Ünverdi, Özhan
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Dr.Öğr.Üyesi
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01.01.09.02. Elektrik- Elektronik Mühendisliği
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144
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| Journal | Count |
|---|---|
| Sensors and Actuators A: Physical | 3 |
| Applied Physics Letters | 2 |
| Carbon | 1 |
| Anadolu Üniversitesi Bilim Ve Teknoloji Dergisi - B Teorik Bilimler | 1 |
| Journal of Alloys and Compounds | 1 |
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12 results
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Article Citation - WoS: 38Citation - Scopus: 38CVD graphene/SiC UV photodetector with enhanced spectral responsivity and response speed(Elsevier B.V., 2023) Ala K. Jehad; Mehmet Fidan; Özhan Ünverdi; Cem Celebi; Fidan, Mehmet; Celebi, Cem; Jehad, Ala K.; Unverdi, OzhanA 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. All rights reserved.Article Citation - WoS: 4Citation - Scopus: 4Light-induced modification of the Schottky barrier height in graphene/Si based near-infrared photodiodes(ELSEVIER, 2022) M. Fidan; G. Donmez; A. Yanilmaz; O. Unverdi; C. Celebi; Donmez, G.; Fidan, M.; Celebi, C.; Yanilmaz, A.; Unverdi, O.The impact of light on the Schottky barrier height (SBH) in p-type graphene/n-type Si (p-Gr/n-Si) based nearinfrared photodiodes is investigated. Hall effect and optoelectronic transport measurements carried out under illumination of 905 nm wavelength light showed that zero-bias SBH in such photodiodes can be effectively tuned in a range between 0.7 and 0.9 eV consistent with the variation in their open-circuit voltage. Shockley-Read-Hall model which considers the charge recombination through mid-gap and interface states at the p-Gr/n-Si heterojunction is used to explain the experimentally observed nonlinear dependence of SBH on the incident light. Light induced tunability of SBH at the graphene/semiconductor heterojunction is of great importance especially for the development of new generation optically driven devices in which graphene acts as a functioning element.Article Citation - WoS: 5Citation - Scopus: 4Graphene/SOI-based self-powered Schottky barrier photodiode array(AIP Publishing, 2022) A. Yanilmaz; M. Fidan; O. Unverdi; C. Celebi; Fidan, M.; Çelebi, C.; Yanilmaz, A.; Unverdi, O.We have fabricated a four-element graphene/silicon on insulator (SOI) based Schottky barrier photodiode array (PDA) and investigated its optoelectronic device performance. In our device design monolayer graphene is utilized as a common electrode on a lithographically defined linear array of n-type Si channels on a SOI substrate. As revealed by wavelength resolved photocurrent spectroscopy measurements each element in the PDA structure exhibited a maximum spectral responsivity of around 0.1 A/W under a self-powered operational mode. Time-dependent photocurrent spectroscopy measurements showed excellent photocurrent reversibility of the device with ~1.36 and ~1.27 mu s rise time and fall time respectively. Each element in the array displayed an average specific detectivity of around 1.3 x 10(12) Jones and a substantially small noise equivalent power of ~0.14 pW/Hz(-1/2). The study presented here is expected to offer exciting opportunities in terms of high value-added graphene/Si based PDA device applications such as multi-wavelength light measurement level metering high-speed photometry and position/motion detection.Article Citation - WoS: 4Citation - Scopus: 5The role of charge distribution on the friction coefficients of epitaxial graphene grown on the Si-terminated and C-terminated faces of SiC(Elsevier Ltd, 2021) Yasemin Keskin; Özhan Ünverdi; Dogan Erbahar; Ismet I. Kaya; Cem Celebi; Çelebi, Cem; Ünverdi, Özhan; Keskin, Yasemin; Erbahar, Dogan; Kaya, İsmet İnönüThe friction coefficients of single-layer epitaxial graphene grown on the Si-terminated and C-terminated faces of Silicon Carbide (SiC) substrate were measured under ambient conditions using Friction Force Microscope (FFM). The lateral friction force measurements acquired in the applied normal force range between 4.0 and 16.0 nN showed that the friction coefficient of graphene on the C-terminated face of SiC is about two times smaller than the one grown on its Si-terminated face. The lateral friction was found to be decreased as the average of root mean square roughness increases suggesting the observed difference in the friction coefficients cannot be related to the roughness of the graphene layers. DFT calculations demonstrated that the altered periodicity of charge distribution on graphene due to the specific interactions with two distinct polar faces of SiC substrate might explain the observed difference in the friction coefficients. © 2021 Elsevier B.V. All rights reserved.Article Citation - WoS: 1Passive matrix Schottky barrier 2D photodiode array on graphene/SOI platform(Springer Science and Business Media Deutschland GmbH, 2025) Alper Yanilmaz; Özhan Ünverdi; Cem Celebi; Yanilmaz, Alper; Ünverdi, Özhan; Çelebi, CemWe fabricated 4 × 4 pixel two-dimensional (2D) photodiode array (PDA) out of monolayer graphene and n-type silicon (n-Si) electrodes on a silicon-on-insulator (SOI) substrate. Our device design is based on passive matrix sensor array architecture consisting of individual graphene and silicon electrodes aligned perpendicular to each other. I-V measurements conducted at room temperature to reveal the electronic characteristics of graphene and Si junction in the device structure. The spectral responsivity respond speed and the optical crosstalk of each G/Si pixels in the array have been determined by wavelength resolved and time dependent photocurrent spectroscopy measurements. Micro-Raman mapping measurements were conducted to examine the surface coverage of graphene electrode on each pixel. The results of Micro-Raman mapping measurements were correlated with the corresponding photocurrent data acquired under light illumination. We believe that this work constitutes a significant potential in integrating variety of 2D materials and SOI technology into next generation image sensing and multiple pixel light detection applications. © 2025 Elsevier B.V. All rights reserved.Article Citation - WoS: 21Citation - Scopus: 21Three-dimensional interaction force and tunneling current spectroscopy of point defects on rutile TiO2(110)(American Institute of Physics Inc. subs@aip.org, 2016) Mehmet Z. Baykara; Harry Mönig; Todd C. Schwendemann; Özhan Ünverdi; Eric I. Altman; Udo Dietmar Schwarz; Schwarz, Udo D.; Altman, Eric I.; Uenverdi, Oezhan; Baykara, Mehmet Z.; Ünverdi, Özhan; Mönig, Harry; Moenig, Harry; Schwendemann, Todd C.The extent to which point defects affect the local chemical reactivity and electronic properties of an oxide surface was evaluated with picometer resolution in all three spatial dimensions using simultaneous atomic force/scanning tunneling microscopy measurements performed on the (110) face of rutile TiO2. Oxygen atoms were imaged as protrusions in both data channels corresponding to a rarely observed imaging mode for this prototypical metal oxide surface. Three-dimensional spectroscopy of interaction forces and tunneling currents was performed on individual surface and subsurface defects as a function of tip-sample distance. An interstitial defect assigned to a subsurface hydrogen atom is found to have a distinct effect on the local density of electronic states on the surface but no detectable influence on the tip-sample interaction force. Meanwhile spectroscopic data acquired on an oxygen vacancy highlight the role of the probe tip in chemical reactivity measurements. © 2018 Elsevier B.V. All rights reserved.Article NANOTRIBOLOGICAL PROPERTIES OF EPITAXIAL GRAPHENE GROWN ON CTERMINATED FACE OF SILICON CARBIDE SEMICONDUCTOR(2018) Özhan ÜNVERDİ; Ünverdi, ÖzhanThe frictional properties of mono-layer and multilayer epitaxial graphene grown on the C terminated face of SiC has beeninvestigated by using atomic force microscopy measurements. Epitaxially grown graphene samples were characterized byRaman spectroscopy measurements. Atomic force microscopy has been employed in ambient conditions for frictionmeasurements using pre-calibrated cantilevers. Both Raman spectroscopy and atomic force microscopy analysis showed thatthe number of defects which increases consistent with increasing number of graphene layers plays an important role on thetribological properties of epitaxial graphene.Article Citation - WoS: 5Citation - Scopus: 5Self-powered photodetector array based on individual graphene electrode and silicon-on-insulator integration(Elsevier B.V., 2023) Alper Yanilmaz; Özhan Ünverdi; Cem Celebi; Yanilmaz, Alper; Unverdi, Ozhan; Celebi, 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. All rights reserved.Article Citation - WoS: 1Citation - Scopus: 1Comparison of the Photoresponse Characteristics for 4H-SiC Schottky Barrier UV Photodetector with Graphene and Ni/Cr Electrode(Elsevier, 2026) Dulcel, Atilla Mert; Gozek, Melike; Celebi, Cem; Unverdi, OzhanGr/4H-SiC and Ni/Cr/4H-SiC Schottky junction UV photodetectors were fabricated and investigated to reveal the effect of electrode materials on the device performance such as spectral response and response speed. I-V characterization, spectral response, and response speed (on-off) measurements were conducted for the UV wavelength range between 200 and 400 nm. The maximum photo-responsivity was obtained as 0.081 A/W for Gr/4H-SiC and 0.041 A/W for Ni/Cr/4H-SiC at a wavelength of 260 nm. This result was attributed to the higher optical transmittance of the graphene electrode compared to the semitransparent Ni/Cr electrode. Zero bias response speed measurements were done under 280 nm wavelength UV light pulsed at different frequencies such as 100 Hz, 500 Hz, and 1000 Hz. The Gr/4H-SiC and Ni/Cr/4H-SiC photodetectors show distinctly different decay times of 5.04 ms and 305.1 mu s, respectively, while their rise times were found to be similar. This observation has been explained by the inclination of graphene to act as a trap site for photogenerated holes.Article Citation - WoS: 10Citation - Scopus: 9High voltage response of graphene/4H-SiC UV photodetector with low level detection(Elsevier Ltd, 2023) Ala K. Jehad; Özhan Ünverdi; Cem Celebi; Unverdi, Ozhan; Celebi, Cem; Jehad, Ala K.A 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. © 2023 Elsevier B.V. All rights reserved.
