Formation of silicon nanocrystals by laser processing of silicon rich oxides

Ankara : The Department of Physics and the Graduate School of Engineering and Science of Bilkent University, 2012.Thesis (Master's) -- Bilkent University, 2012.Includes bibliographical references leaves 77-81.Silicon nanocrystals are well known to exhibit strong luminescence in the visible. Extension of this into a nanocrystal network would be beneficial for many applications. In the light of recent advances on exciton-plasmon interactions and photovoltaic cells, there is renewed interest in the use of nanostructures. Due to quantum confinement, silicon nanoclusters with increased band gaps, are promising for down conversion light and enhanced emission on plasmonic surfaces. Conventional techniques utilize high-temperature processing to obtain the Si-SiO2 phase separation which uses high thermal budget, not suitable for localized applications not compatible with glass substrates or thin-film stacked structures. An alternative approach capable of avoiding high temperature processing is laser irradiation of substochiometric amorphous silicon oxides. In this work, continuous-wave laser processing of Si-rich oxide thin films with varying Si content were performed in order to obtain Si nanocrystals embedded in silica. The role of composition, dwell times and power densities were investigated for Si-SiO2 phase separation. We present cw laser processing of PECVD grown and sputtered SiOx films. XPS, RBS and ERDA techniques were used for the stoichiometry analysis of different composition as grown samples and their optical properties were determined through ellipsometry analysis. Processing was performed with an Ar+ laser at 488 nm. The structural changes due to processing were investigated by Raman and photoluminescence spectroscopy. It has been shown that silicon nanocrystals formation depends both on precursor gas composition (hydrogen-diluted SiH4 and N2O or CO2 gases) and on laser power density. PECVD grown hydrogenated SiOx films were compared with sputtered films with and without hydrogen to identify the role of hydrogen for phase separation.Gündoğdu, SinanM.S

Correlation of TEM data with confined phonons to determine strain and size of Ge nanocrystals embedded in SixNy matrix

Models that use phonon confinement fail to provide consistent results for nanocrystal sizes in differing dielectric matrices due to varying stress experienced by nanocrystals in different dielectric environments. In cases where direct measurement of stress is difficult, the possibility of stress saturation as a function of size opens up a window for the use of phonon confinement to determine size. We report on a test of this possibility in Ge: SixNy system. Ge nanocrystals (NCs) embedded in silicon nitride matrix have been fabricated using plasma enhanced chemical vapor deposition (PECVD) followed by post annealing in Ar ambient. Nanocrystal size dependence of Raman spectra was studied taking into account associated stress and an improved phonon confinement approach. Our analysis show same stress for NCs which have sizes below 7.0 nm allowing the use of phonon confinement to determine the nanocrystal size. The results are compared with TEM data and good agreement is observed

Ge nanocrystals embedded in ultrathin Si3N4 multilayers with SiO2 barriers

Multilayers of germanium nanocrystals (NCs) embedded in thin films of silicon nitride matrix separated with SiO2 barriers have been fabricated using plasma enhanced chemical vapor deposition (PECVD). SiGeN/SiO2 alternating bilayers have been grown on quartz and Si substrates followed by post annealing in Ar ambient from 600 to 900 degrees C. High resolution transmission electron microscopy (HRTEM) as well as Raman spectroscopy show good crystallinity of Ge confined to SiGeN layers in samples annealed at 900 degrees C. Strong compressive stress for SiGeN/SiO2 structures were observed through Raman spectroscopy. Size, as well as NC-NC distance were controlled along the growth direction for multilayer samples by varying the thickness of bilayers. Visible photoluminescence (PL) at 2.3 and 3.1 eV with NC size dependent intensity is observed and possible origin of PL is discussed.UNAM-National Nanotechnology Research Center at Bilkent Universit

Protective Effect of Carvacrol against Paclitaxel-Induced Ototoxicity in Rat Model

Objective: This study aimed to explore whether carvacrol (CV) had a protective effect on paclitaxel-induced ototoxicity from biochemical, functional, and histopathological perspectives.Methods: Forty Wistar albino male rats were randomly separated into five groups of eight rats. Group 1 was the control group, so Paclitaxel or CV was not administered. Group 2 was administered i.p. CV at 25 mg/kg once a week; Group 3, was administered i.p. paclitaxel at 5 mg/kg once a week; Group 4 was administered i.p. paclitaxel at 5 mg/kg followed (30 min later) by CV at 25 mg/kg once a week; and Group 5 was administered i.p. CV at 25 mg/kg followed (1 day later) by paclitaxel at 5 mg/kg. once a week. The drugs were administered intraperitoneally once a week for four consecutive weeks, and distortion product otoacoustic emissions (DPOAE) tests were performed at the beginning of the study before the first drug administration and at the end of the study after the last drug administration. All rats were sacrificed, and cochleae were removed for biochemical and histopathological analysis.Results: Biochemical data indicated that paclitaxel caused oxidative stress in the cochlea. Histopathological findings revealed the loss of outer hair cells in the organ of Corti (CO) and moderate degenerative changes in the stria vascularis (SV). It was observed that DPOAE measurements were significantly reduced at high frequencies. In groups which CV was administered together with paclitaxel, these biochemical, histopathological, and functional changes were favorably reversed.Conclusion: CV may have a protective effect against paclitaxel-induced ototoxicity when given

Uzun dalgaboylu kuantum çağlayan lazerlerin optik ve ışıl dinamikleri

Cataloged from PDF version of article.Thesis (Ph.D.): Bilkent University, Department of Physics, İhsan Doğramacı Bilkent University, 2018.Includes bibliographical references (leaves 85-95).Quantum Cascade Lasers (QCLs) are coherent light sources that make use of intraband transitions of wavefunction engineered semiconductor quantum wells. They have been designed to emit light in a wide spectral range; from mid-wave infrared to terahertz. Long wave QCLs are a subject of interest for some applications such as remote detection of harmful chemicals. These applications demand higher optical powers at room temperature. In this thesis we demonstrate simulation, design, fabrication and characterization of long-wave QCLs that emit light around 9.2 m. To increase optical power and enhance thermal performance, we explore the optical and thermal properties of QCLs. Thermal characteristics of QCLs are analyzed by nite element methods. We developed a spectral technique that relies on analysis of Fabry-Perot modes to measure cavity temperatures experimentally. By combining the simulations and experimental results we scrutinized the thermal properties of QCLs, and estimated the active region thermal conductivity. To increase the optical power, we conducted optical calculations and investigated the sources of loss. As a result of a search for alternative electrical passivation materials, we fabricated HfO2 passivated lasers and demonstrated about to two-fold reduction in optical loss and increase in optical power.by Sinan Gündoğdu.Ph.D

Facet cooling in high-power InGaAs/AlGaAs lasers

Several factors limit the reliable output power of a semiconductor laser under CW operation, such as carrier leakage, thermal effects, and catastrophic optical mirror damage (COMD). Ever higher operating powers may be possible if the COMD can be avoided. Despite exotic facet engineering and progress in non-absorbing mirrors, the temperature rise at the facets puts a strain on the long-term reliability of these diodes. Although thermoelectrically isolating the heat source away from the facets with non-injected windows helps lower the facet temperature, data suggests the farther the heat source is from the facets, the lower the temperature. In this letter, we show that longer non-injected sections lead to cooler windows and biasing this section to transparency eliminates the optical loss. We report on the facet temperature reduction that reaches below the bulk temperature in high power InGaAs/AlGaAs lasers under QCW operation with electrically isolated and biased windows. Acting as transparent optical interconnects, biased sections connect the active cavity to the facets. This approach can be applied to a wide range of semiconductor lasers to improve device reliability as well as enabling the monolithic integration of lasers in photonic integrated circuits.Ermaksan A.Ş

Edge mode bifurcations of two-dimensional topological lasers

© 2020 Optical Society of America. Topological lasers are of growing interest as a way to achieve disorder-robust single-mode lasing using arrays of coupled resonators. We study lasing in a two-dimensional coupled resonator lattice exhibiting transitions between trivial and topological phases, which allows us to systematically characterize the lasing modes throughout a topological phase. We show that, unlike conventional topological robustness that requires a sufficiently large bulk band gap, bifurcations in topological edge mode lasing can occur even when the band gap is maximized.We show that linear mode bifurcations from single-mode to multi-mode lasing can occur deep within the topological phase, sensitive to both the pump shape and lattice geometry. We suggest ways to suppress these bifurcations and preserve single-edge mode lasing11sci