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Critical behavior of the spontaneous polarization and the dielectric susceptibility close to the cubic-tetragonal transition in BaTiO_(3)

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Journal of Advanced Dielectrics 2015年第3期5卷 50-53页

作者： H.Yurtseven F.Oguz Department of Physics Faculty of Arts and Sciences Middle East Technical University06531 AnkaraTurkey Micro and Nanotechnology Program Graduate School of Natural Applied Sciences Middle East Technical University 06800 AnkaraTurkey

Using Landau mean field model,the spontaneous polarization and the dielectric susceptibility are analyzed as functions of temperature and pressure close to the cubic-tetragonal(ferroelectric-paraelectric)transition in BaTiO_(3).From the analysis of the dielectric susceptibility and the spontaneous polarization,the critical exponents are deduced in the classical and quantum limits for BaTiO_(3).From the critical behavior of the dielectric susceptibility,the spontaneous polarization can be described for the ferro-electric–paraelectric(cubic to tetragonal)transition between 4 and 8 GPa at constant temperatures of 0 to 200K in BaTiO_(3) within the Landau mean field model given here.

关键词： Dielectric susceptibility spontaneous polarization ferroelectric-paraelectric transition Landau mean field model BaTiO_(3)

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The Effect of Post-Growth Thermal Annealing on CdZnTe Crystals Grown by Vertical Gradient Freeze Technique

The Effect of Post-Growth Thermal Annealing on CdZnTe Crysta...

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2021 IEEE Nuclear Science Symposium and Medical Imaging Conference, NSS/MIC 2021

作者： Unal, M. Balbasi, O.B. Karaman, M.C. Celik, G. Genc, A.M. Parlak, M. Turan, R. Middle East Technical University Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences Ankara06800 Turkey Middle East Technical University Crystal Growth Laboratory Ankara06800 Turkey Middle East Technical University Department of Physics Ankara06800 Turkey

ISBN: (纸本)9781665421133

Te inclusions are the primary performance degrading defects in CdZnTe crystals. Because of the unique properties of CdZnTe, it is not possible to eliminate Te inclusions during the growth process. However, annealing process could decrease the concentration of Te inclusions by dissolving or migrating them at elevated temperatures. Annealing under Cd atmosphere could dissolve Te inclusions by introducing new Cd atoms into CdZnTe crystals and compensating excess Te atoms inside of inclusions. However, implemented Cd atoms can form interstitial Cd defects easily;thus, annealing under Te atmosphere is needed to eliminate intestinal Cd defects created by Cd-annealing. In this study, annealing under different atmospheres is studied, and changes in Te inclusion concentrations are observed for various crystals. Additionally, it is observed that the resistivity of doped crystals decreases significantly after Cd-annealing, which is restored with Te-annealing. On the other hand, undoped CdZnTe crystals act differently after annealing steps. © 2021 IEEE.

关键词： Annealing

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Sensitivity Enhancement in Resonant microbolometers with Dual Mode Operation

Sensitivity Enhancement in Resonant Microbolometers with Dua...

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IEEE International Symposium on Applications of Ferroelectrics (ISAF)

作者： Onurcan Kaya Kıvanç Azgın Middle East Technical University (METU) Ankara Turkey Graduate School of Natural and Applied Sciences - Micro and Nanotechnology Program METU Ankara Turkey

ISBN: (数字)9781728164304

ISBN: (纸本)9781728167534

This study proposes a novel “vibration amplitude” based sensitivity enhancement method for a resonant-plate microbolometer, which operates in two different mode shapes simultaneously (dual-mode). Thanks to nonlinear cross-coupling between the two resonant modes, the proposed method boosts up the sensitivity of the second operational mode by 25.6% compared to conventional amplitude-based sensing.

关键词： Resonant frequency Vibrations Sensitivity Phase frequency detectors Shape Phase locked loops Temperature sensors

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\(640\times 512\) Extended Short Wavelength Infrared In0.83Ga0.17As Focal Plane Array

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IEEE Journal of Quantum Electronics 2014年第12期50卷 957-964页

作者： Yetkin Arslan Fikri Oguz Cengiz Besikci Quantum Devices and Nanophotonics Research Laboratory Middle East Technical University Ankara Turkey Micro and Nanotechnology Program Graduate School of Natural and Applied Sciences Middle East Technical University Ankara Turkey

We present the detailed characteristics of solid source molecular beam epitaxy (MBE) grown large format (640 × 512) extended short wavelength infrared In0.83Ga0.17As sensor with desirable performance at both pixel and focal plane array (FPA) levels. The FPA pixels in the mesa structure grown on a graded AlInAs buffer layer with 2.65-μm, 300-K cutoff wavelength exhibited 300and 200-K peak detectivities as high as ~2.5×10 10 and ~1×1012 cmHz 1/2 /W, which are both equivalent to the theoretical limits set by the Johnson noise of the detector. Dark current analysis of the pixels displayed no considerable tunneling component with the dark current being dominated by generation-recombination and shunt leakage mechanisms >200 K up to a reverse bias voltage of 3 V. Moreover, the noise measurements displayed no 1/f noise in the FPA pixels. In spite of the large lattice mismatch, the FPA yielded very good response linearity, as well as impressively good responsivity nonuniformity and pixel operability of 5.5% and 99.8%, respectively. The results showing the feasibility of both solid source MBE and graded AlInAs buffer for the growth of extended InGaAs photodetectors are very encouraging for the large format FPA implementation of these detectors with desirable imaging performance and an extended cutoff wavelength as high as ~2.7 μm.

关键词： Detectors Dark current Temperature measurement Indium gallium arsenide Buffer layers Substrates Doping

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A MEMS Based Lens microscanner for Resolution Enhancement of Infrared Imaging Systems

A MEMS Based Lens Microscanner for Resolution Enhancement of...

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IEEE SENSORS

作者： Ahmet Sözak Ertuğ Şimşek Kıvanç Azgın Graduate School of Natural and Applied Sciences Micro and Nanotechnology Program Middle East Technical University Department of Mechanical Engineering Middle East Technical University Ankara Turkey

The aim of this study is to demonstrate a micro Electro Mechanical Systems (MEMS) based in-plane (x and y axes) lens scanner to improve the resolution of Long Wave Infrared Optical Systems (8-12μm wavelength). The proposed actuator consists of a 2 axis decoupled stage and has 4 separate V-Shaped (Chevron) thermal actuators which provide sufficient force and displacement to position the lens within required time. Miniaturization of lens has been achieved by using an aspherical surface and optimization of optical system parameters. Actuators have 40 micrometers displacement capability which is sufficient for sub-pixel image motion on the detector. Frequency of actuation is 25 Hz which corresponds to construction of 1 frame per 40 milliseconds.

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Optimization of a microbolometer pixel for operation in the sub-1 THz region 12

Optimization of a microbolometer pixel for operation in the ...

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Terahertz Emitters, Receivers, and Applications XII 2021

作者： Demirörs, Oytun Atik, Bahar Dirican, Emrah Altan, Hakan Esentürk, Okan Ylldlrlm, Mustafa Akln, Tayfun METU MEMS Center Ankara06510 Turkey Ankara06800 Turkey Graduate School of Natural and Applied Sciences Micro and Nanotechnology METU Ankara06800 Turkey Department of Physics METU Ankara06800 Turkey Department of Electrical and Electronic Engineering METU Ankara06800 Turkey

ISBN: (数字)9781510644939

ISBN: (纸本)9781510644922

Imaging in the Terahertz (THz) region has drawn attention in recent years, but the nature of the THz frequency regime causes some drawbacks in imaging such as long wavelength, high cost, and low emission levels at room temperature. Because of the high atmospheric absorption of THz waves, fabrication of a microbolometer pixel that works in the sub-1 THz frequency regime is necessary. Large pixel pitches due to longer wavelengths and the resulting higher thermal mass pose a difficult challenge. Due to those limitations, a unique design of an absorber is essential for THz microbolometers. This study investigates the use of absorbers based on novel materials and alloys with the goal of developing efficient absorbers in small pixel pitches. First, thin layer metal absorbers typically used in commercial IR microbolometers are characterized in terms of absorption performance in the sub-1 THz region. Thin films based on metal alloys such as TiAlV show a markedly lower absorption in this region than in the IR. To improve the performance of these absorbing layers and reduce pixel pitches. Use of effective media based on the mixture of dielectric materials and metals with patterned thin films are investigated to develop unique absorbing thin layers. It is seen that with the use of an effective medium whose complex dielectric constant is tailored appropriately, efficient absorption of sub-1 THz radiation can be achieved. © 2021 SPIE.

关键词： Thin films

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Dependence of plasmonic enhancement of photocurrent in a-Si: H on the position and thickness of SiNx spacer layers

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Physica Status Solidi (C) Current Topics in Solid State Physics 2015年第9-11期12卷 1220-1223页

作者： Saleh, Zaki M. Nasser, Hisham Özkol, Engin Bek, Alpan Turan, Raşit METU Ankara Turkey Department of Physics Arab American University-Jenin Jenin Palestine Department of Physics Middle East Technical University Ankara Turkey Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences METU Ankara Turkey

Plasmonic interfaces integrated to the front, back and both surfaces of photovoltaic thin films show different degrees of enhancement of light trapping. Enhancements in the spectral dependence of photocurrent normalized to the power of excitation light are used as an indicator of enhanced light trapping. In a previous study, we obtained enhancement in the spectral range of 600-700 nm by integrating 100-nm Ag nanoparticles to the back surface of a-Si:H with a critical dependence on the SiNx spacer layer thickness. In this study, we compare the enhancement in photocurrent due to plasmonic interfaces integrated to the front, back and both front and back surfaces of the a-Si:H absorber. Interfaces integrated to the back result in the largest enhancement in photocurrent while those integrated to the front give the lowest. The marginal enhancements due to two interfaces appear to be mainly due to the back interface. While plasmonics effects may not account for the total enhancement in photocurrent, it explains the relative enhancement in the spectral range of 550-700 rather well. For all configurations, the enhancement in the spectral dependence of photocurrent is accompanied by broadening into the red of the localized surface plasmon resonance (LSPR). © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

关键词： Plasmonics

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Investigation of doped and undoped VOxfilms for terahertz microbolometer applications 12

Investigation of doped and undoped VOxfilms for terahertz mi...

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Terahertz Emitters, Receivers, and Applications XII 2021

作者： Atik, Bahar Dirican, Emrah Demirörs, Oytun Altan, Hakan Esentürk, Okan Ylldlrlm, Mustafa Akln, Tayfun METU MEMS Center Ankara06510 Turkey Dept. of Chemistry METU Ankara06800 Turkey Graduate School of Natural and Applied Sciences Micro and Nanotechnology METU Ankara06800 Turkey Dept. of Physics METU Ankara06800 Turkey Dept. of Electrical and Electronic Eng. METU Ankara06800 Turkey

ISBN: (纸本)9781510644922

The microbolometer technology has proved its potential in the Infrared (IR) region due to its low fabrication costs, and room temperature operation, making this technology desirable to be used in various applications, and this interest has recently expanded into the Terahertz (THz) region as well. The detection in microbolometers is achieved through the absorption of THz radiation which subsequently heats up and is sensed by the temperature sensitive material at the core of the device. This temperature sensitive material is typically based on VOx, which exhibits a sufficient change in resistance with temperature. While this temperature sensitive material is useful in the IR, the low energy of the THz wave compared to the background radiation makes it a challenge to operate the device at room temperature and show a large change in resistance with respect to the slight change in temperature. Metal doped VOx films can show a better performance however these effects are not well understood in the THz region. In this study, Tungsten (W) doped and undoped VOx films are fabricated and then analyzed using Time Domain THz Spectroscopy. The DC electrical properties of the films as well as their optical behaviors in the region of 0.2-2.0 THz are analyzed as a function of temperature. The metal doping is seen to affect the overall electrical and optical response of the film. Understanding this dependence is key to achieving a better film for applications in the THz region. © 2021 SPIE.

关键词： Terahertz waves

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Enhancement of four-wave mixing via interference of multiple plasmonic conversion paths

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Physical Review B 2016年第3期93卷 035410-035410页

作者： Shailendra K. Singh M. Kurtulus Abak Mehmet Emre Tasgin Institute of Nuclear Sciences Hacettepe University 06800 Ankara Turkey The Center for Solar Energy Research and Applications (GUNAM) Ortadoğu Teknik Üniversitesi 06800 Ankara Turkey Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences Ortadoğu Teknik Üniversitesi 06800 Ankara Turkey

Recent experiments demonstrate that plasmonic resonators can enhance the four-wave mixing (FWM) process by several orders of magnitude, due to the localization of the incident fields. We show that, when the plasmonic resonator is coupled to two quantum emitters, a three orders of magnitude enhancement can be obtained on top of the enhancement due to the localization. We explicitly demonstrate—on an expression for the steady-state FWM amplitude—how the presence of a Fano resonance leads to the cancellation of nonresonant terms in a FWM process. A cancellation in the denominator gives rise to an enhancement in the nonlinearity. The explicit demonstration we present here guides one to a method for achieving even larger enhancement factors by introducing additional coupling terms. The method is also applicable to Fano resonances induced by all-plasmonic couplings, which are easier to control in experiments.

关键词： Nonlinear optics Plasmonics Cavity resonators

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The Effect of Post-Growth Thermal Annealing on CdZnTe Crystals Grown by Vertical Gradient Freeze Technique

The Effect of Post-Growth Thermal Annealing on CdZnTe Crysta...

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IEEE Symposium on Nuclear Science (NSS/MIC)

作者： M. Ünal Ö.B. Balbaşı M.C. Karaman G. Çelik A.M. Genç M. Parlak R. Turan Micro and Nanotechnology Program of Graduate School of Natural and Applied Sciences Middle East Technical University Ankara Turkey Crystal Growth Laboratory Middle East Technical University Ankara Turkey Department of Physics Middle East Technical University Ankara Turkey

ISBN: (数字)9781665421133

ISBN: (纸本)9781665421140

Te inclusions are the primary performance degrading defects in CdZnTe crystals. Because of the unique properties of CdZnTe, it is not possible to eliminate Te inclusions during the growth process. However, annealing process could decrease the concentration of Te inclusions by dissolving or migrating them at elevated temperatures. Annealing under Cd atmosphere could dissolve Te inclusions by introducing new Cd atoms into CdZnTe crystals and compensating excess Te atoms inside of inclusions. However, implemented Cd atoms can form interstitial Cd defects easily; thus, annealing under Te atmosphere is needed to eliminate intestinal Cd defects created by Cd-annealing. In this study, annealing under different atmospheres is studied, and changes in Te inclusion concentrations are observed for various crystals. Additionally, it is observed that the resistivity of doped crystals decreases significantly after Cd-annealing, which is restored with Te-annealing. On the other hand, undoped CdZnTe crystals act differently after annealing steps.

关键词： Annealing Temperature Clouds Atmosphere Crystals Conductivity Biomedical imaging

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