Master Degree / Yüksek Lisans Tezleri
Permanent URI for this collectionhttps://hdl.handle.net/11147/3008
Browse
11 results
Search Results
Master Thesis Investigation of the Photo-Response of Graphene Silicon Photodetector in the Ultraviolet Region(01. Izmir Institute of Technology, 2024) Kaplan, Çiçek; Çelebi, Cem; Ünverdi, ÖzhanIn this thesis, we focus on the optoelectronic properties of p-type graphene and n-type Silicon (Gr/Si) Schottky barrier photodiode according to the number of layers in the ultraviolet region (UV). The I-V measurements were conducted at an applied bias voltage between -0.5 and 0.5 V for each Gr/Si heterojunction. The I-V measurements taken under dark conditions showed that all Gr/Si samples with 2-, 4- and 6-layers graphene electrodes exhibited rectifying Schottky junction character, but all device's reverse saturation currents (I0) were different. Schottky barrier heights (ΦB) of the samples with 2-, 4- and 6-layers graphene electrodes were determined using the I0 values obtained from I-V measurements. Compared to the ΦB value of the sample with 2 layers graphene electrode, the ΦB of the sample with 4 layers of graphene electrode increased to ~0.82 eV, and then ΦB was found to decrease to ~0.79 eV for the 6-layer graphene electrode. Additionally, photo-response measurements were carried out at zero bias voltage and in the wavelength range of 250 – 400 nm to determine the spectral response (R) of the devices in the UV region. Compared to the device with 2 layers graphene electrode, R of the sample with 4 layers graphene electrode increased by 3 times. The result obtained revealed that using 4-layer graphene as a light-transparent electrode, Gr/Si Schottky barrier photodiode is the most applicable option for sensitive detection of light in the UV region.Master Thesis The Impact of Adsorbates on the Optoelectronic Properties of Graphene/Silicon Based Schottky Barrier Photodiodes(01. Izmir Institute of Technology, 2020) Şahan, Nusret; Çelebi, CemThe aim of this study is to investigate the effect of atmospheric adsorbates on the electronic and optoelectronic properties of graphene/n-type Silicon (Gr/n-Si) based Schottky barrier photodiodes. Wavelength resolved photocurrent spectroscopy and transient photocurrent spectroscopy measurements conducted under high-vacuum conditions revealed that the adsorbates cause hole doping in graphene and hence increase the zero-bias Schottky barrier height of the Gr/n-Si heterojunction from 0.71 to 0.78 eV. Adsorbate induced increment in the barrier height promotes the separation of photo-excited charge carriers at the depletion region of the heterojunction and leads to an improvement in the maximum spectral response (e.g., from 0.39 to 0.46 A W^-1) and response speed of the Gr/n-Si photodiode in the near-infrared region. The experimentally obtained results are expected to give an insight into the adsorbate induced variations in the rectification and photo-response characters of the heterojunctions of graphene and other 2D materials with different semiconductors.Master Thesis Fabrication and Characterization of Graphene/Silicon Based Schottky Photodiode(Izmir Institute of Technology, 2019) Dönmez, Gülçin; Çelebi, Cem; Sarı, EmreThis thesis focused on fabrication and characterization of CVD grown p-type graphene and n-type Si Schottky junction photodiode with rectification behavior. The device operated at wavelength range between 390 and 1100 nm at self-powered mode. The device was encapsulated with Epoxy Resin to prevent graphene from atmospheric adsorbates. The electronic and optoelectronic characterizations of the devices were done before and after coating the devices with ER. By encapsulation stability of the device was enhanced in terms of photoresponsivity. The maximum obtained photoresponsivity value of the bare device was 0.56 A/W. Also, time-resolved photocurrent spectroscopy measurements showed that the devices exhibited enhanced photodetector performance in terms of photo-switching characteristics. Furthermore, electrical characteristics of Gr/n-Si Schottky photodiode under various illumination power densities with 850 nm wavelength were investigated. The short circuit current showed linear response to power density. However, open circuit voltage exhibited two phased slow and fast increment with increased power density. Hall effect measurements were conducted in order to investigate hole carrier concentration and mobility of the graphene on n-Si. With increasing the power density the carrier concentration increased and the mobility decreased. Besides, light induced manipulation of the Schottky barrier height of Gr/n-Si photodiode was studied. Schottky barrier height of the graphene measured by KPFM method as 0.4 eV. With increasing power density we found that Schottky barrier height of the device increased from 0.4 eV to 0.5 eV and showed similar trend with the change in open circuit voltage.Master Thesis Graphene Transfer Approaches With Different Support Materials on the Substrates With Cavities(Izmir Institute of Technology, 2019) Duman, Sinem; Balantekin, Müjdat; Çelebi, CemA micro capacitive sensor characteristically embraces a thin conductive membrane which is freely suspended above an immovable counter electrode in a parallel plate geometry. Such capacitive structures are found in broad range of applications as a transducer like capacitive micro-machined ultrasonic transducer (CMUT), pressure sensor, resonator and biological or chemical material sensing element. The input can be an ultrasound wave, pressure, chemical or biological mass attachment which result in the deflection of the membrane. Emerging nano materials have shown great potential as candidates for generation of nano and micro electromechanical systems (NEMS, MEMS). Among these nano materials, graphene is regarded as a promising material because of its ultra low mass, thickness, high surface to volume ratio, flexibility, and extraordinary electrical and mechanical properties. However, the transfer of graphene on substrates with micro scale cavities is challenging since the fabrication of large area membranes with a smaller air gap often results in membrane tearing or collapse driven by capillary or electrostatic forces. This study presents a research on the fabrication and the characterization of graphene membranes to be used in micro capacitive sensor applications. Substrates which span a large array of circular and hexagonal micro cavities between 2-100 μm in diameter are fabricated. Graphene transfer with different support materials are studied to fabricate graphene micro membranes. Up to 5 μm diameter membranes on 300 nm deep cavities are demonstrated via scanning electron microscope (SEM) and atomic force microscope (AFM) tools.Master Thesis Thermal Performance of Graphene Coating on Copper(Izmir Institute of Technology, 2019) Ersavaş, Gizem; Toprak, Kasım; Çelebi, Cem; Toprak, Kasım; Çelebi, CemOver heat is always a problem for electronic devices because the locally generated heat cannot be transferred appropriately to the corresponding heat sink fast enough. This situation leads to affect materials’ structures, mechanical properties and conductivities badly. In order to avoid this problem, high thermal conductivity materials are used to dissipate the heat quickly. Thanks to the development of technology, the size of the electronic devices is reduced day by day. This also shrinks the size of the interconnect components. So this situation leads to researchers to investigate nano-sized interconnect components and copper, which is a widely used material, is one of them. Copper is one of the preferred metals for electronic devices because of high thermal conductivity, easy processability, and high use in daily life and industry. For example, copper components, which is used in electronic, are getting so thin and must carry so much current. And that causes to increase friction. Thus heat is occurred. Consequently, cooling problems have arisen. And if the material’s cooling problem won’t be solved then the material can be damaged. It is thought that to overcome this problem, coating with a high thermal conductivity material such as graphene, the thermal conductivity can be improved. In this study, thermal performance of graphene-coated copper were investigated numerically and experimentally. This study consist of two main sections. The first part, MD simulation code was created using C++ programming language to investigate thermal conductivity of copper, different number of graphene layers and these graphene layers were coated on copper in different length, width, height and temperature. In the second part, the thermal performance of pure copper, annealed copper, a layer of graphene-coated copper, and multilayer graphene-coated copper was studied by the experimental setup at three different temperatures and volume flow rates.Master Thesis Nanotribological Properties of Graphene Grown on Silicon Carbide Semiconductor(Izmir Institute of Technology, 2018) Keskin, Yasemin; Çelebi, Cem; Ünverdi, ÖzhanIn this thesis, nanotribological properties of single and multilayer graphene grown on two sides of the Silicon Carbide (SiC) semiconductors were investigated. For this purpose, epitaxial growth technique was used to obtain single-layer graphene on both C-face and Si-face. This thesis consists of two purposes: One of them is to investigate the nanotribological properties of the single and multilayer graphene grown on C-face of SiC and the other one is to compare nanotribological properties of the single layer graphene on two sides of SiC. Graphene, a two-dimensional semi-metal material, was grown epitaxially on the SiC surface under ultra-high vacuum conditions. In epitaxial method, direct current heating is applied to the SiC substrate to vaporize Si atoms from the surface. As the Si atoms evaporate, the remaining C atoms form a graphene layers on top. When single layer graphene is formed on the Si-face, multilayer graphene is formed on the C-face at the same parameters. For this reason, two different samples of graphene were needed in order to compare the tribological properties of the single layer graphene grown on both Si-face and C-face for the secondary objective. A capping method was used to control the rate of Si atoms evaporating from the SiC surface. By this way, single layer graphene on the C-face was obtained too. Number of layers were determined by Raman Spectroscopy. Nanotribological characterizations were done with Atomic Force Microscopy. The experimental results showed that single layer graphene on the Si-face has higher friction coefficient compared to single layer graphene on the C-face. It has been found that the single layer graphene (0.02) formed on the C-surface has a lower coefficient of friction than the multilayer graphene (0.82). It is expected that with the support of the theoretical studies on this results will increase the interest in this study by means of these results are new and original for the literature.Master Thesis The Effect of Atmospheric Gases on the Electrical Stability of Graphene(Izmir Institute of Technology, 2017) Kalkan, Sırrı Batuhan; Çelebi, CemIn 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.Master Thesis Synthesis and Nitrogen Doping of Graphene by Chemical Vapor Deposition(Izmir Institute of Technology, 2017) Yanılmaz, Alper; Çelebi, Cem; Adem, UmutControllable carrier transport due to charged impurities in the graphene lattice is still lacking. Doping of graphene by foreign atoms leads to modify its band structure and electro chemical properties. Among numerous potential dopants, nitrogen (N2) is considered to be an excellent candidate to form strong valence bonds with carbon atoms, which would provide n or p-doping according to bonding character of charged-impurity atom. Exposure of graphene lattice to nitrogen gas leads to a change in the carrier concentration and opens a bandgap due to symmetry breaking. Furthermore, this seems to be an effective way to customize the properties of graphene and exploit its potential for various applications. This thesis focuses on the growth of graphene by low pressure chemical vapor deposition (LPCVD) and doping it with N2 by using N2 plasma treatment. Here, copper foil was used as the catalytic substrate to grow large area graphene at LPCVD system. The grown graphene was transferred onto SiO2, Au (111) and Sapphire substrates. The effect of different plasma time and power on doping process was investigated while keeping the N2 flow rates constant by using N2 plasma. The nitrogen doped graphene (N-graphene) was characterized via Raman Spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy/spectroscopy (STM/STS), Kelvin probe force microscopy (KPFM). Raman mapping of N-graphene was also conducted to show the homogeneity of N2 incorporation into graphitic lattice. STM results were theoretically modelled by using density functional theory (DFT). Our results provide the opportunity to produce N-graphene with homogenous and effective doping which would be valuable in electronic and optoelectronic applications.Master Thesis Self-Organized Network of Silicon Oxide on Epitaxial Graphene(Izmir Institute of Technology, 2017) Özkendir, Dilce; Çelebi, CemIn this thesis, I studied the formation and characterization of self-organized hexagonal-shaped SiO2 wrinkle structures on epitaxial graphene that was grown on SiC substrate. Monolayer graphene was grown by annealing the SiC substrate at high temperatures under ultra-high vacuum conditions. Following the growth process, SiO2 thin film was deposited on epitaxial graphene layer at different deposition temperatures by thermal evaporation method. We found that SiO2 film wrinkles on epitaxial graphene. The origin of the hexagonal shaped wrinkle structures were derived from the thermal expansion coefficient difference between epitaxial graphene and SiO2 thin film. The mesh density of these SiO2 hexagonal wrinkle structures was controlled by changing the cooling rate of the substrate after the thin film deposition. To make a comparison, SiO2 thin film was also deposited on CVD grown graphene and on bare SiC substrate. Unlike on the bare SiC surface, SiO2 thin film on epitaxial graphene exhibited a self-assembled network of hexagonally shaped wrinkles due to thermally induced compressive strain between the two materials. The observed network of wrinkles were found to be comprised of line shaped primary and secondary types of protrusions with distinct topographic characteristics as determined by optical microscopy, Scanning Electron Microscopy and Atomic Force Microscopy measurements. The wrinkle to wrinkle spacing and mesh density of the wrinkle network were modified simply by changing the SiO2 deposition temperature. Our experimental results imply that epitaxial graphene with its high chemical inertness on SiC offers a great potential to be used as a conventional substrate in the realm of thin film metrology.Master Thesis Developing Epitaxial Graphene Electrodes for Silicon Carbide Based Optoelectronic Devices(Izmir Institute of Technology, 2015) Kuşdemir, Erdi; Çelebi, Cem; Sevinçli, HaldunIn this thesis work, I studied the fabrication and characterization of graphene-semiconductor-graphene ultraviolet photodetector based on the rectifying character of Schottky junction at the interface between epitaxial graphene and silicon carbide semiconductor. As-grown single layer epitaxial graphene is interdigitated as transparent conductive electrode to probe photo-generated charge carriers in a semi-insulating 4H-silicon carbide substrate. The fabricated device exhibits the typical current-voltage characteristics of a conventional metal-semiconductor-metal type photodetector with low leakage current. Time-resolved photocurrent measurements suggest an excellent photocurrent reversibility and high response speed of the device. The measurements performed for different illumination wavelengths showed that the sample reveals higher responsivity values when it is exposed to the light with 254 nm wavelength. The obtained results imply that epitaxial graphene can be used readily as transparent conductive electrode for SiC based optoelectronic device applications. Finally, in the last chapter, I discuss how the photoresponsivity of the graphene-semiconductor-graphene photodetector can be enhanced by CdTe/CdS quantum dots. The drop casted CdTe/CdS quantum dots have been shown to increase the photoconductivity of the device. The thickness of the quantum dots is found to effect the enhancement factor of the photoresponsivity of the device.