Master Degree / Yüksek Lisans Tezleri

Permanent URI for this collectionhttps://hdl.handle.net/11147/3008

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Publisher: search.filters.publisher.Izmir Institute of TechnologySubject: search.filters.subject.Photoconductivity

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  • Master Thesis
    Multiple exciton generation in graphene nanostructures
    (Izmir Institute of Technology, 2014) Yıldırım, Jülide; Çakır, Özgür; Çakır, Özgür; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    This thesis comprises a theoretical study on the role of the inverse-Auger process in graphene nanostructures. Inverse-Auger effect (IAE) is the formation of a multitude of low energy excitons from a single exciton of higher energy. Its mechanism is the conversion of the kinetic energy of the high energy carriers to new excitons via Coulomb interaction. Bulk graphene has zero band gap energy and has two Dirac points which is linearly dependent crystal momentum. Due to quantum confinement, graphene nanoribbons and graphene flakes or the structures having periodically holes develop a band gap. The emergence of a band gap makes these structures eligible for solar cell applications. In bulk structures, due to translational symmetry momentum is conserved which leads to a decreased IAE. However, in nanostructures, in addition to the relaxation of momentum conservation condition, the Coulomb interaction between the carriers increases which leads to an enhanced IAE. In this thesis, a theoretical analysis of inverse-Auger effect is carried out for graphene and armchair graphene nanoribbons. Tight binding method is employed to obtain the electronic structure and to calculate the Coulomb matrix elements for the inverse-Auger effect in this structures. According to our calculations, inverse- Auger effect in the bulk graphene provides the formation of new excitons at a rate which is approximately linearly proportional to the energy of an electron at the conduction band.
  • Master Thesis
    The Effects of Deposition Conditions on the Low Energy Absorption Spectrum of Microcrystalline Silicon Thin Films Prepared by Hwcvd Method
    (Izmir Institute of Technology, 2005) Işık, Nebile; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    The optical and electronic properties of hydrogenated microcrystalline silicon films deposited by HWCVD method were investigated using steady state photoconductivity (SSPC), dual beam photoconductivity (DBP) and transmission spectroscopy methods to understand the effects of deposition conditions such as silane concentration and filament temperature on the low absorption coefficient spectrum, alpha (h.). The alpha (h.) spectrum obtained from the detailed optical calculation using the relative DBP and transmission spectra were compared with that independently measured on the same samples using photothermal deflection spectroscopy (PDS) and constant photocurrent method (CPM) techniques. The results were found to be in agreement with those of PDS and CPM at higher energy part of spectrum. On the other hand some differences exist among the spectra at lower energies. These differences were discussed to be consistent with underlying the physics of these methods.The effect of silane concentration on the sub-bandgap alpha (h.) spectrum was found to be substantial. At highest SC of 10% the alpha (h.) spectrum similar to that of a-Si: H is obtained. As SC decreases to 7%, microcrystalline phase becomes dominant.Further decrease of SC, the low energy alpha (h.) decreases and given a minimum around SC of 5%. For the lower SC.s, highly crystalline .c-Si: H films are obtained but the alpha (h.) values increases to higher values indicating an increase in the defect densities present in the microstructure.The effect of filament temperature was investigated for a constant SC of 10%. It was found that at 1700 C and 1800 C, fully amorphous films are obtained but 1800 C results in higher alpha (h.) values at lower energies. At 1880 C, microcrystalline phase becomes dominant and the alpha (h.) spectrum becomes similar to that of single crystal silicon.Finally, due to inhomogeneous microstructure of uec-Si: H, there are left fringes on calculated alpha(h.) spectrum on same samples. The degree of the inhomogeneity was investigated by front and back ac illumination of DBP measurements. It was found that there exists a substantial differences on the spectra measured on the same sample indicating importance of inhomogeneous film growth on optoelectronic measurements and its evaluation.
  • Master Thesis
    Low Temperature Photoconductivity of Hydrogenated Amorphous Silicon (a-Si:h) Thin Flims
    (Izmir Institute of Technology, 2003) Erdoğan, Gökhan; Güneş, Mehmet; Güneş, Mehmet; 04.05. Department of Pyhsics; 04. Faculty of Science; 01. Izmir Institute of Technology
    In this study low temperature photoconductivity of undoped hydrogenated amorphous silicon(a-Si:H) thin films have been studied to investigate the effect of native and Staebler-Wronski defects. The study covers undoped a-Si:H films prepared by various deposition techniques such as DC glow discharge, RF-PECVD with and without H-dilution, RF magnetron sputtering and hot-wire(HW) CVD.In the annealed state, the samples were characterized using temperature dependence of dark conductivity, steady-state photoconductivity, .ph, versus light intensity at room temperature and steady-state photoconductivity versus temperature down to 90 0K at three different intensities. Activation energy ,EF, of the samples changes from 0.60 eV to 1.0 eV. .ph shows a few orders of magnitude higher values from the dark conductivity and its magnitude is sample dependent due to differences in deposition conditions. The intensity dependence of .ph ,., (.ph . F.) is close to unity and varies between 0.70 to 0.90, indicating recombination kinetics through the midgap defect states in the bandgap of a-Si:H. Low temperature photoconductivity versus 1000/T spectrum shows three distinctly different regions. In Region I, .ph decreases with temperature until a transition temperature. Then Region II begins, where .ph begins to increase resulting a peak in spectrum or remains to be unchanged until a second transition temperature to Region III, where .ph continuously decreases with T. Transition temperatures and the degree of increase in .ph in Region II is sample dependent. These results indicate the presence of at least two different types of midgap defect states in the bandgap and exponential tail state present in the annealed state.In the light soaked state, Staebler-Wronski effect (SWE) was investigated after exposing the samples to white light illumination of a few suns intensity. The characterization involves dark conductivity and steady-state photoconductivity at room temperature and .ph versus temperature down to 90 0K for different intensities. Dark conductivity values decreased a certain factor indicating a slight shift in EF through midgap. .ph values decreased substantially from its annealed values due to creation of Steabler-Wronski defects in the bandgap. The intensity dependence of .ph become almost equal and close to unity for all the films even it shows slight variation in the annealed state. The shape of low temperature photoconductivity spectra becomes almost the same for all samples even drastic differences were observed in the annealed state. The spectrum is mainly dominated by only two regions.Region I dominates from room temperature down to 170 0K, where .ph decreases with a constant slope as T decreases. After that temperature, Region II sets in. .ph remains to be constant until temperature used in this study. Region III can only be detected at higher intensity and temperatures lower than 90 0K. Results indicate that more defects around the midgap are created by light, which decrease .ph and relatively less defects are created away from midgap and closer to band edge, which improve .ph instead of decreasing it as temperature decreases. The defect states in Region I responsible for decreasing .ph are more likely that they are neutral silicon dangling bond defects ,D0, and those in Region II responsible for increasing .ph are non-D0 defect states. They act as photosensitising defects with a very low capture cross-sections for electrons. They could be charged silicon dangling bonds ,D+ and D-, or floating bonds results in defect models proposed for a-Si:H.