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The main purpose of this study is focused on the defects within the layers of GaN over AlN thin films which were grown on the substrates of silicon (111) and LiAlO2 (100), respectively. The growth of three sapphire crystals is also addressed. During the epitaxial growth period, the Al pre-deposition time is very important for the defect on AlN/Si, and due to the high stacking fault energy, the partial dislocations in the AlN layers can not be observed. Cracks were found in the GaN films because of the great thermal mismatch between GaN itself and AlN which is up to 25%. Although the lattice mismatch between GaN and LiAlO2 is low, the thermal stress effect still could not be avoided. Cracks also occurred in the GaN films which were grown on LiAlO2 substrates. For the sapphire pulled along [100] direction, the crystal has fewer amounts of bubbles at the lower rotation rate. For the sapphire pulled along [001] direction, the crystal with 4.5 mm/hr pulling rate has cracks along [001] direction, but the crystal with 4.0 mm/hr pulling rate has no cracks inside.

"New high performance optical systems require highly durable, broadband window materials in larger sizes than are currently available. To meet this need, a low cost edge-bonding process was developed for producing large area sapphire windows from smaller, melt-grown blanks. The method uses a polycrystalline alumina interlayer to promote diffusion and single crystal conversion at the interface between two sapphire substrates and produce high strength bonds. The goal of the current research effort is to determine the optimal alumina composition for maximum bond strength. Polycrystalline alumina fillets containing various chemical additives were prepared by tape casting for use in bonding trials. Oriented sapphire blanks were edge bonded in a furnace with special fixtures to accurately align and apply a load to the components during heat treatment. This approach is consistent with and builds upon the methods used in the previous edge-bonding studies. Flexure strength of the bonded samples, as compared to monolithic sapphire, was used as the performance metric. Additional bonding runs were carried out using the highest performing fillet composition in order to provide a sufficient number of specimens to conduct a Weibull analysis of the failure probability of the bonded material as a function of applied stress. A high purity alumina composition containing 3 wt.% SiO2, 0.05 wt.% MgO, and 0.05 wt.% Ti, produced the highest strength bonds. This composition yielded an average fracture strength of 255 MPa (37 kpsi), a Weibull modulus of 8.2, and a characteristic strength of 269 MPa (39 kpsi). These results compare favorably to monolithic sapphire specimens which yielded an average fracture strength of 284 MPa (41 kpsi)."

The research has been analyzed by using scanning electron and transmission electron microscopy (SEM and TEM).To research microstructure in single crystal alumina induced by Vickers indentation and the effect of alumina microstructure induced by deformation twinning. Sapphire is a kind of alumina single crystal called Corundum. It has fine machinery, optical, chemistry and anti-radiationary nature and been used widespread in industry world in recent years. Mechanical twinning is the main mode of plastic deformation. It has two types (1) basal twinning (2) rhombohedral twinning. When author pressed 20 N on (0001) plane, it produces B (0001), R1 ( 102) and R1 ¡¨ ( 104) in cross- section. But when pressing 20 N in (1 10) plane, it produces B (0001), B 1 ( 101) and R2 ( 012) in cross-section. In order to understand the role of pressure direction in twinning at room temperature, this research will discuss what effects will be produced twinning microstructure of alumina sample and derivation of dislocation microstructure by using scanning electron and transmission electron microscopy.

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A single crystal sapphire optical fiber has been developed with an optical cladding that is used to reduce the number of modes that propagate in the fiber. This fiber is the first single crystal sapphire photonic crystal fiber ever produced. Fabrication of the optical cladding reduces the number of modes in the fiber by lowering the effective refractive index around the core, which limits the amount of loss. Different fiber designs were analyzed using Comsol Multiphysics to find the modal volumes of each. The MIT Photonic Bands modeling program was used to see if the first photonic band gap fiber could be achieved theoretically. The fibers were qualified using far field pattern and photodetector measurements as well as gas sensing experiments. The fibers were then exposed to a harsh environment of 1000 °C with a coating of alumina to test the resistance to scattering of the fiber. The fibers were also examined using materials characterization equipment to see how the harsh environments impacted the optical and mechanical stability of the bundled fiber. / Ph. D.

This is a study of the structural and electronic behavior and properties of graphene on α-quartz and α-sapphire using Density Functional Theory. We construct initial structures using the above 2 substrates, place a layer of graphene on them and subsequently allow the atoms to relax. After relaxation we study any structural changes, band structures, density of states, charge density to determine the electronic properties of the entire structure. Eventually this study will help in the search for good substrates for graphene based transistors. / text

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Šiame darbe sprendžiama struktūrinių intarpų kiekio safyro gardelėje temperatūrinės priklausomybės problema. Darbo apimtis 38 lapai. Darbą sudaro įvadas, dvi dalys, išvados, naudotos literatūros sąrašas ir priedai. Darbe pateikiami 23 paveikslai. Įvade trumpai pateikiama, kodėl pasirinkta ši darbo tema, iškeliama darbo problema, tikslai, uždaviniai ir metodai. Pirmoje dalyje pateikiama teorinė medžiaga, kurioje apžvelgiama kietųjų kūnų klasifikacija, medžiagų struktūra, kristalų defektai, struktūros tyrimo metodai, Brego dėsnio panaudojimas defektų tyrimui Antroje dalyje supažindinama su darbo metodika, prietaisais ir priemonėmis kurios naudojamos atliekant tyrimą. Pateikiami tyrimo rezultatai. Panaudojant minkštosios rentgeno spinduliuotės sukeltą vakansijų generavimą, ateityje galima tikėtis naujų prietaisų gamybos technologijų. Prieduose pateikiama kompaktinė plokštelė. / Temperature dependency on structural interspace quantity of sapphire in grating, is surveyed in this work. The size of the work is 38 pages. The paper consists of an introduction, two major parts, conclusions, a list of bibliography, and appendixes. There are 23 pictures enclosed. The theme, problems, aims, goals, and methods are shortly presented in the introductory part. The first part introduces theoretical material that reviews classification of solid-state, composition agent, defects of crystals, composition research methods, Bregg law use researching defects The second part presents methodology of the work, and devices used in the research. It contains the results of the research. In future using vacancies generation evoked by irradiated soft x-rays the development of the new producing technologies can be hoped. Appendixes contain the CD.