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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: 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: 3Citation - Scopus: 3Fabrication of Cdsexte1-X Thin Films by Sequential Growth Using Double Sources(Elsevier, 2021) Demir, M.; Gullu, H. H.; Terlemezoglu, M.; Parlak, M.CdSexTe(1-x) (CST) ternary thin films were fabricated by stacking thermally evaporated CdSe and electron beam evaporated CdTe layers. The final structure was achieved in a stoichiometric form of approximately Cd:Se:Te = 50:25:25. The post-annealing processes at 300, 400, and 450 degrees C were applied to trigger the compound formation of CST thin films. The X-ray diffraction (XRD) profiles revealed that CdTe and CdSe have major peaks at 23.9 degrees and 25.5 degrees corresponds to (111) direction in cubic zinc-blend structure. Raman modes of CdTe were observed at 140 and 168 cm(-1), while Raman modes of CdSe films were detected at 208 and 417 cm(-1). The post-annealing process was found to be an effective method in order to combine both diffraction peaks and the vibrational modes of CdTe and CdSe, consequently to form CST ternary alloy. Transmission spectroscopy analysis revealed that CST films have direct band gap value of 1.6 eV.Article Citation - WoS: 1Citation - Scopus: 1Material Characterization of Thermally Evaporated Znsn2te4< Thin Films(Elsevier Gmbh, Urban & Fischer verlag, 2019) Gullu, H. H.Polycrystalline and stoichiometric ZnSn2Te4 (ZST) thin films were deposited on glass substrates by sequential evaporation of elemental powder sources. The deposited films were annealed in nitrogen atmosphere at annealing temperature ranging 100-300 degrees C. Under post-annealing treatments, the composition, structural, surface morphological, optical and electrical characteristics of the films were investigated. Annealing treatments lead to maintain the structural characteristics with the possible change in atomic concentration of the constituent elements in limit of detection and crystallinity of the films increased with increasing annealing temperature. Grainy surface morphology was observed in as-grown and annealed films and densely packed appearance of the surface of the samples indicates uniform deposition of the film over the entire substrate surface. Under the aim of visible light harvesting in the applications of thin film photovoltaics, normal-incidence transmittance measurements were performed and the direct band gap values were found in the range of 1.8-2.1 eV. Temperature dependent conductivity characteristics of the films were investigated under dark condition and the observed conductivity profiles were found in Arrhenius behavior with temperature dominated by the thermionic emission model.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.Article Citation - WoS: 20Citation - Scopus: 24Temperature Dependence of Band Gaps in Sputtered Snse Thin Films(Pergamon-elsevier Science Ltd, 2019) Delice, S.; Isik, M.; Gullu, H. H.; Terlemezoglu, M.; Surucu, O. Bayrakli; Parlak, M.; Gasanly, N. M.Temperature-dependent transmission experiments were performed for tin selenide (SnSe) thin films deposited by rf magnetron sputtering method in between 10 and 300 K and in the wavelength region of 400-1000 nm. Transmission spectra exhibited sharp decrease near the absorption edge around 900 nm. The transmittance spectra were analyzed using Tauc relation and first derivative spectroscopy techniques to get band gap energy of the SnSe thin films. Both of the applied methods resulted in existence of two band gaps with energies around 1.34 and 1.56 eV. The origin of these band gaps was investigated and it was assigned to the splitting of valence band into two bands due to spin-orbit interaction. Alteration of these band gap values due to varying sample temperature of the thin films were also explored in the study. It was seen that the gap energy values increased almost linearly with decreasing temperature as expected according to theoretical knowledge.Conference Object Citation - WoS: 4Citation - Scopus: 3Temperature-dependent material characterization of CuZnSe2 thin films(Elsevier Science Sa, 2020) Gullu, H. H.; Surucu, O.; Terlemezoglu, M.; Isik, M.; Ercelebi, C.; Gasanly, N. M.; Parlak, M.In the present work, CuZnSe2 (CZSe) thin films were co-deposited by magnetron sputtering of ZnSe and Cu targets. The structural analyses resulted in the stoichiometric elemental composition and polycrystalline nature without secondary phase contribution in the film structure. Optical and electrical properties of CZSe thin films were investigated using temperature-dependent optical transmission and electrical conductivity measurements. The band gap energy values were obtained using transmittance spectra under the light of expression relating absorption coefficient to incident photon energy. Band gap energy values were found in decreasing behavior from 2.31 to 2.27 eV with increase in temperature from 10 to 300 K. Temperature-band gap dependency was evaluated by Varshni and O'Donnell models to detail the optical parameters of the thin films. The experimental dark and photoconductivity values were investigated by thermionic emission model over the grain boundary potential. Room temperature conductivity values were obtained in between 0.91 and 4.65 ( x 10(-4) Omega(-1)cm(-1)) under various illumination intensities. Three different linear conductivity regions were observed in the temperature dependent profile. These linear regions were analyzed to extract the activation energy values.Article Citation - WoS: 8Citation - Scopus: 9Investigation of Electrical Properties of In/Znin2< Diode(indian Acad Sciences, 2019) Gullu, H. H.In/ZnIn2Te4/n-Si/Ag diode structure was fabricated by the thermal deposition of a ZnIn2Te4 thin film on n-Si wafer substrate with Ag metal back contact. The structural characteristics of the film were investigated in terms of composition, X-ray diffraction and topographic measurements. The diode structure was completed by evaporating In metal on the film surface as a top contact. The diode parameters as saturation current, barrier height, ideality factor and series resistance values were determined from the semi-logarithmic forward bias current-voltage characteristics of the diode. According to the assumption of the thermionic emission model, the ideality factor was found higher than unity and it was also observed that the barrier height and ideality factor showed a temperature-dependent profile resulting from the non-ideality in the current-voltage behaviour of the diode. As a result, the model was modified by considering inhomogeneous barrier formation and Gaussian distribution was expected to be dominant on 1.37 eV mean barrier height with a deviation of 0.18. In addition, the voltage dependence of these Gaussian diode parameters was investigated. The forward and reverse bias capacitance and conductance measurements showed that there was a slight change in capacitance values with frequency whereas the conductance values decreased with increase in frequency. In addition to the current-voltage analysis, the distribution of density of interface states and the values of series resistance were evaluated as a function of bias voltage and frequency.Article Citation - WoS: 5Citation - Scopus: 5Improvement of Electrical Characteristics of Snse/Si Heterostructure by Integration of Si Nanowires(Elsevier, 2021) Coskun, E.; Gullu, H. H.; Emir, C.; Parlak, M.In this study, the effects of the nanowire geometry on Si wafer substrate were investigated for the SnSe/Si-nanowire heterojunction device and the obtained results were compared with the one fabricated on planar Si surface. Nanowires on Si surface were produced by metal-assisted etching method and the SnSe film layer was deposited by thermal evaporation technique. On both Si and glass surfaces, deposited film shows polycrystalline and single SnSe phase. From optical transmission measurements, optical band gap of this film was determined as 1.36 eV in a good agreement with the literature. All SnSe/Si heterostructures were found in a p-n diode behavior and the ideality factor and series resistance values were calculated as 2.40, 547 Omega, and 3.71, 1.57 x 10(3) Omega, for SnSe/Si-nanowire and SnSe/Si heterojunctions, respectively. As a result, an improvement in device characteristics concerning the planar Si structure was found by utilizing Si nanowire structure.Article Citation - WoS: 25Citation - Scopus: 26Determination of Current Transport Characteristics in Au-cu/Cuo Schottky Diodes(Elsevier, 2019) Surucu, O. Bayrakli; Gullu, H. H.; Terlemezoglu, M.; Yildiz, D. E.; Parlak, M.In this study, the material properties of CuO thin films fabricated by sputtering technique and electrical properties of CuO/n-Si structure were reported. Temperature-dependent current-voltage (I-V) measurement was carried out to determine the detail electrical characteristics of this structure. The anomaly in thermionic emission (TE) model related to barrier height inhomogeneity at the interface was obtained from the forward bias I-V analysis. The current transport mechanism at the junction was determined under the assumption of TE with Gaussian distribution of barrier height. In this analysis, standard deviation and mean zero bias barrier height were evaluated as 0.176 and 1.48 eV, respectively. Depending on the change in the diode parameters with temperature, Richardson constant was recalculated as 110.20 Acm(-2)K(-2) with the help of modified Richardson plot. In addition, density of states at the interface were determined by using the forward bias I-V results.
