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10 May 2010

Volume 96, Issue 19, Articles (19xxxx)

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Appl. Phys. Lett. 96, 193101 (2010); http://dx.doi.org/10.1063/1.3425776 (3 pages)

J. J. Zhang, N. Hrauda, H. Groiss, A. Rastelli, J. Stangl, F. Schäffler, O. G. Schmidt, and G. Bauer
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Strain engineering in Si via closely stacked, site-controlled SiGe islands

J. J. Zhang, N. Hrauda, H. Groiss, A. Rastelli, J. Stangl, F. Schäffler, O. G. Schmidt, and G. Bauer

Appl. Phys. Lett. 96, 193101 (2010); http://dx.doi.org/10.1063/1.3425776 (3 pages) | Cited 10 times

Online Publication Date: 10 May 2010

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The authors report on the fabrication and detailed structural characterization of ordered arrays of vertically stacked SiGe/Si(001) island pairs. By a proper choice of growth parameters, islands which have both large sizes and high Ge fraction are obtained in the upper layer. Finite element method calculations of the strain distribution reveal that (i) the Si spacer between a pair of islands can act as a lateral quantum dot molecule made of four nearby dots for electrons and (ii) the tensile strain in a Si cap deposited on top of the stack is significantly enhanced with respect to a single layer.
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68.55.ag Semiconductors
68.60.Bs Mechanical and acoustical properties

Dielectric constants of atomically thin silicon channels with double gate

Hiroyuki Kageshima and Akira Fujiwara

Appl. Phys. Lett. 96, 193102 (2010); http://dx.doi.org/10.1063/1.3427364 (3 pages) | Cited 3 times

Online Publication Date: 10 May 2010

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Dielectric constants of Si (111) nanofilms with the double gate are studied in the full inversion regime by using the first-principles calculation. The calculations show that the dielectric constants are significantly smaller than that of the bulk. Further, the dielectric constants depend on the conduction type as well as on the film thickness. They also oscillate with a 2-bilayer-thickness for the p-channel case as the film thickness decreases. The suppressed dielectric constants are found in the channel center as well as in the channel surface. These findings open the way to artificial control of the dielectric constant in semiconductor nanostructures.
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77.22.Ch Permittivity (dielectric function)
73.61.Cw Elemental semiconductors

Tunable fishnet metamaterials infiltrated by liquid crystals

Alexander Minovich, Dragomir N. Neshev, David A. Powell, Ilya V. Shadrivov, and Yuri S. Kivshar

Appl. Phys. Lett. 96, 193103 (2010); http://dx.doi.org/10.1063/1.3427429 (3 pages) | Cited 19 times

Online Publication Date: 10 May 2010

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We analyze numerically the optical response and effective macroscopic parameters of fishnet metamaterials infiltrated with a nematic liquid crystal. We show that even a small amount of liquid crystal can provide tuning of the structures due to reorientation of the liquid crystal director. This enables switchable optical metamaterials, where the refractive index can be switched from positive to negative by an external field. This tuning is primarily determined by the shift in the cut-off wavelength of the holes, with only a small influence due to the change in plasmon dispersion.
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42.70.Df Liquid crystals

Incorporation of the dopants Si and Be into GaAs nanowires

M. Hilse, M. Ramsteiner, S. Breuer, L. Geelhaar, and H. Riechert

Appl. Phys. Lett. 96, 193104 (2010); http://dx.doi.org/10.1063/1.3428358 (3 pages) | Cited 13 times

Online Publication Date: 10 May 2010

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We studied the doping with Si and Be of GaAs nanowires (NWRs) grown by molecular beam epitaxy. Regarding the NW morphology, no influence was observed for Si doping but high Be doping concentrations cause a kinking and tapering of the NWRs. We investigated local vibrational modes by means of resonant Raman scattering to determine the incorporation sites of the dopant atoms. For Si doping, both donors on Ga sites and acceptors on As sites have been observed. Be was found to be incorporated as an acceptor on Ga sites. However, at high doping concentration, Be is also incorporated on interstitial sites.
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61.72.uj III-V and II-VI semiconductors
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
78.30.Fs III-V and II-VI semiconductors
61.72.jj Interstitials

Exchange bias in single-crystalline CuO nanowires

C. Díaz-Guerra, M. Vila, and J. Piqueras

Appl. Phys. Lett. 96, 193105 (2010); http://dx.doi.org/10.1063/1.3428658 (3 pages) | Cited 8 times

Online Publication Date: 10 May 2010

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Exchange anisotropy has been observed and investigated in single-crystalline CuO nanowires grown by thermal oxidation of Cu. The exchange bias field decreases by increasing temperature and can be tuned by the strength of the cooling field. A training effect has also been observed. The obtained results can be understood in terms of a phenomenological core-shell model, where the core of the CuO nanowire shows antiferromagnetic behavior and the surrounding shell behaves as a spin glass-like system due to uncompensated surface spins.
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75.75.-c Magnetic properties of nanostructures
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)
75.30.Et Exchange and superexchange interactions
75.30.Gw Magnetic anisotropy
75.50.Ee Antiferromagnetics
75.50.Lk Spin glasses and other random magnets

Large scale computer simulations of strain distribution and electron effective masses in silicon 〈100〉 nanowires

Christian Tuma and Alessandro Curioni

Appl. Phys. Lett. 96, 193106 (2010); http://dx.doi.org/10.1063/1.3428660 (3 pages) | Cited 2 times

Online Publication Date: 10 May 2010

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A multiscale method is proposed to analyze the internal redistribution of tensile strain applied to silicon 〈100〉 nanowires and its effect on electron effective masses m∗. Nonperiodic, realistic models of unprecedented size containing up to 2.2×107 atoms (652×26×26 nm3) allow the identification of nonuniform redistribution patterns specific to the constraints applied to impose external strain. Depending on how the external strain is imposed, silicon nanowires can show m∗ behavior similar to strained bulk silicon, or, as a function of nanowire size, can display intrinsic strain large enough that external strain hardly reduces m∗ further. For nanowire cross section sizes smaller than 8×8 nm2 quantum confinement leads to an increase in m∗ which cannot be compensated for by tensile strain.
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73.21.Hb Quantum wires
62.23.Hj Nanowires
61.46.Km Structure of nanowires and nanorods (long, free or loosely attached, quantum wires and quantum rods, but not gate-isolated embedded quantum wires)

Effective recombination velocity of textured surfaces

Kanglin Xiong, Shulong Lu, Desheng Jiang, Jianrong Dong, and Hui Yang

Appl. Phys. Lett. 96, 193107 (2010); http://dx.doi.org/10.1063/1.3396078 (3 pages) | Cited 2 times

Online Publication Date: 11 May 2010

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Surface texturization is an effective way to enhance the absorption of light for optoelectronic devices but it also aggravates the surface recombination by enlarging the surface area. In order to evaluate the influence of texture structures on the surface recombination, an effective surface recombination velocity is defined which is assumed to have an equivalent recombination effect on a flat surface. Based on numerical and analytical calculation, the dependences of effective surface recombination on the pattern geometry, the surface recombination velocity, and the diffusion length are analyzed.
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73.25.+i Surface conductivity and carrier phenomena
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Ultraviolet optical near-fields of microspheres imprinted in phase change films

J. Siegel, D. Puerto, J. Solis, F. J. García de Abajo, C. N. Afonso, M. Longo, C. Wiemer, M. Fanciulli, P. Kühler, M. Mosbacher, and P. Leiderer

Appl. Phys. Lett. 96, 193108 (2010); http://dx.doi.org/10.1063/1.3428582 (3 pages) | Cited 3 times

Online Publication Date: 12 May 2010

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We report an experimental method for directly imaging optical near-fields of dielectric microspheres upon illumination with ultraviolet nanosecond laser pulses. The intensity distribution is imprinted in chalcogenide films leaving behind a characteristic fingerprint with features below 200 nm in size, which we read out with high-resolution field emission scanning electron microscopy. The experimental results are well matched by a rigorous solution of Maxwell’s equations. Compared to previous works using infrared femtosecond laser pulses, the use of ultraviolet nanosecond pulses is identified to be superior in terms of minimum recordable features size and surface roughness of the imprint.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
68.37.Vj Field emission and field-ion microscopy
68.35.B- Structure of clean surfaces (and surface reconstruction)

Electrical current flow at conductive nanowires formed in GaN thin films by a dislocation template technique

Shin-ichi Amma, Yuki Tokumoto, Keiichi Edagawa, Naoya Shibata, Teruyasu Mizoguchi, Takahisa Yamamoto, and Yuichi Ikuhara

Appl. Phys. Lett. 96, 193109 (2010); http://dx.doi.org/10.1063/1.3429604 (3 pages) | Cited 5 times

Online Publication Date: 13 May 2010

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Conductive nanowires were fabricated in GaN thin film by selectively doping of Al along threading dislocations. Electrical current flow localized at the nanowires was directly measured by a contact mode atomic force microscope. The current flow at the nanowires was considered to be Frenkel–Poole emission mode, suggesting the existence of the deep acceptor level along the nanowires as a possible cause of the current flow. The results obtained in this study show the possibility for fabricating nanowires using pipe-diffusion at dislocations in solid thin films.
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73.63.Nm Quantum wires
72.20.Ht High-field and nonlinear effects
71.55.Eq III-V semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.uj III-V and II-VI semiconductors
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

Cyclotron resonance of two-dimensional electron system affected by neighboring quantum dot layer

K. Takehana, Y. Imanaka, T. Takamasu, and M. Henini

Appl. Phys. Lett. 96, 193110 (2010); http://dx.doi.org/10.1063/1.3430062 (3 pages)

Online Publication Date: 13 May 2010

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We have investigated the cyclotron resonance (CR) in a two-dimensional electron system (2DES) separated by a thin barrier layer from InAs self-assembled quantum dots (QDs). Two absorption peaks of CR of the 2DES were clearly observed, indicating coexistence of free electrons with high mobility and trapped electrons within local potential minima caused by the neighboring QDs. The CR of the trapped electrons changes its intensity, corresponding to the charge state of the QDs. This is the first optical measurements to show that the charge state of the QDs affects on the 2DES in the role as a floating gate.
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76.40.+b Diamagnetic and cyclotron resonances
72.20.Fr Low-field transport and mobility; piezoresistance
73.21.La Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.63.Hs Quantum wells
78.67.De Quantum wells

Transversal electrostatic strength of patterned collector affecting alignment of electrospun nanofibers

Marek Pokorny, Krzysztof Niedoba, and Vladimir Velebny

Appl. Phys. Lett. 96, 193111 (2010); http://dx.doi.org/10.1063/1.3430507 (3 pages) | Cited 3 times

Online Publication Date: 13 May 2010

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Conventional electrospinning produces randomly ordered nanofibers, however many potential applications require precisely defined morphology and anisotropic properties of nanofibrous materials. In this work, a collector consisting of two segments has been used to prepare uniaxially ordered nanofibers. Optimized collector design comes from both analytical calculations of electrostatic strength in the vicinity of the electrodes and from experimental verification by polyvinyl alcohol nanofibers fabrication. The transversal electric strength is a key factor in achieving very well aligned nanofibers. The results presented here help provide a better understanding of the electrically induced alignment mechanism.
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81.16.-c Methods of micro- and nanofabrication and processing
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
73.63.Bd Nanocrystalline materials

Defect reduction in silicon nanoparticles by low-temperature vacuum annealing

S. Niesar, A. R. Stegner, R. N. Pereira, M. Hoeb, H. Wiggers, M. S. Brandt, and M. Stutzmann

Appl. Phys. Lett. 96, 193112 (2010); http://dx.doi.org/10.1063/1.3428359 (3 pages) | Cited 9 times

Online Publication Date: 13 May 2010

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Using electron paramagnetic resonance, we find that vacuum annealing at 200 °C leads to a significant reduction in the silicon dangling bond (Si-db) defect density in silicon nanoparticles (Si-NPs). The best improvement of the Si-db density by a factor of 10 is obtained when the vacuum annealing is combined with an etching step in hydrofluoric acid (HF), whereas HF etching alone only removes the Si-dbs at the Si/SiO2 interface. The reduction in the Si-db defect density is confirmed by photothermal deflection spectroscopy and photoconductivity measurements on thin Si-NPs films.
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61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.72.Cc Kinetics of defect formation and annealing
76.30.-v Electron paramagnetic resonance and relaxation
73.63.Bd Nanocrystalline materials
76.30.Mi Color centers and other defects
81.65.Cf Surface cleaning, etching, patterning

Nanoelectromechanical torsion switch of low operation voltage for nonvolatile memory application

Wenfeng Xiang and Chengkuo Lee

Appl. Phys. Lett. 96, 193113 (2010); http://dx.doi.org/10.1063/1.3428781 (3 pages) | Cited 8 times

Online Publication Date: 13 May 2010

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Nanoelectromechanical torsion switches are fabricated by using focused ion beam milling on silicon-on-insulator substrate. The device layer thickness of the substrate is 220 nm. A 9 μm long and 1.5 μm wide suspended silicon cantilever is mechanically connected to peripheral silicon device layer via a silicon torsion spring with the length of 2.4 μm and width of 530 nm. After hydrofluoric-acid vapor releasing, the silicon cantilever shows downward deflection. The pull-in voltage is about 5.5 V and the ratio of current measured at the ON/OFF states is over 1000. Moreover, the simulated data of pull-in voltage of torsion switch is in agreement with the experimental result, which will contribute to design of an optimal nanoelectromechanical torsion switch with a driven voltage as low as 1.2 V. According to the preliminary results, this torsion switch with low driven voltage has a great potential for high density non-volatile memory application.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

Simple modeling of self-oscillations in nanoelectromechanical systems

A. Lazarus, T. Barois, S. Perisanu, P. Poncharal, P. Manneville, E. de Langre, S. T. Purcell, P. Vincent, and A. Ayari

Appl. Phys. Lett. 96, 193114 (2010); http://dx.doi.org/10.1063/1.3396191 (3 pages) | Cited 2 times

Online Publication Date: 14 May 2010

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We present here a simple analytical model for self-oscillations in nanoelectromechanical systems. We show that a field emission self-oscillator can be described by a lumped electrical circuit and that this approach is generalizable to other electromechanical oscillator devices. The analytical model is supported by dynamical simulations where the electrostatic parameters are obtained by finite element computations.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
84.30.Ng Oscillators, pulse generators, and function generators
02.70.Dh Finite-element and Galerkin methods

Two-dimensional graphene superlattice made with partial hydrogenation

Ming Yang, Argo Nurbawono, Chun Zhang, Yuan Ping Feng, and Ariando

Appl. Phys. Lett. 96, 193115 (2010); http://dx.doi.org/10.1063/1.3425664 (3 pages) | Cited 14 times

Online Publication Date: 14 May 2010

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Electronic properties of two-dimensional 2D graphene superlattice made with partial hydrogenation were thoroughly studied via density functional tight binding approach which incorporates the tight-binding method into the density functional formalism. The 2D pattern of hydrogen atoms on graphene was found to have great effects on electronic structures of graphene superlattice. In particular, the edges of the 2D pattern, armchair or zigzag, are essential for the energy band gap opening, and the energy band gap sensitively depends on the shape, size, and the 2D periodicity of the pattern. Based on these findings, we suggested that the 2D graphene superlattice could be used in fabricating graphene quantum dots or heterojunctions without the need for cutting or etching.
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73.22.Pr Electronic structure of graphene
73.21.Cd Superlattices
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.15.Ap Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)

ZnO–ZnTe nanocone heterojunctions

Sang Hyun Lee, X.-G. Zhang, Barton Smith, Sung Seok A. Seo, Zane W. Bell, and Jun Xu

Appl. Phys. Lett. 96, 193116 (2010); http://dx.doi.org/10.1063/1.3430604 (3 pages) | Cited 7 times

Online Publication Date: 14 May 2010

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We report heterojunctions made of vertically aligned ZnO–ZnTe nanocones synthesized using a combination of thermal vapor deposition and pulsed-laser deposition. ZnO nanocones and nanorods were controlled by utilizing the growth rate difference between central and boundary sites of precursor domains. The p–n heterojunctions were subsequently formed by growing ZnTe as shells on the nanocone surface. Structural and electric characteristics indicate that nanocones are more feasible than nanorods for forming heterojunction. Furthermore, theoretical modeling demonstrates that the nanocone-based junction exhibits an electrostatic potential profile that is much more effective for carrier transport than the electrostatic potential for the nanorod-based junction.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
73.63.Bd Nanocrystalline materials
81.05.Dz II-VI semiconductors
81.07.Bc Nanocrystalline materials
81.15.Fg Pulsed laser ablation deposition
61.46.-w Structure of nanoscale materials

Using graded barriers to control the optical properties of ZnO/Zn0.7Mg0.3O quantum wells with an intrinsic internal electric field

C. R. Hall, L. V. Dao, K. Koike, S. Sasa, H. H. Tan, M. Inoue, M. Yano, C. Jagadish, and J. A. Davis

Appl. Phys. Lett. 96, 193117 (2010); http://dx.doi.org/10.1063/1.3428430 (3 pages) | Cited 11 times

Online Publication Date: 14 May 2010

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Quantum wells with graded barriers are demonstrated as a means to control both the transition energy and electron-hole wave function overlap for quantum wells with an intrinsic internal electric field. In the case of c-axis grown ZnO/ZnMgO quantum wells, the graded barriers are produced by stepping the magnesium composition during the growth process. Four quantum wells with different structures are examined, where each well has similar transition energy, yet a wide range of wave function overlaps are observed. Photoluminescence and time resolved photoluminescence show good agreement with calculations.
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78.67.De Quantum wells
81.07.St Quantum wells
78.55.Et II-VI semiconductors
78.47.jd Time resolved luminescence
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