The recent explosion in the application of networking technology in many environments and on many scales has been facilitated by advances in both electronic and optical technology. The use of optical fibre transmission has increased greatly the data capacity of networks and at the same time reduced their cost. The increase in speed and complexity and the reduction in cost associated with VLSI electronic technology has brought about cheaper and more intelligent network components. In particular, faster and denser electronic memory means that buffering within the network is now prevalent and the increased speed of electronic gates has enabled a finer granularity of switching. The multiplexing strategy, Asynchronous Transfer Mode (ATM), takes advantage of these developments to integrate a variety of high bandwidth streams onto a single network whilst minimising inter-stream interference. The growth in transmission bandwidth requirements can be met by the increased capacity of glass fibre, but optical technology is insufficiently advanced for the more complex tasks involved in switching. Unfortunately, the buffering and control of data are currently impractical in the optical domain. Interconnects, which form the basis of all communications switches, can benefit from optical technology, particularly where they are large. The reduced complexity of an optical solution has many advantages. However, when compared to their electronic counterparts, many of the components used to build such devices have a relatively long path reconfiguration period during which no data can be transferred and this can seriously reduce the usable bandwidth of the interconnect. This dissertation shows how and where optical components can best be used in the construction of a hybrid opto-electronic ATM switch. It shows how the large bandwidth associated with optical interconnects can be used to overcome the problems of matching inputs to outputs during the arbitration process. It describes three new scheduling schemes for such a switch that take into account the reconfiguration penalty. Simulations involving a single switch and two switches in series are performed to measure the effects the schemes have on throughput and delay, and a new switching strategy, based on these schemes, is proposed especially for switches constructed from optical components. Finally, a simulation of a passive optical network shows how the granularity of scheduling, and therefore the quantity of control information, in such systems can be reduced using a similar scheme.

GaAs was introduced as a thermoelectric material in a micromachined thermal detector. GaAs/Au thermocouples were fabricated on air-bridges with lengths ranging from 40 to 650 <I>μ</I>m by anisotropic etching to undercut the GaAs substrate with photoresist patterns delineated by optical lithography. The devices have been characterized with a 10.6 <I>μ</I>m CO<SUB>2</SUB> laser. The responsivity and time constant for various lengths were determined in both air and vacuum conditions. The time constants ranging from 50 <I>μ</I>s up to 2.2 ms were among the fastest for micromachined thermal detectors; the lower responsitivities were inevitable trade-offs. An analytical thermal transport model of the air-bridges was used to evaluate the device performance and deduce useful physical parameters from the experimental data such as the heat-transfer coefficient. The spectral response in the wavelength band from 1 to 12 <I>μ</I>m, measured by using a monochromator, also highlighted the importance of antireflection coatings on the responsivity. To achieve a fast response time without sacrificing the responsivity of the thermal detectors, antenna-coupled designs were considered. A small free-standing element with a short time constant was coupled to an antenna which collected the incoming radiation and provided gain to the self-heating element. Polysilicon air-bridges, instead of metallic ones, were demonstrated for the first time in such a system and provided a practical level of performance which was measured in the linear-polarized CO<SUB>2</SUB> laser system. Log-periodic and bow-tie antenna designs were chosen to facilitate a broadband response in the mid-infrared regime. The devices were able to operate in both thermocouple and bolometer modes both of which exhibited a higher responsivity than previously reported studies. The time constant was kept as short as 4 <I>μ</I>s. Measurements made at 10.2<I>μ</I>m wavelength indicated that the linearly polarized antennas had an on-axis cross-polarization ratio of about -4.5 dB and the antenna patterns in free-space had a directivity of approximately 5-9 dB. The deduced coupling efficiency was 34%. Measured frequency responses for modulated optical signals also agreed with the thermal transport models.

The GaN system has been of growing interest in the past decade for its potential as a blue-light emitter, and has rapidly achieved commercial importance, but the system also has very attractive properties for high power microwave-frequency transistors, notably formation of a two-dimensional electron gas at the AlGaN/GaN heterointerface, high electron velocity and a large breakdown electric field. The commercialization of such devices is now imminent. Ohmic contacts to GaN and AlGaN/GaN heterostructures are an essential part of the production of all optical and electronic devices from the GaN system, but the requirements are particularly stringent for high power microwave devices, and the contacts are still not well understood or optimized. To understand such contacts, and hence establish the major factors influencing electrical behaviour, studies of the microstructure of various contacts are required. These can then be linked with observed variations in contact resistance and morphology. The many techniques of transmission electron microscopy (ТЕМ) are the most powerful way to characterize contact microstructures, and here ТЕМ studies of several different Al/Ti-based metal contact structures to n-GaN and AlGaN/GaN wafers as a function of annealing temperature are reported. ТЕМ techniques include bright field and dark field imaging of metal microstructures, selected area diffraction identifications of metal and interfacial grains, chemical mapping to establish phase distributions using energy-filtered ТЕМ and energy-dispersive X-ray mapping, and high resolution lattice imaging to establish the identity and morphology of reaction phases at the metal/nitride interface. Links between contact structure or processing conditions and the resulting electrical behaviour are thus proposed. Thin, patchy nitride phases (such as α-AIN and TiN) were observed at the metal/AlGaN interface in good contacts, and a thick (8nm) uniform AlTi2N interfacial layer was found to cause poor contact behaviour.

This thesis describes the microstructural characterization (using transmission electron microscopy [TEM], high resolution X-ray diffraction [HRXRD] and atomic force microscopy [AFM]) of non-polar (11-20) and semi-polar (11-22) GaN epitaxial layers. They were grown under a range of conditions on sapphire by metal-organic vapour phase epitaxy (MOVPE). The majority of the dislocations were partial dislocations bounding BSFs. As a consequence, defects were confined to the c-plane. A 3D-2D growth transition (where 3D islands are grown initially, then coalesced) reduced the defect density of non-polar GaN. Optimised coalescence conditions (i.e. a V/III ratio of 50) produced smooth films with a low RMS roughness. The effect of a 3D-2D growth transition on semi-polar material was less clear, but the microstructure was similar to non-polar GaN, in that the majority of dislocations were partials bounding BSFs. Since the growth conditions for the semi-polar epilayers were not optimised, surfaces were rough. Quantum wells grown on these templates were found to follow the surface roughness resulting in large fluctuations in quantum well width (of a few nanometres) and subsequent broadening of peaks in the photoluminescence spectra. To reduce the defect density, epitaxial laterial overgrowth (ELOG), SiN_x interlayers and ScN interlayers were applied to non-polar and semi-polar films. ELOG was effective at reducing both BSF and dislocation density in both examples. Thick SiN_x interlayers in non-polar GaN reduced dislocation density by a factor of 500 and BSF density by a factor of 5, compared to the template. An 8.5 nm ScN interlayer in semi-polar GaN reduced the dislocation density by a factor of 200 and BSF density by a factor of 14.

Most of the previous attempts to make displays with weak anchoring conditions relied upon there being a weak interaction between the liquid crystal materials and the alignment layer. In this thesis, perfluorinated additives are used to make a slippery surface in order to induce weak anchoring conditions. This slippery surface approach via perfluorinated additives has the advantage that it can be utilized in current display systems without the need to change any of the mechanical components. The perfluorinated additives were designed to have an affinity with either the host LCs or the alignment layer in the LCD cells. The additives were tested in twisted nematic (TN) and super twisted nematic (STN) LCD cells and induced weak anchoring conditions depending on the chemical structure of the additives and the host liquid crystal mixture. In each experiment, 0.5% of additive was mixed with the chosen host and did not phase separate in the LCD cells. The response time of the host was reduced by 3.5% to 13% depending upon the host-additive combination used. Studying the relationship between the chemical structure of the additives and their effectiveness in reducing the response time of the host revealed that the structural requirements for an additive to decrease the response time of E7 (one of the nematic hosts used) are as follows: moderate polarity, medium or small molecular size, ester or ether linking groups or hydroxyl functional groups and a mixture of small and medium sized molecules. However, the reduction in the response time of the liquid crystal system was limited in some cases, because the reduction in the turn-on time was outweighed by the increase in the turn-off time of the host LC mixture. The effects of the perfluorinated additives on the flexo-electro optic switching of a chiral nematic liquid crystal were also examined. The perfluorinated additives reduced the response time of the host LC by 8% to 30%. We also found that the performance of the flexo-electro optic switching was mostly influenced by the effect of the additives on the bulk properties of the host LC (as opposed to the effect on the properties at the substrate). In conclusion the perfluorinated additives reduced the response time of the host by inducing a slippery surface in some limited cases and can be used for: providing the weak anchoring necessary for the manufacture of bistable displays; increasing the temperature range of blue phase materials and improving flexo-electro optic switching.

This thesis is part of a project entitled 'One thousand emitters per square millimetres', an EPSRC project funded under the Basic Technology Programme. The project is jointly run between Heriot-Watt University, The Institute of Photonics at the University of Strathclyde, Imperial College London and the University of Sheffield. The overall aim of the project is to design, manufacture and package a UV-LED array for a variety ofbio-medical applications. The objective of the research work presented here is the support to other partners in the assembly of a microlens array on top of a UV-LED array. This work focuses on the design, manufacture and characterisation of a packaging structure that places a microlens array on top of a micro-UV-LED array. The required lateral tolerance was defined to be less than 2/lm. Two approaches were considered and systematically characterised. In the static approach, the microlens array is to be accurately placed and fixed on the LED array. In the dynamic approach, the microlens array is moveable in the vertical as well as lateral directions. Post processing of the lens and LED arrays has been carried out using a modified UV-LIGA process. The microlens array rests on four posts, to reduce lateral as well as vertical contact area for the static approach, hence reducing the probability of misalignment. Using electroplating of electrodes that are situated on the micro-UV-LED array structure, vertical alignment of the microlens array with sub-micron accuracy has been demonstrated. The dynamic approach features a vertical electrostatic and a lateral magnetic actuator that can be driven simultaneously. Both actuation methods exert a force on the microlens array without the need of a physical contact, which eases the fabrication and assembly process. As a restoring means, gel bumps are currently employed which exhibit the necessary isotropic elasticity for the lateral and vertical movement. The electrostatic actuator is able to achieve displacements of over 70/lm and shows good repeatability with a standard deviation of 0.43/lm at a mean value of 24.75/lm. The magnetic actuator achieves lateral movement exceeding 50/lm. Both actuation methods have been demonstrated to offer sub-micron displacement accuracy. Due to the viscoelastic properties of the gel bumps and with appropriate equipment, accuracies in, the order of tens ofnanometres are feasible.­¬«

No description available.

In - V semiconductors are indispensable for, today's optoelectronic devices such as semiconductor lasers and optical amplifiers in the 1.3 urn wavelength band used for fibre optic communication systems. This has led to the invention of a dilute nitride GaInNAs that is lattice matched to GaAs for such applications. The research reported in this thesis is aimed to design, processinA and characterisation of 1.3 ,. ;,f •...... " ." urn GaInNAs optoelectronic devices, the operation of which is based on either longitudinal or vertical transport. Most of the attention was paid to the development of electrically driven Vertical Cavity Semiconductor Optical Amplifiers (VCSOAs). VCSOAs have a number .of advantages over Semiconductor Optical Amplifiers (SOAs), such as high coupling efficiency to optical fib er, polarisation independent gain, and low power consumption due to a small active volume. Previous devices have been limited by low output power and thermal problems. Furthermore, optical and electrical properties of the Distributed Bragg Mirrors (DBRs), commercial Vertical Cavity Surface Emitting Lasers (VCSELs), Hot Electron Light Emitting and Lasing in Semiconductor Heterostructures (HELLISH)-VCSOAs and Finally TOP-HAT HELLISH-VCSOAs have been studied.

No description available.

No description available.