Nanotechnology Research Center (NANOTAM)

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  • ItemOpen Access
    Radiation shielding tests of crosslinked polystyrene-b-polyethyleneglycol block copolymers blended with nanostructured selenium dioxide and boron nitride particles
    (MDPI, 2022-02-01) Cinan, Zehra Merve; Erol, Burcu; Baskan, Taylan; Mutlu, Saliha; Ortaç, Bülend; Savaskan Yilmaz, Sevil; Yilmaz, Ahmet Hakan; Ortaç, Bülend
    In this work, gamma-ray shielding features of crosslinked polystyrene-b-polyethyleneglycol block copolymers (PS-b-PEG) blended with nanostructured selenium dioxide (SeO2 ) and boron ni-tride (BN) particles were studied. This research details several radiation shielding factors i.e., mass attenuation coefficient (µm ), linear attenuation coefficient (µL ), radiation protection efficiency (RPE), half-value layer (HVL), tenth-value layer (TVL), and mean free path (MFP). The irradiation properties of our nanocomposites were investigated with rays from the152 Eu source (in the energy intervals from 121.780 keV to 1408.010 keV) in a high-purity germanium (HPGe) detector system, and analyzed with GammaVision software. Moreover, all radiation shielding factors were determined by theoretical calculus and compared with the experimental results. In addition, the morphological and thermal characterization of all nanocomposites was surveyed with various techniques i.e., nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and transmission electron microscopy (TEM). Acceptable compatibility was revealed and observed in all nanocomposites between the experimental and theoretical results. The PS-b-PEG copolymer and nanostructured SeO2 and BN particles exerted a significant effect in enhancing the resistance of the nanocomposites, and the samples with high additive rates exhibited better resistance than the other nanocomposites. From the achieved outcomes, it can be deduced that our polymer-based nanocomposites can be utilized as a good choice in the gamma-irradiation-shielding discipline. © 2022 by the authors. Licensee MDPI, Basel, Switzerland.
  • ItemOpen Access
    Uncovering the non-radiative thermal characteristics of a passive radiative cooler under real operating conditions
    (Institute of Physics Publishing Ltd., 2022-12-12) Koçer, Hasan; Durna, Yılmaz; Işık, Halil; Soydan, Mahmut Can; Khalichi, Bahram; Ghobadi, Amir; Kurt, H.; Özbay, Ekmel; Koçer, Hasan; Durna, Yılmaz; Işık, Halil; Soydan, Mahmut Can; Khalichi, Bahram; Ghobadi, Amir; Özbay, Ekmel
    Passive radiative cooling (PasRadCool), which emits thermal energy from objects to deep cold space through atmospheric transparency, offers complementary and alternative green energy solutions for passive cooling of buildings, clothing, and renewable energy harvesting. Depending on the spectral emissive/absorptive properties of the unit under test (UUT), radiative heat exchanges occur between the UUT, atmosphere, and sun, while at the same time non-radiative heat exchange occurs. The performance of the PasRadCool is determined by the combined thermal and thermodynamic effects of both exchange mechanisms. Although the non-radiative heat exchange, which consists of conductive and convective processes to the outer surfaces of the UUT and the surrounding air fluid, is very sensitive to environmental changes, the actual performance is not fully determined since this feature is considered statically in many studies. Herein, we propose a method that reveals the non-radiative thermal characteristics of the PasRadCool under real operating conditions. With a photonic radiative cooler structure, which we manufacture as a proof of concept, we perform nighttime field test measurements in varying non-radiative thermal conditions. The proposed method extracts the time-dependent non-radiative heat transfer coefficient of the UUT as accurately as possible. We also confirm that our experimental result shows good agreement with both numerical and analytical methods. The proposed approach, which highlights the realistic thermal management of PasRadCool, is not specific to the circumstances of our study and can be applied to all PasRadCool situations with different geometry, material, and environmental conditions.
  • ItemOpen Access
    Subwavelength densely packed disordered semiconductor metasurface units for photoelectrochemical hydrogen generation
    (American Chemical Society, 2022-03-10) Ulusoy Ghobadi, T. Gamze; Ghobadi, Amir; Odabaşı, Oğuz; Karadaş, Ferdi; Özbay, Ekmel; Ulusoy Ghobadi, T. Gamze; Ghobadi, Amir; Karadaş, Ferdi; Özbay, Ekmel
    For most semiconductors, especially the visible-light-absorbing ones, the carrier diffusion length is significantly shorter than the light penetration depth, limiting their photoactivities. This limitation could be mitigated through the use of subwavelength semiconductor-based metasurfaces and metamaterials. In this paper, a large-scale compatible metasurface photocathode, made of densely packed disordered p-type chromium oxide (CrOX), is developed to be utilized in photoelectrochemical (PEC) hydrogen generation. For this purpose, first, tightly packed random Cr nanorods are fabricated using an oblique angle deposition technique. Afterward, an annealing step is applied to the sample to transform these metallic units into a semiconducting p-type CrOX-based metasurface. Based on the experimental characterization results and numerical simulations, the proposed design can provide strong light-matter interactions in an ultra-broadband-wavelength range, mainly due to its multidimensional random geometry and ultrasmall gap sizes. Finally, to substantiate the activity of the CrOXnanorods, a core-crown geometry is developed where the NiOXcapping layer catalyzes the hydrogen evolution reaction (HER). The proposed heterostructure metasurface absorber can impose photocurrent values as large as 50 μA cm-2with a photocurrent spectral response extended up to 500 nm. Moreover, the electrode shows outstanding operation under light irradiation for 9 hours. This work demonstrates a simple, scalable design strategy to fabricate low-cost and stable photocathodes for PEC hydrogen evolution. © 2022 American Chemical Society. All rights reserved.
  • ItemOpen Access
    Design and robustness improvement of high-performance LNA using 0.15 μm GaN technology for X-band applications
    (John Wiley & Sons Ltd., 2022-07) Zafar, Salahuddin; Çankaya Akoğlu, Büşra; Aras, Erdem; Yılmaz, Doğan; Nawaz, Muhammad İmran; Kashif, Ahsanullah; Özbay, Ekmel; Zafar, Salahuddin; Çankaya Akoğlu, Büşra; Aras, Erdem; Yılmaz, Doğan; Nawaz, Muhammad İmran; Kashif, Ahsanullah; Özbay, Ekmel
    In this paper, we present a highly robust GaN-based X-band low-noise amplifier (LNA) showing promising small-signal and noise performance as well as good linearity. The LNA is fabricated using in-house 0.15 μm AlGaN/GaN on a SiC HEMT process. Owing to the optimum choice of HEMT topologies and simultaneous matching technique, LNA achieves a noise figure better than 2 dB, output power at 1 dB gain compression higher than 19 dB, input and output reflection coefficients better than −9 and −11 dB, respectively. The small-signal gain of LNA is more than 19 dB for the whole band, and NF has a minimum of 1.74 dB at 10.2 GHz. LNA obtains an OIP3 up to 34.2 dBm and survives input power as high as 42 dBm. Survivability is investigated in terms of gain compression and forward gate current. Reverse recovery time (RRT), a crucial parameter for radar front-ends, is explored with respect to the RC time constant and trap phenomenon. The analysis shows that the significant contribution in RRT is due to traps while the RC time constant is in the nanoseconds range. Moreover, this study also addresses the requirement and choice of a DC gate feed resistor for the subsequent stages in a multi-stage design. The size of the designed LNA chip is 3 mm (Formula presented.) 1.2 mm only.
  • ItemOpen Access
    Polarization insensitive phase change material-based nanoantenna array for thermally tunable infrared applications
    (Institute of Electrical and Electronics Engineers, 2022-09-21) Khalichi, Bahram; Omam, Zahra Rahimian; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Özbay, Ekmel; Khalichi, Bahram; Omam, Zahra Rahimian; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Özbay, Ekmel
    Thermal radiation management is an emerging application of metamaterials owing to their exotic electromagnetic radiative properties. Herein, a thermally tunable phase change material-based nanoantenna array is reported to manipulate electromagnetic waves for potential applications in radiative cooling and multispectral camouflage from thermal infrared detectors. The simulation results show that the proposed nanoantenna array possesses high reflectance exceeding at least 60% within the 3−5 µm and 8−12 µm wavelength ranges, indicating low thermal emissivity, while the reflectance value increases as the temperature rises. Additionally, the wavelength-selective nanoantenna emitter operates with high absorption and therefore emission within the non-atmospheric window (5−8 µm). The thermally tuning feature leads to further controlling the absorption and, therefore, the emission performance of the nanoantenna and corresponding infrared signatures detected by thermal cameras.
  • ItemOpen Access
    A Transmissive all-dielectric metasurface-based nanoantenna array for selectively manipulation of thermal radiation
    (Institute of Electrical and Electronics Engineers, 2022-09-21) Khalichi, Bahram; Omam, Zahra Rahimian; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Özbay, Ekmel; Khalichi, Bahram; Omam, Zahra Rahimian; Osgouei, Ataollah Kalantari; Ghobadi, Amir; Özbay, Ekmel
    In this study, a wavelength-selective thermal nanoantenna emitter based on metamaterial design with heat radiation signature management and radiative cooling property is proposed. The design can be considered as a multifunctional window by reducing the heat signature and releasing the heat energy within the non-atmospheric window. The approach relies on the indium tin oxide cubic-shaped unit cell coated on a flexible and transparent substrate (polystyrene). The spectral behaviors of the proposed structure are obtained using the finite difference time domain method, where the power calculation model is utilized to demonstrate the radiative cooling efficiency and low power detection on infrared cameras.
  • ItemOpen Access
    Transmissive terahertz metasurfaces with vanadium dioxide split-rings and grids for switchable asymmetric polarization manipulation
    (Nature Research, 2022-12) Serebryannikov, Andriy E.; Lakhtakia, Akhlesh; Vandenbosch, Guy A. E.; Özbay, Ekmel; Özbay, Ekmel
    Metasurfaces containing arrays of thermally tunable metal-free (double-)split-ring meta-atoms and metal-free grids made of vanadium dioxide (VO2), a phase-change material can deliver switching between (1) polarization manipulation in transmission mode as well as related asymmetric transmission and (2) other functionalities in the terahertz regime, especially when operation in the transmission mode is needed to be conserved for both phases of VO2. As the meta-atom arrays function as arrays of metallic subwavelength resonators for the metallic phase of VO2, but as transmissive phase screens for the insulator phase of VO2, numerical simulations of double- and triple-array metasurfaces strongly indicate extreme scenarios of functionality switching also when the resulting structure comprises only VO2 meta-atoms and VO2 grids. More switching scenarios are achievable when only one meta-atom array or one grid is made of VO2 components. They are enabled by the efficient coupling of the geometrically identical resonator arrays/grids that are made of the materials that strongly differ in terms of conductivity, i.e. Cu and VO2 in the metallic phase. © 2022, The Author(s).
  • ItemOpen Access
    A study on GaN-based betavoltaic batteries
    (Institute of Physics Publishing Ltd., 2022-10-27) Toprak, Ahmet; Yılmaz, Doğan; Özbay, Ekmel; 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.
  • ItemOpen Access
    DC and RF performance of lateral AlGaN/GaN FinFET with ultrathin gate dielectric
    (Institute of Physics Publishing Ltd., 2022-06-23) Yılmaz, Doğan; Odabaşı, O.; Salkım, Gurur; Urfalı, Emirhan; Akoğlu, Büşra Çankaya; Özbay, Ekmel; Altındal, Ş.; Yılmaz, Doğan; Salkım, Gurur; Urfalı, Emirhan; Akoğlu, Büşra Çankaya; Özbay, Ekmel
    In this study, an enhancement-mode (E-mode) GaN high electron mobility transistor (HEMT) with lateral tri-gate structure field effect transistor (FinFET) is proposed. To passivate the fin width, while keeping the normally-off performance of the FinFET intact, an ultrathin aluminium-oxide/sapphire (Al2O3) gate dielectric is proposed (in a basic single-finger 0.125 mm device). Later, the DC and radio frequency (RF) performances of the proposed FinFET designs (with optimized fin width and Al2O3 thickness) are compared with that of conventional planar HEMT. DC and RF measurements are performed using power transistors in ten-fingers configuration, with a total gate periphery of 2.5 mm. The effect of Fin structure and Al2O3 thickness on the electrical performance of HEMTs, including threshold voltage (Vth) shift, transconductance (gm) linearity, small-signal gain, cut off frequency (f t), output power (Pout), and power-added efficiency (PAE) are investigated. Based on our findings, FinFET configuration imposes normally-off functionality with a Vth = 0.2 V, while the planar architecture has a Vth = −3.7 V. Originating from passivation property of the alumina layer, the FinFET design exhibits two orders of magnitude smaller drain and gate leakage currents compared to the planar case. Moreover, large signal RF measurements reveals an improved Pout density by over 50% compared to planar device, attributed to reduced thermal resistance in FinFETs stemming from additional lateral heat spreading of sidewall gates. Owing to its superior DC and RF performance, the proposed FinFET design with ultrathin gate dielectric could bear the potential of reliable operating for microwave power applications, by further scaling of the gate length.
  • ItemOpen Access
    Fractional fourier transform meets transformer encoder
    (Institute of Electrical and Electronics Engineers, 2022-10-28) Şahinuç, Furkan; Koç, Aykut; Şahinuç, Furkan; Koç, Aykut
    Utilizing signal processing tools in deep learning models has been drawing increasing attention. Fourier transform (FT), one of the most popular signal processing tools, is employed in many deep learning models. Transformer-based sequential input processing models have also started to make use of FT. In the existing FNet model, it is shown that replacing the attention layer, which is computationally expensive, with FT accelerates model training without sacrificing task performances significantly. We further improve this idea by introducing the fractional Fourier transform (FrFT) into the transformer architecture. As a parameterized transform with a fraction order, FrFT provides an opportunity to access any intermediate domain between time and frequency and find better-performing transformation domains. According to the needs of downstream tasks, a suitable fractional order can be used in our proposed model FrFNet. Our experiments on downstream tasks show that FrFNet leads to performance improvements over the ordinary FNet.
  • ItemOpen Access
    Selective glucose sensing under physiological pH with flexible and binder-free prussian blue coated carbon cloth electrodes
    (Wiley-VCH Verlag GmbH & Co. KGaA, 2022-01-27) Oglou, Ramadan Chalil; Ulusoy Ghobadi, T. Gamze; Özbay, Ekmel; Karadaş, Ferdi; Oglou, Ramadan Chalil; Ulusoy Ghobadi, T. Gamze; Özbay, Ekmel; Karadaş, Ferdi
    The frequent detection of physiological glucose levels from human blood or sweat requires the development of low-cost electrodes with high sensitivity and selectivity. In this work, we prepared a series of Prussian blue (PB) modified carbon cloth (CC) electrodes with different cyanoferrate groups. We achieved a sensitivity as high as 145.43 μA mm−1cm−2 in a 0.1–6.5 mm concentration range with a response time below 2 s under physiological pH. The electrodes exhibited a superior selectivity of glucose in the presence of interfering agents, including sucrose, lactose, NaCl, ascorbic acid, and uric acid. The electrodes also showed outstanding long-term stability over 15 days. Furthermore, we performed comprehensive electrochemical and characterization studies to elucidate the role of the cyanoferrate group on the morphologic and electronic properties of non-enzymatic glucose sensors.
  • ItemOpen Access
    ER Stress-induced sphingosine-1-phosphate lyase phosphorylation potentiates the mitochondrial unfolded protein response
    (American Society for Biochemistry and Molecular Biology Inc., 2022-10) Yıldırım, Aslı Dilber; Citir, Mevlut; Doğan, Aslı Ekin; Veli, Zehra; Yıldırım, Zehra; Tufanli, Ozlem; Traynor-Kaplan, Alexis; Schultz, Carsten; Erbay, Ebru; Yıldırım, Aslı Dilber; Doğan, Aslı Ekin; Veli, Zehra; Yıldırım, Zehra
    The unfolded protein response (UPR) is an elaborate signaling network that evolved to maintain proteostasis in the endoplasmic reticulum (ER) and mitochondria (mt). These organelles are functionally and physically associated, and consequently, their stress responses are often intertwined. It is unclear how these two adaptive stress responses are coordinated during ER stress. The inositol-requiring enzyme-1 (IRE1), a central ER stress sensor and proximal regulator of the UPRER, harbors dual kinase and endoribonuclease (RNase) activities. IRE1 RNase activity initiates the transcriptional layer of the UPRER, but IRE1’s kinase substrate(s) and their functions are largely unknown. Here, we discovered that sphingosine 1-phosphate (S1P) lyase (SPL), the enzyme that degrades S1P, is a substrate for the mammalian IRE1 kinase. Our data show that IRE1-dependent SPL phosphorylation inhibits SPL’s enzymatic activity, resulting in increased intracellular S1P levels. S1P has previously been shown to induce the activation of mitochondrial UPR (UPRmt) in nematodes. We determined that IRE1 kinase-dependent S1P induction during ER stress potentiates UPRmt signaling in mammalian cells. Phosphorylation of eukaryotic translation initiation factor 2α (eif2α) is recognized as a critical molecular event for UPRmt activation in mammalian cells. Our data further demonstrate that inhibition of the IRE1-SPL axis abrogates the activation of two eif2α kinases, namely double-stranded RNA-activated protein kinase (PKR) and PKR–like ER kinase upon ER stress. These findings show that the IRE1-SPL axis plays a central role in coordinating the adaptive responses of ER and mitochondria to ER stress in mammalian cells. © 2022 THE AUTHORS.
  • ItemOpen Access
    Ultrathin interfacial layer and pre-gate annealing to suppress virtual gate formation in GaN-based transistors: The impact of trapping and fluorine inclusion
    (Institute of Electrical and Electronics Engineers, 2022-08-31) Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Özbay, Ekmel; Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Özbay, Ekmel
    In AlGaN/GaN high electron mobility transistors (HEMTs), the long-term operation of the device is adversely affected by threshold voltage ( Vth ) instability and current collapse. In this letter, using structural and electrical analyses, the impact of trapping and fluorine (F) inclusion on the device operation is scrutinized. It is found that SiNx interfacial layer significantly reduced the formation of defects, during the ohmic annealing process. Moreover, the incorporation of F ions into GaN bulk, during the gate etch process, triggers the virtual gate phenomenon. This effect has also been mitigated via the pre-gate annealing (PGA) process. As a result of these modifications, a stable operation with minimized lag performance has been achieved.
  • ItemOpen Access
    Impact of the low temperature ohmic contact process on DC and forward gate bias stress operation of GaN HEMT devices
    (Institute of Electrical and Electronics Engineers, 2022-08-17) Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Ünal, Yakup; Salkım, Gurur; Başar, Güneş; Bütün, Bayram; Özbay, Ekmel; Odabaşı, Oğuz; Ghobadi, Amir; Ghobadi, Türkan Gamze Ulusoy; Ünal, Yakup; Salkım, Gurur; Başar, Güneş; Bütün, Bayram; Özbay, Ekmel
    In AlGaN/GaN high electron mobility transistors (HEMTs), high temperature processes (such as ohmic annealing with >800°C value) could deform the crystal structure and induce trap states within the bulk and surface. Expanded defect densities cause crucial problems, such as threshold voltage ( Vth ) instability, current collapse, and high leakages. In this work, a low temperature ohmic contact process (630°C, 10 minutes) is adopted with recess etch, and contact resistances <0.1Ω ⋅ mm with low sheet resistances are achieved. The positive impact of this low thermal budget process on surface morphology, DC operation, long-term stability, and forward gate bias stress of the device is studied.
  • ItemOpen Access
    High efficiency 35 GHz MMICs based on 0.2 μm AlGaN/GaN HEMT technology
    (Cambridge University Press, 2022-06-16) Akoğlu, Büşra Çankaya; Sütbaş, Batuhan; Özbay, Ekmel
    In this paper, two high efficiency monolithic microwave integrated circuits (MMICs) are demonstrated using NANOTAM's in-house Ka-band fabrication technology. AlGaN/GaN HEMTs with 0.2 μm gate lengths are characterized, and an output power density of 2.9 W/mm is achieved at 35 GHz. A three-stage driver amplifier MMIC is designed, which has a measured gain higher than 19.3 dB across the frequency band of 33–36 GHz. The driver amplifier exhibits 31.9 dB output power and 26.5% power-added efficiency (PAE) at 35 GHz using 20 V supply voltage with 30% duty cycle. Another two-stage MMIC is realized as a power amplifier with a total output gate periphery of 1.8 mm. The output power and PAE of the power amplifier are measured as 3.91 W and 26.3%, respectively, at 35 GHz using 20 V supply voltage with 30% duty cycle. The high efficiency MMICs presented in this paper exhibit the capabilities of NANOTAM's 0.2 μm AlGaN/GaN on SiC technology.
  • ItemOpen Access
    21.2 mV/K high-performance Ni(50 nm)-Au(100 nm)/Ga2O3/p-Si vertical MOS type diode and the temperature sensing characteristics with a novel drive mode
    (Institute of Electrical and Electronics Engineers, 2022-11-09) Çiçek, O.; Arslan, E.; Altındal, Ş.; Badali, Y.; Özbay, Ekmel; Özbay, Ekmel
    Sensitivity ( S ) and drive mode are crucial issues for the vertical metal-oxide-semiconductor (MOS) type diode applied in temperature sensing. In this study, experimentally, we indicated that the S values of the Ni(50 nm) - Au(100 nm) /Ga2O3/ p -Si vertical MOS type diode, using the measured capacitance–voltage ( Cm – V ) outputs, are obtained with a novel drive mode. We applied the constant capacitance mode to drive the silicon thermo-diodes as well as constant current mode, and constant voltage mode, which are known as two different methods in the literature. Meanwhile, the S value is 21.2 mV/K at 1 nF. This value is the highest value proven in the literature excepting the cryogenic temperature region, and near room temperature. This study provided an original structure for the silicon thermo-diodes and a novel way to drive them.
  • ItemOpen Access
    Membrane-mediated interactions between disk-like inclusions adsorbed on vesicles
    (Frontiers Media S.A., 2022-10-17) Alizadeh-Haghighi, Elnaz; Karaei Shiraz, Arash; Bahrami, Amir Houshang; Alizadeh-Haghighi, Elnaz; Karaei Shiraz, Arash; Bahrami, Amir Houshang
    Self-assembly of membrane inclusions plays a key role in biological processes such as cellular signalling and trafficking and has potential applications for designing interfacial devices such as sensors and actuators. Despite intensive studies of curvature-mediated interactions, how membrane curvature modulates interactions between flat disk-like inclusions, adsorbed on vesicles, remains unknown. We use Monte Carlo simulations of a triangulated vesicle with simulated annealing to explore curvature-mediated interactions between disk-like rigid inclusions, induced by membrane elastic energy. We distinguish two distinct short and long-range curvature-mediated interactions for disk distances below and above the vesicle diameter. We observe short-range neutral interactions in the limit of small disks, where the vesicle appears as a flat bilayer to the disks. Beyond a certain size of disk-like inclusions, we find a transition from neutral to attractive short-range forces. Consistent with experiments, we also show that upon deflating vesicles, previously-attracted disks experience repulsive interactions. Our findings show how the vesicle curvature and the relative size between the disks and the vesicle determine the character of membrane-mediated interactions between adsorbed disk-like inclusions. Copyright © 2022 Alizadeh-Haghighi, Karaei Shiraz and Bahrami.
  • ItemOpen Access
    Analysis of HfO2 and ZrO2 as high-K dielectric for CMOS nano devices
    (Institute of Electrical and Electronics Engineers, 2022-05-16) Hasan, T.; Zafar, Salahuddin; Özbay, Ekmel; Kashif, A. U.; Zafar, Salahuddin; Özbay, Ekmel
    An analysis has been made on high-K dielectrics (HfO 2 and ZrO 2) for the CMOS process up to 14 nm FAB technology node. The aim is to study the reduction in gate leakage current for Nano-scale devices. High-K Dielectric having K ≥ 20 is beneficial for CMOS Nano-devices, reducing the gate leakage current when EOT ≤ 0.5 nm. MOS structure with high-K, i.e., HfO 2 and ZrO 2 , has been simulated in SILVACO T-CAD to consider as gate stack: metal/oxide/p-Si for the different FAB nodes; 45, 32, 22 & 14 nm. SiO 2 is considered a reference to optimize the MOS structure with high-K dielectric. As a result, 7–8 times the higher physical gate oxide layer is achieved compared to SiO 2 , which has a significant impact on minimizing the gate leakage current.
  • ItemOpen Access
    Thermally switchable, bifunctional, scalable, mid-infrared metasurfaces with VO2 grids capable of versatile polarization manipulation and asymmetric transmission
    (Optica Publishing Group (formerly OSA), 2022-12-01) Serebryannikov, Andriy E.; Lakhtakia, Akhlesh; Özbay, Ekmel; Özbay, Ekmel
    We conceptualized three-array scalable bifunctional metasurfaces comprising only three thin strip grids and numerically determined their characteristics in the mid-infrared spectral regime for switchable operation scenarios involving polarization manipulation and related diodelike asymmetric transmission (AT) as one of two functionalities. A few or all of the grids were taken to be made of VO2, a bifunctionality-enabling phase-change material; there are no layers and/or meta-atoms comprising simultaneously both metal and VO2. For each proposed metasurface, two effective structures and, therefore, two different functionalities exist, corresponding to the metallic and insulating phases of VO2. The achieved scenarios of functionality switching significantly depend on the way in which VO2 is incorporated into the metasurface. Switchable bands of polarization manipulation are up to 40 THz wide. The AT band can be modulated when Fabry-Perot (anti-) resonances come into play. Besides, transmission regimes with the cross-polarized component insensitive to VO2 phase change are possible, as well as the ones with all co- and cross-polarized components having the same magnitude for both linear polarizations of the incident wave. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement.
  • ItemOpen Access
    Lithography-free metamaterial absorbers: Opinion
    (The Optical Society, 2022-02-01) Ghobadi, Amir; Ulusoy Ghobadi, Türkan Gamze; Özbay, Ekmel; Ghobadi, Amir; Ulusoy Ghobadi, Türkan Gamze; Özbay, Ekmel
    Although advancement in nanofabrication provides the opportunity to realize nanoscale geometries with high resolutions, the scalability and repeatability issues limit their large-scale applications. Lithography-free metamaterial absorbers (LFMAs) are a potential route for the upscaling of these designs. With restricted freedom in their synthesis, the importance of the proper material choice is emphasized. Herein, we provide a comprehensive overview of the recently developed LFMAs, from both design and material perspectives, while considering their most promising applications. © 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement