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1 Oct 2012

Volume 101, Issue 14, Articles (14xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 101, 141101 (2012); http://dx.doi.org/10.1063/1.4742864 (4 pages)

Po-Hsun Huang, Michael Ian Lapsley, Daniel Ahmed, Yuchao Chen, Lin Wang, and Tony Jun Huang
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Approaching single-photon detection in near-infrared region

Da Li, Yi Jiang, Yujie J. Ding, Ioulia B. Zotova, and Narasimha S. Prasad

Appl. Phys. Lett. 101, 141126 (2012); http://dx.doi.org/10.1063/1.4757885 (3 pages)

Online Publication Date: 5 October 2012

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We have implemented a single-photon detection system in the 1.55-μm region, based on frequency up-conversion in MgO-doped periodically poled LiNbO3 waveguide. We have subsequently reached a record-low dark count rate of 45 counts per second. The detectable signal photon rate, i.e., the rate for counting the up-converted photons corrected by the dark counts, noises, and losses, reaches a record-low value of 81 photons per second. Through free-space coupling, we have eliminated the dark counts induced by parametric fluorescence.
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42.79.Gn Optical waveguides and couplers
78.55.-m Photoluminescence, properties and materials

Two-dimensional distributed-feedback in InGaAs/GaAs quantum structure lattice arrays

K. Y. Cheng, Chien-Chia Cheng, and K. C. Hsieh

Appl. Phys. Lett. 101, 141127 (2012); http://dx.doi.org/10.1063/1.4757988 (3 pages)

Online Publication Date: 5 October 2012

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The spontaneous emission properties of an artificial semiconductor lattice—quantum structure lattice (QSL)—with in-plane two-dimensional distributed-feedback (DFB) are reported. Quantum box arrays fabricated in strained InGaAs/GaAs quantum wells are used as lattice units in the QSL. The photoluminescence (PL) peak wavelength of the QSL shows the characteristic weak temperature dependence of a DFB structure (0.024 nm/K) between 77 and 150 K. The surface emission of the QSL array shows a collimated radiation pattern. The peak PL intensity has maximum values centered within ±30° of the surface normal and dropped off beyond that angle rapidly.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors

Optical super-resolution by high-index liquid-immersed microspheres

Arash Darafsheh, Gary F. Walsh, Luca Dal Negro, and Vasily N. Astratov

Appl. Phys. Lett. 101, 141128 (2012); http://dx.doi.org/10.1063/1.4757600 (4 pages) | Cited 1 time

Online Publication Date: 5 October 2012

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It is experimentally shown that barium titanate glass microspheres with diameters (D) in the range 2–220 μm and with high refractive index (n ∼ 1.9–2.1) can be used for super-resolution imaging of liquid-immersed nanostructures. Using micron-scale microspheres, we demonstrate an ability to discern the shape of a pattern with a minimum feature size of ∼λ/7, where λ is the illumination wavelength. For spheres with D > 50 μm, the discernible feature sizes were found to increase to ∼λ/4. Detailed data on the resolution, magnification, and field-of-view are presented. This imaging technique can be used in biomedical microscopy, microfluidics, and nanophotonics applications.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
42.30.-d Imaging and optical processing
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Path derivation for a wave scattered model to estimate height correlation function of rough surfaces

M. Zamani, S. M. Fazeli, M. Salami, S. Vasheghani Farahani, and G. R. Jafari

Appl. Phys. Lett. 101, 141601 (2012); http://dx.doi.org/10.1063/1.4756695 (4 pages)

Online Publication Date: 1 October 2012

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The long standing problem on finding the height correlation function is studied by the inverse scattering problem. We propose a method in the frame work of Kirchhoff theory which we call “path derivation of scattered wave” in order to obtain an expression for direct measurements of the height correlation function. This would provide adequate insight to rough surfaces. The efficiency of this method is due to the fact that the height correlation function could be estimated directly by measurements of the scattered intensity on a suggested path. The model is tested numerically and an experimental setup is suggested.
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41.20.Jb Electromagnetic wave propagation; radiowave propagation
68.35.B- Structure of clean surfaces (and surface reconstruction)

Stoichiometric controlling of boron carbide thin films by using boron-carbon dual-targets

Song Zhang, Wenzhong Lu, Chuanbin Wang, Qiang Shen, and Lianmeng Zhang

Appl. Phys. Lett. 101, 141602 (2012); http://dx.doi.org/10.1063/1.4754628 (4 pages)

Online Publication Date: 2 October 2012

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The stoichiometry of boron carbide thin films was controlled in the range of 0.1–8.9 via pulsed laser deposition by using boron-carbon dual-targets. The amorphous films sized 50 nm in thickness. The reaction rate of boron and carbon atoms increased with the increasing of target rotating speed. Carbon atoms preferentially substituted boron atoms within the chain site as sp2 hybridization at lower carbon concentrations and then within the icosahedrons site as sp3 hybridization at higher carbon concentrations.
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68.55.jd Thickness
81.15.Fg Pulsed laser ablation deposition

Electron transporting water-gated thin film transistors

Abdullah Al Naim and Martin Grell

Appl. Phys. Lett. 101, 141603 (2012); http://dx.doi.org/10.1063/1.4757131 (4 pages) | Cited 2 times

Online Publication Date: 2 October 2012

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We demonstrate an electron-transporting water-gated thin film transistor, using thermally converted precursor-route zinc-oxide (ZnO) intrinsic semiconductors with hexamethyldisilazene (HMDS) hydrophobic surface modification. Water gated HMDS-ZnO thin film transistors (TFT) display low threshold and high electron mobility. ZnO films constitute an attractive alternative to organic semiconductors for TFT transducers in sensor applications for waterborne analytes. Despite the use of an electrolyte as gate medium, the gate geometry (shape of gate electrode and distance between gate electrode and TFT channel) is relevant for optimum performance of water-gated TFTs.
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85.30.Tv Field effect devices

Implication of exotic topography depths of surface nanopits in scanning tunneling microscopy of HgCdTe

F. X. Zha, M. S. Li, J. Shao, Q. Y. Wang, X. R. Ren, K. An, X. L. Zhao, and X. C. Shen

Appl. Phys. Lett. 101, 141604 (2012); http://dx.doi.org/10.1063/1.4756938 (4 pages)

Online Publication Date: 4 October 2012

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The vacancy doped p-type narrow band semiconductor of Hg0.73Cd0.27Te was characterized by ultra-high vacuum scanning tunneling microscopy (STM) and spectroscopy. The imaging displays surprisingly different topographies for positive and negative imaging biases, between which the observed surface pits with the negative bias are few tens nanometers shallower than their counterparts with the positive bias. The effect is subjected to two different local tunneling mechanisms which are not solely dictated by surface electronic properties. The observation contrasts with the conventional geometric view of the nanometer-scale corrugation of STM topography.
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68.35.bg Semiconductors
68.55.J- Morphology of films
61.46.-w Structure of nanoscale materials
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Direct imaging and spectral identification of the interfaces in organic semiconductor-ferromagnet heterojunction

D. H. Wei, Chia-Hao Wang, Hui-Ching Chang, Yuet-Loy Chan, Chih-Hao Lee, and Yao-Jane Hsu

Appl. Phys. Lett. 101, 141605 (2012); http://dx.doi.org/10.1063/1.4756943 (4 pages)

Online Publication Date: 4 October 2012

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Using x-ray spectromicroscopy, we studied the interface of pentacene (Pn)-cobalt (Co) heterojunction fabricated by thermal evaporation in an ultra-high vacuum environment. Through element-specific images and absorption spectroscopic analysis, we found evidences suggesting that part of the Co top layer penetrates into Pn film and hybridizes with molecules. As the Curie temperature of infiltrated Co clusters is dimension sensitive, the Co penetration would create an ill-defined interfacial region whose magnetization depends on the temperature and depth of cobalt penetration. The magnetic complexity at Pn/Co interface was found reduced after inserting a thin Cu layer between Pn and Co.
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78.70.Dm X-ray absorption spectra
68.37.Yz X-ray microscopy
75.30.Kz Magnetic phase boundaries (including classical and quantum magnetic transitions, metamagnetism, etc.)
75.50.Cc Other ferromagnetic metals and alloys
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)

First principles calculations of the band offset at SrTiO3−TiO2 interfaces

Nunzio Roberto D'Amico, Giovanni Cantele, and Domenico Ninno

Appl. Phys. Lett. 101, 141606 (2012); http://dx.doi.org/10.1063/1.4757281 (4 pages)

Online Publication Date: 4 October 2012

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We report on first principles calculations of the properties of the epitaxial SrTiO3−TiO2 (anatase) heterojunction, with an emphasis on the electronic band profile and lineup at the interface. The valence and conduction band offsets are calculated as a function of the number of anatase layers deposited onto the SrTiO3, as well as of the position of an oxygen vacancy with respect to the interface. It is shown that the presence of oxygen vacancies in the SrTiO3 is a way to effectively lower the barrier heights at the interface. Our results are in agreement with recent experiments reporting nearly zero band offset.
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73.21.-b Electron states and collective excitations in multilayers, quantum wells, mesoscopic, and nanoscale systems
61.72.jd Vacancies
71.15.-m Methods of electronic structure calculations
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Scaling behavior of GaAs and GaMnAs quantum rings grown by droplet epitaxy

E. Placidi, F. Arciprete, A. Balzarotti, and F. Patella

Appl. Phys. Lett. 101, 141901 (2012); http://dx.doi.org/10.1063/1.4756896 (3 pages)

Online Publication Date: 1 October 2012

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The transition from the liquid phase of Ga droplets to the formation of GaAs and GaMnAs quantum rings has been studied as a function of temperature. We show that different aggregation processes involve the GaAs (GaMnAs) island and the droplet formation. Furthermore, the aspect ratio of the islands exhibits an anomalous scaling law related to a tendency to aggregate in the vertical direction.
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68.55.ag Semiconductors
68.55.J- Morphology of films
81.05.Ea III-V semiconductors
81.07.Ta Quantum dots
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
64.70.D- Solid-liquid transitions

Spatial power combination within fan-shaped region using anisotropic zero-index metamaterials

Qiang Cheng, Ben Geng Cai, Wei Xiang Jiang, Hui Feng Ma, and Tie Jun Cui

Appl. Phys. Lett. 101, 141902 (2012); http://dx.doi.org/10.1063/1.4756908 (3 pages)

Online Publication Date: 1 October 2012

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We show that the radially anisotropic zero-index metamaterials can be used to realize spatial power combination within fan-shaped regions. When the radial component of the permittivity or permeability tensor tends to zero, the coherent omnidirectional radiations will occur when sources are within the fan-shaped metamaterial backed with perfect magnetic conductors (PMCs), which leads to spatial power combination in finite areas. As the included angle of the PMC corner reflector decreases, the powers from internal sources can still be efficiently combined, which has potential applications in radio frequency engineering.
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42.70.-a Optical materials
77.22.Ch Permittivity (dielectric function)

Highly promising electrooptic material: Distorted helix ferroelectric liquid crystal with a specific tilt angle

I. Abdulhalim

Appl. Phys. Lett. 101, 141903 (2012); http://dx.doi.org/10.1063/1.4756917 (5 pages) | Cited 1 time

Online Publication Date: 2 October 2012

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Short pitch deformable helix ferroelectric liquid crystals are shown to become optically isotropic at certain critical tilt angle with an average dielectric constant (ε1+ε2+ε3)/3. For uniaxial medium, this angle equals θd = cos−1(1/math), and it depends weakly on the local biaxial anisotropy. The degeneracy is removed upon the application of a small electric field and the medium becomes optically biaxial. A high contrast fast electrooptic modulation and display devices can be built using such materials even when aligned in a multi-domain. For tilt angles very near the degeneracy angle, ultrasensitivity of the eigenaxes rotation to the applied field is found.
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78.20.Jq Electro-optical 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
77.22.Ch Permittivity (dielectric function)
77.80.Dj Domain structure; hysteresis
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals

Enhanced carrier-carrier interaction in optically pumped hydrogenated nanocrystalline silicon

Thomas W. Roger, Wei He, Igor V. Yurkevich, and Andrey Kaplan

Appl. Phys. Lett. 101, 141904 (2012); http://dx.doi.org/10.1063/1.4755760 (4 pages) | Cited 1 time

Online Publication Date: 2 October 2012

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A femtosecond pump-probe setup was used to measure the time resolved reflectivity of hydrogenated amorphous silicon containing crystalline silicon nanoparticles at eight different incidence angles. Results fitted with the Drude model found a scattering rate of Γ = 2−1+1.2×1015 s−1 at a corresponding carrier concentration of ∼ 1020 cm−3. The observed scattering rate is attributed to enhanced carrier-carrier interaction in optically pumped nanocrystals.
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78.47.J- Ultrafast spectroscopy (<1 psec)
78.47.D- Time resolved spectroscopy (>1 psec)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
82.30.-b Specific chemical reactions; reaction mechanisms

Nonlinear optical mass sensor with an optomechanical microresonator

Jin-Jin Li and Ka-Di Zhu

Appl. Phys. Lett. 101, 141905 (2012); http://dx.doi.org/10.1063/1.4757004 (4 pages) | Cited 1 time

Online Publication Date: 3 October 2012

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Mechanical oscillators have been widely used in mass sensing due to the direct relationship of the frequency-shift and mass-change. A conventional method for detecting the mass of deposited sample is to exploit the frequency variation of mechanical resonator in the linear optical regime. In the present letter, we demonstrate a nonlinear optical mass sensor with an optomechanical microresonator which will provide a route towards the use of cavity optomechanical system in biomedical sensors, deposition monitors, chemical reaction monitors and other nonlinear measurement fields. Based on the current experimental conditions, the nonlinear optical mass sensor proposed here can be achieved by experiments.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
84.30.Ng Oscillators, pulse generators, and function generators

Kinetics of interdiffusion in semiconductor ternary quantum dots

Xu Han, Sumeet C. Pandey, and Dimitrios Maroudas

Appl. Phys. Lett. 101, 141906 (2012); http://dx.doi.org/10.1063/1.4757148 (5 pages) | Cited 1 time

Online Publication Date: 3 October 2012

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We develop a one-dimensional transient model of species transport in ternary quantum dots (QDs) and implement the model for analysis of group-VI species interdiffusion kinetics in ZnSe1−xSx nanocrystals. We use the model results for the interpretation of x-ray photoelectron spectroscopy data for the group-VI species near-surface concentration evolution during thermal annealing of ZnSe1−xSx QDs with a ZnSe/ZnS core/shell initial configuration. The model provides an excellent fitting of the experimental data, which is used to derive transport coefficients and thermal activation barriers for diffusion in the ternary QDs.
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66.30.Ny Chemical interdiffusion; diffusion barriers
79.60.Bm Clean metal, semiconductor, and insulator surfaces
82.40.-g Chemical kinetics and reactions: special regimes and techniques
61.72.Cc Kinetics of defect formation and annealing

Nanocluster Si sensitized Er luminescence: Excitation mechanisms and critical factors for population inversion

In Yong Kim, Kyung Joong Kim, and Jung H. Shin

Appl. Phys. Lett. 101, 141907 (2012); http://dx.doi.org/10.1063/1.4757001 (4 pages)

Online Publication Date: 4 October 2012

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Luminescence from Er3+ ions sensitized by nanocluster Si is investigated using finite-element, Monte-Carlo simulations. We find that we can reproduce and explain many conflicting results that have been reported using only a simple Förster-type interaction. In particular, we show that Er-Er energy migration plays a major role in Er3+ excitation such quantities such as excitation distance and sensitized fraction depend on optically active Er fraction and pumping power. Based on simulation results, we identify optically active fraction as the critical factor and suggest a multi-layered structure as being ideal for achieving population inversion.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
71.35.-y Excitons and related phenomena
78.55.Ap Elemental semiconductors

Ellipsometry applied to phase transitions and relaxation phenomena in Ni2MnGa ferromagnetic shape memory alloy

A. Dejneka, V. Zablotskii, M. Tyunina, L. Jastrabik, J. I. Pérez-Landazábal, V. Recarte, V. Sánchez-Alarcos, and V. A. Chernenko

Appl. Phys. Lett. 101, 141908 (2012); http://dx.doi.org/10.1063/1.4757393 (5 pages)

Online Publication Date: 4 October 2012

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The temperature dependences of the refractive index, n(T), calculated from ellipsometric data during the premartensitic and martensite transitions in a Ni2MnGa ferromagnetic shape memory alloy allow to detect all critical temperatures of the sample. The performed measurements reveal the behavior of a surface layer of a few tens nanometers depth due to the light absorption. Optical measurements reveal the earlier onset of premartensitic transition at the sample surface and shows unknown new features like time and temperature dependent effects related to the crystal surface. The underlying mechanisms of the observed temperature changes of the equilibrium n0 may be related to temperature dependent interactions between free electrons and phonons as well as between electrons and defects. A giant isothermal creep (up to 20%) of the refractive index was found for temperatures below T0 = 315 K. Below T0 the creep amplitude grows with the temperature difference T0-T. The creep disappears and the relaxation time becomes zero at temperatures above T0.
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81.30.Kf Martensitic transformations
75.75.-c Magnetic properties of nanostructures
62.20.Hg Creep
75.50.Cc Other ferromagnetic metals and alloys
76.50.+g Ferromagnetic, antiferromagnetic, and ferrimagnetic resonances; spin-wave resonance
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Infrared absorption coefficients of vertically aligned carbon nanotube films

H. Ye, X. J. Wang, W. Lin, C. P. Wong, and Z. M. Zhang

Appl. Phys. Lett. 101, 141909 (2012); http://dx.doi.org/10.1063/1.4757395 (4 pages)

Online Publication Date: 4 October 2012

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Vertically aligned carbon nanotube (VACNT) films have promising applications in solar cells, radiometers, and photonic devices. We have fabricated VACNT films with thicknesses of 10, 36, 45, and 120 μm on silicon substrates. The absorption coefficient of each film is obtained from the measured regular transmittance from 1 to 19 μm wavelengths, using a ratio method. The degree of alignment of the VACNT films is estimated by comparison of experiments with an analysis based on the effective medium theory. The effect of thickness on the absorption coefficient of the VACNT films is elucidated according to their morphologies.
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78.67.Ch Nanotubes
88.40.J- Types of solar cells
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Growth and characterization of LuAs films and nanostructures

E. M. Krivoy, H. P. Nair, A. M. Crook, S. Rahimi, S. J. Maddox, R. Salas, D. A. Ferrer, V. D. Dasika, D. Akinwande, and S. R. Bank

Appl. Phys. Lett. 101, 141910 (2012); http://dx.doi.org/10.1063/1.4757605 (4 pages) | Cited 1 time

Online Publication Date: 4 October 2012

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We report the growth and characterization of nearly lattice-matched LuAs/GaAs heterostructures. Electrical conductivity, optical transmission, and reflectivity measurements of epitaxial LuAs films indicate that LuAs is semimetallic, with a room-temperature resistivity of 90 μΩ cm. Cross-sectional transmission electron microscopy confirms that LuAs nucleates as self-assembled nanoparticles, which can be overgrown with high-quality GaAs. The growth and material properties are very similar to those of the more established ErAs/GaAs system; however, we observe important differences in the magnitude and wavelength of the peak optical transparency, making LuAs superior for certain device applications, particularly for thick epitaxially embedded Ohmic contacts that are transparent in the near-IR telecommunications window around 1.3 μm.
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81.16.-c Methods of micro- and nanofabrication and processing
68.55.at Other materials
73.40.Ns Metal-nonmetal contacts
73.61.At Metal and metallic alloys
78.66.Bz Metals and metallic alloys
81.05.Bx Metals, semimetals, and alloys

Electromechanical instabilities of thermoplastics: Theory and in situ observation

Qiming Wang, Xiaofan Niu, Qibing Pei, Michael D. Dickey, and Xuanhe Zhao

Appl. Phys. Lett. 101, 141911 (2012); http://dx.doi.org/10.1063/1.4757867 (4 pages)

Online Publication Date: 5 October 2012

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Thermoplastics under voltages are used in diverse applications ranging from insulating cables to organic capacitors. Electromechanical instabilities have been proposed as a mechanism that causes electrical breakdown of thermoplastics. However, existing experiments cannot provide direct observations of the instability process, and existing theories for the instabilities generally assume thermoplastics are mechanically unconstrained. Here, we report in situ observations of electromechanical instabilities in various thermoplastics. A theory is formulated for electromechanical instabilities of thermoplastics under different mechanical constraints. We find that the instabilities generally occur in thermoplastics when temperature is above their glass transition temperatures and electric field reaches a critical value. The critical electric field for the instabilities scales with square root of yield stress of the thermoplastic and depends on its Young's modulus and hardening property.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
64.70.pj Polymers

Crystallographic direction dependence of direct current field induced strain and phase transitions in Na0.5Bi0.5TiO3-x%BaTiO3 single crystals near the morphotropic phase boundary

Chengtao Luo, Wenwei Ge, Qinhui Zhang, Jiefang Li, Haosu Luo, and D. Viehland

Appl. Phys. Lett. 101, 141912 (2012); http://dx.doi.org/10.1063/1.4757877 (4 pages) | Cited 3 times

Online Publication Date: 5 October 2012

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The crystallographic dependence of the DC electric (E) field induced strain and phase transition in Na0.5Bi0.5TiO3-5.6%BaTiO3 (NBT-5.6%BT) single crystals has been investigated. An induced transition between pseudocubic and tetragonal structures was observed under E = 10 kV/cm for fields applied along the 〈001〉 direction, whereas an induced transition between pseudocubic and rhombohedral structures was observed when the E-field was applied along 〈111〉. Our results show near the morphotropic phase boundary that the phase stability of NBT-x%BT is dependent on the crystallographic direction along which E was applied.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
81.30.Hd Constant-composition solid-solid phase transformations: polymorphic, massive, and order-disorder
64.70.K- Solid-solid transitions
61.66.Fn Inorganic compounds

Giant domain wall response of highly twinned ferroelastic materials

W. Schranz, H. Kabelka, A. Sarras, and M. Burock

Appl. Phys. Lett. 101, 141913 (2012); http://dx.doi.org/10.1063/1.4757992 (4 pages)

Online Publication Date: 5 October 2012

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Many ferroelastic crystals display at sufficiently low measurement frequencies a huge elastic softening below Tc which is caused by domain wall motion. Materials range from perovskites to iron based superconductors and shape memory materials. We present a model—based on Landau-Ginzburg theory including long range elastic interaction between needle shaped ferroelastic domains—to describe the observed superelastic softening. The theory predicts that the domain wall contribution to the elastic susceptibility is different for improper and proper ferroelastic materials. A test of the theory against experimental data on SrTiO3, KMnF3, LaAlO3, LaAlO3,La1-xNdxP5O14, and NH4HC2O4·mathH2O yields excellent agreement.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
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Localized surface plasmon-enhanced ultraviolet electroluminescence from n-ZnO/i-ZnO/p-GaN heterojunction light-emitting diodes via optimizing the thickness of MgO spacer layer

W. Z. Liu, H. Y. Xu, L. X. Zhang, C. Zhang, J. G. Ma, J. N. Wang, and Y. C. Liu

Appl. Phys. Lett. 101, 142101 (2012); http://dx.doi.org/10.1063/1.4757127 (5 pages)

Online Publication Date: 1 October 2012

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Localized surface plasmon (LSP)-enhanced ultraviolet light-emitting diodes were manufactured by introducing Ag nanoparticles and MgO spacer layer into n-ZnO/i-ZnO/p-GaN heterostructures. By optimizing the MgO thickness, which can suppress the undesired charge transfer and nonradiative Förster resonant energy transfer between Ag and ZnO, a 7-fold electroluminescence enhancement was achieved. Time-resolved and temperature-dependent photoluminescence measurements reveal that both spontaneous emission rate and internal quantum efficiency are increased as a result of coupling between ZnO excitons and Ag LSPs, and simple calculations, based on experimental data, also indicate that most of LSP's energy can be converted into the photon energy.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Temperature dependent photon echoes of a GaN thin film

J. R. Schneck, E. Dimakis, J. Woodward, S. Erramilli, T. D. Moustakas, and L. D. Ziegler

Appl. Phys. Lett. 101, 142102 (2012); http://dx.doi.org/10.1063/1.4754091 (4 pages)

Online Publication Date: 1 October 2012

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UV photon echoes from a ∼120 nm GaN thin film exhibit a biexponential decay attributed to the coherence loss of slowly and rapidly decaying excitons corresponding to excitons with different degrees of localization or trapping in this material. Both exciton populations are strongly inhomogeneously broadened. For one exciton type, T2 is ∼300 fs at 10 K and ≤25 fs at 300 K; for the other, T2 is pulse-width limited (≤25 fs) at all temperatures. At low temperatures, coherence decay predominantly results from impurity and defect site interactions. At higher temperatures, strong exciton-LO phonon coupling dominates dephasing.
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78.47.jf Photon echoes
71.35.-y Excitons and related phenomena
78.40.Fy Semiconductors
78.66.Fd III-V semiconductors
63.20.kk Phonon interactions with other quasiparticles

Charge sensing in a Si/SiGe quantum dot with a radio frequency superconducting single-electron transistor

Mingyun Yuan, Zhen Yang, D. E. Savage, M. G. Lagally, M. A. Eriksson, and A. J. Rimberg

Appl. Phys. Lett. 101, 142103 (2012); http://dx.doi.org/10.1063/1.4754827 (3 pages)

Online Publication Date: 2 October 2012

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We report the operation of a radio frequency superconducting single-electron transistor (rf-SSET) as a charge sensor for single and double Si/SiGe quantum dots (QDs). Real-time electron tunneling events are observed from the reflected signal of the rf-SSET with a charge sensitivity of 4×10−6 e/math, which demonstrates a fast charge detection time of a few tens of microseconds. Measurements of the reflected power are used to map out the stability diagram of the double quantum dot.
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85.35.Gv Single electron devices
85.25.-j Superconducting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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