Physics / Fizik

Permanent URI for this collectionhttps://hdl.handle.net/11147/6

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Author: search.filters.author.Çelebi, CemInstitution Author: search.filters.institutionAuthor.Kalkan, Sırrı Batuhan

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Now showing 1 - 5 of 5
  • Article
    Citation - WoS: 2
    Citation - Scopus: 2
    Impact of Encapsulation Method on the Adsorbate Induced Electrical Instability of Monolayer Graphene
    (AVS Science and Technology Society, 2019) Kalkan, Sırrı Batuhan; Yanılmaz, Alper; Çelebi, Cem
    Monolayer graphene transferred onto a set of silicon carbide (SiC) substrates was encapsulated with a thin SiO2 film in order to prevent its interaction with atmospheric adsorbates. The encapsulation of graphene samples was realized by using two different thin film growth methods such as thermal evaporation (TE) and state-of-the-art pulsed electron deposition (PED). The encapsulation efficiency of these two techniques on the structural and electrical characteristics of graphene was compared with each other. Scanning electron microscopy (SEM) analysis showed that unlike the SiO2 thin film grown with PED, structural defects like cracks were readily formed on TE grown films due to the lack of surface wettability. The electronic transport measurements revealed that the electrical resistivity of graphene has been increased by two orders of magnitude, and the carrier mobility has been subsequently decreased upon the encapsulation process with the PED method. However, in-vacuum transient photocurrent spectroscopy (TPS) measurements conducted for short periods and a few cycles showed that the graphene layer encapsulated with the PED grown SiO2 film is electrically far more stable than the one encapsulated with TE grown SiO2 film. The results of TPS measurements were related to the SEM images to unravel the mechanism behind the improved electrical stability of graphene samples encapsulated with the PED grown SiO2 film.
  • Article
    Citation - WoS: 6
    Citation - Scopus: 7
    Epitaxial Graphene Thermistor for Cryogenic Temperatures
    (Elsevier, 2018) Kalkan, Sırrı Batuhan; Yiğen, Seren; Çelebi, Cem
    The thermal responsivity of monolayer epitaxial graphene grown on the Si-face surface of semi-insulating SiC substrate is investigated as a function of temperature below 300 K. The measurements showed that adsorption/desorption of atmospheric adsorbates can randomly modify the electrical characteristics of graphene which is indeed undesirable for consistent temperature sensing operations. Therefore, in order to avoid the interaction between graphene layer and adsorbates, the grown graphene layer is encapsulated with a thin SiO2 film deposited by Pulsed Electron Deposition technique. Temperature dependent resistance measurement of encapsulated graphene exhibited a clear thermistor type behavior with negative temperature coefficient resistance character. Both the sensitivity and transient thermal responsivity of the SiO2/graphene/SiC sample were found to be enhanced greatly especially for the temperatures lower than 225 K. The experimentally obtained results suggest that SiO2 encapsulated epitaxial graphene on SiC can be used readily as an energy efficient and stable temperature sensing element in cryogenic applications.
  • Article
    Citation - WoS: 28
    Citation - Scopus: 30
    P3HT-graphene bilayer electrode for Schottky junction photodetectors
    (IOP Publishing Ltd., 2018) Aydın, Hasan; Kalkan, Sırrı Batuhan; Varlıklı, Canan; Çelebi, Cem
    We have investigated the effect of a poly (3-hexylthiophene-2.5-diyl)(P3HT)-graphene bilayer electrode on the photoresponsivity characteristics of Si-based Schottky photodetectors. P3HT, which is known to be an electron donor and absorb light in the visible spectrum, was placed on CVD grown graphene by dip-coating method. The results of the UV-vis and Raman spectroscopy measurements have been evaluated to confirm the optical and electronic modification of graphene by the P3HT thin film. Current-voltage measurements of graphene/Si and P3HT-graphene/Si revealed rectification behavior confirming a Schottky junction formation at the graphene/Si interface. Time-resolved photocurrent spectroscopy measurements showed the devices had excellent durability and a fast response speed. We found that the maximum spectral photoresponsivity of the P3HT-graphene/Si photodetector increased more than three orders of magnitude compared to that of the bare graphene/Si photodetector. The observed increment in the photoresponsivity of the P3HT-graphene/Si samples was attributed to the charge transfer doping from P3HT to graphene within the spectral range between near-ultraviolet and near-infrared. Furthermore, the P3HT-graphene electrode was found to improve the specific detectivity and noise equivalent power of graphene/Si photodetectors. The obtained results showed that the P3HT-graphene bilayer electrodes significantly improved the photoresponsivity characteristics of our samples and thus can be used as a functional component in Si-based optoelectronic device applications.
  • Article
    Citation - WoS: 11
    Citation - Scopus: 11
    The Effect of Adsorbates on the Electrical Stability of Graphene Studied by Transient Photocurrent Spectroscopy
    (American Institute of Physics, 2018) Kalkan, Sırrı Batuhan; Aydın, H.; Özkendir, Dicle; Çelebi, Cem
    Adsorbate induced variations in the electrical conductivity of graphene layers with two different types of charge carriers are investigated by using the Transient Photocurrent Spectroscopy (TPS) measurement technique. In-vacuum TPS measurements taken for a duration of 5 ks revealed that the adsorption/desorption of atmospheric adsorbates leads to more than a 110% increment and a 45% decrement in the conductivity of epitaxial graphene (n-type) and chemical vapor deposition graphene (p-type) layers on semi-insulating silicon carbide (SiC) substrates, respectively. The graphene layers on SiC are encapsulated and passivated with a thin SiO2 film grown by the Pulsed Electron Deposition method. The measurements conducted for short periods and a few cycles showed that the encapsulation process completely suppresses the time dependent conductivity instability of graphene independent of its charge carrier type. The obtained results are used to construct an experimental model for identifying adsorbate related conductivity variations in graphene and also in other 2D materials with an inherently high surface-to-volume ratio.
  • Master Thesis
    The Effect of Atmospheric Gases on the Electrical Stability of Graphene
    (Izmir Institute of Technology, 2017) Kalkan, Sırrı Batuhan; Çelebi, Cem
    In this thesis, adsorbate induced variations in the electrical conductivity of graphene layers with two different types of charge carriers are investigated experimentally by using Transient Photocurrent Spectroscopy (TPS) method. In-vacuum TPS measurements taken for a duration of 5 ks, revealed that the adsorption/desorption of atmospheric adsorbates leads to a 45 % incerment and 110 % decrement in the conductivity of CVD graphene (p--type) and epitaxial graphene (n-type) layers on semi-insulation (SI) Silicon Carbide (SiC) substrates, respectively. The graphene layers on SI-SiC substrates are encapsulated and passivated with thin SiO2 film grown by Thermal Evaporation and Pulsed Electron Deposition (PED) techniques. The mesurements conducted for short periods and a few cycles showed that the thermal passivation of graphene layers is insufficient. However, the PED encapsulation process completely suppresses the time-dependent conductivity instability of graphene independent of its charge carrier type. The obtained results are used the construct an experimental model for identifying adsorbate related conductivity variations in graphene and also in other 2D materials with inherently high surface-to-volume ratio.