Fizik

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Browsing Fizik by Author "Aktaş, Gülen, 1951- ."

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    Archaelogical provenancing of marble by pore size analysis, using phase transition porosimetry
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1999., 1999.) Öztürk, M. Kaan.; Aktaş, Gülen, 1951- .; Özbal, Hadi.
    Determination of the source of a particular marble sample is an ongoing subject of study among archaeometrists. In this study, categorization according to pore size distribution is suggested. The employed method for pore size analysis is Phase Transition Porosimetry (PTP). The principles on which PTP works are described. PTP requires a computer system for data taking and data processing. The electronic data-acquisition system and the control software are discussed extensively. After construction of a dilatometric Phase Transition Porosimeter, marble samples collected from field have been studied with this method and the results are given. The precision of PTP in low-porosity materials are discussed using the results.
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    D.C. photoconductivity in As2Se3 thin films
    (Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2003., 2003.) Atak, Kaan.; Aktaş, Gülen, 1951- .
    In this thesis, the D.C. photoconductivity in amorphous thin films of As2Se3 is investigated. The temperature dependence of dark, and photo currents are explored at the interval 240 - 360 K. Three regimes are observed in the photoconductivity at the given range of temperature. The results are criticized according to the and other data and found to be in accordance with them.
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    Determination of density of states of n-type hydrogenated amorphous silicon by phase shift analysis of modulated photocurrent method
    (Thesis (M.S.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 1995., 1995.) Kocabıyık, Ferhat.; Aktaş, Gülen, 1951- .
    Determination of density of states of hydrogenated amorphous siliconby phase shift analysis of modulated photocurrent.Determination of the energetic distribution of localized statesdue to random network and structural defects within the mobility gap of amorphous semi conductorsis of great importance for understanding the transport properties of these materials. Since there is no structural order,the band states can be only determined experimentally.In this thesis, the density of states (DOS) distribution in the energy gapof Hydrogenated n-type amorphous silicon is determined. For this purpose a The Phase shift analysis of modulated photocurrent (PSAMP) method is studied. The validity of the PSAMP theorem for the doped a-H:Si is discussed. The theorem is verified for this type of semiconductors and the distribution of the states is determined for a range of 400 meV.
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    Electro optical characterization of a silicon microsphere
    (Thesis (M.S.)-Bogazici University. Institute for Graduate Studies in Science and Engineering, 2006., 2006.) Ayaz, Ulaş Kemal.; Aktaş, Gülen, 1951- .; Serpengüzel, Ali.
    In today’s world, with the development of optoelectronics, metal interconnections are no longer limiting factor for the performance of electronic systems. Replacing the metal interconnections by optical interconnections could provide low power dissipation, low latencies, and high bandwiths. Such optical interconnections rely on the integration of micro-photonics and microelectronics. Having high quality factors, optical microsphere resonators are ideal circuit elements for wavelength division multiplexing. Silicon, as a common semiconductor the building block of the integrated circuits, also is a very important component with its optical properties. We have experimentally observed the shifts in resonance wavelengths of an electrically driven silicon microsphere of 1000 microns in diameter, in the near IR. We have used a distributed feedback (DFB) laser at 1475nm, and applied dc voltages ranging from -17V to 9V to the microsphere and observed the respected shifts in the resonance wavelengths around 0.005 nm to 0.080 nm.
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    Frequency-resolved photoconductivity studies in amorphous chalcogenide films
    (Thesis (Ph.D.) - Bogazici University. Institute for Graduate Studies in Science and Engineering, 2000., 2000.) Akat, Ercüment.; Aktaş, Gülen, 1951- .
    Frequency-resolved photocurrent measurements (FRPC) were carried out on amorphous chalcogenide films such as AsSeSb AsSe and AsSeTe, by a method called frequency-resolved spectroscopy (FRS) as a function of temperature (between 230 K and 395 K), of excitation intensity (between 40 and 600 ) and in the frequency range of 10 Hz to 1.1x104 Hz, yielding the lifetime distribution directly. The results indicate that the recombination is through distant pairs, since the lifetime does depend on the excitation intensity, through ; as at 295 K and at 395 K, rather than by geminate (twin) pairs. It can also be concluded that there is a continuous distribution of states in the mobility gap implied by the value of the exponent 0.72 (in ), which is between 0.5 and 1.0, the figures that according to Rose (1978), correspond respectively to monomolecular and bimolecular recombination. The temperature exponent in the relationship has turned out to be between 7 and 10 for a frequency of some (=200 Hz); and 11 and 13 for (=3322 Hz); the value of the exponent showing a diminishing behaviour as the generation rate (intensity of excitation) is elevated. Finally, the exponent in the relationship is found to depend on both temperature and the generation rate in a manner such that the photocurrent will be more sensitive (inversely) to the frequency of excitation as either one of the above variables or both are lowered.