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Article Citation - WoS: 23Citation - Scopus: 23Exploring Temperature-Dependent Bandgap and Urbach Energies in Cdte Thin Films for Optoelectronic Applications(Elsevier, 2024) Surucu, O.; Surucu, G.; Gasanly, N. M.; Parlak, M.; Isik, M.This study examines CdTe thin films deposited via RF magnetron sputtering, focusing on structural and optical properties. X-ray diffraction, Raman spectroscopy, and SEM assessed structural characteristics. Optical properties were analyzed through transmittance measurements from 10 to 300 K. Tauc plots and Varshni modeling revealed a temperature-dependent bandgap, increasing from 1.49 eV at room temperature to 1.57 eV at 10 K. Urbach energy rose from 82.7 to 93.7 meV with temperature. These results are essential for applications where temperature affects CdTe-based device performance.Conference Object Citation - WoS: 10Citation - Scopus: 10Investigation of Carrier Transport Mechanisms in the Cu-Zn Based Hetero-Structure Grown by Sputtering Technique(Canadian Science Publishing, 2018) Gullu, H. H.; Terlemezoglu, M.; Bayrakli, O.; Yildiz, D. E.; Parlak, M.In this paper, we present results of the electrical characterization of n-Si/p-Cu-Zn-Se hetero-structure. Sputtered film was found in Se-rich behavior with tetragonal polycrystalline nature along with (112) preferred orientation. The band gap energy for direct optical transitions was obtained as 2.65 eV. The results of the conductivity measurements indicated p-type behavior and carrier transport mechanism was modelled according to thermionic emission theory. Detailed electrical characterization of this structure was carried out with the help of temperature-dependent current-voltage measurements in the temperature range of 220-360 K, room temperature, and frequency-dependent capacitance-voltage and conductance-voltage measurements. The anomaly in current-voltage characteristics was related to barrier height inhomogeneity at the interface and modified by the assumption of Gaussian distribution of barrier height, in which mean barrier height and standard deviation at zero bias were found as 2.11 and 0.24 eV, respectively. Moreover, Richardson constant value was determined as 141.95 Acm(-2)K(-2) by means of modified Richardson plot.Article Citation - WoS: 22Citation - Scopus: 24Synthesis and Temperature-Tuned Band Gap Characteristics of Magnetron Sputtered Znte Thin Films(Elsevier, 2020) Isik, M.; Gullu, H. H.; Parlak, M.; Gasanly, N. M.Zinc telluride (ZnTe) is one of the attractive semiconducting compounds used in various optoelectronic devices. The usage of ZnTe in optoelectronic applications directs researchers to search its optical characteristics in great detail. For this purpose, structural and optical properties of magnetron sputtered ZnTe thin films were studied by means of x-ray diffraction and transmission spectroscopy measurements. Structural analyses indicated that ZnTe thin films having cubic crystalline structure were successfully grown on soda-lime glass substrates. Transmittance spectra in the 400-1000 nm were recorded in between 10 and 300 K temperature region. The analyses of absorption coefficient spectra resulted in band gap energies decreasing from around 2.31 (10 K) to 2.26 eV (300 K). Temperature dependency of gap energy was studied by Varshni and O'Donnell-Chen relations to determine various optical parameters like absolute zero temperature band gap energy, change of gap energy with temperature, phonon energy.Article Citation - WoS: 6Citation - Scopus: 7Construction of Self-Assembled Vertical Nanoflakes on Cztsse Thin Films(Iop Publishing Ltd, 2019) Terlemezoglu, M.; Surucu, O. Bayrakli; Colakoglu, T.; Abak, M. K.; Gullu, H. H.; Ercelebi, C.; Parlak, M.Cu2ZnSn(S, Se)(4) (CZTSSe) is a promising alternative absorber material to achieve high power conversion efficiencies, besides its property of involving low-cost and earth-abundant elements when compared to Cu(In, Ga) Se-2 (CIGS) and cadmium telluride (CdTe), to be used in solar cell technology. In this study, a novel fabrication technique was developed by utilizing RF sputtering deposition of CZTSSe thin films having a surface decorated with self-assembled nanoflakes. The formation of nanoflakes was investigated by detailed spectroscopic method of analysis in the effect of each stacked layer deposition in an optimized sequence and the size of nanoflakes by an accurate control of sputtering process including film thickness. Moreover, the effects of substrate temperature on the formation of nanoflakes on the film surface were discussed at a fixed deposition route. One of the main advantages arising from the film surface with self-assembled nanoflakes is the efficient light trapping which decreases the surface reflectance. As a result of the detailed production and characterization studies, it was observed that there was a possibility of repeatable and controllable fabrication sequence for the preparation of CZTSSe thin films with self-textured surfaces yielding low surface reflectance.Article Citation - WoS: 2Citation - Scopus: 3Electrical Characterization of Zninse2 Thin-Film Heterojunction(Springer, 2019) Gullu, H. H.; Parlak, M.ZnInSe2/Cu0.5Ag0.5InSe2 diode structures have been fabricated by thermal evaporation of stacked layers on indium tin oxide-coated glass substrates. Temperature-dependent dark current-voltage measurements were carried out to extract the diode parameters and to determine the dominant conduction mechanisms in the forward- and reverse-bias regions. The heterostructure showed three order of magnitude rectifying behavior with a barrier height of 0.72 eV and ideality factor of 2.16 at room temperature. In the high forward-bias region, the series and shunt resistances were calculated with the help of parasitic resistance relations, yielding room-temperature values of 9.54 x 10(2) Omega cm(2) and 1.23 x 10(3) Omega cm(2), respectively. According to the analysis of the current flow in the forward-bias region, abnormal thermionic emission due to the variation of the ideality factor with temperature and space-charge-limited current processes were determined to be the dominant conduction mechanisms in this heterostructure. In the reverse-bias region, the tunneling mechanism was found to be effective in the leakage current flow with trap density of 10(6) cm(-3). Spectral photocurrent measurements were carried out to investigate the spectral working range of the device structure. The main photocurrent peaks observed in the spectrum corresponded to the band-edge values of the active thin-film layers.Article Citation - WoS: 11Citation - Scopus: 12Structural and Temperature-Tuned Bandgap Characteristics of Thermally Evaporated β-in2< Thin Films(Springer, 2021) Surucu, O.; Isik, M.; Terlemezoglu, M.; Gasanly, N. M.; Parlak, M.In2S3 is one of the attractive compounds taking remarkable interest in optoelectronic device applications. The present study reports the structural and optical characteristics of thermally evaporated beta-In2S3 thin films. The crystalline structure of the thin films was found as cubic taking into account the observed diffraction peaks in the X-ray diffraction pattern. The atomic compositional ratio of constituent elements was obtained as consistent with chemical formula of In2S3. Three peaks around 275, 309 and 369 cm(-1) were observed in the Raman spectrum. Temperature-tuned bandgap energy characteristics of the In2S3 thin films were revealed from the investigation of transmittance spectra obtained at various temperatures between 10 and 300 K. The analyses of the transmittance spectra indicated that direct bandgap energy of the In2S3 thin films decreases from 2.40 eV (at 10 K) to 2.37 eV (at 300 K) with the increase of measurement temperature. The bandgap energy vs. temperature relation was investigated by means of Varshni optical model. The fitting of the experimental data under the light of theoretical expression revealed the absolute zero bandgap energy, the rate of change of bandgap energy and Debye temperature.Article Citation - WoS: 3Citation - Scopus: 4Analysis of Temperature-Dependent Transmittance Spectra of Zn0.5in0.5< (zis) Thin Films(Springer, 2019) Isik, M.; Gullu, H. H.; Delice, S.; Gasanly, N. M.; Parlak, M.Temperature-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 - WoS: 4Citation - Scopus: 4Growth and Optical Characterization of Sn0.6sb0.4< Layer Single Crystals for Optoelectronic Applications(Elsevier Sci Ltd, 2022) Bektas, T.; Terlemezoglu, M.; Surucu, O.; Isik, M.; Parlak, M.SnSe compound is an attractive semiconductor material due to its usage in photovoltaic applications. The sub-stitution of Sb in the SnSe compound presents a remarkable advantage especially in point of tuning optical characteristics. The present paper reports the structural and optical properties of Sn1-xSbxSe (x = 0.4) layered single crystals grown by the vertical Bridgman method. To the best of our knowledge, this work is the first investigation of the Sn0.6Sb0.4Se crystal grown with the vertical Bridgman technique. X-ray diffraction (XRD) pattern of the grown crystal indicated the well crystalline structure of the grown crystals. Lattice strain and interplanar spacing of the crystal structure were determined using the XRD pattern. Scanning electron micro-scope images allowed to the observation of the layer crystal structure. The layer crystalline structure shows 2D material properties and provides 2D applications. Optical properties were revealed by carrying out Raman, ellipsometry and transmission measurements. Raman modes, refractive index, extinction coefficient, and dielectric spectra, band gap energy of the crystal were presented throughout the paper. The obtained results indicated that Sn1-xSbxSe (x = 0.4) layer single crystals may be an alternative potential for photovoltaic and optoelectronic applications.Article Citation - WoS: 13Citation - Scopus: 15Investigation of electrical characteristics of Ag/ZnO/Si sandwich structure(Springer, 2019) Gullu, H. H.; Surucu, O. Bayrakli; Terlemezoglu, M.; Yildiz, D. E.; Parlak, M.In this study, temperature-dependent current-voltage (I-V), frequency-dependent capacitance-voltage (C-V) and conductance-voltage (G/omega-V) measurements are carried out for the electrical characterization of a zinc oxide (ZnO) thin film-based diode. The sandwich structure in the form of Ag/ZnO/Si/Al is investigated at temperatures between 220 and 360 K and in the frequency region of 1 kHz-1 MHz. ZnO thin film layer is deposited on a p-Si wafer substrate as a transparent conductive oxide layer by taking into consideration possible electronic applications with intrinsic attractive material properties. At each temperature step, the I-V curves showed about two orders of magnitude rectifying behavior and, according to the Schottky diode relation, the saturation current, zero-bias barrier height and ideality factor were extracted as a function of the temperature. In the case of non-ideal diode characteristics due to the inhomogeneties in the diode as observed from the characteristics of the calculated parameters, effective barrier height values are evaluated. In addition, based on the existence of the interface layer, density of interface states in the band gap region and parasitic resistances were determined by the capacitance measurements.Article Citation - WoS: 2Citation - Scopus: 1Investigation of Conductivity Characteristics of Zn-In Thin Films(World Scientific Publ Co Pte Ltd, 2020) Gullu, H. H.; Parlak, M.Zn-In-Se thin films were fabricated on the ultrasonically cleaned glass substrates masked with clover-shaped geometry by thermal evaporation of its elemental sources. Temperature-dependent conductivity characteristics of the films were investigated under dark and illuminated conditions. The semiconductor type of the films was found as n-type by thermal probe test. According to the van der Pauw technique, the dark electrical conductivity analyses showed that the variations of conductivity of unannealed and annealed at 300 degrees C samples are in exponential dependence of temperature. These conductivity profiles were found to be dominated by the thermionic emission at high temperature region whereas their behaviors at low temperatures were modeled by hopping theory. On the contrary, as a result of the further annealing temperatures, the surface of the samples showed semi-metallic characteristics with deviating from expected Arrhenius behavior. In addition, the temperature-dependent photoconductivity of the films was analyzed under different illumination intensities and the results were explained by the supra-linear characteristic based on the two-center recombination model.
