Browsing by Author "Ünverdi, Özhan"

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Citation - WoS: 3
Citation - Scopus: 3
Enhancing the photo-response characteristics of graphene/n-Si based Schottky barrier photodiodes by increasing the number of graphene layers
(AVS Science and Technology Society, 2022) Mehmet Fidan; Özhan Ünverdi; Cem Celebi; Fidan, Mehmet; celebi, Cem; uenverdi, Oezhan; Ünverdi, Özhan
The impact of the number of graphene layers on the spectral responsivity and response speed of graphene/n-type Si (Gr/n-Si)-based Schottky barrier photodiodes is investigated. Gr/n-Si photodiode devices are fabricated by transferring chemical vapor deposition-grown graphene layers one by one on n-Si substrates reaching up to three graphene layers. The devices show a clear rectifying Schottky character and have a maximum responsivity at a peak wavelength of 905 nm. Wavelength-resolved and time-dependent photocurrent measurements demonstrated that both spectral responsivity and response speed are enhanced as the number of graphene layers is increased from 1 to 3 on n-Si substrates. For example the spectral responsivity and the response speed of the fabricated device were found to be improved by about 15% (e.g. from 0.65 to 0.75 AW-1) and 50% (e.g. 14 to 7 μs) respectively when three graphene layers are used as the hole-collecting cathode electrode. The experimentally obtained results showed that the device parameters such as spectral responsivity and response speed of Gr/n-Si Schottky barrier photodiodes can be boosted simply by increasing the number of graphene layers on n-Si substrates. © 2022 Elsevier B.V. All rights reserved.
NANOTRIBOLOGICAL PROPERTIES OF EPITAXIAL GRAPHENE GROWN ON CTERMINATED FACE OF SILICON CARBIDE SEMICONDUCTOR
(2018) Özhan ÜNVERDİ; Ünverdi, Özhan
The 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.
Citation - WoS: 1
Passive 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, Cem
We 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.
Citation - WoS: 4
Citation - Scopus: 5
The 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 İnönü
The 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.
Citation - WoS: 21
Citation - Scopus: 21
Three-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.