Güllü, Hasan Hüseyin
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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.
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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WoS Researcher ID
Sustainable Development Goals
1NO POVERTY
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2ZERO HUNGER
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3GOOD HEALTH AND WELL-BEING
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4QUALITY EDUCATION
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5GENDER EQUALITY
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6CLEAN WATER AND SANITATION
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7AFFORDABLE AND CLEAN ENERGY
5
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8DECENT WORK AND ECONOMIC GROWTH
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9INDUSTRY, INNOVATION AND INFRASTRUCTURE
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10REDUCED INEQUALITIES
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11SUSTAINABLE CITIES AND COMMUNITIES
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12RESPONSIBLE CONSUMPTION AND PRODUCTION
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13CLIMATE ACTION
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14LIFE BELOW WATER
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15LIFE ON LAND
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16PEACE, JUSTICE AND STRONG INSTITUTIONS
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17PARTNERSHIPS FOR THE GOALS
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Scholarly Output
55
Articles
52
Views / Downloads
183/125
Supervised MSc Theses
1
Supervised PhD Theses
0
WoS Citation Count
886
Scopus Citation Count
912
Patents
0
Projects
0
WoS Citations per Publication
16.11
Scopus Citations per Publication
16.58
Open Access Source
8
Supervised Theses
1
| Journal | Count |
|---|---|
| Journal of Materials Science: Materials in Electronics | 16 |
| Physica B: Condensed Matter | 7 |
| Bulletin of Materials Science | 3 |
| Materials Science in Semiconductor Processing | 3 |
| Optik | 3 |
Current Page: 1 / 5
Scopus Quartile Distribution
Competency Cloud
44 results
Scholarly Output Search Results
Now showing 1 - 10 of 44
Article Citation - WoS: 23Citation - Scopus: 25Synthesis 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.; Bayrakli Sürücü, O.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: 77Citation - Scopus: 79Electrical Properties of Al/Pcbm:zno Heterojunction for Photodiode Application(Elsevier Science Sa, 2020) Gullu, H. H.; Yildiz, D. E.; Kocyigit, A.; Yildirim, M.In this paper, the electrical characteristics of spin-coated PCBM:ZnO interlayered Al/PCBM:ZnO/Si diode are investigated under the aim of photodiode application. Under dark condition, the diode shows about four orders in magnitude rectification rate and diode illumination results in efficient rectification with increase in intensity. The analysis of current-voltage curve results a non-ideal diode characteristics according to the thermionic emission model due to the existence of parasitic resistances and interface states. The measured current-voltage values are used to extract the barrier height and ideality factor under dark and illumination conditions. Under illumination, photo-generated carriers contribute to the current flow and linear photo-conductivity behavior in photo-current measurements with illumination shows the possible use of hybrid PCBM:ZnO layer in Si-based photodiodes. In addition, change in the series and shunt resistance values under illumination is found to be effective in this light-sensing behavior of the diode. This characteristic is also observed from the typical on/off illumination switching behavior for the photodiodes in transient photo-current, photo-capacitance and photo-conductance measurements with the quick response to the illumination. The deviations from ideality are also discussed by means of distribution of interface states and series resistance depending on the applied frequency and bias voltage. (C) 2020 Elsevier B.V. All rights reserved.Article Citation - WoS: 42Citation - Scopus: 41Analysis of Forward and Reverse Biased Current-Voltage Characteristics of Al/Al2< Schottky Diode With Atomic Layer Deposited Al2o3< Thin Film Interlayer(Springer, 2019) Gullu, H. H.; Yildiz, D. E.The dark current-voltage (I-V) characteristics of Al/Al2O3/n-Si Schottky diode are investigated in a wide temperature range of 260-360 K. The diode shows four orders of magnitude rectification. In forward and reverse bias regions, the temperature-dependent I-Vcharacteristics are detailed in terms of diode parameters and dominant conduction mechanisms. Due to the existence of Al2O3 film layer and series resistance in the diode structure, current flow under the forward bias is observed in a deviation from pure exponential characteristics. The diode parameters are estimated from thermionic emission model with non-unity ideality factor, and this non-ideal behavior is resulted in the ideality factors greater than two. In addition to these values, zero-bias barrier height is found to be strongly temperature dependent, and this variation indicates a presence of inhomogeneties in the barrier according to Gaussian distribution (GD) approximation. This fact is investigated plotting characteristic plot of this model and by extracting mean barrier height with its standard deviation. In order to complete the work on the forward I-V region, the carrier transport characteristics of the diode are explained on the basis of thermionic emission mechanism with a GD of the harrier heights. In accordance with this approximation, the conventional Richardson plot exhibits non-linearity behavior and modified current relation based on GD model is used to calculate mean barrier height and Richardson constant. In addition, the values of parasitic resistances are determined using Ohm's law as a function of temperature for all bias voltage spectra. In the reverse bias region, Poole-Frenkel effect is found to be dominant on the conduction associated with the barrier lowering, and barrier height in the emission process from the trapped states, and high-frequency dielectric constant of Al2O3 film layer is calculated.Article Citation - WoS: 33Citation - Scopus: 32Capacitance, Conductance, and Dielectric Characteristics of Al/Tio2< Diode(Springer, 2021) Gullu, H. H.; Yildiz, D. E.In this study, electrical properties of the Al/TiO2/p-Si diode structure with an atomic layer deposited TiO2 interface layer are investigated by current-voltage (I-V), capacitance-voltage (C - V), and conductance-voltage (G - V) measurements. It shows a rectifying behavior with about four order of rectification factor, and barrier height and ideality factor are calculated from the rectification curve. Dielectric parameters are determined from frequency-dependent C - V and G - V relations. The experimental results show that both of these curves are in a strong response to the frequency and bias voltage. They are found in decreasing behavior with increasing frequency, and both of them increase with increase in bias voltage although there are different increasing trends. At reversed bias voltage region, barrier potential, Fermi level energy, and interface charge carrier contribution are evaluated by using 1/C-2 - V plot. Series resistance values are also calculated under the variation of frequency and voltage. Thus, the capacitive characteristics of the diode are corrected by eliminating series resistance contribution together with the possible effect on interface charge carriers. Detailed information is obtained by determining electronic parameters affected by interface states over a wide frequency range (1 kHz to 1 MHz). At this point, strong response to the frequency is observed for the dielectric constant. Under the effect of interfacial polarization at low-frequency region, interface charge contribution to the capacitive response of the diode is obtained. Further analysis is performed on electrical modulus and impedance values derived from experimental dielectric data. Existence of interfacial layer capacitance is detailed by extracting distribution of interface charges from capacitance and conductance profiles of the diode under the effect of frequency.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: 9Citation - Scopus: 9Optical Band Gap and Dispersion of Optical Constants of Cu-Ga Thin Films(Elsevier Gmbh, 2019) Isik, M.; Gullu, H. H.; Coskun, E.; Gasanly, N. M.Thermally deposited Cu-Ga-S thin films were optically characterized by means of experimental techniques of transmission measurements. The analyses of transmittance spectra were accomplished by derivative spectrophotometry analyses to get gap energies of thin films. The transmittance spectra of thin films annealed at different temperatures presented interference fringes which were analyzed by Swanepoel envelope method. The wavelength dependencies of optical parameters; refractive index (n), real part of complex dielectric function (epsilon(re)) and extinction coefficient (k) were reported in the weak absorption region. The photon energy dependencies of n and epsilon(re) were analyzed using single-oscillator and Spitzer-Fan models, respectively.Article Citation - WoS: 13Citation - Scopus: 12A Study on Electrical Properties of Au/4h-sic Schottky Diode Under Illumination(Springer, 2021) Yildiz, D. E.; Karadeniz, S.; Gullu, H. H.Y In this work, a metal-semiconductor diode in the form of Au/4H-SiC is fabricated, and the electrical properties of this device are systematically examined under dark and different illumination intensities. To perform this, the currentvoltage (I-V) characteristics of the Schottky-type diode are analyzed at room temperature. The performance parameters such as saturation current (I-0), barrier height (Phi(B)), ideality factor (n) and series resistance (R-s) are found to be illumination dependent. The reverse biased I - V characteristics under incident light indicate high photo-sensitivity as compared to the response at forward bias. Thus, this result is investigated in detail according to both Schottky and Poole-Frenkel effects. It is found that the Poole-Frenkel mechanism is dominant in the reverse biased region. The Au/4H-SiC Schottky junction has a strong photo-current response to the different illumination intensities and transient photocurrent characteristics of the fabricated device are studied at the illumination intensities of 50 and 100 mW/cm(2). All experimental results indicate that the Au/4H-SiC Schottky diode, with a valuable response to the illumination together with change in illumination intensity, can be used for optoelectronic applications.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.
