Browsing by Author "Toprak, Ahmet"

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Open Access
Colorimetric and near-absolute polarization-insensitive refractive-index sensing in all-dielectric guided-mode resonance based metasurface
(American Chemical Society, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Gökbayrak, Murat; Toprak, Ahmet; Bütün, Bayram; Özbay, Ekmel
Colorimetric detection of target molecules with insensitivity to incident-light polarization has attracted considerable attention in recent years. This resulted from the ability to provide rapid output and reduced assay times as a result of color changes upon altering the environment that are easily distinguishable by the naked eye. In this paper, we propose a highly sensitive refractive-index sensor, utilizing the excitation of guided modes of a novel two-dimensional periodically modulated dielectric grating-waveguide structure. The optimized nanosensor can numerically excite guided-mode resonances with an ultranarrow linewidth (full width at half-maximum) of 0.58 nm. Sensitivity is numerically investigated by considering the deposition of dielectric layers on the structure. For a layer thickness of 30 nm, the maximum sensitivity reached as high as 110 nm/refractive index unit (RIU), resulting in a very high figure of merit of 190. The fabricated devices with 30 nm aluminum oxide and zinc oxide coatings achieved a maximum sensitivity of 235.2 nm/RIU with a linewidth of 19 nm. Colorimetric detection with polarization insensitivity is confirmed practically by a simple optical microscope. Samples with different coatings have been observed to have clearly distinct colors, while the color of each sample is nearly identical upon azimuthal rotation. Excellent agreement is obtained between the numerical and experimental results regarding the spectral position of the resonances and sensitivity. The proposed device is, therefore, highly promising in efficient, highly sensitive, almost lossless, and compact molecular diagnostics in the field of biomedicine with personalized, label-free, early point-of-care diagnosis and field analysis, drug detection, and environmental monitoring.
Open Access
Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface
(American Chemical Society, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, Ekmel
Precise control and stabilization of the operating temperature environment of spacecraft and satellites during their life cycle is of paramount importance to increase device reliabilities and reduce the thermomechanical constraints. Optical solar reflectors are the physical interface between the spacecraft and space, and they are broadband mirrors for the solar spectrum, while having strong thermal emission in the mid-infrared part of the electromagnetic spectrum. Strong light–matter interactions in metamaterials and metasurfaces offer significant advantages compared to the conventional methods in performance, weight, launch, and assembly costs. However, the fabrication complexity of these metastructures due to necessitating lithography hinders their upscaling, reproducibility, large-area compatibility, and mass production. In this regard, we propose a facile, lithography-free fabrication route, exploiting oblique deposition to design a metasurface based on disordered and densely packed Indium Tin Oxide (ITO) nanorod forests. The excellent light trapping capability of the nanorod forests, randomness in the geometrical dimensions of these nanorods, combined with the lossy plasmonic nature of ITO in the thermal-infrared range led to strong coupling of thermal radiation to broad plasmonic resonances and, consequently, an experimental emissivity of 0.968, in a very wide range from 2.5 to 25 μm. In the solar spectrum, the low-loss dielectric characteristic of ITO resulted in an experimental solar absorptivity as small as 0.168. Our proposed design with high throughput, robustness, low cost, and high performance, therefore, shows great promise not only for space missions, but also for promoting environmentally friendly passive radiative cooling for our planet and thermal imaging in the field of security labeling.
Open Access
Disordered and densely packed ITO nanorods as an excellent lithography-free optical solar reflector metasurface for the radiative cooling of spacecraft
(SPIE, 2019) Yıldırım, Deniz Umut; Ghobadi, Amir; Soydan, Mahmut Can; Ateşal, Okan; Toprak, Ahmet; Çalışkan, Mehmet Deniz; Özbay, Ekmel
Optical Solar Reflectors (OSRs) form the physical interface between the spacecraft and space and they are essential for the stabilization and uniform distribution of temperature throughout the spacecraft. OSRs need to possess a spectrally selective response of broadband and perfect electromagnetic wave absorption in the thermal-infrared spectral range, while strongly reflecting the solar energy input. In this work, we experimentally show that disordered and densely packed ITO nanorod forests can be used as an excellent top-layer metasurface in a metal-insulator-oxide cavity configuration, and a thermal-emissivity of 0.97 is experimentally realized in the spectral range from 2.5 to 25 μm. The low-loss dielectric response of ITO in the solar spectrum, from 300 nm to 2.5 μm range limited the solar absorptivity to an experimental value of 0.167. These make our proposed design highly promising for its application in space missions due to combining high throughput, robustness, low cost with ultra-high performance.
Open Access
Effect of gate structures on the DC and RF performance of AlGaN/GaN HEMTs
(Institute of Physics Publishing, 2018) Toprak, Ahmet; Osmanoǧlu, Sinan; Öztürk, Mustafa; Yılmaz, Doğan; Cengiz, Ö.; Şen, Ö.; Bütün, Bayram; Özcan, Ş.; Özbay, Ekmel
This work analyzes the effect of various gate structures on the DC and radio frequency (RF) performance of AlGaN/GaN high-electron mobility transistors (HEMTs). AlGaN/GaN HEMT devices with a 3 μm drain-to-source spacing, 125 μm gate width and 0.3 μm gate length in various gate structures were fabricated to achieve the desired frequency response with a robust, high yield, and repeatable process. The maximum drain current (IDS,max), maximum DC transconductance (gm), pinch-off voltage (Vth), current-gain cutoff frequency (fT), maximum oscillation frequency (fmax), and RF characteristics of the devices in terms of the small-signal gain and RF output power (Pout) at 8 GHz were investigated. The results showed that the output power is increased by 1 dB when the gate structure is changed from field plate to gamma gate. The Vth, gm, fT and fmax values are maximized when the thickness of the passivation layer between the gate foot and the gate head is minimized. It is shown that the IDS,max is decreased and Pout is increased when the gate recess etching process is performed.
Open Access
Effect of Si-rich SiXNY multilayer passivation material on the DC electrical characteristics of AlGaN/GaN HEMTs
(Springer, 2023-08-16) Dinçer, Ahmet Serhat; Haliloğlu, Mehmet Taha; Toprak, Ahmet; Altındal, Ş.; Özbay, Ekmel
In this study, the effect of SiXNY bilayer passivation materials on the electrical properties of an AlGaN high electron mobility transistor (HEMT) was investigated. AlGaN/GaN HEMTs were grown on 3-inch silicon carbide by the metal organic chemical vapor deposition method, which is one of the chemical vapor deposition methods. SiXNY passivation materials with two different Si concentrations, which were 50/1 and 70/3 (Silane—SiH4/Ammonia—NH3), were used. The passivation material coating process was carried out with the plasma enhanced chemical vapor deposition (PECVD) system. The first sample was a coated single layer with 70/3 (SiH4/NH3) passivation material at 75 nm and the second sample was coated with bilayer (two layers) passivation materials wherein the first layer was coated with 15 nm 50/1 (SiH4/NH3) and the second layer was coated with 60 nm 70/3 (SiH4/NH3). The obtained results were compared. Experimental results show that the drain leakage current (Id) and gate leakage current (Ig) decreases; current density (Idss) and transconductance (gm) increases with bilayer passivation.
Open Access
Nonalloyed ohmic contact development with n+InGaN regrowth method and analysis of its effect on AlGaN/GaN HEMT devices
(Elsevier Ltd, 2023-03-22) Toprak, Ahmet; Özbay, Ekmel
In this study, the DC performance of AlGaN/GaN based HEMT devices of different geometries (designed to operate in the S, X and Ka-band frequency ranges) with regrown degenerately doped n + In0.12GaN nonalloyed ohmic contacts on different epitaxial structures were investigated. Once the optimal recess etch depth and regrowth thickness for drain and source contacts were determined, the effects of alloyed and nonalloyed ohmic contacts on the maximum drain current (IDS,max), ON‐resistance (Ron), maximum DC transconductance (gm), pinch-off voltage (Vth), drain leakage current (ID,leak), and gate leakage current (IG,leak) were investigated for S, X and Ka-band HEMT devices. The results showed that the use of nonalloyed ohmic contacts resulted in decreasing Rc with a better surface morphology. Additionally, the nonalloyed ohmic contact structure with low contact resistance caused an increase in the IDS,max and gm values by reducing the Ron resistance, and also reducing the ID,leak and IG,leak leakage currents by preventing the surface distortions and trap formations due to the absence of high temperature. Although there was no dramatic change in Vth for S, X and Ka-band HEMT devices, Vth shifts towards positive in S and X-band devices, and towards negative in Ka-band devices.
Open Access
Saturation magnetization change with structure in CoFe2O4 nanostructures prepared from metallic iron and cobalt by wet grinding method
(Gazi Üniversitesi Mühendislik-Mimarlık, 2019) Kaynar, M. B.; Toprak, Ahmet; Özcan, Ş.
Nanocrystalline cobalt ferrite (CoFe2O4) has been synthesized directly from metallic cobalt (Co) and iron (Fe) via wet-milling followed by calcination. The calcination took place in atmosphere at 750 ◦C. After calcination, samples were dry-milled for up to 12 h to investigate the effects of their mean crystallite sizes and microstress on their magnetic properties. The mean crystallite sizes of the samples were calculated from X-ray powder diffraction (XRD) patterns using a Rietveld analysis program (MaudLab). Results show that the calcined sample had a crystallite size of around 60 nm, which decreased to 12 nm after 12 h of drymilling. However, agglomerated nanocrystallites were observed in the transmission electron microscopy (TEM) images of the material. Rietveld analysis also shows an increase in microstrain from 2 x 10-4 to 1.4 x 10-3 after increasing the dry-milling time to 12 h due to crystal defects induced by collisions while drymilling. The electron binding energies of the Co+2 were measured by X-ray photoelectron spectroscopy (XPS) to determine the degree of inversion, which was used to calculate the saturation magnetization. Vibrating sample magnetometer (VSM) measurements revealed that milling decreased the saturation magnetization from 125 emu g-1 to 57 emu g-1 even the calculated saturation magnetization increased from 4.87 µB to 6.17 µB.
Open Access
Selectively dry etched of p-GaN/InAlN heterostructures using BCI3-based plasma for normally-off HEMT technology
(Institute of Physics Publishing Ltd., 2021-12-10) Toprak, Ahmet; Yılmaz, Doğan; Özbay, Ekmel
In this paper, an alternative selective dry etching of p-GaN over InAlN was studied as a function of the ICP source powers, RF chuck powers and process pressures by using inductively coupled plasma reactive ion etching (ICP RIE) system. A recipe using only BCI3-based plasma with a resulting selectivity 13.5 for p-GaN in respect to InAlN was demonstrated. Surface roughness measurements depending on the etching time was performed by atomic force microscope (AFM) measurement and showed that a smooth etched surface with the root-mean-square roughness of 0.45 nm for p-GaN and 0.37 nm for InAlN were achieved. Normally-off p-GaN/InAlN HEMT devices were fabricated and tested by using the BCI3-based plasma we developed.
Open Access
Spectrally selective ultrathin photodetectors using strong interference in nanocavity design
(Institute of Electrical and Electronics Engineers Inc., 2019) Ghobadi, Amir; Demirağ, Yiğit; Hajian, Hodjat; Toprak, Ahmet; Bütün, Bayram; Özbay, Ekmel
Thinning the active layer's thickness of the semiconductor down to a level comparable with the carriers' diffusion length while keeping its absorption high is an ultimate goal to boost the performance of optoelectronic devices. Strong interference in multilayer structures is one of the promising and practical solutions owing to their simple and large-scale compatible fabrication route. These nanocavity designs not only provide near unity absorption, but they can also be designed in a way that a spectrally selective absorption response can be achieved. In this letter, we will demonstrate the functionality of a metal- insulator-semiconductor (MIS) cavity to obtain spectrally selective ultrathin photodetectors. To prove our theoretical and numerical findings, a 4-nm-thick amorphous silicon (aSi)-based MIS cavity is designed, fabricated, and characterized. The experimental results show that the optimized cavity design can act as an efficient visible blind ultraviolet (UV) photodetector. The proposed design shows the responsivity values of 120 and 2.5 mA/W in the UV (λ = 350 nm) and visible (λ = 500 nm) regions, respectively.
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Structural field plate length optimization for high power applications
(IEEE, 2014) Toprak, Ahmet; Kurt, Gökhan; Şen, Özlem A.; Özbay, Ekmel
In this work, we report GaN high-electron-mobility-transistors (HEMTs) on SiC with field plates of various dimensions for optimum performance. 0.6 μm gate length, 3 μm drain source space AlGaN/GaN HEMTs with field-plate lengths of 0.2, 0.3, 0.5 and 0.7 μm have been fabricated. Great enhancement in radio frequency (RF) output power density was achieved with acceptable compromise in small signal gain. When biased at 35 V, at 3 dB gain compression, a continuous wave output power density of 5.2 W/mm, power-added efficiency (PAE) of 33% and small gain of 11.4 dB were obtained at 8 GHz using device with 0.5 μm field plate length and 800 μm gate width without using via hole technology.
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A study on GaN-based betavoltaic batteries
(Institute of Physics Publishing Ltd., 2022-10-27) Toprak, Ahmet; Yılmaz, Doğan; Özbay, Ekmel
In this paper, a GaN-based betavoltaic epitaxial structure was grown by metal–organic chemical vapor deposition and a p-type ohmic contact was studied for different Ni/Au metal thickness ratios, temperature dependent in N2:O2 (1:1) gas atmosphere and different surface treatments for this epitaxial structure. Transfer length method measurements were done after each different process condition in order to check specific contact resistivities. GaN-based betavoltaic batteries were fabricated and a scanning electron microscope (SEM) was used as an electron source to test these devices. For this purpose, devices connected to a printed circuit board were exposed to an electron current of 1.5 nA with 17 keV energy in the SEM. For 1 × 1 mm2 devices, a dark current value of 2.8 pA at 0 V, fill factor of 0.35, maximum power conversion efficiency of 3.92%, and maximum output power of 1 µW were obtained.
Open Access
Visible light nearly perfect absorber: an optimum unit cell arrangement for near absolute polarization insensitivity
(OSA - The Optical Society, 2017) Ghobadi, Amir; Hajian, Hodjat; Gökbayrak, Murat; Dereshgi, Sina Abedini; Toprak, Ahmet; Butun, Bayram; Özbay, Ekmel
In this work, we propose an optimum unit cell arrangement to obtain near absolute polarization insensitivity in a metal-insulator-metal (MIM) based ultra-broadband perfect absorber. Our findings prove that upon utilizing this optimum arrangement, the response of the absorber is retained and unchanged over all arbitrary incidence light polarizations, regardless of the shape of the top metal patch. First, the impact of the geometry of the top nanopatch resonators on the absorption bandwidth of the overall structure is explored. Then, the response of the MIM design for different incidence polarizations and angles is scrutinized. Finally, the proposed design is fabricated and characterized. © 2017 Optical Society of America.