Growth and characterization of group III-nitrides by migration enhanced afterglow epitaxy

The work presented in this thesis investigates the growth and properties of group III- nitride semiconductors that were grown using the Migration Enhanced Afterglow Epitaxy (MEAglow) method. This work was to enhance the understanding of the MEAglow growth process towards the improvement of quality of the layers grown using this technique. The MEAglow technique applies the migration enhanced epitaxy method in a low pressure plasma-based CVD reactor, which has a potential of producing high quality epitaxial group III-nitride layers at relatively low growth temperatures on large deposition areas. The low temperature pulse growth in metal-rich regime, comprising the MME method was employed under growth pressures between 500 mTorr and 3000 mTorr. As the MME method up to this point has been used only for MBE systems, study of the impact of the growth pressure on the materials properties was necessary. In this work the pressure dependence was mapped to an existing surface phase diagram for MBE systems by calculating the number of nitrogen gas phase collisions and the metalorganic bombardment rate, for the specific to the prototype reactor parameters, to a first approximation. This was done in order to achieve an intermediate regime free of metal droplets for growth in metal-rich regime. High quality epitaxial InN layers were accomplished on extremely thin and smooth Ga2O3 buffer layers. These results indicate a potential for the application of Ga2O3 buffers in InN growth. The MEAglow InN layers were further optimized for growth on commercially available GaN buffer layers and excellent two-dimensional growth was achieved for layers grown under metal-rich conditions at 512 °C. Post-growth annealing studies were carried out for InN layers grown at temperatures below 400 °C to study the limiting processes of the removal of excess nitrogen, believed to be a dominant defect in InN films grown in plasma-based systems at very low temperatures. Variations in GaN stoichiometry under certain growth conditions and the effect of similar growth conditions on MEAglow grown InGaN were also examined. The growth of MEAglow InGaN samples on sapphire substrates was optimized to reduce the indium surface segregation and phase separation of the material

Influence of Al Doping on the Morphological, Structural and Gas Sensing Properties of Electrochemically Deposited ZnO Films on Quartz Resonators

The detection of hazardous gases at different concentration levels at low and room temperature is still an actual and challenging task. In this paper, Al-doped ZnO thin films are synthesized by the electrochemical deposition method on the gold electrodes of AT-cut quartz resonators, vibrating at 10 MHz. The average roughness, surface morphology and gas sensing properties are investigated. The average roughness of Al-doped ZnO layers strongly depends on the amount of the doping agent Al2(SO4)3 added to the solution. The structural dependence of these films with varying Al concentrations is evident from the scanning electron microscopy images. The sensing properties to ethanol and ammonia analytes were tested in the range of 0–12,800 ppm. In the analysis of the sensitivity to ammonia, a dependence on the concentration of the added Al2(SO4)3 in the electrochemically deposited layers is also observed, as the most sensitive layer is at 3 × 10−5 M. The sensitivity and the detection limit in case of ammonia are, respectively, 0.03 Hz/ppm and 100 ppm for the optimal doping concentration. The sensitivity depends on the active surface area of the layers, with those with a more developed surface being more sensitive. Al-doped ZnO layers showed a good long-term stability and reproducibility towards ammonia and ethanol gases. In the case of ethanol, the sensitivity is an order lower than that for ammonia, as those deposited with Al2(SO4)3 do not practically react to ethanol

EFFICIENCY AND ECONOMIC GROWTH OF THE INDUSTRIAL BUSINESS IN CONTEMPORARY CONDITIONS

Основополагащи в Стратегия „Европа 2020” и в Национална програма „България 2020” са въпросите, свързани с постигането на интелигентен и устойчив растеж при ефективно и екологосъобразно използване на ресурсите. Взаимоотношението „ефективност – икономически растеж” се интерпретира в съответствие с идеите на тези програми. Разгледани са възможностите за аналитична оценка на капиталовите и човешки ресурси и на икономическия растеж. Извършени са практически изследвания на степента на използване на ресурсите, на нейното отражение върху икономическия растеж на индустриални предприятия от различни сектори, които в значителна степен илюстрират особеностите в развитието на индустриалния бизнес през периода 2007 – 2010 г. Fundamental strategy in the “Europe 2020” and in the National Program “Bulgaria 2020” are issues with achieving smart and sustainable growth through efficient and environmentally friendly use of resources. Relationship “efficiency – economic growth” is interpreted in accordance with the ideas of these programs. The possibilities for analytical evaluation of capital and human resources and economic growth. Were carried out practical research on utilization of resources, its impact on economic growth of industrial enterprises from different sectors, which largely illustrate the peculiarities of development of industrial business in the period 2007-2010

Flexible Polymer–Organic Solar Cells Based on P3HT:PCBM Bulk Heterojunction Active Layer Constructed under Environmental Conditions

In this study, some crucial parameters were determined of flexible polymer–organic solar cells prepared from an active layer blend of poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) mixed in 1:1 mass ratio and deposited from chlorobenzene solution by spin-coating on poly(ethylene terephthalate) (PET)/ITO substrates. Additionally, the positive effect of an electron transport layer (ETL) prepared from zinc oxide nanoparticles (ZnO np) on flexible photovoltaic elements’ performance and stability was investigated. Test devices with above normal architecture and silver back electrodes deposed by magnetron sputtering were constructed under environmental conditions. They were characterized by current-voltage (I–V) measurements, quantum efficiency, impedance spectroscopy, surface morphology, and time–degradation experiments. The control over morphology of active layer thin film was achieved by post-deposition thermal treatment at temperatures of 110–120 °C, which led to optimization of device morphology and electrical parameters. The impedance spectroscopy results of flexible photovoltaic elements were fitted using two R||CPE circuits in series. Polymer–organic solar cells prepared on plastic substrates showed comparable current–voltage characteristics and structural properties but need further device stability improvement according to traditionally constructed cells on glass substrates

EFFICIENCY AND ECONOMIC GROWTH OF INDUSTRIAL BUSINESS UNDER CONTEMPORARY CONDITIONS

Основополагащи в Стратегия „Европа 2020” и в Национална програма „България 2020” са въпросите, свързани с постигането на интелигентен и устойчив растеж при ефективно и екологосъобразно използване на ресурсите. Взаимоотношението „ефективност – икономически растеж” се интерпретира в съответствие с идеите на тези програми. Разгледани са възможностите за аналитична оценка на капиталовите и човешките ресурси и на икономическия растеж. Извършени са практически изследвания на степента на използване на ресурсите, на нейното отражение върху икономическия растеж на индустриални предприятия от различ- ни сектори, които в значителна степен илюстрират особеностите в развитието на индустриалния бизнес през периода 2007 – 2010 г. The issues related to achieving smart and sustainable growth by efficient and environmentally-friendly use of resources are fundamental in „Europe 2020” strategy and „Bulgaria 2020” National Programme. The relationship „efficiency – economic growth” is interpreted in accordance with the ideas of these programs. The possibilities for analytical evaluation of capital and human resources and economic growth are examined. Practical research was carried out on the extent of resource utilization and its impact on economic growth of industrial enterprises from different sectors, which largely illustrate the peculiarities of development of industrial business in the period 2007-2010