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Işık, Mehmet
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Name Variants
Mehmet, Işık
M.,Işık
Isik, Mehmet
Mehmet, Isik
I., Mehmet
I.,Mehmet
Işık,M.
Isik,M.
I.,Mehmet
M.,Isik
Işık, Mehmet
M., Isik
Isik, M.
M.,Işık
Isik, Mehmet
Mehmet, Isik
I., Mehmet
I.,Mehmet
Işık,M.
Isik,M.
I.,Mehmet
M.,Isik
Işık, Mehmet
M., Isik
Isik, M.
Job Title
Profesör Doktor
Email Address
mehmet.isik@atilim.edu.tr
Main Affiliation
Department of Electrical & Electronics Engineering
Status
Former Staff
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Scopus Author ID
Turkish CoHE Profile ID
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Scholarly Output
171
Articles
166
Citation Count
1080
Supervised Theses
3
170 results
Scholarly Output Search Results
Now showing 1 - 10 of 170
Article Citation - WoS: 6Citation - Scopus: 6Optical Properties of Gas Crystals: Combined Study of Temperature-Dependent Band Gap Energy and Oscillator Parameters(Natl inst Science Communication-niscair, 2017) Isik, Mehmet; Tugay, Evrin; Gasanly, Nizami; Department of Electrical & Electronics Engineering; Department of Electrical & Electronics EngineeringOptical parameters of gallium sulfide (GaS) layered single crystals have been found through temperature-dependent transmission and room temperature reflection experiments in the wavelength range of 400-1100 nm. Experimental data demonstrates the coexistence of both optical indirect and direct transitions and the shift of the absorption edges toward lower energies by increasing temperature in the range of 10-300 K. Band gap at zero temperature, average phonon energy and electron phonon coupling parameter for indirect and direct band gap energies have been obtained from the analyses of temperature dependences of band gap energies. At high temperatures kT>> (E-ph), rates of band gap energy change have been found as 0.56 and 0.67 me V/K for E-gi and E-gd, respectively. Furthermore, the dispersion of refractive index has been discussed in terms of the Wemple-DiDomenico single effective oscillator model. The refractive index dispersion parameters, namely oscillator and dispersion energies, oscillator strength and zero-frequency refractive index, have been found to be 4.48 eV, 24.8 eV, 6.99x10(13) m(-2) and 2.56, respectively. The results of the present work will provide an important contribution to the research areas related to the characterization and optoelectronic device fabrication using GaS layered crystals.Article Citation - WoS: 9Citation - Scopus: 9TL and TSC studies on TlGaSe2 layered single crystals(Elsevier, 2013) Isik, M.; Bulur, E.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringDefects in - as grown - TlGaSe2 layered single crystals were investigated using Thermoluminescence (TL) and Thermally Stimulated Currents (TSC) techniques in the temperature range 10-300 K. TL and TSC curves of samples illuminated using a light with energy greater than the band gap of the material, i.e. blue light (similar to 470 nm) at 10 K, exhibited peaks around 27 and 28 K, respectively, when measured by heating up the samples at a rate of 1 K/s. TL and TSC curves were analyzed to characterize the defects responsible for the peaks. Both TL and TSC peaks were observed to be obeying first order kinetics. Thermal activation energies of the peaks were determined using various methods: curve fitting, initial rise, peak shape and different heating rates. For both TL and TSC peaks, thermal activation energy was determined as around 8 meV, implying that they may originate from similar kinds of trapping centers. A distribution of traps (in terms of energy) was experimentally verified by illuminating the sample at different temperatures and measuring the TL curves. As a result of this, the apparent thermal energies were observed to be shifted from similar to 8 to similar to 17 meV by increasing the illumination temperature from 10 to 16 K. (C) 2013 Elsevier B.V. All rights reserved.Article Citation - WoS: 11Citation - Scopus: 11Structural and Temperature-Tuned Optical Characteristics of Bi12geo20< Sillenite Crystals(Elsevier, 2020) Delice, S.; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringSillenite compounds exhibit unique photorefractive and electro-optic characteristics providing attractiveness to these materials in various optoelectronic applications. The present paper aims at investigating one of the members of this family. Structural and optical characteristics of Bi12GeO20 (BGO) were studied by means of x-ray diffraction, Raman spectroscopy and temperature-dependent transmittance measurements. Obtained transmission curves in the wavelength range of 350-1100 nm and at different applied temperatures between 10 and 300 K were employed to find out the absorption coefficient dependence on the photon energy. Tauc relation revealed the presence of an energy gap of 2.49 eV at room temperature. Extension of energy gap up to 2.57 eV due to decreased temperature down to 10 K was deduced by the analysis. In order to have reliable results, the energy gap value was corroborated by utilizing derivative spectral method and well consistency between both methods was indicated. Energy gap change with temperature was also discussed in the study using an empirical formula developed by Varshni. Energy gap at absolute zero and rate of band gap alteration with temperature were determined as 2.57 eV and -2.4 x 10(-4) eV K (- 1), respectively. Taking into account the previously reported studies on investigation of band gap characteristics of BGO, intrinsic Bi-Ge(3+) + V-O(+) defect could be responsible for the revealed energy value of 2.49 eV which is much lower than reported band gap energy of similar to 3.2 eV.Article Citation - WoS: 7Citation - Scopus: 7Temperature-Tuned Bandgap Characteristics of Bi12tio20< Sillenite Single Crystals(Springer, 2021) Isik, M.; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringBi12MO20 (M: Si, Ge, Ti, etc.) compounds are known as sillenites having fascinating photorefractive characteristics. The present paper reports the structural and optical characteristics of one of the members of this family, Bi12TiO20 single crystals, grown by Czochralski method. X-ray diffraction pattern of the crystal presented sharp and intensive peaks associated with planes of cubic crystalline structure with lattice constant of a = 1.0142 nm. The optical properties were studied by means of room temperature Raman and temperature-dependent transmission experiments at various temperatures between 10 and 300 K. Raman spectrum indicated peaks around 127, 162, 191, 219, 261, 289, 321, 497 and 537 cm(-1). The analyses of transmittance spectra indicated the increase of direct bandgap energy from 2.30 to 2.56 eV as temperature was decreased from room temperature to 10 K. The temperature-dependent bandgap characteristics of Bi12TiO20 were analyzed by means of Varshni and O'Donnell-Chen models. The analyses under the light of these models resulted in absolute zero bandgap energy of E-g(0) = 2.56(4) eV, rate of change of bandgap energy of gamma = - 1.11 x 10(-3) eV/K and average phonon energy of < E-ph & rang; = 8.6 meV.Article Citation - WoS: 5Citation - Scopus: 5Investigation of Linear and Nonlinear Optical Properties of Pbwo4 Single Crystal(Elsevier, 2022) Delice, S.; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringIn this manuscript, PbWO4 single crystals having great importance for device applications were studied in terms of linear and nonlinear optical properties. For this purpose, transmission and reflection experiments were per-formed in the wavelength range of 350-1000 nm. X-ray diffraction analysis presented crystalline structure as scheelite type tetragonal. Spectral dependencies of absorption coefficient, skin depth, refractive index, dielectric function were reported. Optical band gap of the crystal was estimated as 3.25 eV from the Tauc and derivative spectral methods. Urbach, critical point, single oscillator and dispersion energies, static refractive index and dielectric constant were revealed for PbWO4 single crystal. First-and third-order nonlinear susceptibilities, and nonlinear refractive index were also computed for the studied crystal.Article Citation - WoS: 5Citation - Scopus: 5Thermoluminescence Properties and Trapping Parameters of Tlgas2 Single Crystals(Elsevier, 2022) Delice, S.; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringTlGaS2 layered single crystals have been an attractive research interest due to their convertible characteristics into 2D structure. In the present paper, structural, optical and thermoluminescence properties of TlGaS2 single crystals were investigated. XRD pattern of the crystal presented five well-defined peaks associated with monoclinic unit cell. Band gap and Urbach energies were found to be 2.57 and 0.25 eV, respectively, from the analyses of transmittance spectrum. Thermoluminescence measurements were carried out above room temperature up to 660 K at various heating rates. One TL peak with peak maximum temperature of 573 K was obtained in the TL spectrum at 1.0 K/s. Curve fitting, initial rise and variable heating rate methods were used for analyses. All of those resulted in presence of a deep trapping level with activation energy around 0.92 eV. Heating rate dependence of the TL peak was also studied and it was indicated that peak maximum temperature shifted to higher temperatures besides decreasing TL intensity as the higher heating rates were employed.Article Citation - WoS: 1Citation - Scopus: 1Optical Parameters of Anisotropic Chain-Structured Tl2ingate4< Crystals by Spectroscopic Ellipsometry(Elsevier Gmbh, 2016) Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringSpectroscopic ellipsometry measurements were performed on Tl2InGaTe4 single crystals in the 1.2-6.2 eV range for orientations of electric field, parallel (E//c) and perpendicular (E perpendicular to c) to optic axis c. Spectral dependence of optical parameters; real and imaginary components of the dielectric function, refractive index and extinction coefficient were obtained from the analyses of experimental data using an ambient-substrate optical model. The analysis of the absorption data calculated using the extinction coefficient showed the existence of indirect transitions in the crystal with an energy band gap of similar to 0.72 eV for both orientations. Interband transition (critical point) energies were revealed using second-energy derivative spectra of the dielectric function. The results showed the presence of five each interband transition structures for E//c and E perpendicular to c orientations. (C) 2016 Elsevier GmbH. All rights reserved.Article Citation - WoS: 18Citation - Scopus: 19Temperature-Tuned Band Gap Characteristics of Inse Layered Semiconductor Single Crystals(Elsevier Sci Ltd, 2020) Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringLayered structured InSe has attracted remarkable attention due to its effective characteristics utilized especially in optoelectronic device technology. This point directs researchers to investigate optical properties of InSe in great detail. The temperature dependent band gap characteristics of InSe and analyses performed on this dependency have been rarely studied in literature. Here, temperature-dependent transmission and room temperature reflection experiments were performed on InSe layered single crystals. The band gap energy was found around 1.22 eV at room temperature and 1.32 eV at 10 K. The temperature-gap energy dependency was analyzed using Varshni and O'Donnell-Chen models to reveal various optical parameters of the crystal. The structural characteristics; crystalline parameters like lattice constants, lattice strain, dislocation density and atomic compositions of InSe were also determined from the analyses of XRD and EDS measurements.Article Citation - WoS: 4Citation - Scopus: 4Study on Thermoluminescence of Tlins2 Layered Crystals Doped With Pr(Elsevier Sci Ltd, 2018) Delice, S.; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringPraseodymium (Pr) doped TlInS2 crystals were studied by means of thermoluminescence (TL) measurements performed below room temperature with various heating rates. Detected TL signal exhibited glow curve consisting in overlapping two TL peaks at temperatures of 35 K (peak A) and 48 K (peak B) for 0.6 K/s heating rate. TL curve was analyzed with curve fitting and initial rise methods. Both of the applied methods resulted in consistent activation energies of 19 and 45 meV. The revealed trap levels were found to be dominated by mixed order of kinetics. Various heating rate dependencies of TL glow curves were also investigated and it was found that while peak A shows usual behavior, peak B exhibits anomalous heating rate behavior. Distribution of trap levels was explored using an experimental method called as T-max-T-stop method. Quasi-continuous distributions with increasing activation energies from 19 to 29 meV (peak A) and from 45 to 53 meV (peak B) were ascribed to trap levels. Effect of Pr doping on the TL response of undoped TlInS2 crystals was discussed in the paper.Article Citation - WoS: 14Citation - Scopus: 14Temperature-Dependent Optical Characteristics of Sputtered Ga-Doped Zno Thin Films(Elsevier, 2021) Gullu, H. H.; Isik, M.; Gasanly, N. M.; Parlak, M.; Department of Electrical & Electronics EngineeringThe present paper reports structural and optical properties of gallium (Ga) doped ZnO thin films (GZO) grown by magnetron sputtering technique. The crystalline properties were determined from X-ray diffraction measurements and analyses pointed out the crystalline structure as hexagonal, crystalline size as 43 nm and strain as 6.9 x 10(-5). Derivative spectroscopy analyses showed that band gap energy of GZO thin films decreases from 3.50 eV (10 K) to 3.45 eV (300 K). Temperature-band gap energy dependency was analyzed using Varshni and O'DonnellChen models. The absolute zero band gap energy, the rate of change of band gap energy with temperature and phonon energy were found as 3.50 eV, -2.8 x 10(-4) eV/K and 15 meV, respectively. The room temperature band gap and Urbach energies were also determined as 3.43 eV and 102 meV, respectively, from the absorption analysis.
