It is challenging to achieve low-resistive ohmic contacts to III-nitride semiconductors due to their wide bandgap. A common way to reduce the contact resistance is to recess the ohmic area prior to metallization. In the minimization of the contact resistance, parameters like the recess depth, anneal temperature and design of the metal stack are commonly optimized. In this work, three other approaches have been evaluated. All experiments were performed on AlGaN/GaN heterostructures. The fabricated ohmic contacts were recess etched, metallized with a Ta/Al/Ta stack, and annealed at 550-575◦C.Firstly, it is shown that the laser writer intensity, transmittance and focus offset during optical lithography affect the contact resistance. The reason is believed to be the variation in the resist profile, which has an impact on the metal coverage. At the optimum intensity/transmittance/focus condition, which generates a relatively medium undercut, a contact resistance of 0.23 Ωmm was obtained.In the second approach, the metal layer of annealed contacts was removed by wet etching, followed by the re-deposition of a metal stack and annealing. The purpose was to increase the amount of N vacancies in the AlGaN, which are responsible for the contact formation. A minimum contact resistance of 0.41 Ωmm was achieved with this method, compared to 0.28 Ωmm with the regular method (without remetallization).In the last approach, the bottom Ta layer was sputtered, whereas evaporation was used in all other cases. The minimum contact resistance was found to be 0.6 Ωmm, which was higher than for the evaporated contacts. The reason was assumed that the thickness of sputtered Ta should be thinner than the evaporated Ta due to its higher density. Moreover, the obtained lower sheet resistance is assumed to caused by the atomic scale damage due to the high energy ions during sputtering. / En utmaning med III-nitrid-halvledare är att uppnå låg-resistivitetskontakter, på grund av deras breda bandgap. Ett konventionellt tillvägagångsätt för att reducera kontaktresistansen är att fördjupa ohmska ytan före metallisering. I strävandet av att minska den ohmska resistansen sker vanligtvis en optimering av följande parametrar, recessddjup, anlöpningstemperatur och metallagersdesign. I detta arbete så har samtliga tre parametrar evaluerats. Alla experiment utfördes på AlGaN/GaNheterostrukturer. De tillverkade ohmska kontakterna var recesssetsade, metalliserade med ett Ta/Al/Ta lager och anlöpt vid 550-575◦C.Den primära undersökningen, visar att laserritar-intensitet, -transmission och fokusförskjutning under optisk litografi inverkar på kontaktresistansen. Anledningen antas vara variation i resistprofilen, vilket påverkar metallbeläggningen. Vid optimal intensitet/transmission/fokus-förhållanden, (som genererar en underskärning), blev den resulterande kontaktresistansen 0.23 Ωmm uppmätt.I en sekundär undersökning, avlägsnas ohmska kontaktens metallager genom våtetsning, följt av en återdeponering av ett nytt metallager, samt anlöpning. Syftet var att öka mängden N-vakanser i AlGaN-lagret, som formar ohmska kontakten. Minsta kontaktresistansen uppmätt var 0.41Wmm, att jämföras med 0.28 Ωmm, som uppnåddes genom den konventionella metoden (utan återmetallisering).Den sista undersökningen jämförde sputtrade med evaporerade bottenlager av Ta, (evaporation användes som standardmetod i de tidigare undersökningarna). Med sputtrning blev den minsta kontakresistansen 0.6 Ωmm, (högre än de evaporerade kontakterna). En hypotetisk förklarning kan vara att det sputtrade Ta-lagret är tunnare än det evaporerade Ta-lagret, på grund av en dess högre densitet. Därutöver, den uppmätta lägre skiktresistansen antas bero på den skada i atomskala som sker vid de höga energi-kollisioner som joner skapar vid sputtrning.

This thesis investigates the impact of doping on the optical and structural characteristics of cerium oxide (ceria) nanoparticles synthesized using chemical precipitation. The dopants selected are samarium and neodymium, which have positive association energy with oxygen vacancies in the ceria host, and negative association lanthanides, holmium and erbium, as well as two metal dopants, aluminum and iron. Characteristics measured are absorption and fluorescence spectra and the diameter and lattice parameter of ceria. Analysis of the characteristics indicates qualitatively that the dopant controls the O-vacancy concentration and the ratio of the two cerium ionization states: Ce+3 and Ce+4. A novel conclusion is proposed that the negative association lanthanide dopants can act as O-vacancies scavengers in ceria while the O-vacancy concentration increases in ceria doped with positive association lanthanide elements. Doped ceria nanoparticles are evaluated in two applications: dissolved oxygen (DO) sensing and up-conversion. In the first application, ceria doped with either Sm or Nd and ceria doped with aluminum have a strong correlation between the fluorescence quenching with the DO concentration in the aqueous solution in which the ceria nanoparticles are suspended. Stern-Volmer constants (KSV) of doped ceria are found to strongly depend upon the O-vacancy concentration and are larger than some of the fluorescent molecular probes currently used to measure DO. The KSV measured between 25-50oC is found to be significantly less temperature dependent as compared to the constants of commercially-available DO molecular probes. In the second application, up-conversion, ceria nanoparticles doped with erbium and an additional lanthanide, either Sm or Nd, are exposed to IR radiation at 780 nm. Visible emission is only observed after the nanoparticles are calcinated at high temperature, greatly diminishing the concentration of O-vacancies. It is concluded that O-vacancies do not play a dominant role in up-conversion, unlike that drawn for down-conversion, where the fluorescence intensity is strongly correlated with the O-vacancy concentration. Correlations between annealing temperatures, dopant, and dopant concentrations with the power dependence of up-conversion on the pump and the origin of the intensities of the visible emission are presented. These studies show the promise of doped ceria nanoparticles. / Ph. D.

Local employment opportunities are hypothesized to influence educational attainment decisions made by high students, measured by the dropout percentage and the post-secondary education percentage. Data from 1990, 1980 and 1970 are used to estimate these relationships in Virginia's 133 school districts. Secretary of Labor Robert Reich's framework, developed in the Work of Nations: Preparing Ourselves for 21st Century Capitalism, is adopted to incorporate changes in the global-labor market. / Master of Science

by Chan, Yuk Fai Weslie. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves 67-70). / Abstracts in English and Chinese. / Acknowledgments --- p.ii / English Abstract --- p.iv / Chinese Abstract --- p.v / Table of Contents --- p.vi / List of Tables --- p.viii / List of Figures --- p.ix / List of Appendices --- p.x / Chapter Chapter 1. --- Introduction --- p.1 / Chapter Chapter 2. --- Theoretical Background --- p.6 / Chapter 2.1 --- Concepts of Beveridge Curve --- p.7 / Chapter 2.2 --- Beveridge Curve Derived from Labori Market Stock-Flow Identities --- p.15 / Chapter 2.2.1 --- Some Basic Labor Market Stork-Flow Identities --- p.15 / Chapter 2.2.2 --- Steady State Properties of Beveridge Curve --- p.19 / Chapter 2.2.3 --- Comparative Static Analysis of Beveridge Curve --- p.20 / Chapter 2.2.4 --- Short Run Dynamics along Beveridge Curve --- p.24 / Chapter 2.3 --- Beveridge Curve Derived from Matching Function Approach --- p.25 / Chapter Chapter 3. --- Empirical Evidences --- p.28 / Chapter 3.1 --- Decomposition of Total Unemployment of Hong Kong --- p.28 / Chapter 3.2 --- Beveridge Curve of Hong Kong --- p.31 / Chapter 3.2.1 --- Time Series Estimation of Hong Kong's Beveridge Curve --- p.32 / Chapter 3.2.2 --- Cross Sectorial Estimation of Hong Kong's Beveridge Curve --- p.33 / Chapter 3.3 --- Natural Unemployment Rate --- p.34 / Chapter 3.4 --- Unemployment-Vacancy Ratio --- p.36 / Chapter 3.4.1 --- Relation between U-V Ratio and K-L ratio --- p.38 / Chapter Chapter 4. --- Conclusion --- p.41 / Tables --- p.42 / Figures --- p.46 / Appendices --- p.56 / Bibliography --- p.64

In this PhD thesis, the physics of vacancies in two-dimensional ordered Heisenberg antiferromagnets is investigated. We use semi-classical methods to study the influence of a single vacancy in long-range ordered states, with a focus on non-collinear order. Here, on a classical level, a magnetic distortion is created as the spins readjust in response to the vacancy. We use the non-collinear $120^\\circ$ state on the frustrated triangular lattice as an example, where we determine the impurity contributions to the magnetization and susceptibility. An important discovery is the vacancy moment not being quantized due to non-universal partial screening. The resulting effective moment $m_0 \\ll S$ can be observed as a fractional prefactor to an impurity-induced Curie response $m_0^2/(3k_BT)$ at finite temperature. This is in sharp contrast to collinearly ordered states. Here the moment is always quantized to the bulk spin value, $m_0=S$. Furthermore, we present a detailed analysis of the vacancy-induced distortion cloud. Due to Goldstone modes, it decays algebraically as $r^{-3}$ with distance $r$ to the vacancy. Using leading-order $1/S$-expansion, we determine the quantum corrections to both size and direction of the distorted magnetic moments. Secondly, we study the same problem in the presence of an external magnetic field $h$, both for the square and triangular lattice. For the triangular lattice we use a biquadratic exchange term $K$ to stabilize a unique ground state from a degenerate manifold. The finite-field vacancy moment $m(h)$ is generated by field-dependent screening clouds, as different non-collinear bulk states evolve with increasing field. These distortion clouds decay exponentially on a magnetic length scale $l_h\\propto 1/h$. Most importantly, we find that the magnetic-field linear-response limit $h \\rightarrow 0^+$ is generically singular for $SU(2)$ ordered local-moment antiferromagnets, as the vacancy moment in zero field differs fundamentally from even an infinitesimal but finite field, $m(h \\rightarrow 0^+)\\neq m_0$. Moreover, a part of the screening cloud itself becomes universally singular. Particularly for spin-flop states, this leads to a semi-classical version of perfect screening. We present general arguments to support these claims, as well as microscopic calculations. Another remarkable result is an impurity-induced quantum phase transition for overcompensated vacancies in the $M=1/3$ plateau phase on the triangular lattice with $K<0$. We close our analysis with a discussion about important limits for finite vacancy concentrations, as well as a possible experimental verification of our predictions.

Submitted by Cristiane Chim (cristiane.chim@ucpel.edu.br) on 2018-07-09T12:42:46Z No. of bitstreams: 1 Letícia Santos da Silva.pdf: 1085574 bytes, checksum: 92098a43a031d086fba6143e663c62d1 (MD5) / Made available in DSpace on 2018-07-09T12:42:46Z (GMT). No. of bitstreams: 1 Letícia Santos da Silva.pdf: 1085574 bytes, checksum: 92098a43a031d086fba6143e663c62d1 (MD5) Previous issue date: 2018-02-28 / This study investigates the perception of the self-declared black quotations students about the system of reserve of places, based on ethnic-racial criteria implanted in the Federal Institute of Education, Science and Technology Sul-rio-grandense (IFSul), campus Bagé. The objective of the research is to analyze the perception of students who entered the job reserve system in 2015 and 2016 and in 2017 were regularly enrolled and spontaneously accepted to participate in the research. Using the theoretical, legal, political and social subsidies that highlight the political-social meanings of affirmative action, a discussion about these actions is carried out as a compensatory public policy focused on ethnic racial minorities in the Brazilian context, elucidating policy actions affirmative approach to education, specifically the one that seeks to guarantee access to higher and technical education for low-income people, blacks and browns, through the quota system, guaranteed by Law No. 12.711 / 12. Many steps were taken towards this achievement, and it is extremely important to highlight the participation of the Black Movement, a movement of social origin that seeks to guarantee the rights of the black population, as well as the construction and visibility of their racial and cultural ethnic identity. This movement is present in the claims that cover public education for blacks, through quotas. The methodological procedures of the research are based on the assumptions of the qualitative research of the case study type, based on the critical dialectical method. The instrument for data collection used was the interview and, for the data analysis, the content analysis methodology was used. The results of the research indicate that this historically excluded group, in its majority, reproduces the discourse of the ruling class in which the entrance in the public institutions must be by the meritocracy, where the ethnic-racial quotas can represent a discriminatory and sometimes racist act. / Este estudo investiga a percepção dos alunos cotistas autodeclarados pretos acerca do sistema de reserva de vagas, com base em critérios étnico-raciais implantados no Instituto Federal de Educação, Ciência e Tecnologia Sul-rio-grandense (IFSul), campus Bagé. A pesquisa tem, por objetivo, analisar a percepção dos estudantes que ingressaram pelo sistema de reserva de vagas no ano de 2015 e 2016 e no ano de 2017 estavam regularmente matriculados, que espontaneamente aceitaram participar da pesquisa. Recorrendo a subsídios teóricos da área jurídica, política e social, que destacam sentidos político-sociais das ações afirmativas, realiza-se a discussão acerca dessas ações, enquanto política pública compensatória voltada às minorias étnicas raciais, no contexto brasileiro, elucidando a política de ações afirmativas voltada à educação, especificamente a que busca garantir o acesso ao ensino superior e técnico às pessoas de baixa renda, pretos e pardos, através do sistema de cotas, garantido na Lei n° 12.711/12. Muitos passos foram dados para esta conquista, sendo de suma importância destacar a participação do Movimento Negro, movimento de origem social que buscar garantir os direitos da população negra, bem como a construção e visibilidade de sua identidade étnica racial e cultural. Este movimento está presente nas reivindicações que abarcam a educação pública para negros, por meio das cotas. Os procedimentos metodológicos da investigação apoiam-se nos pressupostos da pesquisa qualitativa do tipo estudo de caso, embasado no método dialético crítico. O instrumento para a coleta de dados usado foi a entrevista e, para a análise dos dados, utilizou-se a metodologia de análise de conteúdo. Os resultados da pesquisa indicaram que esse grupo historicamente excluído, em sua maioria reproduz o discurso da classe dominante em que o ingresso nas instituições públicas deve ser pela meritocracia, onde as cotas étnico-raciais podem representar um ato discriminatório e por vezes racista.

Entropy, first introduced in thermodynamics, is used in a wide range of fields. Chapter 1 discusses some important theoretical and practical aspects of entropy: what is entropy, is it subjective or objective, and how to properly apply it to living organisms. Chapter 2 presents applications of entropy to evolution. Chapter 3 shows how cellulosic biofuel production can be improved. Chapter 4 shows how lattice vacancies influence the thermodynamic properties of materials. To determine the nature of thermodynamic entropy, Chapters 1 and 2 describe the roots, the conceptual history of entropy, as well as its path of development and application. From the viewpoint of physics, thermal entropy is a measure of useless energy stored in a system resulting from thermal motion of particles. Thermal entropy is a non-negative objective property. The negentropy concept, while mathematically correct, is physically misleading. This dissertation hypothesizes that concepts from thermodynamics and statistical mechanics can be used to define statistical measurements, similar to thermodynamic entropy, to summarize the convergence of processes driven by random inputs subject to deterministic constraints. A primary example discussed here is evolution in biological systems. As discussed in this dissertation, the first and second laws of thermodynamics do not translate directly into parallel laws for the biome. But, the fundamental principles on which thermodynamic entropy is based are also true for information. Based on these principles, it is shown that adaptation and evolution are stochastically deterministic. Chapter 3 discusses the hydrolysis of cellulose to glucose, which is a key reaction in renewable energy from biomass and in mineralization of soil organic matter to CO2. Conditional thermodynamic parameters, ΔhydG', ΔhydH', and ΔhydS', and equilibrium glucose concentrations are reported for the reaction C6H10O5(cellulose) + H2O(l) ⇄ C6H12O6(aq) as functions of temperature from 0 to 100°C. Activity coefficients of aqueous glucose solution were determined as a function of temperature. The results suggest that producing cellulosic biofuels at higher temperatures will result in higher conversion. Chapter 4 presents the data and a theory relating the linear term in the low temperature heat capacity to lattice vacancy concentration. The theory gives a quantitative result for disordered vacancies, but overestimates the contribution from ordered vacancies because ordering leads to a decreased influence of vacancies on heat capacity.

Le numérique prend une place de plus en plus importante dans la vie de tous les jours et les quantités de données échangées explosent ce qui impose de développer des mémoires de plus en plus performantes, enjeu majeur du secteur de la microélectronique. Parmi les mémoires non-volatiles émergentes, les mémoires OxRRAM à base d'oxyde résistif sont particulièrement attrayantes et représentent un candidat potentiel au remplacement des mémoires FLASH (compatibles avec la technologie CMOS, faibles tensions de programmation). Leur structure est simple (Métal-Isolant-Métal) et leur fonctionnement est basé sur une commutation de résistance sous l'effet d'un champ électrique. Si le mécanisme de formation/dissolution d'un filament conducteur de taille nanométrique est reconnu par la communauté, un débat subsiste encore sur la nature et les caractéristiques du/des filaments dans le cas de l'oxyde HfO2 (lacunes d'oxygène, élément métallique). En nous appuyant sur des méthodes de la microscopie électronique en transmission - STEM-HAADF et STEM-EELS - cette thèse apporte des éléments de compréhension par rapport aux modifications d'état physico-chimique qui s'opèrent lors des différentes étapes du fonctionnement d'une mémoire (FORMING, RESET) et ceci à l'échelle nanométrique définie par la taille du filament conducteur. L'empilement TiN/Ti/HfO2/TiN, préparé selon les procédés de la microélectronique, a été intégré dans différentes architectures (1R, 1T1R) avec une électrode supérieure structurée (50 à 200 nm) pour confiner la zone de conduction dans un volume fini compatible avec la MET puis polarisé selon différentes méthodes (C-AFM, banc de mesure et TEM in-situ). Lorsque les effets thermiques sont contrôlés, l'analyse des cartographies chimiques élémentaires montre que le titane de l'électrode supérieure participe au mécanisme de commutation (migration localisée dans la couche HfO2) en plus de la déplétion en oxygène à l'interface HfO2/électrode inférieure et probablement aux joints de grains dans HfO2. / Digital technology is invading our day life and the amount of data is exploding. This implies to develop memories which perform better and better. This is a major issue in microelectronics. Among non-volatile memories, Oxide based resistive RAM are particularly attractive (compatible with CMOS technology, low programming voltage) and are considered as promising candidate for replacing FLASH memories. The stack is simple (M-I-M) and the switching is based on resistance changes under an applied electrical stress. If forming and breaking a nanometer-sized conductive area is commonly accepted as the physical phenomenon involved in the switching mechanism, a debate remains about the nature and the characteristics of the filamentary area (oxygen vacancies, metallic element). Based on transmission electron microscopy methods - STEM-HAADF and STEM-EELS - this thesis work provides, at the scale of the filament (nm), a further understanding about the physico-chemical modifications of the memory cell induced by the operating step (FORMING, RESET). The TiN/Ti/HfO2/TiN stack, processed with microelectronic techniques, was incorporated into different architectures (1R, 1T1R) with a shaped top electrode (diameter 50 to 200 nm) to confine the filament in a volume compatible with TEM and then biased with different methods (C-AFM, measuring bench, in-situ TEM). When thermal effects are under control, the analysis of the EELS elementary maps shows that titanium from the top electrode plays a role in the switching mechanism (local migration in the HfO2 layer) in addition to the oxygen depletion at the HfO2/bottom electrode interface and probably at grain boundaries in HfO2.

This research investigates the specific labour market dynamics that underline the shortage of engineers in the Petrochemical Industry in South Africa. The central argument of this dissertation is that an understanding of a skills shortage requires a distinct knowledge of the internal and external nature of each labour market in which the shortage is being experienced. This dissertation develops a critique of the neoclassical perspective which dominates current analysis of skill shortages. While it is important to understand the external labour market, it is equally important to consider the internal labour market to better identify and understand the specific dynamics that underline a skills shortage in an organisation and industry. Using Engen Refinery as a case study, this dissertation focuses on an in depth examination of the experiences of engineers working at the Refinery. The findings reveal that the dynamics that underline the skill shortage of engineers in the Petrochemical Industry in the South African context are manifold. Skill shortages are a consequence of dynamics in both the external and internal labour market; these I argue are interrelated. / Thesis (M.Soc.Sc.)-University of KwaZulu-Natal, Durban, 2009.

The world economy and industry in turn is driven by technology and innovation at a rapid rate. Worldwide, the information technology (IT) industry is volatile in terms of turnover intentions of IT professionals The unemployment level in South Africa is high according to global standards. The option of contracting as an IT consultant or self-employment is a form of an alternate employment arrangement. This arrangement will benefit the unemployed in South Africa if they choose to skill themselves and pursue a career in self-employment in the IT industry. There are internal and external employment factors that affect an IT professional’s employment arrangement. Moore’s (2000) information technology employee turnover model was adapted as a basis for this research. An external factor, entrepreneurship (self-employment) was introduced to Moore’s model. This research also tests Moore’s (2000) model for its internal factors. The following factors, role ambiguity, role conflict, autonomy, perceived workload, fairness of reward, work exhaustion and entrepreneurship were formulated in the hypotheses to determine which of these factors influences self-employment in IT professionals. Information technology professionals based in Durban were the target respondents in the City of Durban. The survey questionnaire was emailed to respondents using Questionpro. The sample data was based on 123 respondents who completed the survey. The data was then validated for internal consistency using Cronbach alpha ratio generated by the SPSS (version 19.0) software tool. The quantitative research design was chosen. Frequency tables and Pearson’s bivariate correlation coefficient statistics was used in the data analysis phase. The research objective was achieved successfully and the following factors were determined, they are role ambiguity, role conflict, autonomy, work exhaustion and entrepreneurship. The IT industry is volatile with IT professionals constantly re-skilling themselves to be on par with changing technology and innovation that make them very competent and competitive as a result, these IT professionals create a market for self-employment. / Thesis (MBA)-University of KwaZulu-Natal, Durban, 2012.