Browsing by Author "Delice, S."
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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: 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: 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: 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: 1Growth and temperature-tuned band gap characteristics of LiGd(MoO4)2 single crystals for optoelectronic applications(Elsevier Sci Ltd, 2023) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringLiGd(MoO4)2 has been investigated due to its optoelectronic applications, especially for development of lightemitting diodes. In the present paper, LiGd(MoO4)2 single crystals grown by Czochralski method was studied in terms of structural and temperature dependent optical properties. X-ray diffraction analysis showed that the crystal crystallizes in a single phase tetragonal structure. Raman spectrum exhibited six distinguishable peaks around 207, 319, 397, 706, 756 and 890 cm-1. These peaks correspond to vibrational modes of free rotation, symmetrical stretching, symmetric bending, antisymmetric stretching and antisymmetric bending of (MoO4)2tetrahedron. Infrared transmittance spectrum had eight minima around 2114, 2350, 2451, 2854, 2929, 2960, 3545 and 3578 cm-1 which are due to multiphonon absorptions. Spectral change of transmittance curves at various temperature between 10 and 300 K was utilized to elucidate temperature effect on absorption characteristics. Optical band gap of the material was found using Tauc and spectral derivative methods. The band gap value was obtained as 3.09 eV at room temperature and this value increased to 3.22 eV with decreasing temperature down to 10 K. The detailed analysis on the temperature dependency of the band gap was applied by Varshni model. The band gap at 0 K and change of rate of the band gap were estimated as 3.23 eV and -1.45 x 10-3 eV/K, respectively. Room temperature photoluminescence spectrum of the crystal presented a peak around 709 nm which corresponds to red light emission. LiGd(MoO4)2 is a potential candidate for optoelectronic devices emitting red light.Article Citation Count: 0Investigation of defect levels in Bi12SiO20 single crystals by thermally stimulated current measurements(Iop Publishing Ltd, 2021) Işık, Mehmet; Delice, S.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringBi12SiO20 (BSO) single crystal belongs to the sillenite semiconducting family known as defective compounds. The present paper investigates the defect centers in BSO grown by Czochralski method by means of thermally stimulated current (TSC) measurements performed in the 10-260 K range. The TSC glow curve obtained at heating rate of beta = 0.1 K s(-1) presented several peaks associated with intrinsic defect centers. The activation energies of defect centers were revealed as 0.09, 0.15, 0.18, 0.22, 0.34, 0.70 and 0.82 eV accomplishing the curve fit analyses method. The peak maximum temperatures and orders of kinetics of each deconvoluted peak were also determined as an outcome of fitting process. TSC experiments were expanded by making the measurements at various heating rates between 0.1 and 0.3 K s(-1) to get information about the heating rate dependent peak parameters.Article Citation Count: 3Investigation of linear and nonlinear optical properties of PbWO4 single crystal(Elsevier, 2022) Işık, Mehmet; 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 Count: 3Investigation of optical characteristics of PbMoO4 single crystals by spectroscopic ellipsometry(Elsevier Gmbh, 2022) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringIn this study, we investigated the optical properties of PbMoO4 single crystals grown by Czochralski method. Spectroscopic ellipsometry measurements were carried out in the energy region between 1.0 and 5.5 eV at room temperature. X-ray diffraction measurements were achieved for structural characterization. The resulted pattern exhibited one peak belonging to (200) plane. Spectral variations of complex dielectric function, complex refractive index, absorption coefficient and dissipation function were obtained from the analyses of ellipsometry data. Real part of dielectric function increased up to 4.0 eV and then decreased suddenly at above this value. Zero frequency refractive index and dielectric constant were found to be 2.04 and 4.15, respectively. High frequency dielectric constant was determined to be 4.36. Optical band gap of PbMoO4 crystals was calculated as 3.09 eV. Two critical points with energies of 3.57 and 4.34 eV were estimated from the analyses of second-energy derivative spectra of real and imaginary parts of dielectric function. It was determined that [MoO4]2- complexes and charge transfer from Pb2+ ions into the neighboring Mo groups were responsible for these interband transitions. Dissipation function increased with increasing photon energy.Article Citation Count: 17Investigation of optical properties of Bi12GeO20 sillenite crystals by spectroscopic ellipsometry and Raman spectroscopy(Elsevier Sci Ltd, 2020) Işık, Mehmet; Delice, S.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringBi12GeO20 (BGO) compound is one of the fascinating members of sillenites group due to its outstanding photorefractive and photocatalytic characteristics. The present paper aims at investigating optical properties of BGO crystals by means of spectroscopic ellipsometry and Raman spectroscopy measurements. Bi12GeO20 single crystals grown by Czochralski method were structurally characterized by X-ray diffraction (XRD) experiments and the analyses showed that studied crystals have cubic crystalline structure. Raman spectrum exhibited 15 peaks associated with A, E and F modes. Spectroscopic ellipsometry measurement data achieved in the energy region between 1.2 and 6.2 eV were used in the air/sample optical model to get knowledge about complex pseudodielectric constant, pseudorefractive index, pseudoextinction and absorption coefficients of the crystals. Spectral change of real and imaginary part of complex pseudodielectric constant were discussed in detail. Band gap energy of Bi12GeO20 single crystals was calculated to be 3.18 eV using absorption coefficient dependency on photon energy. Critical point energies at which photons are strongly absorbed were determined by utilizing the second energy derivative spectra of components of complex pseudodielectric function. Fitting of both spectra resulted in the presence of four interband transitions with energies of 3.49, 4.11, 4.67 and 5.51 eV.Article Citation Count: 4Investigation of traps distribution in GaS single crystals by thermally stimulated current measurements(Elsevier Sci Ltd, 2021) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThermally stimulated current (TSC) investigations of p-GaS (gallium sulfide) single crystals grown by Bridgman method were achieved by virtue of consecutive experiments carried out at various heating rates in between 0.4 and 1.0 K/s in the temperature range of 10-280 K. One single TSC peak around 148 K and overlapped, incomplete peaks in the end limit temperature of the experiments were observed in the spectrum recorded at constant heating rate of 1.0 K/s. Individual peak was analyzed utilizing curve fitting method. Existence of one trapping level centered at 0.11 eV was revealed by the analyses. Heating rate dependency of obtained TSC curve was also studied and it was shown that TSC intensity decreased besides increase of peak maximum temperature with heating rate. Characteristics feature of trapping mechanism was investigated in detail by employing different stopping temperature between 50 and 110 K. Analyses on T-m-T-stop dependency resulted in a presence of quasi-continuously distributed traps with activation energies ranging from 0.11 to 0.55 eV. The revealed trap was thought to be arising from intrinsic defect possibly created by V-Ga or antisite S-Ga.Article Citation Count: 5Linear and nonlinear optical characteristics of PbMoO4 single crystal for optoelectronic applications(Springer, 2022) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringIn the present paper, we studied linear and nonlinear optical properties of lead molybdate (PbMoO4) single crystals grown by Czochralski method. Raman measurement was performed in the range of 50-1000 cm(-1) and 11 active vibration modes were defined in the spectrum. The nature of the observed modes was discussed in detail. Optical absorption of the material was investigated utilizing room temperature transmission and reflection experiments in the spectral range of 360-1000 nm. Spectral changes of absorption coefficient, skin depth, refractive index, optical conductivity, and complex dielectric function of PbMoO4 crystals were studied. Absorption coefficient and optical conductivity exhibited strong increment around 3.0 eV with increasing photon energy. Cut-off wavelength was determined to be 390 nm from the skin depth spectrum. Tauc and derivative spectral analyses revealed the presence of direct bandgap with the energy of 3.05 eV. The static refractive index and dielectric constant were estimated as 2.25 and 5.08, respectively, using Wemple-DiDomenico single oscillator model. Urbach energy was calculated to be 0.071 eV from the exponential dependence of absorption coefficient to photon energy. Dielectric constant increased to 5.42 with energy up to 2.60 eV and then it exhibited decreasing behavior. Second energy derivative analysis of imaginary dielectric function resulted in existence of a critical point at 3.19 eV. First- and third-order nonlinear susceptibilities and nonlinear refractive index were found to be 0.32, 1.9 x 10(-12) and 31.7 x 10(-12) esu, respectively. These results show that the material may be used in UV filter and sensor applications.Article Citation Count: 14Low temperature thermoluminescence behaviour of Y2O3 nanoparticles(Elsevier, 2019) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringY2O3 nanoparticles were investigated using low temperature thermoluminescence (TL) experiments. TL glow curve recorded at constant heating rate of 0.4 K/s exhibits seven peaks around 19, 62, 91, 115, 162, 196 and 215 K. Activation energies and characteristics of traps responsible for observed curves were revealed under the light of results of initial rise analyses and T-max-T-stop experimental methods. Analyses of TL curves obtained at different stopping temperatures resulted in presence of one quasi-continuously distributed trap with activation energies increasing from 18 to 24 meV and six single trapping centers at 49, 117, 315, 409, 651 and 740 meV. Activation energies of all revealed centers were reported in the present paper. Structural characterization of Y2O3 nanoparticles was accomplished using X-ray diffraction and scanning electron microscopy measurements. (C) 2019 Chinese Society of Rare Earths. Published by Elsevier B.V. All rights reserved.Article Citation Count: 0Low temperature thermoluminescence of quaternary thallium sulfide Tl4InGa3S8(indian Assoc Cultivation Science, 2015) Işık, Mehmet; Isik, M.; Bulur, E.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThermoluminescence measurements have been carried out on Tl4InGa3S8 single crystals in the temperature range of 10-300 K at various heating rates. The observed thermoluminescence spectra have been analyzed applying many methods like curve fitting, initial rise, peak shape and heating rate methods. Thermal cleaning method has been performed on the observed thermoluminescence glow curve to separate the overlapped peaks. Three distinctive trapping centers with activation energies of 13, 44 and 208 meV have been revealed from the results of the analysis. Heating rate dependence and traps distribution investigations have been also undertaken on the most intensive peak. The thermoluminescence mechanisms in the observed traps have been attributed to first order kinetics (slow retrapping) on the strength of the consistency between theoretical assumptions for slow retrapping process and experimental outcomes.Article Citation Count: 6Low-Temperature Thermo luminescence Studies on TlInS2 Layered Single Crystals(Polish Acad Sciences inst Physics, 2014) Işık, Mehmet; Delice, S.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThermoluminescence characteristics of TlInS2 layered single crystals grown by the Bridgman method were investigated in the low temperature range of 10-300 K. The illuminated sample with blue light (approximate to 470 nm) at 10 K was heated at constant heating rate. Curve fitting, initial rise and various heating rate methods were used to determine the activation energy of the trap levels. All applied methods showed good consistency about the presence of five trapping centers located at 14, 19, 350, 420, and 520 meV. Behavior of the TL curve for various heating rates was investigated. Traps distribution has also been studied. The activation energies of the distributed trapping centers were found to be increasing from 14 to 46 meV.Article Citation Count: 0Material and device properties of Si-based Cu0.5Ag0.5InSe2 thin-film heterojunction diode(Springer, 2020) Işık, Mehmet; Isik, M.; Güllü, Hasan Hüseyin; Parlak, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringCu0.5Ag0.5InSe2 (CAIS) thin films were deposited on a glass substrate by sequential sputtering of Cu, Ag, and In2Se3-stacked film layers. Structural characterization showed that the deposited CAIS film satisfies nearly the stoichiometric form with uniform and homogeneous surface structure. The single-phase polycrystalline behavior without any secondary-phase formation was observed from the diffraction profile. The optical properties were investigated using temperature-dependent transmission measurements in the wavelength region of 600-1100 nm and in between 10 and 300 K. In the region of interest, the transmission spectra shifted towards the higher wavelengths as a result of an increase in the sample temperature. The analysis of the absorption data based on the transmission spectra resulted in absorption coefficient values of around 10(5) cm(-1) and the presence of direct allowed optical transition. From the Tauc plots, CAIS samples were found to have three distinct direct optical transitions depending on the possible splitting in the valence band. The obtained room temperature uppermost band gap energy value of 1.09 eV was found in the energy limit of ternary analogues (CuInSe2 and AgInSe2), and also in a good agreement with the previous works in the literature. The dependency of the band gap energy on the temperature was analyzed using fundamental relations. In addition, the electrical characteristics of the film layer were discussed in four-contact conductivity measurements, and room temperature conductivity was observed as 0.8 ohm(-1) cm(-1). Additionally, two activation energy values were found in the temperature-dependent conductivity profile. As a diode application, CAIS/Si heterojunction was fabricated and the main diode parameters were extracted at dark and room temperature conditions.Article Citation Count: 16Optical characteristics of Bi12SiO20 single crystals by spectroscopic ellipsometry(Elsevier Sci Ltd, 2020) Işık, Mehmet; Delice, S.; Nasser, H.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringStructural and optical characteristics of Bi12SiO20 single crystal grown by the Czochralski method were investigated by virtue of X-ray diffraction (XRD) and spectroscopic ellipsometry measurements. XRD analysis indicated that the studied crystal possesses cubic structure with lattice parameters of a = 1.0107 nm. Spectral dependencies of several optical parameters like complex dielectric constant, refractive index, extinction and absorption coefficients were determined using ellipsometry experiments performed in the energy region of 1.2-6.2 eV. The energy band gap of Bi12SiO20 crystals was found to be 3.25 eV by utilizing absorption coefficient analysis. Moreover, critical point energies were calculated as 3.54, 4.02, 4.82 and 5.58 eV from analyses of the second energy derivative spectra of the complex dielectric constant.Article Citation Count: 5Optical properties of TlGaxIn1-xSe2-layered mixed crystals (0.5 ≤ x ≤ 1) by spectroscopic ellipsometry, transmission, and reflection(Taylor & Francis Ltd, 2014) Işık, Mehmet; Delice, S.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringThe layered semiconducting TlGaxIn1-xSe2-mixed crystals (0.5 <= x <= 1) were studied for the first time by spectroscopic ellipsometry measurements in the 1.2-6.2 eV spectral range at room temperature. The spectral dependence of the components of the complex dielectric function, refractive index, and extinction coefficient were revealed using an optical model. The interband transition energies in the studied samples were found from the analysis of the second-energy derivative spectra of the complex dielectric function. The effect of the isomorphic cation substitution (indium for gallium) on critical point energies in TlGaxIn1-xSe2 crystals was established. Moreover, the absorption edge of TlGaxIn1-xSe2 crystals have been studied through the transmission and reflection measurements in the wavelength range of 500-1100 nm. The analysis of absorption data revealed the presence of both optical indirect and direct transitions. It was found that the energy band gaps decrease with the increase of indium content in the studied crystals.Article Citation Count: 1Shallow trapping centers in Bi12GeO20 single crystals by thermally stimulated current measurements(Elsevier, 2022) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics EngineeringBi12GeO20 single crystals were investigated by thermally stimulated current (TSC) experiments performed in the temperature range of 10-290 K. Recorded TSC glow curve exhibited six distinctive peaks with maxima at around 90, 105, 166, 209, 246, 275 K. The analyses of the obtained glow curve were accomplished by curve fitting and initial rise methods. The analysis results were in good agreement that the TSC peaks appeared in the glow curve due to existence of trapping levels with activation energies of 0.10, 0.18, 0.23, 0.53, 0.68 and 0.73 eV. These trapping levels were estimated to be hole traps above valence band. The heating rate dependent TSC glow curves were also obtained for various rates between 0.30 and 0.45 K/s. The changes of TSC intensity, peak maximum temperature and full-widths-half-maximum values with heating rates were studied in detail. TSC intensity decreased and peak maximum temperature increased with increasing heating rate. Determination of defects and trapping/stimulation mechanism of those are significant for technological applications since local states in these materials take critical role for device performance.Article Citation Count: 3Spectroscopic ellipsometry characterization of PbWO4 single crystals(Elsevier, 2022) Işık, Mehmet; Isik, M.; Gasanly, N. M.; Darvishov, N. H.; Bagiev, V. E.; Department of Electrical & Electronics EngineeringOptical characterization of PbWO4 single crystals grown by Czochralski method was achieved in virtue of spectroscopic ellipsometry experiments carried out in the energy region of 1.0-5.6 eV at room temperature. Tetragonal scheelite structure with lattice parameters of a = b = 5.4619 & Aring; and c = 12.0490 & Aring; was determined for the bulk crystal utilizing from XRD analysis. Analyses of the ellipsometry data presented the photon energy dependencies of complex dielectric function of the crystal. The real part of the dielectric function exhibited increasing behavior with energy in the below 4.1 eV and then decreased immediately. Zero frequency refractive index and dielectric constant were determined to be 2.02 and 4.08, respectively, using Wemple and DiDomenico oscillator model. High frequency dielectric constant was calculated as 4.30 by Spitzer-Fan model. Optical band gap of PbWO4 was found to be 3.24 eV from the dielectric relaxation time spectrum. Moreover, existence of two critical points with energies of 3.70 and 4.58 eV was revealed from the analyses of extinction coefficient and second derivative of the dielectric function. These levels were considered to be due to creation of cation exciton (Pb2+ 6s(2) - Pb2+ 6s6p) and transitions in the [WO4](2-) group.