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This dissertation by Nerijus Armakavicius explores the properties of free charge carriers in group III nitrides and graphene, utilizing THz-to-MIR ellipsometry and the optical Hall effect. The research aims to advance understanding of materials that can surpass silicon's limitations in high-frequency and high-power electronics. Key topics include the interaction of epitaxial graphene with substrates and the electrical properties of group III nitrides for applications such as light-emitting diodes and high electron mobility transistors. The study employs advanced optical techniques to characterize carrier mobility and effective mass without altering intrinsic material properties, making significant contributions to material science and electronic engineering. The intended audience includes researchers and professionals in physics, material science, and electronic engineering.

Graphene. --- Ellipsometry. --- Graphene --- Ellipsometry

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This dissertation by Samiran Bairagi focuses on the optical properties of AlN and GaO-based materials, studied using Mueller matrix spectroscopic ellipsometry. It explores how light interacts with these materials, emphasizing circular polarization and bandgap phenomena. The research aims to synthesize high-quality thin films to enhance understanding of their optical characteristics. The study highlights the impact of film morphology on circular polarization and investigates electronic transitions in ZnGaO thin films. The work provides insights into the growth, structure, and optical properties of these films, contributing to the development of advanced photonic technologies. The intended audience includes researchers and professionals in materials science and optics.

Ellipsometry. --- Thin films. --- Ellipsometry --- Thin films

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Ellipsometry. --- Nanotechnology. --- Surfaces (Physics)

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The investigation of the interfacial phase transitions in fluid systems with short-range intermetallic interactions are of great interest. The phenomena were studied in two systems exhibiting a liquid-liquid miscibility gap: at the fluid/wall interface in fluid KxKCl1-x and at the fluid/vacuum interface of the Ga1 xBix alloys. To characterize the interfacial changes of the ultra thin films (composition, thickness and their evolution with time) the spectroscopic ellipsometry was performed over a wide spectral range. Whereas in the experiments on KxKCl1-x an existing ellipsometer could be used, a completely new UHV-apparatus including the in-situ phase modulation ellipsometer had to be developed for Ga1 xBix alloys. For the KxKCl1-x system new results on complete wetting at solid-liquid coexistence as well as in the homogenous liquid phase (prewetting) are presented. The spectra show the typical F center absorption which indicates that the film is a salt-rich phase. The thickness strongly increases approaching the monotectic from 30 to 440 nm, which is in agreement with the tetra point wetting scenario. For this interpretation a quantitative description of the excess Gibbs energy has been developed. For the Ga1 xBix system the results on complete wetting, surface freezing and oscillatory interfacial instabilities are presented. The high-precision spectra have been recorded approaching the liquid-liquid miscibility. These spectra have been modeled using a Ga-Bi effective medium approximation for the substrate covered by a film of liquid Bi. The measurements give evidence of tetra point wetting in the Ga-Bi system. First ellipsometric study of the surface freezing in Ga-Bi has been performed. Within the miscibility gap a very interesting effect of surface and bulk oscillatory instability was observed. The details of this process at present are not well understood, but a qualitative description is given.

molten salt --- surface phase transition --- spectroscopic ellipsometry --- Ga-Bi alloys

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Photovoltaic cells --- Solar cells --- Ellipsometry. --- Thin films. --- Research.