Browsing by Subject "Crystals"

Now showing 1 - 20 of 35

Open Access
Analysis of defect related optical transitions in biased AlGaN/GaN heterostructures
(2010) Bengi, A.; Lisesivdin, S.B.; Kasap, M.; Mammadov, T.; Ozcelik, S.; Özbay, Ekmel
The optical transitions in AlGaN/GaN heterostructures that are grown by metalorganic chemical vapor deposition (MOCVD) have been investigated in detail by using Hall and room temperature (RT) photoluminescence (PL) measurements. The Hall measurements show that there is two-dimensional electron gas (2DEG) conduction at the AlGaN/GaN heterointerface. PL measurements show that in addition to the characteristic near-band edge (BE) transition, there are blue (BL) and yellow luminescence (YL) bands, free-exciton transition (FE), and a neighboring emission band (NEB). To analyze these transitions in detail, the PL measurements were taken under bias where the applied electric field changed from 0 to 50 V/cm. Due to the applied electric field, band bending occurs and NEB separates into two different peaks as an ultraviolet luminescence (UVL) and Y4 band. Among these bands, only the yellow band is unaffected with the applied electric field. The luminescence intensity change of these bands with an electric field is investigated in detail. As a result, the most probable candidate of the intensity decrease with an increasing electric field is the reduction in the radiative lifetime. © 2010 Elsevier Ltd. All rights reserved.
Open Access
Analysis of electrical characteristics and magnetic field dependences of YBCO step edge and bicrystal grain boundary junctions for rf-SQUID applications
(Institute of Physics, 2004) Fardmanesh, M.; Schubert, J.; Akram, R.; Bick, M.; Banzet, M.; Zander, W.; Zhang, Y.; Krause, H-J.
The dc characteristics and magnetic field dependences of Y-Ba-Cu-O bicrystal grain boundary junctions (BGBJs) and step edge junctions (SEJs) were investigated for fabrication of rf-SQUIDs. Test junctions with up to 8 μm widths as well as the junctions of the two types of junction-based rf-SQUID were studied. The SEJs typically showed lower Jc and higher ρN as compared to the BGBJs, resulting in close IcRN products. All the BGBJs showed classical field dependent Ic following their junction width, resembling Fraunhofer patterns. The field sensitivity of the BGBJs' Uc led to low yield submicron BGBJ rf-SQUIDs partially impaired by the Earth's magnetic field. Two major behaviours of low and high field dependences of Ic were observed for the SEJs. Only the low field-sensitive SEJs resulted in micron size junction rf-SQUIDs not impaired by the Earth's magnetic field. The low field-sensitive SEJs led to low I/f noise magnetically stable rf-SQUIDs appropriate for applications in unshielded environments at 77 K.
Open Access
Anisotropy sensitivity of an acoustic lens with slit aperture
(IEEE, 1993) Atalar, Abdullah; Ishikawa, I.; Ogura, Y.; Tomita, K.
A conventional spherical acoustic lens is modified by restricting its aperture in the form of a slit to provide directional sensitivity. The spacing between the two parallel absorbing sheets forming the slit is adjustable to obtain varying slit widths. The resulting lens can be used in conjunction with V(Z) method to obtain leaky wave velocities of the sample under investigation as a function of direction. The theoretical V(Z) analysis of the lens involves a two-dimensional integral rather than one-dimensional integral of the conventional lens. Single crystal anisotropic materials are chosen as test samples. Reflection coefficients for anisotropic single crystals of given surface cut and orientation are calculated. Numerically evaluated V(Z) curves are used to deduce the surface wave velocity of the object for the given orientation. This is compared with the surface wave velocity directly calculated from the elastic parameters of the object. Results show the compromise between signal-to-noise ratio and angular resolution as the slit width is varied. V(Z) measurement results of a slitted lens are presented to be compared with calculated curves. The new lens is used to measure the acoustic velocity on the (001) surface of GaAs along varying directions with differing slit widths.
Open Access
Band-dropping via coupled photonic crystal waveguides
(Optical Society of American (OSA), 2002) Bayındır, Mehmet; Özbay, Ekmel
We observe the dropping of electromagnetic waves having a specific frequency or a certain frequency band in two-dimensional dielectric photonic crystals. The single frequency is dropped via cavity-waveguide coupling. Tunability of the demultiplexing mode can be achieved by modifying the cavity properties. The band-dropping phenomenon is achieved by introducing interaction between an input planar, or coupled-cavity, waveguide and the output coupled-cavity waveguides (CCWs). The dropping band can be tuned by changing the coupling strength between the localized cavity modes of the output CCWs. We also calculate the transmission spectra and the field patterns by using the finite-difference-time-domain (FDTD) method. Calculated results agree well with the microwave measurements. © 2002 Optical Society of America.
Open Access
BilKristal 4.0: A tool for crystal parameters extraction and defect quantification
(Elsevier, 2015) Okuyan, E.; Okuyan, C.
In this paper, we present a revised version of BilKristal 3.0 tool. Raycast screenshot functionality is added to provide improved visual analysis. We added atomic distance analysis functionality to assess crystalline defects. We improved visualization capabilities by adding high level cut function definitions. Discovered bugs are fixed and small performance optimizations are made. © 2015 Elsevier B.V. All rights reserved.
Open Access
Compact size highly directive antennas based on the SRR metamaterial medium
(Institute of Physics Publishing, 2005) Bulu, I.; Caglayan, H.; Aydin, K.; Özbay, Ekmel
In this work, we studied the far-field properties of the microwave radiation from sources embedded inside the split-ring resonator (SRR) metamaterial medium. Our results showed that the emitted power near the resonance frequency of the SRR structure was confined to a narrow angular region in the far field. The measured radiation patterns showed half-power beamwidths around 14°. The highly directive radiation is obtained with a smaller radiation surface area when compared to the previous results obtained by using photonic crystals. The reduction in the surface area is ten-fold in the case of the SRR metamaterial medium when compared to the photonic crystals. Our results provide means to create compact size highly directive antennas.
Open Access
Development of left-handed composite materials and negative refracting photonic crystals with subwavelength focusing
(SPIE, 2005) Özbay, Ekmel
We review the studies conducted in our group concerning electromagnetic properties of metamaterials and photonic crystals with negative effective index of refraction. In particular, we demonstate the true left handed behavior of a 2D composite metamaterial, by analyzing the electric and magnetic response of the material components systematically. The negative refraction, subwavelength focusing, and flat lens phenomena using 2D dielectric photonic crystals are also presented.
Open Access
The effect of insulator layer thickness on the main electrical parameters in (Ni/Au)/AlxGa1-xN/AIN/GaN heterostructures
(Wiley, 2010) Altindal, S.; Şafak, Y.; Taşçloǧlu I.; Özbay, Ekmel
(Ni/Au)Alx Ga1-x N/AlN/GaN(x = 0.22) heterostructures with and without a passivation layer of the SiNx were fabricated in order to see the effect of the insulator layer on the main electrical parameters such as zero-bias barrier height (BH) (φBO), ideality factor (n), series resistance (Rs) of the structure, and the interface state density (Nss). Some of these parameters were determined from both I-V and admittance (C-V and G/ω-V) measurements at room temperature and at 1 MHz and were compared. The experimental results show that the value of N ss in a Schottky contact without passivation is nearly 1 order of magnitude larger than that in a Schottky contact with SiNx passivation layers. Also, the values of Rs increase with the increasing thickness of the passivation layer. In the forward bias region, the negative values of capacitance are an attractive result of this study. This negative capacitance disappears in presence of the passivation layer. Copyright © 2010 John Wiley & Sons, Ltd.
Open Access
Electric field dependence of radiative recombination lifetimes in polar InGaN/GaN quantum heterostructures
(IEEE, 2009) Sarı, Emre; Nizamoğlu, Sedat; Lee I.-H.; Baek J.-H.; Demir, Hilmi Volkan
We report on external electric field dependence of recombination lifetimes in polar InGaN/GaN quantum heterostructures. In our study, we apply external electric fields one order of magnitude less than and in opposite direction to the polarization-induced electrostatic fields inside the well layers. Under the increasing external electric field, we observe a decrease in carrier lifetimes (τ) and radiative recombination lifetimes (τr), latter showing a weaker dependence. Our results on τr show an agreement with our transfer matrix method based simulation results and demonstrate Fermi's golden rule in polar InGaN/GaN quantum heterostructures dependent on electric field. For our study, we grew 5 pairs of 2.5 nm thick In0.15Ga 0.85N quantum well and 7.5 nm thick GaN barrier layers in a p-i-n diode architecture using metal-organic chemical vapor deposition (MOCVD) on a c-plane sapphire substrate. Devices with 300 μm × 300 μm mesa size were fabricated using standard photolithography, reactive ion etching and metallization steps. We used indium-tin oxide (ITO) based semi-transparent contacts in top (p-GaN) layer for uniform application of electric field across the well layers. The fabricated devices were diced and mounted on a TO-can for compact testing. © 2009 IEEE.
Open Access
Electric field dependent radiative decay kinetics of polar InGaN/GaN quantum heterostructures at low fields
(American Institute of Physics, 2009-05-29) Sari, E.; Nizamoglu, S.; Lee, I. H.; Baek, J. H.; Demir, Hilmi Volkan
Electric field dependent photoluminescence decay kinetics and its radiative component are studied in polar InGaN/GaN quantum heterostructures at low fields. Under externally applied electric field lower than polarization fields, spectrally and time resolved photoluminescence measurements are taken to retrieve internal quantum efficiencies and carrier lifetimes as a function of the applied field. Subsequently, relative behavior of radiative recombination lifetimes is obtained in response to the applied field. In these characterizations of polar InGaN/GaN structures, we observe that both the carrier lifetime and the radiative recombination lifetime decrease with increasing external electric field, with the radiative component exhibiting weaker field dependence.
Open Access
Energy relaxation probed by weak antilocalization measurements in GaN heterostructures
(2009) Cheng H.; Bıyıklı, Necmi; Xie J.; Kurdak Ç.; Morko̧ H.
Energy relaxation and electron-phonon (e-p) interaction are investigated in wurtzite Al0.15Ga0.85 N/AlN/GaN and Al0.83 In0.17 N/AlN/GaN heterostructures with polarization induced two-dimensional electron gases in the Bloch-Grüneisen regime. Weak antilocalization (WAL) and Shubnikov-de Haas measurements were performed on gated Hall bar structures at temperatures down to 0.3 K. We used WAL as a thermometer to measure the electron temperature Te as a function of the dc bias current. We found that the power dissipated per electron, P e, was proportional to Te4 due to piezoelectric acoustic phonon emission by hot electrons. We calculated Pe as a function of Te without any adjustable parameters for both the static and the dynamic screening cases of piezoelectric e-p coupling. In the temperature range of this experiment, the static screening case was expected to be applicable; however, our data was in better agreement with the dynamic screening case. © 2009 American Institute of Physics.
Open Access
Experimental investigation of layer-by-layer metallic photonic crystals
(Institution of Electrical Engineers, 1998-12) Temelkuran, B.; Altug, H.; Özbay, Ekmel
The authors have investigated the transmission properties and defect characteristics of layer-by-layer metallic photonic crystals. They have demonstrated experimentally that the metallicity gap of these crystals extends to an upper band-edge frequency, and no lower edge was detected down to 2 GHz. The defect structures built around these crystals exhibited high transmission peak amplitudes (100%) and high Q factors (2250). The crystals with low filling ratios (around 1-2%) were tested and were still found to possess metallic photonic crystal properties. These crystals exhibited high reflection rates within the metallicity gap and reasonable defect mode characteristics. A power enhancement factor of 190 was measured for the electromagnetic (EM) wave within planar cavity structures, by placing a monopole antenna inside the defect volume. These measurements show that detectors embedded inside a metallic photonic crystal can be used as frequency selective resonant cavity enhanced (RCE) detectors with increased sensitivity and efficiency when compared to conventional detectors.
Open Access
Fabry-Perot-type resonances in metallic photonic crystals
(IEEE, 1998) Serpengüzel, Ali; Savran, Kerim
A metallic photonic crystals (MPC) with a face-centered cubic (FCC) Bravais lattice is investigated in the microwave regime. It was found that the transmission below and above the stop band is unity, which is quite remarkable since the metallic surface would have reflected 99.97% of the incident radiation in the given microwave frequency range. The effect of the incidence angle on the transmission of the MPC was also studied.
Open Access
Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: a first-principles perspective to recent synthesis
(American Institute of Physics Inc., 2018) Kecik D.; Onen, A.; Konuk, M.; Gürbüz, E.; Ersan, F.; Cahangirov, S.; Aktürk, E.; Durgun, Engin; Çıracı, Salim
Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed. © 2018 Author(s).
Open Access
Growth mechanism of 2D Mo2C on Cu via CVD
(American Chemical Society, 2023-7-7) Büke, G. C.; Caylan, Ömer Refet; Oğurtanı, Ö. T.
This study investigates the growth of Mo2C crystals via chemical vapor deposition (CVD) in the presence of a carbon (H2/CH4 gas)-containing environment. The study employs both theoretical and experimental approaches to investigate the vertical and lateral (in-plane) growth of Mo2C crystals. A physico-mathematical consideration is applied to develop an analytical forward model, which incorporates bulk diffusivities, surface diffusivities, and solubility gradients for Mo2C crystal growth. Coupled nonlinear flow equations have been advanced for the Mo-, Cu-, Mo2C layer framework and effectively predicted the Mo2C crystal growth rate for both vertical and lateral directions. Forming the Mo2C crystal height and diameter was directly correlated with copper layer thickness and time using the forward model and then validated by the experiments together with SEM and AFM studies. Studies showed that the Cu layer thickness plays a crucial role in controlling the height of the Mo2C crystal while it is not that critical in changing the lateral dimension of the crystal. Beyond simply enhancing Mo2C crystal growth and property-processing relationship, this study demonstrated the synthesis of designer Mo2C, which can be tailored to the needs of specific applications. This forward model will enable us to further enhance and exploit the family of analogs of materials previously demonstrated by other methods.
Open Access
High sensitivity and multifunctional micro-Hall sensors fabricated using InAlSb/InAsSb/InAlSb heterostructures
(2009) Bando, M.; Ohashi, T.; Dede, M.; Akram, R.; Oral, A.; Park, S.Y.; Shibasaki I.; Handa H.; Sandhu, A.
Further diversification of Hall sensor technology requires development of materials with high electron mobility and an ultrathin conducting layer very close to the material's surface. Here, we describe the magnetoresistive properties of micro-Hall devices fabricated using InAlSb/InAsSb/InAlSb heterostructures where electrical conduction was confined to a 30 nm-InAsSb two-dimensional electron gas layer. The 300 K electron mobility and sheet carrier concentration were 36 500 cm2 V-1 s-1 and 2.5× 1011 cm-2, respectively. The maximum current-related sensitivity was 2 750 V A-1 T-1, which was about an order of magnitude greater than AlGaAs/InGaAs pseudomorphic heterostructures devices. Photolithography was used to fabricate 1 μm×1 μm Hall probes, which were installed into a scanning Hall probe microscope and used to image the surface of a hard disk. © 2009 American Institute of Physics.
Open Access
Highly directional resonant antennas built around photonic crystals
(IEEE, 1999) Özbay, Ekmel; Temelkuran, Burak; Bayındır, Mehmet; Biswas, R.; Sigalas, M. M.; Tuttle, G.; Ho, K. M.
We report a photonic crystal-based resonant antenna with a very high directivity and gain. The layer-by-layer dielectric photonic crystal we used in our experiments was designed to have a three dimensional band gap with a mid-gap frequency around 12 GHz. We used the output port of a microwave network analyzer and a monopole antenna to obtain EM waves. The input port of the network analyzer and a standard gain horn antenna were used to receive the radiated EM field from the monopole antenna. The receiver was kept free to rotate around the antenna. We investigated the radiation characteristics of this monopole antenna, which was inserted into the planar defect structures built around a photonic crystal that consisted of 20 layers. The planar defect was formed by separating the 8th and 9th layers of the structure. In order to suppress the radiation in the backward direction, we intentionally chose one of the mirrors of the cavity to have a higher reflectivity (/spl sim/18-20 dB) than the front mirror.
Open Access
Highly directive radiation and negative refraction using photonic crystals
(Institute of Physics Publishing, 2005) Özbay, Ekmel; Bulu, I.; Aydin, K.; Caglayan H.; Alici, K. B.; Guven, K.
In this article, we present an experimental and numerical study of certain optical properties of two-dimensional dielectric photonic crystals (PCs). By modifying the band structure of a two-dimensional photonic crystal through its crystal parameters, we show how it is possible to confine the angular distribution of radiation from an embedded omnidirectional source. We then demonstrate that the anomalous band dispersions of PCs give rise to completely novel optical phenomena, in particular, the negative refraction of electromagnetic waves at the interface of a PC. We investigate the spectral negative refraction, which utilizes a transverse magnetic (TM)-polarized upper band of a PC, in detail and show that a high degree of isotropy can be achieved for the corresponding effective index of refraction. The presence of nearly a isotropic negative refractive index leads to focusing of omnidirectional sources by a PC slab lens, which can surpass certain limitations of conventional (positive refractive) lenses. These examples indicate the potential of PCs for photonics applications utilizing the band structure.
Open Access
Highly doped silicon micromachined photonic crystals
(IEEE, Piscataway, NJ, United States, 2000) Temelkuran, B.; Bayındır, Mehmet; Özbay, Ekmel; Kavanaugh, J. P.; Sigalas, M. M.; Tuttle, G.
Summary form only given. Photonic crystals are periodic structures with the property of reflecting the electromagnetic (EM) waves in all directions within a certain frequency range. These structures can be used to control and manipulate the behaviour of EM waves. Although earlier work concentrated on building these crystals with dielectric materials, there are certain advantages of introducing metals to photonic crystals. First, metals offer a high rejection rate when compared to the dielectric crystals. Second, for microwave applications, the dimensions of metallic crystals can be kept much smaller than the minimum dimensions needed for a typical dielectric crystal. In the paper, we propose a method for the fabrication of layer-by-layer metallic photonic crystals. A similar method had been used by Ozbay et al. to fabricate dielectric photonic crystals using silicon wafers. We fabricated a new layer-by-layer photonic crystal using highly doped silicon wafers.
Open Access
Investigation of localized coupled-cavity modes in two-dimensional photonic bandgap structures
(IEEE, 2002) Özbay, Ekmel; Bayındır, Mehmet; Bulu, I.; Cubukcu, E.
We present a detailed study of the localized coupled-cavity modes in 2-D dielectric photonic crystals. The transmission, phase, and delay time characteristics of the various coupled-cavity structures are measured and calculated. We observed the eigenmode splitting, waveguiding through the coupled cavities, splitting of electromagnetic waves in waveguide ports, and switching effect in such structures. The corresponding field patterns and the transmission spectra are obtained from the finite-difference-time-domain (FDTD) simulations. We also develop a theory based on the classical wave analog of the tight-binding (TB) approximation in solid state physics. Experimental results are in good agreement with the FDTD simulations and predictions of the TB approximation.