Effects of alternating current voltage amplitude and oxide capacitance on mid-gap interface state defect density extractions in In0.53Ga 0.47As capacitors

This work looks at the effect on mid-gap interface state defect density estimates for In0.53Ga0.47As semiconductor capacitors when different AC voltage amplitudes are selected for a fixed voltage bias step size (100 mV) during room temperature only electrical characterization. Results are presented for Au/Ni/Al2O3/In0.53Ga0.47As/InP metal–oxide–semiconductor capacitors with (1) n-type and p-type semiconductors, (2) different Al2O3 thicknesses, (3) different In0.53Ga0.47As surface passivation concentrations of ammonium sulphide, and (4) different transfer times to the atomic layer deposition chamber after passivation treatment on the semiconductor surface—thereby demonstrating a cross-section of device characteristics. The authors set out to determine the importance of the AC voltage amplitude selection on the interface state defect density extractions and whether this selection has a combined effect with the oxide capacitance. These capacitors are prototypical of the type of gate oxide material stacks that could form equivalent metal–oxide–semiconductor field-effect transistors beyond the 32 nm technology node. The authors do not attempt to achieve the best scaled equivalent oxide thickness in this work, as our focus is on accurately extracting device properties that will allow the investigation and reduction of interface state defect densities at the high-k/III–V semiconductor interface. The operating voltage for future devices will be reduced, potentially leading to an associated reduction in the AC voltage amplitude, which will force a decrease in the signal-to-noise ratio of electrical responses and could therefore result in less accurate impedance measurements. A concern thus arises regarding the accuracy of the electrical property extractions using such impedance measurements for future devices, particularly in relation to the mid-gap interface state defect density estimated from the conductance method and from the combined high–low frequency capacitance–voltage method. The authors apply a fixed voltage step of 100 mV for all voltage sweep measurements at each AC frequency. Each of these measurements is repeated 15 times for the equidistant AC voltage amplitudes between 10 mV and 150 mV. This provides the desired AC voltage amplitude to step size ratios from 1:10 to 3:2. Our results indicate that, although the selection of the oxide capacitance is important both to the success and accuracy of the extraction method, the mid-gap interface state defect density extractions are not overly sensitive to the AC voltage amplitude employed regardless of what oxide capacitance is used in the extractions, particularly in the range from 50% below the voltage sweep step size to 50% above it. Therefore, the use of larger AC voltage amplitudes in this range to achieve a better signal-to-noise ratio during impedance measurements for future low operating voltage devices will not distort the extracted interface state defect density.

Optimisation of Smart Grid performance using centralised and distributed control techniques

A massive change is currently taking place in the manner in which power networks are operated. Traditionally, power networks consisted of large power stations which were controlled from centralised locations. The trend in modern power networks is for generated power to be produced by a diverse array of energy sources which are spread over a large geographical area. As a result, controlling these systems from a centralised controller is impractical. Thus, future power networks will be controlled by a large number of intelligent distributed controllers which must work together to coordinate their actions. The term Smart Grid is the umbrella term used to denote this combination of power systems, artificial intelligence, and communications engineering. This thesis focuses on the application of optimal control techniques to Smart Grids with a focus in particular on iterative distributed MPC. A novel convergence and stability proof for iterative distributed MPC based on the Alternating Direction Method of Multipliers is derived. Distributed and centralised MPC, and an optimised PID controllers' performance are then compared when applied to a highly interconnected, nonlinear, MIMO testbed based on a part of the Nordic power grid. Finally, a novel tuning algorithm is proposed for iterative distributed MPC which simultaneously optimises both the closed loop performance and the communication overhead associated with the desired control.

Multi-scale modelling of atomic layer deposition

High-permittivity ("high-k") dielectric materials are used in the transistor gate stack in integrated circuits. As the thickness of silicon oxide dielectric reduces below 2 nm with continued downscaling, the leakage current because of tunnelling increases, leading to high power consumption and reduced device reliability. Hence, research concentrates on finding materials with high dielectric constant that can be easily integrated into a manufacturing process and show the desired properties as a thin film. Atomic layer deposition (ALD) is used practically to deposit high-k materials like HfO2, ZrO2, and Al2O3 as gate oxides. ALD is a technique for producing conformal layers of material with nanometer-scale thickness, used commercially in non-planar electronics and increasingly in other areas of science and technology. ALD is a type of chemical vapor deposition that depends on self-limiting surface chemistry. In ALD, gaseous precursors are allowed individually into the reactor chamber in alternating pulses. Between each pulse, inert gas is admitted to prevent gas phase reactions. This thesis provides a profound understanding of the ALD of oxides such as HfO2, showing how the chemistry affects the properties of the deposited film. Using multi-scale modelling of ALD, the kinetics of reactions at the growing surface is connected to experimental data. In this thesis, we use density functional theory (DFT) method to simulate more realistic models for the growth of HfO2 from Hf(N(CH3)2)4/H2O and HfCl4/H2O and for Al2O3 from Al(CH3)3/H2O.Three major breakthroughs are discovered. First, a new reaction pathway, ’multiple proton diffusion’, is proposed for the growth of HfO2 from Hf(N(CH3)2)4/H2O.1 As a second major breakthrough, a ’cooperative’ action between adsorbed precursors is shown to play an important role in ALD. By this we mean that previously-inert fragments can become reactive once sufficient molecules adsorb in their neighbourhood during either precursor pulse. As a third breakthrough, the ALD of HfO2 from Hf(N(CH3)2)4 and H2O is implemented for the first time into 3D on-lattice kinetic Monte-Carlo (KMC).2 In this integrated approach (DFT+KMC), retaining the accuracy of the atomistic model in the higher-scale model leads to remarkable breakthroughs in our understanding. The resulting atomistic model allows direct comparison with experimental techniques such as X-ray photoelectron spectroscopy and quartz crystal microbalance.

Ironworking in late medieval Ireland, c. AD. 1200 to 1600

The landscape of late medieval Ireland, like most places in Europe, was characterized by intensified agricultural exploitation, the growth and founding of towns and cities and the construction of large stone edifices, such as castles and monasteries. None of these could have taken place without iron. Axes were needed for clearing woodland, ploughs for turning the soil, saws for wooden buildings and hammers and chisels for the stone ones, all of which could not realistically have been made from any other material. The many battles, waged with ever increasingly sophisticated weaponry, needed a steady supply of iron and steel. During the same period, the European iron industry itself underwent its most fundamental transformation since its inception; at the beginning of the period it was almost exclusively based on small furnaces producing solid blooms and by the turn of the seventeenth century it was largely based on liquid-iron production in blast-furnaces the size of a house. One of the great advantages of studying the archaeology of ironworking is that its main residue, slag, is often produced in copious amounts both during smelting and smithing, is virtually indestructible and has very little secondary use. This means that most sites where ironworking was carried out are readily recognizable as such by the occurrence of this slag. Moreover, visual examination can distinguish between various types of slag, which are often characteristic for the activity from which they derive. The ubiquity of ironworking in the period under study further means that we have large amounts of residues available for study, allowing us to distinguish patterns both inside assemblages and between sites. Disadvantages of the nature of the remains related to ironworking include the poor preservation of the installations used, especially the furnaces, which were often built out of clay and located above ground. Added to this are the many parameters contributing to the formation of the above-mentioned slag, making its composition difficult to connect to a certain technology or activity. Ironworking technology in late medieval Ireland has thus far not been studied in detail. Much of the archaeological literature on the subject is still tainted by the erroneous attribution of the main type of slag, bun-shaped cakes, to smelting activities. The large-scale infrastructure works of the first decade of the twenty-first century have led to an exponential increase in the amount of sites available for study. At the same time, much of the material related to metalworking recovered during these boom-years was subjected to specialist analysis. This has led to a near-complete overhaul of our knowledge of early ironworking in Ireland. Although many of these new insights are quickly seeping into the general literature, no concise overviews on the current understanding of the early Irish ironworking technology have been published to date. The above then presented a unique opportunity to apply these new insights to the extensive body of archaeological data we now possess. The resulting archaeological information was supplemented with, and compared to, that contained in the historical sources relating to Ireland for the same period. This added insights into aspects of the industry often difficult to grasp solely through the archaeological sources, such as the people involved and the trade in iron. Additionally, overviews on several other topics, such as a new distribution map of Irish iron ores and a first analysis of the information on iron smelting and smithing in late medieval western Europe, were compiled to allow this new knowledge on late medieval Irish ironworking to be put into a wider context. Contrary to current views, it appears that it is not smelting technology which differentiates Irish ironworking from the rest of Europe in the late medieval period, but its smithing technology and organisation. The Irish iron-smelting furnaces are generally of the slag-tapping variety, like their other European counterparts. Smithing, on the other hand, is carried out at ground-level until at least the sixteenth century in Ireland, whereas waist-level hearths become the norm further afield from the fourteenth century onwards. Ceramic tuyeres continue to be used as bellows protectors, whereas these are unknown elsewhere on the continent. Moreover, the lack of market centres at different times in late medieval Ireland, led to the appearance of isolated rural forges, a type of site unencountered in other European countries during that period. When these market centres are present, they appear to be the settings where bloom smithing is carried out. In summary, the research below not only offered us the opportunity to give late medieval ironworking the place it deserves in the broader knowledge of Ireland's past, but it also provided both a base for future research within the discipline, as well as a research model applicable to different time periods, geographical areas and, perhaps, different industries..

Quaternary geology of the German North Sea and Western Irish Sea Mud Belt: revised stratigraphies, geotechnical properties, sedimentology and anthropogenic impacts

The climatic development of the Mid to Late Quaternary (last 400,000 years) is characterised by fluctuation between glacial and interglacial periods leading to the present interglacial, the Holocene. In comparison to preceding periods it was believed the Holocene represented a time of relative climatic stability. However, recent work has shown that the Holocene can be divided into cooler periods such as the Little Ice Age alternating with time intervals where climatic conditions ameliorated i.e. Medieval Warm Period, Holocene Thermal Optimum and the present Modern Optimum. In addition, the Holocene is recognised as a period with increasing anthropogenic influence on the environment. Onshore records recording glacial/interglacial cycles as well as anthropogenic effects are limited. However, sites of sediment accumulation on the shallow continental shelf offer the potential to reconstruct these events. Such sites include tunnel valleys and low energy, depositional settings. In this study we interrogated the sediment stratigraphy at such sites in the North Sea and Irish Sea using traditional techniques, as well as novel applications of geotechnical data, to reconstruct the palaeoenvironmental record. Within the German North Sea sector a combination of core, seismic and in-situ Cone Penetration Testing (CPT) data was used to identify sedimentary units, place them within a morphological context, relate them to glacial or interglacial periods stratigraphically, and correlate them across the German North Sea. Subsequently, we were able to revise the Mid to Late Quaternary stratigraphy for the North Sea using this new and novel data. Similarly, Holocene environmental changes were investigated within the Irish Sea at a depositional site with active anthropogenic influence. The methods used included analyses on grain-size distribution, foraminifera, gamma spectrometry, AMS 14C and physical core logging. The investigation revealed a strong fluctuating climatic signal early in the areas history before anthropogenic influence affects the record through trawling.