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.

L'évolution des composants d'électronique de puissance se heurte aujourd'hui aux limites physiques du silicium. L'utilisation des semi-conducteurs à large bande interdite permettraient de dépasser ces limites. Parmi ces nouveaux matériaux, le diamant possède les propriétés les plus intéressantes pour l'électronique de puissance : champ de rupture et conductivité thermique les plus élevés parmi les solides, grandes mobilités des porteurs électriques, possibilité de fonctionnement à haute température. Les substrats de diamant synthétisés actuellement par des méthodes de dépôt en phase vapeur ont des caractéristiques cristallographiques compatibles avec l'exploitation de ces propriétés en électronique de puissance. L'utilisation technologique du diamant reste toutefois difficile ; ses propriétés de dureté et d'inertie chimique rendent son utilisation délicate. L'objet de ces travaux est dans un premier temps d'évaluer les bénéfices que pourrait apporter le diamant en électronique de puissance. Ensuite, différentes étapes technologiques nécessaires à la fabrication de composants sur diamant sont étudiées : dépôts de contacts électriques, dopage et gravure ionique. Enfin, une étude sur la fabrication de diodes Schottky est présentée. Les résultats obtenus permettent d'établir les perspectives à ces travaux et les challenges scientifiques et technologiques qu'il reste à relever. / The evolution of power electronic devices is getting more and more limited by the silicon intrinsic properties. This limitation could be overcome by using wide bandgap semiconductors. Among these materials, diamond properties are the more fitted for power electronics: the highest critical electric field and thermal conductivity amongst the solids, high carriers mobility, high temperature operation possibility. At this time, diamond samples grown by chemical vapour deposition methods exhibit crystallographic properties that are suitable for a use in power electronics. Though, the realization of diamond power devices remains difficult due to its hardness and chemical inertness, among others. First, this work aims at determining the profit that could represent diamond for power electronics. Second, different technologic steps that are necessary to the realisation of electronic devices are studied: ohmic contacts deposition, doping and ion etching. Finally, the first devices we realised, Schottky diodes, are presented. Their characterisation allows establishing new objectives for the future developments of our studies.

abstract: Photovoltaic (PV) energy has shown tremendous improvements in the past few decades showing great promises for future sustainable energy sources. Among all PV energy sources, III-V-based solar cells have demonstrated the highest efficiencies. This dissertation investigates the two different III-V solar cells with low (III-antimonide) and high (III-nitride) bandgaps. III-antimonide semiconductors, particularly aluminum (indium) gallium antimonide alloys, with relatively low bandgaps, are promising candidates for the absorption of long wavelength photons and thermophotovoltaic applications. GaSb and its alloys can be grown metamorphically on non-native substrates such as GaAs allowing for the understanding of different multijunction solar cell designs. The work in this dissertation presents the molecular beam epitaxy growth, crystal quality, and device performance of AlxGa1−xSb solar cells grown on GaAs substrates. The motivation is on the optimization of the growth of AlxGa1−xSb on GaAs (001) substrates to decrease the threading dislocation density resulting from the significant lattice mismatch between GaSb and GaAs. GaSb, Al0.15Ga0.85Sb, and Al0.5Ga0.5Sb cells grown on GaAs substrates demonstrate open-circuit voltages of 0.16, 0.17, and 0.35 V, respectively. In addition, a detailed study is presented to demonstrate the temperature dependence of (Al)GaSb PV cells. III-nitride semiconductors are promising candidates for high-efficiency solar cells due to their inherent properties and pre-existing infrastructures that can be used as a leverage to improve future nitride-based solar cells. However, to unleash the full potential of III-nitride alloys for PV and PV-thermal (PVT) applications, significant progress in growth, design, and device fabrication are required. In this dissertation, first, the performance of ii InGaN solar cells designed for high temperature application (such as PVT) are presented showing robust cell performance up to 600 ⁰C with no significant degradation. In the final section, extremely low-resistance GaN-based tunnel junctions with different structures are demonstrated showing highly efficient tunneling characteristics with negative differential resistance (NDR). To improve the efficiency of optoelectronic devices such as UV emitters the first AlGaN tunnel diode with Zener characteristic is presented. Finally, enabled by GaN tunnel junction, the first tunnel contacted InGaN solar cell with a high VOC value of 2.22 V is demonstrated. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2019

High efficiency solar cells can be attained by the development of two junctions one stacked on top of each other into tandem structures. So that, if a photon is not able to excite an electron-hole pair in the top cell can create a pair in the bottom cell, which has a smaller bandgap. For a two junction tandem device structure, the bandgap of the top cell should be 1.6-1.8eV and for the bottom cell should be 1eV to attain efficiencies in the range of 25%. Cadmium Zinc Telluride which has a tunable bandgap of 1.45- 2.2eV is a candidate for the top cell of the tandem structure. Cadmium Zinc Telluride (Cd1-xZnxTe) films were deposited by co-sputtering of CdTe and ZnTe. Deposition of Cd1-xZnxTe was studied in Ar and Ar/N2 ambient. Characterization of the films was done using transmission response, X-ray diffraction (XRD), Atomic Force Microscopy (AFM), Secondary Electron Microscopy (SEM), current-voltage (I-V) and spectral response measurements. CZT deposited on CdS/SnO2 substrates showed improved performance compared to other heterojunction partners. Doped graphite and copper were utilized as back contacts for CZT devices. Post deposition annealing treatments with ZnCl2 on CZT films were done and their effect on the devices was also studied. The best combination of Voc and Jsc were 530mV and 3.66mA/cm² respectively.

The present thesis is a study of the evolution of defect states in devices based on wide bandgap semiconductors. The attention has been focused on light-emitting diodes based on GaN and Schottky diodes based on SiC, these latter a basic structure for the fabrication of high-power rectifiers and ionising particle detectors. In both cases, we studied the defects and their electronic properties by means of the following experimental techniques: current-voltage (I-V) measurements, in order to investigate the effect of imperfections on the transport properties of the material/device; capacitance-voltage (C-V) measurements, yielding the profile of concentration of charge carriers, and giving information on the influence of defects on this concentration; deep level transient spectroscopy (DLTS), a technique allowing for the identification and characterization of defect-originated electron levels in the gap. I also employed techniques, such as photocurrent spectroscopy (PC), allowing for the characterization of light absorption by the material and/or device versus varying photon energy. In both cases of SiC and GaN, the defect characterization was always interpreted in the framework of its influence on device operation. In the analysed LEDs the defect evolution was connected to the evolution of quantum efficiency, and in the SiC diodes we studied the effects of defect introduction on the charge collection efficiency (CCE) and on the leakage current of the device. Furthermore, for the interpretation of photocurrent spectra, I developed a model describing the generation of photocurrent considering the dispersion relations for the absorption coefficient and refractive index in the various device layers, as well as the internal reflection, transmission and interference phenomena involving the optical field within the device. The research yielded various interesting results: I detected many deep levels introduced by proton- and electron-irradiation in SiC. From the study of their annealing behaviour I concluded that one of these levels is related to a particular lattice defect, the carbon interstitial. By means of the analysis of the introduction rates of the levels and comparisons between proton and electron irradiation, I was able to distinguish between deep levels related to simple intrinsic defects and to defect complexes. In the case of the GaN LED, I found that the evolution of several independent properties are strongly correlated, meaning that a single degradation mechanism is responsible for the observed changes. In particular, I concluded that the degradation of the light emission intensity is due to the generation of defects in the active region of the device.

SiC MESFETs and GaN HEMTs have an enormous potential in high-power amplifiers at microwave frequencies due to their wide bandgap features of high electric breakdown field strength, high electron saturation velocity and high operating temperature. The high power density combined with the comparably high impedance attainable by these devices also offers new possibilities for wideband power microwave systems. In this thesis, Class C switching response of SiC MESFET in TCAD and two different generations of broadband power amplifiers have been designed, fabricated and characterized. Input and output matching networks and shunt feedback topology based on microstrip and lumped components have been designed, to increase the bandwidth and to improve the stability. The first amplifier is a single stage 26-watt using a SiC MESFET covering the frequency from 200-500 MHz is designed and fabricated. Typical results at 50 V drain bias for the whole band are, 22 dB power gain, 43 dBm output power, minimum power added efficiency at P 1dB is 47 % at 200 MHz and maximum 60 % at 500 MHz and the IMD3 level at 10 dB back-off from P 1dB is below ‑45 dBc. The results at 60 V drain bias at 500 MHz are, 24.9 dB power gain, 44.15 dBm output power (26 W) and 66 % PAE. In the second phase, two power amplifiers at 0.7-1.8 GHz without feed back for SiC MESFET and with feedback for GaN HEMT are designed and fabricated (both these transistors were of 10 W). The measured maximum output power for the SiC amplifier at Vd = 48 V was 41.3 dBm (~13.7 W), with a PAE of 32 % and a power gain above 10 dB. At a drain bias of Vd= 66 V at 700 MHz the Pmax was 42.2 dBm (~16.6 W) with a PAE of 34.4 %. The measured results for GaN amplifier are; maximum output power at Vd = 48 V is 40 dBm (~10 W), with a PAE of 34 % and a power gain above 10 dB. The SiC amplifier gives better results than for GaN amplifier for the same 10 W transistor. A comparison between the physical simulations and measured device characteristics has also been carried out. A novel and efficient way to extend the physical simulations to large signal high frequency domain was developed in our group, is further extended to study the class-C switching response of the devices. By the extended technique the switching losses, power density and PAE in the dynamics of the SiC MESFET transistor at four different frequencies of 500 MHz, 1, 2 and 3 GHz during large signal operation and the source of switching losses in the device structure was investigated. The results obtained at 500 MHz are, PAE of 78.3%, a power density of 2.5 W/mm with a switching loss of 0.69 W/mm. Typical results at 3 GHz are, PAE of 53.4 %, a power density of 1.7 W/mm with a switching loss of 1.52 W/mm. / Report code: LIU-TEK-LIC-2008:32

<p>SiC MESFETs and GaN HEMTs have an enormous potential in high-power amplifiers at microwave frequencies due to their wide bandgap features of high electric breakdown field strength, high electron saturation velocity and high operating temperature. The high power density combined with the comparably high impedance attainable by these devices also offers new possibilities for wideband power microwave systems. In this thesis, Class C switching response of SiC MESFET in TCAD and two different generations of broadband power amplifiers have been designed, fabricated and characterized. Input and output matching networks and shunt feedback topology based on microstrip and lumped components have been designed, to increase the bandwidth and to improve the stability. The first amplifier is a single stage 26-watt using a SiC MESFET covering the frequency from 200-500 MHz is designed and fabricated. Typical results at 50 V drain bias for the whole band are, 22 dB power gain, 43 dBm output power, minimum power added efficiency at P 1dB is 47 % at 200 MHz and maximum 60 % at 500 MHz and the IMD3 level at 10 dB back-off from P 1dB is below ‑45 dBc. The results at 60 V drain bias at 500 MHz are, 24.9 dB power gain, 44.15 dBm output power (26 W) and 66 % PAE.</p><p>In the second phase, two power amplifiers at 0.7-1.8 GHz without feed back for SiC MESFET and with feedback for GaN HEMT are designed and fabricated (both these transistors were of 10 W). The measured maximum output power for the SiC amplifier at Vd = 48 V was 41.3 dBm (~13.7 W), with a PAE of 32 % and a power gain above 10 dB. At a drain bias of Vd= 66 V at 700 MHz the Pmax was 42.2 dBm (~16.6 W) with a PAE of 34.4 %. The measured results for GaN amplifier are; maximum output power at Vd = 48 V is 40 dBm (~10 W), with a PAE of 34 % and a power gain above 10 dB. The SiC amplifier gives better results than for GaN amplifier for the same 10 W transistor.</p><p>A comparison between the physical simulations and measured device characteristics has also been carried out. A novel and efficient way to extend the physical simulations to large signal high frequency domain was developed in our group, is further extended to study the class-C switching response of the devices. By the extended technique the switching losses, power density and PAE in the dynamics of the SiC MESFET transistor at four different frequencies of 500 MHz, 1, 2 and 3 GHz during large signal operation and the source of switching losses in the device structure was investigated. The results obtained at 500 MHz are, PAE of 78.3%, a power density of 2.5 W/mm with a switching loss of 0.69 W/mm. Typical results at 3 GHz are, PAE of 53.4 %, a power density of 1.7 W/mm with a switching loss of 1.52 W/mm.</p> / Report code: LIU-TEK-LIC-2008:32

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23909号 / 工博第4996号 / 新制||工||1780(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 安藤 裕一郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

abstract: Wurtzite (In, Ga, Al) N semiconductors, especially InGaN material systems, demonstrate immense promises for the high efficiency thin film photovoltaic (PV) applications for future generation. Their unique and intriguing merits include continuously tunable wide band gap from 0.70 eV to 3.4 eV, strong absorption coefficient on the order of ∼105 cm−1, superior radiation resistance under harsh environment, and high saturation velocities and high mobility. Calculation from the detailed balance model also revealed that in multi-junction (MJ) solar cell device, materials with band gaps higher than 2.4 eV are required to achieve PV efficiencies greater than 50%, which is practically and easily feasible for InGaN materials. Other state-of-art modeling on InGaN solar cells also demonstrate great potential for applications of III-nitride solar cells in four-junction solar cell devices as well as in the integration with a non-III-nitride junction in multi-junction devices. This dissertation first theoretically analyzed loss mechanisms and studied the theoretical limit of PV performance of InGaN solar cells with a semi-analytical model. Then three device design strategies are proposed to study and improve PV performance: band polarization engineering, structural design and band engineering. Moreover, three physical mechanisms related to high temperature performance of InGaN solar cells have been thoroughly investigated: thermal reliability issue, enhanced external quantum efficiency (EQE) and conversion efficiency with rising temperatures and carrier dynamics and localization effects inside nonpolar m-plane InGaN quantum wells (QWs) at high temperatures. In the end several future work will also be proposed. Although still in its infancy, past and projected future progress of device design will ultimately achieve this very goal that III-nitride based solar cells will be indispensable for today and future’s society, technologies and society. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2020

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