published_or_final_version / Physics / Master / Master of Philosophy

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Wide bandgap semiconductors have entered into Naval radar use and will eventually replace vacuum tube and conventional solid-state amplifiers for all modern military radar and communications applications. Gallium Nitride (GaN) High Electron Mobility Transistors (HEMTs) are on the leading edge of wide bandgap technology and have the performance characteristics to dominate in high power â high bandwidth applications. The Defense Advanced Research Projects Agency (DARPA), Office of Naval Research (ONR) and Missile Defense Agency (MDA) are all sponsoring research projects to apply wide bandgap technology. This thesis studies the effects of changing the substrate material of an existing GaN HEMT from sapphire to diamond through the use of commercially available Silvaco software for modeling and simulation. The unparalleled thermal properties of diamond are expected to dramatically decrease device temperatures and increase component lifetimes and reliability.

III–V nitride quantum-confined structures embedded in nanowires (NWs), also known as quantum-disks-in-nanowires (Qdisks-in-NWs), have recently emerged as a new class of nanoscale materials exhibiting outstanding properties for optoelectronic devices and systems. It is promising for circumventing the technology limitation of existing planar epitaxy devices, which are bounded by the lattice-, crystal-structure-, and thermal- matching conditions. This work presents significant advances in the growth of good quality GaN, InGaN and AlGaN Qdisks-in-NWs based on careful optimization of the growth parameters, coupled with a meticulous layer structure and active region design. The NWs were grown, catalyst-free, using plasma assisted molecular beam epitaxy (PAMBE) on silicon (Si) substrates. A 2-step growth scheme was developed to achieve high areal density, dislocation free and vertically aligned NWs on Ti/Si substrates. Numerical modeling of the NWs structures, using the nextnano3 software, showed reduced polarization fields, and, in the presence of Qdisks, exhibited improved quantum-confinement; thus contributing to high carrier radiative-recombination rates. As a result, based on the growth and device structure optimization, the technologically challenging orange and yellow NWs light emitting devices (LEDs) targeting the ‘green-yellow’ gap were demonstrated on scalable, foundry compatible, and low-cost Ti coated Si substrates. The NWs work was also extended to LEDs emitting in the ultraviolet (UV) range with niche applications in environmental cleaning, UV-curing, medicine, and lighting. In this work, we used a Ti (100 nm) interlayer and Qdisks to achieve good quality AlGaN based UV-A (320 - 400 nm) device. To address the issue of UV-absorbing polymer, used in the planarization process, we developed a pendeo-epitaxy technique, for achieving an ultra-thin coalescence of the top p-GaN contact layer, for a self-planarized Qdisks-in-NWs UV-B (280 – 320 nm) LED grown on silicon. This process constitutes a significant advancement in simplifying the UV-B and UV-C fabrication process favoring light extraction. Addressing the issue of poor white light quality in the conventional blue laser diode (LD) and YAG:Ce3+ technology, a number of applications related investigations was conducted. Notably, the orange and yellow emitting InGaN/GaN Qdisks-in-NWs LEDs were implemented as an “active phosphor” to achieve intensity- and bandwidth-tunability for high color-quality solid-state lighting.

by Yun-hung Ng. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references. / Text in English; abstracts in English and Chinese. / by Yun-hung Ng. / Abstract --- p.ii / 論文摘要 --- p.iii / Acknowledgements --- p.iv / Table of Contents --- p.v / List of Figures --- p.ix / List of Tables --- p.xi / Chapter Chapter 1 --- INTRODUCTION --- p.1 / Chapter 1.1 --- Background of Study --- p.1 / Chapter 1.2 --- General Properties of a-SiNx and a-SiOxNy --- p.1 / Chapter 1.3 --- Common Preparation Methods of a-SiNx and a-SiOxNy --- p.2 / Chapter 1.4 --- Applications of a-SiNx in Microelectronics --- p.4 / Chapter 1.5 --- Applications of a-SiOxNy in Microelectronics --- p.6 / References --- p.8 / Chapter Chapter 2 --- METHODOLOGY --- p.10 / Chapter 2.1 --- Introduction --- p.10 / Chapter 2.2 --- Mott Rule --- p.10 / Chapter 2.3 --- Random Mixture Model --- p.11 / Chapter 2.4 --- Random Bonding Model --- p.12 / Chapter 2.5 --- Hasegawa Model --- p.15 / References --- p.20 / Chapter Chapter 3 --- INSTRUMENTATION --- p.21 / Chapter 3.1 --- X-ray Photoelectron Spectroscopy (XPS) --- p.21 / Chapter 3.1.1 --- Fundamental Theory of XPS --- p.21 / Chapter 3.1.2 --- Qualitative Analysis using XPS --- p.25 / Chapter 3.1.2.1 --- Chemical Shift --- p.25 / Chapter 3.1.2.2 --- Angular Effect on XPS --- p.28 / Chapter 3.1.2.3 --- Valence Band Investigation --- p.28 / Chapter 3.1.3 --- Quantitative Analysis using XPS --- p.30 / Chapter 3.1.4 --- Instrumental Setup of XPS --- p.33 / Chapter 3.2 --- Ultraviolet Photoelectron Spectroscopy --- p.37 / Chapter 3.2.1 --- Basic Theory of UPS --- p.37 / Chapter 3.2.2 --- Instrumentation --- p.38 / References --- p.41 / Chapter Chapter 4 --- SHORT RANGE ORDER OF a-SiNx --- p.42 / Chapter 4.1 --- Sample Preparation --- p.42 / Chapter 4.2 --- XPS Analysis of a-SiNx --- p.43 / Chapter 4.2.1 --- Angle Resolved XPS Analysis --- p.43 / Chapter 4.2.2 --- RB Model and RM Model Simulation of a-SiNx --- p.43 / Chapter 4.2.3 --- Intermediate Mixture (IM) Model --- p.50 / Chapter 4.2.4 --- Valence Band Structure of a-SiNx --- p.51 / Chapter 4.3 --- Raman Measurements --- p.54 / Chapter 4.4 --- Photoluminescence of a-SiNx --- p.54 / Chapter 4.5 --- Large Scale Potential Fluctuations in a-SiNx --- p.56 / Chapter 4.6 --- Conclusion --- p.61 / References --- p.62 / Chapter Chapter 5 --- MOTT RULE VERIFICATION OF a-SiOxNy --- p.65 / Chapter 5.1 --- Sample Preparation --- p.65 / Chapter 5.2 --- Validity of Mott Rule on a-SiOxNy --- p.66 / Chapter 5.3 --- Conclusion --- p.73 / References --- p.74 / Chapter Chapter 6 --- SHORT RANGE ORDER OF a-SiOxNy --- p.75 / Chapter 6.1 --- Angle Resolved XPS Analysis --- p.75 / Chapter 6.2 --- Random Bonding Model Simulation of a-SiOxNy --- p.75 / Chapter 6.3 --- Conclusion --- p.79 / References --- p.82 / Chapter Chapter 7 --- CONCLUSIONS --- p.83

Crystalline carbon nitride has been a hot topic for the last ten years because of reports claiming it could work as a photocatalyst for cheap water splitting, a catalyst for difficult reactions inorganic chemistry and the use as a potential two-dimensional semiconductor.The carbon nitride of interest in this project is poly(triazineimide) (PTI), which has a layered structure similar to graphite. Oneof the goals was to examine the synthesis parameters to try tounderstand what makes these crystallites grow. The material was primarily analyzed using scanning electron microscopy and powder x-ray diffraction. The other goal of this project was to examine the physical properties of dissolved PTI. It is currently not understood how PTI behaves in various solvents. The effect on how the freezing point depression varies in different solvents was, therefore, tested.No strong correlations of how the morphology of the produced PTIdiffered with different synthesis parameters. Freezing point measurements suggest that a solution of PTI follows Raoult's law and can be described as a true solution.

Layered van der Waals materials have played a pivotal role in expanding the scope of condensed matter physics by examining the effects of reduced dimensionality in various systems. These include semiconductors, ferromagnets, and charge density wave materials, among many others. Hexagonal boron nitride (hBN) is often used as a passivation/encapsulation layer for air-sensitive materials in optical and electronic studies owing to its effectiveness as a substrate for graphene in transport measurements. In this thesis, samples probed by Raman spectroscopy and as well as those measured through electronic transport were first encapsulated during fabrication. The specific experimental details are found in each corresponding chapter. This thesis aims to characterize several 2-D materials and explore physical phenomena arising from combinations thereof through optical and electronic means. Before delving into the specific research projects, it provides a motivation for each, descriptions of the material(s) involved, and sample fabrication techniques used to assemble the various heterostructures. Topics to be covered include the effects of encapsulation on the transition metal dichalcogenide (TMD) 1T’-MoTe2 subject to elevated temperatures, how the nearly commensurate to commensurate phase transition of another TMD, the charge density wave material 1T-TaS2, in its few-layer form can be tuned electronically, preliminary results of electronic transport in graphene-ferromagnet heterostructures, and an outline of other optical studies on mono- to few-layered forms of related materials and possible future directions that may be pursued.

Boron nitride (BN) materials have unique properties, which has led to interest in them in the last few years. The deformation of boron nitride materials including hexagonal boron nitride, boron nitride nanosheets (BNNSs) and boron nitride nanotubes have been studied by Raman spectroscopy. Both mechanical and liquid exfoliations were employed to obtain boron nitride nanostructures. Boron nitride glass composites were synthesised and prepared in thin films to be deformed by bending test in-situ Raman spectroscopy. Hexagonal boron nitride in the form of an individual flake and as flakes dispersed in glass matrices has been deformed and Raman measurement shows its response to strain. The shift rates were, -4.2 cm-1/%, -6.5 cm-1/% for exfoliated h-BN flake with thick and thin regions and -7.0 cm-1/%, -2.8 cm-1/% for the h-BN flakes in the h-BN/ glass (I) and glass (II) composites. Boron nitride nanosheets (BNNSs) shows a G band Raman peak at 1367.5 cm-1, and the deformation process of BNNSs/ glass composites gives a shift rate of -7.65 cm-1/% for G band. Boron nitride nanotubes (BNNTs) have a Raman peak with position at 1368 cm-1, and their deformation individually and in composites gives Raman band shift rates of -25.7 cm-1/% and -23.6 cm-1/%. Glass matrices shows compressive stresses on boron nitride fillers and this was found as an upshift in the frequencies of G band peak of boron nitride materials. Grüneisen parameters of boron nitride (BN) were used to calculate the residual strains in glass matrices of BNNSs nanocomposites as well as to estimate the band shift rates which found to be in agreement with the experimental shift rate of bulk BN and BNNTs.