Physics / Fizik
Permanent URI for this collectionhttps://hdl.handle.net/11147/6
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Article Citation - WoS: 9Citation - Scopus: 9High Voltage Response of Graphene/4h-sic Uv Photodetector With Low Level Detection(Elsevier, 2023) Jehad, Ala K.; Ünverdi, Özhan; Çelebi, CemA self-powered graphene/silicon carbide (G/4H-SiC) ultraviolet photodetector of a p-i-n like-structure with high voltage response has been fabricated to detect and measure low intensity ultraviolet (UV) radiation. Bilayer graphene sheet grown by chemical vapor deposition (CVD) method was transferred on the top of an epilayer structure of n-/n+ 4 H-SiC. In this structure, two Schottky contacts were formed: one at G/ n- 4H-SiC interface and the other at bulk-4 H-SiC/Cr/Au interface. The photodetector's characteristic measurements revealed low dark current of - 0.58 nA and spectral voltage responsivity of - 0.75 V/W at 300 nm wavelength. Under low level UV illumination of 300 nm wavelength, the photodetector exhibited a leakage current and a photogenerated response voltage of 1.1 nA and 10 mV, respectively. The time-dependent photovoltage measurements displayed a rapid photovoltage response with rise and decay times of -74 ns and - 580 ns, respectively. This novel device holds promise for applications requiring sensitive and self-powered UV detection.Article Citation - WoS: 35Citation - Scopus: 35Cvd Graphene/Sic Uv Photodetector With Enhanced Spectral Responsivity and Response Speed(Elsevier, 2023) Jehad, Ala K.; Fidan, Mehmet; Ünverdi, Özhan; Çelebi, CemA self-powered, high-performance graphene/Silicon Carbide (G/4H-SiC) ultraviolet Schottky junction photodetector has been fabricated, and the effect of using monolayer and bilayer graphene on the device performance parameters was investigated. P-type graphene sheets were grown by the chemical vapor deposition (CVD) method, while 4H-SiC material consists of an epilayer structure of n-/n+ on bulk n-SiC. Two photodetector devices have been studied, one with monolayer graphene (MLG) and the other with bilayer graphene (BLG). The proposed photodetector structure reveals the highest spectral responsivity known of a G/4H-SiC UV photodetector so far. Electronic and optoelectronic characterizations were done under an ultraviolet wavelength range from 240 to 350 nm. The results show two spectral responsivity maxima (Rmax) at 285 nm and 300 nm wavelengths. Exhibiting two maxima in spectral responsivity and detectivity is caused by the constructive and destructive interference effects of multiple reflections at the SiC epilayer's interfaces. The photodetector devices exhibit high spectral responsivity (R ? 0.09 AW?1), maximum detectivity (D* ? 2.9 × 1012 Jones), and minimum noise equivalent power (NEP ? 0.17 pWHz-1/2) in both devices. Using bilayer graphene instead of monolayer showed no significant change in both the photogenerated current and the spectral responsivity due to the higher absorption coefficient of bilayer graphene, however, it exhibited a significant improvement in the response speed. The response speed was found to increase by 50 % when bilayer graphene was used as a hole collecting electrode in the G/4H-SiC junction. This is because bilayer graphene creates a narrower depletion layer and higher electric field, which promotes efficient charge separation and recombination. © 2023 Elsevier B.V.Article Citation - WoS: 22Citation - Scopus: 26Flexible Metal/Semiconductor Type Photodetectors Based on Manganese Doped Zno Nanorods(Elsevier, 2023) Karagöz, Emine; Altaf, Çiğdem Tuç; Yaman, Ecenaz; Yıldırım, İpek Deniz; Erdem, Emre; Çelebi, Cem; Fidan, Mehmet; Sankir, Mehmet; Demirci Sankir, NurdanHigh-performance flexible photodetectors are one of the most interesting research areas due to their great possibilities for a variety of applications such as portable and wearable optoelectronics. This study verifies the performance of flexible metal/semiconductor/metal-type photodetector based on pristine and manganese doped ZnO nanorods (ZnO-NRs) prepared in two different concentrations of zinc precursors and manganese dopant at low temperatures. The photodetectors having ZnO-NRs with high aspect ratios were investigated by various material characterization techniques such as electron paramagnetic resonance and photoluminescence spectroscopy to confirm the relationship between defect concentrations and photodetector performance parameters. It has been calculated that the detectivity (D*) and responsivity (R) of the ZnO nanorod-based photodetectors increased 20 and 18 folds, respectively by increasing the concentration of zinc precursor. Besides the D* and R values of the photodetectors, prepared by the 16.5 mM zinc precursor, increased 18 and 4.5-fold, respectively, after manganese doping. We confirmed that even a very low concentration of zinc precursor could produce a photodetector with high performance in photo-response characteristics, flexibility, and stability against 10,000 cycles of convex/concave bending.(c) 2023 Elsevier B.V. All rights reserved.Article Citation - WoS: 5Citation - Scopus: 5Self-Powered Photodetector Array Based on Individual Graphene Electrode and Silicon-On Integration(Elsevier, 2023) Yanılmaz, Alper; Ünverdi, Özhan; Çelebi, CemOne of the key limitations for the device performance of the silicon (Si) based photodetector arrays is the optical crosstalk effect encountered between photoactive elements as well. The scope of this work is to reduce optical crosstalk and thus increasing the device performances with graphene and Si integration. This paper presents the design, fabrication process, and performance evaluation of self-powered individual Graphene/Silicon on Insulator (GSOI) based Schottky barrier photodiode array (PDA) devices. A 4-element GSOI Schottky barrier PDA with separate graphene electrodes is fabricated to examine possible optical crosstalk encountered between each diode in the array structure. Here, monolayer graphene is utilized as hole collecting separate electrode on individually arrayed n-type Si on SOI substrate by photolithography technique. Each diode in the array exhibited a clear rectifying Schottky character. Photoresponse characterizations revealed that all diodes had excellent device performance even in self-powered mode in terms of an Ilight/Idark ratio up to 104, a responsivity of ∼0.12 A/W, a specific detectivity of around 1.6 × 1012 Jones, and a response speed of ∼1.32 μs at 660 nm wavelength. As revealed by optical crosstalk measurement, the device with pixel pitch of 1.5 mm had a total crosstalk of about 0.10% (−60 dB) per array. These results showed that the optical crosstalk between neighboring n-Si elements can be greatly minimized when graphene is used as separated electrode on arrayed Si on SOI substrate. Our study is expected give an insight into the performance characteristics of GSOI PDA devices which have great potential to be used in many technological applications such as multi-wavelength light measurement, level metering, high-speed photometry and position/motion detection. © 2023 Elsevier B.V.Article Citation - WoS: 20Citation - Scopus: 23Surface Free Energy and Wettability Properties of Transparent Conducting Oxide-Based Films With Ag Interlayer(Elsevier, 2021) Özbay, Salih; Erdoğan, Nursev; Erden, Fuat; Ekmekçioğlu, Merve; Rakop, Büşra; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiITO, ZTO, AZO and Ag are commonly used in transparent conducting oxide (TCO)/metal/TCO electrodes to form multilayered thin films on a suitable substrate. A detailed surface free energy (SFE) knowledge of these films is critical to design desirable TCO-based sandwich structures. In this study, TCO/Ag/TCO multilayer thin films were coated onto glass substrates using ITO, ZTO, AZO and Ag targets by magnetron sputtering. The wettability properties of TCO, Ag interlayer and TCO/Ag/TCO were evaluated by contact angle measurements of seven different liquids having various surface tension values. The dispersive and polar components of SFE were calculated using geometric and harmonic mean approaches. The acidic and basic components of SFE were calculated using van Oss-Chaudhury-Good method. Following this, the work of adhesion values between TCO films and Ag interlayer were estimated using SFE values of the films. The results show that the SFE components of the surfaces differ depending on the TCO type, the total SFE values of TCO/Ag/TCO films were lower than that of TCO films, and AZO/Ag adhesion is stronger than the other TCO/Ag structures. The reasons behind these differences were discussed by evaluating the SFE, XRD, AFM and SEM analysis simultaneously.Article Citation - WoS: 1Citation - Scopus: 1Substrate Effects on Electrical Parameters of Dirac Fermions in Graphene(Elsevier, 2021) Tıraş, Engin; Ardalı, Şükrü; Fırat, Hakan Asaf; Arslan, Engin; Özbay, EkmelThe substrate effects on the electronic transport properties of single-layer graphene on TiO2/Si substrate have been studied. The Hall mobility, sheet carrier density, and transport lifetime were obtained from the temperature-dependent Hall measurements, while the in-plane effective mass, quantum lifetime was obtained from the temperature-dependent variation of the Shubnikov de Haas (SdH) oscillations that were made at 1.8 to 45 K temperature range and up to the magnetic field of 11 T. The measurement results showed that in SLG/TiO2/ Si sample, there were 2.36 +/- 0.12x1016 m-3 amounts of 3D carriers coming from the substrate. In our previous studies, 3D carrier densities were measured as 6.07x1016 m-3 and zero for SLG/SiO2/Si and SLG/SiC sample, respectively. This result shows that the 3D carriers formed in the structure are significantly changed by a substrate. The scattering mechanisms were determined using the zt/zq ratio. The ratio values obtained as 3.66. This value obtained was compared with the values we found for SLG/SiC (zt/zq=1.36) sample and SLG/TiO2/Si (zt/zq=3.08) sample our previous study. The results show that small-angle scattering is dominant in SLG/SiC sample, but large-angle scattering is dominant in SLG/SiO2/Si and SLG/TiO2/Si samples. The charged impurity scattering is the dominant scattering mechanism in SLG/TiO2/Si and SLG/SiO2/Si samples, whereas in SLG/SiC samples, a short-range scattering mechanism such as lattice defects can be said to affect the electronic transport.Article Citation - WoS: 12Citation - Scopus: 17Junction Area Dependent Performance of Graphene/Silicon Based Self-Powered Schottky Photodiodes(Elsevier, 2021) Fidan, Mehmet; Ünverdi, Özhan; Çelebi, CemThis work reports the impact of junction area on the device performance parameters of Graphene/n-Silicon (Gr/n-Si) based Schottky photodiodes. Herein, three batches of Gr/n-Si photodiode samples were produced based on various sized CVD grown monolayer graphene layers transferred on individual n-Si substrates. The fabricated devices exhibited strong Schottky diode character and had high spectral sensi-tivity at 905 nm peak wavelength. The optoelectronic measurements showed that the spectral response of Gr/n-Si Schottky photodiodes has a linear dependence on the active junction area. The sample with 20 mm(2) junction area reached a spectral response of 0.76 AW(-1), which is the highest value reported in the literature for self-powered Gr/n-Si Schottky photodiodes without the modification of graphene electrode. In contrast to their spectral responsivities, the response speed of the samples were found to be lowered as a function of the junction area. The experimental results demonstrated that the device performance of Gr/n-Si Schottky photodiodes can be modified simply by changing the size of the graphene electrode on n-Si without need of external doping of graphene layer or engineering Gr/n-Si interface. This study may serve towards the standardization of junction area for the development of high performance Gr/Si based optoelectronic devices such as solar cells and photodetectors operating in between the ultraviolet and near-infrared spectral region. (C) 2021 Elsevier B.V. All rights reserved.Article Citation - WoS: 9Citation - Scopus: 9A Family-nonuniversal U(1)′ model for excited beryllium decays(Elsevier, 2021) Puliçe, BeyhanExcited beryllium has been observed to decay into electron-positron pairs with a 6.8 sigma anomaly. The process is properly explained by a 17 MeV proto-phobic vector boson. In present work, we consider a family-nonuniversal U(1)' that is populated by a U(1)' gauge boson Z ' and a scalar field S, charged under U(1)' and singlet under the Standard Model (SM) gauge symmetry. The SM chiral fermion and scalar fields are charged under U(1)' and we provide them to satisfy the anomaly-free conditions. The Cabibbo-Kobayashi-Maskawa (CKM) matrix is reproduced correctly by higher-dimension Yukawa interactions facilitated by S. The vector and axial-vector current couplings of the Z ' boson to the first generation of fermions do satisfy all the bounds from the various experimental data. The Z ' boson can have kinetic mixing with the hypercharge gauge boson and S can directly couple to the SM-like Higgs field. The kinetic mixing of Z ' with the hypercharge gauge boson, as we show by a detailed analysis, generates the observed beryllium anomaly. We find that beryllium anomaly can be properly explained by a MeV-scale sector with a minimal new field content. The minimal model we construct forms a framework in which various anomalous SM decays can be discussed.Article Citation - WoS: 63Citation - Scopus: 73Angular Analysis and Branching Fraction Measurement of the Decay B-0 -> K*(0)mu(+)mu(-)(Elsevier, 2013) Demir, Durmuş Ali; Karapınar, GülerThe angular distributions and the differential branching fraction of the decay B-0 -> K*(892)(0)mu(+)mu(-) are studied using a data sample corresponding to an integrated luminosity of 5.2 fb(-1) collected with the CMS detector at the LHC in pp collisions at root s = 7 TeV. From more than 400 signal decays, the forward-backward asymmetry of the muons, the K*(892)(0) longitudinal polarization fraction, and the differential branching fraction are determined as a function of the square of the dimuon invariant mass. The measurements are in good agreement with standard model predictions. (C) 2013 CERN. Published by Elsevier B.V. All rights reserved.Article Citation - WoS: 39Citation - Scopus: 46Surface Free Energy Analysis of Ito/Au Multilayer Thin Films on Polycarbonate Substrate by Apparent Contact Angle Measurements(Elsevier, 2020) Özbay, Salih; Erdoğan, Nursev; Erden, Fuat; Ekmekçioğlu, Merve; Özdemir, Mehtap; Aygün, Gülnur; Özyüzer, LütfiA detailed surface free energy (SFE) knowledge of transparent conducing oxide (TCO)/metal/TCO electrodes is necessary for their applications related to surface wettability. However, SFE analysis of these surfaces has not been performed systematically previously. In this study, ITO and ITO/Au/ITO multilayer thin films were coated onto O-2 plasma treated polycarbonate (PC) substrates by magnetron sputtering. The wettability characteristics of untreated PC, O-2 plasma treated PC, ITO, Au interlayer, and ITO/Au/ITO multilayer thin films were evaluated by apparent contact angle measurements of nine different test liquids having various surface tensions. Following this, Lifshitz-van der Waals, acidic, basic, dispersive, and polar components of SFE were calculated using acidbase, geometric and harmonic mean approaches. In the present study, in which the significance of calculation methods and selected liquid pairs on SFE parameters were investigated, the effect of Au interlayer presence on SFE parameters were also evaluated simultaneously. The results showed that the total SFE values of ITO/Au/ITO multilayer thin films were found to be higher than that of ITO surface. The reasons behind this difference were discussed in terms of SFE components obtained using various liquid pairs by different methods. The results were also supported with XRD, XPS, AFM, and TEM analysis.