Gallium nitride and its alloys are direct band gap semiconductors with a wide variety of applications. Of particular importance are light emitting diodes and laser diodes. Due to the lack of suitable lattice-matched substrates, epitaxial layers contain a high density of defects such as dislocations. To reduce their number and to design a device with desired specifications, multilayered systems with varying composition (and thus material properties) are grown. Theoretical modelling is a useful tool for gaining understanding of various phenomena and materials properties. The scope of the present work is wide. It ranges from a continuum theory of dislocations treated within the linear elasticity theory, connects the continuum and atomistic level modelling for the case of the critical thickness of thin epitaxial layers, and covers some issues of simulating the electronic structure of III-nitride alloys by means of the first principle methods. The first part of this work discusses several topics involving dislocation theory. The objectives were: (i) to apply general elasticity approaches known from the literature to the specific case of wurtzite materials, (ii) to extend and summarise theoretical studies of the critical thickness in heteroepitaxy. Subsequently, (iii) to develop an improved geometrical model for threading dislocation density reduction during the growth of thick GaN films. The second part of this thesis employs first principles techniques (iv) to investigate the electronic structure of binary compounds (GaN, AlN, InN) and correlate these with experimentally available N K-edge electron energy loss near edge structure (ELNES) data, (v) to apply the special quasi-random structures method to ternary III-nitride wurtzite alloys aiming to develop a methodology for modelling wurtzite alloys and to get quantitative agreement with experimental N K-edge ELNES structures, and (vi) to theoretically study strain effects on ELNES spectra.

A near infrared waveguide photodetector in Si-based ternary Si₁−x−yGexCy alloy was demonstrated for 0.85~1.06 µm wavelength fiber-optic interconnection system applications. Two sets of detectors with active absorption layer compositions of Si₀.₇₉Ge₀.₂C₀.₀₁ and Si₀.₇₀Ge₀.₂₈C₀.₀₂ were designed. The active absorption layer has a thickness of 120~450 nm. The external quantum efficiency can reach ~3% with a cut-off wavelength of around 1.2 µm. / Singapore-MIT Alliance (SMA)

Study of liquid film dynamics is of significant importance to the understanding and control of various industrial processes that involve spray cooling (condensation), heating (boiling), coating, cleaning and lubrication. For instance, the critical heat flux (CHF) of boiling heat transfer is one of the key parameters ensuring the efficiency and safety of nuclear power plants under both operational and accident conditions, which occurs as the liquid layers (microlayer and macrolayer) near the heater wall lose their integrity. However, an experimental quantification of thin liquid film dynamics is not straightforward, since the measurement at micro-scale is a challenge, and further complicated by the chaotic nature of boiling process. The object of present study is to develop experimental methods for the diagnosis of liquid film dynamics, and to obtain data for the film instability under various conditions. A dedicated test facility was designed and constructed where micro conductive probes and confocal optical sensors were used to measure the thickness and dynamic characteristics of a thin liquid film on various heater surfaces, while a high speed camera was used to get visual observation. Extensive tests were performed to calibrate and verify the two thickness measuring systems. The micro conductive measuring system was proven to have a high reliability and repeatability with maximum system error less than 5µm, while the optical measuring system is capable of recording the film dynamics with spatial resolution of less than 1 mm. The simultaneous measurement on the same liquid film shows that the two techniques are in a good agreement with respect to accuracy, but the optical sensors have a much higher acquisition rate up to 30 kHz, which are more suitable for rapid process. The confocal optical sensors were therefore employed to measure the dynamic thickness of liquid films (ethanol, hexane and water) evaporating on various horizontal heater surfaces (aluminum, copper, silicon, stainless steel and titanium) to investigate the influences of heat flux, the surface and liquid properties on the film instability and the critical thickness. The critical thickness of water film evaporating on various surfaces was measured in the range of 60-150 mm, increasing with the increased contact angle or increased heat flux (evaporating rate) and also with the decreased thermal conductivity of the heater material. The data suggest the conjugate heat transfer nature of the evaporating liquid film dynamics at higher heat fluxes of interest to boiling and burnout. In the case of hexane on the aged titanium surface with contact angle of ~3o, the liquid film is found resilient to rupture, with film oscillations at relatively large amplitude ensuing as the averaged film thickness decreases below 15 µm. To interpret our experimental findings on liquid film evolution and its critical thickness at rupture, a theoretical analysis is also performed to analyze the dynamics of liquid films evaporating on heater surfaces. While the influences of liquid properties, heat flux, and thermal conductivity of heater surface are captured by the simulation of the lubrication theory, influence of the wettability is considered via a minimum free energy criterion. The thinning processes of the liquid films are generally captured by the simulation of the lubrication theory. For the case with ideally uniform heat flux over the heater surface, the instability of the liquid film occurs at the thickness level of tens micro meters, while for the case of non-uniform heating, the critical thicknesses for the film rupture are closer to  the experimental data but still underestimated by the lubrication theory simulation. By introducing the minimum free energy criterion to considering the influence of surface wettability, the obtained critical thicknesses have a good agreement with the experimental ones for both titanium and copper surfaces, with a maximum deviation less than ±10%. The simulations also explain why the critical thickness on a copper surface is thinner than that on a titanium surface. It is because the good thermal conductivity of copper surface leads to uniform temperature distribution on the heat surface, which is responsible for the resilience of the liquid film to rupture. A silicon wafer with an artificial cavity fabricated by Micro Electronic Mechanical System (MEMS) technology was used as a heater to investigate the dynamics of a single bubble in both a thick and thin liquid layer under low heat flux (<60 kW/m2). The maximum departure diameter of an isolated bubble in a thick liquid film was measured to be 3.2 mm which is well predicted by the Fritz equation. However, in a thin liquid layer with its thickness less than the bubble departure diameter, the bubble was stuck on the heater surface with a dry spot beneath. A threshold thickness of the liquid film which enables the dry spot rewettable was obtained, and its value linearly increases with increasing heat flux. In addition, another test section was designed to achieve a constant liquid film flow on a titanium nano-heater surface which helps to successfully carry boiling in the liquid film from low heat flux until CHF. Again, the confocal optical sensor was employed to measure the dynamics of the liquid film on the heater surface under varied heat flux conditions.  A statistical analysis of the measured thickness signals that emerge in a certain period indicates three distinct liquid film thickness ranges: 0~50 µm as microlayer, 50~500 µm as macrolayer, 500~2500 µm as bulk layer. With increasing heat flux, the bulk layer disappears, and then the macrolayer gradually decreases to ~105 µm, beyond which instability of the liquid film may lose its integrity and CHF occurs. In addition, the high-speed camera was applied to directly visualize and record the bubbles dynamics and liquid film evolution. Dry spots were observed under some bubbles occasionally from 313 kW/m2 until CHF with the maximum occupation fraction within 5%.  A dry spot was rewetted either by liquid receding after the rupture of a bubble or by the liquid spreading from bubbles’ growth in the vicinity. This implies that the bubbles’ behavior (growth and rupture) and their interactions in particular are of paramount importance to the integrity of liquid film under nucleate boiling regime. / QC 20111205 / VR-2005-5729, MSWI

Les procédés de dépôt de couches minces optiques par voie physique ou par voie sol-gel présentent par nature des limitations pour la réalisation de revêtements épais (>1 µm) et ont alors recours à des empilements multicouches pour la préparation de miroirs diélectriques ou de polariseurs. C'est pour ces raisons qu'il est intéressant d'étudier les conditions permettant d'augmenter l'épaisseur critique des films sol-gel notamment. Après avoir étudié la capacité d'empilements des couches colloïdales, trois principaux paramètres ont été identifiés permettant d'augmenter l'épaisseur critique d'empilements monomatériaux et multimatériaux. Ces paramètres sont : l'épaisseur déposée,les interactions chimiques entre les particules et le temps de séchage du film. Ils influencent la microstructure des empilements et par conséquent les propriétés optiques et mécaniques. En contrôlant tous ces paramètres, nous avons montré qu'il est possible de préparer des empilements colloïdaux de fortes épaisseurs ouvrent la voie à la préparation sol-gel de miroirs de hautes performances et/ou de polariseurs. En outre, un développement spécifique de méthodes de caractérisations optiques et mécaniques a été nécessaire pour l'étude de ces films sol-gel, à la fois minces et fragiles. / Main optical deposition processes, physical vapor deposition or sol-gel, exhibit difficulties of achieving thick coatings (>1 µm) and to build multilayer stacks (dielectric mirrors, polarizers). For these reasons, we have studied the conditions to enable a significative increase of deposited sol-gel films thickness. Three main parameters have been evidenced enabling the control of the stacking ability : single layer deposited thickness, chemical interactions beetween nanoparticles and coating drying time. We have shown that these parameters depend on the sol composition and on deposition conditions (process) and that the microstructure of single material stacking is influenced. Optical and mechanical properties of sol-gel films have been studied and optimized regarding these different material and process parameters. For this reason, optical and mechanical characterization techniques have been specifically developed and can now be used for fragile and thin film characterization. In controlling all these parameters, it is now possible to prepare multilayer colloidal stack with high thicknesses enabling the fabrication of high-performance mirrors and polarizers.

abstract: Group III-nitride semiconductors have been commercially used in the fabrication of light-emitting diodes and laser diodes, covering the ultraviolet-visible-infrared spectral range and exhibit unique properties suitable for modern optoelectronic applications. InGaN ternary alloys have energy band gaps ranging from 0.7 to 3.4 eV. It has a great potential in the application for high efficient solar cells. AlGaN ternary alloys have energy band gaps ranging from 3.4 to 6.2 eV. These alloys have a great potential in the application of deep ultra violet laser diodes. However, there are still many issues with these materials that remain to be solved. In this dissertation, several issues concerning structural, electronic, and optical properties of III-nitrides have been investigated using transmission electron microscopy. First, the microstructure of In_xGa_1-xN (<italic>x</italic> = 0.22, 0.46, 0.60, and 0.67) films grown by metal-modulated epitaxy on GaN buffer /sapphire substrates is studied. The effect of indium composition on the structure of InGaN films and strain relaxation is carefully analyzed. High luminescence intensity, low defect density, and uniform full misfit strain relaxation are observed for <italic>x</italic> = 0.67. Second, the properties of high-indium-content InGaN thin films using a new molecular beam epitaxy method have been studied for applications in solar cell technologies. This method uses a high quality AlN buffer with large lattice mismatch that results in a critical thickness below one lattice parameter. Finally, the effect of different substrates and number of gallium sources on the microstructure of AlGaN-based deep ultraviolet laser has been studied. It is found that defects in epitaxial layer are greatly reduced when the structure is deposited on a single crystal AlN substrate. Two gallium sources in the growth of multiple quantum wells active region are found to cause a significant improvement in the quality of quantum well structures. / Dissertation/Thesis / Doctoral Dissertation Physics 2014

Materiais frágeis, tal como o silício, têm sido utilizados em sistemas microeletromecânicos, semicondutores e dispositivos ópticos infravermelhos. Estes materiais são considerados de difícil usinabilidade devido à tendência de sofrerem fraturas. O grande desafio na usinagem dos materiais cristalinos é alcançar uma remoção de material por deformações plásticas (regime dúctil), pois, nessas condições as superfícies ópticas usinadas são geradas sem nenhum dano. Esse regime de usinagem pode ser alcançado em escalas submicrométricas, de forma que, em muitos cristais, as pressões impostas pela ferramenta são altas o suficiente para conduzirem uma transformação de fase do material, favorecendo, assim, a usinagem. Embora pesquisas sobre a relação entre a endentação e a usinagem tenham sido desenvolvidas, a busca por métodos matemáticos com base nas forças e deformações de endentação para serem usados em usinagem de modo a identificar as condições ótimas para remoção de material em regime dúctil não são triviais. O presente trabalho propõe uma relação mais direta com os resultados de endentação para determinar os parâmetros ótimos de usinagem dos materiais frágeis, correlacionando a área da face do endentador em contato e a área efetiva da secção de corte em usinagem. Para isso, ensaios de endentação e experimentos de usinagem com ferramenta de diamante foram realizados em escala micro e nanométrica. O material analisado aqui foi o silício monocristalino (100). Uma matriz experimental foi planejada para as possíveis correlações da variação do ângulo de saída da ferramenta e do avanço de usinagem com as áreas de endentação e o surgimento das trincas e fraturas; forças de usinagem e a pressão de transição frágil-dúctil; tensão residual; espessura crítica de corte e o estado das superfícies usinadas. Em relação às durezas obtidas, foi preciso separá-las em dois estágios: antes do surgimento das trincas durante a endentação e depois desse ponto. Durante a usinagem, a melhor remoção de material em regime dúctil foi obtida na direção mais dura do silício. Os ângulos de saída que proporcionaram resultados desfavoráveis em termos de integridade superficial foram o de -25° e ângulos mais negativos que -60°. A pressão de transição se apresentou de 12 GPa a 13 GPa, sendo que as energias específicas de corte seguiram o mesmo comportamento: 9 j/mm³ a 10 j/mm³ respectivamente. A tensão residual se mostrou inversamente proporcional às forças de usinagem. As espessuras crítica-efetivas de corte variaram de 100 nm a 560 nm. Os valores das espessuras críticas de corte estimadas pelos ensaios de endentação variaram de 200 nm a 530 nm. Portanto, foi possível mostrar que os valores de espessura crítica estimados pelo método proposto, com base nos resultados de endentação, corresponderam muito bem às espessuras críticas obtidas nos experimentos de usinagem. Assim sendo, torna-se possível determinar por meio de tal técnica os valores ótimos de usinagem, podendo ser aplicada para qualquer material cristalino. / Brittle materials, such as silicon, have been used in microelectromechanical systems, semiconductor and infrared optical devices. These types of materials are considered of difficult to machine due to the tendency to suffer fractures. The great challenge in the machining of crystalline materials is to achieve a removal of material by plastic deformations (ductile regime), because in these conditions the machined optical surfaces are generated without any superficial damage. This type of machining can be achieved on a submicrometric machining scale, so that the pressures imposed by the tool are high and lead to a phase transformation of many crystals favoring the machining in ductile regime. Although research on the relationship between microindentation and micromachining has been developed, the search for mathematical methods based on the forces and the deformations of indentation to be used in machining in order to identify the machining conditions under regime ductile are non-trivial. The present work proposes a more direct relationship with the results of the indentation to determine the optimal parameters of the fragile materials, correlating the indenter face area and the cutting section effective area in machining. For this purpose, indentation tests and diamond tool machining experiments were carried out on a micro and nanometric scale. The material analyzed here was monocrystalline silicon (100). An experimental matrix was planned for the possible correlations of the variation of the tool rake angle and of the machining feed with the areas of indentation and the beginning of cracks and fractures; cutting forces and the fragile-ductile transition pressure; residual stress; critical cutting thickness and the state of machined surfaces. In relation to the hardness obtained, it was necessary to separate them in two stages: before the emergence of the cracks during the indentation and after that point. During machining, the best removal of ductile material was obtained in the hardest direction of the silicon. The rake angles which gave unfavorable results in terms of surface integrity were -25° and angles more negative than -60°. The transition pressure reached values from 12 GPa to 13 GPa, and the specific shear energies followed the same behavior: 9 j/mm³ at 10 j/mm³ respectively. The residual stress was inversely proportional to the machining forces. Critical-effective uncut thicknesses ranged from 100 nm to 560 nm. The values of the critical uncut thicknesses estimated by the indentation tests ranged from 200 nm to 530 nm. Therefore, it was possible to show that the critical thickness values estimated by the proposed method, based on indentation results, corresponded very well to the critical thickness obtained in the machining experiments. Thus, it is possible to determine by means of such a technique the optimum values of machining, which can be applied to crystalline material.

Materiais frágeis, tal como o silício, têm sido utilizados em sistemas microeletromecânicos, semicondutores e dispositivos ópticos infravermelhos. Estes materiais são considerados de difícil usinabilidade devido à tendência de sofrerem fraturas. O grande desafio na usinagem dos materiais cristalinos é alcançar uma remoção de material por deformações plásticas (regime dúctil), pois, nessas condições as superfícies ópticas usinadas são geradas sem nenhum dano. Esse regime de usinagem pode ser alcançado em escalas submicrométricas, de forma que, em muitos cristais, as pressões impostas pela ferramenta são altas o suficiente para conduzirem uma transformação de fase do material, favorecendo, assim, a usinagem. Embora pesquisas sobre a relação entre a endentação e a usinagem tenham sido desenvolvidas, a busca por métodos matemáticos com base nas forças e deformações de endentação para serem usados em usinagem de modo a identificar as condições ótimas para remoção de material em regime dúctil não são triviais. O presente trabalho propõe uma relação mais direta com os resultados de endentação para determinar os parâmetros ótimos de usinagem dos materiais frágeis, correlacionando a área da face do endentador em contato e a área efetiva da secção de corte em usinagem. Para isso, ensaios de endentação e experimentos de usinagem com ferramenta de diamante foram realizados em escala micro e nanométrica. O material analisado aqui foi o silício monocristalino (100). Uma matriz experimental foi planejada para as possíveis correlações da variação do ângulo de saída da ferramenta e do avanço de usinagem com as áreas de endentação e o surgimento das trincas e fraturas; forças de usinagem e a pressão de transição frágil-dúctil; tensão residual; espessura crítica de corte e o estado das superfícies usinadas. Em relação às durezas obtidas, foi preciso separá-las em dois estágios: antes do surgimento das trincas durante a endentação e depois desse ponto. Durante a usinagem, a melhor remoção de material em regime dúctil foi obtida na direção mais dura do silício. Os ângulos de saída que proporcionaram resultados desfavoráveis em termos de integridade superficial foram o de -25° e ângulos mais negativos que -60°. A pressão de transição se apresentou de 12 GPa a 13 GPa, sendo que as energias específicas de corte seguiram o mesmo comportamento: 9 j/mm³ a 10 j/mm³ respectivamente. A tensão residual se mostrou inversamente proporcional às forças de usinagem. As espessuras crítica-efetivas de corte variaram de 100 nm a 560 nm. Os valores das espessuras críticas de corte estimadas pelos ensaios de endentação variaram de 200 nm a 530 nm. Portanto, foi possível mostrar que os valores de espessura crítica estimados pelo método proposto, com base nos resultados de endentação, corresponderam muito bem às espessuras críticas obtidas nos experimentos de usinagem. Assim sendo, torna-se possível determinar por meio de tal técnica os valores ótimos de usinagem, podendo ser aplicada para qualquer material cristalino. / Brittle materials, such as silicon, have been used in microelectromechanical systems, semiconductor and infrared optical devices. These types of materials are considered of difficult to machine due to the tendency to suffer fractures. The great challenge in the machining of crystalline materials is to achieve a removal of material by plastic deformations (ductile regime), because in these conditions the machined optical surfaces are generated without any superficial damage. This type of machining can be achieved on a submicrometric machining scale, so that the pressures imposed by the tool are high and lead to a phase transformation of many crystals favoring the machining in ductile regime. Although research on the relationship between microindentation and micromachining has been developed, the search for mathematical methods based on the forces and the deformations of indentation to be used in machining in order to identify the machining conditions under regime ductile are non-trivial. The present work proposes a more direct relationship with the results of the indentation to determine the optimal parameters of the fragile materials, correlating the indenter face area and the cutting section effective area in machining. For this purpose, indentation tests and diamond tool machining experiments were carried out on a micro and nanometric scale. The material analyzed here was monocrystalline silicon (100). An experimental matrix was planned for the possible correlations of the variation of the tool rake angle and of the machining feed with the areas of indentation and the beginning of cracks and fractures; cutting forces and the fragile-ductile transition pressure; residual stress; critical cutting thickness and the state of machined surfaces. In relation to the hardness obtained, it was necessary to separate them in two stages: before the emergence of the cracks during the indentation and after that point. During machining, the best removal of ductile material was obtained in the hardest direction of the silicon. The rake angles which gave unfavorable results in terms of surface integrity were -25° and angles more negative than -60°. The transition pressure reached values from 12 GPa to 13 GPa, and the specific shear energies followed the same behavior: 9 j/mm³ at 10 j/mm³ respectively. The residual stress was inversely proportional to the machining forces. Critical-effective uncut thicknesses ranged from 100 nm to 560 nm. The values of the critical uncut thicknesses estimated by the indentation tests ranged from 200 nm to 530 nm. Therefore, it was possible to show that the critical thickness values estimated by the proposed method, based on indentation results, corresponded very well to the critical thickness obtained in the machining experiments. Thus, it is possible to determine by means of such a technique the optimum values of machining, which can be applied to crystalline material.

The thesis is composed of three parts - theoretical, computational and a project part. The theoretical part deals with heat sharing through conduction, flow and radiation. The computational part is focused on the overall calculation of the heating system to operate smoothly and reliably. Three gas condensing boilers are designed as a source of heat. The heating of the water is solved as a reservoir. It's source of heat is one gas condensation boiler. The project part contains a technical report and the project documentation on the stage of the implementation dossier.

Friction stir welding (FSW) of steel is often performed with an insert made of polycrystalline cubic boron nitride (PCBN). Specifically, MS80 is a grade of PCBN made by Smith MegaDiamond that has been optimized for the FSW process. The PCBN insert is attached to a tungsten carbide (WC) shank by a compression fitting. However, FSW tools manufactured by this method inevitably fail by fracture in the PCBN. Permanently bonding PCBN to WC would likely solve the fracturing problem and increase the life of PCBN FSW tools to be economically viable. Partial transient liquid phase (PTLP) bonding, a process used to join ceramics with thin metallic interlayers, was proposed as a method to permanently bond PCBN to WC. PTLP bonding is often performed using three layers of pure elements. On heating, the two thin outer interlayers melt and bond to the ceramics. Concurrently, these liquid layers diffuse into the thicker refractory core until solidification has occurred isothermally. A procedure was developed to reduce the number of possible three-layer PTLP bonding setups to a small set of ideal setups using logical filters. Steps in this filtering method include a database of all existing binary systems, sessile drop testing of 20 elements, and a routine that calculates maximum interlayer thicknesses. Results of sessile drop testing showed that the PCBN grade required for this research could only be bonded with an alloy of Ti, Cu, Mg, and Sb. Two PTLP bond setups were tested using this special coating on the PCBN, but a successful bond could not be achieved. However, a PTLP bond of WC to WC was successful and proved the usefulness of the filtering procedure for determining PTLP bond setups. This filtering procedure is then set forth in generalized terms that can be used to PTLP bond any material. Also, recommendations for future research to bond this grade of PCBN, or some other grade, to WC are presented.