Güllü, Hasan Hüseyin

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Güllü, Hasan Hüseyin

Name Variants

Gullu,H.H.
H.,Güllü
H.H.Güllü
G., Hasan Huseyin
Güllü, Hasan Hüseyin
H., Gullu
G.,Hasan Huseyin
H.H.Gullu
Hasan Hüseyin, Güllü
G.,Hasan Hüseyin
Hasan Huseyin, Gullu
Gullu, Hasan Huseyin
Güllü,H.H.
Gullu, H. H.
Gullu, Hasan H.

Job Title

Doktor Öğretim Üyesi

Email Address

hasan.gullu@atilim.edu.tr

Main Affiliation

Department of Electrical & Electronics Engineering

Status

Former Staff

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Scholarly Output

55

Articles

52

Citation Count

631

Supervised Theses

1 Scholarly Output Search Results

Now showing 1 - 10 of 55

Citation - WoS: 17
Citation - Scopus: 17
Electrical Characteristics of Organic Heterojunction With an Alternating Benzotriazole and Fluorene Containing Copolymer
(Springer, 2020) Gullu, H. H.; Yildiz, D. E.; Toppare, L.; Cirpan, A.; Department of Electrical & Electronics Engineering
The current-voltage (I - V) and capacitance-voltage (C - V) characteristics of the organic heterojunction diode were investigated in a wide temperature range from 80 to 320 K and frequency range from 10 kHz to 1 MHz, respectively. Alternative to the copolymer partner poly(3-hexylthiophene) (P3HT) to [6,6]phenyl-C61-butyric acid methyl ester (PCBM), poly((9,9-dioctylfluorene)-2,7diyl-(4,7-bis(thien-2-yl)-2-dodecyl-benzo[1,2,3]triazole)) (named as copolymer in this work) was adapted to the bulk-heterojunction layer in the organic diode. Together with the use of Lif/Al bilayer electrode, the diode was fabricated as in the form of Al/LiF/copolymer:PCBM/PEDOT:PSS/ITO/glass. Under the applied bias voltage, this organic-based diode shows two- orders of magnitude rectifying behavior. According to thermionic emission (TE) model, the diode parameters such as saturation current, barrier height and ideality factor were determined and parasitic resistances were also extracted from the conventional ohmic relation. As to the temperature dependency of the diode parameters and their response to the temperature variation, barrier inhomogeneity, surface state and series resistance effects were found in dominant behavior on the current flow. The conduction mechanism was modeled by assuming low-barrier patches around the main barrier that supports TE at low temperatures and their distribution was expressed by a Gaussian function. In addition, series resistance values were detailed depending on temperature using Cheung's model. C - V analysis was performed to evaluate the distribution of surface states at the interface as a function of frequency. Based on the C - V plots, the effects of charges at these traps were observed especially at low frequencies. Additionally, from these results, Fermi level, surface potential and donor concentration values were evaluated in a wide frequency range.
Citation - WoS: 14
Citation - Scopus: 14
Temperature-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 Engineering
The 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.
Citation - WoS: 19
Citation - Scopus: 23
Temperature 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.; Electrical-Electronics Engineering; Department of Electrical & Electronics Engineering
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.
Deposition and Characterization of Znsnse2 Thin-Films Deposited by Using Sintered Stoichiometric Powder
(2019) Sürücü, Özge Bayraklı; Güllü, Hasan Hüseyin; Department of Electrical & Electronics Engineering
In this work, ZnSnSe2 (ZTSe) thin films were deposited using crystalline powder grown by vertical Bridgman-Stockbarger technique. The deposition process was carried out by means of e-beam evaporation on the well-cleaned soda lime glass substrates and keeping them at the substrate temperature of 200°C. The structural, optical and electrical properties of ternary ZTSe thin films were investigated depending on the annealing temperature at 250 and 300°C. X-ray diffraction analysis showed that as-grown films were in amorphous structure, however annealing at 250°C triggered the crystallization on the preferred ternary structure and annealing at 300°C resulted in the changes from amorphous to the polycrystalline structure. Using the compositional analysis, the detail information about the stoichiometry and the segregation mechanisms of the constituent elements in the structure were determined for both as-grown and annealed samples. In addition, they were morphologically characterized using scanning electron microscopy technique. The electrical properties were analyzed using temperature dependent dark- and photo-conductivity measurements. From the variation of electrical conductivity as a function of the ambient temperature, the current transport mechanisms and corresponding activation energies at specific temperature intervals for each sample were determined. The optical properties for the ZTSe thin films were studied depending on the structural changes with annealing.
Citation - WoS: 84
Citation - Scopus: 86
CaXH₃ (X = Mn, Fe, Co) perovskite-type hydrides for hydrogen storage applications
(Wiley, 2020) Surucu, Gokhan; Gencer, Aysenur; Candan, Abdullah; Gullu, Hasan H.; Isik, Mehmet; Department of Electrical & Electronics Engineering
Hydrogen storage is one of the attractive research interests in recent years due to the advantages of hydrogen to be used as energy source. The studies on hydrogen storage applications focus mainly on investigation of hydrogen storage capabilities of newly introduced compounds. The present paper aims at characterization of CaXH3 (X: Mn, Fe, or Co) perovskite-type hydrides for the first time to understand their potential contribution to the hydrogen storage applications. CaXH3 compounds have been investigated by density functional theory studies to reveal their various characteristics and hydrogen storage properties. CaXH3 compounds have been optimized in cubic crystal structure and the lattice constants of studied compounds have been obtained as 3.60, 3.50, and 3.48 angstrom for X: Mn, Fe, and Co compounds, respectively. The optimized structures have negative formation enthalpies pointing out that studied compounds are thermodynamically stable and could be synthesized experimentally. The gravimetric hydrogen storage densities of X: Mn, Fe, and Co compounds were found in as 3.09, 3.06, and 2.97 wt%, respectively. The revealed values for hydrogen storage densities indicate that CaXH3 compounds may be potential candidates for hydrogen storage applications. Moreover, various mechanical parameters of interest compounds like elastic constants, bulk modulus, and Poisson's ratio have been reported throughout the study. These compounds were found mechanically stable with satisfying Born stability criteria. Further analyses based on Cauchy pressure and Pugh criterion, showed that they have brittleness nature and relatively hard materials. In addition, the electronic characteristics, band structures, and associated partial density of states of CaXH3 hydrides have been revealed. The dynamic stability behavior of them was verified based on the phonon dispersion curves.
Citation - WoS: 2
Citation - Scopus: 2
Fabrication of Cdsexte1-X Thin Films by Sequential Growth Using Double Sources
(Elsevier, 2021) Demir, M.; Gullu, H. H.; Terlemezoglu, M.; Parlak, M.; Department of Electrical & Electronics Engineering
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.
Citation - WoS: 3
Citation - Scopus: 4
Analysis of Temperature-Dependent Transmittance Spectra of Zn_0.5in_{0.5< (zis) Thin Films}
(Springer, 2019) Isik, M.; Gullu, H. H.; Delice, S.; Gasanly, N. M.; Parlak, M.; Department of Electrical & Electronics Engineering
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.
Citation - WoS: 1
Citation - Scopus: 1
Material Characterization of Thermally Evaporated Znsn₂te_{4< Thin Films}
(Elsevier Gmbh, Urban & Fischer verlag, 2019) Gullu, H. H.; Department of Electrical & Electronics Engineering
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.
Citation - WoS: 3
Citation - Scopus: 6
Structural and Optical Properties of Thermally Evaporated Cu-Ga (cgs) Thin Films
(Elsevier, 2018) Gullu, H. H.; Isik, M.; Gasanly, N. M.; Department of Electrical & Electronics Engineering
The structural and optical properties of thermally evaporated Cu-Ga-S (CGS) thin films were investigated by Xray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM) and optical transmittance measurements. The effect of annealing temperature on the results of applied techniques was also studied in the present paper. EDS results revealed that each of the elements, Cu, Ga and S are presented in the films and Cu and Ga concentration increases whereas S concentration decreases within the films as annealing temperature is increased. XRD pattern exhibited four diffraction peaks which are well-matched with those of tetragonal CuGaS2 compound. AFM images were recorded to get knowledge about the surface morphology and roughness of deposited thin films. Transmittance measurements were applied in the wavelength region of 300-1000 nm. Analyses of the absorption coefficient derived from transmittance data resulted in presence of three distinct transition regions in each thin films with direct transition type. Crystal-field and spin-orbit splitting energies existing due to valence band splitting were also calculated using quasicubic model.
$cuınse_2$ ve $cugase_2$ İnce Filmlerin Özellikleri Üzerine Karşılaştırmalı Çalışma
(2019) Candan, İdris; Güllü, Hasan Hüseyin; Department of Electrical & Electronics Engineering
Cu(In1-xGax)Se2 (CIGS) yarıiletken ince filmlerin iki kenar noktası olan x=0 ($CuInSe_2$) ve x=1 ($CuGaSe_2$) ince filmleri, Cu, InSe ve GaSe hedeflerden saçtırma yöntemi ile 250 oC sıcaklıkta soda lime cam alttaşlar üzerine kaplandı. In ve Ga oranı ve üretim sonrası ısıl işlemin CuInSe2 (CIS) ve CuGaSe2 (CGS) ince filmlerin özellikleri üzerine etkileri araştırıldı. Üretilmiş filmlerin yapısal özelliklerini incelemek için X-ışını kırınımı (XRD) ve örneklerin bileşenleri enerji dağılımlı X-ışını kırınımı analizi (EDXA) yöntemi kullanılarak analiz edildi. İnce film örneklerinin Raman aktif modlarının tayini için oda sıcaklığında Raman spektroskopi ölçümleri yapıldı. 400 oC sıcaklıkta ısıl işlem uygulanmış CIS ve CGS ince film örneklerinin en aktif modları (A1 modu) en yoğun çizgilerin sırayla 178 cm^{-1} ve 185 $cm^{-1}$ olduğu gözlendi. Bu mod kalkopirit yapıların Raman spektroskopisinde gözlemlenen en güçlü moddur. Isıl işlem uygulanmamış ve 350 oC’ta ısıl işlem uygulanmış CGS örneklerinde 486 $cm^{-1}$ çizgisi gözlenmiş olmasına rağmen bu çizginin yoğunluğu artan ısıl işlem sıcaklığı ile ters orantılı olarak azaldığı ve 400 oC uygulanan ısıl işlem sonrası tamamen yok olduğu gözlenmiştir. Üretilen CIS ve CGS ince filmlerin optik geçirgenlik ölçümleri sonucunda ısıl işlem uygulanmayan filmler ve at 400 oC sıcaklıkta ısıl işlem uygulanan filmler için optik bant aralıklarındaki değişim değerleri CIS için 1.28 eV ile 1.45 eV, CGS için 1.68 eV ile 1.75 eV aralıklarında değiştiği hesaplandı. Numunelerin oda sıcaklığındaki elektriksel iletkenlikleri ısıl işlem öncesi ve 400 oC ısıl işlem sonrasında n-CIS için sırayla 8.6x$10^{-3}$ ve 13.6x$10^{-2}$ $\\;{(Ω.cm)}^{-1}$ , p-CGS için sırayla 1.6 and 1.9 $\\;{(Ω.cm)}^{-1}$ olarak ölçüldü.

Güllü, Hasan Hüseyin

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Scholarly Output

55

Articles

52

Citation Count

631

Supervised Theses

1

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