TCAD numerical simulations have been carried out to study the current-voltage, electrical breakdown, and self-heating characteristics of β-Ga2O3 based metal-oxide field effect transistors (MOSFETs). β-Ga2O3 semiconductor has an ultra-wide bandgap of ~ 4.8 eV, a theoretical critical breakdown field strength, Ec ~ 8 MV/cm, making it an excellent candidate for high-voltage or power electronics applications. The numerical simulations have been benchmarked against experimentally reported data. For modeling impact ionization, which is expected to induce intrinsic avalanche breakdown, the Selberherr’s model has been used with appropriate parameterization. For a device with a gate length of 2 μm, 0.6 μm gate-drain spacing, 3.4 μm source-drain spacing, and 20 nm thick Al2O3 gate insulator, the intrinsic breakdown voltage was found to be ~460 V. While self-heating dramatically affects the output current and conductance, it has an insignificant effect on the breakdown voltage. The use of a thinner epitaxial channel was found to increase the breakdown voltage slightly (by ~30 V).

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At the heart of all power electronic systems lies the semiconductor, responsible for passing large amounts of current at negligible power losses in the on-state, while instantaneously switching to withstand high voltages in the off-state. For decades silicon (Si) has dominated nearly all aspects of electronic systems including power. As importunity for efficiency at higher power and fast switching speeds grows, the environments with which these systems are being tasked to operate in has also increased in rigor. This has placed semiconductors at the forefront of innovation as novel materials are being explored in hopes of meeting the demands for the future of power electronics. This exploration of novel materials for power electronics has come to fruition as the performance limits of narrow bandgap (EG) materials such as Si (1.1 eV) have been reached. The EG is a key measure of a materials ability to operate at high voltages and within high temperature environments. This is due to the direct relationship of the EG to the critical field strength which enables increased performance beyond that of narrow band gap materials such as Si and gallium arsenide. Wide bandgap (WBG) materials such as silicon carbide (SiC) and gallium nitride (GaN) with EG 3.3 eV and 3.4 eV, respectively, have emerged within the power electronics field to offer increased breakdown voltages (VBR) at lower on-resistances. However, ultrawide bandgap (UWBG) devices possess greater potential with superior performance limits in comparison to SiC and GaN. Ga2O3 (4.8 eV) is the only UWBG semiconductor with melt-growth capabilities that has already demonstrated research grade wafers up to 6" in diameter. Ga2O3 is also advantaged by the ability to grow thick, lowly-doped homoepitaxial drift regions from methods such as halide vapor phase epitaxy (HVPE) and metal organic chemical vapor deposition (MOCVD). This makes Ga2O3 a prime candidate for vertical power rectifiers as thick, high quality drift regions are a necessity for high voltage devices such as the PN diode, junction barrier Schottky (JBS) diode, merged-PiN-Schottky (MPS) diode, and Schottky barrier diode (SBD). However, Ga2O3 exhibits a lack of p-type conductive that arises from an absence of dispersion within the valence band maximum. This has caused researchers to abandon the idea of homojunction devices that Si, SiC, and GaN devices benefit from; shifting to a heterojunction approach where NiO (3.7 eV) provides the source of p-type conductivity. This complicates fabrication and device characterization particularly for the Ga2O3 JBS diode where an etched Ga2O3-NiO heterojunction has thus far been unreported throughout the literature. This work investigates the numerous individual aspects that comprise an etched Ga2O3 heterojunction device which include the etching method, post etch damage removal and its impact on electrical performance, and ohmic and Schottky contacts critical for a JBS diode; all culminating in the demonstration of a JBS and MPS diodes. We also report our investigations into co-doping of Ga2O3 that yield degenerately doped epitaxial layers with record mobility (μ) values. While not directly correlated with Ga2O3-NiO heterojunction devices, this study lays the ground work for semi-insulating Ga2O3 depletion into unintentionally doped (UID) n-type Ga2O3. / Doctor of Philosophy / Power semiconductor devices reside at the center of many critical infrastructures that power modern society. These systems include but are not limited to; telecommunications, power supplies, motor drives, and electric trains. The semiconductors embedded within these systems are tasked with passing large amounts of current at negligible power losses in the on-state, while simultaneously withstanding high voltages in the off-state. For decades, the ground breaking discoveries and engineering feats produced by scientist and engineers have propelled the field of power electronics forward. As importunity for efficiency at higher power and fast switching speeds grows, the environments with which these systems are being tasked to operate in has also increased in rigour. These demands cannot be met with traditional silicon (Si) based devices as the material properties have been pushed to their performance limits. This has led to emerging and novel wide and ultrawide bandgap semiconductors such as silicon carbide (SiC), gallium nitride (GaN), and gallium oxide (Ga2O3) becoming a greater presence within the field of high power electronics. Ga2O3 in particular has seen a recent surge in interest within the power electronics communities due to the prospect of meeting the aforementioned demands, aided by a number of advantageous material and electrical properties. Ga2O3 is unlike any other wide or ultrawide bandgap material in that high quality Ga2O3 films known as epitaxial layers can be deposited atop native meltgrown Ga2O3 substrates. This reduces any mismatch or undesirable boundaries between the substrate and epitaxial layers that could otherwise impact device performance. This makes Ga2O3 a prime candidate for vertical power rectifiers, or switches such as a PN diode, junction barrier Schottky (JBS) diode or Schottky barrier diode (SBD). However, there has been no realization of p-type conductivity, or positively charged mobile carriers, within Ga2O3. This makes devices such as the PN and JBS diode difficult, as they rely on both n- and p-type conductivity. Without a source of p-type conductivity, Ga2O3 will be limited to unipolar devices that lack superior breakdown voltages and robustness. This work explores Ga2O3 heterojunction diodes, specifically the JBS diode, where nickel oxide (NiO) is used as the source of p-type conductivity. The need for a heterojunction introduces a host of issues that are otherwise not seen within bipolar semiconductors such as Si, SiC, and GaN. Our work details the analysis of the individual aspects that comprise a Ga2O3 heterojunction barrier Schottky diode including the etching process, etch damage removal, NiO sputtering, and contact formation. Our efforts have provided insight into unexplored areas within the Ga2O3 literature, leading to the first demonstration of a Ga2O3 merged- PiN-Schottky (MPS) diode; a more robust JBS diode capable of handling surge current. This work serves to further Ga2O3 as a viable semiconductor for the future of high power vertical rectifiers.

Die vorliegende Arbeit befasst sich mit der Untersuchung von weitbandlückigen Halbleitern mittels Raman-Spektroskopie. Diese wurde vorwiegend unter Verwendung von Licht einer Wellenlänge von 325 nm im ultravioletten Spektralbereich angeregt. Damit konnten zum einen aufgrund eines erhöhten Streuquerschnittes Messungen zur Probencharakterisierung durchgeführt werden, die mit Anregung im sichtbaren Spektralbereich nicht möglich gewesen wären. Zum anderen wurden bei dieser Anregungswellenlänge auftretende Resonanzeffekte untersucht. Dabei werden zwei verschiedene Materialsysteme behandelt: zum einen Kristalle mit Wurtzitstruktur und zum anderen binäre und ternäre Sesquioxide mit Metallionen der III. Hauptgruppe. An den Kristallen mit Wurtzitstruktur wurde die Streuung des Anregungslichts mit Energie oberhalb der Bandlücke an longitudinal-optischen (LO) Phononen untersucht. Die Streuung an einzelnen LO-Phononen wird unter diesen Anregungsbedingungen von einem Prozess dominiert, der eine elastische Streuung beinhaltet, durch die die Impulserhaltung verletzt wird. Es wurde ein Modell aufgestellt, dass zwischen einer elastischen Streuung an der Oberfläche und an Punktdefekten unterscheidet, und mit Hilfe von Experimenten verifiziert. Weiterhin wurde der Einfluss von Ladungsträgern auf die Energie der LO-Phononen untersucht und es wird eine Anwendung dieser Erkenntnisse zur Charakterisierung der Oberfläche von Zinkoxid vorgestellt. An den binären Oxiden des Galliums und Indiums wurden die Energien der Phononenmoden ermittelt und die resonante Verstärkung bei der verwendeten Anregungswellenlänge untersucht. Für das Galliumoxid wurde dabei insbesondere die Anisotropie des Materials berücksichtigt. Für das Indiumoxid wird dargestellt, dass durch die resonante Anregung alle Phononenmoden beobachtet werden können, was insbesondere auch die Bestimmung der Phononenmoden von Dünnschichtproben ermöglicht. Weiterhin waren Mischkristalle des Galliumoxids Untersuchungsgegenstand, in denen das Gallium teilweise durch Indium oder Aluminium ersetzt wurde. Die Phononenenergien wurden in Abhängigkeit der Zusammensetzung ermittelt und der Einfluss von strukturellen Eigenschaften darauf sowie das Auftreten von Phasenübergängen untersucht.

Gallium Oxide (Ga2O3) has emerged over the last decade as a new up-and-coming alternative to traditional wide bandgap semiconductors. It exists as five polymorphs (α-, β-, γ-, δ-, and ε-Ga2O3), of which β-Ga2O3 is the thermodynamically stable form, and the most extensively studied phase. β-Ga2O3 has a wide bandgap of ~4.8 eV and exhibits a superior figure-of-merit for power devices compared to other wide bandgap materials, such as SiC and GaN. These make β-Ga2O3 a promising candidate in a host of electronic and optoelectronic applications. Recent advances in β-Ga2O3 single crystals growth have also made inexpensive β-Ga2O3 single crystal grown from the melt a possibility in the near future. Despite the plethora of literature on β-Ga2O3-based devices, understanding of contacts to this material --- a device component that fundamentally determines device characteristics — remained lacking. For this research, ohmic and Schottky metal contacts to Sn-doped β-Ga2O3 (-201) single crystal substrates, unintentionally doped (UID) homoepitaxial β-Ga2O3 (010) on Sn-doped β-Ga2O3 grown by molecular beam epitaxy (MBE), and UID heteroepitaxial β-Ga2O3 (-201) epitaxial layers on c-plane sapphire by metal-organic chemical vapor deposition (MOCVD) were investigated. Each of the substrates was characterized for their structural, morphological, electrical, and optical properties, the results will be presented in the following document. Nine metals (Ti, In, Ag, Sn, W, Mo, Sc, Zn, and Zr) with low to moderate work functions were studied as possible ohmic contacts to β-Ga2O3. It was found that select metals displayed either ohmic (Ti and In) or pseudo-ohmic (Ag, Sn and Zr) behavior under certain conditions. However, the morphology was often a problem as many thin film metal contacts dewetted the substrate surface. Ti with a Au capping layer with post-metallization annealing treatment was the only consistently reliable ohmic contact to β-Ga2O3. It was concluded that metal work function is not a dominant factor in forming an ohmic contact to β-Ga2O3 and that limited interfacial reactions appear to play an important role. Prior to a systematic study of Schottky contacts to β-Ga2O3, a comparison of the effects of five different wet chemical surface treatments on the β-Ga2O3 Schottky diodes was made. It was established that a treatment with an organic solvent clean followed by HCl, H2O2 and a deionized water rinse following each step yielded the best results. Schottky diodes based on (-201) β-Ga2O3 substrates and (010) β-Ga2O3 homoepitaxial layers were formed using five different Schottky metals with moderate to high work functions: W, Cu, Ni, Ir, and Pt. Schottky barrier heights (SBHs) calculated from current-voltage (I-V) and capacitance-voltage (C-V) measurements of the five selected metals were typically in the range of 1.0 – 1.3 eV and 1.6 – 2.0 eV, respectively, and showed little dependence on the metal work function. Several diodes also displayed inhomogeneous Schottky barrier behavior at room temperature. The results indicate that bulk or near-surface defects and/or unpassivated surface states may have a more dominant effect on the electrical behavior of these diodes compared to the choice of Schottky metal and its work function. Lastly, working with collaborators at Structured Materials Industries (SMI) Inc., heteroepitaxial films of Ga2O3 were grown on c-plane sapphire (001) using a variety of vapor phase epitaxy methods, including MOVPE, and halide vapor phase epitaxy (HVPE). The stable phase β-Ga2O3 was observed when grown using MOVPE technique, regardless of precursor flow rates, at temperatures ranging between 500 – 850 °C. With HVPE growth techniques, instead of the stable β-phase, we observed the growth of the metastable α- and ε-phases, often a combination of the two. Cross-sectional transmission electron microscopy (TEM) shows the better lattice matched α-phase first growing semi-coherently on the c-plane sapphire substrate, followed by domain matched epitaxy of ε-Ga2O3 on top. Secondary ion mass spectrometry (SIMS) revealed that epilayers forming the ε-phase contain higher concentrations of chlorine, which suggests that compressive stress due to Cl- impurities may play a role in the growth of ε-Ga2O3 despite it being less than thermodynamically favorable.

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23976号 / 人博第1028号 / 新制||人||242(附属図書館) / 2022||人博||1028(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 吉田 寿雄, 教授 田部 勢津久, 教授 中村 敏浩 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

Orientador: Wilson de Figueiredo Jardim / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química / Made available in DSpace on 2018-08-16T20:28:08Z (GMT). No. of bitstreams: 1 Paschoalino_MatheusPaes_D.pdf: 18936349 bytes, checksum: 14276ac91e3b802b1f9fb8852267885f (MD5) Previous issue date: 2010 / Resumo: A fotocatálise heterogênea é uma técnica com grande aplicabilidade comercial quando o fotocatalisador encontra-se na forma suportada. O semicondutor mais estudado como fotocatalisador é o TiO2, principalmente o P25 (Degussa) devido suas excelentes propriedades físicoquímicas e fotocatalíticas. Além do TiO2, óxidos como ZnO e SiO2 também foram amplamente avaliados, enquanto que outros como CuO e Ga2O3 foram muito pouco estudados. Trabalhos recentes mostraram que a eficiência dos fotocatalisadores pode ser prejudicada em função do tipo de suporte ou substrato a que estes são fixados, devido a fatores como ligação catalisadorsuporte ineficiente, além de possíveis alterações morfológicas durante a deposição. Dentro deste contexto os objetivos deste trabalho foram: (a) desenvolver um suporte nanoestruturado ideal para o P25, composto por TiO2/SiO2, que possibilite sua fixação simples e rápida sobre superfícies de vidro e (b) avaliar a eficiência de CuO com alta área superficial na inativação fotocatalítica de E. coli e analisar a atividade fotocatalítica de b-Ga2O3 na degradação de compostos orgânicos. Como resultados obtiveram-se suportes nanoestruturados (100 a 315 m g) com alta adesão em vidro, suportando o P25 eficientemente para degradação de compostos emergentes (hormônios e antibióticos) em diferentes tipos de reatores sob iluminação natural e artificial. Óxidos de cobre com altas áreas superficiais (> 40 m g) mostraram-se eficientes na inativação de E. coli (10 UFC mL) após 1 ou 4 h de irradiação (l > 360 nm). O b-Ga2O3 (1 g L) apresentou-se seletivo para degradação de BTEX (4 mg L) utilizando-se uma lâmpada germicida como fonte de fótons. / Abstract: Heterogeneous photocatalysis is a technique with high commercial applicability when the photocatalyst is supported onto a substrate. The most studied semiconductor used as photocatalyst is the P25 TiO2 (Degussa) due to its excellent physiochemical and photocatalytic properties. Besides TiO2, other oxides such as ZnO and SiO2 were also studied, while CuO and Ga2O3 did not receive similar interest. Recent publications showed that photocatalyst efficiency could be impaired depending of the nature of the support or the substrate used, due to factors like inefficient catalyst-support bond and morphological alterations that can occur during deposition process. In this context, the purposes of this work were: (a) to develop a nanosupport for the P25, synthetizing a TiO2/SiO2 composite capable of fixing it over glass surfaces; (b) to evaluate the high surface area CuO efficiency on the photocatalytical inactivation of E. coli, and (c) to evaluate the photocatalatyc activity of b-Ga2O3 on the degradation of organic compounds. Nanostructured supports were obtained (100 to 315 m g) with high adhesion over glass, supporting P25 efficiently and allowing the degradation of emerging compounds (hormones and antibiotics) using different reactors under natural and artificial ilumination. Copper oxides with high surface area (> 40 m g) showed to be efficient on the E. coli inactivation (10 CFU mL) after 1 or 4 h irradiation (l > 360 nm). b-Ga2O3 (1 g L) was selective for BTEX degradation (4 mg L) using a germicidal lamp as photons source. / Doutorado / Quimica Analitica / Doutor em Ciências

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京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19721号 / 工博第4176号 / 新制||工||1644(附属図書館) / 32757 / 京都大学大学院工学研究科電子工学専攻 / (主査)教授 藤田 静雄, 教授 髙岡 義寛, 准教授 須田 淳 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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