III-nitrides are a promising material system with unique material properties, which allows them to be utilized in a variety of semiconductor devices. III-nitrides grown by NH3-MOMBE are typically grown with high carbon levels (> 1021 cm-3) as a result of the incomplete surface pyrolysis of the metal-organic sources. Recent research has involved the compensating nature of carbon in III-nitrides to produce semi-insulating films, which can provide low-leakage buffer layers in transistor devices. The aim of this work is to investigate the possibility of forming a 2DEG, which utilizes the highly carbon-doped GaN layers grown by NH3-MOMBE to produce low-leakage buffer layers in the fabrication of HEMTs. These low leakage GaN buffers would provide increased HEMT performance, with better pinch-off, higher breakdown voltages and increased power densities. Additionally, methods of controlling and/or reducing the incorporation of carbon will be undertaken in an attempt to broaden the range of possible device applications for NH3-MOMBE. To realize these transistor devices, optimization and improved understanding of the growth conditions for both GaN and AlGaN will be explored with the ultimate goal of determining the feasibility of III-nitride transistors grown by NH3-MOMBE.

Heterostructures of the materials system GaN/AlGaN/GaN were grown by molecular beam epitaxy on 6H-SiC substrates and high electron mobility transistors (HEMTs) were fabricated. For devices with large gate periphery an air bridge technology was developed for the drain contacts of the finger structure. The devices showed DC drain currents of more than 1 A/mm and values of the transconductance between 120 and 140 mS/mm. A power added efficiency of 41 % was measured on devices with a gate length of 1 µm at 2 GHz and 45 V drain bias. Power values of 8 W/mm were obtained. Devices with submicron gates exhibited power values of 6.1 W/mm (7 GHz) and 3.16 W/mm (25 GHz) respectively. The rf dispersion of the drain current is very low, although the devices were not passivated. / Heterostrukturen im Materialsystem GaN/AlGaN/GaN wurden mittels Molekularstrahlepitaxie auf 6H-SiC-Substraten gewachsen und High-Electron-Mobility-Transistoren (HEMTs) daraus hergestellt. Für Bauelemente mit großer Gateperipherie wurde eine Air-Bridge-Technik entwickelt, um die Drainkontakte der Fingerstruktur zu verbinden. Die Bauelemente zeigten Drainströme von mehr als 1 A/mm und Steilheiten zwischen 120 und 140 mS/mm. An Transistoren mit Gatelängen von 1 µm konnten Leistungswirkungsgrade (Power Added Efficiency) von 41 % (bei 2 GHz und 45 V Drain-Source-Spannung) sowie eine Leistung von 8 W/mm erzielt werden. Bauelemente mit Gatelängen im Submikrometerbereich zeigten Leistungswerte von 6,1 W/mm (7 GHz) bzw. 3,16 W/mm (25 GHz). Die Drainstromdispersion ist sehr gering, obwohl die Bauelemente nicht passiviert wurden.

Ce travail porte sur la croissance épitaxiale et la caractérisation optique de boîtes quantiques d'InAs/InP(001) en faible densité en vue de la réalisation de nouveaux composants nanophotoniques émettant à 1,55 µm. Les propriétés structurales et optiques des îlots ont été corrélés pour différents paramètres de croissance d'un système d'épitaxie par jet moléculaire à sources solides. Nos résultats soulignent l'influence des reconstructions de surface d'InAs sur la forme des îlots. Des boîtes, plutôt que des bâtonnets allongés généralement observés,peuvent être directement formées dans des conditions de croissance adéquates. Une transition de forme de bâtonnets vers des boîtes est également démontrée par des traitements postcroissance sous arsenic. Les faibles densités de boîtes sont obtenues pour des faibles épaisseurs d'InAs déposées. Leur émission est facilement contrôlée à 1,55 µm par une procédure d'encapsulation spécifique appelé " double cap ". Quelques propriétés des boîtes individuelles d'InAs/InP sont ensuite évaluées. Les études de micro-photoluminescence révèlent des pics d'émission très fins et distincts autour de 1,55 µm confirmant les propriétés" quasi-atomiques " de ces boîtes uniques. Enfin, nous proposons pour la première fois une méthode à haute résolution spatiale qui permet d'étudier le transport de charges autour d'une boîte unique grâce à une technique de cathodoluminescence à basse tension d'accélération.Une mesure directe de la longueur de diffusion des porteurs avant capture dans une boîte a été obtenue. Ces résultats ouvrent de nouvelles perspectives quant à l'intégration de ces boîtes uniques dans des microcavités optiques pour la réalisation de sources de lumières quantiques à 1,55 µm.

This thesis is about the growth of ZnO thin films for optoelectronic applications. ZnO thin films were grown using plasma assisted molecular beam epitaxy and were studied using various conventional and novel characterisation techniques. The significance of different growth variables on growth efficiency was investigated. The growth rate of ZnO films was found to be linearly dependent on the Zn flux under O-rich growth conditions. Under Zn-rich conditions, the growth rate was dependent on both atomic and molecular oxygen flux. By characterising the oxygen plasma generated using different RF power and aperture plate designs and correlating the results with the growth rates observed, it was found that atomic oxygen was the dominant growth species under all conditions. Molecular oxygen also participated in the growth process, with its importance dependent on the aperture plate design. In addition, an increase in growth temperature was found to monotonically decrease the growth rate. A growth rate of 1.4 Å/s was achieved at a growth temperature of 650 ℃ by using an oxygen flow rate of 1.6 standard cubic centimetres utilising a plasma source with a 276 hole plate operating at 400 W, and a Zn flux 1.4✕10¹⁵ atoms/cm²⋅s. Characterisation of the MBE grown thin films revealed that the qualities of ZnO thin films were dependent on the growth conditions. Experimental evidence suggested that a maximum adatom diffusion rate can be achieved under Zn-rich conditions, giving samples with the best structural quality. O-rich conditions in general led to statistical roughening which resulted in rough and irregular film surfaces. Experimental results also suggested that by increasing the atomic oxygen content and decreasing the ion content of the plasma, the excitonic emission of the ZnO thin films can possibly be improved. It was also found that the conductivity of the films can possibly be reduced by increasing the plasma ion content. By investigating the evolution of the buffer layer surface during the early stages of growth, dislocation nucleation and surface roughening were found to be important strain relief mechanisms in MBE grown ZnO thin films that affected the crystal quality. The usage of LT-buffer layers was found to improve substrate wetting, and was shown to significantly reduce dislocation propagation. Further strain reduction was achieved via the application of a 1 nm MgO buffer layer, and a significant reduction of carrier concentration and improvement in optical quality was subsequently observed. A carrier concentration of <1✕10¹⁶ cm⁻³ and a near band emission full width half maximum of 2 meV was observed for the best sample. The study of electrical characteristics using the variable magnetic field Hall effect confirmed the existence of a degenerate carrier and a bulk carrier in most MBE grown ZnO thin films. The bulk carrier mobility was measured to be ~120 - 150 cm²/Vs for most as-grown samples, comparable to the best reported value. A typical bulk carrier concentration of ~1✕10¹⁶ - 1✕10¹⁸ cm⁻³ was observed for as-grown samples. Annealing was found to increase the mobility of the bulk carrier to ~120 - 225 cm²/Vs and decrease the bulk carrier concentration by two orders of magnitude. Using time resolved photoluminescence, it was found that the radiative recombination in MBE grown ZnO thin films was dominated by excitonic processes, and followed a T³⁄² trend with temperature. A maximum radiative lifetime of 10 ns was observed for as-grown samples. The non-radiative lifetime in ZnO thin films was dominated by the Shockley-Read-Hall recombination processes. The modelling of the temperature dependence of the non-radiative lifetime suggested that an electron trap at ~0.065 eV and a hole trap at ~0.1 eV may be present in these samples. The application of time resolved photoluminescence also allowed the direct observation of carrier freeze-out in these ZnO films at low temperature.

This dissertation shows the properties of lithium niobate and lithium tantalate as a promising substrate for III-nitrides, addresses several problems of integrating compound semiconductor materials on LN and LT. It also suggests some solutions of the addressed problems, including furnace anneals at high temperature. While this furnace anneal improved surface smoothness and III-nitride film adhesion, it also caused the repolarization on the congruent LN (48.39 mole % of Li2O) samples. However, the repolarization was not developed in the stoichiometric LN (49.9 mole % of Li2O) samples during the identical thermal treatment. Also, the structural quality of GaN epitaxial layers showed slight improvement when grown on LT substrates over LN substrates. Conventional epitaxial growth technologies were adapted and modified to implement a successful AlGaN/GaN heterostructure on LN (LT). The heterostructure were analyzed to verify the electrical and material properties using several characterization techniques. Finally, it demonstrates AlGaN/GaN-based HEMT devices on ferroelectric materials that will allow the future development of the multifunctional electrical and optical applications.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23901号 / 工博第4988号 / 新制||工||1779(附属図書館) / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 田中 功, 教授 杉村 博之, 准教授 野瀬 嘉太郎 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

The development of InGaN quantum dots (QDs) is both scientifically challenging and promising for applications in visible spectrum LEDs, lasers, detectors, electroabsorption modulators and photovoltaics. Such QDs are typically grown using the Stranski-Krastanov (SK) growth mode, in which accumulated in-plane compressive strain induces a transition from 2D to 3D growth. This method has a number of inherent limitations, including the unavoidable formation of a 2D wetting layer and the difficulty of controlling the composition, areal density, and size of the dots. In this research, I have developed InGaN QDs by two methods using a plasma-assisted molecular beam epitaxy reactor. In the first method, InGaN QDs were formed by SK growth mode on (0001) GaN/sapphire. In the second, I have addressed the limitations of the SK growth of InGaN QDs by developing a novel alternative method, which was utilized to grow on both (0001) GaN/sapphire and AlN/sapphire. This method relies upon the ability to form thermodynamically stable In-Ga liquid solutions throughout the entire compositional range at relatively low temperatures. Upon simultaneous or sequential deposition of In and Ga on a substrate, the adatoms form a liquid solution, whose composition is controlled by the ratio of the fluxes of the two constituents FIn/(FIn+FGa). Depending on the interfacial free energy between the liquid deposit and substrate, the liquid deposit and vapor, and the vapor and substrate, the liquid deposit forms Inx-Ga1−x nano-droplets on the substrate. These nano-droplets convert into InxGa1−xN QDs upon exposure to nitrogen RF plasma. InGaN QDs produced by both methods were investigated in-situ by reflection high-energy electron diffraction and ex-situ by atomic force microscopy, field emission scanning electron microscopy, transmission electron microscopy, high resolution x-ray diffraction, and grazing incidence small angle x-ray scattering. The optical activity and device potential of the QDs were investigated by photoluminescence measurements and the formation and evaluation of PIN devices (in which the intrinsic region contains QDs embedded within a higher bandgap matrix). InGaN QDs with areal densities ranging from 109 to 1011 cm−2 and diameters ranging from 11 to 39 nm were achieved.

abstract: There has been recent interest in demonstrating solar cells which approach the detailed-balance or thermodynamic efficiency limit in order to establish a model system for which mass-produced solar cells can be designed. Polycrystalline CdS/CdTe heterostructures are currently one of many competing solar cell material systems. Despite being polycrystalline, efficiencies up to 21 % have been demonstrated by the company First Solar. However, this efficiency is still far from the detailed-balance limit of 32.1 % for CdTe. This work explores the use of monocrystalline CdTe/MgCdTe and ZnTe/CdTe/MgCdTe double heterostructures (DHs) grown on (001) InSb substrates by molecular beam epitaxy (MBE) for photovoltaic applications. Undoped CdTe/MgCdTe DHs are first grown in order to determine the material quality of the CdTe epilayer and to optimize the growth conditions. DH samples show strong photoluminescence with over double the intensity as that of a GaAs/AlGaAs DH with an identical layer structure. Time-resolved photoluminescence of the CdTe/MgCdTe DH gives a carrier lifetime of up to 179 ns for a 2 µm thick CdTe layer, which is more than one order of magnitude longer than that of polycrystalline CdTe films. MgCdTe barrier layers are found to be effective at confining photogenerated carriers and have a relatively low interface recombination velocity of 461 cm/s. The optimal growth temperature and Cd/Te flux ratio is determined to be 265 °C and 1.5, respectively. Monocrystalline ZnTe/CdTe/MgCdTe P-n-N DH solar cells are designed, grown, processed into solar cell devices, and characterized. A maximum efficiency of 6.11 % is demonstrated for samples without an anti-reflection coating. The low efficiency is mainly due to the low open-circuit voltage (V_oc), which is attributed to high dark current caused by interface recombination at the ZnTe/CdTe interface. Low-temperature measurements show a linear increase in V_oc with decreasing temperature down to 77 K, which suggests that the room-temperature operation is limited by non-radiative recombination. An open-circuit voltage of 1.22 V and an efficiency of 8.46 % is demonstrated at 77 K. It is expected that a coherently strained MgCdTe/CdTe/MgCdTe DH solar cell design will produce higher efficiency and V_oc compared to the ZnTe/CdTe/MgCdTe design with relaxed ZnTe layer. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2014

abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

Nous avons étudié les propriétés structurales et optiques d'hétérostructures de nitrures d'éléments III. Les croissances ont été réalisées par épitaxie par jets moléculaires assistée plasma d'azote. L'étude a été en particulier axée sur des structures émettant dans l'UV, à des longueurs d'onde sub-300 nm. Pour atteindre cette longueur d'onde à l'aide de couches bidimensionnelles, il est nécessaire d'utiliser des alliages ternaires d'AlGaN à forte teneur en Al (supérieure à 50 %). Nous avons donc tout d'abord corrélé les résultats de techniques de caractérisation aux conditions de croissance. Ainsi, il est possible de contrôler en partie l'homogénéité de l'alliage grâce à la température de croissance et au rapport de flux métal/azote. Nous avons ensuite tenté d'appliquer ces résultats à des hétérostructures à puits quantiques AlGaN/AlGaN. D'autre part, nous avons réalisé des alliages digitaux (super-réseaux à courte période de puits quantiques GaN/AlN). Ceux-ci se sont montré une alternative intéressante aux couches 2D pour la réalisation de dispositifs émetteurs de lumière. Enfin, une part importante du manuscrit s'attache à l'étude des effets d'un recuit après la croissance et à haute température. Cette étude a été menée sur des couches d'AlGaN ainsi que sur des boites quantiques GaN/AlN. Pour les couches d'AlGaN, nous avons constaté une augmentation de l'inhomogénéité des couches après un recuit à 950 °C. Pour les boîtes quantiques GaN, nous avons constaté dès 1000 °C, un décalage vers le bleu de la luminescence. A 1300-1500 °C, des images TEM ont clairement montré la diffusion de Ga dans la matrice AlN au-dessus des boîtes quantiques. / We studied the structural and optical properties of III-nitride heterostructures. The growth were achieved by plasma-assisted molecular beam epitaxy. The study was particularly focused on structures emitting in the UV range at sub-300 nm wavelengths. To achieve this wavelength using two-dimensional layers, it is necessary to use ternary AlGaN alloys with high Al content (above 50%). So we first correlated the results of characterization techniques to growth conditions. As a result, we find that it is possible to partially control the alloy homogeneity with the growth temperature and metal/nitrogen flux ratio. Then, we tried to apply these results to AlGaN/AlGaN quantum well heterostructures . On the other hand, we have made digital alloys (short-period GaN/AlN quantum well superlattices). They appeared as an interesting alternative to 2D layers for the realization of light emitting devices. Finally, an important part of the manuscript focuses on studying the effects of post growth annealing, at high temperature. This study was conducted on AlGaN layers and on GaN/AlN quantum dots. For AlGaN layers, we found an increase in the inhomogeneity of the alloy after annealing at 950 °C. For GaN quantum dots, a blue shift of the luminescence is observed for annealig temperature as low as 1000 °C. At 1300-1500 ° C, TEM images clearly showed the diffusion of Ga in the AlN matrix above the quantum dots.