Microelectronics has enjoyed great success in the past century. As the technology node progresses, the complementary metal-oxide-semiconductor scaling has already reached a wall, and serious challenges in high-bandwidth interconnects and fast-speed signal processing arise. The incorporation of photonics to microelectronics provides potential solutions. The theme of this thesis is focused on the novel applications of travelling-wave microresonators such as microdisks and microrings for the on-chip optical interconnects and signal processing. Challenges arising from these applications including theoretical and experimental ones are addressed. On the theoretical aspect, a modified version of coupled mode theory is offered for the TM-polarization in high index contrast material systems. Through numerical comparisons, it is shown that our modified coupled mode theory is more accurate than all the existing ones. The coupling-induced phase responses are also studied, which is of critical importance to coupled-resonator structures. Different coupling structures are studied by a customized numerical code, revealing that the phase response of symmetric couplers with the symmetry about the wave propagating direction can be simply estimated while the one of asymmetric couplers is more complicated. Mode splitting and scattering loss, which are two important features commonly observed in the spectrum of high-Q microresonators, are also investigated. Our review of the existing analytical approaches shows that they have only achieved partial success. Especially, different models have been proposed for several distinct regimes and cannot be reconciled. In this thesis, a unified approach is developed for the general case to achieve a complete understanding of these two effects. On the experimental aspect, we first develop a new fabrication recipe with a focus on the accurate dimensional control and low-loss performance. HSQ is employed as the electron-beam resist, and the lithography and plasma etching steps are both optimized to achieve vertical and smooth sidewalls. A third-order temperature-insensitive coupled-resonator filter is designed and demonstrated in the silicon-on-insulator (SOI) platform, which serves as a critical building block element in terabit/s on-chip networks. Two design challenges, i.e., a broadband flat-band response and a temperature-insensitive design, are coherently addressed by employing the redundant bandwidth of the filter channel caused by the dispersion as thermal guard band. As a result, the filter can accommodate 21 WDM channels with a data rate up to 100 gigabit/s per wavelength channel, while providing a sufficient thermal guard band to tolerate more than ±15°C temperature fluctuations in the on-chip environment. In this thesis, high-Q microdisk resonators are also proposed to be used as low-loss delay lines for narrowband filters. Pulley coupling scheme is used to selectively couple to one of the radial modes of the microdisk and also to achieve a strong coupling. A first-order tunable narrowband filter based on the microdisk-based delay line is experimentally demonstrated in an SOI platform, which shows a tunable bandwidth from 4.1 GHz to 0.47 GHz with an overall size of 0.05 mm². Finally, to address the challenges for the resonator-based delay lines encountered in the SOI platform, we propose to vertically integrate silicon nitride to the SOI platform, which can potentially have significantly lower propagation loss and higher power handling capability. High-Q silicon nitride microresonators are demonstrated; especially, microresonators with a 16 million intrinsic Q and a moderate size of 240 µm radius are realized, which is one order of magnitude improvement compared to what can be achieved in the SOI platform using the same fabrication technology. We have also successfully grown silicon nitride on top of SOI and a good coupling has been achieved between the silicon nitride and the silicon layers.

Disertacija skirta krūvininkų rekombinacijos tyrimams plačiatarpiuose nitridiniuose puslaidininkiuose bei jų dariniuose. Kompleksiniai eksperimentiniai tyrimai buvo atlikti naudojant kelias skirtingas metodikas. Atlikti krūvininkų dinamikos GaN sluoksniuose tyrimai labai žemų ir aukštų sužadinimų sąlygomis. Pasiūlytas naujas liuminescencijos gesimo kinetikų interpretavimo metodas, siejant liuminescencijos ir šviesa indukuotų dinaminių gardelių kinetikas. Naujas požiūris į geltonosios liuminescencijos juostą GaN sluoksniuose leido susieti geltonosios liuminescencijos intensyvumą su krūvininkų gyvavimo trukme. Skirtingomis technologijomis augintų AlGaN sluoksnių palyginimas suteikė informacijos apie juostos potencialo fliuktuacijas bei krūvininkų gyvavimo trukmę ribojančius veiksnius AlGaN medžiagose. Atskleista naujų krūvininkų dinamikos daugialakštėse AlGaN/AlGaN kvantinėse duobėse ypatumų – vidinio elektrinio lauko bei kvantinės duobės pločio fliuktuacijų sąlygotos lokalizacijos įtaka krūvininkų dinamikai. Dauguma tirtų bandinių buvo auginti naudojant MEMOCVDTM technologiją ir tyrimai patvirtino šios technologijos potencialą siekiant pagerinti medžiagų kokybę. / The thesis is dedicated to carrier recombination investigations in wide-band-gap semiconductors and their structures. The complex experimental studies were performed by combining several different techniques. Carrier dynamics in GaN epilayers were investigated under extremely low and high excitation conditions. A new method for interpreting photoluminescence decay kinetics was suggested by interrelating luminescence and light-induced grating decay transients. The new approach for studies of yellow band in GaN was shown by linking the carrier lifetime with yellow band intensity. Two AlGaN epilayers grown by different novel growth techniques were compared and the factors limiting carrier lifetime were identified. Moreover, more evidence on alloy mixing and band potential fluctuations in AlGaN was provided by our study. Essential knowledge was attained about carrier dynamics in high-Al-content AlGaN/AlGaN multiple quantum well structures: the influence of built-in electric field and carrier localization on carrier dynamics. Most of the samples under study were grown by MEMOCVDTM growth technique, and our study confirmed the high potential of this innovative growth technique for improving material quality.

The thesis is dedicated to carrier recombination investigations in wide-band-gap semiconductors and their structures. The complex experimental studies were performed by combining several different techniques. Carrier dynamics in GaN epilayers were investigated under extremely low and high excitation conditions. A new method for interpreting photoluminescence decay kinetics was suggested by interrelating luminescence and light-induced grating decay transients. The new approach for studies of yellow band in GaN was shown by linking the carrier lifetime with yellow band intensity. Two AlGaN epilayers grown by different novel growth techniques were compared and the factors limiting carrier lifetime were identified. Moreover, more evidence on alloy mixing and band potential fluctuations in AlGaN was provided by our study. Essential knowledge was attained about carrier dynamics in high-Al-content AlGaN/AlGaN multiple quantum well structures: the influence of built-in electric field and carrier localization on carrier dynamics. Most of the samples under study were grown by MEMOCVDTM growth technique, and our study confirmed the high potential of this innovative growth technique for improving material quality. / Disertacija skirta krūvininkų rekombinacijos tyrimams plačiatarpiuose nitridiniuose puslaidininkiuose bei jų dariniuose. Kompleksiniai eksperimentiniai tyrimai buvo atlikti naudojant kelias skirtingas metodikas. Atlikti krūvininkų dinamikos GaN sluoksniuose tyrimai labai žemų ir aukštų sužadinimų sąlygomis. Pasiūlytas naujas liuminescencijos gesimo kinetikų interpretavimo metodas, siejant liuminescencijos ir šviesa indukuotų dinaminių gardelių kinetikas. Naujas požiūris į geltonosios liuminescencijos juostą GaN sluoksniuose leido susieti geltonosios liuminescencijos intensyvumą su krūvininkų gyvavimo trukme. Skirtingomis technologijomis augintų AlGaN sluoksnių palyginimas suteikė informacijos apie juostos potencialo fliuktuacijas bei krūvininkų gyvavimo trukmę ribojančius veiksnius AlGaN medžiagose. Atskleista naujų krūvininkų dinamikos daugialakštėse AlGaN/AlGaN kvantinėse duobėse ypatumų – vidinio elektrinio lauko bei kvantinės duobės pločio fliuktuacijų sąlygotos lokalizacijos įtaka krūvininkų dinamikai. Dauguma tirtų bandinių buvo auginti naudojant MEMOCVDTM technologiją ir tyrimai patvirtino šios technologijos potencialą siekiant pagerinti medžiagų kokybę.

No description available.

No description available.

The growth of thin-film metal nitride materials from elemental metal targets by plasma-assisted pulsed laser deposition (PLD) has been explored and analysed. A new UHV PLD growth system has been installed and assembled and its system elements were calibrated. A series of GaN thin films have been grown to calibrate the system. In-situ RHEED indicated that the films were single crystal and that growth proceeded in a three-dimensional fashion. SEM images showed heavy particulation of film surfaces that was not in evidence for later refractory metal nitride films. This may be connected to the fact that Ga targets were liquid while refractory metals were solid. Most GaN films were not continuous due to insufficient laser fluence. Continuous films did not exhibit photoluminescence. HfN films have been grown by PLD for the first time. Films grown have been shown to have high reflectivity in the visible region and low resistivity. These factors, along with their crystal structure, make them suitable candidates to be used as back-contacts in GaN LEDs and could also serve as buffer layers to enable the integration of GaN and Si technologies. Growth factors affecting the films’ final properties have been investigated. Nitrogen pressure, within the operating range of the plasma source, has been shown to have little effect on HfN films. Substrate temperature has been demonstrated to have more influence on the films’ properties, with 500 °C being established as optimum. ZrN films have also been grown by PLD. Early results indicated that they exhibit reflectivities 50 % ± 5 % lower than those of HfN. However, further growth and characterisation would be required in order to establish this as a fundamental property of ZrN as nitride targets were mostly used in ZrN production. Single-crystal epitaxial GdN and SmN films have been produced by PLD. This represents an improvement in the existing quality of GdN films reported in the literature, which are mostly polycrystalline. In the case of SmN, these are the first epitaxial films of this material to be grown. Film quality has been monitored in-situ by RHEED which has allowed growth to be tailored to produce ever-higher crystal quality. Post-growth analyses by collaborators was also of assistance in improving film growth. Substrate temperatures and nitrogen plasma parameters have been adjusted to find optimum values for each. In addition, laser fluence has been altered to minimise the presence of metal particulates in the films, which interfere with magnetic measurements carried out in analyses. Capping layers of Cr, YSZ or AlN have been deposited on the GdN and SmN prior to removal from vacuum to prevent their degradation upon exposure to atmospheric water vapour. The caps have been steadily improved over the course of this work, extending the lifetime of the nitride films in ambient. However, they remain volatile and this may persist since water vapour can enter the film at the edge regardless of capping quality. Optical transmission has shown an onset of absorption at 1.3 eV for GdN and 1.0 eV for SmN.

Studies have been done to determine rare-earth elements’ optical and luminescent properties using wide bandgap nitride semiconductors as suitable hosts. Research done here will contribute to the information needed to further study rare-earth elements and their unique properties. Thin films of rare-earth elements erbium, terbium, ytterbium, and both erbium and ytterbium doped into AlN are studied by laser excitation. A 532 nm Nd: YAG green laser and 783nm crystal infrared laser are used for excitation in conjunction with a spectrometer to measure photoluminescence. With the 532 nm laser, AlN: Er emits peaks at 554 nm, 561 nm, and 1552 nm, AlN: Tb emits peaks at 549 nm and 562 nm, AlN: Yb emits peaks at 966 nm, and co-doped AlN: ErYb contains peaks including both AlN: Er and AlN: Yb. Energy transfer occurred from Er to Yb resulting in an increased magnitude and peak shift. The 783 nm laser gave peaks at 1563nm for AlN: Er, 1508 nm and 1533 nm for AlN: Tb, and 1567nm for AlN: ErYb. No detectable peaks were given for AlN: Yb. A peak shift was detected in comparison of AlN: Er and AlN: ErYb. A magnetic field of 1000 G was applied to AlN: ErYb resulting in an increase in intensity of the major peak at 561nm with a splitting, creating a secondary peak at 564.5 nm. Biomedical applications can be used from the high penetration ability of lower wavelength lasers and the use of a magnetic field, which is not harmful to the human body. Enhanced green emission in erbium can be useful in future optical, photonic, and electrical devices. / Department of Physics and Astronomy

The phenomenal growth rate for the use of engineering ceramics is attributed to successful scientific responses to industrial demand. These materials are replacing metal and its alloys in diverse applications from cutting tools and heat engine components to integrated circuits. Joining technology plays a vital role in this changing and evolving technology as success and failure comes with breaking new barriers. It is important to improve existing techniques and to develop new techniques that reliably join simple shape components to form complex assemblies or join dissimilar materials such as metal to ceramic. Joining of ceramics is not simple due to their high chemical stability and low coefficient of thermal expansion (CTE). Joining between metal and ceramic is usually carried out at elevated temperatures and upon cooling thermal residual stresses are induced that lead to joint failure or poor strength. Most metal-ceramic joints cannot be used over 500°C primarily due to the low melting temperature of the interlayer. This investigation was concerned with the successful joining for higher temperature applications (above 500°C) of two dissimilar high temperature oxidation and corrosion resistant materials, Fecralloy and silicon nitride. The primary focus was on the effects of process conditions upon the microstructure and mechanical properties of the joint and to also study/identify the joining mechanism. Two novel techniques were employed to join successfully the metal to ceramic. The first was by use of a thin Cu foil that did not remain after joining. Joining occurs by a process that results in partial melting of the Fecralloy interface, where Fe, Cr, Al and Cu reactively infiltrate into the silicon nitride. This liquid mixture causes partial dissolution of the silicon nitride interface, where Si and N diffuse into the Fecralloy. A thin reaction product layer was formed at the silicon nitride interface and our results suggested that this was AIN. The free surface Si and porosity of the silicon nitride along with the eutectic temperatures above 1100°C are all vital for this joining process. The highest average shear strength of a Fecralloy-silicon nitride joint produced by the method was 67.5 MPa. The second route was that of a powder metallurgy one, where cold pressed Ni-Al (1:1 molar) compacts were used to join successfully the Fecralloy to silicon nitride. The formation of NiAl from its constituents is highly exothermic and this is initiated between 500-650°C. The high temperature reached causes partial melting of the Fecralloy interface and dissolution/reactive wetting at the silicon nitride interface. Mostly Fe infiltrates the NiAl improving room temperature ductility, fracture toughness and yield strength. Molten Al from the interlayer reacts and wets the silicon nitride interface with small amount of infiltration and no reaction product forming. The reaction synthesis of NiAl was studied using DTA and TGA, where the effects of Ni particle size and heating rate were investigated. This joining process is highly dependant upon process conditions, the most important of which are applied pressure, heating rate and Ni/A1 particle size. The highest average shear strength attained was 94.30 MPa and this is attributed to good interfacial bonding, high pressure, moderate process temperature and dwell time. The exothermic formation of the NiAl interlayer that is densified and monophase was paramount for this joining process. The Bansal-Doremus kinetic model for evaluating the kinetic parameters from non-isothermal DTA data was shown to be valid. The results obtained were identical to those by other authors who used a different model and approach.

No description available.

While modern epitaxial methods enable precise, monolayer (ML) control of the thin film deposition process, the complexity of certain device structures is ultimately limited by the capability and cost of the fabrication process. The objective of this work is to develop a pathway toward three-dimensional epitaxy (3DE) - the ability to intentionally and dynamically pattern regions of a film during the deposition process - in order to enable novel device concepts unbound by the traditional device fabrication paradigm. This work pioneers UV-assisted metal organic molecular beam epitaxy (MOMBE) as a particularly selective epitaxy technique to create a pathway toward 3DE of a crucial and topical material system - the III-Nitrides. A novel UV-assisted MOMBE system is developed enabling intense UV irradiation of films during growth. High quality, heavily (unintentionally) carbon-doped GaN is successfully grown by NH₃-based MOMBE and for the first time InGaN, AlGaN, and magnesium-doped GaN are demonstrated by NH₃-based MOMBE. Intense UV irradiation of films during NH₃-based MOMBE significantly enhances photo-desorption of species during the growth process, subsequently affecting the resultant InGaN alloy composition, carbon dopant concentration, or magnesium dopant concentration. A digital micromirror device is introduced to pattern incident UV radiation during InGaN growth, demonstrating that the effects of photoexcitation during MOMBE which have been proposed, discovered, and identified by this thesis indeed can be leveraged to deposit an InGaN film that is compositionally patterned within the growth plane. The results demonstrate that the new approach presented herein is possible for the 3DE of III-Nitrides if additional challenges in practical implementation can be overcome.