The continuing need for increased information storage capacity has driven a remarkable increase in areal density over the lifetime of hard disk technologies, built on the physical principles of nano magnetic structures. Understanding how the component nano-granular magnetic recording media operates at ultra-high densities and how novel switching mechanisms, such as heat assisted magnetic recording (HAMR), impact the nature of magnetic data storage is essential. In this thesis current state-of-the-art macro scale modeling methods, built on the principles of the Landau-Lifshitz-Bloch equation, are developed and applied to better understand the physical principles that govern the ultra-high density recording media. The modeling method is shown to be in close agreement with experiment in a number of situations. The dependence of magnetic damping, the combination of intrinsic and extrinsic damping, due to inter-granular interaction is shown to be significant. The form of the damping arises due to a change in the degeneracy of the ferromagnetic frequency of spinwaves, as a function of both increasing magnetostatic and inter-granular exchange interactions. The observed damping results in a nontrivial dependence of the magnetic switching time on intergranular interactions, within the range for the intergranular exchange and saturation magnetisation that is likely in ultra-high density recording media. The nontrivial nature of the switching time should be taken in to consideration when selecting materials for the magnetic grains and inter-granular regions. A detailed investigation of the HAMR process is made, concentrating on the thermodynamic limits of the technology. The nature of HAMR is shown to be far more complex than simply magnetisation reversal over a thermally reduced energy barrier. It is shown that, to achieve the required level of magnetisation reversal a number of factors must be considered. The temperature rise must be to the Curie point or above, invoking the linear reversal mechanism, with a cooling rate that is sufficiently low to allow the temperature of the media to remain higher than the blocking temperature for a period of time significantly larger than the relaxation time of the material. Also, the write field must be sufficiently large not only to reverse the magnetisation, but also to ensure no thermally activated back switching of the magnetisation, as in the concept of thermal writability. Also a new method for approximating the magnetostatic field to a high level of accuracy with a computational runtime that is comparable with the tensor form of the dipole approximation has been developed and tested.

This thesis presents a collection of new novel techniques using the bulk-driven approach, which can lead to performance enhancement in the field of CMOS analogue amplifier design under the very low-supply voltage constraints. In this thesis, three application areas of the bulk-driven approach are focused – at the input-stage of differential pairs, at the source followers, and at the cascode devices. For the input stage of differential pairs, this thesis proposes two new novel circuit design techniques. One of them utilises the concept of the replica-biased scheme in order to solve the non-linearity and latch-up issues, which are the potential problems that come along with the bulk-driven approach. The other proposed circuit design technique utilises the flipped voltage scheme and the Quasi-Floating Gate technique in order to achieve low-power high-speed performances, and furthermore the reversed-biased diode concept to overcome the issue of degraded input impedance characteristics that come along with the bulk-driven approach. Applying the bulk-driven approach in source followers is a new type of circuit blocks in CMOS analogue field, in which to the author’s best knowledge has not been proposed at any literatures in the past. This thesis presents the bulk-driven version of the flipped voltage followers and super source followers, which can lead to eliminating the DC level shift. Furthermore, a technique for programming the DC level shift less than the threshold voltage of a MOSFET, which cannot be achieved by conventional types of source followers, is presented. The effectiveness of the cascode device using the bulk-driven approach is validated by implementing it in a complete schematics design of a fully differential bulk-driven operational transcoductance amplifier (OTA). This proposal leads to solving the lowtranconductance problem of a bulk-driven differential pair, and in effect the open loop gain of the OTA exceeds 60dB using a 0.35μm CMOS technology. The final part of this thesis provides the study result of the input capacitance of a bulk-driven buffer. To verify the use of the BSIM3 MOSFET model in the simulation for predicting the input capacitance, the measurement data of the fabricated device are compared with the postlayout simulation results.

No description available.

This project is primarily concerned with the incorporation of quantum effects into physical models for heterojunction field effect transistors. Several simulations have been developed including a one-dimensional Schottky-gate model which self-consistently solves the effective mass Schrodinger equation with Poisson’s equation. This model employs a fast, accurate and robust solution algorithm based upon an expanded Newton scheme. This work is extended to two-dimensions, permitting charge transport and hence adding the current-continuity equation. All three equations are solved under non-equilibrium conditions. Finally a quasi-two-dimensional HFET model has been written, also including quantum mechanics which produces excellent agreement with measured characteristics. As a rigorous solution of the full two-dimensional Schrodinger equation and corresponding transport equation is very demanding and computationally expensive the problem has been simplified to by assuming the electron wavefunction to take the form of Bloch, or travelling wave solutions is the directions parallel to the heterojunction interface is then solved by taking multiple one-dimensional solutions sampled at various positions throughout the device. This new approach requires alternative solution algorithms to be developed since the conventional schemes are not applicable. This thesis reviews the physics behind semiconductor heterojunctions, discusses the solution schemes used in the models and presents results from the one-dimensional, two-dimensional and quasi-two-dimensional simulations.

Zinc oxide (ZnO) has long been advanced as a low cost, earth-abundant transparent conducting oxide (TCO) with potential as a replacement for high-performance, but costly, indium oxide (In2O3) based materials in a wide range of technological applications. However, despite decades of research and development efforts, ZnO-based materials have still failed to displace the incumbents in any large-scale applications. Given the compelling materials cost advantages of ZnO, it is almost certain that its poor adoption is due to deficits in its technical performance. This thesis aims to fulfill the need for systematic, fundamental work to identify and examine the factors that limit TCO performance, and in particular, those that limit ZnO relative to In2O3. Using spray pyrolysis as the primary deposition method, many different series of ZnO and In2O3 films have been prepared and examined using a range of chemical, structural, and optoelectronic characterization techniques. After essential background information on the basic physics and chemistry of TCOs, as well as a detailed discussion of the chosen deposition and characterization methods, three main classes of performance limitations will be covered: 1) those related to the intrinsic properties of electronic transport in crystalline TCO domains, 2) those arising in the course of impurity doping, and 3) those occurring due to grain boundary effects and the polycrystalline nature of thin film TCO samples. Taken together, these results will show that preparing ZnO-based TCOs with performance approaching that of the best In2O3-based materials, while very likely to be technically possible, will almost certainly involve overcoming significant engineering and process development challenges that, importantly, are not required to make high quality In2O3. Ultimately, whether ZnO will ever find significant, real-world use as a TCO will depend on whether the deep differences between ZnO and In2O3 performance limits that will be highlighted and examined in this thesis can be bridged in a practical and cost-effective manner.

Fe-chalcogenides are of great interest because they have the simplest structure in the Fe-based superconductors, and show low anisotropies, high critical fields and high current densities all of which make these compounds useful for understanding Fe-based superconductors, a new class of superconductors with unusual properties, both for fundamental physics and potential high-field applications. The main part of this thesis concentrates on the fabrication and characterization of FeySe1-xTex (Fe-11) thin films in order to link superconductivity and microstructure in this system. Since sputtering has been rarely used for the deposition of Fe(Se,Te) films, we used sputtering to study the feasibility of this technique for this purpose and to optimize the processing conditions. Phase evolution and texture development as a function of processing conditions are studied, and the optimum conditions for the best quality film are found. Crystallization temperature of the 11-phase was found to be 250°C, and texture is shown to be dependent on substrate, substrate temperature and film thickness which strongly control in-plane alignment. The best texture is obtained at 315°C for films thinner than 50nm on the MgO. Texture of the films becomes poorer on the substrates in the order MgO > LAO > STO > CaF2 > LiF > LSAT, and the relationship between texture and substrate is shown to be more related to the growth mechanism rather than lattice misfit. 2D layer-by-layer growth is found for the LSAT, while the films on the LAO and MgO show 3D island growth, and a combination of both growths is found for the film on the STO. The films grown by in-situ sputtering are shown to have better quality than those grown by the ex-situ. A sharp TC of about 10.2K is obtained for a single crystal Fe1.01Se0.56Te0.44.film with a thickness of 58nm. Most of the deposited films are Fe-rich (y > 1) due to compositional variation in the target and the volatility of Se and Te. Extra Fe is the main detrimental parameter for the suppression of superconductivity in these films showing the necessity of using multiple sources in sputtering. Nano-scale inhomogeneities are also observed in the majority of the films possibly as a result of different Se/Te ratios and Fe3O4 impurity. Substitution of 2% Mn for Fe in the FeySe1-xTex structure leads to the stabilization of tetragonal 11 phase and improvement of crystallographic texture. Mn atoms are shown to occupy interstitial Fe sites in the tetragonal unit cell. I also applied TEM technique to functional materials from external collaborators including Bi2Te3 and CaFe2As2. Studies of Mn-doped Bi2Te3 single crystals reveal that at doping concentrations, Mn incorporates with the Bi2Te3 structure differently. At low doping concentrations (9%Mn), Mn atoms are randomly substituted onto the Bi sites resulting in the variation in both composition and lattice parameter. At high doping concentrations (15%Mn), supersaturation occurs and the extra Mn atoms occupy the interstitial sites in addition to forming nano-scale Mn-rich precipitates. CaFe2As2 single crystals grown by the FeAs-flux method are unstable FeAs-supersaturated crystals containing a high concentration of stacking faults. Annealing at high temperature (700°C) causes the extra FeAs to form needle-shaped precipitates perpendicular to the c-axis by Ca diffusion, and the matrix develops dislocations. Annealing at lower temperature (400°C) results in a number of stacking faults and similar precipitates with low Ca. As a result, the microstructure of the CaFe2As2 single crystals, especially the amount of strain, can be controlled by the annealing temperature resulting in similar effects to applying external pressure.

No description available.

On a global scale, the mass production of organic semiconductors for electronic applications in flexible display technology, among others, is linked to both research and consumer demand. To overcome challenges, the fabrication of the organic electronic devices has moved from high cost small-molecule vapour-phase deposition type techniques, to relatively low cost solution processing of the organic thin film transistors (OTFTs). Initial studies in this work focused on the modelling, prediction and synthesis of two dimensional planar structures. These studies probed the isomeric structures of pentacene. Quantum mechanics/density functional theory calculations found that at least five out of a total of 12 isomers (inclusive) are both air stable and have the same order of magnitude for the hole and electron rates of charge transfer as that of pentacene; suggesting that isomeric candidates of the initial target structure are worthy of thorough investigation. A second study on planar structures focused on novel and rationally designed molecules, six isomers inclusive, in an attempt to develop a challenging but high performing seven fused ring heteroacene. The study gave the interesting result that one structure is suitable for not just OTFTs, but also non linear optics, inferring that highly functional optoelectronic switches are possible from a single OTFT. A small library of rod-like structures were probed to contrast with the nature of the structures investigated so far. These contain linear components that separate the planar aromatic components in the molecules. It was discovered that these structures are highly flexible and soluble in common organic solvents, showing promise for applications as solution processed OTFTs. In summary, key discoveries of the study include (i) world-class candidates could be out-competed by their own isomers; (ii) new heteroacenes conceived could have bifunctional outputs for two separate fields in organic electronics; (iii) novel molecular rods were synthesised and found to have excellent solubility in common organic solvents, thereby enabling large scale fabrication of flexible display technologies.

No description available.

The memristor has been proposed as the fourth circuit element. Among the emerging nano-technologies, the memristor has become a very promising candidate for building storage structures because of its shorter switching time, higher capacity and lower power consumption. In this thesis, I will first introduce a new memristor model with controllable window functions, which is more authentic and flexible than those existing memristor models. Then I will present my novel design of a Memristor Content Addressable Memory (Memristor-CAM) structure that is based on my own design of Memristor-CAM cells. The major contribution of this work is the fuzzy look-up functionality, which is achieved by summing up the current of the matched cell lines in the Memristor-CAM. In addition, this fuzzy look-up functionality of the new Memristor-CAM design could be further extended in order to fit into a lot of practical applications. With the benefits of memristors, this Memristor-CAM storage structure could reduce the power consumption, increase the capacity and improve the performance of computer memory. My new design is tested in a common experimental design that includes computer simulations and circuit emulations. The results of my experiments support the validity of my contributions and allow further analysis and insights on the behaviours of memristors when different settings are applied.