Browsing by Subject "Color centers"

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Open Access
Effects of radiodarkening on the light transmission properties of ytterbium-doped optical fibers
(2021-03) Çamiçi, Hüseyin Can
Optical ﬁbers have been acting in a very crucial role in telecommunication and non-telecom industries like medicine, machining, sensing as well as in lasers and ampliﬁers when they are doped with rare earth ions for their remarkable features such as very good beam quality, high power generation and low cost after the fab-rication of ﬁrst low-loss optical ﬁbers in early 1970s. However, these outstanding properties are deteriorated when optical ﬁbers are exposed to ionizing radiation limiting their use in nuclear power plants and space applications. The main fabrication method of silica optical ﬁbers, Modiﬁed Chemical Vapor Deposition (MCVD), as well as the eﬀects of compositional variations of optical ﬁbers on the gamma radiation resistance were highlighted by this thesis. Optically sound and robust ﬁbers were evaluated through optical and chemical characterization methods and the light guiding abilities of them were measured before and after gamma radiation exposure to reveal its eﬀects. Ytterbium-doped silica optical ﬁbers showed signiﬁcant transmission losses due to the creation of color centers by ionizing radiation exposure which were characterized and their contribution to radiation induced attenuation was described. Growth and recovery of these color centers at room temperature were analysed. Suggestions in terms of chem-ical composition for the fabrication of radiation resistant ﬁbers as well as ﬁber dosimeters were made. The techniques that should be utilized for further recovery of the ﬁbers were proposed.
Open Access
Electrical conduction and dielectric relaxation properties of AlN thin films grown by hollow-cathode plasma-assisted atomic layer deposition
(Institute of Physics Publishing, 2016) Altuntas, H.; Bayrak, T.; Kizir, S.; Haider, A.; Bıyıklı, Necmi
In this study, aluminum nitride (AlN) thin films were deposited at 200 ï¿½C, on p-type silicon substrates utilizing a capacitively coupled hollow-cathode plasma source integrated atomic layer deposition (ALD) reactor. The structural properties of AlN were characterized by grazing incidence x-ray diffraction, by which we confirmed the hexagonal wurtzite single-phase crystalline structure. The films exhibited an optical band edge around ∼5.7 eV. The refractive index and extinction coefficient of the AlN films were measured via a spectroscopic ellipsometer. In addition, to investigate the electrical conduction mechanisms and dielectric properties, Al/AlN/p-Si metal-insulator-semiconductor capacitor structures were fabricated, and current density-voltage and frequency dependent (7 kHz-5 MHz) dielectric constant measurements (within the strong accumulation region) were performed. A peak of dielectric loss was observed at a frequency of 3 MHz and the Cole-Davidson empirical formula was used to determine the relaxation time. It was concluded that the native point defects such as nitrogen vacancies and DX centers formed with the involvement of Si atoms into the AlN layers might have influenced the electrical conduction and dielectric relaxation properties of the plasma-assisted ALD grown AlN films.