Indium Tin Oxide (ITO) films were deposited on a number of semi-conductor materials using reactive r. f sputtering technique to form both rectifying Schottky and ohmic contacts. These contacts were applied in the fabrication of a number of novel optoelectronic devices: Schottky photo-diodes, transparent gate High Electron Mobility Transistors (HEMTs), heterojunction bipolar transistors (HBTs) being used as heterojunction phototransistors (HPTs), light emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs). A number ofthese novel devices were studied in comparatively greater detail; these were the Schottky diode and the HPT. Deposition conditions necessary to produce ITO films with high conductivity and optical transparency over a wide spectral range were studied and optimised. Separate post deposition techniques were developed to produce near ideal rectifying contacts and ohmic contacts with low contact resistance respectively. A thin film of indium (In) was also used to optimise ITO ohmic contacts to n + - GaAs substrates. Near ideal Schottky diodes were realised on n-GaAs substrates using aluminium (AI) and gold (Au) metal contacts. A simulation model was then developed and implemented to study the behaviour of current transport mechanisms over a wide temperature range. Photodiodes with ITO as the Schottky metal contact were fabricated and a study comprising of both their electrical and optical behaviour was undertaken. Relatively large geometry HBTs and HPTs were fabricated using AIGaAs/GaAs, InGaP/GaAs and InPlInGaAs systems respectively; the latter devices were first reported as a result of this study. A comparative study between devices fabricated from these systems were then made. This was followed by an appraisal of the electrical properties of each of their optical counterparts which had ITO emitter contacts. The specific photo responsivity and the spectral responses of these HPTs were analysed. In light of HPTs with transparent ITO emitter ohmic contacts, a brief examination of the merits of vertical versus lateral illumination was also made in this work. Finally a spectral response model was developed to understand and help design optoelectronic detectors comprising of single layer devices (n-GaAs Schottky photo diodes) or multiple semiconductor materials (HPTs using AIGaAs/GaAs or InPlInGaAs systems) to help predict responsivities at a given incident wavelength. As well as material properties of the constituent semiconductors, this model takes into account the specific lateral and vertical geometrical dimensions of the device.

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Thesis advisor: Dunwei Wang / Due to the increasing energy demand from human activities, efficient utilization of renewable energy, such as wind, solar and geothermal energies, becomes necessary and urgent. Photoelectrochemical water splitting offers a great example to convert solar energy and storage it in the term of chemical bond. Seeking suitable photoanode materials becomes the research focus of my study, because the development of photoanode materials significantly lags that of robust photocathode (such as Si). The main challenge is to fabricate an efficient and stable photoanode material which can deliver high photocurrent and sufficient photovoltage which can match well with those of photocathode when made into tandem cell configuration. Hematite (α-Fe2O3) represents a promising metal oxide photoanode material, with a suitable band gap (2.1 eV), low cost and toxicity. Applying nanostructures and appropriate surface modification layers help address existing research challenges. As a result, a much lower turn on potential and greater photocurrent density is achieved. Another photoanode material attracts our attention is tantalum nitride (Ta3N5), with a similar band gap to hematite but much better light absorption properties, shows a poor stability in aqueous electrolyte. For both photoanode materials, thermodynamic and kinetic aspects are studied in details when tested in water splitting devices. These works provide new ideas and insights on the future studies. / Thesis (PhD) — Boston College, 2015. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Chemistry.

We quantify the chemical species present at and reactivity of the tetragonal (100) face of single-crystal methylammonium lead iodide, MAPbI3(100). MAPbI3 is an ABX3 perovskite, experiments utilized the orthogonal reactivity of the A+-site cation, the B2+-site cation, and the X–-site halide anion. Ambient-pressure exposure to BF3 solutions probe the reactivity of interfacial halides. Reactions with p-trifluoromethylanilinium chloride probe the exchange reactivity of the A+-site cation. The ligand 4,4’-bis(trifluoromethyl)-2,2’-bipyridine probe for interfacial B2+-site cations. Fluorine features in x-ray photoelectron spectroscopy (XPS) quantify reaction extents with each solution-phase species. XP spectra reveals adsorption of BF3 indicating surface-available halide anions on tetragonal MAPbI3(100) and preliminary examinations on the (112), (110), and thin-film surfaces. Temperature-programmed desorption (TPD) established a ~200 kJ mol–1 desorption activation energy from tetragonal MAPbI3(100). Adsorption of the fluorinated anilinium cation includes no concomitant adsorption of chlorine as revealed by the absence of Cl 2p features within the limits of XPS detection on the tetragonal (100) and (112) faces with no discernable exchange in preliminary experiments on tetragonal (110). Within detection limits, bipyridine ligand demonstrate no adsorption to tetragonal MAPbI3(100) or (112), while it does demonstrate significant adsorption on the (110) in preliminary experiments. We discuss the present results in the context of interfacial stability, passivation, and reactivity for perovskite-based energy conversion materials and some preliminary investigations into bilayer graphene-based dye sensitized photovoltaic materials.

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