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Copper is abundantly used throughout marine infrastructure, both in its pure and alloyed form. Pure copper is used in electrical cables in offshore infrastructure due to it desirable electrical properties. Copper itself is not corrosion resistant to seawater, therefore extensive sheathing is employed to protect the copper from being exposed to seawater. However, these cables are susceptible to damage, leading to the exposure of copper to seawater. Faults can arise due to a number of reasons involving physical, climatic and/or chemical effects. One particular fault includes water-treeing caused by the dielectric breakdown of cable insulation. This form of fault often results in small faults in the μm range. Though this seems small, such a fault can ultimately lead to cable failure due to the corrosion of the copper core in seawater. Therefore, an effort is made here to investigate a technique which could prevent the corrosion of copper exposed to seawater in this manner, subsequently improving the lifetime of a defective cable. In particular in this thesis, the anodic passivation of copper microelectrodes (consistent with μm sized faults) in model seawater is investigated as a corrosion protection technique. Anodic passivation involves the use of electrochemical methods, where polarising a given metal to positive applied potentials encourages the growth of a protective film. This can be achieved by performing voltammetric or chrono methods. In the case of copper, a partially passive CuCl film forms. The anodic passivation of copper has proven to be complex, however, where dynamic current phenomenon is observed during voltammetry to high positive potentials. This phenomenon refers to current oscillations/noise, which are primarily linked to the formation and dissolution of CuCl. In this thesis, the aim is to develop an understanding of the anodic passivation mechanism for copper microelectrodes in model seawater. The mechanism is investigated primarily with voltammetry to distinguish potential-dependent processes. The effects of electrode diameter, pH, salinity, temperature and the presence and type of gas on anodic passivation is explored. In addition to pure copper microelectrodes, the effects of alloying copper with nickel is also investigated. In particular, copper-nickel alloys constantan and monel are studied. Initial results for copper microelectrodes in 0.5MNaCl(aq) using voltammetry found that CuCl forms initially at approximately -0.11 V vs. saturated calomel electrode (SCE), and is the dominant product observed throughout voltammetry to +5.00 V vs. SCE. The continual presence of CuCl is confirmed by performing in situ Raman spectroscopy. At 0.20 V vs. SCE, the formation of Cu(II) is observed which coincides with the onset of dynamic current phenomena-chaotic current oscillations. It is hypothesised that the Cu(II/I) process is linked to the onset of current oscillations, where Cu(II) causes instabilities in the CuCl film leading to current noise/oscillations. Current oscillations proceed to 5.00 V vs. SCE, where large current spikes are observed (up to 100 μA in magnitude) in addition to baseline current oscillations (around 5-6 μA). The use of a polymer of intrinsic microporosity (PIM) revealed mechanical processes associated with aspects of the current oscillations. The use of PIM revealed a link between the large current spikes and the expulsion of particulate CuCl, hindered by the presence of PIM. This process is referred to as colloidal dissolution. For pure copper, no evidence for anodic gas evolution is observed (high potentials can cause water splitting), and thus not considered in proposed mechanisms for pure copper. The effects of the presence and type of gas in the electrolyte has a profound effect on the anodic passivation of copper, particularly on the current oscillations observed up to 5.00 V vs. SCE. More specifically, the frequency and presence of large current spikes change depending on the presence and type of gaseous solutes in the electrolyte (aqueous 0.5 M NaCl). Vacuum-degassing, and saturation of the electrolyte with O₂, He and H₂ suppresses current spikes, whereas saturation with Ar, N₂ and CO₂ enhance current spike events (and thus colloidal dissolution). Saturation with air gives a mixture of effects owing to N₂ and O₂. It is proposed that colloidal dissolution events occur due to gas bubble nucleation, where bubble nucleation causes strain in the CuCl film. The release of said strain then leads to film breakdown and the expulsion of CuCl particulates. Furthermore, the effects of different gases arelinked to the their ability to act as surfactants. The anodic passivation of copper-nickel alloys is also owed to the formation of CuCl. The anodic behaviour of the alloys constantan and monel proves to be similar at low positive applied potentials (up to 1.80 V vs. SCE), and significantly different at potentials > 1.80 V vs. SCE. The difference being a transition from low-level current oscillations similar to copper (5-6 μA) to high current oscillations in the mA range. This transition occurs at 1.80 V vs. SCE. The potential-dependence of the high current noise is linked to gas evolution due to the formation of a stoichiometric oxidant NiO₂, and thus a Ni(IV/II) process. A mechanism is proposed with support from Pourbaix diagrams and scanning electron microscopy (SEM) imaging, based on gas evolution and rapid breakdown of a CuCl film. Overall, this thesis highlights the complexity of anodic passivation for copper and coppernickel microelectrodes and shows that film formation and dynamic current phenomena are sensitive to changes in electrolyte environment and alloying. New hypotheses are given for the partial passivation of copper and copper-nickel alloys, and novel approaches are used to do so, including the use of PIM and recession analysis by SEM.

This dissertation describes our study on ion migration-driven optoelectronic and structural changes of hybrid metal halide perovskites under light illumination or electric bias, and methods to exploit or inhibit these ion migration processes by implementation of effective passivation approaches. We first critically review recent studies on the photobrightening effect, which has a strong correlation with the extent of photo-induced ion migration processes, and has a substantial impact on performance and lifetime of the perovskite optoelectronic devices. We highlight the sensitivity of this effect to experimental considerations such as atmosphere, photon energy, photon dose, perovskite composition, and morphology, and resolve apparent contradictions in the literature. We then explore the impact of light illumination on triple-cation lead mixed-halide perovskite films, with substantially enhanced luminescence efficiency observed in these materials. We perform a range of experimental characterisations to understand these changes in photo-physical properties of the perovskite materials under light illumination. Although the brightening effect in halide perovskites has the advantage of achieving high luminescence efficiencies, the bandgap instability arising from ion segregation, particularly in systems with high bromide fractions, prevents the full exploitation of the continuous bandgap tunability in perovskite materials. To this end, we demonstrate a passivation approach using potassium halides to both enhance the luminescence efficiency and inhibit ion segregation in hybrid perovskite films. Through photoelectron spectroscopy, and chemical mapping measurements, we indicate that the passivation effects mainly originate from formation of potassium halide inclusions with higher concentration at the perovskite surface. We also explore the changes in properties of perovskite films when biased with an electric field in an operating light-emitting device architecture. Using a combination of in-situ and ex-situ measurements, we monitored the degradation of mixed-halide perovskite light-emitting diodes over time. Through chemical mapping, and optical spectroscopy measurements, we revealed that the degraded performance arises mainly from accumulation of halides at one interface. We demonstrate how the potassium passivation immobilizes excess halide species and thus inhibits these ionic migration effects. The passivated light-emitting diodes show enhanced external quantum efficiency from 0.5 to 4.5% and, most importantly, show significantly enhanced operational stability over the control devices. The half-life of decay for the devices under continuous operation is increased from <1 hour to ~10 hours through passivation, and ~200 hours under pulsed bias operation. This work provides an insight into understanding and controlling ion migration processes and highlight routs to improved performance and operational stability in new device structures.

Crystalline silicon is by far the most prevalent material in the manufacture of solar cells, principally due to their low cost of fabrication. However, depending on the growth method used to produce silicon crystals, unwanted impurities, and defects, can remain. These defects can be electrically active, acting as traps for charge carriers, reducing carrier lifetime, and ultimately leading to reduced efficiency in finished solar cells. In this thesis, two sets of defects in crystalline silicon are studied using electrical techniques - capacitance voltage (C-V) measurements, deep level transient spectroscopy (DLTS), Laplace DLTS, and minority carrier transient spectroscopy (MCTS), with the aim of understanding how their impact on cell efficiency might be reduced. In the first part of this thesis, the electrical activity of transition metal impurities, which can be common contaminants of cheaper silicon that has not been through costly chemical purification processes, is investigated. Although certain transition metals can be removed by gettering, slow diffusing metal atoms can remain, and alternative means of reducing their electrical activity, and hence their effect on cell efficiency, are required. One potential means to achieve this is the use of hydrogen as a passivation tool, as it can bond to metallic impurities, rendering them electrically inactive. The effect of hydrogen on electrical activity has been investigated for two of the slowest diffusing metals, vanadium and molybdenum, in intentionally contaminated silicon treated with a hydrogen plasma. We find that hydrogen passivation occurs in n-type silicon, with an observed reduction in the electrical activity of vanadium atoms, but that no such bonding occurs in p-type silicon for either vanadium or molybdenum. We suggest that this difference is due to the fact that metal and hydrogen atoms are both positively charged in p-type silicon, and experience Coulombic repulsion. The possibility of forcing metal-hydrogen bonding in p-type silicon using optical excitation is then investigated and found to be successful for both vanadium and molybdenum following illuminated annealing at temperatures 800 °C, after which they are not seen to form upon subsequent annealing between 450 and 700 °C. Finally, the influence of nitrogen doping during the float zone growth process on the formation of these thermally activated defects is investigated. We show that the concentration of defects formed during annealing is significantly greater in nitrogen doped than nitrogen lean silicon, and suggest that this effect is due to the differences in the concentration of vacancies between the two materials.

Catalytic Chemical Vapour Deposition (CCVD) of graphene is the most promising production method of large-area, high-quality graphene. However, the required transfer step from the metal growth substrate to the desired dielectric substrate is a major bottleneck for the integrated manufacturing of graphene-based electronic devices. The performance of a typical graphene field effect transistor (GFET) is marred by the presence of graphene grain boundaries, atmospheric doping and polymer residues from the transfer and fabrication steps. We here show that these three factors all interact with each other. The grain boundaries acts as a preferential adsorption site for polymer residues from the transfer step, which reduce the electronic properties of graphene and help to retain atmospheric doping. These polymer residues become embedded below the Al₂O₃ ALD layer during the encapsulation process, which prevents the passivation of the graphene surface and the restoration of its intrinsic electronic properties. The transfer-free catalytic graphene growth, directly at the interface between the metal catalyst and the desired dielectric substrate, is an interesting alternative for the integrated manufacturing process. However, the growth mechanism is poorly understood so far. We here present a detailed study of the feasibility, challenges and mechanism of growing these interface graphene films (IGF). Firstly, we find that with typical metal catalysts -such as nickel or copper-, the commonly used SiO₂/Si substrate roughens at relevant growth temperatures due to the capillary action of the overlaying metal catalyst grain boundaries and its high interface diffusion, which makes this substrate less attractive for IGF growth. Secondly, we investigate both the thermodynamics and kinetics of IGF growth. By modelling the different carbon fluxes through the metal catalyst thin film during the growth process, we find that the limited carbon bulk reservoir plays an important role in the IGF growth mechanism and that supersaturation at the interface is difficult to achieve. For stable catalysts -with no significant mass loss or dewetting occuring at elevated temperatures-, IGF growth is thus limited to precipitation during cooling, while for unstable catalysts isothermal growth is possible. The proposed mechanism is corroborated by systematic experimental results on IGF growth at the metal/sapphire interface. Additionally, our proposed growth mechanism allows for the first time to critically reinterpret the published IGF growth reports in the literature. We highlight the fundamental trade-offs found between growing full coverage, but low-quality, inhomogeneous IGF films by precipitation with stable catalyst and obtaining high-quality, homogeneous IGF films but with no full coverage when using unstable catalysts. Based on our findings and developed understanding of the IGF growth mechanism, we propose a novel engineered catalyst structure for high-quality, homogeneous, full coverage and controllable IGF growth, consisting of a Cu/ Ni/aC/dielectric stack. The copper both blocks the carbon from forming graphene at the free metal surface as well as gradually reduces the carbon solubility in the nickel layer. This pushes the dissolved carbon towards the interface and causes isothermally supersaturation and IGF growth to occur. Various challenges and solutions in the form of copper diffusion barriers are discussed to control the kinetics of this system and improve the resulting IGF quality. This work represents an important milestone for the IGF growth by systematically analysing the fundamental interactions, thermodynamics and kinetics and provides a stepping stone for further research in the area.

Colloidal quantum dot (CQD)-based solar cells are a promising technology that could provide low-cost energy in the future. In this thesis the passivation of PbS CQD surfaces, carrier dynamics in PbS CQD-based solar cell systems, and the electronic structure of a potential photoanode material (CdO) are studied using a variety of photoemission spectroscopies and complementary techniques. The surface passivation of PbS CQDs is investigated using X-ray photoelectron spectroscopy (XPS) and synchrotron radiation-excited depth profiling XPS, which gives information on the chemical composition and oxidation state of the surface. PbS CQDs were studied after treatments including Cd cation exchange, halide (Cl, Br, I) ligand exchanges, short organic ligand (EDT) exchange, and combinations of these treatments. For Cd cation exchanged PbS CQDs a thicker Cd shell results in less oxidation, and an optimum effective shell thickness of 0.1 nm provides suffcient protection against oxidation without introducing a large insulating barrier which can hinder charge transfer from the CQD. For combined ligand treatments, evidence of passivants being etched from the surface is found. This significantly reduces the air stability and device performance, which is found to be strongly connected to the number of passivants present on the PbS CQD surface. Laser-pump photoemission-probe spectroscopy is used to investigate both charge transport at the PbS CQD-ZnO photoanode interface, and band bending and carrier dynamics at the surface of PbS CQD solids. Charge injection was observed from PbS CQDs into ZnO, the dynamics of which are limited by the persistent photoconductivity of the ZnO. For PbS CQD solids passivated with EDT, MPA, PbI2, and a quasiperovskite MAI/PbI2 shell, band bending at the solid-vacuum interface is observed for the first time. Comparison of the charge carrier dynamics for different CQD solids show that the dynamics can occur on timescales between microseconds (MAI/PbI2) and seconds (MPA, PbI2). Oxygen contaminants (observed with XPS), creating deep traps, is suggested as the reason for the slower dynamics. The origin of the two-dimensional electron gas (2DEG) on the surface of CdO is also investigated with angle-resolved photoemission and core-level XPS. Surface adsorbates and interstitial hydrogen are found to donate electrons to the surface, occupying conduction band states. The effects on the occupancy of the 2DEG with removal of adsorbates and diffusion of atomic hydrogen via cracking are explored.

Defects in the functional oxides play an important role in electronic devices like metal oxide semiconductor field effect transistors (MOSFETs) and resistive random-access memories (ReRAMs). The continuous scaling of CMOS has brought the Si MOSFET to its physical technology limit and the replacement of Si channel with Ge channel is required. However, the performance of Ge MOSFETs suffers from Ge/oxide interface quality and reliability problems, which originates from the charge traps and defect states in the oxide or at the Ge/oxide interface. The sub-oxide layers composed of GeII states at the Ge/GeO2 interface seems unavoidable with normal passivation methods like hydrogen treatment, which has poor electrical properties and is related to the reliability problem. On the other hand, ReRAM works by formation and rupture of O vacancy conducting filaments, while how this process happens in atomic scale remains unclear. In this thesis, density functional theory is applied to investigate the defect behaviours in oxides to address existing issues in these electronic devices. In chapter 3, the amorphous atomic structure of doped GeO2 and Ge/GeO2 interface networks are investigated to explain the improved MOSFET reliability observed in experiments. The reliability improvement has been attributed to the passivation of valence alternation pair (VAP) type O deficiency defects by doped rare earth metals. In chapter 4, the oxidation mechanism of GeO2 is investigated by transition state simulation of the intrinsic defect diffusion in the network. It is proposed that GeO2 is oxidized from the Ge substrate through lattice O interstitial diffusion, which is different from SiO2 which is oxidized by O2 molecule diffusion. This new mechanism fully explains the strange isotope tracer experimental results in the literature. In chapter 5, the Fermi level pinning effect is explored for metal semiconductor electrical contacts in Ge MOSFETs. It is found that germanides show much weaker Fermi level pinning than normal metal on top of Ge, which is well explained by the interfacial dangling bond states. These results are important to tune Schottky barrier heights (SBHs) for n-type contacts on Ge for use on Ge high mobility substrates in future CMOS devices. In chapter 6, we investigate the surface and subsurface O vacancy defects in three kinds of stable TiO2 surfaces. The low formation energy under O poor conditions and the +2 charge state being the most stable O vacancy are beneficial to the formation and rupture of conducting filament in ReRAM, which makes TiO2 a good candidate for ReRAM materials. In chapter 7, we investigate hydrogen behaviour in amorphous ZnO. It is found that hydrogen exists as hydrogen pairs trapped at oxygen vacancies and forms Zn-H bonds. This is different from that in c-ZnO, where H acts as shallow donors. The O vacancy/2H complex defect has got defect states in the lower gap region, which is proposed to be the origin of the negative bias light induced stress instability.

Due to an aging population and younger patients presenting with musculoskeletal disorders, there is a need for orthopaedic implants with improved healing rates and longer implant life. Numerous research has developed implant surfaces with micro-topography and biomolecules to imitate the native extra cellular matrix (also known as biomimetic surfaces). This research has utilised such a biomimetic approach by immobilising the cell adhesive peptide, RGD (Arginine-Glycine-Aspartic Acid), to a titanium alloy Ti6Al4V surface. This research polymerised Hyperbranched Polyglycerol (HBPG) from the titanium surface using Ring Opening Multi-Branching Polymerisation (ROMBP). HBPG is a biologically compatible and non-toxic synthetic biopolymer, able to reduce non-specific protein adsorption, increase the titanium surface wetting (hydrophilicity), thereby limiting foreign body reactions. Extensive hydroxyl groups at the periphery of HBPG provides conjugation sites for biomolecule attachment. In this work the RGD peptide was conjugated to the polymer via a siloxane layer. This research developed a novel passivation solution for the preparation of the titanium alloy surface, using a mixture of hydrogen peroxide and nitric acid (a passivation mixture not used in the literature). This novel mixture was shown to etch the titanium surface, producing micro and nano surface features, both of which have been shown to improve cellular function in the literature. The hydrogen peroxide/nitric acid solution showed extensive oxidising ability on titanium, leading to the formation of reactable hydroxyl groups. Contact-angle measurements showed that the novel passivating solution produces a hydrophilic surface similar to that of peroxidation for 12-hours, but achieved in only 2-hours. In conjunction with the etching and oxidising abilities of hydrogen peroxide, the nitric acid reacts with the titanium surface, leading to the formation of a protective titanium oxide layer, enhancing corrosion resistance and improving biocompatibility. Biological investigations with the pre-osteoblast cell line MC3T3-E1 showed greater osteoblast cell attachment and adhesion strength, as well as improved bone matrix mineralisation on the passivated titanium surface functionalised with HBPG and the RGD peptide, compared to the raw and passivated titanium surfaces. Antibacterial testing of HBPG revealed substantially reduced bacterial cell colonies on the passivated/polymerised titanium surface, possibly arising from electrostatic and hydrophobic repulsion. This research has successfully developed a new titanium passivation solution (hydrogen peroxide/nitric acid) that can yield a contact-angle of around 35° in just 2-hours, rivalling the Piranha solution. The successful immobilisation of a cyclic RGD (cyclic-RGDfc) to a titanium surface functionalised with HBPG, has been shown in this research to drastically improve mineralised bone matrix production from the MC3T3-E1 cell line. This indicates earlier osseointegration of the implant may be possible, thereby improving patient healing times.

Passivation of silicon surfaces remains a critical factor in achieving high conversion efficiency in solar cells, particularly in future generations of rear contact cells -the best performing cell geometry to date. In this thesis, passivation is characterised as either intrinsic or extrinsic, depending on the origin of the chemical and field effect passivation components in dielectric layers. Extrinsic passivation, obtained after film deposition or growth, has been shown to improve significantly the passivation quality of dielectric films. Record passivation has been achieved leading to surface recombination velocities below 1.5 cm/s for 1 Ωcm n-type silicon covered with thermal oxide, and 0.15 cm/s in the same material covered with a thermal SiO2/PECVD SiNx double layer. Extrinsic field effect passivation, achieved by means of corona charge and/or ionic species, has been shown to decrease by 3 to 10 times the amount of carrier recombination at a silicon surface. A new parametrisation of interface charge, and electron and hole recombination velocities in a Shockley-Read-Hall extended formalism has been used to model accurately silicon surface recombination without the need to incorporate a term relating to space-charge or surface damage recombination. Such a term is unrealistic in the case of an oxide/silicon interface. A new method to produce extrinsic field effect passivation has been developed in which charge is introduced into dielectric films at high temperature and then permanently quenched in place by cooling to room temperature. This approach was investigated using charge due to one or more of the following species: ions produced by corona discharge, Na+, K+, Cs+, Mg2+ and Ca2+. It was implemented on both single SiO2 and double SiO2/SiNx dielectric layers which were then measured for periods of up to two years. The decay of the passivation was very slow and time constants of the order of 10,000 days were inferred for two systems: 1) corona-charge-embedded into oxide grown on textured FZ-Si, and 2) potassium ions driven into an oxide on planar FZ-Si. The extrinsic field effect passivation methods developed in this work allow more flexibility in the combined optimisation of the optical properties and the chemical passivation properties of dielectric films on semiconductors. Increases in cell Voc, Jsc and η parameters have been observed in simulations and obtained experimentally when extrinsic field effect passivation is applied to the front surface of silicon solar cells. The extrinsic passivation reported here thus represents a major advancement in controlled and stable passivation of silicon surfaces, and shows great potential as a scalable and cost effective passivation technology for solar cells.

Photoconductive microwave switches can be used in place of traditional microwave switches to reconfigure antennas and RF circuits. The switch, which consists of a silicon die placed over a gap in transmission line, is controlled by illumination via a fibre optic cable. Hence there is no requirement to design electrical biasing lines which may affect RF performance. This benefit is the main motivation behind further developing and understanding the photoconductive switch. The second motivation is the growing demand for reconfigurable antennas which necessitate certain switching requirements; one specific area of interest is in cognitive radio applications. However, in order to use such a switch in RF circuitry, the photoconductive nature of the switch must be understood. This is addressed in this thesis presenting and applying analytical equations which dictate the material properties in photoconductive silicon. These equations are then used to generate a 3D EM simulation model to investigate transmission loss in the photoconductive switch. The concept of signal planarity is investigated so as to give some insight into the best way to package the switch. In order to potentially reduce loss and facilitate a packaged device, the fabrication of the switch is investigated. Namely, the treatment of the silicon and the addition of contacts on the silicon are discussed as possible methods to improve switch performance. Lastly, linearity, power handing and switching times are presented for the photoconductive switch. This characterisation is important with regards to understanding which types of application the switch can be used in. In particular the single tone and two tone linearity of the switch is measured these values have not previously been reported for this type of photoconductive switch. The results are encouraging and support further development of the switch into a packaged product to be used in reconfigurable antennas and circuitry.

High-temperature superconductivity (HTS) was supposed to change the world. And it did, albeit less dramatically than previously predicted over 40 years ago. Thin films of HTS materials have been applied in qubits for quantum computing, analog-to-digital converters, and neuromorphic circuits; and are necessary for the most sensitive magnetometers used in geological surveys and medical technology. This technological revolution hinges on Yttrium Barium Copper Oxide, or YBa2Cu3Ox+6 (YBCO). This ceramic oxide from the cuprate family has been the benchmark for “high temperature” superconductors, in which the superconducting state is reached above the boiling temperature of nitrogen. However, its processing and technical implementation in thin films come with challenges. It is a non-stoichiometric crystal, and its superconducting properties are highly dependent on its concentration of oxygen (and therefore the variable oxidation state of copper). Most importantly – and the focus of this dissertation – is its stability as a thin film under 40 nanometers thick: YBCO expands differently from its substrates under thermal cycling, loses oxygen at high temperatures, and in air, reacts to form non-superconducting (and oxygen-depleted) products. The work presented here explores how YBCO thin films fail, and how to protect their surface and preserve thin-film superconductivity. The focus lies on the use of bromine – a halogen that tightly binds to the YBCO surface – as a means of chemical passivation, protecting YBCO from being reduced in the atmosphere without harming the desirable superconducting properties, and also without harming the performance of superconducting devices such as YBCO-based nanoelectronics. We found that thermal stress is not the root cause of failure in superconducting thin films, which led the study into further chemistry. Remarkably, treatment with dilute bromine in ethanol solutions was found to have little effect on the material’s performance and does not destroy the electronic properties of Josephson Junctions. The reaction, while an etching process, only attacks the surface, leaving the underlying material (and written junctions) intact. Through this work, we aim to increase the longevity of HTS nanoelectronics. This dissertation uses the toolkits of chemistry to solve problems in electrical engineering – ultimately inching closer to less expensive, more robust HTS devices.

Kesterite Cu2ZnSnS4 (CZTS), having the Earth-abundant and environment-benign constituents, and stable structure, is regarded as a promising thin-film photovoltaic material. However, the current power conversion efficiency (PCE) of CZTS solar cells is far below the commercialization-viable level. One of the main issues restricting the efficiency is the severe Shockley-Read-Hall (SRH) recombination at the highly defective CdS-buffer/CZTS-absorber heterointerface and within the CZTS absorber layer, giving rise to a large open circuit voltage (Voc) deficit. This thesis aims to mitigate the SRH recombination within the CdS/CZTS heterojunction by novel post-deposition treatment technologies to facilitate the passivation of the local defects. Firstly, the ultrathin intermediate stannic oxide (SnO2) layer was introduced at the CZTS/CdS heterointerface via a solution method. The employment of this layer enabled the effective passivation of the heterointerface, resulting in higher Voc, fill factor (FF) and thus PCE. Secondly, we applied our in-house developed moisture-assisted post-deposition annealing (MAPDA) treatment to modify the heterojunction by manipulating the trace element distributions. This technology enabled Na and K depletion in the CZTS film, which, in turn, facilitated the spontaneous Cd diffusion during the chemical bath deposition process for CdS buffer layer, driving a significant improvement in device performance. The heterojunction modification is attributed to the remarkable mitigation of local deep-level defect and the creation of the preferrable shallow acceptor copper vacancies. Peak efficiency at 9.40 % was obtained using the combined MAPDA and heterojunction air annealing (HJA) treatments, which further optimized the elemental distributions within the heterojunction region. Finally, the nanoscale optoelectronic characterization techniques, including Kelvin probe force microscopy (KPFM) and conductive-atomic force microscopy (C-AFM) were applied to investigate the impact of excess Na and K at the CZTS surface. The significant enhancement of quasi-Fermi level splitting and effective alleviation of SRH recombination through the combined MAPDA and HJA treatments were also revealed by surface photovoltage (SPV) analysis through KPFM. These technologies with first-hand novelty explore new defect passivation routes in kesterite solar cells, which can also be widely applied to other thin-film solar cells.

Perovskite solar cells have been recognised as one of the most promising photovoltaic (PV) technologies due to their high efficiency potential for low-cost manufacturing and flexible and semi-transparent PV applications. This thesis focuses on three aspects of perovskite material engineering to improve the power conversion efficiency (PCE) and the stability of the perovskite solar cells; i) morphology control, ii) surface/bulk passivation and iii) compositional engineering. The first part of the thesis focuses on perovskite crystallisation engineering where acetic acid (Ac) is introduced as an additive in chlorobenzene antisolvent to control the perovskite film crystallisation. It was found that Ac not only reduced the perovskite film roughness and improved the film conductivity but also reduced PbI2 residual. Further studies showed that part of the Ac stays on the perovskite surface and passivates the film due to the interaction of the carboxyl group and undercoordinated Pb ions. An efficiency of over 22% on a 0.159 cm2 device was achieved, while the efficiency of the reference devices was less than 20%. The second part of this thesis employs three different homologous bromides FABr, MABr and PEABr, to passivate perovskite films. Results show that not only MABr and FABr react with PbI2 on the perovskite surface, but they also penetrate the perovskite film, increasing the bandgap and suppressing non-radiative recombination. The highest open-circuit voltage (Voc) achieved was over 1.24 V with an efficiency of 22.1% from the device with a bandgap of 1.63 eV while the reference device without any passivation achieved a Voc of only 1.19 V. For the third part of this thesis, perovskite bandgap engineering was conducted based on the triple-cation mixed-halide perovskite. By tuning the bromide ion fraction in the perovskite film to a lower percentage and combining the homologous bromide passivation technique, a very high efficiency of 23.7% was achieved for the device with a bandgap of 1.56 eV, which is higher than the ~22% regularly achieved for perovskite cells with a bandgap around 1.60 eV. Furthermore, the combined technique was applied to larger area (1 cm2) perovskite solar cells and the champion cell achieved an efficiency of 21.8%.

Rigorous research on perovskite solar cell for more than ten years all over the world has led the rapid development of perovskite technology giving us device efficiency of 25.2%, surpassing the performance of traditional thin-film solar technologies on small scale devices (2). Perovskite material has excellent electronic and optical properties making it greatly suitable for PV application. The device fabrication process being simple with low cost and solution processability has led the researchers to visualize high efficiency scalable devices. Due to solution process fabrication method the defects in the materials causes the devices perform less than anticipated. These defects at the absorber material can come through as grain boundaries, surface defects, interfacial mismatch thus reducing photocurrent, open circuit voltage, fill factor and device stability. However, efforts and concentration has been highly directed on diminishing these detrimental defects from the perovskite crystals. Passivation engineering like interface interlayers, 2D perovskites, 2D/3D hybrid perovskites, secondary layers, grain engineering have been reported as a retaliation to minimize defects. Motivated by the outcomes of these works, a simple secondary growth of passivation layer on top of perovskite surface at the hole transport layer interface is studied here in this work showing enhanced device performance resulted from the treatment. Optical, surface morphological and different electronic study on both perovskite films and perovskite devices have been extensively studied and presented in this work to reveal the properties of the grain boundaries in triple cation mixed perovskite materials for simple planar n-i-p devices. With the secondary treatment of post perovskite deposition, the device performance enhanced to 20.6% efficiency from the control device efficiency of 17.4%, short circuit current density increased from 21.71 mA/cm2 to 24.49 mA/cm2, open circuit voltage improved to 1.13V from 1.11V and fill factor rallied to 0.75 from 0.72. Overall, this study provides elaborate in depth understanding on the grain boundaries, their detrimental effects, and successful passivating route to improve device performance.

In this work, various aspects related to polysilicon (poly-Si) passivating contacts for the next generation of industrial silicon solar cells are investigated. As poly-Si passivating contacts are relatively new for the School of Photovoltaic and Renewable Engineering (SPREE), this thesis explores various methods to synthesize the silicon films as well as the interfacial oxide films and the final hydrogenation step. With respect to silicon deposition, both plasma-enhanced chemical vapour deposition (PECVD) and physical vapour deposition (PVD) are investigated. The thickness and surface passivation uniformity of our PECVD reactor is studied and the best sample positions on the sample carrier are identified. For PECVD a-Si films which were thicker than 40 nm, visible blisters are observed after deposition. The formation of blisters is shown to be related to the surface roughness of the substrate and temperature ramp in the post-deposition treatments. More blisters were formed at Si surfaces with a thermally grown SiO2 layer, which is likely related to reduced adhesion of the films. For PVD, significant damage on the oxide passivation layer of poly-Si passivating contact is found caused by the a-Si sputtering process severely affecting the performance of the passivating contacts. The sputter damage could not be healed by a subsequent hydrogenation treatment by atomic layer deposited aluminum oxide. This damage can be avoided by the use of a PECVD deposited a-Si interface capping layer. With respect to the interface oxide formation various methods are investigated. In addition to the standard thermal oxidation and chemical oxides, also plasma oxidation and silicon oxide grown by ALD is investigated. In particular plasma oxidation is appealing as it has the possibility of incorporating the oxidation process into a PECVD or PVD tool so that in-situ tunnel oxide fabrication can be achieved for poly-Si passivating contacts. The iVoc of poly-Si passivating contacts with plasma oxidation as tunnel oxide growth method shows a high dependence on the plasma oxide thickness in this work. For oxides as thin as 0.61 nm, a low contact resistivity of 0.028 Ω · cm2 and a low minority saturation current density of 6.89 fA/cm2 is achieved which is very promising for industrial applications.

As the scaling of nanoelectronic features continues well below the 5 nm node, conventional patterning mechanism like lithography and etching are unable to create aligned features with low margin errors and low line edge roughness. Atomic layer deposition (ALD) and chemical vapor deposition (CVD) help overcome this challenge. However, ALD typically leads to conformal deposition over the entire surface, without any control on the lateral arrangement of the atoms .Self-assembled monolayers (SAMs) can be used to locally deactivate the ALD growth if they can be selectively deposited. A critical issue with SAMS is that they are typically applied by using wet chemistry and are composed of long chain molecules (1-2nm) which require long deposition times to get molecular alignment which is needed for effective passivation.This work reports effective vapor phase passivants for the application of self-assembled monolayers of small molecules to deactivate the selected ALD growth area. The passivants used are smaller than 1nm in length, create a film with RMS roughness of less than 5 Angstrom, and require low processing temperatures. Using one such passivant, [tetramethyldisilazane (TMDS)], up to 12nm of HfOx was deposited on Si without any deposition on passivated SiCOH and SiO2. Deposition of metals like cobalt was also investigated by ALD. Contact angle was used to characterize the hydrophobicity of the surface, the surface topology was examined by atomic force microscopy (AFM) and the metal/metal oxide growth was characterized by x-ray photoelectron spectroscopy (XPS).

As the most commonly used electrode, leadframe, and package material in electronic packaging, copper (Cu) has severe oxidation problem, preventing wetting during soldering processes and prohibiting successful wire bonding. Thus leads and bond pads of leadframes and packages are often coated with a passivation layers such as palladium, silver, or tin, which suppresses the growth of copper oxides. In this research, I investigated the use of silver-indium ((Ag)In) solid solution as a new alternative coating material for passivation. First, a reaction study of bulk (Ag)-9.5In solid solution disks with Sn was conducted to evaluate possible intermetallic compounds in the Ag-In-Sn system. Next, (Ag)-9.5In solid solution layer was coated on copper substrates by E-beam evaporation. The (Ag)9.5In-passivated Cu substrates and bare Cu substrates were aged at 150°C for up to 1000 hours in air. No oxide was detectable on (Ag)9.5In-passsivated Cu substrates while thick Cu2O3 was grown on bare Cu. This result illustrates that (Ag)9.5In solid solution is an excellent passivation coating material. Then, (Ag)9.5In-passivated Cu substrates were electroplated with tin. As a reference, bare Cu substrates were also plated with tin. These two different types of samples were reflowed and aged for different amount of time. No flux was used. The reaction results were examined by scanning electron microscope/energy dispersive X-ray spectroscopy (SEM/EDX) and compared. The experimental results show that tin wetted and reacted well with (Ag)9.5In-passivated Cu substrates. The growth of intermetallic compound (Cu6Sn5, Cu3Sn) on (Ag)9.5In-passivated Cu substrates is also suppressed compared to the growth on bare Cu substrates. The design of (Ag)In solid solution passivation is an invention and its superior performance is a discovery. It provides an economical, environmentally friendly, and valuable passivation alternative on copper electrodes, leadframes, and packages.

Abstract: Severe localized corrosion was found throughout an aeration tank of a modular wastewater treatment plant. Specifically, pitting, crevice corrosion, galvanic corrosion, and intergranular corrosion (IGC) were observed on the 5000 and 6000 series aluminum alloys used in the tank’s construction. One of the suspected environmental causes of corrosion was an aeration tank process additive, namely a coagulant – either aluminum sulphate (Alum) or polyaluminum chloride (PACl; Isopac). Laboratory scale corrosion cells were designed to isolate, simulate, and evaluate the corrosivity of the coagulants (in relevant quantities) on the metallic alloys of construction. Additionally, due to presence of deposit found throughout the tank (resulting in under-deposit corrosion), fabricated crevices were installed on the tested alloys to help simulate crevice corrosion environments. The solution conditions in the cells were monitored throughout the duration of the immersion testing in order to assess whether the cells were within normal operating conditions of the corroded aeration tank. A series of material characterizations and mass change measurements were performed on the corrosion coupons post-testing to evaluate and rank the corrosivity of the coagulants. Results from the laboratory scale testing revealed that both coagulants led to localized corrosion of the 5000 and 6000 series aluminum alloys – in the form of pitting, crevice corrosion, and IGC. Therefore, it can be concluded that neither Isopac nor Alum should be used in aeration tanks constructed of the aluminum alloys tested in this study. Whilst the immersion testing did not simulate all conditions observed and measured in the corroded tank, the testing did allow for the study of the isolated, corrosive effects of either Isopac or Alum on the tested alloys. It can be reasoned that even though the coagulants led to similar corrosion observed in the tank, the coagulants themselves are not the sole cause of the aforementioned corrosion; ultimately, additional factors and conditions in the aeration tank could be deemed more corrosive than merely the coagulants.

The growth and characterization of nanowires (NW) has inspired tremendous research efforts in a broad range of disciplines. The development of self-assembled NW growth enables the fabrication of defect-free single crystal nanowires. High-mobility III-V semiconductor nanowires have shown particular promise as channels in high-performance field effect transistors (FET). This work helps to advance the development and understanding of NWFET devices by introducing a new gate insulator, new device fabrication methods, and by studying quantum transport effects in new nanostructures: (i) Gate oxides cause a high density of charge traps at the nanowire surface. This reduces transistor performance in NWFETs. In this work, we introduce the organic polymer parylene as a new gate insulator. Using parylene will enable the incorporation of semiconductor surface passivation layers that have been shown to reduce charge trapping and enhance transistor performance. We built a custom chemical-vapor deposition system specifically designed to grow ultra-thin parylene films that can conformally coat single nanowires. Oxygen plasma etching in combination with lithographic methods were used to pattern the parylene thin films and create fully functional wrap-gated NWFETs that exhibit performance on par with oxide-insulated devices. (ii) The gate needs to fully wrap around the nanowire to maximize NWFET performance. In this work, we introduce a fabrication method that allows for the fabrication of multiple independent wrap gates on the same nanowire without additional processing steps. This method was used to create a logic gate and a common-source transistor pair, each on a single nanowire. We introduce a second fabrication method that significantly enhances the control over the gate-segment length, which allows for the creation of shorter wrap gates. (iii) Low-temperature measurements were conducted to advance the understanding of quantum transport phenomena in nanoscale III-V FETs. We report transport features in a GaAs nanowire that we attribute to one-dimensional conductance quantization. We also present the measurements on InAs nano membranes, a novel nanostructure grown by selective area epitaxy. These exhibit signatures attributed to magnetoconductance fluctuations, weak localization/antilocalization and Shubnikov-de Haas oscillations.

Water is one of the most capable and widely used working fluids in heat pipes and thermosiphons. Aluminum is a high conductivity, lightweight, and high strength choice for heat pipe casing material. However, aluminum and water are a heat pipe combination that is not considered viable due to the rapid generation of hydrogen, a noncondensable gas (NCG), resulting from chemical reactions between water and aluminum. A great deal of past research has been done evaluating the compatibility of many pure fluids with different metal heat pipe casings, yielding a large volume of lifetime tests demonstrating success or failure of various combinations. On the other hand, very little research was found analyzing what, if any, progress can be made to take low compatibility combinations and make them more compatible through the use of chemical inhibitors. This is particularly true for the combination of water and aluminum. Recently, inorganic chemicals in an aqueous solution, with the proper concentrations and pH range, have been shown to suppress the unwanted reactions and subsequent hydrogen formation rates in a manner that could prove useful enough to allow aluminum heat pipe casings to operate with aqueous based solutions as a working fluid. The goal of this thesis is to produce, understand, and experimentally show the robustness of using inorganic inhibitors in aqueous heat pipe fluids for aluminum devices. Thermodynamic predictions were made to estimate conditions in which a stable oxide layer of the base metal, as well as the oxidizing inhibitors, will form within the pipe, thus giving a better chance at preventing NCG generation. E-pH, or Pourbaix, diagrams will be generated as a function of temperature to allow for prediction at any given heat pipe operating temperature range. Chemical reactions and processes responsible for NCG suppression will be explained. Inhibitor species’ concentrations will be investigated as an independent variable to determine recommended inhibitor amounts. Aluminum thermosiphons and heat pipes will be experimentally tested to verify efficacy of various inhibitor solutions in different thermosiphon geometries tested under different test conditions. Simple corrosion tests will also be carried out within a vacuumed chamber to determine relative amounts of hydrogen generated when an aluminum sample is in direct, static contact with a test fluid. The results of this thesis will show that while there is good evidence that inorganic inhibitors can significantly reduce the production of NCG in an aluminum-water heat pipe, the complex nature of two phase fluid circulation in such devices still presents serious challenges to the reliability and adoption of inhibitor fluids for critical thermal management needs. Reasons behind this phenomenon will be presented along with pathways for future research. These future work paths are believed to provide great opportunities for overcoming the challenges found in this work as well as potentially expanding the application of inhibiting heat pipe fluids to other active metals such as steel devices. The creation of a dilute aqueous solution which maintains the high latent heat of water but is also compatible with aluminum heat pipes would allow for significantly higher heat transport per device unit mass than currently used aluminum and ammonia heat pipes and provide another option for intermediate temperature heat pipes at low cost.

SiGe is a promising material for channel or contact applications because of its high hole and electron mobility and capacity for both compressive and tensile strain by integration with Ge-rich and Si-rich layers. The high hole mobility of SiGe can be used for p-channel FET as an alternative to Si. The larger lattice constant of SiGe compared to Si can provide tensile or compressive stress into the Si channel thereby enhancing the electron or hole mobility, respectively. Moreover, the multi-gate structure, which utilizes multiple crystalline planes such as (001) and (110) can be employed to overcome the challenges such as SCEs resulting from single-gate MOS devices. In this work, cleaning, passivation, and functionalization of SiGe(001) and (110) surfaces were developed and studied using in-situ XPS, STM, and STS. XPS was utilized to understand the chemical compositions, oxidation states, and thickness of thin films on SiGe(001) and (110) surfaces. STM was used to study the topological structures and bonding configurations on the surfaces. STS was performed to probe the electronic structures such as pinning or unpinning effects by characterizing the density of states. In order to avoid the oxygen and carbon contaminations after ex-situ native oxide removal, a combined wet and dry cleaning was performed. Wet in-situ HF clean method was successful to remove native oxides on SiGe(001) and contained no oxygen. Dry clean of atomic H via a thermal gas cracker method was found to be effective to remove the residual carbon contaminations. Sputter-cleaned SiGe(001) surface was terminated with only Ge dimers while SiGe (110) surface was terminated with both Si and Ge adtoms verified by STM. H2O2(g) was employed to passivate the SiGe surfaces and provide a high nucleation density for the metal ALD process confirmed by XPS. TMA was dosed onto the hydroxyl terminated SiGe surfaces to form a monolayer of Al2O3. TDMAT or TiCl4 was also exposed onto the hydroxyl terminated SiGe surfaces to form a monolayer of TiOx. Al atoms were bonded to one oxygen atom while Ti atoms were bonded to two oxygen atoms on the surfaces studied by STM and XPS. Furthermore, PDA resulted in the formation of selective Si-O-Al or Si-O-Ti on SiGe(001) and (110) surfaces.

In this study, HfO2 [hafnium oxide] thin films are investigated extensively as part of indium gallium zinc oxide (IGZO) thin film transistor (TFT) devices. They are incorporated into the TFTs, both as a gate insulator and a passivation layer. First, the HfO2 [hafnium oxide] films themselves are investigated through an annealing study and through I-V and C-V measurements. Then, HfO2 [hafnium oxide] is suggested as a replacement for commonly used SiO2 [silicon dioxide] gate insulator, as it has a dielectric constant that is 4 – 6 times higher. This higher dielectric constant allows for comparable TFT performance at a lower operation voltage (5 V vs. 20 V). Finally, HfO2 [hafnium oxide] is applied as a passivation layer in IGZO TFTs, and an annealing study is conducted to determine which processing steps will allow for optimal TFT performance. The HfO2 [hafnium oxide] passivation layer proves to show a good level of uniformity. Therefore, taking all results into consideration, both HfO2 [hafnium oxide] gate insulators and passivation layers can be used in conjunction with IGZO TFTs to produce a full electrowetting array, which should prove to be useful in “lab on a chip” studies.

Rear-passivated solar cells demonstrate higher efficiency due to reduced rear surface recombination; however surface passivation and patterning methods used need to be robust, capable of high throughput and readily incorporated in production lines. This thesis examined possible cost-effective improvements in rear local contact formation for silicon solar cells that could be used in manufacturing, with a special focus on the formation of anodic oxide dielectric layers and patterning of these layers. An initial study explored the effects of patterning by laser and chemical etching on the properties of local back surface fields (LBSF) regions formed by aluminium alloying. Chemical etching resulted in 2 - 3 µm thicker LBSF regions and reduced Kirkendall void formation. This understanding inspired the development of a new inkjet etching patterning technique for patterning anodic aluminium oxide (AAO), a proposed new low cost dielectric for silicon solar cells. Point opening diameters of 20 – 30 µm and LBSF regions that were up to 7 µm deep were realised in 600 nm AAO layer. However, properties associated with the clip anodisation process used to form the AAO layer limited the reliability of this patterning process. A novel light-induced anodisation (LIA) technique was developed to address the limitations of clip anodisation. Anodic silicon dioxide (SiO2) layers formed using LIA achieved comparable effective minority carrier lifetimes to thermal SiO2 with a low density of interface states (Dit) of 6 × 10^11 eV-1 cm-2 and leakage currents that were six orders of magnitude lower than thermal SiO2. The Dit of AAO layers formed by LIA of aluminium were measured to be as low as 1 × 10^10 cm-2 eV-1, a value which is lower than any reported aluminium oxide layer deposited by other deposition techniques. Cells fabricated with SiO2/AAO rear passivation layers achieved open circuit voltages of 660 mV. This value is comparable to that achieved by cells passivated with the industrial standard of aluminium oxide/silicon nitride, therefore highlighting the potential of this new low-cost AAO passivation layer and the novel LIA process to reduce the manufacturing cost of rear-passivated silicon solar cells.

Thin-film transistors (TFTs) with channel materials made out of hydrogenated amorphous silicon (a-Si:H) and polycrystalline silicon (poly-Si) have been extensively investigated. Amorphous silicon continues to dominate the large-format display technology; however newer technologies demand a higher performance TFT which a-Si:H cannot deliver due to its low electron mobility, µn ~ 1 cm2/V*s. Metal-oxide materials such as Indium-Gallium-Zinc Oxide (IGZO) have demonstrated semiconductor properties, and are candidates to replace a Si:H for TFT backplane technologies. This work involves the fabrication and characterization of TFTs utilizing a-IGZO deposited by RF sputtering. An overview of the process details and results from recently fabricated IGZO TFTs following designed experiments are presented, followed by analysis of electrical results. The investigated process variables were the thickness of the IGZO channel material, passivation layer material, and annealing conditions. The use of electron-beam deposited Aluminum oxide (alumina or Al2O3) as back-channel passivation material resulted in improved device stability; however ID VG transfer characteristics revealed the influence of back-channel interface traps. Results indicate that an interaction effect between the annealing condition (time/temperature) and the IGZO thickness on the electrical behavior of alumina-passivated devices may be significant. A device model implementing fixed charge and donor-like interface traps that are consistent with oxygen vacancies (OV) resulted in a reasonable match to measured characteristics. Modified annealing conditions have resulted in a reduction of back-channel interface traps, with levels comparable to devices fabricated without the addition of passivation material.

In the crystalline Si solar cell industry, there is a push to reduce module cost through a combination of thinner substrates and increased cell efficiency. Achieving solar cells with sub-100 µm substrates cost-effectively is a formidable task because such thin substrates impose stringent handling requirements and thermal budget due to their flexibility, ease of breakage, and low yield. Moreover, as the substrate thickness decreases the surface passivation quality dictates the performance of the cells. Crystalline Si heterojunction (HJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) have attracted significant interest in recent years due to their excellent surface passivation properties, potential for high efficiency, low thermal budget and low cost. HJ cells with ultra-passivated surfaces showing > 700 mV open-circuit voltages (Voc) and > 20% conversion efficiency have been demonstrated. In these cells, it has been identified that high-quality a-Si:H films deposited by a low-damage plasma process is key to achieving such high cell performance. However, the options for low-damage plasma deposition process are limited. The main objectives of this work are to develop a low-plasma damage a-Si:H thin film deposition process based on remote plasma chemical vapor deposition (RPCVD) and to demonstrate high efficiency HJ solar cells on bulk substrates as well as on ultra-thin silicon and germanium substrates obtained by a novel, low-cost semiconductor-on-metal (SOM) technology. This manuscript presents a detailed description of the RPCVD system and the process leading to the realization of high quality a-Si:H thin films and high efficiency HJ solar cells. First, p-type a-Si:H thin films are developed and optimized, then HJ solar cells are subsequently fabricated on bulk and ultra-thin Si and Ge SOM substrates without intrinsic a-Si:H passivation. Single HJ cells on ~ 500 µm bulk Si and ~25 µm ultra-thin substrates exhibited conversion efficiencies of η = 16% (Voc = 615 mV, Jsc = 34 mA/cm2, and FF = 77%) and η = 11.2% (Voc = 605 mV, Jsc = 29.6 mA/cm2, and FF = 62.8%), respectively. The performance of the ~25 µm cell was further improved to η = 13.4% (Voc = 645 mV, Jsc = 31.4 mA/cm2, and FF = 66.2%) by implementing the dual HJ architecture without front side i-layer passivation. For single HJ cells based on Ge substrates, the results were η = 1.78 % (Voc = 148 mV, Jsc = 35.1 mA/cm2, and FF = 1.78%) on ~500 µm bulk Ge, compared to η =5.3% (Voc = 203 mV, Jsc = 44.7 mA/cm2, and FF = 5.28%) on ~ 50 µm Ge SOM substrates. Respectively, the results obtained on ultra-thin SOM substrates are among the highest reported in literature for based on comparable architecture and substrate thickness. In order to achieve improved cell performance, dual HJ cells with i-layer passivation of both surfaces were fabricated. First, optimized RPCVD-based i-layer films were developed by varying the deposition temperature and H2 dilution ratio (R). It was found that excellent surface passivation on planar substrates with as-deposited minority carrier lifetimes > 1 ms is achievable by using deposition temperature of 200 ºC and moderate dilution ratio 0.5 ≤ R ≤ 1, even without the more rigorous RCA pre-cleaning process typically used in literature for achieving comparable results. Subsequently, dual HJ solar cells with i-layer films were demonstrated on planar and textured bulk Si substrates showing improved conversion efficiencies of η = 17.3% (Voc = 664 mV, Jsc = 34.34 mA/cm2 and FF = 76%) and η = 19.4% (Voc = 643 mV, Jsc = 38.99 mA/cm2, and FF = 77.5%), respectively.

Electrolessly deposited CoWP capping layers have been demonstrated to effectively reduce electromigration of Cu at the interconnect/dielectric-barrier cap interface while reducing resistivity relative to SiCN. However, as device dimensions scale, the need for alternative methods for the selective deposition of sub-5 nm, ultrathin, conformal Co capping layers is apparent. To develop methods for area-selective atomic layer deposition (AS-ALD) of Co caps for next-generation Cu interconnects, the ALD behavior of bis(N-tert-butyl-N’-ethylpropionamidinato) cobalt(II) (CoAMD) is evaluated on Cu, SiO₂, and a porous low-k ( ~2.6) dielectric, CDO. The first and second ALD half reactions of CoAMD on the respective substrates is evaluated with H₂ coreactant by adsorbing the precursor on the substrates under ALD cycling conditions at 265 °C with and without coreactant exposure. The adsorption studies indicate that CoAMD preferentially deposits most on Cu and least on CDO. Further, CoAMD, like other amidinate precursors, readily dissociates on the Cu transition metal surface but the ultimate per-cycle coverage is self-limited by the slow desorption of amidinate ligands and fragments from the Cu surface. Co films deposited by ALD from CoAMD on Cu at 265 °C indicate that Co burrows into the lower energy Cu surface as the film grows in order to reduce the free surface energy. The Cu remains as a surfactant-like layer on the topmost Co surface up to film thicknesses of at least 16 nm. Moreover, considerable intermixing at the Co/Cu interface and Cu concentration several nm into the Co films are observed indicating high surface mobility of the two materials and Cu diffusion at polycrystalline Co grain boundaries. Finally, employing low-tempurature ALD and selectively passivating the dielectric surfaces with OH targeting passivants leads to enhanced selectivity of CoAMD for deposition on Cu versus SiO₂ and CDO. Depositing Co from CoAMD on Cu and CDO at 165 °C after 500 kTorr-s exposure to trimethylchlorosilane at 50 °C leads to a 30:1 preference for Co accumulation on Cu, a twelve times improvement compared to deposition on cleaned Cu and CDO at 265 °C.

The purpose of this thesis is to improve electronic quality of polycrystalline silicon thin film solar cells by plasma hydrogen passivation. This hydrogen passivation study is conducted on two different polycrystalline silicon solar cells, which are SPC (Solid Phase Crystalline) and LPC (Liquid Phase Crystalline) silicon solar cells. Firstly, hydrogen passivation on SPC silicon solar cells is focused on finding impact of process parameters on thicker cells (3, 4, and 5 um) than baseline 2 um cell. It is discovered that higher process temperature improves open-circuit voltage most and the impact increases as cell is getting thicker. Process time and plasma power does not have significant effect on improving open-circuit voltage. These findings can be applied to optimise thicker SPC silicon solar cells which promises higher short-circuit current. Secondly, hydrogen passivation on LPC silicon solar cells is studied. LPC solar cells are thicker (10 um) than SPC cells and have less crystallographic defects. These differences require development of different hydrogen passivation. In LPC cells, both process temperature and process time have substantial impacts on increasing open-circuit voltage but plasma power does not have impacts on it. Additionally, improvements which are associated to n=1 and n=2 are also investigated through Suns-Voc data. Not only n=1 related defects but also n=2 related defects are well-passivated by higher process temperature and longer process time.

Implementation of silicon nitride into the fabrication process of commercial silicon solar cells was a significant development leading to a large performance improvement. A key reason for the commercialization of silicon nitride is its multifunction applications, since it provides both surface passivation and anti-reflection coating. Anodic aluminium oxide (AAO) has several properties which potentially provide multifunctional applications, especially for the solar cell rear design. These properties can be easily adjusted by varying the type and concentration of the electrolyte, and the anodisation voltage. Implementation of AAO layers in the rear surface design improves both the surface passivation and the formation of rear contacts. It is shown that when AAO layers are formed by anodising aluminium over an intervening dielectric layer of either silicon dioxide or silicon nitride on n-type silicon surfaces, the implied open circuit voltage (Voc) of wafers can be increased by up to 40 mV. This thesis also explored several methods for enabling cell metallisation through AAO layers. The first method utilises the pore structure of AAO film, the second facilitates metal contacting by laser doping through high concentrations of aluminium, while the third selectively anodises an aluminium layer by pre patterning the aluminium prior the anodisation. AAO can be simply adopted into different cell structures such as PERL cell, bifacial solar cell and back contact solar cell while providing improved performance and reduced fabrication cost.

Technologies that could further improve commercial silicon solar cell cost effectiveness are explored in this thesis. The screen printed p-type silicon solar cell efficiency is limited by two factors: Al screened rear and defects within p-type silicon bulk. To solve this problem and further improve solar cell performance, AlOx is used as a rear side surface passivation dielectric and hydrogenation processes are tuned to effectively reduce defects within bulk silicon. The first part of the thesis aims to improve the effective lifetime of the silicon wafer by optimising AlOx surface passivation and silicon bulk hydrogenation processes. Laser doping through AlOx/SiNx and metallization to form local contacts are studied. Solar cells are fabricated in the second part of the thesis. AlOx has shown excellent passivation of p-type silicon and suitability for laser doping technology. A high fixed charge density of over 1012 cm-2 and low interface defect density of below 1011 eV-1cm-2 are achieved with ALD AlOx deposited on -OH terminated p-type silicon surfaces. As a result, the surface recombination velocity is controlled at below 8 cm/s. SiNx, as hydrogen source, is added onto AlOx for rear side surface passivation. After hydrogenation under controlled illumination, a significant improvement in silicon bulk lifetime by 500 μs is demonstrated. High iVoc values of 726 mV and 710 mV have been achieved with ALD and PECVD AlOx/ SiNx rear passivated pseudo cell structures. The AlOx passivation dielectric is shown to be an effective dopant source. A peak doping of over 1020 cm-3 is detected by ECV measurement in P+ regions formed by laser doping AlOx. Laser conditions are optimised to reduce damage along the lines caused by Gaussian laser energy distribution across the laser beam. Double side laser doped solar cells with rear AlOx/SiNx passivation are fabricated. The rear reflector is improved, leading to the current density being increased to over 40 mA/cm2 and a solar cell efficiency of above 19% is achieved with the AlOx/SiNx rear passivated solar cell. Further process optimisation by reducing the edge isolation and laser doping damage shows the potential of increasing cell efficiency to over 21%.

Infrared (IR) detectors today are being utilized for a variety of imaging applications such as medical diagnostics, navigation instruments of automobiles and aircrafts, meteorological imaging, night-vision and fog/smoke imaging, surveillance, target acquisition and astronomical/space imaging. These wide ranging implementations have given rise to varied design requirements for these devices such as the required sensitivity, operating temperature, spectral sensitivity, peak wavelength and cost. Type-II band aligned InAs/Ga(In)Sb strained layer superlattice (SLS) material system proposed for the IR detection in the 1970s has been considered in recent years as an interesting alternative to the present day IR detection technologies. Type-II SLS technology possesses mature growth technologies able to achieve uniformity over large areas along with band-gap tunability which results in IR detectors in mid wave (MW), long-wave (LW) and very long-wave infrared (VLWIR) ranges. The large electron effective mass in SLS help in reducing inter-band tunneling and hence allow for longer-wavelength operation in these IR detectors. Large splitting between heavy-hole and light-hole valence subbands due to strain in the SLSs contributes to the suppression of Auger recombination and this enables significantly higher operating temperatures. The commercialization of type-II SLS technology has been hindered by material defects that promote excess dark currents in the bulk and on detector surfaces. During SLS device fabrication the mesa delineation leads to discontinuity of the periodic crystal structure which results in formation of unsatisfied chemical bonds on the etched surfaces and hence enhancing the surface leakage currents. More-over, with the scaling of single pixel dimensions, the performance of focal plane arrays is strongly dependent on surface effects due to the large pixels surface to volume ratio. Hence the reduction of surface leakage currents in LWIR detectors has become a technological necessity and is the objective of this dissertation. This work focuses on performance improvement of InAs/GaSb SLS LWIR detectors by optimization of mesa delineation and surface passivation techniques. The first part of the dissertation works on the development of optimized etching scheme for the 400μm x 400μm area single-pixel SLS detectors and for 25μm x 25μm area focal plane arrays (FPAs). Firstly, optimization of wet chemical etches were carried out on GaSb substrates and reported here is that HCl: H2O2:H2O (1:1:4) solution rendered the smoothest etched surface with a root mean square (RMS) roughness of 1.59nm. This work also reports the results of comparison of mesa sidewall profiles of InAs/GaSb SLS single-pixel and FPA detectors obtained after (a) a conventional BCl3 gas based inductively coupled plasma dry etch, (b) a HCl: H2O2:H2O (1:1:4) solution wet chemical etch and (c) combination of both. It was found that HCl: H2O2:H2O (1:1:4) solution based wet etch was ideal for single pixel mesa delineation, but not on FPA device proportions. In the etch study experiments, the InAs/GaSb SLS structure was looked at as one single entity and as an amalgamation of its constituents InAs and GaSb materials. The second part of this research deals with the development of efficient surface passivation methods for the long wave infrared (LWIR) SLS detectors with pin, pBiBn, and graded bandgap designs. A comparative study of dielectric passivants (Silicon di-oxide and Silicon nitride) versus organic passivant of SU-8 versus chalcogenide passivants (Zinc Sulfide, Ammonium Sulfide and Electro-chemical passivation i.e. deposition of pure sulphur) were carried out on InAs/GaSb SLS LWIR single-pixel detectors. Ammonium sulfide [(NH4)2S] treatment and electrochemical sulfur deposition (ECP) reduced dark current density (Jd) at 77K and applied bias= - 0.1V by (a) by factor of 25 and 200 in InAs/GaSb SLS LWIR detector with p-i-n design and (b) by factor of 3 and 54 in InAs/GaSb SLS LWIR with pBiBn design. In the comparative study of all the sulphur-based passivants of thioacetamide (acid-based and base-based), (NH4)2S and ECP on graded-bandgap design based single-pixel LWIR SLS detectors, ECP showed superior performance of all with the highest surface resistivity (rsurface) of 1.4x105 \u2126-cm at 77K. In the long term stability study of ECP, it showed degradation over time with Jd=0.09A/cm2 right after passivation and Jd =2.18A/cm2 after 4 weeks of passivation. Hence as an alternative to ECP (i.e. sulphur passivation), we propose the utilization of novel chlorine-doped Zinc Telluride (ZnTe:Cl) as a passivation technique. It is for the first time that ZnTe:Cl has been used as a passivation technique on any IR device. This novel ZnTe:Cl passivation technique is implemented on InAs/GaSb SLS LWIR single-pixel detectors with pin and graded-bandgap designs. Though ECP showed a superior rsurface=105\u2126-cm over ZnTe:Cl passivation with rsurface =6700\u2126-cm, ZnTe:Cl passivation had almost no degradation over time. Ergo, an improved ZnTe:Cl with its robust, uniform passivation layer has the potential to replace ECP in FPA fabrication on InAs/GaSb SLS LWIR detectors and there-of take these high-performing SLS-based IR detectors to commercialization.

Initially, advances in the high frequency markets were begun by work in Gallium Arsenide systems. In recent years, however, the focus has shifted to the promise of ever higher power at ever higher frequency with the emergence of wide bandgap group III-V semiconductors, including Gallium Nitride. One area receiving attention is that of novel passivation materials for the active areas of AlGaN/GaN devices. Passivation is a critical issue because surface trapping effects are essentially unavoidable, even with the highest queality epitaxial layers, due to the polarized nature of the material. The question then becomes, which passivation materials offer the best mitigation of surface trapping effects with the least impact on parasitic elements detrimental to device performance. In this work, AlGaN/GaN devices passivated with AlSiN for both high frequency and high power operation are studied. The high frequency devices were fabricated alongside devices passivated with SiN, a standard passivation material, and characterized for both small signal and large signal performance. The AlSiN passivation was found to enhance both small and large signal performance, and so another set of devices was fabricated with high voltage, high power switching as the intended application. These devices were characterized for off-state breakdown, which was more than 4 times that of typical SiN-passivated devices, and time-domain and loadline measurements were performed.

Global photovoltaic (PV) market harvested rapid growth annually during the past two decades. As the primary part of this rapid growth, most of Silicon (Si) wafer-based solar cells employ screen printing(SP) technology for metallization on both polarities of P-type CZ Si wafers and the solar cell efficiency is around 18%. This efficiency is significantly lower than that of high efficiency solar cells developed in research laboratory. The fabrication of high efficiency solar cells in laboratory usually involves multiple expensive processes and high quality silicon material which are not suitable for production. Compared to the complicated and expensive laboratory process for high efficiency solar cell, SP technology is a simple and robust process. However, such technology also has some disadvantages such as poor blue response, high shading issue, low aspect ratio of metal fingers and insufficient rear surface passivation.Selective emitter (SE) technology is one possible way to further increase solar cell efficiency by harvesting more blue light. SE technology is of high interest to the PV industry in recent past. One of the cost effective method of achieving SE is by using laser doping. The laser doped selective emitter (LDSE) solar cell developed at the University of New South Wales (UNSW) combines laser doping and self-aligned light induced plating (LIP) technologies which make it one of the most feasible solutions for industrial selective emitter solar cell structure. The main advantage of LDSE technology is the simultaneously creation of dielectric layer patterning and localized heavy doping without an extra high temperature process or any other masking processes which are required by other technologies. In this thesis, LDSE technology was employed to improve the conversion efficiency of solar cells using a commercial available continuous wave (CW) green laser. The basic laser theory was reviewed and laser induced defects by CW laser were studied. The impact of different laser parameters and dielectric layer combinations on the morphology of laser scanned region was investigated. The process optimization of standard LDSE solar cells was presented. The focus of optimization was given to two key processes: laser doping and light induce plating in the LDSE solar cell process. Wide ranges of parameters were investigated in detail for each process, such that systematic improvements were demonstrated. The influences of different parameters on final solar cell devices were studied through the use of techniques such as light I-V curve, spectral response, scanning electron microscope, focus ion beam etc. As a result of optimizations, a final solar cell device with efficiency >19% was achieved on p-type 1 Ω cm Czochralski (CZ) silicon wafer.The other very important aspect of improving solar cell efficiency is the rear surface design. The aluminum (Al) back surface field (BSF) used in SP solar cells only has moderate passivation effect. Such Al BSF was identified as a limiting factor for standard LDSE solar cells. To overcome this problem, a stack dielectric layer of silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) was developed. Such stack layer demonstrated excellent surface and bulk passivation ability on commercial grade p-type 1 Ω cm CZ Si wafer with carrier lifetime of 670µs and implied open circuit voltage (iVoc) of 735 mV. The significance of rear surface passivation has been realized by researchers worldwide as a key approach for high efficiency solar cell designs and several dielectric layers were successfully developed to passivate Si surface and get high carrier lifetime. However, challenge lies in making local contact opening on the dielectric layers or forming local BSF (LBSF) without massive jeopardize passivation ability using industry suitable approach rather than expensive photolithograph which is normally employed in laboratory. As a local heating process, laser doping could achieve dopants diffusion and dielectric layer patterning in step without causing massive degradation in the passivation ability of dielectric layer. In this thesis, CW laser system was used to create double sided laser doped (DSLD) solar cell by performing phosphorous and boron laser doping to front and rear surface of industry p-type CZ Si wafer passivated by SiNx/SiO2 stack layers on the front surface and SiO2/SiOxNy stack layers on the rear surface. By optimizing the thermal stability of the dielectric layers, emitter sheet resistance and the laser doping parameters, over 700mV iVoc was achieved on CZ Si samples after laser process, prior to metallization. As proof-of-concept, DSLD solar cells were made on CZ samples. Open circuit voltage (Voc) in the range of 660mV was achieved with high short circuit current density. However, at this stage, the solar cells efficiency is limited by low fill factor (FF). The origin of low FF was investigated and possible causes and solutions were discussed. At the end of this thesis, DSLD solar cells with Voc of 680mV was achieved on p-type 1 Ω cm CZ silicon wafers by optimizing rear surface metallization process.

Chromium conversion coatings (CCC) based on Cr(VI) are widely used enhance corrosion resistance of aluminum alloys. However, the conventional CCC system consists of various Cr(VI) compounds, which are toxic and carcinogenic, leading to increasingly restricted usage. The Trivalent Chromium Process (TCP), for example, has proven to be a promising alternative to CCC. Although significant research on the application and performance of TCP has been done, questions such as the film microstructure, the nature of the TCP/substrate interface and the evolution of the TCP film in response to different environments have not been answered.Neutron reflectivity (NR) and x-ray reflectivity (XRR) are used to determine the structure and composition of TCP films on aluminum alloy 2024-T3. A new electro-assisted deposition method was designed. Quantitative NR and XRR analysis confirmed linear film growth. The film composition was determined to be Cr2O3¿¿¿¿iH2O¿¿¿¿x(ZrO2¿¿¿¿jH2O) (i =2.10 ¿¿¿¿ 0.55, j = 1.60 ¿¿¿¿ 0.45 and x = 0.85 ¿¿¿¿ 0.14).In-situ neutron reflectivity was used to observe the structure and evolution of a TCP passive film with a simplified formulation on Al in a NaCl-D2O solution. We observed the evolution of the TCP film on the Al anode and compared the degradation of the Al with and without TCP protection. A dramatic improvement in corrosion resistance of AA2024-T3 is achieved by anodic hardening TCP passive films with Ce(III). The anodic current density is suppressed by the factor of 500 after exposure in the presence of Ce(III) at potentials in metastable pitting region (-580 mV vs. SCE for 4 < pH < 8). Dynamic polarization analysis after step-by-step polarization reveals that Ce(III) increases both the open circuit potential and the pitting potential.In-situ NR is applied to investigate the evolution of structure and composition of a TCP passive film on pure Al under Ce(III) anodic hardening. We observed the formation of a Ce-enriched TCP layer at the film/solution interface during anodic polarization, which extends the pitting potential range and reduces the baseline corrosion current.Similar to the in-situ Ce(III) anodic hardening experiment, in-situ NR is used to investigate the evolution of structure and composition of a TCP passive film under cathodic polarization in the presence of Ce(III). We observed a film evolution at Al/TCP interface; while the in-situ current between the anode and the cathode process shows significant attenuation over 24 hours. Cathodic inhibition was verified by a separate potentiostatic electrochemical experiment. An evolution model based on the in-situ NR data is proposed to explain the cathodic inhibition process.These experiments are the first to measure the response of a passive film on Al in an active corrosion environment. The results dramatically alter the conventional wisdom regarding passive films. TCP film is not a water barrier. In fact the films rapidly imbibe water. Nevertheless the metal remains passive. Protection is due to a very thin barrier layer right at the metal/film interface. The presence of the water in the bulk film actually facilitates Ce(III) penetration, which leads to further improvement in corrosion protection.

III-V compound semiconductors possess advantageous materials properties, such as direct bandgap and high carrier mobility, which make them attractive in optoelectronic, fiber optical communications and high-speed digital circuit applications. Integration of III-V heterostructures using direct wafer bonding has the added benefit of avoiding constraints in lattice parameter mismatch compared to epitaxially grown devices. As a result, wafer bonding is significant in its ability to integrate mismatched materials and circumvent the issues of inferior device performance due to a high density of threading dislocations.In the solar industry, off-axis substrates are commonly used in the growth of III-V epitaxial layers to avoid the formation of antiphase boundaries, which act as deep level non-radiative recombination centers. Previously published studies from our research group showed that an additional sulfur-based passivation technique can reduce the density of surface charge states and improve the conductivity across the interface. However, a research topic that has not been investigated is the effect that miscut substrates have on the performance of direct-bonded III-V devices. In this study, the effect of the wafer offcut angle on the electrical conductivity of III-V solar cell devices is investigated using n-GaAs/n-GaAs wafer-bonded structures.GaAs (001) wafers misoriented towards A are chosen and compared to nominal on-axis substrates. Prior to bonding, the surfaces are treated with either an oxide etch or additional 5-min treatment in aqueous (NH4)2S. Off-axis wafers are positioned face-to-face with a non-zero relative surface misorientation between the tilted (001) planes and low force bonding is initiated at room temperature. The highest degree of misorientation in this study is produced using 6° off-axis wafers and can be represented as a grain boundary with a twin defect at the (1 1 13) plane and a tilt angle of 12° about the common [110] direction. The samples are annealed at 400 °C for two hours to strengthen the bond. It is observed that the electrical conductivity improves considerably with a short rapid thermal processing (RTP) at 600 °C for 2 mins. However, the degree of miscut has a detrimental effect, as both the oxide-etched and sulfur-treated samples exhibit increasingly non-ohmic behavior with greater relative misorientations. A theoretical model that describes the electron tunneling across a grain boundary between semiconductor bicrystals is used to represent the bonded interface and estimate the barrier conduction height. Fitting the zero-bias conductance data over a range of temperatures reveals an increased barrier height for greater offcut angles, with 4° and 6° miscut sulfur-passivated wafers producing a 0.4 eV increase. When compared to on-axis structures, the interface resistance at room temperature increases from 0.01 ohm·cm2 to 3.4 ohm·cm2.High resolution transmission electron microscopy (HRTEM) and scanning transmission electron microscopy (STEM) are used to compare the interface morphology across the range of relative misorientations after the 600 °C RTP. The ratio of well-bonded crystalline regions to amorphous oxide inclusions is consistent across all bonded samples, indicating that the degree of misorientation does not affect the level of interface recrystallization. It is also observed that regions adjacent to the interface undergo a process of atomic redistribution and recrystallize into the same lattice arrangement as the bulk semiconductor.The effect of relative surface misorientation on conductivity is further investigated by fabricating zero-degree relative (001) misoriented bonded samples using 6° miscut substrates and subjecting to the same thermal and sulfur passivation treatment. This can be described as a grain boundary at the (1 1 13) plane without any twin defects about the [110] direction. Current-voltage (I-V) characteristics are comparable to nominal on-axis specimen with the interface resistance measured as 0.02 ohm·cm2. It is concluded that the degree of relative misorientation of (001) planes across the bonded interface has a significant impact on the electrical properties, as illustrated by the two orders of magnitude difference in conductance.Non-ohmic behavior has previously been discovered in direct-bonded n-type GaAs/GaAs structures. In order to pinpoint the source of this inferior conductance, several lines are physically cut into the surface with a diamond scribe to electrically isolate a portion of the metal contacts. I-V measurements are taken at various test points to compare the resistance across the different interfaces. The measured resistance across the metal-metal and metal-semiconductor-metal regions is found to be 0.8 ohms. However, the inclusion of the bonded interface results in a significant increase in resistance to 770 ohms at zero-bias conditions. The non-ohmic behavior is confirmed to be solely attributed to the bonded interface.These results demonstrate the potential usefulness of using off-axis substrates in the fabrication of direct wafer-bonded III-V heterostructures. More importantly, the primary effect on conductivity does not originate from the miscut substrates themselves. Instead, the out-of-plane relative misorientation of the tilted (001) planes is the critical parameter that needs to be controlled within a manufacturing environment in order to achieve acceptable electrical performance in multijunction solar applications.

Screen printed solar cell technology has dominated the photovoltaic (PV) industry for the past couple of decades due to its simplicity, robustness and cost-effectiveness. The draw-back of screen printed solar cells is its relatively low efficiency being 16.5% for mono-crystalline and 15.5% for multi-crystalline silicon solar cells [Mai06]. Much higher efficiency solar cells have been achieved in the laboratory. Unfortunately, the fabrication processes of the high efficiency laboratory solar cells are relatively complicated and excessively costly with the involvement of photolithography and high temperature thermal processes. Consequently, it is believed a new high efficiency solar cell structure on low priced wafer materials with low fabrication costs is required in order to achieve the overall cost reductions in solar cell production. The objective of this thesis work was therefore to realise such a cell design by applying the laser doping (LD) process as the high efficiency selective emitter technology onto well passivated, p-type multi-crystalline wafers together with a suitable self-aligned metallisation plating scheme to fabricate industrial viable high efficiency multi-crystalline solar cells, that is the laser doped p-type multi-crystalline silicon solar cell. An important aspect of laser doping (LD) technology is to provide localised heavily doped regions without subjecting the entire wafer to high temperature processing. This feature makes LD highly suitable for fabricating multi-crystalline solar cells due to the fact that multi-crystalline wafer materials often degrade at high processing temperatures. Several challenges arose adapting the laser doping process to lower quality multi-crystalline materials, especially due to the presence of grain boundaries. The encountered challenges arising from the laser doping process on the multi-crystalline wafers were identified, examined and resolved. It was shown that modifying the laser processing parameters of the laser systems and incorporating a silicon oxy-nitride (SiOxNy) film by using plasma enhanced chemical vapour deposition (PECVD) can overcome the improper melting and doping of the grain boundaries during the LD process. Just as importantly this thesis work demonstrated that a single layer PECVD SiOxNy and a double layer of PECVD SiOxNy/SiNx films are capable of minimising the laser induced defects arising from the thermal expansion mismatch between the silicon and PECVD SiNx. In addition, they also provide excellent surface and bulk passivation for textured, lightly phosphorus diffused, industrial grade, p-type Czochralski (CZ) and multi-crystalline wafers. Moreover, the reasons for the improvement in the post-firing effective minority carrier lifetime (τeff) either due to the surface or the bulk passivation quality were analysed. It was proven that PECVD SiOxNy and dual layer stacked SiOxNy/SiNx films are also suitable as front surface passivating anti-reflection coatings. N-type Float Zone (FZ) wafers were used to determine the surface recombination velocity (SRV) and the implied voltages under various PECVD SiOxNy film deposition conditions. An implied open circuit voltage (iVoc) of 743 mV was extracted from PECVD SiOxNy films suitable for anti-reflection coatings. These films possessed associated effective minority carrier lifetimes (τeff) of greater than 1.9 ms at a minority carrier injection level of 1 x 1015 cm-3, with a SRV value as low as 0.9 cms-1. Average post-firing iVoc of 668mV and 637mV on textured, diffused, commercial grade CZ and multi-crystalline wafers, respectively, were achieved with PECVD SiOxNy films with a refractive index of 2.05. In this thesis, the background theory, and the processing sequence of effective self-aligned electroless and light induced plating (LIP) metallisation schemes are explained and presented. These metallisation schemes have been well known and quantified for many decades; nevertheless, applying these plating techniques onto the standard acid textured, phosphorus diffused, p-type multi-crystalline wafers with a SiNx coated surface can be relatively challenging. The origins of over-plating sites of nickel (Ni) and copper (Cu) on the acid textured, phosphorus diffused, SiNx coated p-type multi-crystalline wafer surfaces were examined. They were found to be more likely to over-plate underneath the wells and along the sharp edges of the acid textured multi-crystalline wafer surfaces. The three main causes of over-plating on acid textured multi-crystalline wafers with SiNx coated surfaces were analysed and verified: (1) the physical nature of PECVD SiNx deposition and the thicknesses of the SiNx film, (2) the surface n-type concentration density, and (3) the topology of the acid textured multi-crystalline wafer surfaces. In addition to increased shading losses, over-plating was also proven to cause shunting mechanisms or form Schottky contacts, which significantly degrades the overall electrical performance of the solar cell devices. Several methods of preventing over-plating problems were experimentally implemented and evaluated: (1) the variations of the plating conditions, (2) the modifications of the front surface dielectric coating, (3) the incorporation of the chemically grown oxide layer beneath the dielectric coating, and (4) the modifications of the topology of the acid textured multi-crystalline wafer surfaces. The outcomes of the experimental results from each of the individual problem solving techniques were analysed and presented in order. It was found that the development of the innovative acid rounding etch and alkali etch processes can be applied after the phosphorus diffusion and prior to the PECVD process to successfully eliminate over-plating problems on acid textured, phosphorus diffused, SiNx coated, p-type multi-crystalline silicon wafers. Finally, multi-crystalline wafers without excess plating on the surface were demonstrated. Finally, having solved the associated challenges, novel industrially viable, high efficiency, laser doped selective emitter silicon solar cells on p-type multi-crystalline wafers were successfully developed, and the achievements of such a cell structure were thoroughly demonstrated. An average efficiency of 17.2% and an average fill factor (FF) of 78% on commercial grade p-type multi-crystalline wafers with nominal resistivity of 1 Ω.cm was reproducibly fabricated.

Although silicon (Si) nanostructures exhibit size dependent light emission, which can be attributed to quantum confinement, the role of surface passivation is not yet fully understood. This understanding is central to the development of nanocrystal-based detectors. This study investigated the growth, surface chemistry, passivation with deuterium (D2), ammonia (ND3) and diborane (B2D6) and the resulting optical properties of Si nanostructures. Si nanocrystals less than 6 nm in diameter are grown on SiO2 surfaces in an ultra high vacuum chamber using hot-wire chemical vapor deposition and the as grown surfaces are exposed to atomic deuterium. Temperature programmed desorption (TPD) spectra show that that the nanocrystals surfaces are covered by a mix of monodeuteride, dideuteride and trideuteride species. The manner of filling of the deuteride states on nanocrystals differs from that for extended surfaces as the formation of the dideuteride and trideuteride species is facilitated by the curvature of the nanocrystal. No photoluminescence (PL) is observed from the as grown unpassivated nanocrystals. As the deuterium dose is increased, the PL intensity also begins to increase. This can be associated with increasing amounts of mono-, di- and trideuteride species on the nanocrystal surface, which results in better passivation of the dangling bonds and relaxing of the reconstructed surface. At high deuterium doses, the surface structure breaks down and amorphization of the top layer of the nanocrystal takes place. Amorphization reduces the PL intensity. Finally, as the nanocrystal size is varied, the PL peak shifts, which is characteristic of quantum confinement. The dangling bonds and the reconstructed bonds at the NC surface are also passivated and transformed with D and NDx by using deuterated ammonia (ND3), which is predissociated over a hot tungsten filament prior to adsorption. At low hot wire ND3 doses PL emission is observed at 1000 nm corresponding to reconstructed surface bonds capped by predominantly monodeuteride and Si-ND2 species. As the hot wire ND3 dose is increased, di- and trideuteride species form and intense PL is observed around 800 nm that does not shift with NC size and is associated with defect levels resulting from NDx insertion into the strained Si-Si bonds forming Si2=ND. The PL intensity at 800 nm increases as the ND3 dose is increased and the intensity increase is correlated to increasing concentrations of deuterides. At extremely high ND3 doses PL intensity decreases due to amorphization of the NC surface. In separate experiments, Si NCs were subjected to dissociative (thermal) exposures of ammonia followed by exposures to atomic deuterium. These NCs exhibited size dependent PL and this can be attributed to the prevention of the formation of Si2=ND species. Finally, deuterium-passivated Si NCs are exposed to BDx radicals formed by dissociating deuterated diborane (B2D6) over a hot tungsten filament and photoluminescence quenching is observed. Temperature programmed desorption spectra reveal the presence of low temperature peaks, which can be attributed to deuterium desorption from surface Si atoms bonded to subsurface boron atoms. The subsurface boron likely enhances nonradiative Auger recombination.

Silicon nanocrystals (SiNCs) have become a heavily researched material over the past several years. Researchers envision that this material can be used in many diverse applications such as electronic devices, non-toxic biological tags, optical devices such as LEDs, lasers or displays, thermoelectrics, and photovoltaic (PV) applications. For many of these proposed applications one needs to properly control the NC size and the surface chemistry via passivation. Current passivation techniques allow for the creation of highly efficient SiNC optical emitters, however the emission of these NCs are fixed in the red- NIR range. To resolve this issue several novel in-flight passivation techniques were investigated. A novel dual-plasma setup which allows for the in-flight passivation of SiNCs through a thermal or LPCVD based nitridation process was developed first. FTIR and XPS analysis were used to study the surface chemistry on of the nitride passivated NCs while TEM was used to investigate whether or not a “shell” was grown on the surface. PL measurements and thermal stability tests were performed on the nitride passivated NCs to gain a further understanding of the stability (in both air as well as other ambients) of the NCs and their surface chemistry. Tunable full color emission from SiNCs was developed for the dual-plasma reactor utilizing CF4 as both an etching and passivating source. F radicals generated in the etching plasma remove Si from the surface of the NC, while at the same time CF2 radicals lead to the formation of a fluorocarbon passivation layer on the NC surface. By controlling the parameters of the reactor (CF4 flow rate, power), the NC size and thus its color can be controlled. Red to green luminescence was observed from SiNCs and is believed to be due to the quantum confinement effect. The blue emission observed from the NCs is appears to be related to oxide related surface states. Despite the defects, high QY was observed from these CF4-etched NCs. The fluorocarbon passivation layer, although stable, prevents further functionalization of the NCs. To counteract this problem another silicon-based dry etch chemistry, SF6 was investigated. Full-color emission was observed from SF6 etched NCs, with QY 2X higher than that of CF4-etched NCs. A maximum QY of nearly 55% at 700 nm was observed after several weeks in air, comparable to that observed with alkyl passivation. The native oxidation of the bare oxidized and SF6-etched NCs were also studied. Results show that the NC oxidation follows the Cabrera-Mott mechanism for low temperature oxidation. Inorganic-NC based LED structures were then investigated. Fabrication processes for the inorganic hole and electron transport layers were developed by RF sputtering and atomic layer deposition (ALD). Thorough characterization was performed on the metal-oxide films (ZnO, TiO2, NiO) to verify their stoichiometry as well as study their optical and electrical properties. Novel inorganic-NC device structures were fabricated. Inorganic NC devices which use a metal-oxide HTL but no ETL, emit light, however their emission is so weak. The addition of an ETL increases the light output by a factor of 4, but the device reproducibility is poor. To improve efficiency two insulating matrix layers were investigated. In both cases, the film deposited on the top of the NC is rough, porous, discontinuous, and potentially full of traps – certainly not the ideal film for a device. Therefore, more work is needed, specifically on the NC layer to improve the structure of the as-deposited NC film, but efficient device structures appear to be possible.

Historically, the mechanism of localized corrosion in CO2 (sweet) environments has been poorly understood. This shortcoming is an obstacle in the development of corrosion control and protection protocols. The purpose of this PhD project was to explore and understand localized sweet corrosion mechanisms through systematic study. An artificial pit cell was developed in order to directly measure the galvanic current resulting from localized corrosion propagation. Thus, galvanic mechanisms of localized CO2 corrosion were elucidated. It was found that two surfaces coexist as termed anode (bare surface in the pit) and cathode (surrounding surface usually covered by corrosion scales) with open circuit potentials (OCP) for these different surfaces being different under the same bulk environments. This potential difference can be the driving force for localized corrosion propagation when the reactions on the two surface balance (a mixed potential is reached). A "gray zone" criterion was determined through experiments and theory to explain localized CO2 corrosion propagation. It was concluded that localized corrosion propagates when the conditions are near the saturation point for iron carbonate, i.e. in the "gray zone". Under this condition, which is neither highly supersaturated nor undersaturated, the pit area stays scale free while the scale remains on the surrounding cathode surface. Electrochemical studies demonstrated that passivation, especially spontaneous passivation, can occur on the cathode surface and that results in a higher open circuit potential on the cathode. Surface analysis using GIXRD and TEM/EDX showed that beneath an iron carbonate film formed first a passive film is formed due to the local high pH conditions underneath the FeCO3 film. The passive film was identified and confirmed to be magnetite, Fe3O4, under the test conditions using X-ray diffraction (XRD) with grazing incidence, its thickness being at the nanometer level, as detected by TEM/EDX. This passive film is responsible for the spontaneous passivation of the surface and causes the more positive open circuit potential compared with that on the bare surface (pit area). In order to confirm the passivation mechanism, a surface pH probe was developed. The surface pH measurements under simulated iron carbonate scale showed a higher pH value, which was high enough to reach passivation as defined by the Pourbaix diagram. An electrochemical model was constructed based on the described galvanic mechanisms of localized CO2 corrosion, having the capability to predict bare surface uniform corrosion, filmed surface passivation and galvanic effects for localized corrosion propagation, in other words, a steady state "worst case" localized corrosion propagation scenario.

Dielectric thin films have a long history in silicon photovoltaics. Due to the specific physical properties, they can function as passivation layer in solar cells. Also, they can be used as antireflection coating layers on top of the devices. They can improve the back surface reflectance if proper dielectric layers combination is used. What’s more, they can protect areas by masking during chemical etching, diffusion, metallization among the whole fabrication process. Crystalline silicon solar cell can be passivated by two ways: one is to deposit dielectric thin films to saturate the dangling bonds; the other is to introduce surface electrical field and repel back the minority carriers. This thesis explores thermally grown SiO2 and sputtered Si3N4(:H) to passivate n-type and thermal evaporation AlF3 to passivate p-type Float Zone silicon wafers, respectively. Sputtering is a cheap passivation method to replace PECVD in industry usage, but all sputtered samples are more likely to have encountered surface damage from neutral Ar and secondary electrons, both coming from the sputtered target. AlF3/SiO2 multi-layer stack is a negative charge combination; p inversion layer will form on the wafer surface. Light trapping is an important part in solar cell research work. In order to enhance the reflectance and improve the absorption possibility of near infrared photons, especially for high efficiency PERL cell application, the back surface structure is optimized in this work. Results show SiO2/Ag is a very good choice to replace SiO2/Al back reflectors. The maximum back surface reflectance is 97.82%. At the same time, SiO2/Ag has excellent internal angle dependence of reflectance, which is beneficial for surface textured cells. A ZnS/MgF2/SiO2/Al(Ag) superlattice can improve the back reflectance, but it is sensitive to incident angle inside the silicon wafer. If planar wafers are used to investigate all kinds of back reflectors, and an 8 degrees incident angle is fixed for typical spectrometry measurement, the results are easy to predict by Wvase software simulation. If a textured surface is considered, the light path inside the silicon wafer is very complicated and hard to calculate and simulate. The best way to evaluate the result is through experiment.

The ability to passivate metallic surfaces under non-UHV conditions is not only of fundamental interests, but also of growing practical importance in catalysis and microelectronics. In this work, the passivation effect of a surface iodine layer on air-exposed Ta for the copper electroless deposition was investigated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Although the passivation effect was seriously weakened by the prolonged air exposure, iodine passivates the Ta substrate under brief air exposure conditions so that enhanced copper wetting and adhesion are observed on I-passivated Ta relative to the untreated surface.

In previous studies, device quality Si-SiO2 interfaces and dielectric bulk films (SiO2) were prepared using a two-step process; i) remote plasma-assisted oxidation (RPAO) to form a superficially interfacial oxide (~0.6 nm) and ii) remote plasma enhanced chemical vapor deposition (RPECVD) to deposit the oxide film. The same approach has been applied to the GaN-SiO2 system. After a 300 oC remote N2/He plasma treatment of the GaN surface, residual C and Cl were reduced below Auger electron spectroscopy (AES) detection, and the AES peak ratio of O KLL and N KLL was ~0.06 or ~0.1 monolayer of oxygen. RPAO of GaN surfaces using O2, N2O, and N2O in N2 source gases were investigated by on-line AES to determine the oxidation kinetics and chemical composition of the interfacial oxide. Without an RPAO step, subcutaneous oxidation of GaN takes place during RPECVD deposition of SiO2, and on-line AES indicates a ~0.6-0.8 nm subcutaneous oxide. Compared to single step SiO2 deposition, significantly reduced interface state density (Dit) was obtained at the GaN-SiO2 interface by independent control of GaN-Ga2O3 interface formation by thin RPAO oxide (~1 nm) and SiO2 film deposition by RPECVD. High-low frequency method and conductance method indicate that Dit of GaN Metal-Oxide-Semiconductor (MOS) sample without RPAO is ~5 times larger than that of the sample with RPAO. For the GaN MOS structure with remote plasma oxidation and nitridation, Dit determined at DCmax was low-to-mid x 1011 cm-2eV-1. Also, we report on high temperature and photo-assisted capacitance-voltage (C-V) characteristics.

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The potential use of sodium propionate as a corrosion inhibitor for mild steel in aqueous media is investigated using a range of electrochemical and surface analytical techniques. The use of sodium propionate for the inhibition of mild steel corrosion is discussed, and the effective pH range of sodium propionate using various buffers is investigated. The effectiveness of sodium propionate as an inhibitor for mild steel pitting corrosion in the presence of various concentrations of CI- is studied. The effect of some oxidants, IO3-, BrO3-, NO32- on the anodic behaviour of mild steel in deaerated 0.01M carboxylate solutions of acetate, propionate, formate, succinate and salicylate is investigated. The critical temperature for effective inhibition of mild steel corrosion with sodium propionate is established, and the chemical composition of the film formed on mild steel surface in sodium propionate solution is studied using surface sensitive Fourier transform infrared spectroscopy FTIR. The efficiency of sodium propionate is compared to that of conventional inhibitors and a mechanism for the inhibition of mild steel corrosion with sodium propionate is proposed.