Chalcogenide glasses have a wide transmission window which can extend from the far-visible to the mid-infrared (IR) range. They have very high optical nonlinearity compared to silica glasses, low phonon energies, photosensitive properties and can be doped with rare earth elements. Chalcogenide glasses have attracted the research for different optical applications such as optical imaging, integrated optics and all-optical switching. In this Project, chalcogenide glasses were Radio-Frequency (RF) sputtered to form two Bragg mirror structures both with a stop-bands in the telecommunications and near-infrared and each comprised of alternating layers of two chalcogenide glass pairs: AS4oSe601Ge17As!sSe65 and GelsAs1SSel7 Te53! Ge17As18Se65 (atomic %). Two alternative techniques: hot embossing and Inductively Coupled Plasma (ICP) etching were used to fabricate rib waveguides in RF sputtered As40Se60 chalcogenide glass thin films on Gel7AslSSe6S chalcogenide glass substrates. Previous work on hot embossing of thermally evaporated thin films has produced rib waveguides with a reported optical loss down to 2.9 dB cm-1. In this work, hot embossing was used to fabricate successfully rib waveguides with optical loss < 0.78 dB cm•1 for the quasi-TE mode and < 0.81 dB cm-1 for the quasi-TM mode. ICP etching was also successfully used to fabricate rib waveguides in similar sputtered films with optical loss < 1.05 dB cm-l for the quasi-TE mode and < 1.12 dB cm-l for the quasi-TM mode. Measurements of optical properties such as linear absorption coefficient (a), linear refractive index (no), bandgap energy (Eg), nonlinear refractive index (n2) and nonlinear absorption coefficient (β2) were carried out on chalcogenide glass samples with compositions varied systematically within the ternary systems As-S-Se and As-Se-Te. These fundamental measurements are important in the further development of any chalcogenide based optical component.

One of the most exciting studies of contemporary physics is that of high temperature superconductor (HTS). Since its discovery, a large body of experimental and theoretical work by various groups has attempted to achieve a common understanding of this phenomenon. One of the main driving forces for applications centres on the possibility of new and improved microwave devices based on HTS materials, mainly due to the large reduction in the surface resistance that HTS has to offer. However, various problems need to be addressed before the use of HTS materials can be justified, for example fundamental issues such as the nonlinearity of these materials with respect to microwave power, which form the basis of this work. Microwave measurements were conducted on four magnetron sputtered and three laser ablated, Icm2 YBCO thin films, grown on LaAI03 and MgO substrates, respectively, employing the dielectric (rutile) resonator and coplanar resonator techniques. The low power response of the films was initially analysed, looking for clues to the underlying pairing mechanisms in these films. Power dependence and microwave intennodulation distortion (lMD) measurements were conducted between 12 K to 60 K to investigate the nonlinear response of the films, both in zero and finite dc (10 mT) fields. The effect of patterning on the microwave response of the films was also studied. From these measurements, it was observed that the microwave losses of these films are extrinsic in nature, probably a consequence of weak links/defects, and the results also show that films fabricated from the same technique can have significantly varying quality.

Over the past 5 years there has been an increase in the number of applications that require devices that operate in the millimetre range (30-300GHz). This demand has driven research into " devices that will operate at frequencies above 100GHz. This performance has been achieved using two main technologies, the Heterojunction Bipolar Transistor (HBT) and the High Electron Mobility Transistor (HEMT). At present it is a HEMT device that holds the record for the highest operating frequency of any transistor. It is this technology that this project concentrates on. In order to fabricate devices that operate at these frequencies two methods are commonly employed. The first is to vary the material of the device, in particular, increasing the indium content of the channel. The second method is to reduce the physical dimensions of the transistors, including reducing the gate length of the device therefore reducing transit time and gate capacitance. Reducing the separation of the source-drain ohmic contacts or employing a self-aligned ohmic strategy reduces the associated parasitic resistances. This project will concentrate on the scaling of the gate length in addition to the reduction of parasitic resistances with the use of self-aligned ohmic contacts. This work includes the realisation of the first self-aligned 120nm T -Gate. GaAs pHEMT fabricated at the University of Glasgow. These devices required the development of two key technologies, the non-annealed ohmic contact and the succinic acid based selective wet etch. The self-aligned devices showed good RF performance with a ft of 150 GHz and a fmax of 180 GHz which compares favourable with results o~ 120nm GaAs pHEMTs previously fabricated at Glasgow. The investigation of gate length scaling to device performance included the development of two lithographic process capable of producing HEMT with a gate length of 50nm and 30nm respectively in addition to a method ~f sample preparation that allows these devices to be analysed using TEM techniques. This work has lead to the realisation of SOnm T -gate metamorphic HEMTs using a PMMAIcopolymer resist stack, these devices displayed an excellent yield, with over 95% of devices working. The uniformity of the gate process was also high with a threshold voltage of - 0.44SV with a standard deviation of O.OOSV. The devices demonstrated an .it of 330GHz and a fmax of 260GHz making these devices some of the fastest transistors that have ever been fabricated on a GaAs substrate. The second lithography process was developed to realise T -gates with a gate length of less than SOnm. This processed used a two stage "bi-lithography" process to minimise the effect of forward s7attering through the resist. The gate footprint was transferred into a Si02 gate by a dry etch process. This lithography process was integrated into a full process flow for lattice matched InP HEMTs Using this process, HEMTs were fabricated with a T-gate of 2Snm. This is the smallest T -gate device that has been fabricated at the University of Glasgow and is comparable with the smallest HEMT devices in the world.

This thesis presents a ne- approach to the fabrication of short gate length HI-V High Electron Mobility Transistors (HEMTs) that reduces the impact of external parasitic elements, and in particular access resistances, upon device performance. This was approached through the development of a self-aligned T-gate process with non-annealed ohmic contacts. The process was used to fabricate both GaAs pseudomorphic HEMT and subsequently lattice matched InP devices. In addition, a new selective recess etch was developed for cap layers containing indium. Characterisation of the self-aligned GaAs pHEMT devices indicated good RF performance with fT = 137GHz and fmax = 182GHz for devices of 120nm gate length, although DC performance was found to be restricted by the unoptimised non-annealed ohmic process. Analysis of the operation of the GaAs pHEMT devices led to the design and growth of an InP material structure incorporating double delta doping to minimise the non-annealed ohmic contact resistance. Using this optimised structure, standard and self-aligned HEMT devices with gates of length 120nm and 70nm were fabricated for comparison. The benefits and limitations of the self-aligned process were highlighted by comparing the performance of the self-aligned and standard devices. The self-aligned 120nm devices had fT = 220GHz and fmax = 255GHz, which rose to fT = 270GHz and fmax = 300GHz for the 70nm devices. Transconductance figures of up to 1500mS/mm were extracted for both. It is concluded that the self-aligned process, although beneficial to device performance at the 120nm, and to a lesser degree the 70nm node, would begin to degrade performance at reduced gate lengths due to increased parasitic gate capacitances. The non-annealed ohmic technology developed in this work provides a route that minimises parasitic resistances and increases performance without the increased parasitic gate capacitances associated with a self-aligned gate approach. A possible solution for the minimisation of parasitic gate capacitances using a self-aligned approach is proposed.

The potential of Calcium-Vanadium garnet loaded binary composites for use in the production of planar frequency agile microwave devices has been investigated. A WR90 rectangular waveguide system using the transmission/reflection technique has been used to compare effective medium theory predicted permittivities and permeabilities for unmagnetised and transversely magnetised composites with actual measured composite properties. Use of the the Bruggeman effective medium theory with manufacturer supplied garnet permittivity and values of garnet permeability calculated using simple empirical models were demonstrated to be as accurate as predictions made using the measured properties of the composites' individual constituents. Errors in predicted material properties for unmagnetised and transversely magnetised samples relative to measured data were less than 5% across almost half of the 8.2 -12.4GHz measurement band and within a worst-case error of 15% across the whole band. A series of end-coupled linear microstrip resonators using garnet-loaded composite substrates has been fabricated and tested. Tunabilities in resonance of up to 3.9% at 9.2GHz have been demonstrated for DC magnetic bias fields applied transverse to the microwave magnetic field component. An entirely new and previously unpublished broadband microstrip technique for the measurement of the effective permittivity and permeability of bulk gyromagnetic and gyromagnetic material loaded composite substrates subject to transverse DC magnetic bias fields is presented. This will have wide application in the design of frequency agile microwave integrated circuits.