Advanced power semiconductor devices have consistently proven to be a major force in pushing the progressive development of power conversion technology. The emerging wide-band-gap (WBG) material based power semiconductor devices are considered as gaming changing devices which can exceed the limit of silicon (Si) and be used to pursue groundbreaking high-frequency, high-efficiency, and high-power-density power conversion. The switching performance of cascode GaN HEMT is studied at first. An accurate behavior-level simulation model is developed with comprehensive consideration of the impacts of parasitics. Then based on the simulation model, detailed loss breakdown and loss mechanism analysis are studied. The cascode GaN HEMT has high turn-on loss due to the reverse recovery charge and junction capacitor charge, and the common source inductance (CSI) of the package; while the turn-off loss is extremely small attributing to unique current source turn off mechanism of the cascode structure. With this unique feature, the critical conduction mode (CRM) soft switching technique is applied to reduce the dominant turn on loss and significantly increase converter efficiency. The switching frequency is successfully pushed to 5MHz while maintaining high efficiency and good thermal performance. Traditional packaging method is becoming a bottle neck to fully utilize the advantages of GaN HEMT. So an investigation of the package influence on the cascode GaN HEMT is also conducted. Several critical parasitic inductance are identified, which cause high turn on loss and high parasitic ringing that may lead to device failure. To solve the issue, the stack-die package is proposed to eliminate all critical parasitic inductance, and as a result, reducing turn on loss by half and avoiding potential failure mode of the cascode GaN device effectively. Utilizing soft switching and enhanced packaging, a GaN-based MHz totem-pole PFC rectifier is demonstrated with 99% peak efficiency and 700 W/in3 power density. The switching frequency of the PFC is more than ten times higher than the state-of-the-art industry product while it achieves best possible efficiency and power density. Integrated power module and integrated PCB winding coupled inductor are all studied and applied in this PFC. Furthermore, the technology of soft switching totem-pole PFC is extended to a bidirectional rectifier/inverter design. By using SiC MOSFETs, both operating voltage and power are dramatically increased so that it is successfully applied into a bidirectional on-board charger (OBC) which achieves significantly improved efficiency and power density comparing to the best of industrial practice. In addition, a novel 2-stage system architecture and control strategy are proposed and demonstrated in the OBC system. As a continued extension, the critical mode based soft switching rectifier/inverter technology is applied to three-phase AC/DC converter. The inherent drawback of critical mode due to variable frequency operation is overcome by the proposed new modulation method with the idea of frequency synchronization. It is the first time that a critical mode based modulation is demonstrated in the most conventional three phase H-bridge AC/DC converter, and with 99% plus efficiency at above 300 kHz switching frequency. / Ph. D.

Reaction sintered silicon nitride (RSSN) was studied as a substitute coating material on the carbon-carbon material (RCC) presently used as a heat shield on the space shuttle, and on advanced carbon-carbon (ACC), a later development. On RCC, RSSN showed potential in a 538°C (1000°F) screening test in which silicon carbide coated material exhibits its highest oxidation rate; RSSN afforded less protection to ACC because of a larger thermal expansion mismatch. Organosilicon densification and metallic silicon sealing methods were studied as means of further increasing the oxidation resistance of the coating, and some improvement was noted when these methods were employed. / Master of Science

High surface area tungsten oxynitride samples have been prepared by the temperature programmed reaction (TPR) of W0₃ with NH₃. All samples were characterized by X-ray diffraction (XRD), nitrogen physisorption, and CO chemisorption. In addition, some sample compositions were determined by elemental analysis. Samples were prepared at various heating rates (β), allowing a Redhead analysis to be carried out giving an activation energy of nitridation of 109 kJ mol⁻¹. A heating rate of 0.016 K S⁻¹ gave optimum synthesis conditions. Solid state intermediates were studied by interruption of the temperature program at various stages. No distinct suboxide phases formed along the synthesis path were found using XRD. An increase in surface area, CO uptake and nitrogen weight content, were found to occur as the reaction progressed. Reactivity experiments showed reasonable hydro deoxygenation (HDO) and hydrodenitrogenation (HDN) activities, but little hydrogenation (HYD) or hydrodesulfurization (HDS) activities. / Master of Science

Silicon nitride (Si₃N₄) has the potential for use in various high-performance applications. However, surface defects such as voids/pits are commonly present on as processed Si₃N₄. When subjected to external forces, fracture originates at such flaws. To reduce or eliminate surface flaws, machining operations are required which constitute a major proportion of production costs. In order to offer an inexpensive alternative to machining and also to enhance the room temperature flexural strength of as fired Si₃N₄, low expansion glaze coatings of lithium aluminosilicate (LAS) and magnesium aluminosilicate (MAS) compositions were developed. Homogeneous and crack-free glaze coatings were successfully formed on as processed Si₃N₄. This ensured formation of compressive surface stresses on the as fired Si₃N₄ which, in turn, led to the reduction of the effects of surface flaws. When compared to the uncoated as fired Si₃N₄, both the glaze coatings helped achieve greater flexural strength. Analyses of the two glazes indicated better strength for the MAS coating compared to the LAS. Wear tests revealed that the MAS glaze exhibited higher wear resistance than the LAS glaze. These differences were attributed to the ability of the magnesium aluminosilicate glaze to achieve greater surface smoothness and better adherence to the substrate than the lithium aluminosilicate. / Master of Science

Nitride-based semiconductor materials make it possible to fabricate optoelectronic devices that operate in the whole electromagnetic range, since the band gaps of these compounds can be modified by doping. Among these materials, the sp2-hybridized boron nitride has properties that make it a potential candidate for integration in devices operating in the short-wavelength limit, under harsh environment conditions, due to the strength of the B-N bond.  Nevertheless, this binary compound has been the less studied material among the nitrides, due to the lack of complete control on the growth process. This thesis is focused on the study of the optical properties of sp2-hybridized boron nitride grown by hot-wall chemical vapor deposition (CVD) method, at the Department of Physics, Chemistry and Biology, at Linköping University, Sweden. The samples received for this study were grown on c-plane aluminum nitride as the buffer layer, which in turn was grown by nitridation on c- plane oriented sapphire, as the substrate material. The first objective of the research presented in this thesis was the development of a suitable ellipsometry model in a spectral region ranging from the infrared to the ultraviolet zones of the electromagnetic spectrum, with the aim of obtaining in the process optical properties such as the index of refraction, the energy of the fundamental electronic interband transition, the frequencies for the optical vibrational modes of the crystal lattice, as well as their broadenings, and the numerical values of the dielectric constants; and on the other hand, structural parameters such as the layers thicknesses, and examine the possibility of the presence of roughness or porosity on the boron nitride layer, which may affect the optical properties, by incorporating their effects into the model. The determination of these parameters, and their relation with the growth process, is important for the future adequate design of heterostructure-based devices that incorporate this material. In particular, emphasis has been put on the modeling of the polar lattice resonance contributions, with the TO- LO model, by using infrared spectroscopic ellipsometry as the characterization technique to study the phonon behavior, in the aforementioned spectral region, of the boron nitride. On the other hand, spectroscopic ellipsometry in the visible-ultraviolet spectral range was used to study the behavior of the material, by combining a Cauchy model, including an Urbach tail for the absorption edge, and a Lorentz oscillator in order to account for the absorption in the material in the UV zone. This first step on the research project was carried out at Linköping University.  The second objective in the research project was to carry out additional studies on the samples received, in order to complement the information provided by the ellipsometry model and to improve the model itself, provided that it was possible. The characterization techniques used were X-ray diffraction, which made it possible to confirm that in fact boron nitride was present in the samples studied, and made it possible to verify the crystalline quality of the aforementioned samples, and in turn relate it to the quality of the ellipsometry spectra previously obtained; the Raman spectroscopy made it possible to further verify and compare the crystalline qualities of the samples received, as well as to obtain the frequency for the Raman active B-N stretching vibration in the basal plane, and to compare this value with that corresponding to the bulk sp2-boron nitride; scanning electron microscopy made it possible to observe the rough surface morphologies of the samples and thus relate them to some of the conclusions derived from the ellipsometry model; and finally cathodoluminescence measurements carried out at low temperature (4 K) allowed to obtain a broad band emission, on all the samples studied, which could be related to native defects inside the boron nitride layers, i.e., boron vacancies. Nevertheless, no trace of a free carrier recombination was observed. Considering that the hexagonal-boron nitride is nowadays considered to be a direct band gap semiconductor, it may be indirectly concluded, in principle, that the dominant phase present in the samples studied was the rhombohedral polytype. Moreover, it can be tentatively concluded that the lack of an observable interband recombination may be due to the indirect band gap nature of the rhombohedral phase of the boron nitride. Spectroscopic ellipsometry does not give a definite answer regarding this issue either, because the samples analyzed were crystalline by nature, thus not being possible to use mathematical expressions for the dielectric function models that incorporate the band gap value as a fitting parameter. Therefore, the nature of the band gap emission in the rhombohedral phase of the boron nitride is still an open research question. On the other hand, luminescent emissions originating from radiative excitonic recombinations were not observed in the cathodoluminescence spectra. This second step of the project was carried out at the Leroy Eyring Center for Solid State Science at Arizona State University.

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.

abstract: Wide bandgap (WBG) semiconductors GaN (3.4 eV), Ga2O3 (4.8 eV) and AlN (6.2 eV), have gained considerable interests for energy-efficient optoelectronic and electronic applications in solid-state lighting, photovoltaics, power conversion, and so on. They can offer unique device performance compared with traditional semiconductors such as Si. Efficient GaN based light-emitting diodes (LEDs) have increasingly displaced incandescent and fluorescent bulbs as the new major light sources for lighting and display. In addition, due to their large bandgap and high critical electrical field, WBG semiconductors are also ideal candidates for efficient power conversion. In this dissertation, two types of devices are demonstrated: optoelectronic and electronic devices. Commercial polar c-plane LEDs suffer from reduced efficiency with increasing current densities, knowns as “efficiency droop”, while nonpolar/semipolar LEDs exhibit a very low efficiency droop. A modified ABC model with weak phase space filling effects is proposed to explain the low droop performance, providing insights for designing droop-free LEDs. The other emerging optoelectronics is nonpolar/semipolar III-nitride intersubband transition (ISBT) based photodetectors in terahertz and far infrared regime due to the large optical phonon energy and band offset, and the potential of room-temperature operation. ISBT properties are systematically studied for devices with different structures parameters. In terms of electronic devices, vertical GaN p-n diodes and Schottky barrier diodes (SBDs) with high breakdown voltages are homoepitaxially grown on GaN bulk substrates with much reduced defect densities and improved device performance. The advantages of the vertical structure over the lateral structure are multifold: smaller chip area, larger current, less sensitivity to surface states, better scalability, and smaller current dispersion. Three methods are proposed to boost the device performances: thick buffer layer design, hydrogen-plasma based edge termination technique, and multiple drift layer design. In addition, newly emerged Ga2O3 and AlN power electronics may outperform GaN devices. Because of the highly anisotropic crystal structure of Ga2O3, anisotropic electrical properties have been observed in Ga2O3 electronics. The first 1-kV-class AlN SBDs are demonstrated on cost-effective sapphire substrates. Several future topics are also proposed including selective-area doping in GaN power devices, vertical AlN power devices, and (Al,Ga,In)2O3 materials and devices. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

<p>The development and implementation of a metal gate technology (alloy, compound, or silicide) into metal-oxide-semiconductor field effect transistors (MOSFETs) is necessary to extend the life of planar CMOS devices and enable further downscaling. This thesis examines possible metal gate materials for improving the performance of the gate stack and discusses process integration as well as improved electrical and physical measurement methodologies, tested on capacitor structures and transistors. </p><p>By using reactive PVD and gradually increasing the N₂/Ar flow ratio, it was found that the work function (on SiO₂) of the TiN_x and ZrN_x metal systems could be modulated ~0.7 eV from low near nMOS work functions to high pMOS work functions. After high-temperature anneals corresponding to junction activation, both metals systems reached mid-gap work function values. The mechanisms behind the work function changes are explained with XPS data and discussed in terms of metal gradients and Fermi level pinning due to extrinsic interface states.</p><p>A modified scheme for improved Fowler-Nordheim tunnelling is also shown, using degenerately doped silicon substrates. In that case, the work functions of ALD/PVD TaN were accurately determined on both SiO₂ and HfO₂ and benchmarked against IPE (Internal Photoemission) results. KFM (Kelvin Force Microscopy) was also used to physically measure the work functions of PVD TiN and Mo deposited on SiO₂; the results agreed well with <i>C-V</i> and <i>I-V</i> data.</p><p>Finally, an appealing combination of novel materials is demonstrated with ALD TiN/Al₂O₃/HfAlO_x/Al₂O₃/strained-SiGe surface channel pMOS devices. The drive current and transconductance were measured to be 30% higher than the Si reference, clearly demonstrating increased mobility and the absence of polydepletion. Finally, using similarly processed transistors with Al₂O₃ dielectric instead, low-temperature water vapour annealing was shown to improve the device characteristics by reducing the negative charge within the ALD Al₂O₃.</p>

We present the results of GaInNAs/GaAs quantum dot structures with GaAsN barrier layers grown by solid source molecular beam epitaxy. Extension of the emission wavelength of GaInNAs quantum dots by ~170nm was observed in samples with GaAsN barriers in place of GaAs. However, optimization of the GaAsN barrier layer thickness is necessary to avoid degradation in luminescence intensity and structural property of the GaInNAs dots. Lasers with GaInNAs quantum dots as active layer were fabricated and room-temperature continuous-wave lasing was observed for the first time. Lasing occurs via the ground state at ~1.2μm, with threshold current density of 2.1kA/cm[superscript 2] and maximum output power of 16mW. These results are significantly better than previously reported values for this quantum-dot system. / Singapore-MIT Alliance (SMA)

The development and implementation of a metal gate technology (alloy, compound, or silicide) into metal-oxide-semiconductor field effect transistors (MOSFETs) is necessary to extend the life of planar CMOS devices and enable further downscaling. This thesis examines possible metal gate materials for improving the performance of the gate stack and discusses process integration as well as improved electrical and physical measurement methodologies, tested on capacitor structures and transistors. By using reactive PVD and gradually increasing the N2/Ar flow ratio, it was found that the work function (on SiO2) of the TiNx and ZrNx metal systems could be modulated ~0.7 eV from low near nMOS work functions to high pMOS work functions. After high-temperature anneals corresponding to junction activation, both metals systems reached mid-gap work function values. The mechanisms behind the work function changes are explained with XPS data and discussed in terms of metal gradients and Fermi level pinning due to extrinsic interface states. A modified scheme for improved Fowler-Nordheim tunnelling is also shown, using degenerately doped silicon substrates. In that case, the work functions of ALD/PVD TaN were accurately determined on both SiO2 and HfO2 and benchmarked against IPE (Internal Photoemission) results. KFM (Kelvin Force Microscopy) was also used to physically measure the work functions of PVD TiN and Mo deposited on SiO2; the results agreed well with C-V and I-V data. Finally, an appealing combination of novel materials is demonstrated with ALD TiN/Al2O3/HfAlOx/Al2O3/strained-SiGe surface channel pMOS devices. The drive current and transconductance were measured to be 30% higher than the Si reference, clearly demonstrating increased mobility and the absence of polydepletion. Finally, using similarly processed transistors with Al2O3 dielectric instead, low-temperature water vapour annealing was shown to improve the device characteristics by reducing the negative charge within the ALD Al2O3.