Graphene and layered two-dimensional (2D) materials have set a new paradigm in modern solid-state physics and technology. In particular their exceptional optical and electronic properties have shown great promise for novel applications in light detection. However, several challenges remain to fully exploit such properties in commercial devices. Such challenges include the limited linear dynamic range (LDR) of graphene-based photodetectors (PDs), the efficient extraction of photoexcited charges and ultimately the environmental stability of such atomically-thin materials. In order to overcome the aforementioned limits, novel approaches to tune the properties of graphene and semiconducting \ce{HfS2} are explored in this work, using chemical functionalisation and laser-irradiation. Intercalation of graphene with \ce{FeCl3} is shown to lead to a highly tunable material, with unprecedented stability in ambient conditions. This material is used to define photo-active junctions with an unprecedented LDR via laser-irradiation. Intercalation with \ce{FeCl3} is also used to demonstrate the first all-graphene position-sensitive photodetector (PSD) promising for novel sensing applications. Finally, laser-irradiation is employed, to perform controlled oxidation of ultra-thin \ce{HfS2}, which leads to induced strain in the material and a consequent spatially-varying bandgap. Such structure is used to demonstrate, for the first time, efficient extraction of photogenerated carriers trough the so-called ``charge-funnel'' effect, paving the way to the development of ultra-thin straintronic devices.

Thesis (MTech (Electrical Engineering))--Peninsula Technikon, Cape Town, 1998 / The purpose of this study was to perform an investigation into advances in the field of opto-electronics and to develop a suitable lightwave sensor, for application in smart structures research. Included in the scope of this research was the theoretical development and analysis of an appropriate technology to lead to an implementation of such sensors. This project forms part of an overall plan to gain experience in optoelectronic (photonic) sensor design. In research done previously in smart structure monitoring, piezo-electric techniques with the usual electrical interconnections was used. In a highly distributed sensor system the problems of electromagnetic interference, the tribo-electric effect and noise could be problematic. In this research, opto electronic techniques were thoroughly researched and an improvement on laser based fibre-optic interferometers was made. A non monochromatic lightwave interferometer was developed from theory and a prototype tested. The results suggests that an interferometric sensor can be operated with a non monochromatic source by using a second interferometer to modulate the frequency spectrum of the light before it is detected by a photodetector. Various test and measurement circuits for improved photodetector performance were evaluated, as well as a study of signal processing techniques that would be of use for an upgrade of the project where specific feature detection and analysis using the sensor is envisaged. A specification for a computer based data acquisition system was developed to do initial tests. The project should continue, with the sensor head being improved and all the necessary signal processing routines programmed into a Labview based data acquisition system.

Thesis (DTech (Electrical Engineering))--Peninsula Technikon, Cape Town, 2002 / In the last decade, the South Amcan Electricity Supply Commission would have had their ability to serve an ever demanding public severely tested. With the dilemma of providing electricity supply through hazardous environmental conditions, and with prospects of supplying power even beyond South Afiican borders, the need for a comprehensive damage and power delivery assessment strategy becomes all the more relevant. The rapid growth being made in the evolution of so called "intelligent" structures, with inherent sensor, actuator and control mechanisms built in can have direct influence on a power distribution network. At least in the foreseeable future, the impact ofphotonic sensors with inherent miniaturization, a foremost candidate in Smart System technology, can play a vital role in damage assessment of a potentially large network such as that found in the supply ofelectricity. Smart systems are nonliving systems that integrate the functions of sensing, actuation, logic and control, to respond adaptively to changes in their condition or environment to which they are exposed, in a useful and usually repetitive manner. Sensors are a fundamental part of the evolution of such systems and form the basis for the topic of this dissertation. The use ofoptical fiber sensors is increasing widely mainly due to their (a) miniature size, (b) remote signal processing ability, and (c) multiplexing capabilities. Because of the above features a variety of optical fiber sensing techniques has evolved over the years having potential for a myriad of applications. In this work a systems model and equations was developed for modeling the propagation of light in a optical waveguide, in order to study a Fabry Perrot sensor topology for application as a miniaturised sensor in a new type of smart structure, namely a smart electrical power system.

This thesis describes the use of charge modulation spectroscopy to investigate the negative and positive charge-induced absorptions in conjugated semiconducting polymers as a way to experimentally compare the wavefunctions of electrons and holes. Interactions between light and charges including fluorescence quenching and photocurrent are also explored. Conjugated polymers have an electronic structure with an energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). In the neutral ground state, there are no optical transitions at photon energies lower than this gap. When an excess charge is added to a conjugated polymer, the charge couples with a local structural reorganisation forming a localised entity known as a polaron. The polaron has two new electronic states within the energy gap symmetrically spaced about the midgap energy. Typically two new optical transitions between the polaronic states are allowed and can be accessed with sub gap energies. In order to probe the sub gap polaron absorptions charges can be added by electrical injection. Electrical injection in a transistor configuration provides a controlled way to measure the absorption of a known number of charges in the solid state and without triplet or singlet absorptions complicating the spectra as observed in photo-induced absorption. By taking advantage of recently developed ambipolar transistors where both holes and electrons can be accumulated in the same device a comparison can be made between the negative and positive polaron wavefunctions. Two polyfluorene polymers were chosen as examples where quantum chemical calculations predict either the same or different wavefunctions for the electron and hole. Poly(9,9-di-n-octylfluorene) (F8) is a hydrocarbon-only polymer which is expected to have similar electron and hole wavefunctions, whereas the related co-polymer, poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT) is expected to have an electron wavefunction that is more localized that the hole. The ambipolar transistors used in this thesis are typified by a dominant contact resistance which introduces difficulties in the charge modulation spectroscopy experiment. New techniques for simultaneous electrical and optical characterisation are developed and new device structures and fabrication processes are introduced in order to overcome a number of artifacts and improve the accuracy of the measurement allowing quantitative comparisons to be made. The increase in transistor or diode current with energy gap illumination and the quenching of fluorescence in the presence of charges is also investigated and a new method for imaging charge trapping and device operation in transistors with luminescent semiconductors is introduced.

III-Nitride materials, which consist of AlN, GaN, InN, and their alloys have become the cornerstone of the third generation compound semiconductor. Planar IIINitride materials are commonly grown on sapphire substrates which impose several limitations such as challenging scalability, rigid substrate, and thermal and lattice mismatch between substrate and material. Semiconductor nanowires can help circumvent this problem because of their inherent capability to relieve strain and grow threading dislocation-free without strict lattice matching requirements, enabling growth on unconventional substrates. This thesis aims to investigate the microscopic characteristics of the nanowires and expand on the possibility of using transparent amorphous substrate for III-nitride nanowire devices. In this work, we performed material growth, characterization, and device fabrication of III-nitride nanowires grown using molecular beam epitaxy on unconventional substrates. We first studied the structural imperfections within quantum-disks-in-nanowire structure grown on silicon and discovered how growth condition could affect the macroscopic photoluminescence behavior of nanowires ensemble. To expand our work on unconventional substrates, we also used an amorphous silica-based substrate as a more economical substrate for our nanowire growth. One of the limitations of growing nanowires on an insulating substrate is the added fabrication complexity required to fabricate a working device. Therefore, we attempted to overcome this limitation by 5 investigating various possible GaN nanowire nucleation layers, which exhibits both transparency and conductivity. We employed various nucleation layers, including a thin TiN/Ti layer, indium tin oxide (ITO), and Ti3C2 MXene. The structural, electrical, and optical characterizations of nanowires grown on different nucleation layers are discussed. From our work, we have established several key processes for transparent nanowire device applications. A nanowire LED emitting at ∼590 nm utilizing TiN/Ti interlayer is presented. We have also established the growth process for n-doped GaN nanowires grown on ITO and Ti3C2 MXene with transmittance above 40 % in the visible wavelength, which is useful for practical applications. This work paves the way for future devices utilizing low-cost substrates, enabling further cost reduction in III-nitride device fabrication.

No description available.

Semiconducting materials have emerged as the cornerstone of modern electronics owing to their extensive device applications. There is a continuous quest to find cost-effective and low-temperature compatible materials for future electronics. The recent reemergence of solution processable halide perovskites have taken the optoelectronics research to new paradigms. Apart from photovoltaics, the versatile characteristics of halide perovskites have resulted in a multitude of applications. This dissertation focuses on various properties and emerging applications particularly, photodetection and thermoelectrics of both hybrid and all-inorganic halide perovskites. It is important to understand the underlying properties of perovskites to further develop this class of materials. One of the major hurdles restricting the practical devices of perovskites is their sensitivity to moisture. A systematic investigation on the effect of humidity on hybrid perovskites revealed different degree of moisture uptake behaviour for micropatterns, films, and single crystals. Degradation pathways and processing limitations of hybrid perovskites are discussed which will aid in designing strategies to overcome these impediments for future large scale device integration. There is a recent surge of reports on doping hybrid perovskites to control its optoelectronic properties but in-depth understanding of these dopants and their ramifications remain unexplored. The effect of doping on the optoelectronic properties of hybrid perovskites is studied and a model is proposed for the observed behavior. Leveraging on the rapid growth of microcrystalline perovskite films, for the first time tunable bifacial perovskite photodetectors were fabricated, operating in both broadband and narrowband regimes. Furthermore, self-biased single crystalline photodetectors based on all-inorganic perovskite were developed with high on-off ratio and low dark current. Halide perovskites are emerging as a new class of materials for thermoelectric applications owing to their ultralow thermal conductivity and decent Seebeck coefficient. Here, halide perovskites are evaluated in terms of composition, stability, and performance tunability to understand their thermoelectric efficacy. Finally, as an alternative to Pb and Sn-based perovskites, a new hybrid was discovered with ultralow thermal conductivity and a general synthetic route to design such hybrids is proposed.

Dual purpose fiber optic microbend loss sensors have been developed for measurement of underwater acoustic wave amplitudes and for detection of the direction of wave propagation. Cylindrical sensing elements with external threads have fibers wound around them. Axial slots, cut along the length of the cylinder and deeper than the threads, provide the microbends. Three different construction schemes for cylindrical sensing elements are built. The dual purpose hydrophones are characterized for frequencies ranging from 15 kHz to 75 kHz. Based on the results, an improved design that uses the wavelength dependence of microbend loss in a single mode fiber is proposed. / Master of Science

In this thesis, three primary findings are presented concernmg optolelectronic properties of AlGaN superlattices. First, we obtain the lowest lateral p-type resistivity and highest lateral p-type mobility to date in the AlGaN material system. Second, we obtain the first experimental results of multi-subband photoluminescence in p-type AlGaN superlattices. Last, we report the first direct measurement of perpendicular electrical transport ( electrical transport perpendicular to the superlattice planes) in AlGaN superlattices. Our research into resistivity and mobility of AlGaN superlattices stems from the fact that p-type AlGaN is highly resistive. To overcome the problem of highly resistive p-type AlGaN, we propose and demonstrate modulation doping in p-type AlGa superlattices. Our measurements yield a low-temperature lateral resistivity and mobility of 0.068 S1 • cm and 36 cm 2 /(V • s), respectively. This is the lowest resistivity and highest mobility recorded to date in p-type AlGaN and results from reduced ionized impurity scattering inherent in modulation doping. The optical properties of AlGaN superlattices are of great interest because they are often used in light-emitting diodes and laser diodes. Specifically, the absorption edge in AlGaN superlattices is typically thought of as being severely red-shifted due to internal electric fields present in AlGaN-based materials. We obtain experimental photoluminescence results on large-period superlattices that indicate that the redshifting of the absorption edge is much less than previously thought due to the combined effects of band-filling and oscillator strength on energy. We develop a computer model based on the self-consistent solution of the Poisson and Schrodinger system of equations. Our model predicts a drastic decrease in spontaneous recombination lifetime with increased transition energy, which is consistent with our experimental data. The perpendicular resistivity of AlGaN superlattices is also of critical importance to the development of AlGaN-based devices. We therefore measured the perpendicular resistivity of an n-type AlGaN superlattice and compared it to bulk n-type GaN. The superlattice has a perpendicular resistivity of 1.2 D • cm while bulk n-type GaN is 0.18 D • cm. We develop a theoretical model based on sequential tunneling and enhanced free carrier concentration to explain our experimental findings. Our model shows that perpendicular resistivity is dominated by two factors; carrier concentration and tunneling probability.

Photovoltaics (PV) are a renewable energy technology experiencing rapidly increasing commercial adoption today. Nonetheless, many proposed PV applications still require higher efficiencies, lower costs and comparable reliability to currently available in commercial devices (typically made from silicon). To enable the rigorous study of a much wider range of materials and novel design concepts, particularly those based on compound thin films, Concentrated Photovoltaics (CPV), cells with bifaciality, a comprehensive modeling framework is developed to couple photon absorption, carrier transport, photon recycling, and thermal transport in PV devices. The universality of this framework manifest itself in approaching various PV related problems as follows: 1) exploring the novel design of wide-Eg GaInP solar cells as an intermediate step to enhance the efficiency of multijunction PV devices; 2) characterizing the open-circuit voltage (VOC) degradation in thin-film vapor liquid solid (TF-VLS) grown InP solar cell through combined device and circuit model for interpreting photoluminescence (PL) image; 3) establishing optic-electric-thermal coupled framework to assess and compare the passive cooling effect for Silicon CPV devices that employ porous soda-lime glass radiative cooler and conventional copper cooler respectively; 4) Investigating and formulating the analytic solution of the optimal design that minimizes combined optical shadowing loss and electrical resistive loss for two types of bifacial PV devices: a) interdigitated back contact (IBC) Silicon heterojunction (SHJ) solar cells and b) Copper Indium Gallium DiSelenide (CIGSe) solar cell with Al2O3 passivation; and 5) Constructing an Neural Network Autoen- coder (NNA) that compresses and reconstructs the J-V characteristics obtained from TCAD simulation and literature for rapid screening and automated classification.