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In engineering education, a project can rarely be completed without the involved students having to read extensively and search for extra information not available in their textbooks, lecture notes, or laboratory manuals. Students have to find extra information for their research-projects and combine them with their knowledge from the other courses. This important objective opens students’ eyes to the realization that the degree by which they have digested the fundamental ideas of their core lessons will dictate their ability to access more knowledge because they appear to face paradoxes when confronting new situations. The merits of teamwork have been sacrificed for the sake of giving the student a very clear idea of the meaning of scientific research and significance of published material. It is expected to aid the student in a future research-oriented career. Teamwork will increase the amount of time spent on out-of-class learning as defined by the student, can be more effective than in-class time, particularly if the focus is learning on higher order learning. The authors believe that the student will be sufficiently exposed to teamwork values during their future design projects.

Bu çalışmada üniversite düzeyinde mesleki eğitim veren meslek yüksek okullarında (MYO) eğitim alanönlisans öğrencilerinin okullarını tercih etme ve okudukları bölümden beklentileri, sektör çalışma kosullarınıalgılamaları ve kariyer beklentileri değerlendirilmesi amaçlanmıştır. Bu amacı gerçekleştirmek için KocaeliÜniversitesi Gölcük MYO öğrencileri üzerinde anket çalışması yapılmıştır. Anket sonucu elde edilen verileristatistiksel olarak analiz edilmiştir. Öğrencilerin sektörün çalışma koşullarını algılamaları ve kariyer beklentilerineyönelik bir değerlendirme yapılmaktadır.

The sense of exhaustion, which affects people andinstitutions to a significant extent, is a constantly explored issue. Work isan important part of life and everyday life, and personal success is closelyrelated to the sense of exhaustion. The feeling that the body's physical andmental energy is over is an event that directly affects the person in thebilateral relationship and individually. In this study, the all dimension ofthe burnout sensation with different dimensions was studied and the effect ofthe individual on the personal success was examined. For this purpose,statistically tested using a generally accepted scale that can measuredepersonalization, emotional burnout and personal success, and the effect ofemotional burnout on personal achievement is examined., Kişileri ve kurumları önemli ölçüde etkileyentükenmişlik duygusu sürekli olarak araştırılan bir konu durumundadır. Çalışmahayatı ve günlük yaşamın önemli bir parçası olan kişisel başarı tükenmişlikduygusu ile yakından ilişkilidir. Kişinin bedensel ve ruhsal olarak enerjisininbittiğini hissetmesi, ikili ilişkilerinde ve bireysel olarak kişiyi direketkileyen bir olay halindedir. Bu çalışma da farklı boyutları olan tükenmişlikduygusunun tüm boyutları ele alınarak bireylerin kişisel başarısı üzerinde kietkisi incelenmeye çalışmıştır. Bu amaçla duyarsızlaşma, duygusal tükenmişlikve kişisel başarıyı ölçebilecek genel kabul görmüş bir ölçek kullanılarak eldeedilen veriler istatistiksel olarak test edilmiş ve duygusal tükenmişlikdüzeyinin kişisel başarı üzerindeki etkisi incelenmiştir.&nbsp

Sağlamlıkları, yüksek enerji radyasyonlarına dirençleri, morötesi dalgaboylarında çalışacak aygıtların üretimine olanak sağlaması gibi özelliklerinden dolayı III-Nitrat tabanlı malzeme sistemleri son yıllarda büyük ilgiyle araştırılmakta olan bir alan olmuştur. Üstün özellikleri sayesinde özellikle uzay uygulamalarında, morötesi aygıtların üretilmesinde, beyaz ışık oluşturulmasında, düşük dalgaboylarında yarıiletken lazer, ve lazer pompalama sistemlerinin geliştirilmesinde bu malzeme sistemleri üzerinde yoğun bir şekilde çalışılmaktadır. Diğer yarıiletken teknolojilerinde de karşılaşılan sürekli daha mükemmel kristal kalitesine sahip malzemenin büyütülmesi ve üretilmesi, özellikle AlGaN yapılarında karşılaşılan yüksek p-tipi Ohmik kontak kalitesi problemleri üzerinde yoğunlaşılmıştır. Buna ek olarak özellikle ışık saçan diyotlarda bulunan ışığın yapının dışına çıkarılması, halen değişik yöntemlerin denendiği bir çalışma konusudur. Bu tezde, yüksek kalitede nitrat tabanlı malzeme üretilmesi, bu malzemelerden yüksek performansa sahip fotodetektörler ve ışık kaynaklarının üretilmesi üzerinde çalışılmıştır. Buna ek olarak, diyotların üzerinde değişik yapılar üzerinde çalışılmış ve ışık saçan diyotla elektriksel olarak pompalanabilen inorganik/organik hybrid lazer sistemlerinin geliştirilmesi konusunda çalışmalar yapılmıştır. Recently in semiconductor market, III-Nitride materials and devices are of much interest due to their mechanical strength, radiation resistance, working in the spectrum from visible down to the deep ultraviolet region and solar-blind device applications. These properties made them strongest candidates for space telecommunication, white light generation, high power lasers and laser pumping light emitting diodes. Since, like other semiconductors, there have been material quality related issues, ongoing research efforts are concentrated on growing high quality crystals and making low p-type ohmic contact. Also, in light emitting device applications, similar to the visible and infrared spectrum components, there are challenging issues like high extraction efficiency and controlled radiation. In this thesis, we worked on growth and characterizations of high quality (In,Al)GaN based semiconductors, fabricating high performance photodiodes and light emitting diodes. We studied different surface modifications and possibilities of obtaining light emitting diode pumped organic/inorganic hybrid laser sources. 159

Özet DÜŞÜK SICAKLIKTA BÜYÜTÜLMÜŞ GaAs TABANLI REZONANT KA VİTE ARTTIRIMLI FOTODİYOTLAR Bayram Bütün Elektrik ve Elektronik Mühendisliği Yüksek Lisans Tez Yöneticisi: Prof. Orhan Aytür Ocak 2004 Uzun mesafe fiber optik haberleşme sistemlerinde, 1.3-1.6 um dalgaboyu aralığında çalışan fotodedektörler çok önemlidir. Gelişmiş üretim ve işleme teknolojisiyle GaAs, fotodedektör teknolojisinde en çok kullanılan yarıiletkenlerden biridir. Bununla birlikte, 870 nm civarinda kesilen soğurum tayfı nedeniyle yüksek dalgaboylarında kullanılamamaktadır. Yakın geçmişte, GaAs yarıiletkenini sözü geçen dalga boylarında da kullanabilmek için, yeni bir büyütme tekniği geliştirildi. Böylece yeni yapı, ışığı 1.7 um dalgaboyuna kadar soğurabiliyor. Bu çalışmamızda, 1.55 um'de çalışan, yüksek hızlı, p-i-n yapısında GaAs tabanlı fotodetektörlerin tasarımı, büyütülmesi, üretimi ve karakterizasyonunu sunuyoruz. Soğuran tabaka olarak düşük sıcaklıkta büyütülmüş GaAs kullanıldı. Işığın emilimi 1.55 um etrafındaki dar bir aralıkta, rezonant kavite yapısı mkullanılarak artırıldı. Taban aynası 15 çift GaAs/AlAs tabakasından oluşan Bragg Aynası şeklinde oluşturuldu. Tüm yapı, emilim bölgesi 200 °C sıcaklığında olmak üzere, moleküler ışın büyütümü tekniğiyle büyütüldü. Üretilen aygıtların rezonansının 1548 nm'de olduğu görüldü. Işığın emilim bölgesinden bir defa geçtiği yapıyla karşılaştırıldığında, rezonanstan kaynaklanan yükseltim faktörünün 7.5 olduğu ölçüldü. 1.55 um de gerçekleştirilen yüksek hız ölçümünde 1 1.2 GHz'e bant genişliğine karşılık gelen, 30 ps darbe tepkisi kaydedildi. Anahtar Sözcükler. GaAs, p-i-n fotodiyot, düşük sıcaklıkta büyütülmüş GaAs, moleküler ışın büyütümü, rezonant kavite artırımı. iv Abstract LOW TEMPERATURE GROWN GaAs BASED RESONANT CAVITY ENHANCED PHOTODIODES Bayram Bütün M. S. in Electrical and Electronics Engineering Supervisor: Prof. Orhan Aytür January 2004 High performance photodetectors operating in the 1.3 - 1.6 um wavelength range are vital components for long-haul optical fiber communication systems. GaAs with its mature fabrication methods is one of the most used semiconductors in photodetector technology, but with a low cut-off wavelength around 870 nm. To use GaAs at longer wavelengths, a new growth technique has been developed, in which GaAs was grown at low temperatures (LT-GaAs), so that it absorbs photons with wavelengths up to 1.7 um. In this work, we report the design, growth, fabrication, and characterization of GaAs-based high-speed p-i-n photodiodes operating at 1.55 um. A LT-GaAs layer was used as the absorption layer and the photoresponse was selectively enhanced at 1.55 um using a resonant cavity detector structure. The bottom mirror of the resonant cavity was formed by a highly reflective 15-pair GaAs/AlAs Bragg mirror. Molecular beam epitaxy was used for wafer growth, where the active LT-GaAs layer was grown at a substrate temperature of 200 °C. The fabricated devices exhibited resonance around 1548 nm. When compared to the efficiency of a conventional single-pass detector, an enhancement factor of 7.5 was achieved. Temporal pulse-response measurements were carried out at 1.55 um. Fast pulse responses with 30 ps pulse-width and a corresponding 3-dB bandwidth of 11.2 GHz was measured. Keywords: GaAs, p-i-n photodiode, low temperature grown GaAs, molecular beam epitaxy, resonance cavity enhancement. 67

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

Gallium nitride (GaN) high-electron-mobility transistor (HEMT) devices, which have wide application potential from power amplifiers to satellite, need to be thoroughly examined in terms of reliability in order to benefit the superior intrinsic properties of the device. The most critical parameter in the device reliability is the hotspot, or Tmax , which occurs somewhere on the subsurface and along the channel of the GaN HEMT, which is optically inaccessible due to optical path disability. Therefore, the Tmax value is underestimated in optical measurements, such as the thermographic IR and Raman methods. With 3-D electrothermal simulations, Tmax is obtained close to reality, but it requires a huge computation load and the complex modeling of semiconductor device physics. In 2-D or 3-D thermal simulations that do not use electrothermal simulations, since the self-heating is mostly modeled with a single heat source, neither the correct Tmax value is obtained nor the effect of bias conditions is considered. To address the aforementioned shortcomings, a hybrid method is demonstrated, which exploits the electrical measurements of GaN HEMT, which RF and reliability engineers often and easily do. It is demonstrated that Tmax can be determined quickly and close to the electrothermal simulations in a GaN HEMT device with a two-heat source method and finite element analysis (FEA) hybrid interaction with respect to various bias conditions. Moreover, the impact of the knee voltage is investigated with different knee-detection techniques. The proposed method provides GaN HEMT reliability engineers with an easy-to-implement alternative to reveal the hotspot location and the value.

This paper presents a method to reveal the channel temperature profile of high electron mobility transistors (HEMTs) in a multi-stage monolithic microwave integrated circuit (MMIC). The device used for this study is a two-stage X-band low-noise amplifier fabricated using 0.15 m GaN-on-SiC technology with 4x50 m and 4x75 m HEMTs at the first and the second stage, respectively. The surface temperature measured through infrared (IR) thermography has a diffraction-limited resolution. Moreover, it is impossible to measure sub-surface Tmax residing inside the two-dimensional electron gas of HEMT using IR thermographic measurements. Finite element analysis (FEA) thermal simulations are performed in this study to acquire the surface and sub-surface temperature profiles of the whole MMIC. IR measurements and FEA simulations are integrated through a correlation-based method verifying the accuracy of the FEA-based temperature profiles. This method leads to accurately finding the hotspots in the MMIC, thus revealing the Tmax of both stages. The correlation method using two filters approach to match the measurements and simulated temperature profiles of all the stages finds its application in MMICs’ high-temperature operating lifetime reliability tests.

In this article, heat generation distribution and maximum device temperature of gallium-nitride (GaN) high-electron-mobility transistors (HEMTs) are investigated by using the 2-D electrothermal and finite-element method (FEM) simulations. Devices with different gate lengths and source-to-drain spacing are investigated. It is observed that the maximum device temperature (TMAX) depends on the drain-to-source spacing and is almost independent of the gate length and that the assumption of a uniform heat generation region, under the gate, is not accurate; this is contrary to conventional calculation methods. Moreover, based on the results, a new approximation is proposed to use in the FEM simulations that can estimate TMAX more accurately. This method does not require physics-based technology computer-aided design (TCAD) simulations and can work with a low mesh density. The performance is compared with prior methods. This work was supported by Turkish Scientific and Technological Research Council, TUBITAK, under 1501 project GaNTURK. The work of Ekmel Özbay was supported in part by the Turkish Academy of Sciences.

Light-Matter Interactions at subwavelength-designed nanostructures have been the subject of intensive study in recent years. The realization of an ideal "blackbody" absorber is an emerging topic in nanophotonics and nanoplasmonics. An ideal black absorber is an object that harvests incoming light with near-unity efficiency. Based on their absorption spectral coverage, they are classified as narrow-band or broadband absorbers. This requirement can be achieved in bulky designs that have a thickness much larger than its light penetration depth as well as antireflective surface texturing.

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 (a-Si)-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 ( $\lambda =\textsf {350}$ nm) and visible ( $\lambda =\textsf {500}$ nm) regions, respectively.

In recent years, sub-wavelength metamaterials-based light perfect absorbers have been the subject of many studies. The most frequently utilized absorber configuration is based on nanostructured plasmonic metals. However, two main drawbacks were raised for this design architecture. One is the fabrication complexity and large scale incompatibility of these nano units. The other one is the inherent limitation of these common metals which mostly operate in the visible frequency range. Recently, strong interference effects in lithography-free planar multilayer designs have been proposed as a solution for tackling these drawbacks. In this paper, we reveal the extraordinary potential of bismuth (Bi) metal in achieving light perfect absorption in a planar design through a broad wavelength regime. For this aim, we adopted a modeling approach based on the transfer matrix method (TMM) to find the ideal conditions for light perfect absorption. According to the findings of our modeling and numerical simulations, it was demonstrated that the use of Bi in the metal-insulator-metal-insulator (MIMI) configuration can simultaneously provide two distinct functionalities; a narrow near unity reflection response and an ultra-broadband near perfect absorption. The reflection behavior can be employed to realize additive color filters in the visible range, while the ultra-broadband absorption response of the design can fully harvest solar irradiation in the visible and near infrared (NIR) ranges. The findings of this paper demonstrate the extraordinary potential of Bi metal for the design of deep sub-wavelength optical devices.

In this work, highly linear AlGaN/GaN laterally gated (or buried gate) high-electron-mobility transistors (HEMTs) are reported. The effect of gate dimensions on source-access resistance and the linearity of laterally gated devices are investigated experimentally in detail for the first time. Transistors with different gate dimensions and conventional planar devices are fabricated using two-step electron beam lithography (EBL). Current–voltage, source-access resistance, small-signal, and two-tone measurements are performed to evaluate the linearity of devices. Contrary to conventional planar HEMTs, the intrinsic transconductance of laterally gated devices monotonically increases with increasing gate voltage, showing a similar behavior as junction field-effect transistors (FETs). The source-access resistance shows a polynomial increase with the drain current, which can be reduced by decreasing the filling ratio of the buried gates. Through the optimization of these two competing factors, i.e., intrinsic transconductance and the source-access resistance, flat transconductance with high linearity is achieved experimentally. The laterally gated structure shows flat transconductance and small-signal power gain over a larger span of gate voltage that is 2.5 times higher than a planar device. Moreover, 6.9-dB improvement in output intercept point (OIP3)/ ${P}_{\text {DC}}$ is achieved. This approach can be used to improve the linearity of AlGaN/GaN HEMTs at the device level.

Mid-infrared (MIR) plasmon-phonon features of heterostructures composing of a plasmonic anisotropic two-dimensional material (A2DM) on a hexagonal boron nitride (hBN) film are analyzed. We derive the exact dispersion relations of plasmon-phonons supported by the heterostructures and demonstrate the possibility of topological transitions of these modes within the second Reststrahlen band of hBN. The topological transitions lead to enhanced local density of plasmon-phonon states, which intensifies the spontaneous emission rate, if the thickness of the hBN layer is appropriately chosen. We also investigate a lateral junction formed by A2DM/hBN and A2DM, demonstrating that one can realize asymmetric guiding, beaming, and unidirectionality of the hybrid guided modes. Our findings demonstrate potential capabilities of the A2DM/hBN heterostructures for active tunable light–matter interactions and asymmetric in-plane polariton nanophotonics in the MIR range.

Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit (FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5-9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found. ispartof: Scientific Reports vol:8 issue:1 ispartof: location:England status: published

Fabrication, characterization, and analysis of an ultra-broadband lithography-free absorber is presented. An over 94% average absorption is experimentally achieved in the wavelength range of 450-1400 nm. This ultra-broadband absorption is obtained by a simple annealed tri-layer metal-insulator-metal (MIM) configuration. The metal used in the structure is Manganese (Mn), which also makes the structure cost-effective. It is shown that the structure retains its high absorption for TM polarization, up to 70 degrees, and, for TE polarization, up to 50 degrees. Moreover, the physical mechanism behind this broadband absorption is explained. Being both lithography-free and cost-effective, the structure is a perfect candidate for large-area and mass production purposes., 8 pages, 3 figures

Date of Conference: 2-7 February 2019 Conference name: SPIE OPTO, 2019 A normally-off InAlN/GaN high electron mobility transistor (HEMT) on Si substrate with a p-GaN gate is reported. Devices are fabricated on two different epitaxial structures, one containing a high resistive GaN buffer layer and one containing an AlGaN back-barrier, and the threshold voltage, drain current density, and buffer leakage current are compared. With the epitaxial structure containing a high resistive GaN layer, normally-off operation with a threshold voltage of +0.5 V is achieved. The threshold voltage is further increased to +2 V with the AlGaN back-barrier, and the buffer leakage current was improved by over an order of magnitude. The Society of Photo-Optical Instrumentation Engineers (SPIE)

Bi-tunable asymmetric light transmission (AT) and nearly perfect resonant absorption functionalities are achieved by a Lorentz-reciprocal metamaterial for the operation at the mid-infrared (MIR) wavelengths and transverse magnetic polarization. The bi-tunable metamaterial with bi-functional features and a total thickness of 1.8 mu m is based on an hBN/graphene/hBN heterostructure that is bounded by a Ge grating on the upper side and a hybrid VO2/Au grating on the lower side. Through analytical calculations, we first investigate how the dispersion characteristics of the high-beta hyperbolic phonon polaritons of hBN can be controlled and hybridized through the insulator (i-VO2) to metal (m-VO2) transition of VO2 in a bare hBN/VO2 heterostructure. Then, at the absence of graphene and owing to the support of the hybridized high-beta modes, a broad and efficient AT with forward-to-backward contrast exceeding 40% is obtained by numerical calculations for the i-VO2 case, as the first functionality of the structure. Moreover, it is found that for the m-VO2 case, the device is no longer transmittive and a nearly perfect resonant absorption response, as the second functionality, is observed for backward illumination. Finally, by introducing multilayer graphene into the structure and considering the intermediate states of VO2 in the calculations, the bi-tunable transmission and absorption characteristics of the device are investigated. We believe the designed metamaterial is well-suited for MIR optical diodes, sensors, and thermal emitters. (c) 2019 Optical Society of America

We theoretically prove in this paper that using planar multilayer graphene-hexagonal boron nitride (hBN) metamaterials (GhMMs) can yield ultrabroadband and high-contrast asymmetric transmission (AT) and asymmetric absorption (AA) of light. The AA and AT features are obtained in the far-infrared (FIR) and mid-infrared (MIR) regions for normally incident light with transverse magnetic polarization. Here, the GhMMs are integrated with two asymmetric gratings of Ge and are composed of alternating multilayers of graphene (11 multilayers) and hBN layers (10 layers). Moreover, the total subwavelength thickness of the hybrid structures is about 3 mu m, being less than half of the free-space wavelength up to nearly 50 THz. This approach-which is similar to the one introduced by Xu and Lezec [Nat. Commun. 5, 4141 (2014)] for a passive hyperbolic metamaterial operating in the visible range-is based on the excitation of high-beta modes of the GhMM with different transmission characteristics. In addition to being ultrabroadband and high-contrast, AT and AA features of the proposed GhMMs can be actively tuned by varying the chemical potential of graphene. Furthermore, it is shown that an on-off switching of AT factor at FIR and selective tunability at MIR frequencies can be obtained via varying mu. Due to its subwavelength and planar configuration and active operation, these multilayer graphene-hBN metamaterials with AT and AA characteristics hold promise for integration with compact optical systems operating in the MIR and FIR ranges and are suitable for applications such as optical diodes, sensors, and thermal emitters. Published under license by AIP Publishing.

In this paper, we report an angstrom-thick atomic layer deposited (ALD) aluminum oxide (Al2O3) dielectric passivation layer for an AlGaN/GaN high electron mobility transistor (HEMT). Our results show a 55% improvement in the gate lag performance of the design and a decrease by half in interface state density upon coating with two cycles of ALD Al2O3. DC characteristics such as current density, threshold voltage, and leakage currents were maintained. ALD Al2O3 passivation layers with thicknesses up to 10 nm were investigated. XPS analyses reveal that the first ALD cycles are sufficient to passivate GaN surface traps. This study demonstrates that efficient passivation can be achieved in atomic-scale with dimensions much thinner than commonly used bulk layers.

Achieving nearly perfect light absorption from the microwave to optical region utilizing metamaterials has begun to play a significant role in photonics and optoelectronics due to their vital applications in thermal emitters, thermal photovoltaics, photovoltaics, sensing, filtering, and photodetection. However, employing passive components in designing perfect absorbers based on metamaterials and photonic crystals imposes some limits on their spectral operation. In order to overcome those limits, extensive research has been conducted on utilizing different materials and mechanisms to obtain active metamaterial light absorbers. In this review paper, we investigate the recent progress in tunable and reconfigurable metamaterial light absorbers through reviewing different active materials and mechanisms, and we provide a perspective for their future development and applications. (C) 2019 Optical Society of America

In this article, a lithography-free multilayer based color filter is realized using a proper series connection of two cavities that shows relatively high efficiency, high color purity, and a wide view angle. The proposed structure is a metal-insulator-metal-insulator-semiconductor (MIMIS) design. To optimize the device performance, at the first step, transfer matrix method (TMM) modeling is utilized to find the right choices of materials for each layer. Simulations are carried out later on to optimize the geometries of the layers to obtain our desired colors. Finally, the optimized devices are fabricated and experimentally characterized to evaluate our modelling findings. The characterization results of the fabricated samples prove the successful formation of efficient and wide view angle color filters. Unlike previously reported FP based designs that act as a band-stop filter in reflection mode (absorbing a narrow frequency range and reflecting the rest of the spectrum), this design generates a specific color by reflecting a narrow spectral range and absorbing the rest of the spectrum. The findings of this work can be extended to other multilayer structures where an efficient connection of cavities in a tandem scheme can propose functionalities that cannot be realized with conventional FP resonators.

We demonstrate the electrical performances of an AlGaN/GaN metal–insulator–semiconductor high electron mobility transistor (MIS-HEMT) with low gate leakage current (Ig). A low gate leakage current as low as the order of 10−11 A/mm was achieved from normally-off MIS-HEMT device (Vth = 2.16 V) with a partially recessed gate, fluorine treatment, and ALD Al2O3 gate dielectric layer. The gate leakage current decrease is attributed to the pre-treatment of the gate region with hydrofluoric acid (HF) and deionized water (DI) solution, which acts to remove the native oxide layer and thus decrease interface traps. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM) analyses demonstrate that the AlGaN surfaces are modified such that the surface roughness and native oxide introduced by the treatments used to achieve normally-off operation are remedied with the use of the pre-treatment.

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.

A hybrid approach for obtaining normally off high electron mobility transistors (HEMTs) combining fluorine treatment, recess etch techniques, and AlGaN buffer was studied. The effects of process variations (recess etch depth and fluorine treatment duration) and epitaxial differences (AlGaN and carbon doped GaN buffers) on the DC characteristics of the normally off HEMTs were investigated. Two different epitaxial structures and three different process variations were compared. Epitaxial structures prepared with an AlGaN buffer showed a higher threshold voltage ( $V_{\mathrm {th}} = + 3.59 ~\text {V}$ ) than those prepared with a GaN buffer ( $V_{\mathrm {th}} = +1.85 ~\text {V}$ ).

In this Letter, we demonstrate an ultra-broadband metamaterial absorber of unrivaled bandwidth (BW) using extraordinary optical response of bismuth (Bi), which is the material selected through our novel analysis. Based on our theoretical model, we investigate the maximum metal–insulator–metal (MIM) cavity BW, achievable by any metal with known n-k data. We show that an ideal metal in such structures should have a positive real permittivity part in the near-infrared (NIR) regime. Contrary to noble and lossy metals utilized by most research groups in the field, this requirement is satisfied only by Bi, whose data greatly adhere to the ideal material properties predicted by our analysis. A Bi nanodisc-based MIM resonator with an absorption above 0.9 in an ultra-broadband range of 800 nm–2390 nm is designed, fabricated, and characterized. To the best of our knowledge, this is the broadest absorption BW reported for a MIM cavity in the NIR with its upper-to-lower absorption edge ratio exceeding best contenders by more than 150%. According to the findings in this Letter, the use of proper materials and dimensions will lead to realization of deep sub-wavelength efficient optical devices.

Beynin yapısındaki sinapsları taklit etmek için üretilen nano boyutlardaki cihazların modellenmesi, insan beyninden ilham alan hesaplama sistemlerinin geliştirilmesinde temel bir rol oynamaktadır. Faz-değişim malzemesi kullanan bu sinaptik aygıtlar, krosbar mimarisi ile paralel olarak bağlandığında, insan beyni ile hız, enerji ve kapladığı hacim bakımından kıyaslanabilir bir performans göstermektedir. Bu çalışmada ise faz-değişim malzemesi kullanılarak üretilen seri bağlanmış hafıza ve seçiciden oluşan tek bir sinaptik aygıtın, elektrotermal özellikleri modellenmiştir. Çalışmanın ilk kısmında, sonlu element yöntemi tabanlı bir simülasyon yapısı sunulmuş, sinaptik cihazın hafıza kısmının elektriksel, ısıl ve olasılıksal faz-değişim dinamikleri modellenmiştir. Ek olarak, hafıza kodlanmasını sağlayan amorf ve kristal hallerinin arasındaki kademeli geçişin nano saniye uzunluğundaki voltaj vuruşlarıyla yapılabildiği gösterilmiştir. İkinci kısımda, sinaptik cihazın seçici kısmının zamanla elektriksel iletkenliğinin değişimi ve doyumu, sıcaklık ve zamana bağlı olarak modellenmiştir. Geliştirdiğimiz model, 8 nm teknolojisi ile üretilmiş cihazlarda ilk defa gözlemlenen, çok hızlı oluşan elektriksel iletkenlik değişiminin doyumu ile yakın uyum göstermiştir. Modeling nanoscale devices that emulate the functionality of synapses of the biological brain is a fundamental operation for developing brain-inspired computational systems. Phase-change material based synaptic devices offer promising performance in speed, spatial and power efficiency metrics, up to human brain level, when connected in a massively parallel crossbar array architecture. In this work, we modeled electrothermal characteristics of a single synaptic device consisting of phase-change material based memory and its selector. First, we proposed a finite element method based simulation framework for modeling electrical, thermal and probabilistic crystallization dynamics of the memory unit. Gradual phase transitions that form device memory between amorphous and crystalline states are studied under nanosecond voltage pulses. Second, we implemented time and temperature dependent resistance drift saturation model for phase-change material based selector device. Our model is in close agreement with the ultrafast saturation phenomena which is observed for the first time in fabricated devices with 8 nm node technology. 89

In this paper, we propose an electrically tunable mid-infrared plasmonic-phononic absoiber with omnidirectional and polarization insensitive nearly perfect resonant absorption characteristics. The absorber consists of a graphene/hexagonal boron nitride (hBN)/graphene multilayer on top of a gold bottom reflector separated by a dielectric spacer. The graphene/hBN/graphene multilayer is patterned as a hole array in square lattice. We analytically and numerically prove that, due to the support of hybrid plasmon-phononpolaritons, nearly perfect multi-resonant absorption peaks with high quality factors are obtained both inside and outside of the Reststrahlen band of hBN. As a result of the hybridization of graphene plasmons with the hyperbolic phonon polaritons of hBN, the high quality resonant absorptions of the metamaterial are almost unaffected by decreasing the phenomenological electron relaxation time of graphene. Moreover, the obtained resonances can be effectively tuned in practice due to the continuity of the graphene layers in the hole array metamatenal. These features make the graphene-hBN metamaterial a skeptical design for practical purposes and mid-infrared multi-functional operations such as sensing. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

Orta kızılötesi kuantum çağlayan lazer teknolojisi, 1994 yılında icadından beri hızla gelişmiştir. Kuantum çağlayan lazer teknolojisi küçük, karmaşık olmayan ve verimli olduğundan, çeşitli uygulamalar için en çok kullanılan orta kızılötesi lazer kaynağı haline gelmiştir. Kuantum çağlayan lazerlerinin özel uygulama alanları serbest alan iletişimi, kimyasal spektroskopi, çevresel izleme ve kızılötesi karşı önlemlerdir. Bu çalışma teorik arka plan incelemesi, teorik analizler, kuantum çağlayan lazer tasarımı, kuantum mekanik, termal ve optik simülasyonlar, cihaz imalatı, optimizasyonu ve deneysel karakterizasyonu içeren orta kızılötesi kuantum çağlayan lazeri üzerinde kapsamlı bir analiz sunmaktadır. Bu çalışma sonucunda orta kızılötesi kuantum çağlayan lazer başarıyla geliştirilmiş ve üretilmiştir. Mid-infrared quantum cascade laser technology has been developed rapidly since it was demonstrated in 1994. Since quantum cascade laser technology is small, robust and efficient, they have become commonly used mid-infrared laser source for various applications. The particular application fields of quantum cascade lasers are free space communication, chemical spectroscopy, environmental monitoring and infrared countermeasure. This work presents a comprehensive study over mid-infrared quantum cascade laser including review of theoretical background, theoretical analyses, active layer design, quantum mechanical, thermal and optical simulations, device fabrication, optimization and experimental characterization. As a result of this work mid-infrared quantum cascade laser have been successfully demonstrated. 126

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.

In this paper, we propose a methodology to maximize the absorption bandwidth of a metal-insulator-metal (MIM) based absorber. The proposed structure is made of a Cr-Al2O3-Cr multilayer design. At the initial step, the optimum MIM planar design is fabricated and optically characterized. The results show absorption above 0.9 from 400 nm to 850 nm. Afterward, the transfer matrix method is used to find the optimal condition for the perfect light absorption in an ultra-broadband frequency range. This modeling approach predicts that changing the filling fraction of the top Cr layer can extend light absorption toward longer wavelengths. We experimentally proved that the use of proper top Cr thickness and annealing temperature leads to a nearly perfect light absorption from 400 nm to 1150 nm, which is much broader than that of a planar design. Therefore, while keeping the overall process lithography-free, the absorption functionality of the design can be significantly improved. The results presented here can serve as a beacon for future performance-enhanced multilayer designs where a simple fabrication step can boost the overall device response without changing its overall thickness and fabrication simplicity. (c) 2018 Chinese Laser Press