Browsing by Subject "Semiconductor"
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Item Open Access 2D anisotropic photonic crystals of hollow semiconductor nanorod with liquid crystals(2013) Karaomerlioglu F.; Şimsek, Şevket; Mamedov, Amirullah M.; Özbay, EkmelPhotonic crystals (PCs) have many applications in order to control light-wave propagation. A novel type of two-dimensional anisotropic PC is investigated band gap and optical properties as a hollow semiconductor nanorod with nematicliquid crystals (LC). The PC structure composed of an anisotropic nematicLC in semiconductor square hollow nanorod is designed using the plane wave expansion (PWE) method and finite-difference time-domain (FDTD) method. It has been used 5CB (4-pentyl-4'-cyanobiphenyl) as LC core, and Tellurium (Te) as square hollow nanorod material.The PC with hollow Tenanorod with nematicLC is compared with the PC with solid Tenanorodand the PC with hollow Tenanorod. © (2013) Trans Tech Publications, Switzerland.Item Open Access Applied Materials uses operations research to design its service and parts network(Institute for Operations Research and the Management Sciences (INFORMS), 2010) Şen, A.; Bhatia, D.; Doǧan, K.Applied Materials Inc. is the global leader in nanomanufacturing technology solutions. It has a broad portfolio of innovative equipment, service, and software products and supports its customers worldwide with an extensive service and parts network with more than 100 locations. At the end of 2006, Applied Materials decided to evaluate and rationalize the design of its North American network. It set up a detailed optimization model (including 50,000 parts) to develop a network and distribution strategy. To our knowledge, this is the first large-scale multiechelon network-design model that incorporates safety stock inventory costs while considering the effects of lead time and risk pooling. The company used the model's recommendations to reduce costs while maintaining or improving its service to customers. The recommendations included simplifying the distribution network by consolidating depot locations for specific customers and skipping an echelon for others, thus leading to a projected inventory reduction of $10 million. The company is currently implementing these recommendations and has already eliminated five depots. Applied Materials estimates that during the first year of implementation, inventory reductions of $5.24 million and total savings of $1.1 million can be attributed to these network changes.Item Open Access Atomic layer deposition: an enabling technology for the growth of functional nanoscale semiconductors(Institute of Physics Publishing, 2017) Bıyıklı, Necmi; Haider A.In this paper, we present the progress in the growth of nanoscale semiconductors grown via atomic layer deposition (ALD). After the adoption by semiconductor chip industry, ALD became a widespread tool to grow functional films and conformal ultra-thin coatings for various applications. Based on self-limiting and ligand-exchange-based surface reactions, ALD enabled the low-temperature growth of nanoscale dielectric, metal, and semiconductor materials. Being able to deposit wafer-scale uniform semiconductor films at relatively low-temperatures, with sub-monolayer thickness control and ultimate conformality, makes ALD attractive for semiconductor device applications. Towards this end, precursors and low-temperature growth recipes are developed to deposit crystalline thin films for compound and elemental semiconductors. Conventional thermal ALD as well as plasma-assisted and radical-enhanced techniques have been exploited to achieve device-compatible film quality. Metal-oxides, III-nitrides, sulfides, and selenides are among the most popular semiconductor material families studied via ALD technology. Besides thin films, ALD can grow nanostructured semiconductors as well using either template-assisted growth methods or bottom-up controlled nucleation mechanisms. Among the demonstrated semiconductor nanostructures are nanoparticles, nano/quantum-dots, nanowires, nanotubes, nanofibers, nanopillars, hollow and core-shell versions of the afore-mentioned nanostructures, and 2D materials including transition metal dichalcogenides and graphene. ALD-grown nanoscale semiconductor materials find applications in a vast amount of applications including functional coatings, catalysis and photocatalysis, renewable energy conversion and storage, chemical sensing, opto-electronics, and flexible electronics. In this review, we give an overview of the current state-of-the-art in ALD-based nanoscale semiconductor research including the already demonstrated and future applications.Item Open Access Colloidal semiconductor nanocrystals(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi VolkanIn this chapter, we review colloidal semiconductor nanocrystals (NCs) and their remarkable size-dependent properties. We emphasize on colloidal nanoplatelets and explain how they differ from NCs of other classes.Item Open Access Cu doping induced structural and optical properties of bimetallic oxide nanodots by the vertical spark generation(Polish Academy of Sciences, 2019-05) Güngör, T.; Güngör, E.; Çalışkan, Deniz; Özbay, EkmelUndoped ZnO and Cu doped ZnO nanodots (NDs) were synthesized by the modified sparking technique with the Zn and Cu metal electrode pairs such as Zn–Zn, Zn–Cu and Cu–Cu. The effect of deposition geometry on the structural, optical properties and band gap energy were examined. The X-ray diffraction (XRD) analysis demonstrates that the nanodots have the hexagonal wurtzite structure, and visible considerable shift in the peaks position can be linked with the influence of Cu. However, when copper electrode was used, some copper oxide phases, metallic copper and paramelaconite phases were observed. From the results, the average diameters of metal oxide nanodots are about 25 nm and 50 nm which were obtained by using Cu–Cu and for Zn–Zn electrodes respectively from the scanning electron microscopy (SEM) analysis. When the Zn–Cu electrode pairs were used, the mixture of nanorod and nanodots appeared. It was observed that the island growth occurs in the horizontal geometry of electrode pairs and the growth metal oxide species are more strongly bonded to each other than to the substrate. But, these nanodots have more uniform distribution in the vertical geometry of electrodes. Optical studies indicated that the band gap decreased (red shift) when the Cu electrode was used.Item Open Access Electron momentum and energy relaxation rates in GaN and AlN in the high-field transport regime(The American Physical Society, 2003) Bulutay, C.; Ridley, B. K.; Zakhleniuk, N. A.Momentum and energy relaxation characteristics of electrons in the conduction band of GaN and AlN are investigated using two different theoretical approaches corresponding to two high electric-field regimes, one up to 1-2 MV/ cm values for incoherent dynamics, and the other at even higher fields for coherent dynamics where semiballistic and ballistic processes become important. For the former, ensemble Monte Carlo technique is utilized to evaluate these rates as a function of electron energy up to an electric-field value of 1 MV/cm (2 MV/cm) for GaN (AlN). Momentum and energy relaxation rates within this incoherent transport regime in the presence of all standard scattering mechanisms are computed as well as the average drift velocity as a function of the applied field. Major scattering mechanisms are identified as polar optical phonon (POP) scattering and the optical deformation potential (ODP) scattering. Roughly, up to fields where the steady-state electron velocity attains its peak value, the POP mechanism dominates, whereas at higher fields ODP mechanism takes over. Next, aiming to characterize coherent dynamics, the total out-scattering rate from a quantum state (chosen along a high-symmetry direction) due to these two scattering mechanisms are then computed using a first-principles full-band approach. In the case of POP scattering, momentum relaxation rate differs from the total out-scattering rate from that state; close to the conduction-band minimum, momentum relaxation rate is significantly lower than the scattering rate because of forward-scattering character of the intravalley POP emission., However, close to the zone boundary the difference between these two rates diminishes due to isotropic nature of intervalley scatterings. Finally, a simple estimate for the velocity-field behavior in the coherent transport regime is attempted, displaying a negative differential mobility due to the negative band effective mass along the electric-field direction.Item Open Access Electronic and optical properties of atomic layer-deposited ZnO and TiO2(Springer New York LLC, 2018) Ates, H.; Bolat, S.; Oruc, F.; Okyay, Ali KemalMetal oxides are attractive for thin film optoelectronic applications. Due to their wide energy bandgaps, ZnO and TiO2 are being investigated by many researchers. Here, we have studied the electrical and optical properties of ZnO and TiO2 as a function of deposition and post-annealing conditions. Atomic layer deposition (ALD) is a novel thin film deposition technique where the growth conditions can be controlled down to atomic precision. ALD-grown ZnO films are shown to exhibit tunable optical absorption properties in the visible and infrared region. Furthermore, the growth temperature and post-annealing conditions of ZnO and TiO2 affect the electrical properties which are investigated using ALD-grown metal oxide as the electron transport channel on thin film field-effect devices.Item Open Access High-conducting magnetic nanowires obtained from uniform titanium-covered carbon nanotubes(American Physical Society, 2004) Daǧ, Sefa; Durgun, Engin; Çıracı, SalimWe have shown that a semiconducting single-wall carbon nanotube (SWNT) can be covered uniformly by titanium atoms and form a complex but regular atomic structure. The circular cross section changes to a squarelike form, and the system becomes metallic with high state density at the Fermi level and with high quantum ballistic conductance. Metallicity is induced not only by the metal-metal coupling, but also by the band-gap closing of SWNT's at the corners of the square. Even more interesting is that uniform titanium-covered tubes have magnetic ground state with significant net magnetic moment. Our results have been obtained by the first-principles pseudopotential plane-wave calculations within the density-functional theory.Item Open Access Initial stages of Pt growth on Ge (001) studied by scanning tunneling microscopy and density functional theory(American Physical Society, 2004) Gurlu, O.; Zandvliet, H. J. W.; Poelsema, B.; Dag, S.; Çıracı, SalimWe have studied the initial stages of submonolayer Pt growth on the Ge(001). We have observed several stable and meta-stable adsorption configurations of Pt atoms at various temperatures. Calculations indicate relatively high binding energies of Pt atoms onto the Ge lattice, at different adsorption sites. Our results show that through-the-substrate bonding (concerted bonding) of two Pt atoms is more favored on Ge(001) surface then a direct Pt-Pt bond. Both our experiments and calculations indicate the breaking of Ge-Ge bonds on the surface in the vicinity of Pt adsorbates. We have also observed the spontaneous generation of 2 + 1 dimer vacancy defects at room temperature that cause the ejection of Ge atoms onto the surface. Finally we have studied the diffusion of Pt atoms into the bulk as a result of annealing and found out that they get trapped at subsurface sites.Item Open Access Metal nanoring and tube formation on carbon nanotubes(American Physical Society, 2002) Bagci, V. M. K.; Gülseren, O.; Yildirim, T.; Gedik, Z.; Ciracil, S.The structural and electronic properties of aluminum-covered single-wall carbon nanotubes (SWNT's) are studied from first principles for a large number of coverages. Aluminum-aluminum interaction, that is stronger than aluminum-tube interaction, prevents uniform metal coverage, and hence gives rise to the clustering. However, a stable aluminum ring and aluminum nanotube with well defined patterns can also form around the semiconducting SWNT's and lead to metallization. The persistent current in the Al nanoring is discussed to show that a high magnetic field can be induced at the center of SWNT.Item Open Access Oxygenation of carbon nanotubes: Atomic structure, energetics, and electronic structure(American Physical Society, 2003) Dag, S.; Gülseren, O.; Yildirim, T.; Çıracı, SalimThis paper presents an extensive and systematic analysis of the oxygenation of semiconducting and metallic single-wall carbon nanotubes by using the first principles pseudopotential plane wave method. Our study involves the physisorption of oxygen molecules, chemisorption of oxygen atoms and formation of an oxide, and deals with the equilibrium binding geometry and corresponding electronic energy structure. The binding energies of an oxygen molecule physisorbed at different sites are determined by calculating short and long range interactions. The triplet state of the physisorbed oxygen molecule is energetically favorable, whereas the nonmagnetic (spin paired) state yields a relatively stronger binding energy. An oxygen atom is adsorbed on top of the carbon-carbon bond. The zigzag bonds of the nanotubes are weakened and eventually are replaced by a carbon-oxygen-carbon bridge bond. Chemisorption of atomic oxygen and physisorption of an oxygen molecule modify the electronic energy structure of the bare tube in different ways. For a different coverage and pattern, self-consistent field electronic energy structure calculations using the optimized physisorption geometry corresponding to the triplet ground state result in a small energy gap between unoccupied oxygen levels and the top of the valence band of the semiconducting carbon nanotube. These results invalidate the hole doping of the semiconducting carbon nanotube upon the physisorption of oxygen.Item Open Access Persistent currents in helical structures(American Physical Society, 2004) Iskin, M.; Kulik, I. O.The recent discovery of mesoscopic electronic structures, in particular the carbon nanotubes, made necessary an investigation of what effect a helical symmetry of the conductor (metal or semiconductor) may have on the persistent current oscillations, We investigate persistent currents in helical structures which are nondecaying in time, not requiring a voltage bias, dissipationless stationary flow of electrons in a normal-metallic or semiconducting cylinder or circular wire of mesoscopic dimension. In the presence of magnetic flux along the toroidal structure, helical symmetry couples circular and longitudinal currents to each other. Our calculations suggest that circular persistent currents in these structures have two components with periods Φ0 and Φ0/s (s is an integer specific to any geometry). However, resultant circular persistent current oscillations have Φ0 period.Item Open Access A photometric investigation of ultra-efficient LEDs with high color rendering index and high luminous efficacy employing nanocrystal quantum dot luminophores(Optical Society of America, 2009-12-24) Erdem, T.; Nizamoglu, S.; Sun, X. W.; Demir, Hilmi VolkanWe report a photometric study of ultra-efficient light emitting diodes (LEDs) that exhibit superior color rendering index (CRI) and luminous efficacy of optical radiation (LER) using semiconductor quantum dot nanocrystal (NC) luminophores. Over 200 million systematically varied NC-LED designs have been simulated to understand feasible performance in terms of CRI vs. LER. We evaluated the effects of design parameters including peak emission wavelength, full-width-at-half-maximum, and relative amplitudes of each NC color component on LED performance. Warm-white LEDs with CRI >90 and LER >380 lm/W at a correlated color temperature of 3000 K are shown to be achieved using nanocrystal luminophores. (C) 2009 Optical Society of AmericaItem Open Access Quantum dot integrated LEDs using photonic and excitonic color conversion(Elsevier, 2011-11-23) Demir, Hilmi Volkan; Nizamoglu, S.; Erdem, T.; Mutlugun, E.; Gaponik N.; Eychmüller A.This review summarizes advantages, recent progress and challenges related to the quickly evolving research field of colloidal quantum dot integrated LEDs based on color conversion. We start with presenting a short general introduction to the field of solid state lighting and color conversion phenomena, which are essential for defining the requirements for high-quality general lighting. Subsequently we continue with a brief description of the synthesis of nanocrystal quantum dots and their optical properties together with the advantages of utilizing them in LEDs as color convertors. Following this basic background, we review the recent efforts on quantum dot integrated color-conversion LEDs, Förster resonance energy transfer (FRET) converted LEDs and FRET-enhanced LEDs. Finally, we conclude with a future outlook on semiconductor lighting and quantum dot integrated LEDs.Item Open Access Radiative Dark-Bright Instability and Critical Casimir Effect in DQW Exciton Condensates(Elsevier, 2011) Hakioglu, T.; Özgün E.It is already well known that radiative interband interaction in the excitonic normal liquid in semiconducting double quantum wells is responsible for a negligible splitting between the energies of the dark and bright excitons enabling us to consider a four fold spin degeneracy. This has also lead many workers to naively consider the same degeneracy in studying the condensate. On the other hand, the non-perturbative aspects of this interaction in the condensed phase, e.g. its consequences on the order parameter and the dark-bright mixture in the ground state have not been explored. In this work, we demonstrate that the ground state concentrations of the dark and the bright exciton condensates are dramatically different beyond a sharp interband coupling threshold where the contribution of the bright component in the ground state vanishes. This shows that the effect of the radiative interband interaction on the condensate is nonperturbative. We also observe in the free energy a discontinuous derivative with respect to the layer separation at the entrance to the condensed phase, indicating a strong critical Casimir force. An estimate of its strength shows that it is measurable. Measuring the Casimir force is challenging, but at the same time it has a conclusive power about the presence of the long sought for condensed phase.Item Open Access Robust ground state and artificial gauge in DQW exciton condensates under weak magnetic field(Elsevier B.V., 2014) Hakioǧlu, Tuğrul; Özgün, Ege; Günay, MehmetAn exciton condensate is a vast playground in studying a number of symmetries that are of high interest in the recent developments in topological condensed matter physics. In double quantum wells (DQWs) they pose highly nonconventional properties due to the pairing of non-identical fermions with a spin dependent order parameter. Here, we demonstrate a new feature in these systems: the robustness of the ground state to weak external magnetic field and the appearance of the artificial spinor gauge fields beyond a critical field strength where negative energy pair-breaking quasi particle excitations, i.e. de-excitation pockets (DX-pockets), are created in certain k regions. The DX-pockets are the Kramers symmetry broken analogs of the negative energy pockets examined in the 1960s by Sarma. They respect a disk or a shell-topology in k-space or a mixture between them depending on the magnetic field strength and the electron-hole density mismatch. The Berry connection between the artificial spinor gauge field and the TKNN number is made. This field describes a collection of pure spin vortices in real space when the magnetic field has only inplane components.Item Open Access Room temperature scanning Hall probe microscopy using GaAs/AlGaAs and Bi micro-hall probes(Elsevier Science B.V., 2002) Sandhu, A.; Masuda, H.; Oral, A.; Yamada, A.; Konagai, M.A room temperature scanning Hall probe microscope system utilizing GaAs/AlGaAs and bismuth micro-Hall probes was used for magnetic imaging of ferromagnetic domain structures on the surfaces of crystalline thin film garnets and permanent magnets. The Bi micro-Hall probes had dimensions ranging between 0.25 and 2.8μm2 and were fabricated using a combination of optical lithography and focused ion beam milling. The use of bismuth was found to overcome surface depletion effects associated with semiconducting micro-Hall probes. Our experiments demonstrated that Bi is a practical choice of material for fabricating sub-micron sized Hall sensors.Item Open Access Solid-solution of Cd(formula)Zn(formula)S nanocrystals in the channels of mesostructured silica films(Bilkent University, 2006) Akdoğan, YaşarMesostructured silica can be used as a reaction medium to produce solidsolution of Cd1-xZnxS nanocrystals as thin films. These films were synthesized from oligo(ethylene oxide) non-ionic surfactant (CH3(CH2)11(OCH2CH2)10OH, (C12EO10)), cadmium and zinc nitrate salts ([Cd(H2O)4](NO3)2 and [Zn(H2O)6](NO3)2), water, and tetramethylorthosilicate (TMOS, as silica source) mixtures using a liquid crystalline templating (LCT) approach and metal containing liquid crystalline (MLC) mesophase. Metal ion to surfactant mole ratio was 1.0 which determines the stability and structure of the mesostructured silica. The mesostructured silica film has a 3D hexagonal structure with oriented channels. The silica pore size can be controlled by controlling ageing temperature and time. The pore diameter of the silica channels that aged at room temperature (RT) for two days is 4.7 nm and the one aged at 2500 C for 30 minutes is 3.3 nm. Cd(II) and Zn(II) incorporated film samples can be reacted at RT under H2S atmosphere to produce zinc blend, Cd1-xZnxS nanocrystals (nano-Cd1-xZnxS-meso-SiO2) in the channels of the mesostructured silica. The band gaps of the nano-Cd1- xZnxS-meso-SiO2 vary between 2.6 eV for CdS and 4.1 eV for ZnS. The Cd (II) rich nanoparticles are larger (4.4 nm) than Zn (II) rich nanoparticles (3.1 nm). The silica wall thickness that can be controlled by ageing at different temperatures confines the growth of the Cd1-xZnxS nanocrystals in the pores. By controlling the size of the silica channel between 4.7 and 3.3 nm, one can control the band-gap of the CdS nanocrystals between 2.6 and 2.8 eV.Item Open Access Systematic study of adsorption of single atoms on a carbon nanotube(American Physical Society, 2003) Durgun, Engin; Dag, S.; Bagci, V. M. K.; Gülseren, O.; Yildirim, T.; Çıracı, SalimWe studied the adsorption of single atoms on a semiconducting and metallic single-wall carbon nanotube from first principles for a large number of foreign atoms. The stable adsorption sites, binding energy, and the resulting electronic properties are analyzed. The character of the bonding and associated physical properties exhibit dramatic variations depending on the type of the adsorbed atom. While the atoms of good conducting metals, such as Cu and Au, form very weak bonding, atoms such as Ti, Sc, Nb, and Ta are adsorbed with relatively high binding energy. Most of the adsorbed transition-metal atoms excluding Ni, Pd, and Pt have a magnetic ground state with a significant magnetic moment. Our results suggest that carbon nanotubes can be functionalized in different ways by their coverage with different atoms, showing interesting applications such as one-dimensional nanomagnets or nanoconductors and conducting connects, etc.Item Open Access Theoretical study of Ga-based nanowires and the interaction of Ga with single-wall carbon nanotubes(American Physical Society, 2004) Durgun, Engin; Dag, S.; Çıracı, SalimGallium displays physical properties which can make it a potential element to produce metallic nanowires and high-conducting interconnects in nanoelectronics. Using first-principles pseudopotential plane method we showed that Ga can form stable metallic linear and zigzag monatomic chain structures. The interaction between individual Ga atom and single-wall carbon nanotube (SWNT) leads to a chemisorption bond involving charge transfer. Doping of SWNT with Ga atom gives rise to donor states. Owing to a significant interaction between individual Ga atom and SWNT, continuous Ga coverage of the tube can be achieved. Ga nanowires produced by the coating of carbon nanotube templates are found to be stable and high conducting.