficiencies and short lifetimes. Despite the ideal properties of InGaN for water splitting, there are still very few reports utilising these semiconductors. This thesis investigates GaN and InGaN based structures for water splitting. Initially focussing on the fabrication of nanorods via the use of a self-organised nickel mask, where diameter and height of the structures have been optimised. As a result, the surface area of the device increases dramatically leading to an enhancement in photocurrent compared to as-grown planar devices. Alongside this, the fabricated nanostructures allow for an enhancement in electron-hole separation and an increase in the hydrogen generation rate. The lifetime of the fabricated devices is also discussed. Prolonged exposure of the nanostructured devices results in the degradation and etching of the InGaN material. The addition of a secondary semiconductor material, NiO, acts as a reaction site for photogenerated holes preventing the oxidation and dissolution of InGaN devices in the experimental electrolytes, increasing the device lifetime. Furthermore, a photoelectrochemical etch technique is implemented to create a porous device structure. The nanoporous network in the structure shortens the required diffusion length of the photogenerated carriers to values close to that of InGaN. An enhancement in photocurrent and hydrogen production has been observed due to the nanoporous structure

Optimisation of high-efficiency UV and visible light sources utilising lateral localisation in InAlN and InGaN based nano-structure devices

III-nitride semiconductor materials (including GaN, InN and AlN and their alloys), have the capability to emit light at wavelengths spanning from the near IR to the deep UV. However, understanding these materials is challenging due to the presence of strong polarisation fields and large difference in optimum growth temperature between binary compounds are two such examples. InAlN is perhaps the least well understood III-N alloy. It has potential be applied for optoelectronic devices operating in the UV spectral range. However, the variation of band-gap with alloy composition, particularly in the low In content regime, is not understood. In this work, a strongly composition dependent bowing parameter has been observed for ~100 nm thick InxAl1−xN epitaxial layers with 0 ≤ x ≤ 0.224, grown by metalorganic vapour phase epitaxy (MOVPE), prepared on AlN/Al2O3-templates. Also a double absorption edge was observed for InAlN with x < 0.01, attributed to crystal-field splitting of the highest valence band states. These results indicate that the ordering of the valence bands is changed at much lower In contents than linear interpolation of the valence band parameters would predict. Coupling our results with the published literature data the band-gap and bowing parameter of InAlN across the full composition range were determined. Additionally, applying the InAlN band-gap data with those for other alloys the refractive index of III-N alloys is predicted using an Adachi model resulting in a very good agreement with previous experimental data where available. For InAlN/AlGaN multi-quantum-wells (MQWs) excited by photoluminescence (PL) and emitting between 300-350 nm, high apparent internal quantum efficiencies (IQE) (IPL(300 K)/IPL(T)max) of up to 70% were obtained. This is attributed to the exceptionally strong carrier localisation in this material, which is also manifested by a high Stokes shift (0.52 eV) of the luminescence. A non-monotonic dependence of luminescence efficiency on indium content with a maximum at about 18% In was explained as a trade-off between a strain relaxation for higher indium contents and a type I to type II band line-up conversion for low In content alloys. Nanoscale materials have attracted a lot of attention due to their ability to decrease dislocations as well as build-in field reduction. In the second part of this thesis, GaN nanostructures, were used as templates for InGaN MQW growth targeting nano-LED structures. Two nano-structuring methods were examined; using GaN nano-columns (NCs) following an etch regrowth methodology, and selective area aperture growth (SAG). In the former case we determined the optimal etch conditions for the GaN columns and conditions for overgrowth InGaN QWS. The rod tops formed semipolar facets. InGaN QWs grown on these pyramids were found to be extremely thin leading to difficulties in obtaining PL in our case. Using the SAG approach, nano-pyramids were formed in nano-apertures, with good uniformity. InGaN QWs exhibited blue PL, which cathodoluminescence (CL) showed to be made up of two spectral features, attributed to the pyramid nano-facets and pyramid apex tips, respectively

Optical studies of cubic III-nitride structures

The properties of cubic nitrides grown by molecular beam epitaxy (MBE) on GaAs (001) have been studied using optical and electrical techniques. The aim of these studies was the improvement of the growth techniques in order to improve the quality of grown nitrides intended for bulk substrate and optoelectronic device applications. We have also characterised hexagonal nanocolumn structures incorporating indium. Firstly, bulk films of cubic AlxGa1-xN with aluminium fractions (x) spanning the entire composition range were tested using time-integrated and time-resolved photoluminescence (PL) plus reflectivity measurements. Strong PL emission was recorded from the samples, with improved intensity for higher aluminium concentrations. Temperature dependent and time-resolved PL showed the increasing role of carrier localisation at larger AlN fractions. The reflectivity results showed a near-steady increase in the bandgap energy with increasing AlN content. Alternative interpretations that did and did not involve a transition from direct-gap to indirect-gap behaviour in cubic AlxGa1-xN were considered. We next looked at cubic AlxGa1-xN/GaN/AlxGa1-xN single quantum well (QW) structures with varying AlN content in the barrier regions. The PL studies indicated that carrier escape from the QWs and non-radiative recombination at layer interfaces were limiting factors for strong well emission. Higher AlN concentration in the barriers appeared to exacerbate these problems. The doping of cubic GaN with silicon (n-type) and magnesium (p-type) was also studied. For Mg-doped GaN, a strong blue band emission was noted in the PL spectrum, which became more intense at higher doping levels. The Mg-doped GaN layers had low conductivity and their mobility could not be measured due to strong compensation effects. The cubic film had similar time-resolved PL properties for the blue band emission compared to hexagonal Mg:GaN. These results suggested that the blue band was the result of recombination between a shallow Mg acceptor and deep donor, believed to be a complex including a nitrogen vacancy and an Mg atom. This complex was also associated with the compensation effect seen in the electrical measurements. With the Si-doped cubic GaN, we observed PL spectra that were consistent with other sources. Thicker layers of GaN:Si did not have measurable mobility. This was likely caused by the rough surface structure that was imaged using a scanning electron microscope. The thin layer had a very smooth surface in comparison. The mobility of sub-micron thickness layers with a carrier concentrations of n = 2.0×1018cm-3 and n = 9.0×1017cm-3 were μ = 3.9cm2/Vs and μ = 9.5cm2/Vs respectively. The mobility values and structural issues indicated that growth improvements were needed to reduce scattering defects. In addition to cubic structures, we have considered nanocolumn growth of InGaN and InN. InxGa1-xN nanocolumns were grown on Si (111) by MBE with a nominal indium concentration of x = 0.5. PL emission was obtained from samples grown at higher temperature, but overall intensity was low. A second set of samples, where nanocolumn growth was followed by growth of a continuous coalesced film exhibited much stronger PL emission, which was attributed to the elimination of a phase separated core-shell structure in the nanocolumns. Next, a coalesced InxGa1-xN structure with vertically varying indium fraction was characterised. PL readings showed evidence of successful concentration grading. Finally, the PL spectra of coalesced InN layers were recorded, for which a specialised infrared PL system needed to be used. The results highlighted how increased growth temperature and indium flux can improve PL properties. For the binary alloy however, coalescence growth can decrease PL intensity compared to the nanocolumns stage