The global energy market is continuously changing due to changes in demand and fuel availability. Amongst the technologies considered as capable of fulfilling these future energy requirements, Photovoltaics (PV) are one of the most promising. Currently the majority of the PV market is fulfilled by crystalline Silicon (c-Si) solar cell technology, the so called 1st generation PV. Although c-Si technology is well established there is still a lot to be done to fully exploit its potential. The cost of the devices, and their efficiencies, must be improved to allow PV to become the energy source of the future. The surface of the c-Si device is one of the most important parts of the solar cell as the surface defines the electrical and the optical properties of the device. The surface is responsible for light reflection and charge carrier recombination. The standard surface finish is a thin film layer of silicon nitride deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD). In this thesis an alternative technique of coating preparation is presented. The HiTUS sputtering tool, utilising a remote plasma source, was used to deposit the surface coating. The remote plasma source is unique for solar cells application. Sputtering is a versatile process allowing growth of different films by simply changing the target and/or the deposition atmosphere. Apart from silicon nitride, alternative materials to it were also investigated including: aluminium nitride (this was the first use of the material in solar cells) silicon carbide, and silicon carbonitride. All the materials were successfully used to prepare solar cells apart from the silicon carbide, which was not used due to too high a refractive index. Screen printed solar cells with a silicon nitride coating deposited in HiTUS were prepared with an efficiency of 15.14%. The coating was deposited without the use of silane, a hazardous precursor used in the PECVD process, and without substrate heating. The elimination of both offers potential processing advantages. By applying substrate heating it was found possible to improve the surface passivation and thus improve the spectral response of the solar cell for short wavelengths. These results show that HiTUS can deposit good quality ARC for silicon solar cells. It offers optical improvement of the ARC s properties, compared to an industrial standard, by using the DL-ARC high/low refractive index coating. This coating, unlike the silicon nitride silica stack, is applicable to encapsulated cells. The surface passivation levels obtained allowed a good blue current response.

Novel photovoltaic cells receive considerable attention from researchers as evidenced by high numbers of published articles. Different types of materials are currently being examined in order to reduce the cost and improve the efficiency of solar cells. Essentially, solar cells are constructed by placing layers of light absorber between electron and hole transport materials. Electricity generation by solar cells involves multiple processes. These processes require an understanding of the physical properties of the surfaces and interfaces of the materials. In this thesis, materials for novel photovoltaic cells are studied by X ray photoelectron spectroscopy (XPS), a surface and interface characterisation technique. The materials studied in this thesis are colloidal quantum dots (CQDs) of the core/shell systems CdTe/CdSe and PbS/CdS, and CQDs that have been surface passivated using Cl- (CdTe/Cl) and CdS (CdTe/CdSe/CdS and PbS/CdS). Moreover, CsSnI3, a perovskite material, is also studied in both bulk and thin film form. CQDs can be used as light absorbers in solar cells while CsSnI3 can be employed as the hole transport material. The role of the core shell structure and surface passivation treatment is to improve or maintain charge transport as well as acting as a protective layer to the CQDs. Depth profiling synchrotron radiation XPS is used to determine these structures. In the CdTe/CdSe samples, the elemental ratio between Se (shell) and Te (core) increases with decreasing sampling depth, demonstrating the presence of a CdSe shell located at the surface of the CQDs. The shell thicknesses of the core-shell systems are estimated from XPS and show that the addition of the third thin shell (of CdS) protects the CQD during ligand exchange. Cl- passivation is shown to reduce the energy the valence band maximum and the energy gap of CdTe CQDs. This is associated with the passivation of midgap trap states due to the removal of dangling bonds at the surface of CQDs. Surface passivation is shown to improve the stability of CQDs to air exposure. This is indicated by a significant reduction of the surface oxide species in the passivated PbS/CdS samples. In the unpassivated core-only PbS samples, however, oxidation rapidly occurs which affects the electronic states required for charge transport in solar cells. XPS studies of CsSnI3 show that this material is reactive to air exposure. Surface preparation techniques are performed to remove the contamination layer and reveal the physical properties of the perovskite itself. This is confirmed by the elemental ratios from XPS. The metallic character of CsSnI3 is also observed in the valence band spectra as evidenced by the appearance of the Fermi edge.

Energy traps at the silicon surface originating from discontinuities in the lattice is detrimental to the performance of solar cells. Acting as recombination centers, they offer a location where the charge carriers may easily return to their original energy band after excitation. Surface passivation is an effective method to combat this and can be done either by suppressing traps (lowering trap density) or by forming an electric field, preventing the carriers from reaching the defect states. Silicon oxide, SiO2, and aluminum oxide, Al2O3, are two materials which have previously been shown to provide good passivating qualities. In this thesis, SiO2 and Al2O3 have been used both as single layers and in a stack configuration to passivate the surface of crystalline silicon (c-Si). Using a response surface methodology approach, temperature optimization with respect to deposition and annealing temperature has been conducted for SiO2/Al2O3 stacks deposited with plasma-enhanced atomic layer deposition, PEALD. It was shown that the same deposition temperature (Tdeposition = 140 °C, Tanneal = 395 °C) could be used for both materials and provide good passivation with an effective surface recombination velocity, Seff, of 5.3 cm/s (1Ωcm n-type Si wafers). From FTIR measurements, an increase in hydroxyl groups was seen as the SiO2 deposition temperature increased while the opposite was observed for Al2O3 which also showed fewer carbon related impurities with increasing temperature. Increasing the SiO2 temperature strongly affected the fixed charge density, causing it to decrease and even switch polarity. The fixed charge density could also be controlled by varying the thickness of the intermediate SiO2 layer. At a thickness of 1-2 nm, a minimum in the effective lifetime was observed and was correlated to Si close to flat-band conditions. N-type wafers showed a larger negative fixed charge density than p-type wafers which results in stronger field-effect passivation. For phosphorous doped emitters (200 Ω/sq on 10 Ωcm p-type wafer), it was seen that SiO2/Al2O3 stacks with a SiNx anti-reflection coating performed better than SiO2 or Al2O3 single layers. By depositing SiO2 at 130 °C in SiO2/Al2O3 stacks and annealing at 450 °C, an implied open circuit voltage (iVoc) of 710 mV was measured (AM1.5G) together with an implied fill factor (iFF) of 84.1% and a recombination parameter (J0) of 19.2 fA/cm2. Al2O3 single layer showed an extremely low J0 of 10 fA/cm2 but suffered from a decreased iFF and strong injection dependent lifetimes which originates from an inversion layer. ALD ozone processes were successfully developed for SiO2 and Al2O3.  The deposition rate per cycle for SiO2 was found to be only ~0.175 Ǻ/cycle (PEALD ~1.1 Ǻ/cycle), making it rather unpractical for use outside of research. Single layer SiO2 deposited with ozone showed, similarly to a plasma process, almost no surface passivation. Al2O3 however proved to be highly passivating on its own with a τeff = 3.8 ms, Seff = 1.2 cm/s (1 Ωcm n-type) after depositing at 250 °C. Studies on the effect of annealing showed that an annealing temperature of 450 °C is necessary to completely activate the passivation. The low Seff values were attributed to a very high negative fixed charge density ~1013 cm-2 together with strong chemical passivation.

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Der Graben-Gate-MOSFET ist eine herausragende Bauelementarchitektur unter den vertikalen Bauelementen auf GaN-Basis, die derzeit für die nächste Generation der Leistungselektronik untersucht werden. Ein niedriges Reststromniveau im Aus-Zustand bei hoher Drain-Spannung ist für vertikale Transistoren von großer Bedeutung, da es ein entscheidendes Merkmal für eine hohe Durchbruchspannung und Zuverlässigkeit der Bauelemente ist. Die Drain-Restströme im Aus-Zustand haben ihren Ursprung in verschiedenen Quellen im vertikalen Trench-Gate-MOSFET. Neben dem Trench-Gate-Modul können auch die Reststrompfade an der trockengeätzten Seitenwand des lateralen Kantenabschlusses erheblich zum Drain-Reststrom im Aus-Zustand beitragen. In diesem Bericht wird der Einfluss jedes relevanten Prozessschritts auf den Drain-Reststrom im Aus-Zustand anhand spezifischer Teststrukturen auf hochwertigem epitaktischem GaN-Material, welche den lateralen Kantenabschluss des MOSFETs nachbilden, untersucht. Die elektrische Charakterisierung zeigt die Empfindlichkeit des Reststroms gegenüber plasmabezogenen Prozessen. Es wird eine Technologie der Randterminierung vorgestellt, die zu einem niedrigen Reststrom führt und gleichzeitig dicke dielektrische Schichten aus plasma-unterstützter Abscheidung enthält, die für die Herstellung einer Feldplattenstruktur über dem Kantenabschluss vorgesehen sind. / The trench gate MOSFET represents a prominent device architecture among the GaN based vertical devices currently investigated for the next generation of power electronics. A low leakage current level in off-state under high drain bias is of great importance for vertical transistors since it is a crucial feature for high breakdown voltage and device reliability. The off-state drain leakage originates from different sources in the vertical trench gate MOSFET. Besides the trench gate module, the leakage paths at the dry-etched sidewall of the lateral edge termination can also significantly contribute to the off-state drain-current. In this report, the influence of each relevant process step on the drain leakage current in off-state is investigated utilizing specific test structures on high-quality GaN epitaxial material which mimic the lateral edge termination of the MOSFET. Electrical characterization reveals the sensitivity of the leakage current to plasma-related processes. A termination technology is presented that results in low leakage current while including thick dielectric layers from plasma-assisted deposition as intended for fabrication of a field plate structure over the edge termination.

L'optomécanique est la science des interactions entre la lumière et les mouvements mécaniques. Ce rapport de thèse décrit des expériences réalisées avec des microdisques fabriqué dans différents résonateurs semi-conducteurs III-V: l'Arséniure de Gallium (GaAs), l'Arséniure d'Aluminium Gallium (AlGaAs) et l'Arséniure d'Indium Phosphide (InGaP). Ces matériaux sont compatibles avec les fonctionnalités de l’optoélectronique et procurent un couplage optomécanique géant. Pour améliorer les performances des résonateurs en GaAs, nous avons développé des méthodes de traitement de surface permettant de réduire la dissipation optique par un facteur dix et ainsi d'atteindre un facteur de qualité de six millions. En plus de ces études sur le GaAs, nous avons réalisés une étude comparative des interactions optomecaniques dans des microdisques d'InGaP et d'AlGaAs, et nous avons mis en évidences leurs résonances optomécaniques. Finalement, nous avons réalisé des mesures de force avec des résonateurs en GaAs, démontrant un nouveau principe de détection basé sur notre étude de leur la trajectoire dans l'espace de phase et leur bruit de phase / Optomechanics studies the interaction between light and mechanical motion. This PhD thesis reports on optomechanical experiments carried with miniature disk resonators fabricated out of distinct III-V semiconductors: Gallium Arsenide (GaAs), Aluminium Gallium Arsenide (AlGaAs) and Indium Gallium Phosphide (InGaP). These materials are compliant with optoelectronics functionalities and provide giant optomechanical coupling. In order to boost performances of GaAs resonators, we implemented surface control techniques and obtained a ten-fold reduction of optical dissipation, attaining a Q of six million. On top of GaAs, we performed a comparative investigation of optomechanical interactions in InGaP and AlGaAs disk resonators, and demonstrated their operation as optomechanical oscillators. Finally, we carried out optomechanical force sensing experiments with GaAs resonators, analyzing a new sensing principle in light of the phase space trajectory and phase noise of the corresponding oscillators

III-V nanostructures have attracted substantial research effort due to their interesting physical properties and their applications in new generation of ultrafast and high efficiency nanoscale electronic and photonic components. The advances in nanofabrication methods including growth/synthesis have opened up new possibilities of realizing one dimensional (1D) nanostructures as building blocks of future nanoscale devices. For processing of semiconductor nanostructure devices, simplicity, cost effectiveness, and device efficiency are key factors. A number of methods are being pursued to fabricate high quality III-V nanopillar/nanowires, quantum dots and nano disks. Further, high optical quality nanostructures in these materials together with precise control of shapes, sizes and array geometries make them attractive for a wide range of optoelectronic/photonic devices. This thesis work is focused on top-down approaches for fabrication of high optical quality nanostructures in III-V materials. Dense and uniform arrays of nanopillars are fabricated by dry etching using self-assembly of colloidal SiO2 particles for masking. The physico-chemistry of etching and the effect of etch-mask parameters are investigated to control the shape, aspect ratios and spatial coverage of the nanopillar arrays. The optimization of etch parameters and the utilization of erosion of etch masks is evaluated to obtain desired pillar shapes from cylindrical to conical. Using this fabrication method, high quality nanopillar arrays were realized in several InP-based and GaAs-based structures, including quantum wells and multilayer heterostructures. Optical properties of these pillars are investigated using different optical spectroscopic techniques. These nanopillars, single and in arrays, show excellent photoluminescence (PL) at room temperature and the measured PL line-widths are comparable to the as-grown wafer, indicating the high quality of the fabricated nanostructures. The substrate-free InP nanopillars have carrier life times similar to reference epitaxial layers, yet an another indicator of high material quality. InGaAs layer, beneath the pillars is shown to provide several useful functions. It effectively blocks the PL from the InP substrate, serves as a sacrificial layer for generation of free pillars, and as a “detector” in cathodoluminescence (CL) measurements. Diffusion lengths independently determined by time resolved photoluminescence (TRPL) and CL measurements are consistent, and carrier feeding to low bandgap InGaAs layer is evidenced by CL data. Total reflectivity measurements show that nanopillar arrays provide broadband antireflection making them good candidates for photovoltaic applications.  A novel post etch, sulfur-oleylamine (S-OA) based chemical process is developed to etch III-V materials with monolayer precision, in an inverse epitaxial manner along with simultaneous surface passivation. The process is applied to push the limits of top-down fabrication and InP-based high optical quality nanowires with aspect ratios more than 50, and nanostructures with new topologies (nanowire meshes and in-plane wires) are demonstrated.  The optimized process technique is used to fabricate nanopillars in InP-based multilayers (InP/InGaAsP/InP and InP/InGaAs/InP). Such multilayer nanopillars are not only attractive for broad-band absorption in solar cells, but are also ideal to generate high optical quality nanodisks of these materials. Finally, the utility of a soft stamping technique to transfer free nanopillars/wires and nanodisks onto Si substrate is demonstrated. These nanostructures transferred onto Si with controlled densities, from low to high, could provide a new route for material integration on Si. / <p>QC 20130205</p>

Engineering new solid propellant materials requires optimization of several factors, to include energy density, burn rate, sensitivity, and environmental impact. Equally important is the need for materials that will maintain their mechanical properties and thermal stability during long periods of storage. The nanoscale materials considered in this dissertation are proposed metal additives that may enhance energy density and improve combustion in a composite rocket motor. Density Functional Theory methods are used to determine cluster geometries, bond strengths, and energy densities. The ground-state geometries and electron affinities (EAs) for MnxO?: x = 3, 4, y = 1, 2 clusters were calculated with GGA, and estimates for the vertical detachment energies compare well with experimental results. It was found that the presence of oxygen influences the overall cluster moment and spin configuration, stabilizing ferrimagnetic and antiferromagnetic isomers. The calculated EAs range from 1.29-1.84 eV, which is considerably lower than the 3.0-5.0 eV EAs characteristic of current propellant oxidizers. Their use as solid propellant additives is limited. The structures and bonding of a range of Al-cyclopentadienyl cluster compounds were studied with multilayer quantum mechanics/molecular mechanics (QM:MM) methods. The organometallic Al-ligand bonds are generally 55-85 kcal/mol and are much stronger than Al-Al interactions. This suggests that thermal decomposition in these clusters will proceed via the loss of surface metal-ligand units. The energy density of the large clusters is calculated to be nearly 60% that of pure aluminum. These organometallic cluster systems may provide a route to extremely rapid Al combustion in solid rocket motors. Lastly, the properties of COOH-terminated passivating agents were modeled with the GPW method. It is confirmed that fluorinated polymers bind to both Al(111) and Al(100) at two Al surface sites. The oligomers HCOOH, CH3CH2COOH, and CF3CF2COOH chemisorb onto Al(111) with adsorption energies of 10-45 kcal/mol. The preferred contact angle for the organic chains is 65-85 degrees, and adsorption energy weakens slightly with increasing chain length. Despite their relatively weak adsorption energies, fluorinated polymers have elevated melting temperatures, making them good passivation materials for micron-scale Al fuel particles.

Aufkonvertierende (upconverting; UC) Nanomaterialien bilden eine neue Klasse nichtlinearer lumineszenter Reporter, die nah-infrarotes (NIR) Anregungslicht in Photonen von höherer Energie umwandeln. Das effizienteste bekannte UC-System bildet hierbei β-NaYF4: 20%Yb(III), 2%Er(III) mikrokristallines Bulkmaterial, für welches UC-Quantenausbeuten (ΦUC) von 10 % berichtet werden, während ΦUC von Nanokristallen (nanocrystals; NC) um mehrere Größenordnungen niedriger sein können. Um die Effizienz von UC-Nanomaterialien zu erhöhen, werden NC üblicherweise mit inerten Schalen versehen. In dieser Arbeit werden mehrere verschiedene Bulkmaterialien spektroskopisch untersucht, um ein Vergleichsmaterial auszuwählen, das als Maßstab für alle folgenden, vergleichbaren Messungen an NC dient. Die Oberfläche von ultrakleinen (3.7±0.5) nm NC wird mit Schalen von bis zu 10 nm Dicke versehen, um die optimale Schalendicke für vollständige Oberflächenpassivierung zu identifizieren, allerdings weisen die Ergebnisse auf eine mögliche Kern-Schale-Durchmischung hin. In einer zweiten Studie werden die unterschiedlichen Dotanden, Er(III) und Yb(III), auf ihre optischen Eigenschaften sowie die Einflüsse von Energietransfer (ET) und von ihrer Umgebung spektroskopisch untersucht. Dabei kann klar zwischen Oberflächeneffekten und oberflächenunabhängigen Volumeneffekten unterschieden werden. Die Ergebnisse werden durch ein einfaches Monte-Carlo-Modell gestützt, durch das die größen- und leistungsdichte-(P-)abhängigen Populierungsdynamiken der strahlenden Banden von Er(III) vorhergesagt werden können. Zuletzt werden durch eine verbesserte Synthesemethode UCNC mit stark verbesserten Lumineszenzeigenschaften hergestellt, mit denen bei vergleichsweise niedrigen P die gleichen ΦUC wie beim Bulkmaterial erreicht werden. Dies liefert einen Einblick in vielfältige Anwendungsmöglichkeiten für UCNC. / Upconversion (UC) nanomaterials are an emerging new class of non-linear luminescent reporters which convert near-infrared (NIR) excitation light into higher-energy photons. The most efficient known UC material is the β-NaYF4: 20%Yb(III), 2%Er(III) bulk (microcrystalline) phosphor with reported UC quantum yields (ΦUC) of 10 %, while ΦUC of nanocrystals (NC) can be several orders of magnitude lower. Strategies to improve the efficiency of UC nanomaterials include surface passivation with inert shells. In this work, several different bulk materials are compared to select one benchmark material for comparisons with NC analyzed with the same measurement techniques. The surface of ultrasmall (3.7 ± 0.5) nm NC is coated with inert shells of up to 10 nm thickness to identify an optimal shell thickness for complete surface passivation, but the results suggest core-shell intermixing. To distinguish between the different dopant ions, Er(III) and Yb(III), and the effect of energy transfer (ET) in a second study, single- and co-doped UCNC are investigated spectroscopically and the influence of their environment is determined thoroughly. Herein, a clear distinction between surface-related and surface-independent, volume-related effects is achieved and the results are emphasized by the use of a simple random walk model which accurately predicts size- and power density (P)-dependent population dynamics of the emissive bands of Er(III). Finally, utilizing an improved synthesis technique, UCNC with enhanced luminescence properties are produced, reaching the same ΦUC as the benchmarked bulk material at reasonably low P, providing an insight into numerous possible applications of UCNC.

<p>III-V nanowire (NW) ensemble devices were fabricated using novel approaches to address key NW optoelectronic issues concerning electrical contacts, doping, surface effects and underlying electrostatics physics.</p> <p>NWs were first embedded in a filling medium, thus achieving low sheet resistance front contacts while preventing shunts. Various filling materials were assessed for porosity, surface roughness and thermal stability, giving Cyclotene as an ideal filing material. Sonication was also introduced as a novel method to achieve perfect planarization.</p> <p>The presence of the Cyclotene also enabled the NWs to be characterized precisely and easily by secondary ion mass spectrometry (SIMS) to give the NW dopant concentration with excellent spatial resolution. Additionally, SIMS characterization demonstrated the ability to characterize the height uniformity of individual segments in a heterostructure NW ensemble.</p> <p>The focus of the work shifted towards surface effects on NW device performance. Therefore, Poisson's equation was solved to provide a comprehensive model of NW surface depletion as a function of interface state density, NW radius and doping density. Underlying physics was examined where surface depletion was found to significantly reduce the conductivity of thin NWs, leading to carrier inversion for some.</p> <p>This model was then applied in conjunction with a transport model to fit current-voltage curves of an AlInP-passivated GaAs NW ensemble device. A 55% decrease in surface state density was achieved upon passivation, corresponding to an impressive four order of magnitude increase in the effective carrier concentration. Additionally, conventional and time-resolved photoluminescence measurements showed intensity and carrier lifetime improvement greater than 20x upon passivation.</p> <p>Finally, the model was extended to describe radial pn junction NWs with surface depletion to give radial energy band profiles for any arbitrary set of NW parameters. Specific cases were analyzed to extract pertinent underlying physics, while the built-in potential was optimized for the design for an optimal device.</p> / Doctor of Philosophy (PhD)