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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

First principles simulations of the structure and incorporation of point defects in diamond

Atumi, Mohammed Khalifa January 2014 (has links)
Diamond is a radiation hard, a wide band-gap semiconductor, with high intrinsic carrier mobilities and high thermal-conductivity, allowing it to be used in extreme radiation environments, high temperatures and high power electronic devices. Additionally, based upon key defects, diamond has also recently emerged as a candidate material for a range of quantum-based applications including quantum information, single photon sources and high-sensitivity magnetometry. Importantly, the growth of diamond both in bulk and film form, has also radically improved over the past decade, so that use of high-quality diamond in a wide range of application is becoming more viable. Diamond synthesis, especially in the context of this thesis from the gas phase via chemical vapour deposition, is only partially understood. The defects which are incorporated during synthesis are specific to the growth method, and some key defects exhibit orientational polarisation relative to the growth surface orientation. Both the structure and polarisation of these defects are key witnesses to the growth mechanisms, and therefore developing atomistic structures is a key step towards a more comprehensive growth model. In this thesis, quantum chemical methods based upon density functional theory are used to determine the structure and incorporation mechanisms for key defect centres. It is crucial that quantum-mechanically based methods are used to provide both sufficient quantitative accuracy and to obtain the electronic properties key to comparison with the relevant experimental data, such as required for electron paramagnetic resonance centres including substitutional and interstitial nitrogen, nitrogen-vacancy and nitrogen-vacancy-hydrogen, and silicon containing centres. For the interstitial centres, it is shown that the models proposed from interpretation of the experimental data for the WAR9 and WAR10 centres are most probably incorrect, as is that of the WAR2 hydrogen-related centre. In contrast, the structures of the P1 epr centre, as well as the NV, NVH and SiV centres are not in dispute, but by simulating these centres in the upper most layers of (110), (111) and (001) diamond surfaces it is shown here that experimentally observed 100% polarisation of the N-related centres, and partial polarisation of the SiV complex can be explained for the (110) surface. The polarisation of defects can give information about how the defects incorporated during diamond growth, which in turn gives some indication of diamond growth mechanisms.

Fabrication and nano-scale characterisation of ferroelectric thin films

Karunakaran Ponon, Nikhil January 2014 (has links)
This thesis focuses on the fabrication and characterisation of BaTiO3 thin films. One of the aims is to deposit amorphous BaTiO3 films on conductive thin films through sputtering at temperatures compatible with semiconductor manufacturing, followed by post deposition annealing to crystallise these films. However, rapid thermal processing (RTP) is known to create pinholes and cracks due to thermal mismatches between the electrode and insulator, causing degradation of the film quality. Initial focus was to develop thin film electrodes which can withstand process temperatures above 800 C. Deposition conditions, including the nitrogen flow rate relative to that of argon during deposition were optimised to obtain TiNx with least resistivity and excellent material properties through reactive sputtering. TiNx films deposited at various nitrogen flow rates were then annealed in a non-oxidising condition and their properties were thoroughly studied. Films deposited at the highest nitrogen flow rate (95%) showed least variation in resistivity and showed excellent material properties even after a high temperature anneal. BaTiO3 films of varying thicknesses were deposited on TiNx using RF-sputtering and subjected to RTP at various temperatures. It was found that there exists a critical thickness for each RTP temperature below which BaTiO3 films are pinhole free. A process was then developed by depositing and annealing multiple layers of BaTiO3 films, with the thickness of each deposition less than the critical thickness. It was observed that the multi-layered films are stable and pinhole free with a smooth surface while the single layers of equivalent thicknesses showed cracked surfaces. Current-atomic force microscopy studies showed leakage current through large pinholes in single-layered films, whereas the pinholes were not the leakage path for multi-layered films. Metal-insulator-metal capacitor structures were also fabricated using BaTiO3 with TiNx top and bottom electrodes and the fringing effects in leakage characteristics were studied. Finally, the polarisation reversal mechanism in BaTiO3 was investigated using piezoresponse force spectroscopy (PFS). It was experimentally demonstrated that the polarisation reversal in these materials is a two-step process, which involves polarisation rotation and switching when the applied electric field is not parallel to the crystallographic orientation of the grain. However, it is a single step switching when the polarisation and the electric field are parallel, as widely perceived. The two step polarisation reversal was found to help [101] and [111] oriented grains to switch at a lower electric field compared to [001] grains.

Electrical and material properties of thin film perovskites

Appleby, Daniel Joseph Robert January 2014 (has links)
This thesis presents a study of negative capacitance in the robust perovskite BaTiO3. Negative capacitance is an unstable state in ferroelectrics, which explains why there is a lack of experimental evidence in the literature. A positive capacitance in a series capacitor configuration allows stabilisation of negative capacitance. The key finding is the stabilisation of negative capacitance at room temperature in BaTiO3. Temperature constraints in back-end-of-line processing should be at 500 °C or below in order to avoid diffusion of dopants and to inhibit high resistivity silicide phases. Three deposition techniques, pulsed laser deposition, atomic layer deposition and sputter deposition are used to investigate the material and electrical properties of perovskites for back-end integration within this temperature constraint. SrTiO3, Ba0.8Sr0.2TiO3 and BaTiO3 are all explored as possible solutions for tunable capacitance under low temperature processing. Evidence is shown for SrTiO3 displaying fully crystallised structures through pulsed laser deposition at 500 °C growth temperature. A refined model of effective oxide thickness is used to calculate interfacial layers that impact metal-oxide-semiconductor capacitors. The model is applied to SrTiO3 metal-insulator-metal capacitors in terms of a dead layer. Calculation of the dead layer thickness, which has been previously unattainable using solely the series capacitance model, is carried out using the effective oxide thickness model. However, transmission electron microscopy images suggest that a physical layer of ‘dead’ material is abesnt in the capacitors. The results support the hypothesis of an intrinsic explanation to the dead layer phenomenon. ii Finally, pulsed laser deposited BaTiO3 is explored in terms of ferroelectricity when integrated with Si using Pt/Ti/SiO2/Si substrates. Here, a mixed phase relationship is shown in the films of BaTiO3 in which the cubic phase, responsible for paraelectricity, dominates at room temperature. Increasing film thickness also correlates with higher remnant polarization in the films. The result confirms a size driven phase transition in thin film BaTiO3 which has preveously been studied on perovskite free-standing films or nanoparticles.

he surface structure and composition of various elemental and III - IV semiconductors

Bayliss, Colin R. January 1974 (has links)
No description available.

Anodic oxides on GaAs for device applications

Bayraktaroglu, Burhan January 1977 (has links)
No description available.

Physical problems relating to semiconductor devices

Bassett, R. J. January 1970 (has links)
No description available.

Physical process in inter-band and inter-subband mid-infrared photonic devices

Aldukhayel, Abdullah Mohammed January 2014 (has links)
Mid-infrared light emitting diodes and quantum cascade lasers are of increasing interest due to their promising applications. They can be used in detecting and monitoring pollutant gases such as methane (CH4) and carbon dioxide (C02), Such devices are preferred for these purposes due to their potential for high sensitivity for detecting gases, long device lifetime, and potential low-cost. Mid-infrared light emitting diodes emitting at a wavelength of 3.7 !-lm based on the pentanary alloy GaInAsSbP engineered to provide a favourable band structure for the suppression of non-radiative Auger recombination were investigated. Temperature dependence measurements were made to investigate the performance of these LEDs. Hydrostatic pressure measurements at room temperature and at 100 K were used to tune the band gap towards resonance with the spin-orbit splitting to inv,estigate the influence of the hot-hole (CHSH) Auger process on LED performance. Analysis of the resulting electroluminescence showed that while Auger recombination related to hot electrons occurs, it confirms that the nonradiative Auger recombination process involving the spin-orbit split-off band (CHSH) is suppressed under ambient conditions. In order to identify the performance limitations of InGaAsI AIAs(Sb) quantum cascade lasers, experimental investigations of the temperature and pressure dependenGies of the threshold current (Ith) were undertaken. Using the theoretically estimated optical phonon current (Iph) and calculated carrier leakage (I leak) as a function of pressure the measured pressure dependence of the threshold current showed that electron scattering from the upper laser level into the L valley minima gives rise to the increase in Ith with pressure and temperature. It was found that this carrier leakage path accounts for approximately 3 % of Ith at R T and is negligible at 100 K. However, it is shown that even this small leakage current causes strong temperature sensitivity of the devices and limits their maximum operating temperature.

Characterisation of compound semiconductors for next generation photonic devices

Batool, Zahida January 2014 (has links)
This thesis focuses on exploring novel III-V bismide alloys due to their potential of engineering the band structure in such a way that the spin-orbit splitting energy is greater than the band gap (LIsa> Eg) which helps to suppress important non-radiative Auger recombination losses. These losses reduce the efficiency of near-IR telecommunication lasers (1.55I1m) as well as limit the performance of mid-IR light emitting devices. A number of optical characterisation techniques have been applied to study the detailed band' structure of bismide alloys to provide feedback for band structure modelling and growth optimisation. PR studies on the GaBixAsl-x/GaAs material system with up to 10.4% Bi, showed a strong Eg reduction accompanied by an increase in LIsa with increasing Bi content, with a cross-over of Eg and LIsa at Bi~9.4. ±0.2%. RT PL emission has been measured at telecom wavelength (1.5 11m) with Bi ~ 10.4 %. Power dependent PL studies showed tentative evidence for the suppression of CHSH Auger losses in the 10.4% Bi containing sample. The InGaAsBi/lnP alloy has been explored for its potential for cheap and efficient mid-IR photonic devices. PL, PR and absorption revealed the cross-over (Llso~Eg) in this material system between ~ 3.4 - 4% Bi. Temperature dependent PR studies for the highest Bi containing InGaAsBi (~5%) and GaAsBi (~10.4%) epilayers were used to tune the band gap into resonance LIsa ~ Eg and an increase in the line width of the PR spectrum around the temperature of the resonance is found. Another alloy GaAsBiN/GaAs has been suggested to provide a lattice matched material to GaAs with widely tailored Eg while offering improved conduction and valence band offsets. PR measurements on GaAsBiN/GaAs showed a reduction in Eg around -141 ± 22meV/%N at a fixed Bi composition and also a decrease in the temperature coefficient with increasing N.

Integration of oxynitride barriers by reactive RF sputtering for use in magnetic tunnel junctions

Atcheson, Gwénaël Yves Peter January 2014 (has links)
Magnetic field sensors were developed with use of a SFI Shamrock sputtering tool. GMR spin-valve type sensors were developed in CIP geometry requiring no additional processing once deposited, and TMR magnetic tunnel junctions were developed in CPP. This meant the additional development of a photolithographic process in order to produce working devices. The MTJs used MgO as a tunneling barrier in reference devices, and a novel oxynitride barrier in experimental devices deposited by reactive RF sputtering from an elemental target in a variable argon/oxygen/nitrogen atmosphere. Devices produced show potential for further development.

FePd thin films as a replacement for FePt thin films as the hard bias magnet in the read head for hard disk drives

Egan, Patrick W. January 2014 (has links)
The purpose of the research was commercial in that it was carried out to ascertain if the more expensive FePt thin film could be replaced by the more cost effective FePd thin film as the hard biasing magnet in the read head of a hard disk drive. Magnetron sputtering was used to deposit the thin films. The magnetic properties and crystal structure of both FePd alloy thin films and FePd multilayers were investigated using VSM, B-H Looper and XRD. Room temperature deposition together with post annealing and heated substrate deposition of both the FePd alloy and FePd multilayers was carried out. In addition the effect of seed layers, seed layer thickness and the effect of heated deposition on the grain size was also studied. The magnetic properties and crystal structure of FePd alloy thin films and the Fe/Pd multilayer thin films are affected by the deposition temperature, seed layers and seed layer thickness and also by the thickness of the Fe and Pd layers in the Fe/Pd multilayer. FePd alloy and Fe/Pd multilayer thin films were produced with in-plane magnetisation and a saturation magnetisation of 1T or better but none of the films produced have a coecivity of 3500 Oe. The 36nm Fe50Pd50 alloy film with an Bnm Ru seed layer deposited at a temperature of 450°C showed the largest He of 2500 Oe but with an Ms of only 0.79T. Therefore the FePd thin film alloy or multilayers were unable to produce the required He. FePd thin films are therefore not a viable replacement for the FePt thin films currently used as the hard biasing magnet in the read head of HDD. Similarly the temperatures used to produce the FePd films with the largest He values are too high for the read head manufacturing process, because temperatures above 350°C would cause other layers in the read head to degrade.

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