The assistance of thin film deposition with low-energy ion bombardment influences their final properties significantly. Especially, the application of so-called hyperthermal ions (energy <100 eV) is capable to modify the characteristics of the growing film without generating a large number of irradiation induced defects. The nitrogen ion beam assisted molecular beam epitaxy (ion energy <25 eV) is used to deposit GaN thin films on (0001)-oriented 6H-SiC substrates at 700 C. The films are studied in situ by reflection high energy electron diffraction, ex situ by X-ray diffraction, scanning tunnelling microscopy, and high-resolution transmission electron microscopy. It is demonstrated that the film growth mode can be controlled by varying the ion to atom ratio, where 2D films are characterized by a smooth topography, a high crystalline quality, low biaxial stress, and low defect density. Typical structural defects in the GaN thin films were identified as basal plane stacking faults, low-angle grain boundaries forming between w-GaN and z-GaN and twin boundaries. The misfit strain between the GaN thin films and substrates is relieved by the generation of edge dislocations in the first and second monolayers of GaN thin films and of misfit interfacial dislocations. It can be demonstrated that the low-energy nitrogen ion assisted molecular beam epitaxy is a technique to produce thin GaN films of high crystalline quality.

Power device robustness refers to the device capability of withstanding abnormal events in power electronics applications, which is one of the key device capabilities that are desired in numerous applications. While the current robustness test methods and qualification standards are developed across the 70 years of Silicon (Si) device history, their applicability to the recent wide bandgap (WBG) power devices is questionable. While the market of WBG power devices has exceeded $1 billion and is fast growing, there are many knowledge gaps regarding their robustness, including the failure or degradation physics, testing methods, and lifetime extraction. This dissertation work studies the robustness of Gallium Nitride (GaN) power device. The structures of many GaN power devices are fundamentally different from Si or Silicon Carbide (SiC) power devices, leading to numerous open questions on GaN power device robustness. Based on the device structure, this dissertation is divided into two parts: The first half discusses the robustness of lateral GaN high electron mobility transistor (HEMT), which recently sees rapid adoption among wide range of applications such as the power adapter and chargers, data center, and photovoltaic panels. The absence of p-n junction between the source and drain of GaN HEMT results in the lack of avalanche mechanism. This raises a concern on the device capability of withstanding surge-energy or overvoltage stress, which hinders the penetration of GaN HEMTs in broader applications. To address this concern, the study begins with conducting the single-event unclamped inductive switching (UIS) test on two mainstream commercial p-gate GaN HEMTs with the Ohmic- and Schottky-type gate contacts, where the GaN HEMT is found to withstand surge energy through a resonant energy transfer between the device capacitance and the loop inductance. The failure mechanism is identified to be a pure electrical breakdown determined by device transient breakdown voltage (BV). The BV of GaN HEMT is further found to be "dynamic" from the switching tests with various pulse widths and frequencies, which is further explained by the time-dependent buffer trapping. This dynamic BV (BVDYN) phenomenon indicates that the static or single-pulse test may not reveal the true BV of GaN HEMT in high frequency switching applications. To address this gap, a novel testbed based on a zero-voltage-switching converter with an active clamping circuit is developed to enable the stable switching with kilovolt overvoltage and megahertz frequency. The overvoltage failure boundaries and failure mechanisms of four commercial p-gate GaN HEMTs from multiple vendors are explored. In addition to the frequency-dependent BVDYN, two new failure mechanisms are observed in some devices, which are attributable to the serious carrier trapping in GaN HEMTs under the high-frequency overvoltage switching. At last, based on the findings in the high frequency overvoltage test (HFOT), a physics-based lifetime model for commercial GaN HEMTs utilizing the device on resistance (RON) shift is established and validated by experimental results. Overall, the switching-based test methodology and experimental results provide critical references for the overvoltage protection and qualification of GaN power HEMTs. The second half of the dissertation discusses the robustness of the vertical GaN fin-channel junction field effect transistor (Fin-JFET), a promising pre-commercialized GaN power device with the p-n junction embedded between the gate and drain which enables the avalanche breakdown. The robustness study on GaN JFET follows similar test approaches as Si metal-oxide-semiconductor field-effect transistor (MOSFET) with two key interests: the avalanche and short circuit capabilities. The avalanche breakdown is first explored via the single-event and repetitive UIS tests and under various gate drivers, from which an interesting "avalanche-through-fin-channel" mechanism is discovered. By leveraging this avalanche path, the electro-thermal stress migrates from the main blocking p-n junction to the n-GaN fin channel, resulting in a very favorable failure-to-open-circuit signature. The single-pulse critical avalanche energy density (EAVA) of vertical GaN Fin-JFET is measured to be as high as 10 J/cm2, which is much higher than the Si MOSFET and comparable to the SiC MOSFET. The short circuit capability is explored utilizing the hard-switching fault on the 650-V rated GaN Fin-JFET, with a gate driving circuit identical to the switching application to best mimic device operation in converters. The short circuit withstanding time is measured to be 30.5 µs at an input voltage of 400 V, 17.0 µs at 600 V, and 11.6 µs at 800 V, all among the longest reported for 600-700 V normally-off transistors. In addition, the failure-to-open-circuit signature is also shown in the single-event and repetitive short circuit tests; all devices retain the avalanche breakdown after failure, which is highly desirable for system applications. These results suggest that, while GaN HEMT is already available in market, vertical GaN Fin-JFET shows superior avalanche and short-circuit robustness and thereby can unlock great potential of GaN devices for applications like automotive powertrains, motor drives, and grids. / Doctor of Philosophy / In recent years, many power electronics applications such as data centers and electric vehicles have witnessed a rapid increase in the adoption of wide bandgap (WBG) power devices. The Gallium Nitride (GaN) device is one of the most attractive candidates in WBG devices, owing to its good tradeoff between breakdown voltage and on resistance, as well as the small gate charge that enables high frequency switching. For power devices, their robustness against overvoltage and overcurrent stresses is as important as their performance under normal operations. However, the new material, new device structure, and new device physics in GaN power devices brought up many open knowledge gaps in their robustness study, particularly under the dynamic operation in switching circuits. This dissertation presents the work in exploring the robustness of GaN power devices. Based on the device structure, the discussion is divided in two parts: The first half of the dissertation focuses on the overvoltage robustness of the lateral GaN High Electron Mobility Transistor (HEMT), the commercially available device covering 30 to 900 V voltage classes. A key feature of this device is the lack of p-n junction between source and drain, leading to an absence of avalanche capability. The study is conducted on mainstream, commercial p-gate GaN HEMTs, with a combination of circuit testing, microscale failure analysis, and physics-based device simulation. The main contribution is on three aspects: identifying the single-event and high-frequency repetitive overvoltage boundaries of GaN HEMT, unveiling the failure and degradation mechanisms under transient overvoltage conditions, and providing guidelines to GaN HEMT device users with proper robustness test methodology for device qualification and screening. The second half of the dissertation focuses on the robustness of vertical GaN fin-channel junction field effect transistor (Fin-JFET), a promising pre-commercial GaN power device with the p-n junction implemented between the source and drain. The robustness tests follow the classic approaches deployed for Silicon power devices, where both the avalanche and short circuit capabilities are investigated. From the single-event and repetitive test results, the GaN JFET shows excellent avalanche robustness with a desirable failure-to-open-circuit behavior, as well as a critical avalanche energy (EAVA) of 10 J/cm2 that is higher than the Silicon metal-oxide-semiconductor field-effect transistor (MOSFET) and comparable to the Silicon Carbide MOSFET. For a 650-V rated GaN Fin-JFET, a record high 30.5 μs short circuit time is demonstrated under the hard-switching fault condition at 400 V input voltage. Overall, the results show great potential of GaN power devices for the power electronics applications that involve more stressful operation conditions for devices.

Gallium nitride (GaN) based high electron mobility transistors (HEMTs) are promising for power electronic applications due to their high breakdown voltage and power efficiency compared to Si-based power devices. As known, the design of the HEMT has high impact on the performance of the devices. In this project various GaN HEMTs on SiC substrate with different design configurations are characterized and investigated. These HEMTs are designed and fabricated by the Research Institutes of Sweden (RISE). The important properties of the HEMTs such as contact resistance, current density, capacitance, and breakdown voltage are characterized and emphasized. The uniformity of the contact resistance of the devices located across a 4’’ wafer is investigated, which reveals the lowest contact resistance of 4.3Ω·mm at the center of the wafer. The highest maximum current density of the devices is 1.15A/mm, and the maximum current scales with the gate dimensions of the devices. The gate capacitance of the devices is between 0.1 and 0.6pF under 1MHz. The gate insulation breakdown voltage of the devices is above 40V and the drain to source breakdown voltage is higher than 360V. Based on the results, discussions about the effects of the designs on the device performance are provided. Suggestions for further improvement of the device performance are given. / Galliumnitrid (GaN) baserade högelektronmobilitetstransistorer (HEMTs) är lovande för kraftelektroniska applikationer på grund av deras höga nedbrytningsspänning och effektivitet jämfört med Si-baserade kraftenheter. Som känt har designen av HEMT stor inverkan på enheternas prestanda. I detta projekt karakteriseras och undersöks olika GaN HEMTs på SiC-substrat med olika designkonfigurationer. Dessa HEMTs är designade och tillverkade av Sveriges forskningsins titut (RISE). De viktiga egenskaperna hos HEMTs såsom kontaktmotstånd, strömtäthet, kapacitans och nedbrytningsspänning karakteriseras och betonas. Enhetligheten i kontaktmotståndet för enheterna som är placerade över en 4'' skiva undersöks, vilket avslöjar det lägsta kontaktmotståndet på 4.3 Ω·mm i mitten av skivan. Den högsta maximala strömtätheten för enheterna är 1.15A/mm, och den maximala strömskalan med enheternas grindmått. Portens kapacitans för enheterna är mellan 0.1 och 0.6pF under 1MHz. Enhetsspänningen för grindisoleringen för enheterna är över 40V och avloppsspänningen till källan är högre än 360V. Baserat på resultaten ges diskussioner om designens effekter på enhetens prestanda. Förslag för ytterligare förbättring av enhetens prestanda ges.

Silicon photonics (SiP) has evolved into a mature platform for cost-effective low power compact integrated photonic microsystems for many applications. There is a looming capacity crunch for telecommunications infrastructure to overcome the data-hungry future, driven by streaming and the exponential increase in data traffic from consumer-driven products. To increase data capacity, researchers are now looking at the wavelength window of the thulium-doped fiber amplifier (TDFA), centered near 2 µm as an attractive new transmission window for optical communications, motivated by the demonstrations of low loss, low nonlinearity, and high bandwidth transmission. Large-scale implementation of SiP telecommunication infrastructure will require light sources (lasers) and amplifiers to generate signals and boost transmitted and/or received signals, respectively. Silicon (Si) and silicon nitride (Si3N4) have become the leading photonic integrated circuit (PIC) material platforms, due to their low-cost and wafer-scale production of high-performance circuits. Silicon does however have a number of limitations as a photonic material, including that it is not an ideal light-emitting/amplifying material. This proposed research pertains to the fabrication of on-chip silicon and silicon nitride lasers and amplifiers to be used in a newly accessible optical communications window of the TDFA band, which is a significant step towards compact PICs for the telecommunication networks. Tellurium oxide (TeO2) is an interesting host material due to its large linear and non-linear refractive indices, low material losses and large rare-earth dopant solubility showing good performance for compact low-loss waveguides and on-chip light sources and amplifiers. Chapter 1 provides an overview of silicon photonics in the context of particularly rare earth lasers and amplifiers, operating at extended wavelengths enabled by the Thulium doped fiber amplifier. Chapter 2 presents a theoretical performance of waveguides and microresonators as the efficient structure for laser and amplifiers applications designed for optimized use in Erbium and Thulium doped fiber amplifier wavelength bands. Then spectroscopic study thulium (Tm3+) has been studied as the rare earth element for Thulium doped fiber amplifier wavelength bands. Chapter 3 presents an experimental study of TeO2:Tm3+ coated Si3N4 waveguide amplifiers with internal net gains of up to 15 dB total in a 5-cm long spiral waveguide. Chapter 4 provides a study of TeO2:Tm3+ -coated Si3N4 waveguide lasers with up to 16 mW double-sided on-chip output power. Chapter 5 presents an experimental study of low loss and high-quality factor silicon microring resonators coated with TeO2 for active, passive, and nonlinear applications. Chapter 6 represents the first demonstration of an integrated rare-earth silicon laser, with high performance, including single-mode emission, a lasing threshold of 4 mW, and bidirectional on-chip output powers of around 1 mW. Further results with a different design are presented showing lasers with more than 2 mW of double-sided on-chip output power, threshold pump powers of < 1 mW and lasing at wavelengths over a range of > 100 nm. Importantly, a simple, low-cost design was used which is compatible with silicon photonics foundry processes and enables wafer scale integration of such lasers in SiP PICs using robust materials. Chapter 7 summarizes the thesis and provides paths for future work. / Dissertation / Doctor of Engineering (DEng)

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