This work establishes a solid foundation for the use of indirect band gap semiconductors for light emitting application and presents the work on development of white light emitting diodes (LEDs) in silicon carbide (SiC). Novel laser doping has been utilized to fabricate white light emitting diodes in 6H-SiC (n-type N) and 4H-SiC (p-type Al) wafers. The emission of different colors to ultimately generate white light is tailored on the basis of donor acceptor pair (DAP) recombination mechanism for luminescence. A Q-switched Nd:YAG pulse laser (1064 nm wavelength) was used to carry out the doping experiments. The p and n regions of the white SiC LED were fabricated by laser doping an n-type 6H-SiC and p-type 4H-SiC wafer substrates with respective dopants. Cr, B and Al were used as p-type dopants (acceptors) while N and Se were used as n-type dopants (donors). Deep and shallow donor and acceptor impurity level states formed by these dopants tailor the color properties for pure white light emission. The electromagnetic field of lasers and non-equilibrium doping conditions enable laser doping of SiC with increased dopant diffusivity and enhanced solid solubility. A thermal model is utilized to determine the laser doping parameters for temperature distribution at various depths of the wafer and a diffusion model is presented including the effects of Fick's diffusion, laser electromagnetic field and thermal stresses due to localized laser heating on the mass flux of dopant atoms. The dopant diffusivity is calculated as a function of temperature at different depths of the wafer based on measured dopant concentration profile. The maximum diffusivities achieved in this study are 4.61x10-10 cm2/s at 2898 K and 6.92x10-12 cm2/s at 3046 K for Cr in 6H-SiC and 4H-SiC respectively. Secondary ion mass spectrometric (SIMS) analysis showed the concentration profile of Cr in SiC having a penetration depth ranging from 80 nm in p-type 4H-SiC to 1.5 [micro]m in n-type 6H-SiC substrates respectively. The SIMS data revealed enhanced solid solubility (2.29x1019 cm-3 in 6H-SiC and 1.42x1919 cm-3 in 4H-SiC) beyond the equilibrium limit (3x1017 cm-3 in 6H-SiC above 2500 [degrees]C) for Cr in SiC. It also revealed similar effects for Al and N. The roughness, surface chemistry and crystalline integrity of the doped sample were examined by optical interferometer, energy dispersive X-ray spectrometry (EDS) and transmission electron microscopy (TEM) respectively. Inspite of the larger atomic size of Cr compared to Si and C, the non-equilibrium conditions during laser doping allow effective incorporation of dopant atoms into the SiC lattice without causing any damage to the surface or crystal lattice. Deep Level Transient Spectroscopy (DLTS) confirmed the deep level acceptor state of Cr with activation energies of Ev+0.80 eV in 4H-SiC and Ev+0.45 eV in 6H-SiC. The Hall Effect measurements showed the hole concentration to be 1.98x1019 cm-3 which is almost twice the average Cr concentration (1x1019 cm-3) obtained from the SIMS data. These data confirmed that almost all of the Cr atoms were completely activated to the double acceptor state by the laser doping process without requiring any subsequent annealing step. Electroluminescence studies showed blue (460-498 nm), blue-green (500-520 nm) green (521-575 nm), and orange (650-690 nm) wavelengths due to radiative recombination transitions between donor-acceptors pairs of N-Al, N-B, N-Cr and Cr-Al respectively, while a prominent violet (408 nm) wavelength was observed due to transitions from the nitrogen level to the valence band level. The red (698-738 nm) luminescence was mainly due to metastable mid-bandgap states, however under high injection current it was due to the quantum mechanical phenomenon pertaining to band broadening and overlapping. This RGB combination produced a broadband white light spectrum extending from 380 to 900 nm. The color space tri-stimulus values for 4H-SiC doped with Cr and N were X = 0.3322, Y = 0.3320 and Z = 0.3358 as per 1931 CIE (International Commission on Illumination) corresponding to a color rendering index of 96.56 and the color temperature of 5510 K. And for 6H-SiC n-type doped with Cr and Al, the color space tri-stimulus values are X = 0.3322, Y = 0.3320 and Z = 0.3358. The CCT was 5338 K, which is very close to the incandescent lamp (or black body) and lies between bright midday sun (5200 K) and average daylight (5500 K) while CRI was 98.32. Similar white LED's were also fabricated using Cr, Al, Se as one set of dopants and B, Al, N as another.

Silicon carbide has many properties that make it a promising and desirable material for diverse applications. One such application for silicon carbide wafers is as a transparent cryogenic probe card. This thesis briefly describes the design of a probe card based on a silicon carbide wafer substrate. It includes a description of electroplating fundamentals and demonstrates the feasibility of electroplating copper onto a wafer for the formation of bond pads between the substrate and external PCB ring. The process for electroplating copper with good adhesion and quality based on metal alloy formation, current control, and materials selection is outlined. Results of this process are also presented. This work also demonstrates the ability to machine silicon carbide using electrical discharge machining, abrasive water jet machining, and diamond bit milling, proving diamond grinding to be the most versatile of the described methods for machining intricate patterns into the wafers.

付記する学位プログラム名: 京都大学卓越大学院プログラム「先端光・電子デバイス創成学」 / 京都大学 / 新制・課程博士 / 博士(工学) / 甲第24623号 / 工博第5129号 / 新制||工||1980(附属図書館) / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 木本 恒暢, 教授 川上 養一, 准教授 浅野 卓 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This thesis presents the first use of a novel direct-write, non-line-of-sight, two-photon photoelectrochemical etching technique for etching of single crystal silicon carbide substrates. The use of this technique has resulted in structuring of 3-dimensional structures in high quality single crystal silicon carbide wafers. The 3-dimensional structures demonstrated cannot be formed by any single or combination of traditional silicon carbide machining techniques. This thesis outlines the development of the optical, electrical, and diagnostic components required to achieve two-photon photoelectrochemical etching in silicon carbide. The diagnostic sub-assemblies --a single pixel confocal detector assembly and an in-situ optical microscope assembly-- and their design is also discussed. Several etched structures using the two-photon photoelectrochemical etching technique are presented.

No description available.

Electric and more-electric vehicle proliferation continues unabated as government mandates worldwide demand fuel economies in excess of what conventional internal combustion engines are capable of. Vehicle electrification, to any degree, is perceived to be the means by which automotive companies may meet these targets. Electrification introduces a myriad of problems including cost, weight and reliability, all of which must be addressed in their own right. The rapid commercialisation of wide-bandgap semiconductor materials which, as a whole, exhibit properties superior to ubiquitous Silicon, provides the opportunity for power electronic converter minimisation and efficiency maximisation, easing the challenge of meeting current and incoming standards. This thesis concerns itself with the design methodology of a highly power dense converter, as applied to a three-phase inverter. By using figures of merit, simple modelling techniques and novel discrete component selection tools, a converter is designed that is capable of switching 30kW of electric power at 100kHz in a small package. Testing results show that the converter, with a simple forced air heatsinking solution, can effectively switch 9kW of power and is capable of reaching 15kW. Given the temperature rise of one phase leg of the inverter relative to the others, a superior heatsink design would allow the inverter to reach its rated power levels. / Dissertation / Master of Applied Science (MASc)

The development and implementation of silicon carbine (SiC) devices is steadily increasing facilitating the electrification of aircrafts. In this thesis, a printed circuit board (PCB) based heavy copper busbar design and verification are introduced for a SiC based 250 kW multiphase drive system operated at 40,000 ft. Finite-element analysis (FEA) simulation studies of the PCB busbar are conducted to optimize the electric field intensity. Busbar modeling technic is also discussed to derive the current distribution and extract the loss. The measured partial discharge inception voltage (PDIV), switching transients and converter-level validations are provided for insulation, thermal and commutation loop verifications. As the part of the inverter system, the integrated gate driver is designed with SPI communication to drive the wide bandgap SiC power modules. With feature of drain-to-source current sensing feature, the gate driver could also provide over-current protection to fast-switching SiC power modules. The converter level verification is performed under single, dual, and quadruple three-phase inverter system for aviation motor drive to evaluate the overall performance of the powertrain converter. The outcomes of this research contribute to the advancement of electric aircraft technology by leveraging the benefits of SiC devices and optimizing busbar design, providing valuable insights and guidelines for engineers and researchers involved in the development and optimization of power electronic systems for all-electric aircraft applications. / Master of Science / This research focuses on the design and verification of PCB (Printed Circuit Board) busbars for a multiphase SiC-based all-electric aircraft powertrain converter. Silicon Carbide (SiC) devices, known for their high efficiency and fast-switching capabilities, are used in the converter to enhance its performance. The goal is to develop an optimized busbar design that ensures efficient power distribution and minimizes energy losses in this advanced aviation powertrain system. The study explores different aspects of busbar insulation design and analyzes busbar current distribution and loss extraction using simulation and modeling techniques. Additionally, gate driver design and communication network are investigated to drive and protect the wide bandgap SiC devices and to ensure the overall performance of the powertrain converter. The converter level verification is also performed under single, dual, and quadruple three-phase inverter system for aviation motor drive. The findings of this research contribute to the advancement of electric aircraft technology, utilizing SiC devices and optimized busbar design, and provide valuable insights for engineers and researchers working on power electronic systems for electric aircraft applications.

<p>  </p> <p>The development of a high-temperature heat exchanger made of silicon carbide (SiC) required the development of processing and joining technologies for the fabrication and integration of a prototype. Traditional ceramic forming techniques such as dry powder compaction, tape casting, or injection molding cannot effectively process complex and micron-size parts such as those required by heat exchangers to generate high surface area for improved thermal efficiency. Ceramic co-extrusion has been a successful fabrication technique to produce small structures, ceramic piezoelectric, and fibrous monolithic.</p> <p><br></p> <p>The co-extrusion process is unique in its ability to create micron-size features in two dimensions through multiple reduction steps. Using this process, the heat exchanger channels are developed to create a section with a high surface area to enhance the heat transfer between fluids.</p> <p><br></p> <p>Ceramic co-extrusion requires the development of ceramic/polymer binder systems based on SiC powder, fugitive thermoplastic binders, and low molecular weight polymeric species as processing aids. The thermoplastic binders mixed with SiC powder provided molding and extrusion capabilities to build the heat exchanger prototype. Afterward, a binder removal process and sintering were performed to densify the final component. The presence of cracks is common when working with ceramic/polymer binder systems. Ten different SiC ceramic/polymer binder systems were developed and evaluated to understand the mechanisms that generate cracks and lower the mechanical strengths of components.</p> <p><br></p> <p>A SiC heat exchanger is comprised of a main core where the fluids exchange energy and the manifolds that direct both cold and hot fluids to the respective set of channels. The integration of these components is challenging because of the high degree of covalent bonding and low self-diffusivity of SiC. Welding and other integration methods common in metals are not feasible due to the high melting point of SiC (2730 °C). Reaction bonding is a technique that has displayed the potential to integrate SiC parts by recreating the reaction of silicon (Si) and carbon (C) on an interlayer between SiC components. This work presents the development of a pressureless joining technique for SiC by reaction bonding using SiC/C loaded ceramic suspensions and the methodology to create a successful bonding region between SiC components. The approaches studied varied the thickness in the joint region to study its mechanical strength, and crystalline structure.</p>

Film thickness, free carrier concentration and free carrier mobility are critical figures of merit for silicon carbide epitaxial growth. Room temperature Fourier Transform Infrared (FTIR) reflection spectroscopy can estimate these parameters non-destructively and is capable of high-resolution wafer mapping. Commercially available equipment has greatly simplified the application of this technique by coupling a high performance automated spectrometer with model-based data analysis and interpretation based on the personal computer. While powerful numerical techniques run fast and efficient on modern computers, it is essential that low-order, well-conditioned models are needed. The observed reflectance spectrum is the result of reflection and refraction of light at different interfaces due to constructive and destructive interference. The estimation of film thickness and free carrier concentration for single epitaxial layers has been improved by studying the Longitudinal Optical Phonon Plasmon (LPP) coupled modes. However, the addition of multiple layers introduces many degrees of freedom, which complicates parameter extraction. The multiple epitaxial layer stacks studied were intended for Metal Semiconductor Field Effect Transistor (MESFET?s) on both conducting and semi-insulating substrates. The thickness estimation of the n-channel in the MESFET stack on semi-insulating substrate is improved by preconditioning the curve fit for plasma frequency obtained from doping estimation from capacitance voltage profiling or by observing an LPP- peak.

Creation of semi-insulating layers in SiC is highly desirable for high voltage device fabrication. Specifically PiN diodes can be fabricated with a compensated semi-insulating layer that would be capable of blocking a large reverse voltage. Semi-insulating (SI) behavior in SiC has been traditionally achieved via passivation of shallow dopants with vanadium-related deep levels. Degraded electrical properties of SiC devices result from the use of vanadium compensated SiC because unintentional formation of additional defects due to vanadium segregation and stress generation in the material occur. In this work, the possibility of low doped or SI epilayers via engineering of the boron related defects in SiC is investigated. High temperature treatment (up to 2000°C) of boron doped samples is used to stimulate boron diffusion and formation of deep boron centers in concentration sufficient for compensation of shallow dopants, without simultaneous formation of undesirable shallow boron levels. High temperature annealing of both epitaxial layers in-situ doped with boron and boron implanted 4H-SiC is investigated. The possibility of diffusion from highly boron doped substrate is also investigated. The diffusion profiles are modeled and the diffusion coefficients extracted to give information about diffusion mechanisms. The boron D-center was observed using photoluminescence (PL) after high temperature annealing of the implanted samples. Clear temperature dependence of the creation of the D-center was observed. Compensated material was revealed after an Inductively Coupled Plasma (ICP) etch on an epi-over grown sample.