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13 Jul 2009

Volume 95, Issue 2, Articles (02xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 95, 023701 (2009); http://dx.doi.org/10.1063/1.3173808 (3 pages)

G. Devès, S. Roudeau, A. Carmona, S. Lavielle, K. Gionnet, G. Déléris, and R. Ortega
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Tunneling magnetoresistance sensors for high resolutive particle detection

C. Albon, A. Weddemann, A. Auge, K. Rott, and A. Hütten

Appl. Phys. Lett. 95, 023101 (2009); http://dx.doi.org/10.1063/1.3179241 (3 pages) | Cited 9 times

Online Publication Date: 13 July 2009

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Arrays of tunnel magnetoresistance sensors based on MgO as insulating layer are employed to detect magnetic microbeads. For single bead detection, elliptically shaped sensors of axis lengths of 400 and 100 nm are used. Due to high shape anisotropy a linear response of the sensor signal in a magnetic field range between −500 and 500 Oe can be reported. By performing static detection measurements of magnetic microbeads, a distinct signal shape correlated with the position of beads in respect to the sensor can be observed. The experimental data are compared to micromagnetic simulations carried out on a trilayer model.
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85.75.Ss Magnetic field sensors using spin polarized transport

Self-assembled Mn5Ge3 nanomagnets close to the surface and deep inside a Ge1−xMnx epilayer

R. T. Lechner, V. Holý, S. Ahlers, D. Bougeard, J. Stangl, A. Trampert, A. Navarro-Quezada, and G. Bauer

Appl. Phys. Lett. 95, 023102 (2009); http://dx.doi.org/10.1063/1.3159827 (3 pages) | Cited 7 times

Online Publication Date: 13 July 2009

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Under defined growth conditions ferromagnetic hexagonal Mn5Ge3 precipitates are formed in cubic Ge1−xMnx epilayers. To study the topotaxial relationship of these nanomagnets we perform x-ray diffraction experiments in coplanar as well as in grazing incidence geometries at synchrotron sources. Additionally, to the well defined topotaxial relation derived for buried nanomagnets deep within the Ge layer, we found an additional class of Mn5Ge3 precipitates very close to the surface, with larger inclusion diameter and several different crystallographic orientations with respect to the buried ones.
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75.70.Ak Magnetic properties of monolayers and thin films
61.50.-f Structure of bulk crystals
75.50.Cc Other ferromagnetic metals and alloys
75.50.Tt Fine-particle systems; nanocrystalline materials
75.75.-c Magnetic properties of nanostructures
81.16.Dn Self-assembly
61.72.Qq Microscopic defects (voids, inclusions, etc.)

On the complex behavior of strain relaxation in (In,Ga)As/GaAs(001) quantum dot molecules

M. Hanke, M. Dubslaff, M. Schmidbauer, Zh. M. Wang, Yu. I. Mazur, P. M. Lytvyn, J. H. Lee, and G. J. Salamo

Appl. Phys. Lett. 95, 023103 (2009); http://dx.doi.org/10.1063/1.3176409 (3 pages)

Online Publication Date: 13 July 2009

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A detailed growth scenario of surface quantum dot molecules (QDM) in the system (In,Ga)As/GaAs(001) has been investigated in terms of shape and elastic strain evolution. QDMs are grown by a combined approach using droplet epitaxy for initial homoepitaxial GaAs mounds, which subsequently serve as nucleation spots for surrounding (In,Ga)As surface quantum dots. Atomic force micrographs trace a detailed pathway toward the final QDM containing up to six quantum dots with perfect inherent symmetry. Synchrotron-based grazing incidence diffraction together with grazing incidence small angle x-ray scattering reveal a relaxation behavior, which for all growth stages comprises a strained lattice along [math10] and partial elastic relaxation along [110]. Numerical finite element calculations on the three-dimensional strain profile support the experimental findings.
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68.65.Hb Quantum dots (patterned in quantum wells)
68.37.Ps Atomic force microscopy (AFM)
78.70.Ck X-ray scattering
68.60.Bs Mechanical and acoustical properties

A nonvolatile memory capacitor based on Au nanocrystals with HfO2 tunneling and blocking layers

V. Mikhelashvili, B. Meyler, S. Yoffis, J. Salzman, M. Garbrecht, T. Cohen-Hyams, W. D. Kaplan, and G. Eisenstein

Appl. Phys. Lett. 95, 023104 (2009); http://dx.doi.org/10.1063/1.3176411 (3 pages) | Cited 12 times

Online Publication Date: 13 July 2009

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We report on a nonvolatile memory capacitor based on gold nanocrystals serving as charge storage elements located between two HfO2 films acting as the tunneling and control layers. The capacitor has an equivalent oxide thicknesses of 7 nm and exhibits a large hysteresis in the C-V characteristics of 1 and 9 V for gate voltage sweeps of ±1 and ±7 V, respectively, with no frequency dependence in the range of 10 kHz to 1 MHz. The storage charge density is ∼ 1.2×1013 cm−2 and the flat band voltage shift is stable for write/erases operations with a voltage swing of ±5 V for over 18 h.
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84.30.Sk Pulse and digital circuits
73.40.Gk Tunneling
84.32.Tt Capacitors
68.55.-a Thin film structure and morphology
61.46.Hk Nanocrystals

Nonlinear optical switching behavior of Au nanocubes and nano-octahedra investigated by femtosecond Z-scan measurements

Yih Hong Lee, Yongli Yan, Lakshminarayana Polavarapu, and Qing-Hua Xu

Appl. Phys. Lett. 95, 023105 (2009); http://dx.doi.org/10.1063/1.3177064 (3 pages) | Cited 13 times

Online Publication Date: 13 July 2009

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Two types of gold nanoparticles, nanocubes and nano-octahedra were prepared and their nonlinear absorption properties were investigated by using femtosecond open aperture Z-scan experiments. Both nanocubes and nano-octahedra exhibited a shift from saturable absorption (SA) to reverse saturable absorption (RSA) at higher excitation intensities. The SA behavior was ascribed to ground state plasmon band bleaching and RSA was ascribed to excited state absorption and two-photon absorption of these materials. The competition of different processes is responsible for the observed switching behavior.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons
42.50.Gy Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption

An electrical evaluation method for the silicidation of silicon nanowires

X. Tang (唐晓慧), N. Reckinger, V. Bayot, D. Flandre, E. Dubois, D. A. Yarekha, G. Larrieu, A. Lecestre, J. Ratajczak, N. Breil, V. Passi, and J.-P. Raskin

Appl. Phys. Lett. 95, 023106 (2009); http://dx.doi.org/10.1063/1.3171929 (3 pages)

Online Publication Date: 13 July 2009

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Physical and electrical properties of PtSi nanowires (NWs) fabricated on a silicon-on-insulator wafer are investigated. The Si consumption rule in NW silicidation is consistent with that of planar process. The cross-sectional area ratio between PtSi NW and Si NW is about 1.5:1. An electrical method is used to evaluate the silicidation degree of NWs. According to the dependence of the current passing through the NW on the backside substrate voltage, we can determine whether the Si NW is fully or partially silicided. The electrical evaluation results are in agreement with transmission electron microscopy inspections.
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73.61.At Metal and metallic alloys
68.65.Hb Quantum dots (patterned in quantum wells)
81.16.-c Methods of micro- and nanofabrication and processing

Electronic properties of zigzag graphene nanoribbons on Si(001)

Zhuhua Zhang and Wanlin Guo

Appl. Phys. Lett. 95, 023107 (2009); http://dx.doi.org/10.1063/1.3179426 (3 pages) | Cited 11 times

Online Publication Date: 14 July 2009

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We show by first-principles calculations that the electronic properties of zigzag graphene nanoribbons (Z-GNRs) adsorbed on Si(001) substrate strongly depend on ribbon width and adsorption orientation. Only narrow Z-GNRs with even rows of zigzag chains across their width adsorbed perpendicularly to the Si dimer rows possess an energy gap, while wider Z-GNRs are metallic due to width-dependent interface hybridization. The Z-GNRs can be metastably adsorbed parallel to the Si dimer rows, but show uniform metallic nature independent of ribbon width due to adsorption-induced dangling-bond states on the Si surface.
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68.43.Mn Adsorption kinetics
71.20.Mq Elemental semiconductors
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Tx Fullerenes and related materials; intercalation compounds

Strain-induced localized states within the matrix continuum of self-assembled quantum dots

Voicu Popescu, Gabriel Bester, and Alex Zunger

Appl. Phys. Lett. 95, 023108 (2009); http://dx.doi.org/10.1063/1.3159875 (3 pages) | Cited 6 times

Online Publication Date: 14 July 2009

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Quantum dot-based infrared detectors often involve transitions from confined states of the dot to states above the minimum of the conduction band continuum of the matrix. We discuss the existence of two types of resonant states within this continuum in self-assembled dots: (i) virtual bound states, which characterize square wells even without strain and (ii) strain-induced localized states. The latter emerge due to the appearance of “potential wings” near the dot, related to the curvature of the dots. While states (i) do couple to the continuum, states (ii) are sheltered by the wings, giving rise to sharp absorption peaks.
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73.21.La Quantum dots
73.20.Fz Weak or Anderson localization
85.60.Gz Photodetectors (including infrared and CCD detectors)
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots

Destruction of graphene by metal adatoms

D. W. Boukhvalov and M. I. Katsnelson

Appl. Phys. Lett. 95, 023109 (2009); http://dx.doi.org/10.1063/1.3160551 (3 pages) | Cited 20 times

Online Publication Date: 14 July 2009

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The formation energies for mono- and bivacancies in graphene in the presence of adatoms of various metals and small metallic clusters have been calculated. It is shown that transition metal impurities such as iron, nickel, and, especially, cobalt reduce dramatically the vacancy formation energies whereas gold impurities have almost no effect on characteristics of the vacancies. These results highlight that special measures are required in order to protect graphene from damage by transition metal leads.
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces
61.72.jd Vacancies
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)

Resonance control of membrane reflectors with effective index engineering

Hongjun Yang, Santhad Chuwongin, Zexuan Qiang, Li Chen, Huiqing Pang, Zhenqiang Ma, and Weidong Zhou

Appl. Phys. Lett. 95, 023110 (2009); http://dx.doi.org/10.1063/1.3182801 (3 pages) | Cited 4 times

Online Publication Date: 14 July 2009

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We report here broadband membrane reflectors based on Fano resonances in patterned silicon nanomembranes. Resonance control of the reflectors was realized either by partially removing buried oxide layer underneath the device layer or by controlled SiO2 film deposition on top of the devices. Both blueshifts and redshifts were demonstrated with a turning range of 50 nm for a center wavelength of 1550 nm. These results demonstrate practical postprocess means for Fano resonance engineering for both narrowband filters and ultracompact broadband reflectors.
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42.82.-m Integrated optics
42.79.Ci Filters, zone plates, and polarizers
42.70.-a Optical materials
42.79.Wc Optical coatings

Subnanometric Si film reactive diffusion on Ni

A. Portavoce, B. Lalmi, G. Tréglia, C. Girardeaux, D. Mangelinck, B. Aufray, and J. Bernardini

Appl. Phys. Lett. 95, 023111 (2009); http://dx.doi.org/10.1063/1.3177187 (3 pages) | Cited 3 times

Online Publication Date: 14 July 2009

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The dissolution of 3–5 Si ML on Ni has been studied using in situ ultrahigh vacuum Auger electron spectroscopy (AES). The AES signal shows delays and kinetic changes in the dissolution process. These observations, combined with atomistic kinetic Monte Carlo simulations, considering an fcc Ni–Si nonregular solid solution, show that the AES signal should correspond to a successive apparition of the Ni-silicides present in the Ni–Si phase diagram at low temperatures, starting with the Si-richer phase.
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66.30.H- Self-diffusion and ionic conduction in nonmetals
81.30.Dz Phase diagrams of other materials
64.75.Bc Solubility

Ordered Si nanoaperture arrays for the measurement of ion currents across lipid membranes

Svetoslav Koynov, Martin S. Brandt, and Martin Stutzmann

Appl. Phys. Lett. 95, 023112 (2009); http://dx.doi.org/10.1063/1.3171931 (3 pages) | Cited 1 time

Online Publication Date: 14 July 2009

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Confined arrays of highly ordered microchannels in silicon with apertures as small as 100 nm and flat ridges between them are manufactured using electrochemically defined Si macropores whose orifices are reduced by deposition of dielectric SiOx thin films. The formation of continuous fluid lipid membranes over these channel arrays is achieved by vesicle fusion. This structure should allow a large choice of lipid membranes to be investigated by spatially resolved ion current measurements.
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87.16.dp Transport, including channels, pores, and lateral diffusion
87.16.Vy Ion channels
81.16.-c Methods of micro- and nanofabrication and processing
82.45.Yz Nanostructured materials in electrochemistry
87.14.Cc Lipids

Size-dependent modulation of carrier mobility in top-down fabricated silicon nanowires

Lidija Sekaric, Oki Gunawan, Amlan Majumdar, Xiao Hu Liu, Dana Weinstein, and Jeffrey W. Sleight

Appl. Phys. Lett. 95, 023113 (2009); http://dx.doi.org/10.1063/1.3177331 (3 pages) | Cited 4 times

Online Publication Date: 14 July 2009

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We have investigated the size dependence of field-effect mobility in top-down fabricated Si nanowires (NWs). We find that electron mobility increases while hole mobility decreases with the NW width. The observed trends are opposite of what we expect based on facet-dominated transport. We simulate charge densities and investigate the effect of gate stack-induced stress in an effort to explain these trends. We find that the use of piezoresistive coefficients for bulk or thin-film Si does not give sufficient change in mobility to reverse the facet-driven mobility trend. We suggest further investigation into the contribution of one-dimensional NW corner effects.
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73.63.Nm Quantum wires
62.23.Hj Nanowires
62.20.de Elastic moduli
81.40.Jj Elasticity and anelasticity, stress-strain relations
72.20.Fr Low-field transport and mobility; piezoresistance

Broadband electromagnetic-wave absorption by FeCo/C nanocapsules

Z. Han, D. Li, H. Wang, X. G. Liu, J. Li, D. Y. Geng, and Z. D. Zhang

Appl. Phys. Lett. 95, 023114 (2009); http://dx.doi.org/10.1063/1.3177067 (3 pages) | Cited 19 times

Online Publication Date: 15 July 2009

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Electromagnetic-wave absorption by FeCo/C nanocapsules has been investigated. In contrast to earlier reported materials, including other nanocapsules, the absorption amplitude of FeCo/C nanocapsules is found not to decrease with increasing absorption-layer thickness. A reflection loss (RL) exceeding −20 dB can be obtained for all frequencies within the 2–18 GHz range by choosing an appropriate layer thickness between 1.6 and 8.5 mm. The broadest bandwidth (RL values exceeding −10 dB) from 10 to 18 GHz, covering half of the X-band and the whole Ku-band, is obtained for a 2 mm layer.
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61.46.-w Structure of nanoscale materials
81.16.-c Methods of micro- and nanofabrication and processing
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
77.22.Ch Permittivity (dielectric function)
77.22.Gm Dielectric loss and relaxation
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

Interface reconstruction in V-oxide heterostructures determined by x-ray absorption spectroscopy

H. Wadati, D. G. Hawthorn, J. Geck, T. Z. Regier, R. I. R. Blyth, T. Higuchi, Y. Hotta, Y. Hikita, H. Y. Hwang, and G. A. Sawatzky

Appl. Phys. Lett. 95, 023115 (2009); http://dx.doi.org/10.1063/1.3177328 (3 pages) | Cited 4 times

Online Publication Date: 15 July 2009

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We present an x-ray absorption study of the dependence of the V oxidation state on the thickness of LaVO3 (LVO) and capping LaAlO3 (LAO) layers in the multilayer structure of LVO sandwiched between LAO. We found that the change of the valence of V as a function of LAO layer thickness can be qualitatively explained by a transition between electronically reconstructed interfaces and a chemical reconstruction. The change as a function of LVO layer thickness is complicated by the presence of a considerable amount of V4+ in the bulk of the thicker LVO layers.
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68.65.Ac Multilayers
73.40.-c Electronic transport in interface structures
78.70.Dm X-ray absorption spectra
68.35.Ct Interface structure and roughness

Template-directed self-assembled magnetic nanostructures for probe recording

P. Kappenberger, F. Luo, L. J. Heyderman, H. H. Solak, C. Padeste, C. Brombacher, D. Makarov, T. V. Ashworth, L. Philippe, H. J. Hug, and M. Albrecht

Appl. Phys. Lett. 95, 023116 (2009); http://dx.doi.org/10.1063/1.3176937 (3 pages) | Cited 7 times

Online Publication Date: 16 July 2009

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We employ a template-directed self-assembly process to arrange nanospheres as small as 20 nm in a polymer resist template with regular hole arrays with periods down to 42 nm produced using extreme ultraviolet interference lithography. To demonstrate magnetic probe recording employing magnetic tips, an array of magnetic caps with perpendicular magnetic anisotropy was then created by depositing Co/Pt multilayer films on 60 nm nanospheres, arranged on a square lattice with 100 nm period. The magnetic nanocaps can be switched individually in a controlled fashion, with recognition of a successful switching event realized by measurement of a force-distance curve.
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81.16.Dn Self-assembly
81.16.Nd Micro- and nanolithography
85.40.Hp Lithography, masks and pattern transfer
75.30.Gw Magnetic anisotropy
81.07.Bc Nanocrystalline materials

Enhanced photoluminescence in gold nanoparticles doped ferroelectric liquid crystals

A. Kumar, J. Prakash, D. S. Mehta, A. M. Biradar, and W. Haase

Appl. Phys. Lett. 95, 023117 (2009); http://dx.doi.org/10.1063/1.3179577 (3 pages) | Cited 26 times

Online Publication Date: 17 July 2009

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We report the characterization and photoluminescence (PL) of newly synthesized deformed helix ferroelectric liquid crystal (DHFLC) material having short pitch and high spontaneous polarization. We observed ninefold enhancement in PL intensity in gold nanoparticles doped DHFLC material. This enhancement in the PL intensity has been attributed to the coupling of localized surface plasmon resonance from metal nanoparticles with DHFLC molecules, resulting in the increase in excitation and emission rate of the liquid crystal molecules in the localized electromagnetic field. These studies would provide a cutting edge tool in the realization of enhanced photoluminescent liquid crystal display devices.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
61.30.-v Liquid crystals
77.80.-e Ferroelectricity and antiferroelectricity
81.16.-c Methods of micro- and nanofabrication and processing
81.07.Bc Nanocrystalline materials

Optical and nonlinear absorption properties of Na doped ZnO nanoparticle dispersions

B. Karthikeyan, C. S. Suchand Sandeep, T. Pandiyarajan, P. Venkatesan, and Reji Philip

Appl. Phys. Lett. 95, 023118 (2009); http://dx.doi.org/10.1063/1.3182302 (3 pages) | Cited 10 times

Online Publication Date: 17 July 2009

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We report linear and nonlinear optical properties of the biologically important Na doped ZnO nanoparticle dispersions. Interesting morphological changes involving a spherical to flowerlike transition have been observed with Na doping. Optical absorption measurements show an exciton absorption around 368 nm. Photoluminescence measurements reveal exciton recombination emission, along with shallow and deep trap emissions. The increased intensity of shallow trap emission with Na doping is attributed to oxygen deficiency and shape changes associated with doping. Nonlinear optical measurements show a predominantly two-photon induced, excited state absorption, when excited with 532 nm, 5 ns laser pulses, indicating potential optical limiting applications.
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78.55.Et II-VI semiconductors
78.40.Ha Other nonmetallic inorganics
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters

A level set simulation for ordering of quantum dots via cleaved-edge overgrowth

X. B. Niu, E. Uccelli, A. Fontcuberta i Morral, and C. Ratsch

Appl. Phys. Lett. 95, 023119 (2009); http://dx.doi.org/10.1063/1.3182730 (3 pages) | Cited 1 time

Online Publication Date: 17 July 2009

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Cleaved-edge overgrowth (CEO) is a promising technique to obtain ordered arrays of quantum dots, where the size and position of the dots can be controlled very well. We present level set simulations for CEO. Our simulations illustrate how the quality of the CEO technique depends on the potential energy surface (PES) for adatom diffusion, and thus suggest how variations of the PES can potentially improve the uniformity of quantum dot arrays.
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68.65.Hb Quantum dots (patterned in quantum wells)
66.30.-h Diffusion in solids

High-field transport and velocity saturation in graphene

Jyotsna Chauhan and Jing Guo

Appl. Phys. Lett. 95, 023120 (2009); http://dx.doi.org/10.1063/1.3182740 (3 pages) | Cited 13 times

Online Publication Date: 17 July 2009

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High-field transport in graphene is studied by the Monte Carlo simulation. The results indicate velocity and current saturation in agreement with a recent experiment [ I. Meric, M. Y. Han, A. F. Young, B. Oezyilmaz, P. Kim, and K. Shepard, Nat. Nanotechnol. 3, 654 (2008) ]. The saturation current scales as the square root of the charge density, or equivalently, the square root of the gate overdrive voltage, which is qualitatively different from silicon field-effect transistors. By analytical fitting to the numerical simulation results, a simple expression of the field-dependent mobility is obtained at different strengths of charged impurity scattering.
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72.20.Ht High-field and nonlinear effects
73.63.-b Electronic transport in nanoscale materials and structures
72.80.Rj Fullerenes and related materials
72.20.Dp General theory, scattering mechanisms
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