No description available.

No description available.

Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is larger than that of Silicon (Si), Silicon Carbide (SiC), and Gallium Nitride (GaN). A benefit of this ultra-wide bandgap is the high-temperature stability due to the low intrinsic carrier concentration. Another benefit is the high critical electric field (Ec), which is estimated to be from 6 MV/cm to 8 MV/cm in Ga2O3. This allows for a superior Baliga's figure of merit (BFOM) of unipolar Ga2O3 power devices, i.e., they potentially can achieve a smaller specific on-resistance (RON,SP) as compared to the Si, SiC, and GaN devices with the same breakdown voltage (BV). The above prospects make Ga2O3 devices the promising candidates for next-generation power electronics. This dissertation explores the design, fabrication, characterization, and packaging of vertical β-Ga2O3 Schottky barrier diodes (SBDs) and P-N diodes. The power SBDs allow for a small forward voltage and a fast switching speed; thus, it is ubiquitously utilized in power electronics systems. Meanwhile, the Ga2O3 power P-N diodes have the benefit of smaller leakage current, and the diode structure could be a building block for many advanced diodes and transistors. Hence, the study of Ga2O3 Schottky and P-N diodes is expected to provide the foundation for developing a series of Ga2O3 power devices. Firstly, vertical Ga2O3 Schottky and P-N diodes with a novel edge termination (ET), the multi-layer Nickel Oxide (NiO) junction termination extension (JTE), are fabricated on Ga2O3 substrates. This multi-JTE NiO structure decreases the peak electric field (Epeak) at the triple point of device edge when the Ga2O3 diodes are reversely biased. For SBDs, BV reach 2.5 kV, the 1-D junction field reaches 3.08 MV/cm, and the BFOM exceeds 1 GW/cm2. For P-N diodes, BV reaches 3.3 kV, the junction field reaches 4.2 MV/cm, and the BFOM reaches 2.6 GW/cm2. These results are among the highest in Ga2O3 power devices and are comparable to the state-of-the-art vertical GaN Schottky and P-N diodes. Notably, all these diodes are small-area devices. Secondly, large-area (3 mm×3 mm anode size) Ga2O3 Schottky and P-N diodes with high current capability are fabricated to explore the packaging, thermal management, and switching characteristics of Ga2O3 diodes. The same ET is applied for the large-area P-N diode. The fabricated large-area P-N diodes have a turn-on voltage of 2 V, a differential on-resistance (Ron) of 0.2 Ω, and they can reach at least 15 A when measured in the pulse mode. The BV of large-area Ga2O3 P-N diodes varies due to the fabrication non-uniformity, but the best device achieves a BV of 1.6 kV, standing among the highest values reported for large-area Ga2O3 diodes. Also, the large-area Ga2O3 SBDs with similar current rating but with a FP ET are fabricated mainly for the packaging and thermal management studies. Thirdly, medium-area Ga2O3 P-N diodes with a current over 1 A and a higher yield of BV are fabricated to evaluate the JTE's capacitance and switching characteristics. The JTE accounts for only ~11% of the junction capacitance of this 1 A diode, and the percentage is expected to be even smaller for higher-current diodes. The turn-on/off speed and reverse recovery time of the diode are comparable to commercial SiC Schottky barrier diodes under the on-wafer switching test. These results show the viability of NiO JTE for enabling a fast switching speed in high-voltage Ga2O3 power devices. Fourthly, the fabricated large-area Ga2O3 diodes are packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity (kT) and better reliability as compared to the solder joint. Due to the low kT of Ga2O3 material, junction-side-cooled (JSC) packaging configuration is necessary for Ga2O3 devices. For the packaged device, its junction-to-case thermal resistance (RθJC) is measured in the bottom-side-cooled (BSC) and junction-side-cooled (JSC) configuration by the transient dual interface method according to the JEDEC 51-14 standard. The RθJC of the junction- and bottom-cooled Ga2O3 SBD is measured to be 0.5 K/W and 1.43 K/W, respectively. The former RθJC is lower than that of similarly-rated commercial SiC SBDs. This manifests the significance of JSC packaging for the thermal management of Ga2O3 devices. Fifthly, to evaluate the electrothermal robustness of the packaged Ga2O3 devices, the surge current capability of JSC packaged Ga2O3 SBDs are measured. The Ga2O3 SBDs with proper packaging show high surge current capabilities. The double-side-cooled (DSC) large-area Ga2O3 SBDs can sustain a peak surge current over 60 A, with a ratio between the peak surge current and the rated current superior to that of similarly-rated commercial SiC SBDs. These results show the excellent ruggedness of Ga2O3 power devices. Finally, a Ga2O3 integrated diode module consisting of four single-diode sub-modules is designed and fabricated. For many power electronics applications, high current is desired; however, for emerging semiconductors, the current upscaling is difficult by directly increasing the device area because of the limitation of heat extraction capability and the limited material/processing yield. Here we explore the paralleling of multiple Ga2O3 P-N diodes to increase the current level. For each sub-module, the JSC packaging structure is used for heat extraction, and a metal post is sintered to the anode for electric field (E-field) management. RθJC is measured to be 1 W/K for each sub-module. On-board double-pulsed test is performed for both the sub-module and the full module. The sub-module and full module demonstrate 400 V, 10 A and 150 V, 70 A switching capabilities, respectively. This is the first demonstration of Ga2O3 power module and shows a promising approach to upscale of the power level of Ga2O3 power electronics. In addition to Ga2O3 device study, a research is conducted to explore the chip size (Achip) minimization for wide-bandgap (WBG) and ultra-wide bandgap (UWBG) power devices. Achip optimization is particularly critical for WBG and UWBG power devices and modules due to the high material cost. This work presents a new, holistic, electrothermal approach to optimize Achip for a given set of target specifications including BV, conduction current (I0), and switching frequency (f). The conduction and switching losses of the device are considered, as well as the heat dissipation in the chip and its package. For a given BV and I0, the optimal Achip, Wdr, and Ndr show strong dependence on f and thermal management. Our approach offers more accurate cost analysis and design guidelines for power modules. In summary, this dissertation covers the design, fabrication, characterization, and packaging of Ga2O3 Schottky and P-N diodes, with the aim to advance Ga2O3 devices to power electronics applications. This dissertation addresses many knowledge gaps on Ga2O3 devices, including the voltage upscaling (ET), current upscaling (large-area device fabrication, packaging, and thermal management), and their concurrence (module demonstration), as well as the circuit-level switching characterizations. / Doctor of Philosophy / Power electronics is the processing of electric energy using solid-state electronics. It is ubiquitously used in consumer electronics, data centers, electric vehicles, electricity grids, and renewable energy systems. Advanced power device technologies are paramount to improving the performance of power electronic systems. Power device design centers on the concurrent realization of low on-resistance (RON), high breakdown voltage (BV), and small turn-on/turn-off power losses. A key driver for advancement of power devices is the semiconductor material. Over the last decade, power devices based on wide-bandgap semiconductors like SiC and GaN have enabled tremendous performance advancements in power electronic systems. On the horizon, Ga2O3 is an emerging semiconductor with an ultra-wide bandgap (UWBG) of 4.5–4.9 eV, which is higher than that of Si, SiC, and GaN. Benefitted from this larger bandgap, the theoretical performance of Ga2O3 power devices is superior to the Si, SiC, and GaN counterparts. Hence, Ga2O3 devices are regarded as the promising candidates for next-generation power electronics. The power diode is an important component in power circuits, and the diode structure is usually a building block for power transistors. Small-area (0.1 A current level) Ga2O3 Schottky barrier diodes (SBDs) and P-N diodes are first designed and fabricated with a novel ET, the NiO JTE, reaching a high BV from 2.5 to 3.5 kV and junction E-field up to 4.2 MV/cm. Subsequently, large-area (>15 A current level) Ga2O3 diodes are fabricated with the same ET, achieving a BV of 1.6 kV, which is among the highest BV demonstrated in large-area Ga2O3 devices. In addition, on-wafer switching tests are performed on the medium-area (1 A current level) Ga2O3 P-N diodes, and their turn-on/off speed and reverse recovery time are comparable to commercial SiC Schottky barrier diodes. In addition to voltage upscaling, current upscaling is also a key challenge for Ga2O3 power devices. To overcome the low thermal conductivity (kT) of Ga2O3, junction side cooling (JSC) packaging is used to increase the heat extraction capability of Ga2O3 diode, enabling the demonstration of a junction-to-case thermal resistance comparable to that of similarly-rated, commercial SiC diode. Benefitted from this enhanced heat extraction, the packaged Ga2O3 diodes show an excellent surge current robustness. Finally, a Ga2O3 integrated diode module consisting of four single-diode sub-modules is designed, fabricated, and tested in the on-board switching circuits up to 70 A and 400 V. This is the first demonstration of a Ga2O3 power module. In summary, this dissertation covers the design, fabrication, characterization, and packaging of Ga2O3 power diodes with the aim to advance Ga2O3 devices to power electronics applications. This dissertation addresses many knowledge gaps on the voltage upscaling, current upscaling, and the circuit-level switching characteristics of Ga2O3 power devices and modules and thus pave the road for their power electronics applications.

No description available.

No description available.

No description available.

Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is higher than the bandgap of Silicon (Si), Silicon Carbide (SiC), and Gallium Nitride (GaN). A benefit of this wide-bandgap is the high critical electric field of Ga2O3, which is estimated to be from 5 MV/cm to 9 MV/cm. This allows a higher Baliga’s figure of merit (BFOM), i.e., unipolar Ga2O3 devices potentially possess a smaller specific on-resistance (Ron,sp) as compared to the Si, SiC, and GaN devices with the same breakdown voltage (BV). This prospect makes Ga2O3 devices promising candidates for next-generation power electronics. This thesis explores the design, fabrication, and packaging of vertical Ga2O3 Schottky barrier diodes (SBDs). The power SBD allows for a small forward voltage and a fast switching speed; thus, it is ubiquitously utilized in power electronics systems. It is also a building block for many advanced power transistors. Hence, the study of Ga2O3 SBDs is expected to pave the way for developing a series of Ga2O3 power devices. In this work, a vertical β-Ga2O3 SBD with a novel edge termination, which is the small-angle beveled field plate (SABFP), is fabricated on thinned Ga2O3 substrates. This SABFP structure decreases the peak electric field (Epeak) at the triple point when the Ga2O3 SBD is reverse biased, resulting in a BV of 1.1 kV and an Epeak of 3.5 MV/cm. This device demonstrates a BFOM of 0.6 GW/cm2, which is among the highest in β-Ga2O3 power devices and is comparable to the state-of-the-art vertical GaN SBDs. The high-temperature characteristics of Ga2O3 SBDs with a 45o beveled angle sidewall edge termination are studied at temperatures up to 600 K. As compared to the state-of-the-art SiC and GaN SBDs with a similar blocking voltage, the vertical Ga2O3 SBDs are capable of operating at higher temperatures and show a smaller leakage current increase with temperature. The leakage current mechanisms were also revealed at various temperatures and reverse biases. A new fabrication method of a dielectric field plate and Ga2O3 mesa of a medium angle (10o~30o) is achieved by controlling the adhesion between the photoresist (PR) and the dielectric surface. As compared to the small-angle termination, this medium-angle edge termination can allow a superior yield and uniformity in device fabrication, at the same time maintaining the major functionalities of beveled edge termination. Good surface morphology of the field plates and Ga2O3 mesa of the medium angle 10o~30o sidewall angle is verified by atomic force microscopy. Finally, large-area Ga2O3 SBDs are fabricated and packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity and better reliability as compared to the solder joint. The metal finish on the anode and cathode has been optimized for silver sintering. Large-area, packaged Ga2O3 SBDs with an anode size of 3×3 mm2 are prototyped. They show a forward current of over 5 A, a current on/off ratio of ~109, and a BV of 190 V. To the best of the author’s knowledge, this is the first experimental demonstration of a large-area, packaged Ga2O3 power device. / M.S. / Power electronics is the processing of electric energy using solid-state electronics. It is ubiquitously used in consumer electronics, data centers, electric vehicles, electricity grids, and renewable energy systems. Advanced power device technologies are paramount to improving the performance of power electronic systems. Power device design centers on the concurrent realization of low on-resistance (RON), high breakdown voltage (BV), and small turn-on/turn-off power losses. The performance of power devices hinges on semiconductor material properties. Over the last several years, power devices based on wide-bandgap semiconductors like Silicon Carbide (SiC) and Gallium Nitride (GaN) have enabled tremendous performance advancements in power electronic systems. Gallium Oxide (Ga2O3) is an ultra-wide bandgap semiconductor with a bandgap of 4.5–4.9 eV, which is higher than the bandgap of Silicon (Si), SiC, and GaN. As a benefit of this wide bandgap, the theoretical performance of Ga2O3 devices is superior to the Si, SiC, and GaN counterparts. Hence, Ga2O3 devices are regarded as promising candidates for next-generation power electronics. This thesis explores the design, fabrication, and packaging of vertical Ga2O3 Schottky barrier diodes (SBDs). The power SBD allows a small forward voltage and a fast switching speed; thus, it is extensively utilized in power electronics systems. It is also a building block for many advanced power transistors. First, a vertical β-Ga2O3 SBD with a novel edge termination is fabricated. This edge termination structure reduces the peak electric field (Epeak) in the device and enhances the BV. The fabricated device shows one of the highest figure of merits in β-Ga2O3 power devices. Next, the high-temperature characteristics of the fabricated Ga2O3 SBDs are studied at temperatures up to 600 K. The leakage current mechanisms were also revealed at various temperatures and reverse biases. Finally, large-area Ga2O3 SBDs are fabricated and packaged using silver sintering as the die attach. The sintered silver joint has higher thermal conductivity and better reliability as compared to the conventional solder joint. The packaged Ga2O3 SBDs show a forward current of over 5 A and a BV of 190 V. To the best of the author’s knowledge, this is the first experimental demonstration of a large-area, packaged Ga2O3 power device.

The global lighting source energy consumption accounts for about 22% of the total electricity generated. New high-efficiency solid-state light sources are needed to reduce the ever increasing demand for energy. Single-phased emitter-based composed of transparent conducting oxides (TCOs) nanocrystals and fluorescent dyes can potentially revolutionize the typical composition of phosphors, the processing technology founded on the binding of dye acceptors on the surface of nanocrystals, and the configurations of the light-emitting diodes (LEDs) and electroluminescence devices. The hybrid white-light-emitting nanomaterial is based on the expanded spectral range of the donor-acceptor pair (DAP) emission originated from the γ-gallium oxide nanocrystals via Förster resonance energy transfer (FRET) to the surface-anchored fluorescent dyes. The emission of the nanocrystals and the sensitized emission of the chromophore act in sync as an internal relaxation upon the excitation of the γ–gallium oxide nanocrystals. It extends the lifetime of the secondary fluorescent dye chromophore and the internal relaxation within this hybrid complex act as a sign for a quasi single chromophore. The model system of white-light-emitting nanostructure system developed based on this technology is the γ–gallium oxide nanocrystals-Rhodamine B lactone (RBL) hybrid complex. The sufficient energy transfer efficiency of 31.51% within this system allowed for the generation of white-light emission with the CIE coordinates of (0.3328, 0.3380) at 5483 K. The relative electronic energy differences of the individual components within the hybrid systems based on theoretical computation suggested that the luminance of the nanocomposite comprised of RBL is dominantly mediated by FRET. The production of white-light-emitting diode (WLED) based on this technology have been demonstrated by solution deposition of the hybrid nanomaterials to the commercially available ultraviolet (UV) LED due to the versatility and chemical compatibility of the developed phosphors.

The global lighting source energy consumption accounts for about 22% of the total electricity generated. New high-efficiency solid-state light sources are needed to reduce the ever increasing demand for energy. Single-phased emitter-based composed of transparent conducting oxides (TCOs) nanocrystals and fluorescent dyes can potentially revolutionize the typical composition of phosphors, the processing technology founded on the binding of dye acceptors on the surface of nanocrystals, and the configurations of the light-emitting diodes (LEDs) and electroluminescence devices. The hybrid white-light-emitting nanomaterial is based on the expanded spectral range of the donor-acceptor pair (DAP) emission originated from the γ-gallium oxide nanocrystals via Förster resonance energy transfer (FRET) to the surface-anchored fluorescent dyes. The emission of the nanocrystals and the sensitized emission of the chromophore act in sync as an internal relaxation upon the excitation of the γ–gallium oxide nanocrystals. It extends the lifetime of the secondary fluorescent dye chromophore and the internal relaxation within this hybrid complex act as a sign for a quasi single chromophore. The model system of white-light-emitting nanostructure system developed based on this technology is the γ–gallium oxide nanocrystals-Rhodamine B lactone (RBL) hybrid complex. The sufficient energy transfer efficiency of 31.51% within this system allowed for the generation of white-light emission with the CIE coordinates of (0.3328, 0.3380) at 5483 K. The relative electronic energy differences of the individual components within the hybrid systems based on theoretical computation suggested that the luminance of the nanocomposite comprised of RBL is dominantly mediated by FRET. The production of white-light-emitting diode (WLED) based on this technology have been demonstrated by solution deposition of the hybrid nanomaterials to the commercially available ultraviolet (UV) LED due to the versatility and chemical compatibility of the developed phosphors.

It is well known that nanostructures possess unique electronic, optical, magnetic, ferroelectric and piezoelectric properties that are often superior to traditional bulk materials. In particular, one dimensional (1D) nanostructured inorganic materials including nanofibres, nanotubes and nanobelts have attracted considerable attention due to their distinctive geometries, novel physical and chemical properties, combined effects and their applications to numerous areas. Metal ion doping is a promising technique which can be utilized to control the properties of materials by intentionally introducing impurities or defects into a material. γ-Alumina (Al2O3), is one of the most important oxides due to its high surface area, mesoporous properties, chemical and thermal properties and its broad applications in adsorbents, composite materials, ceramics, catalysts and catalyst supports. γ-Alumina has been studied intensively over a long period of time. Recently, considerable work has been carried out on the synthesis of 1D γ-alumina nanostructures under various hydrothermal conditions; however, research on the doping of alumina nanostructures has not been forthcoming. Boehmite (γ-AlOOH) is a crucial precursor for the preparation of γ-Alumina and the morphology and size of the resultant alumina can be manipulated by controlling the growth of AlOOH. Gallium (Ga) is in the same group in the periodic table as aluminum. β-Gallium (III) oxide (β-Ga2O3), a wide band gap semiconductor, has long been known to exhibit conduction, luminescence and catalytic properties. Numerous techniques have been employed on the synthesis of gallium oxide in the early research. However, these techniques are plagued by inevitable problems. It is of great interest to explore the synthesis of gallium oxide via a low temperature hydrothermal route, which is economically efficient and environmentally friendly. The overall objectives of this study were: 1) the investigation of the effect of dopants on the morphology, size and properties of metal ion doped 1D alumina nanostructures by introducing dopant to the AlOOH structure; 2) the investigation of impacts of hydrothermal conditions and surfactants on the crystal growth of gallium oxide nanostructures. To achieve the above objectives, trivalent metal elements such as iron, gallium and yttrium were employed as dopants in the study of doped alumina nanostructures. In addition, the effect of various parameters that may affect the growth of gallium oxide crystals including temperature, pH, and the experimental procedure as well as different types of surfactants were systematically investigated. The main contributions of this study are: 1) the systematic and in-depth investigation of the crystal growth and the morphology control of iron, gallium and yttrium doped boehmite (AlOOH) under varying hydrothermal conditions, as a result, a new soft-chemistry synthesis route for the preparation of one dimensional alumina/boehmite nanofibres and nanotubes was invented; 2) systematic investigation of the crystal growth and morphology and size changes of gallium oxide hydroxide (GaOOH) under varying hydrothermal conditions with and without surfactant at low temperature; We invented a green hydrothermal route for the preparation of α-GaOOH or β-GaOOH micro- to nano-scaled particles; invented a simple hydrothermal route for the direct preparation of γ-Ga2O3 from aqueous media at low temperature without any calcination. The study provided detailed synthesis routes as well as quantitative property data of final products which are necessary for their potential industrial applications in the future. The following are the main areas and findings presented in the study: • Fe doped boehmite nanostructures This work was undertaken at 120ºC using PEO surfactant through a hydrothermal synthesis route by adding fresh iron doped aluminium hydrate at regular intervals of 2 days. The effect of dopant iron, iron percentage and experimental procedure on the morphology and size of boehmite were systematically studied. Iron doped boehmite nanofibres were formed in all samples with iron contents no more than 10%. Nanosheets and nanotubes together with an iron rich phase were formed in 20% iron doped boehmite sample. A change in synthesis procedure resulted in the formation of hematite large crystals. The resultant nanomaterials were characterized by a combination of XRD, TEM, EDX, SAED and N2 adsorption analysis. • Growth of pure boehmite nanofibres/nanotubes The growth of pure boehmite nanofibres/nanotubes under different hydrothermal conditions at 100ºC with and without PEO surfactant was systematically studied to provide further information for the following studies of the growth of Ga and Y doped boehmite. Results showed that adding fresh aluminium hydrate precipitate in a regular interval resulted in the formation of a mixture of long and short 1D boehmite nanostructures rather than the formation of relatively longer nanofibres/nanotubes. The detailed discussion and mechanism on the growth of boehmite nanostructure were presented. The resultant boehmite samples were also characterized by N2 adsorption to provide further information on the surface properties to support the proposed mechanism. • Ga doped boehmite nanostructures Based on this study on the growth of pure boehmite nanofibre/nanotubes, gallium doped boehmite nanotubes were prepared via hydrothermal treatment at 100ºC in the presence of PEO surfactant without adding any fresh aluminium hydrate precipitate during the hydrothermal treatment. The effect of dopant gallium, gallium percentage, temperature and experimental procedure on the morphology and size of boehmite was systematically studied. Various morphologies of boehmite nanostructures were formed with the increase in the doping gallium content and the change in synthesis procedure. The resultant gallium doped boehmite nanostructures were characterized by TEM, XRD, EDX, SAED, N2 adsorption and TGA. • Y doped boehmite nanostructures Following the same synthesis route as that for gallium doped boehmite, yttrium doped boehmite nanostructures were prepared at 100ºC in the presence of PEO surfactant. From the study on iron and gallium doped boehmite nanostructures, it was noted both iron and gallium cannot grow with boehmite nanostructure if iron nitrate and gallium nitrate were not mixed with aluminium nitrate before dissolving in water, in particular, gallium and aluminium are 100% miscible. Therefore, it’s not necessary to study the mixing procedure or synthesis route on the formation of yttrium doped boehmite nanostructures in this work. The effect of dopant yttrium, yttrium percentage, temperature and surfactant on the morphology and size of boehmite were systematically studied. Nanofibres were formed in all samples with varying doped Y% treated at 100ºC; large Y(OH)3 crystals were also formed at high doping Y percentage. Treatment at elevated temperatures resulted in remarkable changes in size and morphology for samples with the same doping Y content. The resultant yttrium doped boehmite nanostructures were characterized by TEM, XRD, EDX, SAED, N2 adsorption and TGA. • The synthesis of Gallium oxide hydroxide and gallium oxide with surfactant In this study, the growth of gallium oxide hydroxide under various hydrothermal conditions in the presence of different types of surfactants was systematically studied. Nano- to micro-sized gallium oxide hydroxide was prepared. The effect of surfactant and synthesis procedure on the morphology of the resultant gallium oxide hydroxide was studied. β-gallium oxide nanorods were derived from gallium oxide hydroxide by calcination at 900ºC and the initial morphology was retained. γ-gallium oxide nanotubes up to 65 nm in length, with internal and external diameters of around 0.8 and 3.0 nm, were synthesized directly in solution with and without surfactant. The resultant nano- to micro-sized structures were characterized by XRD, TEM, SAED, EDX and N2 adsorption. • The synthesis of gallium oxide hydroxide without surfactant The aim of this study is to explore a green synthesis route for the preparation of gallium oxide hydroxide or gallium oxide via hydrothermal treatment at low temperature. Micro to nano sized GaOOH nanorods and particles were prepared under varying hydrothermal conditions without any surfactant. The resultant GaOOH nanomaterials were characterized by XRD, TEM, SAED, EDX, TG and FT-IR. The growth mechanism of GaOOH crystals was proposed.