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Bragg grating writing in acoustically excited optical fiber

Roberson A. Oliveira, Kevin Cook, John Canning, and Alexandre A. P. Pohl

Appl. Phys. Lett. 97, 041101 (2010); doi:10.1063/1.3467041 (3 pages)

Online Publication Date: 26 July 2010

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The inscription of Bragg gratings in optical fiber under acoustic excitation is proposed and demonstrated. A permanent change in the refractive index as the period of the grating is modulated with periodic acoustic excitation during UV grating writing leads to sampling of the structure and the production of spectral combs. A physical contribution to index change through stress profiling reveals a complicated picture of UV-induced index change, well beyond simple localized defect and structural changes. The method offers the potential for tuning and tailoring conventional uniform gratings during the writing process using adjustable acoustic wave functions, an approach to control the writing process.

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42.81.Bm	Fabrication, cladding, and splicing
43.35.Sx	Acoustooptical effects, optoacoustics, acoustical visualization, acoustical microscopy, and acoustical holography
42.79.Dj	Gratings

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Mechanical and thermal stability of plasmonic emitters on flexible polyimide substrates

Ismail Emre Araci, Veysi Demir, Aleksandr Kropachev, Terje Skotheim, Robert A. Norwood, and N. Peyghambarian

Appl. Phys. Lett. 97, 041102 (2010); doi:10.1063/1.3462795 (3 pages)

Online Publication Date: 26 July 2010

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We have fabricated Ag/SiO₂/Ag plasmonic thermal infrared (IR) emitters with different hexagonal lattice periods on flexible polyimide (PI) membranes. The high temperature resistance and comparable coefficient of thermal expansion with metallic films enabled the fabrication of plasmonic IR emitters on PI films. We have investigated the effects of temperature and mechanical strain on the spectral and structural properties of the flexible emitter. The low heat conductivity and low thermal mass of thin free standing PI substrates enables IR emitters with low power consumption (<1 W) and fast switching time (in the order of ten milliseconds).

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68.35.Gy	Mechanical properties; surface strains
68.35.Ct	Interface structure and roughness
66.70.Hk	Glasses and polymers
68.60.Dv	Thermal stability; thermal effects
68.60.Bs	Mechanical and acoustical properties
78.67.-n	Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

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Efficient electron spin injection in MnAs-based spin-light-emitting-diodes up to room temperature

E. D. Fraser, S. Hegde, L. Schweidenback, A. H. Russ, A. Petrou, H. Luo, and G. Kioseoglou

Appl. Phys. Lett. 97, 041103 (2010); doi:10.1063/1.3464966 (3 pages)

Online Publication Date: 26 July 2010

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Studies of ferromagnetic MnAs in recent years have revealed a wide range of properties desirable for spintronic applications. Previously studied MnAs spin-light-emitting-diodes exhibited a low value of spin injection into the device active region. In this work, we have investigated injection of spin polarized electrons from MnAs into AlGaAs(n)/GaAs(i)/AlGaAs(p) n-i-p structures. The band-edge electroluminescence emitted from these devices has a saturation circular polarization of 26% at 7 K and B = 2 T. Using optical pumping measurements the corresponding electron spin polarization was determined to be 52%. Emission persists up to room temperature, with a saturation circular polarization of 6% at B = 2 T.

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85.60.Jb

Light-emitting devices

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Femtosecond laser fabrication of high reflectivity micromirrors

D. Brodoceanu, G. D. Cole, N. Kiesel, M. Aspelmeyer, and D. Bäuerle

Appl. Phys. Lett. 97, 041104 (2010); doi:10.1063/1.3467846 (3 pages)

Online Publication Date: 26 July 2010

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High-quality freestanding micromirrors consisting of 40 dielectric layers on silicon have been fabricated by ultrashort-pulse laser ablation in combination with laser-assisted wet chemical etching. Backside material removal enables direct access to both faces of the dielectric coating. The amplitude reflectance of the micromirrors has been determined by Fabry–Pérot interferometry; a finesse in excess of 8900±700, corresponding to a reflectivity exceeding 99.95%, has been found. The mechanical quality factor, Q, of the microresonators, measured at 20 K, is determined to be between 5000 and 6000.

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42.79.Bh	Lenses, prisms and mirrors
42.65.Re	Ultrafast processes; optical pulse generation and pulse compression
07.60.Ly	Interferometers
42.82.Cr	Fabrication techniques; lithography, pattern transfer
42.79.Wc	Optical coatings

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Bias-induced offset effect overlapped on bipolar-resistance effect based on Co/SiO₂/Si structure

Chong Qi Yu and Hui Wang

Appl. Phys. Lett. 97, 041105 (2010); doi:10.1063/1.3469932 (3 pages)

Online Publication Date: 26 July 2010

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Recent study shows the resistance of a metal-oxide-semiconductor (MOS) structure can be controlled by a laser via a bipolar-resistance effect (BRE). Based on this BRE phenomenon, we find an overlapped offset effect which is induced by an external bias applying to the structure. This offset effect features with a moveable equilibrium point of BRE, suggesting a combined control to the resistance and adding functionality to the MOS-based photoelectric devices.

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73.40.Qv	Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
85.60.-q	Optoelectronic devices
61.80.Ba	Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.82.Fk	Semiconductors
72.40.+w	Photoconduction and photovoltaic effects

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Spatially resolved characterization of InGaAs/GaAs quantum dot structures by scanning spreading resistance microscopy

T. Hakkarainen, O. Douhéret, S. Anand, L. Fu, H. H. Tan, and C. Jagadish

Appl. Phys. Lett. 97, 041106 (2010); doi:10.1063/1.3467138 (3 pages)

Online Publication Date: 27 July 2010

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Cross-sectional scanning spreading resistance microscopy (SSRM) is used to investigate stacked InGaAs/GaAs quantum dot (QD) structures with different doping schemes. Spatially resolved imaging of the QDs by SSRM is demonstrated. The SSRM contrast obtained for the QD layers is found to depend on doping in the structure. In the undoped structures both QD-layers and QDs within the layers could be resolved, while in the doped structures the QD layers appear more or less uniformly broadened. The origin of the SSRM contrast in the QD layer in the different samples is discussed and correlated with doping schemes.

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81.07.Ta	Quantum dots
61.72.U-	Doping and impurity implantation

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Electro-optic switching element for dielectric-loaded surface plasmon polariton waveguides

A. V. Krasavin and A. V. Zayats

Appl. Phys. Lett. 97, 041107 (2010); doi:10.1063/1.3464552 (3 pages)

Online Publication Date: 27 July 2010

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We present three-dimensional numerical modeling of an active electronically controlled switching element for fully-functional plasmonic circuits based on dielectric-loaded surface plasmon polariton waveguides. It has been demonstrated that the transmission of the guided mode through a highly wavelength-selective waveguide ring resonator (WRR) can be efficiently controlled with very small refractive index changes of the order of 10⁻³, achievable through the electro-optic effect in nonlinear materials. Furthermore, we have introduced a figure of merit for such active plasmonic elements and optimized the active WRR performance in terms of its sensitivity and size. These results shows the potential to create high performance 600 nm radius plasmonic WRR switches.

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42.79.Gn

Optical waveguides and couplers

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Programmable purification of type-I polarization-entanglement

Simone Cialdi, Davide Brivio, and Matteo G. A. Paris

Appl. Phys. Lett. 97, 041108 (2010); doi:10.1063/1.3467832 (3 pages)

Online Publication Date: 27 July 2010

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We suggest and demonstrate a scheme to compensate spatial and spectral decoherence effects in the generation of polarization entangled states by type-I parametric downconversion. In our device, a programmable spatial light modulator imposes a polarization dependent phase-shift on different spatial sections of the overall downconversion output and this effect is exploited to realize an effective purification technique for polarization entanglement.

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42.65.Lm	Parametric down conversion and production of entangled photons
03.67.Mn	Entanglement measures, witnesses, and other characterizations
42.79.Hp	Optical processors, correlators, and modulators
42.50.-p	Quantum optics

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Mach–Zehnder interferometry method for decoupling electro-optic and piezoelectric effects in poled polymer films

Charles Greenlee, Anael Guilmo, Ayodeji Opadeyi, Roland Himmelhuber, Robert A. Norwood, Mahmoud Fallahi, Jingdong Luo, Su Huang, Xing-Hua Zhou, Alex K.-Y. Jen, and Nasser Peyghambarian

Appl. Phys. Lett. 97, 041109 (2010); doi:10.1063/1.3467847 (3 pages)

Online Publication Date: 27 July 2010

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A Mach–Zehnder interferometer (MZI) is utilized to decouple the electro-optic and piezoelectric tensor effects occurring in a poled polymer film. This method has significant advantages over the commonly used Teng-Man reflection ellipsometry technique by allowing for the independent determination of the Pockel’s coefficients r₁₃ and r₃₃ and the piezoelectric coefficient d₃₃. The r₃₃ value of a guest host polymer that consists of AJLZ53 amorphous polycarbonate was found to be 122.69 pm/V and 123.03 pm/V using the MZI and reflection ellipsometry method, respectively. The r₃₃ data fits well to the dispersion of the second order susceptibility tensor based on the two-level model approximation.

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78.66.Qn	Polymers; organic compounds
78.20.Jq	Electro-optical effects
78.20.hb	Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
42.70.Jk	Polymers and organics
42.65.An	Optical susceptibility, hyperpolarizability
07.60.Ly	Interferometers

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Freestanding spherical silicon nanocrystals: A model system for studying confined excitons

B. Goller, S. Polisski, H. Wiggers, and D. Kovalev

Appl. Phys. Lett. 97, 041110 (2010); doi:10.1063/1.3470103 (3 pages)

Online Publication Date: 27 July 2010

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We report on the light emitting properties of freestanding hydrogen-terminated spherical silicon nanocrystals. The nanocrystals exhibit size-dependent tunable light emission properties. Many light emission properties of this system are found to be very similar to those known for other systems containing silicon nanocrystals. However, we found specific emission properties of this system ascribed to the spherical shape of silicon nanocrystals and their spatial separation. We attributed all observations to the spatial confinement of excitons within the crystalline core of the indirect band gap silicon nanocrystals.

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78.67.Hc	Quantum dots
71.35.-y	Excitons and related phenomena
78.30.Am	Elemental semiconductors and insulators
78.55.Ap	Elemental semiconductors
78.66.Db	Elemental semiconductors and insulators
78.40.Fy	Semiconductors

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Room temperature terahertz electro-optic modulation by excitons in carbon nanotubes

T. Ogawa, S. Watanabe, N. Minami, and R. Shimano

Appl. Phys. Lett. 97, 041111 (2010); doi:10.1063/1.3470105 (3 pages)

Online Publication Date: 27 July 2010

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We have investigated the electroabsorption of aligned micelle-wrapped single-walled carbon nanotubes induced by intense terahertz (THz) pulses by using THz-pump and optical-probe spectroscopy at room temperature. The electroabsorption signal is enhanced at exciton resonances near the optical communication wavelength. The signal showed a quadratic dependence on the THz electric field and instantaneously followed the THz oscillation ( ∼ 1 ps). The observed ultrafast electro-optic modulation with a THz bandwidth suggests the potential functionality of carbon nanotubes in high-speed electro-optic devices.

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71.35.-y	Excitons and related phenomena
81.07.De	Nanotubes
78.67.Ch	Nanotubes
78.47.J-	Ultrafast spectroscopy (<1 psec)

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Robust one-way modes in gyromagnetic photonic crystal waveguides with different interfaces

Jin-Xin Fu, Rong-Juan Liu, and Zhi-Yuan Li

Appl. Phys. Lett. 97, 041112 (2010); doi:10.1063/1.3470873 (3 pages)

Online Publication Date: 27 July 2010

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We experimentally realize a one-way waveguide made from a gyromagnetic photonic crystal (GPC) and a normal dielectric photonic crystal in their common photonic band gap. Due to the time reversal symmetry breaking of the GPC under a dc magnetic field, the edge modes exhibit a one-way property. We investigate the unidirectional propagation character originating from the chiral edge states at different widths of the waveguide. Both the forward modes, which are robust to various obstacles, and the backward modes show an obvious frequency shift for different waveguide widths. Numerical simulations by means of the transfer-matrix method and multiple scattering methods agree well with the experimental data.

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42.55.Tv	Photonic crystal lasers and coherent effects
42.79.Gn	Optical waveguides and couplers

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Efficient unidirectional generation of surface plasmon polaritons with asymmetric single-nanoslit

Jianjun Chen, Zhi Li, Song Yue, and Qihuang Gong

Appl. Phys. Lett. 97, 041113 (2010); doi:10.1063/1.3472251 (3 pages)

Online Publication Date: 28 July 2010

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An asymmetric single-nanoslit composed of a conventional nanoslit with a nanogroove next to it in a metallic film is proposed to generate unidirectional surface plasmon polaritons (SPPs) efficiently with back-side illumination. Numerical simulations show that due to the different interference processes of SPPs to opposite directions, efficient unidirectional SPP generation can be achieved. Experimentally, an extinction ratio of about 30:1 for SPPs to opposite directions and a generation efficiency of about 1.8 times that of the symmetrical case are demonstrated at wavelength of 830 nm with the lateral dimension of the asymmetric single-nanoslit of only 370 nm.

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73.20.Mf	Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
71.36.+c	Polaritons (including photon-phonon and photon-magnon interactions)
78.66.Bz	Metals and metallic alloys

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Subwavelength focusing of light by a tapered microtube

Jian Fu, Hongtao Dong, and Wei Fang

Appl. Phys. Lett. 97, 041114 (2010); doi:10.1063/1.3472224 (3 pages)

Online Publication Date: 29 July 2010

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We propose a mechanism for subwavelength focusing at optical frequencies based on the use of a tapered microtube fabricated from a glass capillary tube. Using coherent illumination at 671 nm and a near-field scanning optical microscope probe which was mounted on a three-axis piezonanopositioning stage, a sequence of two-dimensional intensity profiles were obtained. Our experimental results reveal the smallest focal spot with a near diffraction-limited full width at half-maximum of ∼ 435 nm (0.65λ) at a distance of ∼ 1.47 μm (2.2λ) from the output end face of microtube. The experimental results are in agreement with our numerical simulation.

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42.79.-e

Optical elements, devices, and systems

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Polarization asymmetry and optical modal gain saturation via carrier–photon interaction in ZnO

Bumjin Kim, Heedae Kim, Sungkyun Park, Kwangseuk Kyhm, and Chaeryong Cho

Appl. Phys. Lett. 97, 041115 (2010); doi:10.1063/1.3473729 (3 pages)

Online Publication Date: 29 July 2010

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The polarization dependence of modal gain was examined in ZnO using a variable stripe length method, where the transverse electric (TE) mode gain was dominant over the transverse magnetic (TM) mode gain due to the polarization asymmetry of the wurtzite structure. Modal gain saturation was also investigated using a modal gain contour map for the wavelength and stripe length. The TE modal gain and degree of polarization increased with increasing stripe length up to a threshold length ( ∼ 100 μm). At longer stripe lengths, rapid reduction in the carrier density along the stripe resulted in gain saturation and a spectral redshift.

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78.66.Hf	II-VI semiconductors
68.55.-a	Thin film structure and morphology
42.70.-a	Optical materials

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Spin modulation in semiconductor lasers

Jeongsu Lee, William Falls, Rafal Oszwałdowski, and Igor Žutić

Appl. Phys. Lett. 97, 041116 (2010); doi:10.1063/1.3473759 (3 pages)

Online Publication Date: 29 July 2010

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We provide an analytic study of the dynamics of semiconductor lasers with injection (pump) of spin-polarized electrons, previously considered in the steady-state regime. Using complementary approaches of quasistatic and small signal analyses, we elucidate how the spin modulation in semiconductor lasers can improve performance, as compared to the conventional (spin-unpolarized) counterparts. We reveal that the spin-polarized injection can lead to an enhanced bandwidth and desirable switching properties of spin-lasers.

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42.55.Px	Semiconductor lasers; laser diodes
42.60.Fc	Modulation, tuning, and mode locking

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Photonic crystal reflectance switching by dye electrophoresis

Joshua D. Krabbe and Michael J. Brett

Appl. Phys. Lett. 97, 041117 (2010); doi:10.1063/1.3473827 (3 pages)

Online Publication Date: 29 July 2010

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Reflectivity of a photonic crystal device fabricated by glancing angle deposition may be reversibly altered by infiltration with an absorbing dye solution. An electric field controls the dye ion motion through the photonic crystal. Rapid reflectance changes up to 0.4 in the crystal’s optical band gap are demonstrated. The time evolution of the dye movement process is examined and its operation described. This work may have applications for a passive optical display.

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42.65.Pc	Optical bistability, multistability, and switching, including local field effects
42.70.Qs	Photonic bandgap materials

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Plane and hemispherical potential structures in magnetically expanding plasmas

Kazunori Takahashi, Yuichi Igarashi, and Tamiya Fujiwara

Appl. Phys. Lett. 97, 041501 (2010); doi:10.1063/1.3467857 (3 pages)

Online Publication Date: 28 July 2010

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Two-dimensional potential structures are measured for different gas pressure in expanding argon plasma using permanent magnets, where the magnetic field is about 100 G in the source and several gauss in the diffusion chamber. The plane potential drop is observed near the source exit for 0.35 mTorr, while the potential structure becomes hemispherical when increasing up to 1 mTorr; the hemispherical structure results in the radial divergence of the ion beam. It is found that the trajectories of the accelerated ions and the electrons overcoming the potential drop are dominated by the potential structure and magnetic-field lines, respectively.

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52.30.Cv

Magnetohydrodynamics (including electron magnetohydrodynamics)

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Recombination effects during expansion into vacuum in laser produced Sn plasma

Russell A. Burdt, Yoshifumi Ueno, Yezheng Tao, Sam Yuspeh, Mark S. Tillack, and Farrokh Najmabadi

Appl. Phys. Lett. 97, 041502 (2010); doi:10.1063/1.3473817 (3 pages)

Online Publication Date: 30 July 2010

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The distance over which the charge state distribution evolves during the expansion of laser produced Sn plasma in vacuum is investigated experimentally. This distance is found to be less than 6 cm with a planar target irradiated by a 1.064 μm laser at 8.3×10¹¹ W/cm² but greater than 60 cm when a 10.6 μm laser at 2.5×10¹⁰ W/cm² is used. The difference is attributed to the laser wavelength dependence of the coronal electron density and the subsequent recombination processes during expansion. Important implications to the extreme ultraviolet x-ray source application are discussed specifically.

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52.50.Jm	Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.25.-b	Plasma properties

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Dielectric barrier discharge control of a turbulent boundary layer in a supersonic flow

S. Im, H. Do, and M. A. Cappelli

Appl. Phys. Lett. 97, 041503 (2010); doi:10.1063/1.3473820 (3 pages)

Online Publication Date: 30 July 2010

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We demonstrate effective manipulation of a turbulent boundary layer at Mach 4.7 conditions using a surface dielectric barrier discharge (DBD) actuator. The freestream conditions of low static pressure (1 kPa) and temperature (60 K) are conducive to the visualization of flow features using Rayleigh scattering from condensed CO₂ particles. The boundary layer thinning is observed when spanwise momentum is induced by the low power (6.8 W), low frequency (28 kHz) single actuator pair oriented parallel to the freestream flow.

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52.80.-s	Electric discharges
47.27.nb	Boundary layer turbulence
47.80.Jk	Flow visualization and imaging
52.38.Bv	Rayleigh scattering; stimulated Brillouin and Raman scattering

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Percolation and polaritonic effects in periodic planar nanostructures evolving from holes to islands

Y. Peng, T. Paudel, W.-C. Chen, W. J. Padilla, Z. F. Ren, and K. Kempa

Appl. Phys. Lett. 97, 041901 (2010); doi:10.1063/1.3462935 (3 pages)

Online Publication Date: 26 July 2010

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We study interaction of the electromagnetic radiation with a series of thin film periodic nanostructures evolving from holes to islands. We show, through model calculations, simulations, and experiments, that the responses of these structures evolve accordingly, with two topologically distinct spectral types for holes and islands. We find also, that the response at the transitional pattern is singular. We show that the corresponding effective dielectric function follows the critical behavior predicted by the percolation theory and thus the hole-to-island structural evolution in this series is a topological analog of the percolation problem, with the percolation threshold at the transitional pattern.

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64.60.ah	Percolation
61.46.-w	Structure of nanoscale materials
71.45.Gm	Exchange, correlation, dielectric and magnetic response functions, plasmons
02.40.Pc	General topology
68.55.-a	Thin film structure and morphology
71.36.+c	Polaritons (including photon-phonon and photon-magnon interactions)
41.20.Jb	Electromagnetic wave propagation; radiowave propagation

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Hydrostatic pressure decreases the proton mobility in the hydrated BaZr_0.9Y_0.1O₃ proton conductor

Qianli Chen, Artur Braun, Alejandro Ovalle, Cristian-Daniel Savaniu, Thomas Graule, and Nikolai Bagdassarov

Appl. Phys. Lett. 97, 041902 (2010); doi:10.1063/1.3464162 (3 pages)

Online Publication Date: 27 July 2010

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Impedance spectroscopy on the hydrated proton conductor BaZr_0.9Y_0.1O₃ at high temperatures shows that the bulk proton conductivity activation energy E_b scales with the strain parameter ε, as achieved by hydrostatic pressures up to 2 GPa, suggesting that large lattices favor proton diffusivity. At high temperature, E_b increases upon pressure by 40%. The grain boundary activation energy Eg is around twice as E_b, indicating higher proton mobility in grain boundaries as a result of pressure induced sintering. An expanded lattice with strain parameter ε>1 should have lower E_b, suggesting that thin films expansive tensile strain could have larger proton conductivity.

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66.30.hp	Molecular crystals
81.05.-t	Specific materials: fabrication, treatment, testing, and analysis
62.50.-p	High-pressure effects in solids and liquids
82.80.Fk	Electrochemical methods
81.40.Lm	Deformation, plasticity, and creep
81.05.Je	Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)

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Goos–Hänchen shift of the reflection from nonlinear nanocomposites with electric field tunability

Dongliang Gao and Lei Gao

Appl. Phys. Lett. 97, 041903 (2010); doi:10.1063/1.3470000 (3 pages)

Online Publication Date: 27 July 2010

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A structure of nonlinear metal-dielectric nanocomposites is proposed to control the lateral shift of the reflected beam by tuning an applied electric field. Based on spectral representation theory and stationary phase method, the lateral shift is found to exhibit hysteretic effects including single optical bistability, double optical bistability, and optical tristability. As a consequence, it can be manipulated via the applied field. Moreover, the shift is strongly dependent on the volume fraction, the shape of the metallic particles, and the incident angle. Numerical simulations based on Gaussian waves are in good agreement with our theoretical calculations.

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78.67.Sc	Nanoaggregates; nanocomposites
77.84.Lf	Composite materials
42.65.Pc	Optical bistability, multistability, and switching, including local field effects

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Effect of edge smoothening on the extinction spectra of metal nanoparticles

J. Jayabalan, Asha Singh, and Rama Chari

Appl. Phys. Lett. 97, 041904 (2010); doi:10.1063/1.3464321 (3 pages)

Online Publication Date: 28 July 2010

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To calculate the extinction spectrum of a metal nanoparticle, it is common to use a regular shape which is close to the experimental one. We show that, to model a real metal particle, it is essential to remove sharp corners and tips and smoothen the bounding surface. An efficient and simple method to smoothen the tips and corners of the model shape of the particle is presented. The extinction calculated using smoothed particles predicts more accurately the extinction spectrum of as grown particles as well as the changes in the extinction spectrum during melting and reshaping of the particle.

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68.37.-d	Microscopy of surfaces, interfaces, and thin films
61.46.Df	Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
78.67.Bf	Nanocrystals, nanoparticles, and nanoclusters
64.70.dj	Melting of specific substances
73.22.Lp	Collective excitations
78.20.Ci	Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

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Silver induced faceting of Si(112)

T. Nabbefeld, C. Wiethoff, F.-J. Meyer zu Heringdorf, and M. Horn-von Hoegen

Appl. Phys. Lett. 97, 041905 (2010); doi:10.1063/1.3464555 (3 pages)

Online Publication Date: 28 July 2010

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Si nanowires grown in ultrahigh vacuum by metal-catalyzed vapor-liquid-solid epitaxy are known to exhibit sidewalls with {112}-type orientation. For some metals the sidewalls show pronounced faceting. Ag induced faceting on Si(112) surfaces was studied in situ by spot-profile-analyzing low energy electron diffraction and ex situ atomic force microscopy. The (112) surface decomposes into (115)- and (111)-( math

×

)-facets, both of which are Ag terminated. The width of the facets is kinetically limited and varies between 6 nm at T<550 °C and 30 nm at T = 690 °C.

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68.65.La	Quantum wires (patterned in quantum wells)
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)
68.35.bg	Semiconductors
81.07.Gf	Nanowires
81.16.Hc	Catalytic methods
82.65.+r	Surface and interface chemistry; heterogeneous catalysis at surfaces

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Real-time observation of anisotropic strain relaxation by three-dimensional reciprocal space mapping during InGaAs/GaAs (001) growth

Hidetoshi Suzuki, Takuo Sasaki, Akihisa Sai, Yoshio Ohshita, Itaru Kamiya, Masafumi Yamaguchi, Masamitu Takahasi, and Seiji Fujikawa

Appl. Phys. Lett. 97, 041906 (2010); doi:10.1063/1.3458695 (3 pages)

Online Publication Date: 28 July 2010

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Real-time three-dimensional reciprocal space mapping (3D-RSM) measurement during In_0.12Ga_0.88As/GaAs(001) molecular beam epitaxial growth has been performed to investigate anisotropy in relaxation processes along [110] and [ math

10] directions caused by α and β misfit dislocations (MDs). Anisotropies, strain relaxation, and crystal quality in both directions were simultaneously evaluated via the position and broadness of 022 diffraction in 3D-RSM. In the small-thickness region, strain relaxation caused by α-MDs is higher than that caused by β-MDs, and therefore crystal quality along [110] is worse than that along [ math

10]. Rapid relaxation along both [110] and [ math

10] directions occurs at almost the same thickness. After rapid relaxation, anisotropy in strain relaxation gradually decreases, whereas crystal quality along [ math

10] direction, presumably due to β-MDs, becomes better that along [110] direction and the ratio does not decay with thickness.

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81.05.Ea	III-V semiconductors
81.15.Hi	Molecular, atomic, ion, and chemical beam epitaxy
68.55.ag	Semiconductors
61.72.Lk	Linear defects: dislocations, disclinations
61.66.Fn	Inorganic compounds

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Strong circular photogalvanic effect in ZnO epitaxial films

Q. Zhang, X. Q. Wang, C. M. Yin, F. J. Xu, N. Tang, B. Shen, Y. H. Chen, K. Chang, W. K. Ge, Y. Ishitani, and A. Yoshikawa

Appl. Phys. Lett. 97, 041907 (2010); doi:10.1063/1.3467835 (3 pages)

Online Publication Date: 28 July 2010

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We report a strong circular photogalvanic effect (CPGE) in ZnO epitaxial films under interband excitation. It is observed that CPGE current is as large as 100 nA/W in ZnO, which is about one order in magnitude higher than that in InN film while the CPGE currents in GaN films are not detectable. The possible reasons for the above observations are the strong spin orbit coupling in ZnO or the inversed valence band structure of ZnO.

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73.50.Pz	Photoconduction and photovoltaic effects
71.70.Ej	Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
71.20.Nr	Semiconductor compounds
73.61.Ga	II-VI semiconductors

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Observation of ultrasonic backward beam displacement in transmission through a solid having superimposed periodicity

Sarah W. Herbison, John M. Vander Weide, and Nico F. Declercq

Appl. Phys. Lett. 97, 041908 (2010); doi:10.1063/1.3469998 (3 pages)

Online Publication Date: 28 July 2010

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The backward displacement of an ultrasonic beam on a periodically corrugated liquid–solid interface was originally observed in 1976. Additional study of the phenomenon has occurred recently, as further studies of acoustic and elastic wave interaction with periodic surfaces and structures have been driven by increasing interest in phononic crystals. Until now, the phenomenon has only been observed in reflection. This study reports results from experiments that investigate whether a backward beam displacement can also occur in transmission through the interior of a solid. The results indicate that a backward shift may in fact be observed in all fields generated. Therefore, ultrasonic backward beam displacements are not only a reflection phenomenon but may also be observed in transmission.

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68.35.Iv	Acoustical properties
68.35.Ja	Surface and interface dynamics and vibrations
43.20.El	Reflection, refraction, diffraction of acoustic waves
43.35.Pt	Surface waves in solids and liquids

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Carrier dynamics in isoelectronic ZnSe_1−xO_x semiconductors

Y. C. Lin, H. L. Chung, W. C. Chou, W. K. Chen, W. H. Chang, C. Y. Chen, and J. I. Chyi

Appl. Phys. Lett. 97, 041909 (2010); doi:10.1063/1.3473776 (3 pages)

Online Publication Date: 29 July 2010

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This study explores the effects of both Oxygen and temperature on the carrier dynamics of isoelectronic ZnSe_1−xO_x (x = 0.027 and 0.053) semiconductors using photoluminescence (PL) and time-resolved PL spectroscopy. We find that the Kohlrausch law is highly consistent with the complex decay traces induced by isoelectronic O traps, and the mechanism of carrier recombination undergoes a complicated change from trapped to free excitons with the increase in temperature. Complex recombination processes are clarified using the relaxation model based on various decay channels. These findings are consistent with the initial fall in the stretching exponent β followed by its monotonic increase with increasing temperature.

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72.20.Jv	Charge carriers: generation, recombination, lifetime, and trapping
78.47.D-	Time resolved spectroscopy (>1 psec)
72.80.Ey	III-V and II-VI semiconductors
78.55.Et	II-VI semiconductors
81.05.Dz	II-VI semiconductors
71.35.-y	Excitons and related phenomena

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Type II heterostructures formed by zinc-blende inclusions in InP and GaAs wurtzite nanowires

J.-M. Jancu, K. Gauthron, L. Largeau, G. Patriarche, J.-C. Harmand, and P. Voisin

Appl. Phys. Lett. 97, 041910 (2010); doi:10.1063/1.3473779 (3 pages)

Online Publication Date: 29 July 2010

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Crystal phase heterostructures, consisting of homogeneous composition zinc-blende inclusions in wurtzite InP and GaAs nanowires are investigated theoretically in the frame of the extended-basis tight-binding approach. Increased band gap for the wurtzite phase and staggered type II band alignment are predicted for both materials. Comparison of theoretical results with microphotoluminescence measurements on single InP nanowires yields fair semiquantitative agreement.

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73.21.Hb	Quantum wires
71.15.Ap	Basis sets (LCAO, plane-wave, APW, etc.) and related methodology (scattering methods, ASA, linearized methods, etc.)
78.67.Uh	Nanowires
78.55.Cr	III-V semiconductors
61.72.Qq	Microscopic defects (voids, inclusions, etc.)
78.67.Lt	Quantum wires

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Fluorine-vacancy defects in fluorine-implanted silicon studied by electron paramagnetic resonance

T. Umeda, J. Isoya, T. Ohshima, S. Onoda, N. Morishita, K. Okonogi, and S. Shiratake

Appl. Phys. Lett. 97, 041911 (2010); doi:10.1063/1.3473763 (3 pages)

Online Publication Date: 30 July 2010

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An electron paramagnetic resonance (EPR) study on fluorine-vacancy defects (F_nV_m) in fluorine-implanted silicon is demonstrated. Fluorine implantation is an important technology for Si microdevices and EPR measurements showed that this process created a variety of F_nV_m defects of different sizes (V₂, V₄, and V₅). In F_nV_m, a Si–F bond exhibited a different chemical nature compared to a Si–H bond in hydrogen-vacancy complexes. The most primitive defect was FV₂ (F0 center) and the final types were F_nV₅ (F1 center) and F_nV₂ (F2 center) which increased in annealing processes as low temperature as 200 °C.

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76.30.Lh	Other ions and impurities
61.72.jd	Vacancies

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Oxygen vacancy estimation of high k metal gate using thermal dynamic model

H. L. Chang and M. S. Liang

Appl. Phys. Lett. 97, 041912 (2010); doi:10.1063/1.3473772 (3 pages)

Online Publication Date: 30 July 2010

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Oxygen vacancies are electronic defects in materials. In a metal oxide system, the distribution of such vacancies is determined by the oxygen affinity. This study predicts the oxygen vacancy concentration in a high-k/metal gate system using a developed thermal dynamic model. A system with Ti:N = 2 has a 200 mV lower flat band voltage than an N rich metal. The Gibbs free energy of formation of oxygen vacancies, ∼ 0.5 eV, is derived from flab band voltage shifts and created neutral oxygen vacancy. The oxygen vacancy model based on estimating thermal dynamics is proposed.

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73.40.Qv	Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
84.32.Tt	Capacitors
61.72.jd	Vacancies
65.40.G-	Other thermodynamical quantities

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Self-diffusivity as a function of density and temperature in crystalline solids and compensating rules for self-diffusion parameters in carbon subgroup crystals

A. N. Papathanassiou, I. Sakellis, and J. Grammatikakis

Appl. Phys. Lett. 97, 041913 (2010); doi:10.1063/1.3467203 (3 pages)

Online Publication Date: 30 July 2010

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The self-diffusion coefficient of crystalline solids as a function of density and temperature may derive from thermodynamics concepts and an earlier elastic thermodynamic point defect model [ Varotsos and Alexopoulos, Phys. Rev. B 15, 4111 (1977) ; Phys. Rev. B 18, 2683 (1978)] . Compensation laws ruling self-diffusion parameters in carbon-subgroup crystals obtained from theoretical calculations are predicted, as well.

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66.30.-h	Diffusion in solids
81.05.U-	Carbon/carbon-based materials
62.20.dq	Other elastic constants
61.72.J-	Point defects and defect clusters

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Local structure and site substitution in amorphous and quasicrystalline Zr–Ti–Ni–(Cu) alloys

S. Mechler, G. Schumacher, V. Koteski, H. Riesemeier, F. Schäfers, and H.-E. Mahnke

Appl. Phys. Lett. 97, 041914 (2010); doi:10.1063/1.3467265 (3 pages)

Online Publication Date: 30 July 2010

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Local structures of amorphous and quasicrystalline phases in Zr–Ti–Ni and Zr–Ti–Ni–Cu alloys have been studied by means of x-ray absorption spectroscopy. The amorphous phases show a high degree of icosahedral short-range order. In all investigated systems local order around Cu and Ni atoms was found to be nearly identical pointing to site substitution of Ni by Cu as the reason for an improved glass forming ability when Ni is partly substituted by Cu. The results strengthen an icosahedral cluster based approach for the structure of metallic glasses and indicate the importance of the medium-range order on glass formation.

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75.50.Kj	Amorphous and quasicrystalline magnetic materials
75.40.-s	Critical-point effects, specific heats, short-range order
61.43.Dq	Amorphous semiconductors, metals, and alloys
61.05.cj	X-ray absorption spectroscopy: EXAFS, NEXAFS, XANES, etc.
61.43.Fs	Glasses

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Observation of the forbidden doublet optical phonon in Raman spectra of strained Si for stress analysis

Vladimir Poborchii, Tetsuya Tada, and Toshihiko Kanayama

Appl. Phys. Lett. 97, 041915 (2010); doi:10.1063/1.3474604 (3 pages)

Online Publication Date: 30 July 2010

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Using high numerical aperture lenses, we detected doublet optical phonon, forbidden by selection rules, in Raman spectra of Si strained in the (001) plane (bulk Si as well as strained Si device structure grown on SiGe). This allowed us to quantitatively determine stress and its distribution in strained Si with the ∼ 10% accuracy, assuming symmetric biaxial stress. At the same time, we demonstrate some deviations of the real stress from the assumed model. For better accuracy, one has to consider these deviations as well as a possibility of improvement of available Si phonon deformation potential values.

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78.30.Am	Elemental semiconductors and insulators
78.20.hb	Piezo-optical, elasto-optical, acousto-optical, and photoelastic effects
63.20.-e	Phonons in crystal lattices

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From ZnS nanobelts to ZnO/ZnS heterostructures: Microscopy analysis and their tunable optical property

Wei Liu, Rongming Wang, and Ning Wang

Appl. Phys. Lett. 97, 041916 (2010); doi:10.1063/1.3474609 (3 pages)

Online Publication Date: 30 July 2010

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Ultrathin ZnS nanobelts were assembled by ultrasmall nanocrystals with oriented self assembly, which were then transformed into their single-crystal counterparts by thermal treatment in N₂. ZnO/ZnS heterostructures were also obtained at elevated temperature in open air, where well aligned ZnO nanocones were grown along the ZnS nanobelts with a definite orientation relationship described as [0001]_ZnS∥[0001]_ZnO and (10 math

0)_ZnS∥(10 math

0)_ZnO. Mixed transition and tuned band gaps of the heterostructure were experimentally observed, which was in good agreement with the theoretical predications and can be interpreted based on the model of a strained staggered type-II band structure.

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64.70.Nd	Structural transitions in nanoscale materials
73.22.-f	Electronic structure of nanoscale materials and related systems
61.46.Hk	Nanocrystals
78.67.Bf	Nanocrystals, nanoparticles, and nanoclusters
81.40.Gh	Other heat and thermomechanical treatments
64.70.kg	Semiconductors

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Fabry–Pérot and whispering gallery modes enhanced luminescence from an individual hexagonal ZnO nanocolumn

N. W. Wang, Y. H. Yang, and G. W. Yang

Appl. Phys. Lett. 97, 041917 (2010); doi:10.1063/1.3474611 (3 pages)

Online Publication Date: 30 July 2010

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Strongly enhanced luminescence was observed from an individual hexagonal ZnO nanocolumn using the monochromatic cathodeluminescence equipment attached at a scanning electron microscopy. The intense luminescence emissions are focused on two regions, i.e., the profile and the center of the nanocolumn, respectively. The luminescence from the center region is attributed to the Fabry–Pérot (FP) mode enhanced emission, and the one from the hexagonal profile is attributed to the whispering gallery (WG) mode enhanced emission when the individual ZnO nanocolumn is regarded as an optical resonator. The FP enhanced emission is much stronger than the WG enhanced one.

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81.05.Dz	II-VI semiconductors
78.60.Hk	Cathodoluminescence, ionoluminescence
78.67.Bf	Nanocrystals, nanoparticles, and nanoclusters
79.70.+q	Field emission, ionization, evaporation, and desorption

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Symmetry and electronic states of Mn²⁺ in ZnS nanowires with mixed hexagonal and cubic stacking

Li Chen, Demid Kirilenko, Andre Stesmans, Xuan Sang Nguyen, Koen Binnemans, Bart Goderis, Johan Vanacken, Oleg Lebedev, Gustaaf Van Tendeloo, and Victor V. Moshchalkov

Appl. Phys. Lett. 97, 041918 (2010); doi:10.1063/1.3475017 (3 pages)

Online Publication Date: 30 July 2010

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Electron spin resonance and electronic spectroscopy techniques were used to study the symmetry and electronic structure of Mn²⁺ dopants in solvothermally synthesized ZnS nanowires. The average diameter of ∼ 5 nm leads to the observable quantum confinement effects in the photoluminescence excitation spectra. The results clearly demonstrate the three symmetry locations of Mn²⁺ incorporation. Together with the inferred Mn²⁺ center densities, these data indicate a much higher efficiency of Mn²⁺ substitution in the nanowire sample with about two times larger diameter.

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73.22.-f	Electronic structure of nanoscale materials and related systems
78.67.Uh	Nanowires
81.07.Gf	Nanowires
78.55.Et	II-VI semiconductors
81.05.Dz	II-VI semiconductors
76.30.Fc	Iron group (3d) ions and impurities (Ti-Cu)

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Orientational photoreactive effects in nematic liquid crystals on silver sulfide thin films

Akira Emoto, Kazuya Maeda, Kunihiko Tanaka, Nobuhiro Kawatsuki, and Hiroshi Ono

Appl. Phys. Lett. 97, 041919 (2010); doi:10.1063/1.3475019 (3 pages)

Online Publication Date: 30 July 2010

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We present a highly functionalized photorefractive device consisting of fullerene (C₆₀)-doped nematic liquid crystals (NLCs) on silver sulfide thin films. Either stable holograms or real-time holography is possible depending on the polarity of an external electric field. When certain direction of external field is applied to the device under irradiation by interference field, photoexcited charges are recombined with silver ions at the boundary between the silver sulfide and NLCs. The resulting interface structures reorient the NLC molecules and holographic gratings are stably memorized. When the polarity of the external field is reversed, normal photorefractive holographic gratings are formed.

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78.20.Mg	Photorefractive effects
78.15.+e	Optical properties of fluid materials, supercritical fluids and liquid crystals
61.30.Gd	Orientational order of liquid crystals; electric and magnetic field effects on order
42.70.Df	Liquid crystals
42.70.Nq	Other nonlinear optical materials; photorefractive and semiconductor materials

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Resistance switching in polycrystalline BiFeO₃ thin films

Kuibo Yin, Mi Li, Yiwei Liu, Congli He, Fei Zhuge, Bin Chen, Wei Lu, Xiaoqing Pan, and Run-Wei Li

Appl. Phys. Lett. 97, 042101 (2010); doi:10.1063/1.3467838 (3 pages)

Online Publication Date: 26 July 2010

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We report resistance switching effects in polycrystalline pure BiFeO₃ films prepared by a sol-gel method. By current-voltage and conductive atomic force microscope (c-AFM) measurements, resistance switching effects are observed in BiFeO₃ films annealed at and above 650 °C. A fresh sample can be transformed into a low-resistive state by applying a high positive voltage without forming process and then be switched to a high-resistive state by applying a negative voltage. Both c-AFM and retention results suggest that the redistribution of oxygen vacancies in grain boundaries could play a key role on the resistance switching in the polycrystalline pure BiFeO₃ films.

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73.61.Ng	Insulators
71.30.+h	Metal-insulator transitions and other electronic transitions
72.60.+g	Mixed conductivity and conductivity transitions
68.55.Ln	Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Mm	Grain and twin boundaries
61.72.jd	Vacancies
75.85.+t	Magnetoelectric effects, multiferroics

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Photoemission of Si 1s→2p_z transition in GaAs/AlGaAs quantum well for zero-dimensional states infrared detection

Thomas Antoni, Mathieu Carras, Xavier Marcadet, Borge Vinter, and Vincent Berger

Appl. Phys. Lett. 97, 042102 (2010); doi:10.1063/1.3456553 (3 pages)

Online Publication Date: 26 July 2010

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Intersubband absorption in a GaAs/AlGaAs quantum well usually happens between two two-dimensional (2D) electronic states. However for sufficiently low doping levels, the electrons can be entirely distributed on the silicon localized states. We demonstrate that depending on the temperature, absorption in such structures is dominated either by 2D quantum well states or by zero-dimensional localized Si states. We find a regime where the localized states are the main contributors to the photocurrent, this is expected to strongly impact the behavior of quantum well infrared photodetectors.

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85.60.Gz	Photodetectors (including infrared and CCD detectors)
72.40.+w	Photoconduction and photovoltaic effects
07.57.Kp	Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors

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Ni-catalyzed growth of silicon wire arrays for a Schottky diode

Sang-Won Jee, Joondong Kim, Jin-Young Jung, Han-Don Um, Syed Abdul Moiz, Bongyoung Yoo, Hyung Koun Cho, Yun Chang Park, and Jung-Ho Lee

Appl. Phys. Lett. 97, 042103 (2010); doi:10.1063/1.3467839 (3 pages)

Online Publication Date: 27 July 2010

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Vertically grown Si wire arrays were fabricated on a large scale by the Ni-catalyzed vapor-liquid-solid method. A single Si wire has a length of several tens of micrometers with a pure Si stem and a NiSi₂ tip. The NiSi₂ tip was spontaneously formed on a Si wire due to a slight lattice mismatch relative to Si. Further, this system provides a Schottky contact having a rectifying ratio of ∼ 10² with a low leakage current of about 2.88×10⁻¹⁰ A. The growth mechanism of vertical Si wires and the performance of a Schottky diode are discussed.

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85.30.Kk	Junction diodes
85.30.Hi	Surface barrier, boundary, and point contact devices
73.30.+y	Surface double layers, Schottky barriers, and work functions

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Proposal for a ferromagnetic multiwell spin oscillator

Christian Ertler, Walter Pötz, and Jaroslav Fabian

Appl. Phys. Lett. 97, 042104 (2010); doi:10.1063/1.3469999 (3 pages)

Online Publication Date: 27 July 2010

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The highly nonlinear coupling of transport and magnetic properties in a multiwell heterostructure, which comprises ferromagnetic quantum wells made of diluted magnetic semiconductors, is theoretically investigated. The interplay of resonant tunneling and carrier-mediated ferromagnetism in the magnetic wells induces very robust, self-sustained current and magnetization oscillations. Over a large window of steady bias voltages the spin polarization of the collector current is oscillating between positive and negative values, realizing a spin oscillator device.

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84.30.Ng

Oscillators, pulse generators, and function generators

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Improved photoelectrochemical responses of Si and Ti codoped α-Fe₂O₃ photoanode films

Minglong Zhang, Wenjun Luo, Zhaosheng Li, Tao Yu, and Zhigang Zou

Appl. Phys. Lett. 97, 042105 (2010); doi:10.1063/1.3470109 (3 pages)

Online Publication Date: 27 July 2010

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We studied photoelectrochemical performance of the undoped, Si-doped, Ti-doped, and codoped α-Fe₂O₃ film prepared by ultrasonic spray pyrolysis. Since the ions radius of Si⁴⁺<Fe³⁺<Ti⁴⁺, Si and Ti codoping can balance the ion radius difference between Fe³⁺ and Si⁴⁺ (or Ti⁴⁺) and increase the donor concentration. Their donor concentrations, calculated from slopes of the Mott–Schottky plots, are 9.10×10¹⁸ cm⁻³, 1.89×10²⁰ cm⁻³, 2.04×10²⁰ cm⁻³, and 7.06×10²⁰ cm⁻³, respectively. Incident photon to current efficiency of the codoped film is 34% at 365 nm and 0.6 V versus Ag/AgCl, much higher than 10%, 20%, and 22% for the undoped, Si-doped, and Ti-doped film, respectively.

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82.50.-m	Photochemistry
82.45.-h	Electrochemistry and electrophoresis
82.30.Lp	Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.47.Jk	Photoelectrochemical cells, photoelectrochromic and other hybrid electrochemical energy storage devices
68.55.A-	Nucleation and growth

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Large Seebeck coefficients and thermoelectric power factor of La-doped SrTiO₃ thin films

Bharat Jalan and Susanne Stemmer

Appl. Phys. Lett. 97, 042106 (2010); doi:10.1063/1.3471398 (3 pages)

Online Publication Date: 27 July 2010

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Seebeck coefficients and conductivity of La-doped SrTiO₃ thin films grown by molecular beam epitaxy were measured as a function of carrier concentration. At low carrier concentrations, thin films show very high Seebeck coefficients (up to 980 μV K⁻¹). The maximum thermoelectric power factor was 39 μWcm⁻¹ K⁻² at a carrier concentration of 7×10²⁰ cm⁻³. La-delta-doped superlattices were also characterized and exhibited Seebeck coefficients of ∼ 500 μV K⁻¹. The results are discussed in the context of reports of enhanced Seebeck coefficients in delta-doped SrTiO₃ superlattices.

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73.61.Ng	Insulators
73.50.Lw	Thermoelectric effects
73.63.-b	Electronic transport in nanoscale materials and structures

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Monolithic n-type conductivity on low temperature grown freestanding ultrananocrystalline diamond films

P. T. Joseph, N. H. Tai, and I. N. Lin

Appl. Phys. Lett. 97, 042107 (2010); doi:10.1063/1.3472204 (3 pages)

Online Publication Date: 28 July 2010

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We report monolithic n-type conductivity on low-temperature (<570 °C) grown ultrananocrystalline diamond (UNCD) films by Li-diffusion (about 255 nm) from LiNbO₃ substrates. Low resistivity of 1.2 Ω cm with carrier concentration of −2×10²⁰ cm⁻³ is obtained on freestanding UNCD films. The films bonded to Cu-tape show very low turn-on field of 4.2 V/μm with emission current density of above 0.3 mA/cm² at a low applied filed of 10 V/μm. The n-type conductivity of low-temperature Li-diffused UNCD films overwhelms that of the high-temperature ( ≥ 800 °C) nitrogen doped ones and will make a significant impact to diamond-based electronics.

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73.61.Cw	Elemental semiconductors
61.72.uf	Ge and Si
81.05.Cy	Elemental semiconductors
68.55.ag	Semiconductors

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p-type doping and codoping of ZnO based on nitrogen is ineffective: An ab initio clue

Ying Cui and Fabien Bruneval

Appl. Phys. Lett. 97, 042108 (2010); doi:10.1063/1.3473762 (3 pages)

Online Publication Date: 28 July 2010

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Whereas nitrogen is regarded as the best candidate to induce p-type doping in ZnO, the practical results are mainly negative either for monodoping or codoping. We perform first-principles calculations to investigate the p-type nature of boron and nitrogen codoped ZnO. The p-type character can be obtained with the proper BN₃ substitutional clusters. We propose a mechanism that explains the difficulties to synthesize p-type ZnO samples: the formation of N₂ molecules substituting oxygen, which are donors, energetically prevails over any codoping cluster. This mechanism is very general and explains the experimental instability of N-based doping and codoping versus time and temperature.

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61.72.uj

III-V and II-VI semiconductors

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Direct observation of microscopic change induced by oxygen vacancy drift in amorphous TiO₂ thin films

Hu Young Jeong, Jeong Yong Lee, and Sung-Yool Choi

Appl. Phys. Lett. 97, 042109 (2010); doi:10.1063/1.3467854 (3 pages)

Online Publication Date: 28 July 2010

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To clarify the resistive switching and failure mechanisms in Al/amorphous TiO₂/Al devices we investigate the microscopic change in amorphous titanium oxide films and interface layers after the set process according to film deposition temperatures. For low temperature (<150 °C) samples, the thickness of top interface layer decreased after the set process due to the dissociation of a top interface layer by uniform migration of oxygen vacancies. Meanwhile, for high temperature samples, crystalline TiO phases emerged in the failed state, meaning the formation of conducting paths from the local clustering of oxygen vacancies in nonhomogeneous titanium oxide film.

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68.55.-a	Thin film structure and morphology
82.30.Lp	Decomposition reactions (pyrolysis, dissociation, and fragmentation)
61.43.-j	Disordered solids
61.72.jd	Vacancies

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The effects of cap layers on electrical properties of indium nitride films

Wei Liu, Rayson Jen Ngee Tan, Chew Beng Soh, and Soo Jin Chua

Appl. Phys. Lett. 97, 042110 (2010); doi:10.1063/1.3475400 (3 pages)

Online Publication Date: 29 July 2010

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The unintentional n-type doping in the indium nitride thin films was investigated. The electron density decreases from 3.5×10¹⁹ to 9×10¹⁸ cm⁻³ and the mobility increases from 4 to 457 cm² V⁻¹ s⁻¹ when the thickness increases from 50 to 350 nm. This can be explained by assuming the film consists of a surface accumulation layer and a bulk layer. It was found that the accumulation layer can be eliminated by capping the surface with silicon nitride, GaN or zinc nitride of 2 nm each, respectively; while an AlN cap layer will cause the formation of two-dimensional electron gas at the AlN/InN interface.

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73.61.Ey	III-V semiconductors
73.50.Dn	Low-field transport and mobility; piezoresistance
61.72.uj	III-V and II-VI semiconductors
73.21.-b	Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems

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Modeling of tunnel junctions for high efficiency solar cells

John R. Hauser, Zach Carlin, and S. M. Bedair

Appl. Phys. Lett. 97, 042111 (2010); doi:10.1063/1.3469942 (3 pages)

Online Publication Date: 29 July 2010

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Ultrahigh efficiency, in the range of 40%, can be achieved in multijunction solar cells operating at high solar concentrations, larger than 100 suns. Critical to this approach are high band gap tunnel junctions that serve as electrically low loss interconnections between the cells. The purpose of this work is to theoretically model such wide band gap tunnel junctions and to explore the advantages of a staggered band line up for improving the peak tunnel current. Theoretical results are calculated for heterojunction diodes made of n⁺-InGaP/p⁺-AlGaAs over a range of doping levels. The results illustrate the advantage of a conduction band discontinuity in achieving low interconnect resistance for multijunction solar cells.

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88.40.H-	Solar cells (photovoltaics)
85.30.Kk	Junction diodes

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Tuning carrier type and density in Bi₂Se₃ by Ca-doping

Zhiyong Wang, Tao Lin, Peng Wei, Xinfei Liu, Randy Dumas, Kai Liu, and Jing Shi

Appl. Phys. Lett. 97, 042112 (2010); doi:10.1063/1.3473778 (3 pages)

Online Publication Date: 30 July 2010

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The carrier type and density in Bi₂Se₃ single crystals are systematically tuned by introducing a calcium (Ca) dopant. A carrier density of ∼ 1×10¹⁷ cm⁻³ which corresponds to ∼ 25 meV in the Fermi energy is obtained in both n- and p-type materials. Electrical transport properties show that the insulating behavior is achieved in low carrier density crystals. In addition, both the band gap and reduced effective mass of carriers are determined.

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72.20.Fr	Low-field transport and mobility; piezoresistance
61.72.up	Other materials
71.20.Ps	Other inorganic compounds
71.18.+y	Fermi surface: calculations and measurements; effective mass, g factor

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Hysteresis in the resistance of a graphene device induced by charge modulation in the substrate

J. C. Brant, J. Leon, T. C. Barbosa, E. N. D. Araujo, B. S. Archanjo, F. Plentz, and E. S. Alves

Appl. Phys. Lett. 97, 042113 (2010); doi:10.1063/1.3473815 (3 pages)

Online Publication Date: 30 July 2010

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We have fabricated graphene devices on lightly doped Si substrates and show that pronounced changes in resistance versus gate voltage, R(V_g), characteristics of these devices at 77 K are induced by the variation in the charge distribution in substrate with both gate voltage and illumination. The R(V_g) of the graphene devices in the dark shows remarkable changes as the carriers in the underlying substrate go through accumulation, depletion, and inversion regimes. We demonstrate the possibility of using a graphene device as an optical-latch.

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85.35.-p

Nanoelectronic devices

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Mobility improvement in nanowire junctionless transistors by uniaxial strain

Jean-Pierre Raskin, Jean-Pierre Colinge, Isabelle Ferain, Abhinav Kranti, Chi-Woo Lee, Nima Dehdashti Akhavan, Ran Yan, Pedram Razavi, and Ran Yu

Appl. Phys. Lett. 97, 042114 (2010); doi:10.1063/1.3474608 (3 pages)

Online Publication Date: 30 July 2010

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Improvement of current drive in n- and p-type silicon junctionless metal-oxide-semiconductor-field-effect-transistors (MOSFETs) using strain is demonstrated. Junctionless transistors have heavily doped channels with doping concentrations in excess of 10¹⁹ cm⁻³ and feature bulk conduction, as opposed to surface channel conduction. The extracted piezoresistance coefficients are in good agreement with the piezoresistive theory and the published coefficients for bulk silicon even for 10 nm thick silicon nanowires as narrow as 20 nm. These experimental results demonstrate the possibility of enhancing mobility in heavily doped silicon junctionless MOSFETs using strain technology.

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85.30.Tv	Field effect devices
81.07.Gf	Nanowires
85.40.Ry	Impurity doping, diffusion and ion implantation technology

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Flux pinning with a magnetic nanorod array

K. Kim, A. E. Ozmetin, D. G. Naugle, and I. F. Lyuksyutov

Appl. Phys. Lett. 97, 042501 (2010); doi:10.1063/1.3467452 (3 pages)

Online Publication Date: 26 July 2010

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We report measurements of the phase diagram and critical current of a PbBi superconducting film with an embedded square array of Ni nanorods. Strong hysteresis of the superconducting properties and an increase in the critical current which varies from several times to several orders of magnitude depending on the magnetic field and temperature is found.

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74.25.Wx	Vortex pinning (includes mechanisms and flux creep)
74.62.-c	Transition temperature variations, phase diagrams
74.25.Sv	Critical currents

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Co doping enhanced giant magnetocaloric effect in Mn_1−xCo_xAs films epitaxied on GaAs (001)

P. F. Xu, S. H. Nie, K. K. Meng, S. L. Wang, L. Chen, and J. H. Zhao

Appl. Phys. Lett. 97, 042502 (2010); doi:10.1063/1.3467467 (3 pages)

Online Publication Date: 26 July 2010

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A giant magnetocaloric effect was found in series of Mn_1−xCo_xAs films epitaxied on GaAs (001). The maximum magnetic entropy change caused by a magnetic field of 4 T is as large as 25 J/kg K around room temperature, which is about twice the value of pure MnAs film. The observed small thermal hysteresis is more suitable for practical application. Growing of layered Mn_1−xCo_xAs films with Co concentration changing gradually may draw layered active magnetic regenerator refrigerators closer to practical application. Our experimental result may provide the possibility for the combination of magnetocaloric effect and microelectronic circuitry.

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75.30.Sg	Magnetocaloric effect, magnetic cooling
81.15.Hi	Molecular, atomic, ion, and chemical beam epitaxy
68.60.-p	Physical properties of thin films, nonelectronic
65.40.gd	Entropy

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Probing magnetization reversal process in ferromagnetic disk by superconductor-ferromagnet junction

S. R. Bakaul, B. L. Wu, G. C. Han, and Y. H. Wu

Appl. Phys. Lett. 97, 042503 (2010); doi:10.1063/1.3463474 (3 pages)

Online Publication Date: 27 July 2010

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We use lateral superconductor (SC)-ferromagnet (FM)-SC device to probe the magnetization reversal process of micron sized FM disk. Upon decreasing external magnetic field from saturated state, a buckling pattern forms first. The onset of buckling pattern and its switching to vortex state are studied with the help of Andreev conductance of the SC-FM interface below the critical temperature (T_c) of SC and the magnetoresistances both below and above the T_c. In the latter case, micromagnetic modeling has been carried out to interpret the experimental results by taking into account the current distributions when the electrodes are at different states.

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75.60.Jk	Magnetization reversal mechanisms
75.78.Cd	Micromagnetic simulations
75.50.Bb	Fe and its alloys
75.47.Np	Metals and alloys
72.15.Gd	Galvanomagnetic and other magnetotransport effects

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Mediating distribution of magnetic Co ions by Cr-codoping in (Co,Cr): ZnO thin films

Wensheng Yan, Zhihu Sun, Qinghua Liu, Tao Yao, Qinghua Jiang, Fengchun Hu, Yuanyuan Li, Jingfu He, Yanhua Peng, and Shiqiang Wei

Appl. Phys. Lett. 97, 042504 (2010); doi:10.1063/1.3470090 (3 pages)

Online Publication Date: 27 July 2010

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The control over the distribution of magnetic ions in a host is crucial for the functionality of magnetically doped semiconductors. Herein, (Co,Cr)-codoped ZnO shows a possibility of Cr-codoping engineering in mediating the distribution of magnetic Co ions via manipulating the charge state of the Co ions. The x-ray absorption fine structure analyses at Co K-edge indicate that a secondary phase of metallic Co clusters is formed in the Zn_0.92Co_0.08O film. However, Cr-codoping suppresses the formation of Co clusters, so that all doped Co ions occupy the substitutional sites in ZnO. The ability of Cr in mediating Co distribution, as revealed by first-principles calculations, arises from the strong hybridization between the Co 3d states and the donor band induced by substitutional Cr ions, which facilitates the charge transfer from the donor band to the Co 3d states, changes the charge state of Co ions and modifies the electrostatic interactions among Co ions.

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68.55.Ln	Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.uj	III-V and II-VI semiconductors
75.70.Ak	Magnetic properties of monolayers and thin films
75.30.Hx	Magnetic impurity interactions
75.50.Pp	Magnetic semiconductors
78.70.Dm	X-ray absorption spectra

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Rotatable magnetic anisotropy of CoO/Fe/Ag(001) in ultrathin regime of the CoO layer

J. S. Park, J. Wu, E. Arenholz, M. Liberati, A. Scholl, Y. Meng, Chanyong Hwang, and Z. Q. Qiu

Appl. Phys. Lett. 97, 042505 (2010); doi:10.1063/1.3451464 (3 pages)

Online Publication Date: 28 July 2010

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CoO/Fe thin films were grown epitaxially onto vicinal Ag(001) and investigated using magneto-optic Kerr effect, x-ray magnetic circular dichroism (XMCD), and x-ray magnetic linear dichroism (XMLD) techniques. We show that the CoO film in the ultrathin regime does not induce a uniaxial magnetic anisotropy but a coercivity enhancement. This result provides a mechanism for the microscopic origin of the rotatable magnetic anisotropy. XMLD measurement further reveals that the underlying mechanism is that the CoO spins are totally rotatable in the ultrathin regime to follow the Fe magnetization.

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75.70.Ak	Magnetic properties of monolayers and thin films
78.20.Ls	Magneto-optical effects
75.30.Gw	Magnetic anisotropy
75.50.Vv	High coercivity materials

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Temperature dependence of the magnetoelectric effect in Pb(Zr_0.2Ti_0.8)O₃/La_0.8Sr_0.2MnO₃ multiferroic heterostructures

C. A. F. Vaz, Y. Segal, J. Hoffman, R. D. Grober, F. J. Walker, and C. H. Ahn

Appl. Phys. Lett. 97, 042506 (2010); doi:10.1063/1.3472259 (3 pages)

Online Publication Date: 29 July 2010

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The magnetoelectric response of Pb(Zr_0.2Ti_0.8)O₃/La_0.8Sr_0.2MnO₃ (PZT/LSMO) artificial multiferroic heterostructures as a function of temperature, electric, and magnetic field, shows that the largest magnetoelectric coupling is attained at temperatures near the magnetic critical point of LSMO, at ∼ 180 K (−13.5 Oe cm kV⁻¹). The magnetoelectric coupling displays a strong temperature dependence, changing sign at 150 K and saturating to positive values below ∼ 100 K (+6 Oe cm kV⁻¹). The magnetoelectric curve switches hysteretically between two states in response to the ferroelectric switching. The peak in the magnetoelectric response coincides with the observation of on/off switching of magnetism in LSMO near the critical region, where the sensitivity to electric field is largest, making it a promising approach for device applications.

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75.85.+t	Magnetoelectric effects, multiferroics
77.84.Bw	Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.80.Fm	Switching phenomena

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Magnetization dynamics of a CrO₂ grain studied by micro-Hall magnetometry

P. Das, F. Porrati, S. Wirth, A. Bajpai, M. Huth, Y. Ohno, H. Ohno, and J. Müller

Appl. Phys. Lett. 97, 042507 (2010); doi:10.1063/1.3467870 (3 pages)

Online Publication Date: 29 July 2010

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Micro-Hall magnetometry is employed to study the magnetization dynamics of a single, micron-size CrO₂ grain. With this technique, we track the motion of a single domain wall, which allows us to probe the distribution of imperfections throughout the material. An external magnetic field along the grain’s easy magnetization direction induces magnetization reversal, giving rise to a series of sharp jumps in magnetization. Supported by micromagnetic simulations, we identify the transition to a state with a single cross-tie domain wall, where pinning/depinning of the wall results in stochastic Barkhausen jumps.

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75.78.Fg	Dynamics of domain structures
75.78.Cd	Micromagnetic simulations
75.60.Jk	Magnetization reversal mechanisms
75.60.Ch	Domain walls and domain structure
75.60.Ej	Magnetization curves, hysteresis, Barkhausen and related effects
72.20.My	Galvanomagnetic and other magnetotransport effects

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Observation of spin-glass behavior in antiperovskite compound SnCFe₃

B. S. Wang, P. Tong, Y. P. Sun, X. B. Zhu, Z. R. Yang, W. H. Song, and J. M. Dai

Appl. Phys. Lett. 97, 042508 (2010); doi:10.1063/1.3469924 (3 pages)

Online Publication Date: 29 July 2010

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We investigated the physical properties of antiperovskite compound SnCFe₃ by comprehensive magnetic measurements. The strong irreversibility is clearly observed from zero-field-cooled and field-cooled magnetizations. The peaks of ac susceptibility display strong dependences on the frequency and magnetic field. Both the magnetic relaxation effects and related analysis indicate a typical spin-glass (SG) behavior in SnCFe₃. The corresponding characteristic parameters are obtained: the freezing temperature T₀ = 20.3 K, the dynamical exponent zν = 9.441, and the flipping time τ₀ = 2.42×10⁻¹¹ s. Furthermore, the Sn deficiency affects significantly the SG behavior and results in a sharp decrease in T₀.

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75.30.Cr	Saturation moments and magnetic susceptibilities
75.60.Ej	Magnetization curves, hysteresis, Barkhausen and related effects
75.50.Lk	Spin glasses and other random magnets
76.50.+g	Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance

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In situ growth of superconducting NdFeAs(O,F) thin films by molecular beam epitaxy

T. Kawaguchi, H. Uemura, T. Ohno, M. Tabuchi, T. Ujihara, K. Takenaka, Y. Takeda, and H. Ikuta

Appl. Phys. Lett. 97, 042509 (2010); doi:10.1063/1.3464171 (3 pages)

Online Publication Date: 30 July 2010

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Superconducting NdFeAs(O,F) thin films were grown on GaAs substrates by molecular beam epitaxy. Films grown with a sufficiently long growth time exhibited a clear superconducting transition with an onset temperature up to 48 K and zero resistance temperature up to 42 K without the need of an ex situ annealing process. Electron probe microanalysis and Hall coefficient measurements indicated that the superconducting films are doped with fluorine, and depth-profile analysis by Auger electron spectroscopy revealed the formation of a NdOF layer near the surface, which is probably connected with the fluorine doping.

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74.78.-w	Superconducting films and low-dimensional structures
68.55.A-	Nucleation and growth
81.15.Hi	Molecular, atomic, ion, and chemical beam epitaxy
74.25.F-	Transport properties
61.72.Cc	Kinetics of defect formation and annealing
61.72.up	Other materials

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Magnetic behavior of SnO₂ nanosheets at room temperature

Cen Wang, Mingyuan Ge, and J. Z. Jiang

Appl. Phys. Lett. 97, 042510 (2010); doi:10.1063/1.3473764 (3 pages)

Online Publication Date: 30 July 2010

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Magnetic behavior of SnO₂ nanosheets has been investigated both experimentally and theoretically. It is found that SnO₂ nanosheets with a thickness of 1.5–13 nm show ferromagnetic state at ambient temperature. First-principles calculations reveal that oxygen atoms at O-terminated surface in SnO₂ sheet are spin polarized, resulting in magnetic moment of 1.6 μ_B for pure SnO₂ nanosheet while Sn atoms just beneath the O-terminated surface contribute only −0.082 μ_B. This study will stimulate more investigations for understanding the origin of ferromagnetic TM-doped SnO₂ (TM = 3d transition metals) in particular case and TM-doped semiconductor oxides in general at ambient temperature and trigger further studies for magnetic behavior of low-dimensional systems which are nonmagnetic in bulk state.

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75.75.-c	Magnetic properties of nanostructures
75.50.Dd	Nonmetallic ferromagnetic materials
75.50.Tt	Fine-particle systems; nanocrystalline materials
71.15.-m	Methods of electronic structure calculations
72.25.-b	Spin polarized transport
75.30.Cr	Saturation moments and magnetic susceptibilities

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Matching effect and dynamic phases of vortex matter in Bi₂Sr₂CaCu₂O₈ nanoribbon with a periodic array of holes

S. Avci, Z. L. Xiao, J. Hua, A. Imre, R. Divan, J. Pearson, U. Welp, W. K. Kwok, and G. W. Crabtree

Appl. Phys. Lett. 97, 042511 (2010); doi:10.1063/1.3473783 (3 pages)

Online Publication Date: 30 July 2010

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We report investigations on the dynamics of vortex matter with periodic pinning in crystalline Bi₂Sr₂CaCu₂O₈ nanoribbons containing an array of nanoscale holes. We found that the matching effect is enhanced near the melting field and persists to higher fields beyond the melting line. We attribute this enhancement to the existence of a soft-solid phase and a mixture of solid-liquid phases near the melting line, enabling the vortices to pin more effectively. We observed distinct regions in the voltage-current curves attributed to transitions of various dynamic phases which also account for the driving current dependent appearance of the matching effect.

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74.25.Wx	Vortex pinning (includes mechanisms and flux creep)
74.72.-h	Cuprate superconductors
74.78.Na	Mesoscopic and nanoscale systems

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Magnetization reversal and anisotropic magnetoresistance behavior in bicomponent antidot nanostructures

D. Tripathy, P. Vavassori, J. M. Porro, A. O. Adeyeye, and N. Singh

Appl. Phys. Lett. 97, 042512 (2010); doi:10.1063/1.3474802 (3 pages)

Online Publication Date: 30 July 2010

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We report on the magnetic properties and anisotropic magnetoresistance behavior of engineered bicomponent array of “holes” with alternating diameters embedded in a continuous permalloy film. We observed that the magnetization reversal is significantly altered when compared to a homogeneous array of antidots, and exhibits strong dependence on the permalloy thickness. Our results demonstrate that the use of two antidot sublattices greatly enhances the parameters available for engineering the behavior of antidot nanostructures.

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75.75.-c	Magnetic properties of nanostructures
75.60.Jk	Magnetization reversal mechanisms
75.47.Np	Metals and alloys
72.15.Gd	Galvanomagnetic and other magnetotransport effects
75.50.Tt	Fine-particle systems; nanocrystalline materials

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Ferroelectric and antiferroelectric properties of AgNbO₃ films fabricated on (001), (110), and (111)SrTiO₃ substrates by pulsed laser deposition

Hiroyuki Sakurai, Seiji Yamazoe, and Takahiro Wada

Appl. Phys. Lett. 97, 042901 (2010); doi:10.1063/1.3467137 (3 pages)

Online Publication Date: 26 July 2010

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We fabricated 001, 110, and 111 oriented AgNbO₃ (AN) films on (001)SrTiO₃ (STO), (110)STO, and (111)STO substrates by pulsed laser deposition, respectively. Scanning electron microscope images showed that the surface textures of the AN films on the (001), (110), and (111)STO substrates took the form of a lattice-shaped pattern, a striped pattern, and trigonal pyramid-like structures, respectively. The AN film on the (001)STO had the largest relative dielectric constant of 748 in all films. The P-E hysteresis showed that the AN film on (001)STO exhibited antiferroelectricity and the other AN films deposited on (110) and (111)STO showed a ferroelectric property.

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77.55.fj	Niobate- and tantalate-based films
68.55.aj	Insulators
77.22.Ch	Permittivity (dielectric function)
68.55.jm	Texture
81.15.Fg	Pulsed laser ablation deposition
77.80.-e	Ferroelectricity and antiferroelectricity

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Web-like domain structure formation in barium titanate single crystals

B. J. Rodriguez, L. M. Eng, and A. Gruverman

Appl. Phys. Lett. 97, 042902 (2010); doi:10.1063/1.3467871 (3 pages)

Online Publication Date: 28 July 2010

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The evolution of ferroelectric domain structure in Remeika-grown BaTiO₃ crystals under an inhomogeneous electric field has been investigated using piezoresponse force microscopy. The ac imaging bias was found to affect the metastable polarization state and lead to the formation of a web-like domain structure. It is suggested that this behavior is due to the existence of subsurface domains arising in single crystals with a layered structure.

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77.80.Dj

Domain structure; hysteresis

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The influences of surface treatment and gas annealing conditions on the inversion behaviors of the atomic-layer-deposition Al₂O₃/n-In_0.53Ga_0.47As metal-oxide-semiconductor capacitor

H. D. Trinh, E. Y. Chang, P. W. Wu, Y. Y. Wong, C. T. Chang, Y. F. Hsieh, C. C. Yu, H. Q. Nguyen, Y. C. Lin, K. L. Lin, and M. K. Hudait

Appl. Phys. Lett. 97, 042903 (2010); doi:10.1063/1.3467813 (3 pages)

Online Publication Date: 29 July 2010

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The inversion behaviors of atomic-layer-deposition Al₂O₃/n-In_0.53Ga_0.47As metal-oxide-semiconductor capacitors are studied by various surface treatments and postdeposition annealing using different gases. By using the combination of wet sulfide and dry trimethyl aluminum surface treatment along with pure hydrogen annealing, a strong inversion capacitance-voltage (C-V) response is observed, indicating a remarkable reduction in interface trap state density (D_it) at lower half-part of In_0.53Ga_0.47As band gap. This low D_it was confirmed by the temperature independent C-V stretch-out and horizontal C-V curves. The x-ray photoelectron spectroscopy spectra further confirm the effectiveness of hydrogen annealing on the reduction of native oxides.

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84.32.Tt

Capacitors

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Scaling and universality of ac conductivity and dielectric response in disordered materials under pressure

I. Sakellis, A. N. Papathanassiou, and J. Grammatikakis

Appl. Phys. Lett. 97, 042904 (2010); doi:10.1063/1.3466904 (3 pages)

Online Publication Date: 29 July 2010

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Scaling of physical quantities describing the ac response is a common feature of disordered matter, while in some cases, empirical functions have been proposed. The exact forms of analytic functions that scale complex conductivity and permittivity are derived in this work by exploring the effect of pressure at various temperatures on the ac conductivity of polypyrrole. The scaling behavior is visualized within the frame of percolation and self-similarity.

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72.80.Le	Polymers; organic compounds (including organic semiconductors)
81.05.Lg	Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
77.22.Ch	Permittivity (dielectric function)

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Space charges and size effects in semiconducting ferroelectric BaTiO₃/SrTiO₃ superlattices

Y. Y. Liu and J. Y. Li

Appl. Phys. Lett. 97, 042905 (2010); doi:10.1063/1.3473821 (3 pages)

Online Publication Date: 30 July 2010

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A continuum theory is developed to investigate the effects of space charges and size on ferroelectric superlattices, treating the ferroelectric perovskites as wide band-gap semiconductors instead of insulators, making it possible to couple the distribution of polarization and space charges through Maxwell’s equation. It is found that the space charges tend to accumulate near the superlattice interface, resulting in large electric field near the interface, and thus enhanced polarization and asymmetric hysteresis loop. It also eliminates the uniform polarization that would exist in the superlattice in the absence of space charges. The effect of superlattice thickness is also investigated, and good agreements with experiments are observed.

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77.22.Jp	Dielectric breakdown and space-charge effects
72.30.+q	High-frequency effects; plasma effects
77.22.Ej	Polarization and depolarization
73.63.-b	Electronic transport in nanoscale materials and structures
77.80.Dj	Domain structure; hysteresis
77.84.Cg	PZT ceramics and other titanates

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In situ investigation of self-induced GaN nanowire nucleation on Si

C. Chèze, L. Geelhaar, A. Trampert, and H. Riechert

Appl. Phys. Lett. 97, 043101 (2010); doi:10.1063/1.3464956 (3 pages)

Online Publication Date: 26 July 2010

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The nucleation of GaN nanowires grown by molecular beam epitaxy on bare Si(111) and Si(001) has been investigated in situ by reflection high-energy electron diffraction (RHEED) and line-of-sight quadrupole mass spectrometry. On either substrate, the incorporation rate of Ga increases in two steps to steady-state conditions, and the RHEED transmission pattern of GaN appears only in the second stage. Ex situ transmission electron microscopy on samples from both stages grown on Si(001) revealed that the nanowire nucleation is strongly affected by the simultaneous nitridation of the Si substrate.

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81.07.Gf	Nanowires
81.05.Ea	III-V semiconductors

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The effect of annealing on the photoconductivity of carbon nanofiber/TiO₂ core-shell nanowires for use in dye-sensitized solar cells

Caitlin Rochford, Zhuang-Zhi Li, Javier Baca, Jianwei Liu, Jun Li, and Judy Wu

Appl. Phys. Lett. 97, 043102 (2010); doi:10.1063/1.3464965 (3 pages)

Online Publication Date: 26 July 2010

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Electrical transport properties and photoresponse of individual TiO₂-coated carbon nanofibers were studied in an attempt to elucidate the limiting factors of core-shell nanowire-based dye-sensitized solar cells (DSSC). The role of the semiconductor shell microstructure was investigated by comparing as grown and thermally annealed samples. Steady state I-V and transient photoconductivity measurements suggest that improving the microstructure leads to reduced resistivity and contact resistance, a decrease in charge traps, improved surface stoichiometry for dye adsorption, and reduced absorption of visible light by the semiconductor, all of which may improve nanowire-based DSSC performance.

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73.61.Le	Other inorganic semiconductors
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)
78.56.-a	Photoconduction and photovoltaic effects
88.40.H-	Solar cells (photovoltaics)
61.66.Bi	Elemental solids
61.66.Dk	Alloys
68.43.Mn	Adsorption kinetics
61.72.Cc	Kinetics of defect formation and annealing
72.40.+w	Photoconduction and photovoltaic effects
61.46.Df	Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
73.40.Cg	Contact resistance, contact potential

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Sum-frequency generation in doubly resonant GaP photonic crystal nanocavities

Kelley Rivoire, Ziliang Lin, Fariba Hatami, and Jelena Vučković

Appl. Phys. Lett. 97, 043103 (2010); doi:10.1063/1.3469936 (3 pages)

Online Publication Date: 26 July 2010

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We demonstrate and characterize continuous wave χ⁽²⁾ sum-frequency generation in gallium phosphide photonic crystal nanocavities. We use two confined modes of the nanocavity in the wavelength range 1500–1600 nm to enhance conversion efficiency. Our results show that these nanocavities can serve as integrated light sources across a range of wavelengths, and are promising for on-chip upconversion of weak intensity telecommunication wavelengths signals to visible wavelengths.

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42.65.Ky	Frequency conversion; harmonic generation, including higher-order harmonic generation
42.65.An	Optical susceptibility, hyperpolarizability
42.70.Qs	Photonic bandgap materials

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Gapless insulator and a band gap scaling law in semihydrogenated graphene

A. R. Wright, T. E. O’Brien, D. Beaven, and C. Zhang

Appl. Phys. Lett. 97, 043104 (2010); doi:10.1063/1.3469941 (3 pages)

Online Publication Date: 26 July 2010

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We demonstrate two unusual electronic properties of semihydrogenated graphene with variable sized A- or B-hydrogenated domains within the tight-binding formalism as follows: (i) a universal band gap scaling law which states that the band gap depends linearly upon the ratio of the number of A- to B-hydrogenated atoms, N_A/N_B, reaching zero gap at N_A = N_B, but independent of the domain size, and (ii) an insulating state with zero band gap at N_A = N_B, a rare phenomenon in nature. We confirm this gapless insulator state by the zero optical conductance at low frequencies.

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73.22.Pr	Electronic structure of graphene
78.20.Ci	Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.67.Wj	Optical properties of graphene
81.05.ue	Graphene

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Coupling of single InGaAs quantum dots to the plasmon resonance of a metal nanocrystal

A. Urbańczyk, G. J. Hamhuis, and R. Nötzel

Appl. Phys. Lett. 97, 043105 (2010); doi:10.1063/1.3467853 (3 pages)

Online Publication Date: 27 July 2010

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The authors report the coupling of single InGaAs quantum dots (QDs) to the surface plasmon resonance of a metal nanocrystal. Clear enhancement of the photoluminescence (PL) in the spectral region of the surface plasmon resonance is observed which splits up into distinct emission lines from single QDs in micro-PL. The hybrid metal-semiconductor structure is grown by molecular beam epitaxy on GaAs (100) utilizing the concept of self-organized anisotropic strain engineering for realizing ordered arrays with nanometer-scale precise positioning of the metal nanocrystals with respect to the QDs.

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73.21.La	Quantum dots
73.20.Mf	Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
81.15.Hi	Molecular, atomic, ion, and chemical beam epitaxy

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High efficiency mesoporous titanium oxide PbS quantum dot solar cells at low temperature

Tong Ju, Rebekah L. Graham, Guangmei Zhai, Yvonne W. Rodriguez, Alison J. Breeze, Lily Yang, Glenn B. Alers, and Sue A. Carter

Appl. Phys. Lett. 97, 043106 (2010); doi:10.1063/1.3459146 (3 pages)

Online Publication Date: 28 July 2010

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Efficient charge transport is demonstrated in TiO₂/PbS quantum dot solar cells where the PbS absorber ( ∼ 1.1 eV band gap) is deposited by dip coating and ethanedithiol ligand exchange, with power efficiencies above 3% at AM1.5. An increase in power efficiency occurs as the device temperature is lowered to 170 K, with a open-circuit voltage of 0.66 V, short-circuit current density of 28.6 mA/cm² and fill factor of 42.4%. This remarkable temperature dependence is due to a large increase in charge transport between the PbS quantum dots with decreasing temperature.

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88.40.H-	Solar cells (photovoltaics)
85.35.Ds	Quantum interference devices

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Characteristics of high-k Al₂O₃ dielectric using ozone-based atomic layer deposition for dual-gated graphene devices

B. Lee, G. Mordi, M. J. Kim, Y. J. Chabal, E. M. Vogel, R. M. Wallace, K. J. Cho, L. Colombo, and J. Kim

Appl. Phys. Lett. 97, 043107 (2010); doi:10.1063/1.3467454 (3 pages)

Online Publication Date: 28 July 2010

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We present characteristics of dual-gated graphene devices with an Al₂O₃ gate dielectric formed by an O₃-based atomic-layer-deposition process. Raman spectra reveal that a O₃ process at 25 °C on single-layered graphene introduces the least amount defects, while a substantial number of defects appear at 200 °C. This graphene device with O₃-based Al₂O₃ dielectric demonstrates a heterojunction within the graphene sheet when applying V_TG and V_BG and possesses good dielectric properties with minimal chemical doping, including a high dielectric constant ∼ 8, low hysteresis width ∼ 0.2 V, and low leakage current and a carrier mobility of 5000 cm²/V s 25 °C in ambient.

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77.55.D-	High-permittivity gate dielectric films
78.30.Na	Fullerenes and related materials
77.80.Dj	Domain structure; hysteresis
85.30.Tv	Field effect devices
77.22.Ch	Permittivity (dielectric function)
77.84.Bw	Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.

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Engineering the band gap of SiC nanotubes with a transverse electric field

G. Alfieri and T. Kimoto

Appl. Phys. Lett. 97, 043108 (2010); doi:10.1063/1.3469944 (3 pages)

Online Publication Date: 28 July 2010

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The effects of a transverse electric field, of intensities ranging from 0 to 0.20 V/Å, on the band gap width of SiC nanotubes (SiCNTs) are investigated by means of density functional theory. A decrease in the band gap width, as a function of the electric field, is observed and the mechanism for such reduction, as well as its dependence on the nanotube diameter, is analyzed. An empirical model to describe the field dependence of the band gap is also proposed.

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73.22.-f	Electronic structure of nanoscale materials and related systems
81.07.De	Nanotubes
71.20.Nr	Semiconductor compounds
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections

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Soft-solution route to ZnO nanowall array with low threshold power density

Eue-Soon Jang, Xiaoyuan Chen, Jung-Hee Won, Jae-Hun Chung, Du-Jeon Jang, Young-Woon Kim, and Jin-Ho Choy

Appl. Phys. Lett. 97, 043109 (2010); doi:10.1063/1.3466910 (3 pages)

Online Publication Date: 29 July 2010

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ZnO nanowall array (ZNWA) has been directionally grown on the buffer layer of ZnO nanoparticles dip-coated on Si-wafer under a soft solution process. Nanowalls on substrate are in most suitable shape and orientation not only as an optical trap but also as an optical waveguide due to their unique growth habit, math

⪢V_[0001] ≈ math

. Consequently, the stimulated emission at 384 nm through nanowalls is generated by the threshold power density of only 25 kW/cm². Such UV lasing properties are superior to those of previously reported ZnO nanorod arrays. Moreover, there is no green (defect) emission due to the mild procedure to synthesize ZNWA.

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81.16.Be	Chemical synthesis methods
78.67.-n	Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.45.+h	Stimulated emission

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The temperature dependence of the impedance of alkanethiol self-assembled monolayers

Brigid O’Brien, Harmonie Sahalov, and Peter C. Searson

Appl. Phys. Lett. 97, 043110 (2010); doi:10.1063/1.3469911 (3 pages)

Online Publication Date: 29 July 2010

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The impedance of alkanethiol self-assembled monolayers at the solid/liquid interface exhibits unusual behavior. The capacitance C_m is independent of applied potential, however, the resistance R_m decreases exponentially at potentials negative to the potential of zero charge (PZC). At potentials positive to the PZC, the monolayer resistance is very large and independent of temperature, whereas at potentials negative to the PZC, the monolayer resistance shows Arrhenius behavior with an activation energy of 0.20 eV. We provide evidence that this effect is related to field-induced torque on the alkanethiol molecules that leads to increasing ion penetration into the monolayer.

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73.40.-c	Electronic transport in interface structures
68.47.Pe	Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces

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Optimizing transistor performance of percolating carbon nanotube networks

V. K. Sangwan, A. Behnam, V. W. Ballarotto, M. S. Fuhrer, Ant Ural, and E. D. Williams

Appl. Phys. Lett. 97, 043111 (2010); doi:10.1063/1.3469930 (3 pages)

Online Publication Date: 29 July 2010

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In percolating networks of mixed metallic and semiconducting carbon nanotubes (CNTs), there is a tradeoff between high on-current (dense networks) and high on/off ratio (sparse networks). Experiments on transistors and Monte Carlo simulations were performed to determine the scaling behavior of device resistivity as a function of channel length (L) for CNT density (p) between 0.04 and 1.29 CNTs/μm² in the on- and off-states (nanotube root mean square length of 5 μm). Optimized devices with field-effect mobility up to 50 cm²/V s at on/off ratio >10³ were obtained at channel width W = 50 μm and L>70 μm for p = 0.54–0.81 CNTs/μm².

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85.30.Tv	Field effect devices
81.07.De	Nanotubes
85.35.Kt	Nanotube devices

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Miniband formation in InGaAs quantum dot superlattice

Takeyoshi Sugaya, Takeru Amano, Masahiko Mori, and Shigeru Niki

Appl. Phys. Lett. 97, 043112 (2010); doi:10.1063/1.3470108 (3 pages)

Online Publication Date: 29 July 2010

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We report the formation of a miniband in a quantum dot (QD) superlattice with 20-stack InGaAs QD layers, which we confirmed by the excitation power dependence in photoluminescence (PL) measurements. The PL peak of a QD superlattice shifts to a shorter wavelength as the excitation power is increased. The blueshifted energy of the PL peak is 8 meV for a QD superlattice with an interdot spacing of 3.5 nm, whereas the blueshift is not observed for a multistacked QD structure with an interdot spacing of 15 nm. The blueshifted energy becomes larger as the interdot spacing is reduced.

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78.55.Cr	III-V semiconductors
78.66.Fd	III-V semiconductors
78.47.da	Excited states
68.65.Hb	Quantum dots (patterned in quantum wells)
68.65.Cd	Superlattices

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Scanning Raman spectroscopy of graphene antidot lattices: Evidence for systematic p-type doping

S. Heydrich, M. Hirmer, C. Preis, T. Korn, J. Eroms, D. Weiss, and C. Schüller

Appl. Phys. Lett. 97, 043113 (2010); doi:10.1063/1.3474613 (3 pages)

Online Publication Date: 30 July 2010

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We have investigated antidot lattices, which were prepared on exfoliated graphene single layers via electron-beam lithography and ion etching, by means of scanning Raman spectroscopy. The peak positions, peak widths, and intensities of the characteristic phonon modes of the carbon lattice have been studied systematically in a series of samples. In the patterned samples, we found a systematic stiffening of the G band phonon mode, accompanied by a line narrowing, while the 2D two-phonon mode energies are found to be linearly correlated with the G mode energies. We interpret this as evidence for p-type doping of the nanostructured graphene.

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63.22.Rc	Phonons in graphene
81.05.ue	Graphene
78.30.Am	Elemental semiconductors and insulators

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Cobalt dependence of the magneto-optical response in magnetoplasmonic nanodisks

J. B. González-Díaz, B. Sepúlveda, A. García-Martín, and G. Armelles

Appl. Phys. Lett. 97, 043114 (2010); doi:10.1063/1.3474617 (3 pages)

Online Publication Date: 30 July 2010

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In this work, we study the plasmon-mediated enhancement of the magneto-optical (MO) activity in the case of Au/Co/Au nanodisks as a function of the Co content. We show that both the spectral shape and amplitude of the complex polar Kerr rotation are modified by the relative amount of gold (diamagnetic) and cobalt (ferromagnetic). We demonstrate that such shape modification is dictated by the predominance of the absorptive or dispersive part of the MO constant in each material, which is intimately related to the origin of their MO activity.

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78.20.Ls	Magneto-optical effects
78.66.Bz	Metals and metallic alloys
73.22.Lp	Collective excitations

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Intrinsic half-metallic BN–C nanotubes

Bing Huang, Chen Si, Hoonkyung Lee, Lu Zhao, Jian Wu, Bing-Lin Gu, and Wenhui Duan

Appl. Phys. Lett. 97, 043115 (2010); doi:10.1063/1.3474620 (3 pages)

Online Publication Date: 30 July 2010

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Using spin-polarized density functional theory calculations, we demonstrate that hybrid BN–C nanotubes (BN-CNTs) have diverse electronic and magnetic properties depending on their percentage of carbon and BN components. Typically, a BN-CNT is converted from a nonmagnetic semiconductor to a spin-polarized metal and then to a nonmagnetic semiconductor by increasing the ratio of BN component. The intrinsic half-metallicity could be achieved when the percentage of carbon component in the tube is within a certain ratio, and is insensitive to the tube curvature. Our findings suggest that BN-CNTs may offer unique opportunities for developing nanoscale spintronic materials.

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73.21.-b	Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
71.15.Mb	Density functional theory, local density approximation, gradient and other corrections
73.63.Fg	Nanotubes
75.25.-j	Spin arrangements in magnetically ordered materials (including neutron and spin-polarized electron studies, synchrotron-source x-ray scattering, etc.)
75.30.Cr	Saturation moments and magnetic susceptibilities
75.75.-c	Magnetic properties of nanostructures

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Memory bistable mechanisms of organic memory devices

Ching-Ting Lee, Li-Zhen Yu, and Hung-Chun Chen

Appl. Phys. Lett. 97, 043301 (2010); doi:10.1063/1.3467050 (3 pages)

Online Publication Date: 26 July 2010

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To investigate the memory bistable mechanisms of organic memory devices, the structure of [top Au anode/9,10-di(2-naphthyl)anthracene (ADN) active layer/bottom Au cathode] was deposited using a thermal deposition system. The Au atoms migrated into the ADN active layer was observed from the secondary ion mass spectrometry. The density of 9.6×10¹⁶ cm⁻³ and energy level of 0.553 eV of the induced trapping centers caused by the migrated Au atoms in the ADN active layer were calculated. The induced trapping centers did not influence the carrier injection barrier height between Au and ADN active layer. Therefore, the memory bistable behaviors of the organic memory devices were attributed to the induced trapping centers. The energy diagram was established to verify the mechanisms.

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85.30.-z

Semiconductor devices

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Doping kinetics of organic semiconductors investigated by field-effect transistors

F. Maddalena, E. J. Meijer, K. Asadi, D. M. de Leeuw, and P. W. M. Blom

Appl. Phys. Lett. 97, 043302 (2010); doi:10.1063/1.3466903 (3 pages)

Online Publication Date: 27 July 2010

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The kinetics of acid doping of the semiconductor regioregular poly-3-hexylthiophene with vaporized chlorosilane have been investigated using field-effect transistors. The dopant density has been derived as a function of temperature and exposure time from the shift in the pinch-off voltage, being the gate bias where current starts to flow. The doping kinetics are perfectly described by empirical stretched exponential time dependence with a saturation dopant density of 1±0.5×10²⁶ m⁻³ and a thermally activated relaxation time. We show that a similar relationship holds for previously reported kinetics of poly-thienylene-vinylene doped with molecular oxygen.

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61.72.U-	Doping and impurity implantation
85.30.Tv	Field effect devices
81.05.Fb	Organic semiconductors

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Conjugated polyelectrolytes for organic light emitting transistors

Jung Hwa Seo, Ebinazar B. Namdas, Andrea Gutacker, Alan J. Heeger, and Guillermo C. Bazan

Appl. Phys. Lett. 97, 043303 (2010); doi:10.1063/1.3467856 (3 pages)

Online Publication Date: 28 July 2010

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We report on solution-processed light emitting field-effect transistors (LEFETs) that incorporate symmetric high work function (WF) source and drain metal electrodes. A key architectural design is the incorporation of a conjugated polyelectrolyte (CPE) electron injection layer atop the emissive layer. The device structure also comprises a hole-transporting layer underneath the emissive layer. Both holes and electrons are injected from stable, high WF metal though the CPE layer leading to electroluminescence near the electron-injecting electrode. With the benefits of the simplicity in device fabrication, the LEFETs incorporating CPEs are interesting structures for integrated organic optoelectronic devices.

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85.60.Jb	Light-emitting devices
85.30.Tv	Field effect devices

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Interfacial dipole dynamics of light-emitting diodes incorporating a poly(amidoamine) dendrimer monolayer

Gustaf Winroth, Oliver Fenwick, Mark A. Scott, Daniel Yip, Stefan Howorka, and Franco Cacialli

Appl. Phys. Lett. 97, 043304 (2010); doi:10.1063/1.3459964 (3 pages)

Online Publication Date: 30 July 2010

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We investigate the effects induced by a nonconjugated self-assembled monolayer of poly(amidoamine) (PAMAM) on the work function of thermally evaporated Al-electrodes. We compare results from Kelvin probe measurement of the electrodes work function with electroabsorption characterization of the built-in voltage of polymer light-emitting diodes incorporating them. Interestingly, we find that the PAMAM monolayer decreases the work function by 0.4 eV in a Kelvin probe measurement but that the dipole induced by the PAMAM monolayer is altered by biasing, and that it exhibits a hysteretic behavior in a cyclic voltage scan.

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73.30.+y	Surface double layers, Schottky barriers, and work functions
68.47.Pe	Langmuir-Blodgett films on solids; polymers on surfaces; biological molecules on surfaces
85.60.Jb	Light-emitting devices

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Carrier hopping in disordered semiconducting polymers: How accurate is the Miller–Abrahams model?

Nenad Vukmirović and Lin-Wang Wang

Appl. Phys. Lett. 97, 043305 (2010); doi:10.1063/1.3474618 (3 pages)

Online Publication Date: 30 July 2010

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We performed direct calculations of carrier hopping rates in strongly disordered conjugated polymers based on the atomic structure of the system, the corresponding electronic states and their coupling to all phonon modes. We found that the dependence of hopping rates on distance and the dependence of the mobility on temperature are significantly different than the ones stemming from the simple Miller–Abrahams model, regardless of the choice of the parameters in the model. A model that satisfactorily describes the hopping rates in the system and avoids the explicit calculation of electron–phonon coupling constants was then proposed and verified. Our results indicate that, in addition to electronic density of states, the phonon density of states and the spatial overlap of the wave functions are the quantities necessary to properly describe carrier hopping in disordered conjugated polymers.

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71.20.Rv	Polymers and organic compounds
72.15.Nj	Collective modes (e.g., in one-dimensional conductors)
72.20.Jv	Charge carriers: generation, recombination, lifetime, and trapping
71.38.-k	Polarons and electron-phonon interactions
73.22.-f	Electronic structure of nanoscale materials and related systems

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Photoinduced charge transfer and photovoltaic energy conversion in self-assembled N,N′-dioctyl-3,4,9,10-perylenedicarboximide nanoribbons

S. Karak, S. K. Ray, and A. Dhar

Appl. Phys. Lett. 97, 043306 (2010); doi:10.1063/1.3475024 (3 pages)

Online Publication Date: 30 July 2010

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One-dimensional nanostructures like nanowires, nanoribbons, or nanobelts are attractive building blocks for optoelectronic device applications. Here we report the self-assembly of several micrometer long n-type organic nanoribbons with 100–200 nm of width from N,N′-dioctyl-3,4,9,10-perylenedicarboximide molecules. Significant change in absorption spectra above 550 nm indicates strong π-π stacking along the long axis of nanoribbons. Photovoltaic properties were investigated by mixing them with two well studied donor materials. Strong photoluminescence (PL) quenching and significant decrease in PL life time surmise efficient photoinduced charge transfer and exciton dissociation between nanostructures and donor materials, which leads to a power conversion efficiency of about 1%.

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72.40.+w	Photoconduction and photovoltaic effects
78.55.Kz	Solid organic materials
71.35.-y	Excitons and related phenomena
81.16.Dn	Self-assembly

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High current densities in a highly photoluminescent organic single-crystal light-emitting transistor

Kosuke Sawabe, Taishi Takenobu, Satria Zulkarnaen Bisri, Takeshi Yamao, Shu Hotta, and Yoshihiro Iwasa

Appl. Phys. Lett. 97, 043307 (2010); doi:10.1063/1.3466915 (3 pages)

Online Publication Date: 30 July 2010

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We report the improvement of electron transport in 5,5″-bis(biphenylyl)-2,2′:5′,2″-terthiophene (BP3T) single crystals on a field-effect transistor configuration by systematically investigating the effects of device aging under pure nitrogen and optimizing the organic dielectric layer-fabrication process. We reduced the effect of electron traps and achieved extremely high current densities up to 10 kA/cm², which is one or two orders of magnitude greater than the current densities achieved in previous devices.

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85.30.Tv	Field effect devices
85.60.Jb	Light-emitting devices

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High capacity and excellent cycling stability of branched cobalt oxide nanowires as Li-insertion materials

Cheng Chao Li, Xiao Ming Yin, Li Bao Chen, Qiu Hong Li, and Tai Hong Wang

Appl. Phys. Lett. 97, 043501 (2010); doi:10.1063/1.3469997 (3 pages)

Online Publication Date: 27 July 2010

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In this paper, the authors report the high-performance lithium storage properties of branched cobalt oxide nanowires synthesized through a topotactic conversion route. Compared with other nanosized materials, such branched nanostructures not only inherit the advantages of high surface-to-volume ratio and good dispersion but also show better electronic contact and conduction between nanobuilding units. Both features facilitate charge transport and insertion/extraction of Li ions and lead to higher capacity and better cycling stability compared with other transition metal oxide nanomaterials. Under the condition of high current density of 100 mA/g, the reversible capacity after 50 cycles is up to 1043 mAhg⁻¹ with an average fading rate of 0.15% per cycle.

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73.63.Bd	Nanocrystalline materials
81.16.-c	Methods of micro- and nanofabrication and processing
81.07.Gf	Nanowires
82.47.Aa	Lithium-ion batteries

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Accurate formula for conversion of tunneling current in dynamic atomic force spectroscopy

John E. Sader and Yoshiaki Sugimoto

Appl. Phys. Lett. 97, 043502 (2010); doi:10.1063/1.3464165 (3 pages)

Online Publication Date: 28 July 2010

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Recent developments in frequency modulation atomic force microscopy enable simultaneous measurement of frequency shift and time-averaged tunneling current. Determination of the interaction force is facilitated using an analytical formula, valid for arbitrary oscillation amplitudes [ Sader and Jarvis, Appl. Phys. Lett. 84, 1801 (2004)] . Here we present the complementary formula for evaluation of the instantaneous tunneling current from the time-averaged tunneling current. This simple and accurate formula is valid for any oscillation amplitude and current law. The resulting theoretical framework allows for simultaneous measurement of the instantaneous tunneling current and interaction force in dynamic atomic force microscopy.

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68.37.Ef	Scanning tunneling microscopy (including chemistry induced with STM)
07.79.Cz	Scanning tunneling microscopes
07.79.Lh	Atomic force microscopes

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Ge-stabilized tetragonal ZrO₂ as gate dielectric for Ge metal-oxide-semiconductor capacitors fabricated on Si substrate

Yung-Hsien Wu, Min-Lin Wu, Jia-Rong Wu, and Lun-Lun Chen

Appl. Phys. Lett. 97, 043503 (2010); doi:10.1063/1.3455904 (3 pages)

Online Publication Date: 28 July 2010

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A Ge-stabilized tetragonal ZrO₂ (t-ZrO₂) film formed by incorporating Ge atoms thermally driven from an underlying Ge layer into a ZrO₂ film was investigated as the gate dielectric for Ge metal-oxide-semiconductor (MOS) capacitors fabricated on a Si substrate. A sole t-ZrO₂ film on Ge is not eligible for the gate dielectric because of the poor interface quality. By using a thermally-grown ultrathin GeO₂ film as an interfacial layer, the t-ZrO₂/GeO₂/Ge stack shows improved interface characteristics and a permittivity (κ) value of 36.6 for the t-ZrO₂. In addition, the stack also demonstrates good leakage current since the amorphous GeO₂ layer terminates grain boundary channels in the crystalline ZrO₂. Further leakage current suppression can be achieved by a H₂ annealing of the t-ZrO₂/GeO₂/Ge stack since the defects at grain boundaries can be effectively passivated, which makes a leakage current of 1.08×10⁻⁶ A/cm² at V_FB−1 V for effective oxide thickness of 1.66 nm and paves an alternative avenue to develop a high-performance crystalline gate dielectric for Ge MOS devices.

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84.32.Tt	Capacitors
77.22.Ch	Permittivity (dielectric function)
61.72.Mm	Grain and twin boundaries
81.65.Rv	Passivation
77.55.-g	Dielectric thin films

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Time dependent behavior of a localized electron at a heterojunction boundary of graphene

Min Seok Jang, Hyungjun Kim, Harry A. Atwater, and William A. Goddard, III

Appl. Phys. Lett. 97, 043504 (2010); doi:10.1063/1.3454909 (3 pages)

Online Publication Date: 29 July 2010

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We develop a finite-difference time-domain (FDTD) method for simulating the dynamics of graphene electrons, denoted GraFDTD. We then use GraFDTD to study the temporal behavior of a single localized electron wave packet, showing that it exhibits optical-like dynamics including the Goos–Hänchen effect [F. Goos and H. Hänchen, Ann. Phys. 436, 333 (1947)] at a heterojunction, but the behavior is quantitatively different than for electromagnetic waves. This suggests issues that must be addressed in designing graphene-based electronic devices analogous to optical devices. GraFDTD should be useful for studying such complex time-dependent behavior of a quasiparticle in graphene.

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73.21.-b	Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
73.40.Lq	Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
71.55.Jv	Disordered structures; amorphous and glassy solids

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Efficient capacitive transduction of high-frequency micromechanical resonators by intrinsic cancellation of parasitic feedthrough capacitances

Julien Arcamone, Eric Colinet, Antoine Niel, and Eric Ollier

Appl. Phys. Lett. 97, 043505 (2010); doi:10.1063/1.3472217 (3 pages)

Online Publication Date: 29 July 2010

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Parasitic feedthrough capacitances represent a generic issue for capacitively transduced microelectromechanical resonators. Those parasitic capacitances degrade the output signal’s magnitude and phase by increasing the feedthrough signal and attenuating the resonance peak because of the resulting antiresonance peak. Whereas classical capacitive actuation/detection schemes only partially circumvent this issue, this work presents a specific, balanced, set-up that intrinsically cancels the effects of feedthrough capacitances. The resonator can recover its intrinsic purely RLC behavior thanks to this method which preferentially applies to given bulk modes, such as Lamé for plates and elliptic for disks. It has been experimentally tested on 100 MHz plates and disks.

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85.85.+j

Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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On the origin of efficient electron injection at phosphonate-functionalized polyfluorene/aluminum interface in efficient polymer light-emitting diodes

Baohua Zhang, Chuanjiang Qin, Xiaodi Niu, Zhiyuan Xie, Yanxiang Cheng, Lixiang Wang, and Xinglin Li

Appl. Phys. Lett. 97, 043506 (2010); doi:10.1063/1.3475021 (3 pages)

Online Publication Date: 29 July 2010

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Ultraviolet and x-ray photoelectron spectroscopy are used to explore the underlying mechanism of efficient electron injection at phosphonate-functionalized polyfluorene (PF-EP)/Al interface in polymer light-emitting diodes (PLEDs). It is proposed that both the favorable interfacial dipoles of phosphonate groups and intense coordination interaction between the phosphonate groups and Al at PF-EP/Al interface are responsible for the efficient electron injection. As a result, the PLEDs with PF-EP/Al cathode demonstrate superior performance compared to the PLEDs with Ba or Ca cathode due to efficient electron injection and suppressed exciton-quenching at cathode interface.

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85.60.Jb

Light-emitting devices

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Temperature stability of intersubband transitions in AlN/GaN quantum wells

Kristian Berland, Martin Stattin, Rashid Farivar, D. M. S. Sultan, Per Hyldgaard, Anders Larsson, Shu Min Wang, and Thorvald G. Andersson

Appl. Phys. Lett. 97, 043507 (2010); doi:10.1063/1.3456528 (3 pages)

Online Publication Date: 29 July 2010

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Temperature dependence of intersubband transitions in AlN/GaN multiple quantum wells grown with molecular beam epitaxy is investigated both by absorption studies at different temperatures and modeling of conduction-band electrons. For the absorption study, the sample is heated in increments up to 400 °C. The self-consistent Schrödinger–Poisson modeling includes temperature effects of the band gap and the influence of thermal expansion on the piezoelectric field. We find that the intersubband absorption energy decreases only by ∼ 6 meV at 400 °C relative to its room temperature value.

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81.05.Ea	III-V semiconductors
81.15.Hi	Molecular, atomic, ion, and chemical beam epitaxy
78.67.De	Quantum wells
78.66.Fd	III-V semiconductors
78.55.Cr	III-V semiconductors
77.55.H-	Piezoelectric and electrostrictive films

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Valley filter in strain engineered graphene

T. Fujita, M. B. A. Jalil, and S. G. Tan

Appl. Phys. Lett. 97, 043508 (2010); doi:10.1063/1.3473725 (3 pages)

Online Publication Date: 30 July 2010

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We propose a simple, yet highly efficient and robust device for producing valley polarized current in graphene. The device comprises of two distinct components; a region of uniform uniaxial strain, adjacent to an out-of-plane magnetic barrier configuration formed by patterned ferromagnetic stripes. We show that when the amount of strain, magnetic field strength, and Fermi level are properly tuned, the output current can be made to consist of only a single valley contribution. Perfect valley filtering is achievable within experimentally accessible parameters.

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73.63.-b	Electronic transport in nanoscale materials and structures
73.21.-b	Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
68.65.Pq	Graphene films
85.35.-p	Nanoelectronic devices
62.25.-g	Mechanical properties of nanoscale systems
73.22.Pr	Electronic structure of graphene

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Differential conductance as a promising approach for rapid DNA sequencing with nanopore-embedded electrodes

Yuhui He, Lubing Shao, Ralph H. Scheicher, Anton Grigoriev, Rajeev Ahuja, Shibing Long, Zhuoyu Ji, Zhaoan Yu, and Ming Liu

Appl. Phys. Lett. 97, 043701 (2010); doi:10.1063/1.3467194 (3 pages)

Online Publication Date: 26 July 2010

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We propose an approach for nanopore-based DNA sequencing using characteristic transverse differential conductance. Molecular dynamics and electron transport simulations show that the transverse differential conductance during the translocation of DNA through the nanopore is distinguishable enough for the detection of the base sequence and can withstand electrical noise caused by DNA structure fluctuation. Our findings demonstrate several advantages of the transverse conductance approach, which may lead to important applications in rapid genome sequencing.

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87.80.St	Genomic techniques
87.15.Tt	Electrophoresis
87.15.-v	Biomolecules: structure and physical properties
87.15.Pc	Electronic and electrical properties

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Giant magnetoimpedance-based microchannel system for quick and parallel genotyping of human papilloma virus type 16/18

Hao Yang, Lei Chen, Chong Lei, Ju Zhang, Ding Li, Zhi-Min Zhou, Chen-Chen Bao, Heng-Yao Hu, Xiang Chen, Feng Cui, Shuang-Xi Zhang, Yong Zhou, and Da-Xiang Cui

Appl. Phys. Lett. 97, 043702 (2010); doi:10.1063/1.3467833 (3 pages)

Online Publication Date: 26 July 2010

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Quick and parallel genotyping of human papilloma virus (HPV) type 16/18 is carried out by a specially designed giant magnetoimpedance (GMI) based microchannel system. Micropatterned soft magnetic ribbon exhibiting large GMI ratio serves as the biosensor element. HPV genotyping can be determined by the changes in GMI ratio in corresponding detection region after hybridization. The result shows that this system has great potential in future clinical diagnostics and can be easily extended to other biomedical applications based on molecular recognition.

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87.85.Ox	Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)
85.75.-d	Magnetoelectronics; spintronics: devices exploiting spin polarized transport or integrated magnetic fields
85.70.Kh	Magnetic thin film devices: magnetic heads (magnetoresistive, inductive, etc.); domain-motion devices, etc.
87.80.Ek	Mechanical and micromechanical techniques
85.85.+j	Micro- and nano-electromechanical systems (MEMS/NEMS) and devices

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Plasmon-enhanced microalgal growth in miniphotobioreactors

S. Torkamani, S. N. Wani, Y. J. Tang, and R. Sureshkumar

Appl. Phys. Lett. 97, 043703 (2010); doi:10.1063/1.3467263 (3 pages)

Online Publication Date: 27 July 2010

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Photoactivity of green microalgae is nonmonotonic across the electromagnetic spectrum. Experiments on Chlamydomonas reinhardtii (green alga) and Cyanothece 51142 (green-blue alga) show that wavelength specific backscattering in the blue region of the spectrum from Ag nanoparticles, caused by localized surface plasmon resonance, can promote algal growth by more than 30%. The wavelength and light flux of the backscattered field can be controlled by varying the geometric features and/or concentration of the nanoparticles.

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87.50.W-	Optical/infrared radiation effects
82.50.Hp	Processes caused by visible and UV light
78.68.+m	Optical properties of surfaces
78.40.Kc	Metals, semimetals, and alloys

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Application of electroosmotically-driven solution displacement for on-chip probing and characterization of protein adsorption

Szu-Wei Tang, Chien-Hsiang Chang, and Hsien-Hung Wei

Appl. Phys. Lett. 97, 043704 (2010); doi:10.1063/1.3459688 (3 pages)

Online Publication Date: 28 July 2010

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An alternative strategy for realizing on-chip characterization of protein adsorption is demonstrated using solution displacement with electro-osmotic flow in microchannels. The idea is illustrated by studying adsorption of bovine serum albumin (BSA) on polydimethylsiloxane surfaces. Through tracking the zeta potential change using the current monitoring technique, we quantify how the surface coverage of BSA varies with time and the BSA concentration. The Langmuir kinetic model is employed to capture the transient behavior of the adsorption and to determine both the adsorption and desorption rate constants. The Langmuir isotherm is also established in line to account for the sorption equilibrium.

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87.15.np	Dissolution
87.14.E-	Proteins
68.43.Mn	Adsorption kinetics
87.50.ch	Electrophoresis/dielectrophoresis and other mechanical effects
47.60.Dx	Flows in ducts and channels

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Integration of plasma-assisted surface chemical modification, soft lithography, and protein surface activation for single-cell patterning

Q. Cheng and K. Komvopoulos

Appl. Phys. Lett. 97, 043705 (2010); doi:10.1063/1.3462326 (3 pages)

Online Publication Date: 28 July 2010

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Surface patterning for single-cell culture was accomplished by combining plasma-assisted surface chemical modification, soft lithography, and protein-induced surface activation. Hydrophilic patterns were produced on Parylene C films deposited on glass substrates by oxygen plasma treatment through the windows of polydimethylsiloxane shadow masks. After incubation first with Pluronic F108 solution and then serum medium overnight, surface seeding with mesenchymal stem cells in serum medium resulted in single-cell patterning. The present method provides a means of surface patterning with direct implications in single-cell culture.

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87.17.-d	Cell processes
87.85.J-	Biomaterials
87.15.R-	Reactions and kinetics

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Label-free discrimination of normal and pulmonary cancer tissues using multiphoton fluorescence ratiometric microscopy

Chun-Chin Wang (王俊欽), Ruei-Jr Wu (吳瑞智), Sung-Jan Lin (林頌然), Yang-Fang Chen (陳永芳), and Chen-Yuan Dong (董成淵)

Appl. Phys. Lett. 97, 043706 (2010); doi:10.1063/1.3460913 (3 pages)

Online Publication Date: 29 July 2010

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We performed multiphoton excited autofluorescence and second harmonic generation microscopy for the distinction of normal, lung adenocarcinoma (LAC), and squamous cell carcinoma (SCC) specimens. In addition to morphological distinction, we derived quantitative metrics of cellular redox ratios for cancer discrimination. Specifically, the redox ratios of paired normal/SCC and normal/LAC specimens were found to be 0.53±0.05/0.41±0.06 and 0.56±0.02/0.35±0.06, respectively. The lower redox ratios in cancer specimens, indicating an increase in metabolic activity. These results show that the combination of morphological multiphoton imaging along with redox ratio indices can be used for the discrimination of normal and pulmonary cancer tissues.

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87.64.mn	Multiphoton
87.64.kv	Fluorescence
87.50.wf	Biophysical mechanisms of interaction
87.19.xj	Cancer

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Impedance spectroscopy on stretchable microelectrode arrays

I. R. Minev and S. P. Lacour

Appl. Phys. Lett. 97, 043707 (2010); doi:10.1063/1.3469919 (3 pages)

Online Publication Date: 29 July 2010

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We are reporting on the fabrication and electrical characterization of stretchable polydimethylsiloxane (PDMS) based microelectrode arrays for recording of neuronal action potentials. Electrodes are immersed in phosphate buffered saline solution. Their electrical impedance spectra are recorded as a function of uniaxial macroscopic strain applied using a custom built automated stretcher. We propose a model to account for the physical phenomena responsible for electrical conduction in the bulk electrode and at the metal–electrolyte interface. Electrodes with active area dimensions of 100×140 μm² maintain an impedance of approximately 1 MΩ at 1 kHz when cycled to 20% strain.

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82.45.Fk	Electrodes
87.80.Kc	Electrochemical techniques

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Design of acoustic devices with isotropic material via conformal transformation

Chunyu Ren, Zhihai Xiang, and Zhangzhi Cen

Appl. Phys. Lett. 97, 044101 (2010); doi:10.1063/1.3467852 (3 pages)

Online Publication Date: 26 July 2010

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The known realization of transformation acoustic devices requires exotic material parameters and has been hampered by limited available materials. By merging the conformal mapping and the embedded coordinate transformation method, we demonstrate that transformation-based acoustic devices can be made from isotropic materials, which greatly facilitates their implementation. The acoustic directional antenna based on such approach is presented and validated by full-wave simulations using the finite element method.

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43.58.-e	Acoustical measurements and instrumentation
43.38.-p	Transduction; acoustical devices for the generation and reproduction of sound

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Self-erasing and rewritable wettability patterns on ZnO thin films

Ville Kekkonen, Antti Hakola, Timo Kajava, Elina Sahramo, Jari Malm, Maarit Karppinen, and Robin H. A. Ras

Appl. Phys. Lett. 97, 044102 (2010); doi:10.1063/1.3460915 (3 pages)

Online Publication Date: 27 July 2010

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Self-erasing patterns allow a substrate to be patterned multiple times or could store temporary information for secret communications, and are mostly based on photochromic molecules to change the color of the pattern. Herein we demonstrate self-erasing patterns of wettability on thin ZnO films made by atomic layer deposition. Hydrophilic patterns are written using UV light and decay spontaneously, i.e. become hydrophobic, or are erased aided by vacuum conditions or heat. We demonstrate that these patterns can be applied for channels to confine flow of water without physical walls.

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68.08.Bc	Wetting
61.82.Fk	Semiconductors
81.65.Cf	Surface cleaning, etching, patterning
81.05.Dz	II-VI semiconductors
61.80.Ba	Ultraviolet, visible, and infrared radiation effects (including laser radiation)

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Position and mass determination of multiple particles using cantilever based mass sensors

Søren Dohn, Silvan Schmid, Fabien Amiot, and Anja Boisen

Appl. Phys. Lett. 97, 044103 (2010); doi:10.1063/1.3473761 (3 pages)

Online Publication Date: 29 July 2010

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Resonant microcantilevers are highly sensitive to added masses and have the potential to be used as mass-spectrometers. However, making the detection of individual added masses quantitative requires the position determination for each added mass. We derive expressions relating the position and mass of several added particles to the resonant frequencies of a cantilever, and an identification procedure valid for particles with different masses is proposed. The identification procedure is tested by calculating positions and mass of multiple microparticles with similar mass positioned on individual microcantilevers. Excellent agreement is observed between calculated and measured positions and calculated and theoretical masses.

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29.40.-n	Radiation detectors
07.10.Cm	Micromechanical devices and systems

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Voltage tunable photodetecting properties of La_0.4Ca_0.6MnO₃ films grown on miscut LaSrAlO₄ substrates

X. M. Li, K. Zhao, H. Ni, S. Q. Zhao, W. F. Xiang, Z. Q. Lu, Z. J. Yue, F. Wang, Yu-Chau Kong, and H. K. Wong

Appl. Phys. Lett. 97, 044104 (2010); doi:10.1063/1.3473781 (3 pages)

Online Publication Date: 29 July 2010

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We report the voltage tunable photodetecting properties of a La_0.4Ca_0.6MnO₃ film grown on miscut LaSrAlO₄ (001) substrates under ultraviolet pulsed laser irradiation at ambient temperature. The photovoltage and photocurrent peak sensitivities can be tuned in the range of 0.295–0.786 V/mJ and 0.172–0.314 A/mJ, respectively, when the applied bias is changed from −20 to +20 V, indicating that the vicinal manganite film can be used as an electric tunable ultraviolet photodetector. A possible mechanism based on the high resolution transmission electron microscopy is introduced to explain the experiment results.

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85.60.Gz	Photodetectors (including infrared and CCD detectors)
81.15.Fg	Pulsed laser ablation deposition
68.55.ag	Semiconductors

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Evading surface and detector frequency noise in harmonic oscillator measurements of force gradients

Eric W. Moore, SangGap Lee, Steven A. Hickman, Lee E. Harrell, and John A. Marohn

Appl. Phys. Lett. 97, 044105 (2010); doi:10.1063/1.3465906 (3 pages)

Online Publication Date: 30 July 2010

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We introduce and demonstrate a method of measuring small force gradients acting on a harmonic oscillator in which the force-gradient signal of interest is used to parametrically up-convert a forced oscillation below resonance into an amplitude signal at the oscillator’s resonance frequency. The approach, which we demonstrate in a mechanically detected electron spin resonance experiment, allows the force-gradient signal to evade detector frequency noise by converting a slowly modulated frequency signal into an amplitude signal.

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46.80.+j	Measurement methods and techniques in continuum mechanics of solids
07.10.Pz	Instruments for strain, force, and torque
07.07.Df	Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

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Comment on “Nanometer resolution piezoresponse force microscopy to study deep submicron ferroelectric and ferroelastic domains” [ Appl. Phys. Lett. 94, 162903 (2009) ]

A. H. G. Vlooswijk, Gustau Catalan, and Beatriz Noheda

Appl. Phys. Lett. 97, 046101 (2010); doi:10.1063/1.3467005 (2 pages) | Cited 1 time

Online Publication Date: 29 July 2010

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Abstract Unavailable

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77.80.Dj	Domain structure; hysteresis
77.55.F-	High-permittivity capacitive films
81.40.Jj	Elasticity and anelasticity, stress-strain relations
62.20.D-	Elasticity
68.55.A-	Nucleation and growth

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Response to “Comment on ‘Nanometer resolution piezoreponse force microscopy to study deep submicron ferroelectric and ferroelastic domains’ ” [ Appl. Phys. Lett. 97, 046101 (2010) ]

C. Durkan, D. P. Chu, and Y. Ivry

Appl. Phys. Lett. 97, 046102 (2010); doi:10.1063/1.3467006 (1 page)

Online Publication Date: 29 July 2010

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Abstract Unavailable

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77.65.-j	Piezoelectricity and electromechanical effects
62.20.D-	Elasticity

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Publisher's Note: “Effects of cathode buffer layers on the efficiency of bulk-heterojunction solar cells” [ Appl. Phys. Lett. 96, 263506 (2010) ]

Ching-Chun Chang, Chi-Feng Lin, Jian-Ming Chiou, Tzung-Han Ho, Yian Tai, Jiun-Haw Lee, Yang-Fang Chen, Juen-Kai Wang, Li-Chyong Chen, and Kuei-Hsien Chen

Appl. Phys. Lett. 97, 049901 (2010); doi:10.1063/1.3473732 (1 page)

Online Publication Date: 26 July 2010

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Abstract Unavailable

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99.10.Fg	Publisher's note
88.40.H-	Solar cells (photovoltaics)

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Volume 97, Issue 4, Articles (04xxxx)