Browsing by Author "Fidan, Mehmet"

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Performance enhancement of graphene/silicon based near-infrared Schottky photodiodes
(Izmir Institute of Technology, 2022-07) Fidan, Mehmet; Çelebi, Cem
This thesis presents an experimental investigation on the performance enhancement of graphene/silicon based near-infrared Schottky photodiodes. The photodiode devices were fabricated by transferring CVD graphene layers onto n-type silicon (n-Si) substrates. The samples exhibited strong Schottky diode character and had high spectral sensitivity at 905 nm peak wavelength. The Schottky contact characteristics of the samples (e.g., barrier height, ideality factor and sheet resistance) were determined by analyzing the current-voltage measurement data. All the samples demonstrated a clear photovoltaic activity under light illumination. The Schottky barrier height (SBH) in Gr/n-Si photodiodes was tuned as a function of light power density. Light power density driven modification of the SBH was correlated with the variation in the measured open-circuit voltage. The impact of junction area and number of graphene layers on the spectral responsivity and response speed of Gr/n-Si based Schottky photodiodes were also investigated. Firstly, three batches of Gr/n-Si photodiode samples with junction area of 4 mm2, 12 mm2 and 20 mm2 were produced by transferring monolayer CVD graphene on individual n-Si substrates. The sample with 20 mm2 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 was found to be lowered as a function of the junction area. After that, we increased the number of graphene layers on n-Si. 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. This thesis showed that the device performance of Gr/n-Si Schottky photodiodes can be modified simply by changing the size of graphene electrode and/or as well as the number of graphene layers on n-Si without need of external doping of graphene layer or engineering Gr/n-Si interface.
Structural and magnetic characterization of nitrogen ion implanted stainless steel and CoCrMo alloys
(Izmir Institute of Technology, 2014) Fidan, Mehmet; Öztürk, Orhan
Ion beam surface modification methods can be used to create hard and wear resistant surface layers with enhanced corrosion resistance on austenitic stainless steels (SS) and CoCr base alloys using nitrogen ions. This is mainly due to the formation of high N content phase, Î³N, at relatively low substrate temperatures from about 350 to 450 ÂºC. This surface layer is known as an expanded austenite layer. Different N contents and diffusion rates depending on grain orientations as well as anisotropic lattice expansion and high residual stresses are some peculiar properties associated with the formation of this phase. Another peculiar feature of the expanded austenite phase is related to its magnetic character. In this study, new data related to the magnetic nature of the expanded austenite layers on austenitic stainless steel (304 SS) and CoCrMo alloy by nitrogen plasma immersion ion implantation (PIII) are presented. Magnetic behaviour, nitrogen distribution, implanted layer phases, surface topography, and surface hardness were studied with a combination of experimental techniques involving magnetic force microscopy, SIMS, XRD, SEM, AFM and nanoindentation method. The experimental analyses indicate that the low temperature samples clearly show the formation of the expanded austenite phase, while the decomposition of this metastable phase into CrN precipitates occurs at higher temperatures. As a function of the processing temperature, phase evolution stage for both alloys follows the same trend: (1) initial stage of the expanded phase, Î³N, formation; (2) its full development, and (3) its decomposition into CrN precipitates and the Cr-depleted matrix, fcc Î³-(Co,Mo) for CoCrMo and bcc Î±-(Fe,Ni) for 304 SS. MFM imaging reveals distinct, stripe-like ferromagnetic domains for the fully developed expanded austenite layers both on 304 SS and CoCrMo alloys. Weak domain structures are observed for the CoCrMo samples treated at low and high processing temperatures. The images also provide strong evidence for grain orientation dependence of magnetic properties. The ferromagnetic state for the Î³N phase observed here is mainly linked to large lattice expansions due to high N content.