Browsing by Author "Isik, M."
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Article Citation Count: 5Absorption edge and optical constants of Tl2Ga2S3Se crystals from reflection and transmission, and ellipsometric measurements(Elsevier, 2012) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThe optical properties of Tl2Ga2S3Se layered crystalline semiconductors were investigated from transmission, reflection and ellipsometric measurements. The experimental results of the room temperature transmission and reflection measurements performed in the wavelength range of 400-1100 nm showed the presence of both indirect and direct transitions in the band structure of the crystals with 2.38 and 2.62 eV band gap energies. Spectroscopic ellipsometry measurements on Tl2Ga2S3Se crystals were carried out on the layer-plane (0 0 1) surfaces with light polarization E perpendicular to c* in the 1.20-4.70 eV spectral range at room temperature. The real and imaginary parts of the dielectric function as well as refractive and absorption indices were found as a result of analysis of ellipsometric data. The Wemple-DiDomenico single-effective-oscillator model was used to study the dispersion of the refractive index in the below band gap energy range. The structures of critical points have been characterized from the second derivative spectra of the dielectric function. The analysis revealed four interband transition structures with 3.14, 3.40, 3.86 and 4.50 eV critical point energies. (C) 2012 Elsevier B.V. All rights reserved.Article Citation Count: 2Analysis of temperature-dependent transmittance spectra of Zn0.5In0.5Se (ZIS) thin films(Springer, 2019) Işık, Mehmet; Gullu, H. H.; Güllü, Hasan Hüseyin; Gasanly, N. M.; Parlak, M.; Department of Electrical & Electronics EngineeringTemperature-dependent transmission experiments of ZnInSe thin films deposited by thermal evaporation method were performed in the spectral range of 550-950nm and in temperature range of 10-300K. Transmission spectra shifted towards higher wavelengths (lower energies) with increasing temperature. Transmission data were analyzed using Tauc relation and derivative spectroscopy. Analysis with Tauc relation was resulted in three different energy levels for the room temperature band gap values of material as 1.594, 1.735 and 1.830eV. The spectrum of first wavelength derivative of transmittance exhibited two maxima positions at 1.632 and 1.814eV and one minima around 1.741eV. The determined energies from both methods were in good agreement with each other. The presence of three band gap energy levels were associated to valence band splitting due to crystal-field and spin-orbit splitting. Temperature dependence of the band gap energies were also analyzed using Varshni relation and gap energy value at absolute zero and the rate of change of gap energy with temperature were determined.Article Citation Count: 0Analysis of Thermoluminescence Glow Peaks in β-Irradiated TlGaSeS Crystals(Polish Acad Sciences inst Physics, 2016) Işık, Mehmet; Yildirim, T.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThermoluminescence properties of TlGaSeS layered single crystals were investigated in the temperature range of 280-720 K. Thermoluminescence glow curve exhibited three peaks with maximum temperatures of approximate to 370, 437, and 490 K. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of the trapping centers. All applied methods resulted with energies around 0.82, 0.91, and 0.99 eV. Dose dependence of the thermoluminescence intensity was also examined for the doses in the range of 0.7-457.6 Gy. Peak maximum intensity of the observed peak around 370 K showed an increase up to a certain dose and then a decrease at higher doses. This non-monotonic dose dependence was discussed under the light of a reported model in which different kinds of competition between radiative and nonradiative recombination centers during excitation or heating stages of the thermoluminescence process are explained.Article Citation Count: 0Annealing Effect on Dark Electrical Conductivity and Photoconductivity of Ga-In-Se Thin Films(Polish Acad Sciences inst Physics, 2018) Işık, Mehmet; Gullu, H. H.; Güllü, Hasan Hüseyin; Department of Electrical & Electronics EngineeringDark-conductivity and photoconductivity properties of thermally evaporated Ga-In-Se (GIS) thin films were investigated in the temperature range of 80-430 K. All measurements were performed on as-grown and annealed GIS thin films at 300 and 400 degrees C to get information about the effect of the annealing temperature on the conductivity properties. Room temperature conductivity was obtained as 1.8 x 10(-8) Omega(-1) cm(-1) for as-grown films and increased to 3.6 x 10(-4) Omega(-1) cm(-1) for annealed films at 400 degrees C. Analysis of the dark-conductivity data of as-grown films revealed nearly intrinsic type of conductivity with 1.70 eV band gap energy. Temperature dependent dark conductivity curves exhibited two regions in the 260-360 and 370-430 K for both of annealed GIS films. Conductivity activation energies were found as 0.05, 0.16 and 0.05, 0.56 eV for films annealed at temperatures of 300 and 400 degrees C, respectively. The dependence of photoconductivity on illumination intensity was also studied in the range from 17 to 113 mW/cm(2).Article Citation Count: 5Annealing effect on the low temperature thermoluminescence properties of GaSe single crystals(Elsevier Science Bv, 2014) Işık, Mehmet; Hadibrata, W.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringTrapping centers in as-grown GaSe single crystals have been investigated by thermoluminescence (TL) measurements in the temperature range of 30-300 K. The analysis of the observed peaks in TL glow curve to determine the activation energies of the associated centers were accomplished using curve fitting, initial rise and peak shape methods. Activation energies of the revealed four trapping centers obtained from various methods were in good agreement with each other on the energy values of 0.14, 0.18, 0.24 and 037 eV. The annealing effect on the TL properties of the GaSe single crystals was also studied for the annealing temperature of 500 degrees C. It was observed that annealing significantly decreased the U intensity and shifted the observed peaks to lower temperature resulting with smaller activation energy values. The distribution of the trapping centers with most intensive peak was also studied on both as-grown and annealed crystals. (C) 2014 Elsevier B.V. All rights reserved.Article Citation Count: 3Characteristic features of thermoluminescence in neodymium-doped gallium sulfide(Wiley, 2018) Işık, Mehmet; Isik, M.; Ahmedova, F.; Guseinov, A.; Gasanly, N.; Department of Electrical & Electronics EngineeringThe thermoluminescence (TL) of neodymium-doped gallium sulfide (GaS:Nd) single crystals was measured from 10 K to room temperature with various heating rates between 0.2 and 1.0 K/sec. Two peaks centered at 70.9 K and 116.0 K were observed when using a heating rate of 0.8 K/sec. Initial rise and curve fitting methods were used to obtain information on trap activation energies. Activation energies of 94 and 216 meV were found for two analyzable peaks. The heating rate dependencies of TL intensities revealed that one of the observed peaks showed normal behavior according to the one trap-one recombination model, whereas the other model showed anomalous heating rate behavior. TL experiments were also carried out at different illumination temperatures from 10 to 32 K; maximum peak temperature remained almost the same at various illumination temperatures. This behavior indicated that the revealed trapping centers are single, discrete levels. The TL glow curves of undoped GaS crystals were also investigated and the effect of Nd doping on the TL characteristics of crystals is discussed in the manuscript.Article Citation Count: 0Characterization of linear and nonlinear optical properties of NaBi(WO4)2 crystal by spectroscopic ellipsometry(Elsevier, 2024) Işık, Mehmet; Guler, I.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringNaBi(WO4)2 compound has been a material of considerable attention in optoelectronic applications. The present research, in which we examined the linear and nonlinear optical properties of NaBi(WO4)2 crystal using the spectroscopic ellipsometry method, elucidates the optical behavior of the crystal in detail. Our work provides a sensitive approach to determine the spectral characteristic of the crystal. The spectral dependence of various optical parameters such as refractive index, extinction coefficient, dielectric function and absorption coefficient was reported in the range of 1.2-5.0 eV. Optical values such as bandgap energy, critical point energy, single oscillator parameters were obtained as a result of the analyses. In addition to linear optical properties, we also investigated the nonlinear optical behavior of NaBi(WO4)2 and shed new light on the potential applications of the crystal. Absorbance and photoluminescence spectra of the crystal were also reported to characterize optical, electronic and emission behavior of the compound. Our findings may form the basis for a number of technological applications such as optoelectronic devices, frequency conversion, and optical sensors. This research contributes to a better understanding of the optical properties of NaBi(WO4)2 crystal, highlighting the material's role in future optical and electronic technologies.Article Citation Count: 8Characterization of trap centers in Gd2O3 nanoparticles by low temperature thermoluminescence measurements(Elsevier Gmbh, 2018) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringTrapping centers in Gd2O3 nanoparticles were investigated using thermoluminescence (TL) measurements in the below room temperature region of 10-280 K. Seven peaks having peak maximum temperatures between 30 and 252 K were observed in the TL spectra measured at constant heating rate of 0.3 K/s. Activation energies, order of kinetics and frequency factors were reported using three different analysis techniques: curve fitting, initial rise and peak shape methods. Activation energies of the trapping centers were found between 0.012 eV and 0.79 eV. Most of the TL transitions associated with observed peaks were found as dominated by mixed order of kinetics. Structural characterization of used nanoparticles was achieved using x-ray diffraction and scanning electron microscopy experiments. (C) 2017 Elsevier GmbH. All rights reserved.Article Citation Count: 0Compositional Dependence of Optical Modes Frequencies in T1GaxIn1-xS2 Layered Mixed Crystals (0 ≤ x ≤ 1)(Polish Acad Sciences inst Physics, 2014) Işık, Mehmet; Gasanly, N. M.; Korkmaz, F.; Department of Electrical & Electronics EngineeringThe infrared transmittance and Raman scattering spectra in TlGaxIn1-xS2 (0 <= x <= 1) layered mixed crystals grown by the Bridgman method were studied in the frequency ranges of 400-2000 and 250-400 cm(-1), respectively. The bands observed at room temperature in IR transmittance spectra of TlGaxIn1-xS2 were interpreted in terms of multiphonon absorption processes. The dependences of the frequencies of IR- and Raman-active modes on the composition of TlGaxIn1-xS2 mixed crystals were also established. The structural characterization of the mixed crystals was investigated by means of X-ray diffraction measurements and compositional dependence of lattice parameters was revealed.Article Citation Count: 15Deep Traps Distribution in TlInS2 Layered Crystals(Polish Acad Sciences inst Physics, 2009) Işık, Mehmet; Gasanly, N. M.; Ozkan, H.; Department of Electrical & Electronics EngineeringThe trap centers and distributions in TlInS2 were studied in the temperature range of 100-300 K by using thermally stimulated currents technique. Experimental evidence was found for the presence of three trapping centers with activation energies 400, 570, and 650 meV. Their capture cross-sections were determined as 6.3 x 10(-16), 2.7 x 10(-12), and 1.8 x 10(-11) cm(2), respectively. It was concluded that in these centers retrapping is negligible as confirmed by the good agreement between the experimental results and the theoretical predictions of the model that assumes slow retrapping. An exponential distribution of hole traps was revealed from the analysis of the thermally stimulated current data obtained at different light excitation temperatures. This experimental technique provided a value of 800 meV/decade for the trap distribution.Article Citation Count: 7Defect characterization in Bi12GeO20 single crystals by thermoluminescence(Elsevier, 2021) Işık, Mehmet; Isik, M.; Sarigul, N.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringBi12GeO20 single crystal grown by Czochralski method was investigated in terms of thermoluminescence (TL) properties. TL experiments were performed for various heating rates between 1 and 6 K/s in the temperature region of 300-675 K. One TL peak with peak maximum temperature of 557 K was observed in the TL spectrum as constant heating rate of 1 K/s was employed. Curve fitting, initial rise and variable heating rate methods were applied to calculate the activation energy of trap level corresponding to this TL peak. Analyses resulted in a presence of one trap center having mean activation energy of 0.78 eV. Heating rate characteristics of revealed trap center was also explored and theoretically well-known behavior that TL intensity decreases and peak maximum temperature increases with heating rates was observed for the trap level. Distribution of trapping levels was studied by thermally cleaning process for different T-stop between 425 and 525 K. Quasi-continuously distributed trapping levels were revealed with mean activation energies ranging from 0.78 to 1.26 eV. Moreover, absorption analysis revealed an optical transition taking place between a defect level and conduction band with an energy difference of 2.51 eV. These results are in good agreement for the presence of intrinsic defects above valence band in Bi12GeO20 crystals.Article Citation Count: 0Defect characterization of Ga4Se3S layered single crystals by thermoluminescence(indian Acad Sciences, 2016) Işık, Mehmet; Delice, S.; Gasanly, N.; Department of Electrical & Electronics EngineeringTrapping centres in undoped Ga4Se3S single crystals grown by Bridgman method were characterized for the first time by thermoluminescence (TL) measurements carried out in the low-temperature range of 15-300 K. After illuminating the sample with blue light (similar to 470 nm) at 15 K, TL glow curve exhibited one peak around 74 K when measured with a heating rate of 0.4 K/s. The results of the various analysis methods were in good agreement about the presence of one trapping centre with an activation energy of 27 meV. Analysis of curve fitting method indicated that mixed order of kinetics dominates the trapping process. Heating rate dependence and distribution of the traps associated with the observed TL peak were also studied. The shift of peak maximum temperature from 74 to 113 K with increasing rate from 0.4 to 1.2 K/s was revealed. Distribution of traps was investigated using an experimental technique based on cleaning the centres giving emission at lower temperatures. Activation energies of the levels were observed to be increasing from 27 to 40 meV by rising the stopping temperature from 15 to 36 K.Article Citation Count: 7The defect state of Yb-doped ZnO nanoparticles using thermoluminescence study(Elsevier Sci Ltd, 2019) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics EngineeringShallow trapping centers in Yb-doped ZnO nanoparticles were determined using thermoluminescence (TL) measurements applied in the 10-300 K temperature region. Undoped and Yb-doped ZnO nanoparticles were synthesized by sol-gel method. TL glow curve of undoped nano-particles presented three peaks around 56, 108 and 150 K whereas one additional peak around 83 K was observed in the TL curve of Yb-doped ZnO nano-particles. The increase of Yb concentration in the nanoparticles increased the TL intensity of this additional peak. Activation energies of interstitial defect centers were found as 20, 82 and 105 meV while energy of trapping center existing due to Yb-doping was obtained as 72 meV using curve fitting and initial rise methods.Article Citation Count: 6Determination of mechanical properties of Bi12TiO20 crystals by nanoindentation(Elsevier Sci Ltd, 2022) Işık, Mehmet; Gasanly, N. M.; Rustamov, F. A.; Department of Electrical & Electronics EngineeringBi12TiO20 (BTO) single crystal was grown by Czochralski method and investigated mechanically by nano-indentation measurements. X-ray diffraction pattern of the crystal presented one intensive peak around 37.95 degrees associated with (330) plane of cubic crystalline structure. Nanoindentation experiments were performed at various loads between 5 and 100 mN. Hardness and Young's modulus of the crystal were determined by Oliver-Pharr method. The hardness-load dependency exhibited behavior of indentation size effect. True hardness value of BTO crystal was revealed as 4.4 GPa. Young's modulus decreased with increase of load and load-independent Young's modulus was found around 93 GPa at high loads. The load-dependent elastic and plastic deformation components were calculated and it was observed that the dominant component in BTO single crystal is plastic deformation at the applied loads. The present paper reports for the first time the mechanical characteristics of the BTO single crystal by carrying out nanoindentation experiments.Article Citation Count: 8Determination of optical parameters of Ga0.75In0.25Se layered crystals(Wiley-v C H verlag Gmbh, 2012) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThe optical properties of the Ga0.75In0.25Se crystals have been investigated by means of transmission and reflection measurements in the wavelength range of 380-1100 nm. The analysis of the results performed at room temperature revealed the presence of optical indirect transtions with band gap energy of 1.89 eV. The variation of the band gap energy as a function of temperature was also studied in the temperature range of 10-300 K. The rate of change of band gap energy (? = 6.2 x 10(4) eV/K) and absolute zero value of the band gap (Egi(0) = 2.01 eV) were reported. The wavelength dependence of the refractive index was analyzed using Wemple and DiDomenico, Sellmeier and Cauchy models to find the oscillator energy, dispersion energy, oscillator strength and zero-frequency refractive index values. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)Article Citation Count: 5Determination of trapping parameters of thermoluminescent glow peaks of semiconducting Tl2Ga2S3Se crystals(Pergamon-elsevier Science Ltd, 2015) Işık, Mehmet; Yildirim, T.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThermoluminescence (TL) properties of Tl2Ga2S3Se layered single crystals were researched in the temperature range of 290-770 K. U glow curve exhibited two peaks with maximum temperatures of similar to 373 and 478 K. Curve fitting, initial rise and peak shape methods were used to determine the activation energies of the trapping centers associated with these peaks. Applied methods were in good agreement with the energies of 780 and 950 meV. Capture cross sections and attempt-to-escape frequencies of the trapping centers were reported. An energy level diagram showing transitions in the band gap of the crystal was plotted under the light of the results of the present work and previously reported papers on photoluminescence, thermoluminescence and thermally stimulated current measurements carried out below room temperature. (C) 2015 Elsevier Ltd. All rights reserved.Article Citation Count: 4Dielectric functions and interband critical points of anisotropic chain structured TlSe single crystals(Amer inst Physics, 2012) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics EngineeringSpectroscopic ellipsometry measurements were carried out on TlSe single crystals for orientations of electric field, parallel (E parallel to c), and perpendicular (E perpendicular to c) to optic axis c. The experiments were performed in the 1.2-6.2 eV spectral range at room temperature. The real and imaginary parts of the pseudodielectric function as well as pseudorefractive index and pseudoextinction coefficient were calculated from the analysis of ellipsometric data under the light of ambient-substrate optical model. The energies of interband transitions (critical points) have been found from the analysis on second derivative spectra of the pseudodielectric function. The analysis revealed four and five interband transition structures for E parallel to c and E perpendicular to c configurations, respectively. The obtained critical point energies were assigned tentatively to interband transitions using the available electronic energy band structure given in literature. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4761963]Article Citation Count: 9Effect of heating rate on thermoluminescence characteristics of Y2O3 nanoparticles(Elsevier, 2019) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThe present paper reports the results of heating rate dependencies of thermoluminescence (TL) peaks observed for Y2O3 nanoparticles in the below room temperature region. TL glow curve presented six peaks around 62.5, 91.3, 114.5, 162.7, 196.0 and 214.9 K for heating rate of 0.4 K/s. The increase of heating rate resulted in increase in peak maximum temperature and decrease in peak maximum intensity as expected according to theoretical information. Peak maximum temperature-heating rate dependencies of observed peaks were analysed according to exponential dependency relation. Curve fit and initial rise methods were applied on thermally cleaned individual peaks and activation energies of associated trap centers, frequency factors and order of kinetics were obtained from the analyses. Activation energy values of the revealed trapping centers found from both methods were in good agreement with each other. Moreover, lattice parameters, crystalline size and micro-strain of nanoparticles were investigated by means of x-ray diffraction measurements.Article Citation Count: 10Effect of temperature on band gap of PbWO4 single crystals grown by Czochralski method(Iop Publishing Ltd, 2022) Işık, Mehmet; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringThe structural and optical properties of PbWO4 single crystals grown by Czochralski method and investigated by x-ray diffraction (XRD) and transmission experiments. XRD pattern presented well-defined and intensive peaks related with tetragonal scheelite structure. Transmission experiments were accomplished for the first time at various temperatures between 10 and 300 K on PbWO4 single crystals to reveal variation of band gap with temperature. Derivative spectroscopy method presented the experimentally observed band gap energy as increasing from 3.20 to 3.35 eV when the temperature was decreased to 10 K from room temperature. The revealed energy was associated with transition taking place between delocalized and trap levels. Temperature-band gap energy plot was analyzed by Varshni and Bose-Einstein models. The fitting processes under the light of these models revealed optical characteristics of absolute zero experimentally observed band gap, variation rate of gap energy with temperature and Debye temperature of PbWO4 single crystal.Article Citation Count: 5Effect of thallium (Tl) substitution for indium (In) on ellipsometric characteristics of TlInSe2 single crystals(Elsevier Sci Ltd, 2021) Işık, Mehmet; Gasanly, N. M.; Department of Electrical & Electronics EngineeringTlMeSe2 (Me: Tl, In) semiconducting compounds exhibiting chain structure have been attractive ternary materials in various technological devices. In the TlMeSe2 structure, Tl1+ is monovalent while Me3+ is trivalent ions. The present paper reports the results of spectroscopic ellipsometry measurements performed on Tl1+(Tl0.2In0.8)3+Se2 (abbreviated as Tl1.2In0.8Se2) single crystals which were grown by substituting thallium for indium. The measurements were performed for orientations of E//c and Etc (E: electric field and c: optical axis). The analyses of ellipsometry data considering air-sample optical model presented the spectral dependencies of dielectric function, refractive index and extinction coefficient in the 1.2-5.0 eV range. Critical point energies of studied single crystal were obtained by fitting second-energy derivative spectra of dielectric function. The determined energies were compared with those of TlInSe2 to understand the effect of thalliumindium substitution in the compound. The crystal structure and atomic compositions of the constituent elements were also reported throughout the paper.