Department of Electrical and Electronics Engineering

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  • ItemOpen Access
    High external quantum efficiency light-emitting diodes enabled by advanced heterostructures of type-ii nanoplatelets
    (American Chemical Society, 2023-03-13) Durmusoglu, Emek G.; Hu, Sujuan; Hernandez-Martinez, Pedro Ludwig; Izmir, Merve; Shabani,Farzan; Guo, Min; Gao, Huayu; Isik, Furkan; Delikanli, Savas; Sharma, Vijay Kumar; Liu, Baiquan; Demir, Hilmi Volkan; Demir, Hilmi Volkan; Shabani, Farzan; Işık, Furkan; Delikanli, Savaş
    Colloidal quantum wells (CQWs), also known as nanoplatelets (NPLs), are exciting material systems for numerous photonic applications, including lasers and light-emitting diodes (LEDs). Although many successful type-I NPL-LEDs with high device performance have been demonstrated, type-II NPLs are not fully exploited for LED applications, even with alloyed type-II NPLs with enhanced optical properties. Here, we present the development of CdSe/CdTe/CdSe core/crown/crown (multi-crowned) type-II NPLs and systematic investigation of their optical properties, including their comparison with the traditional core/crown counterparts. Unlike traditional type-II NPLs such as CdSe/CdTe, CdTe/CdSe, and CdSe/CdSexTe1–x core/crown heterostructures, here the proposed advanced heterostructure reaps the benefits of having two type-II transition channels, resulting in a high quantum yield (QY) of 83% and a long fluorescence lifetime of 73.3 ns. These type-II transitions were confirmed experimentally by optical measurements and theoretically using electron and hole wave function modeling. Computational study shows that the multi-crowned NPLs provide a better-distributed hole wave function along the CdTe crown, while the electron wave function is delocalized in the CdSe core and CdSe crown layers. As a proof-of-concept demonstration, NPL-LEDs based on these multi-crowned NPLs were designed and fabricated with a record high external quantum efficiency (EQE) of 7.83% among type-II NPL-LEDs. These findings are expected to induce advanced designs of NPL heterostructures to reach a fascinating level of performance, especially in LEDs and lasers.
  • ItemOpen Access
    Differential entropy of the conditional expectation under additive gaussian voise
    (Institute of Electrical and Electronics Engineers, 2022) Atalik, Arda; Köse, Alper; Gastpar, Michael; Atalik, Arda
    The conditional mean is a fundamental and important quantity whose applications include the theories of estimation and rate-distortion. It is also notoriously difficult to work with. This paper establishes novel bounds on the differential entropy of the conditional mean in the case of finite-variance input signals and additive Gaussian noise. The main result is a new lower bound in terms of the differential entropies of the input signal and the noisy observation. The main results are also extended to the vector Gaussian channel and to the natural exponential family. Various other properties such as upper bounds, asymptotics, Taylor series expansion, and connection to Fisher Information are obtained. Two applications of the lower bound in the remote-source coding and CEO problem are discussed.
  • ItemOpen Access
    Bimodal interventional instrument markers for magnetic particle imaging and magnetic resonance imaging—a proof-of-concept study
    (MDPI, 2022-05-02) Wegner, Franz; Lüdtke-Buzug, Kerstin; Cremers, Sjef; Friedrich, Thomas; Sieren, Malte M.; Haegele, Julian; Koch, Martin A.; Borm, Paul; Buzug, Thorsten M.; Barkhausen, Joerg; Ahlborg, Mandy; Sarıtaş, Emine Ülkü; Sarıtaş, Emine Ülkü
    The purpose of this work was to develop instrument markers that are visible in both magnetic particle imaging (MPI) and magnetic resonance imaging (MRI). The instrument markers were based on two different magnetic nanoparticle types (synthesized in-house KLB and commercial Bayoxide E8706). Coatings containing one of both particle types were fabricated and measured with a magnetic particle spectrometer (MPS) to estimate their MPI performance. Coatings based on both particle types were then applied on a segment of a nonmetallic guidewire. Imaging experiments were conducted using a commercial, preclinical MPI scanner and a preclinical 1 tesla MRI system. MPI image reconstruction was performed based on system matrices measured with dried KLB and Bayoxide E8706 coatings. The bimodal markers were clearly visible in both methods. They caused circular signal voids in MRI and areas of high signal intensity in MPI. Both the signal voids as well as the areas of high signal intensity were larger than the real marker size. Images that were reconstructed with a Bayoxide E8706 system matrix did not show sufficient MPI signal. Instrument markers with bimodal visibility are essential for the perspective of monitoring cardiovascular interventions with MPI/MRI hybrid systems. © 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
    A deblurring model for X-space MPI based on coded calibration scenes
    (Infinite Science Publishing, 2022) Ergun, Esen; Arola, Abdullah Ömer; Saritas, Emine Ulku; Ergun, Esen; Arola, Abdullah Ömer; Saritas, Emine Ulku
    X-space reconstructions suffer from blurring caused by the point spread function (PSF) of the Magnetic Particle Imaging (MPI) system. Here, we propose a deep learning method for deblurring x-space reconstructed images. Our proposed method learns an end-to-end mapping between the gridding-reconstructed collinear images from two partitions of a Lissajous trajectory and the underlying magnetic nanoparticle (MNP) distribution. This nonlinear mapping is learned using measurements from a coded calibration scene (CCS) to speed up the training process. Numerical experiments show that our learning-based method can successfully deblur x-space reconstructed images across a broad range of measurement signal-to-noise ratios (SNR) following training at a moderate SNR.
  • ItemOpen Access
    MNP characterization and signal prediction using a model-based dictionary
    (Infinite Science Publishing, 2022-03-21) Alpman, Asli; Utkur, Mustafa; Saritas, Emine Ulku; Alpman, Asli; Utkur, Mustafa; Saritas, Emine Ulku
    Magnetic Particle Imaging (MPI) utilizes the nonlinear magnetic response of magnetic nanoparticles (MNPs) for signal localization. Accurate modeling of the magnetization behavior of MNPs is crucial for understanding their MPI signal responses. In this work, we propose a model-based dictionary approach using a coupled Brown-Néel rotation model. With experimental results on a Magnetic Particle Spectrometer (MPS), we show that this approach can successfully characterize MNPs and predict their signal responses.
  • ItemOpen Access
    Engineered ultraviolet InGaN/AlGaN multiple-quantum-well structures for maximizing cathodoluminescence efficiency
    (American Institute of Physics Inc., 2022-01-04) Zheng, Haiyang; Sharma, Vijay Kumar; Sharma, Vijay Kumar
    We demonstrate a systematic way to understand and select the accelerating voltage for maximizing cathodoluminescence (CL) by correlating the carrier diffusion length with the efficiency of ultraviolet (UV) InGaN/AlGaN multiple quantum wells (MQWs). We showed that the absorption of MQWs benefits from the absorbed energy within the diffusion length below the MQWs. With this understanding, we have achieved good agreement between the experimental data of and the Monte Carlo (CASINO) simulations on the dependence of acceleration voltage and QW number on InGaN/AlGaN MQW structures. These findings indicate that CL-based UV generation from carefully engineered III-N MQW structures with an appropriate number of QWs is highly promising. The understanding and application of this work can be extended to electron-beam pumped devices emitting in deep-UV (200-280 nm) wavelengths. © 2022 Author(s).
  • 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
    A z-gradient array coil with a dedicated active-shielded array coil for MRI
    (John Wiley and Sons Inc., 2022-08-02) Takrimi, Manouchehr; Atalar, Ergin; Takrimi, Manouchehr; Atalar, Ergin
    Purpose: An array-based z-gradient coil with a set of programmable power amplifiers can outperform a conventional z-gradient coil and make it highly customizable with a broader range of tunable features. Methods: A dynamically adjustable imaging volume can be achieved using a pair of independent arrays and a modified optimization procedure based on analytic equations. Two modes of operation are provided: (a) standard mode that resembles a conventional coil; (b) advanced mode, where all performance parameters can be adjusted employing a controllable feeding mechanism. Commercial software is used to demonstrate the validity and feasibility of the proposed coil. Results: Primary and shield array diameters are 24 and 30 cm, both of which comprise 12 bundles of 10 turns copper wires. Maximum feeding voltage/current is 250 V/100 A for all array elements. Four distinct magnetic profiles are provided: (a) conventional profile with 140 mm diameter spherical region of interest, 120 mT/m gradient, and up to 4500 T/m/s slew rate; (b) profile of 200 mT/m, 70 mm region of interest, and up to 6900 T/m/s slew rate; (c) 60 mm axially shifted 70 mm region of interest with 120 mT/m strength and 3600 T/m/s slew rate; and (d) profile of 370 mT/m, 120 mm region of interest, and 3700 T/m/s slew rate when the active shield is reverse fed. Conclusion: By using an active-shielded gradient array coil, the magnetic field profile of the imaging volume can be adjusted dynamically, and it can provide new features and a wide range of field profiles for diverse applications in MRI.
  • ItemOpen Access
    Nonlinear droop compensation for current waveforms in MRI gradient systems
    (John Wiley and Sons Inc, 2022-03-28) Babaloo, Reza; Atalar, Ergin; Babaloo, Reza; Atalar, Ergin
    Purpose: Providing accurate gradient currents is challenging due to the gradient chain nonlinearities, arising from gradient power amplifiers and power supply stages. This work introduces a new characterization approach that takes the amplifier and power supply into account, resulting in a nonlinear model that compensates for the current droop. Methods: The gradient power amplifier and power supply stage were characterized by a modified state-space averaging technique. The resulting nonlinear model was inverted and used in feedforward to control the gradient coil current. A custom-built two-channel z-gradient coil was driven by high-switching (1 MHz), low-cost amplifiers (<$200) using linear and nonlinear controllers. High-resolution (<80 ps) pulse-width-modulation signals were used to drive the amplifiers. MRI experiments were performed to validate the nonlinear controller's effectiveness. Results: The simulation results validated the functionality of the state-space averaging method in characterizing the gradient system. The performance of linear and nonlinear controllers in generating a trapezoidal current waveform was compared in simulations and experiments. The integral errors between the desired waveform and waveforms generated by linear and nonlinear controllers were 1.9% and 0.13%, respectively, confirming the capability of the nonlinear controller to compensate for the current droop. Phantom images validated the nonlinear controller's ability to correct droop-induced distortions. Conclusion: Benchtop measurements and MRI experiments demonstrated that the proposed nonlinear characterization and digitally implemented feedforward controller could drive gradient coils with droop-free current waveforms (without a feedback loop). In experiments, the nonlinear controller outperformed the linear controller by a 14-fold reduction in the integral error of a test waveform.
  • ItemOpen Access
    Simultaneous temperature and viscosity estimation capability via magnetic nanoparticle relaxation
    (Wiley-Blackwell Publishing, Inc., 2022-04) Utkur, Mustafa; Sarıtaş, Emine Ülkü; Utkur, Mustafa; Sarıtaş, Emine Ülkü
    Purpose: Magnetic particle imaging (MPI) is emerging as a highly promising imaging modality. Magnetic nanoparticles (MNPs) are used as imaging tracers in MPI, and their relaxation behavior provides the foundation for its functional imaging capability. Since MNPs are also utilized in magnetic fluid hyperthermia (MFH) and MPI enables localized MFH, temperature mapping arises as an important application area of MPI. To achieve accurate temperature estimations, however, one must also take into account the confounding effects of viscosity on the MPI signal. In this work, we analyze the effects of temperature and viscosity on MNP relaxation and determine temperature and viscosity sensitivities of relaxation time constant estimations via TAURUS (TAU estimation via Recovery of Underlying mirror Symmetry) at a wide range of operating points to empower simultaneous mapping of these two parameters. Methods: A total of 15 samples were prepared to reach four target viscosity levels (0.9–3.6 mPa (Formula presented.) s) at five different temperatures (25–45 (Formula presented.) C). Experiments were performed on a magnetic particle spectrometer (MPS) setup at 60 different operating points at drive field amplitudes ranging between 5 and 25 mT and frequencies ranging between 1 and 7 kHz. To enable these extensive experiments, an in-house arbitrary-waveform MPS setup with temperature-controlled heating capability was developed. The operating points were divided into four groups with comparable signal levels to maximize signal gain during rapid signal acquisition. The relaxation time constants were estimated via TAURUS, by restoring the underlying mirror symmetry property of the positive and negative half cycles of the time-domain MNP response. The relative time constants with respect to the drive field period, (Formula presented.), were computed to enable quantitative comparison across different operating points. At each operating point, a linear fit was performed to (Formula presented.) as a function of each functional parameter (i.e., temperature or viscosity). The slopes of these linear fits were utilized to compute the temperature and viscosity sensitivities of TAURUS. Results: Except for outlier behaviors at 1 kHz, the following global trends were observed: (Formula presented.) decreases with drive field amplitude, increases with drive field frequency, decreases with temperature, and increases with viscosity. The temperature sensitivity varies slowly across the operating points and reaches a maximum value of 1.18%/ (Formula presented.) C. In contrast, viscosity sensitivity is high at low frequencies around 1 kHz with a maximum value of 13.4%/(mPa (Formula presented.) s) but rapidly falls after 3 kHz. These results suggest that the simultaneous estimation of temperature and viscosity can be achieved by performing measurements at two different drive field settings that provide complementary temperature/viscosity sensitivities. Alternatively, temperature estimation alone can be achieved with a single measurement at drive field frequencies above 3 kHz, where viscosity sensitivity is minimized. Conclusions: This work demonstrates highly promising temperature and viscosity sensitivities for TAURUS, highlighting its potential for simultaneous estimation of these two environmental parameters via MNP relaxation. The findings of this work reveal the potential of a hybrid MPI–MFH system for real-time monitored and localized thermal ablation treatment of cancer.
  • ItemOpen Access
    Automated parameter selection for accelerated mri reconstruction via low-rank modeling of local k-space neighborhoods
    (Elsevier GmbH, 2022-02-01) Ilıcak, Efe; Sarıtaş, Emine Ülkü; Çukur, Tolga; Ilıcak, Efe; Sarıtaş, Emine Ülkü; Çukur, Tolga
    Purpose: Image quality in accelerated MRI rests on careful selection of various reconstruction parameters. A common yet tedious and error-prone practice is to hand-tune each parameter to attain visually appealing reconstructions. Here, we propose a parameter tuning strategy to automate hybrid parallel imaging (PI) – compressed sensing (CS) reconstructions via low-rank modeling of local k-space neighborhoods (LORAKS) supplemented with sparsity regularization in wavelet and total variation (TV) domains. Methods: For low-rank regularization, we leverage a soft-thresholding operation based on singular values for matrix rank selection in LORAKS. For sparsity regularization, we employ Stein's unbiased risk estimate criterion to select the wavelet regularization parameter and local standard deviation of reconstructions to select the TV regularization parameter. Comprehensive demonstrations are presented on a numerical brain phantom and in vivo brain and knee acquisitions. Quantitative assessments are performed via PSNR, SSIM and NMSE metrics. Results: The proposed hybrid PI-CS method improves reconstruction quality compared to PI-only techniques, and it achieves on par image quality to reconstructions with brute-force optimization of reconstruction parameters. These results are prominent across several different datasets and the range of examined acceleration rates. Conclusion: A data-driven parameter tuning strategy to automate hybrid PI-CS reconstructions is presented. The proposed method achieves reliable reconstructions of accelerated multi-coil MRI datasets without the need for exhaustive hand-tuning of reconstruction parameters. © 2022
  • ItemOpen Access
    Trimmed multilevel fast multipole algorithm for D-type volume ıntegral equations
    (Institute of Electrical and Electronics Engineers, 2022-09-21) Topözlü, Halil; Karaosmanoğlu, B.; Ertük, Vakur Behçet; Topözlü, Halil; Ertük, Vakur Behçet
    In this work, we present a trimming scheme for the multilevel tree structure of multilevel fast multipole algorithm (MLFMA), which is applied on D-type volume integral equations. With this approach, the number of iterations and the durations of matrix-vector multiplications are significantly reduced for the solution of multi-scale volumetric problems. The trimming operation is performed on rows and columns of the impedance matrix. In order to eliminate the matrix columns, the current coefficients are estimated via machine learning techniques. The implementation particularly provides significant acceleration for the iterative solutions of electrically large volumetric problems.
  • 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
    Management of electroluminescence from silver-doped colloidal quantum well light-emitting diodes
    (Cell Press, 2022-05-18) Liu, Baiquan; Sharma, Manoj; Yu, Junhong; Wang, Lin; Shendre, Sushant; Sharma, Ashma; Izmir, Merve; Delikanlı, Savaş; Altıntaş, Yemliha; Dang, Cuong; Sun, Handong; Demir, Hilmi Volkan; Delikanlı, Savaş; Altıntaş, Yemliha; Demir, Hilmi Volkan
    Impurity doping is a promising strategy to afford colloidal nanocrystals exhibiting novel optical, catalytic, and electronic characteristics. However, some significant properties of noble metal-doped nanocrystals (NMD-NCs) remain unknown. Here, we report the electroluminescence (EL) from NMD-NCs. By doping silver impurity into cadmium selenide colloidal quantum wells (CQWs), dual-emission emitters are achieved and a light-emitting diode (LED) with a luminance of 1,339 cd m−2 is reported. In addition, the proposed energy gap engineering to manage exciton recombination is a feasible scheme for tunable EL emissions (e.g., the dopant emission is tuned from 606 to 761 nm). Furthermore, an organic-inorganic hybrid white LED based on CQWs is realized, reaching a color rendering index of 82. Moreover, flexible CQW-LEDs are reported. The findings present a step to unveil the EL property of NMD-NCs, which can be extended to other noble metal impurities, and pave the pathway for NMD-NCs as a class of electronic materials for EL applications. © 2022 The Authors