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22 Jan 2007

Volume 90, Issue 4, Articles (04xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 90, 043507 (2007); http://dx.doi.org/10.1063/1.2435508 (3 pages)

D. Y. Kim and A. J. Steckl
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Integrated cantilever sensors with a torsional resonance mode for ultraresoluble on-the-spot bio/chemical detection

Dazhong Jin, Xinxin Li, Hanhan Bao, Zhixiang Zhang, Yuelin Wang, Haitao Yu, and Guomin Zuo

Appl. Phys. Lett. 90, 041901 (2007); http://dx.doi.org/10.1063/1.2433753 (3 pages) | Cited 11 times

Online Publication Date: 22 January 2007

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Torsion-mode resonance is built in an integrated cantilever sensor for ultraresoluble detection of specifically bio/chemical mass adsorption. The superior mass resolution of the torsion-mode cantilever to a conventional bending-mode one is verified by energy-dissipation analysis and Q-factor simulation. With integrated transverse piezoresistance for frequency-shift signal readout and Lorentz force for resonance excitation, the torsion-mode sensor is optimally designed for high sensitivity. The microfabricated torsion-mode sensor is measured with a high close-loop Q factor in air. By Allan-variance analysis for the measured frequency stability, 23 fg resolution is obtained for the torsion-mode sensor, which is much improved compared to the 313 fg for the conventional flexure-mode sensor. The torsional sensor is used to recognize biotin-avidin specific combination, resulting in 443 Hz frequency shift for 50 μM streptavidin solution.
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87.80.-y Biophysical techniques (research methods)
87.14.E- Proteins
87.15.N- Properties of solutions of macromolecules
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
82.80.-d Chemical analysis and related physical methods of analysis
07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing

Anharmonic decay of subterahertz coherent acoustic phonons in GaN

Tzu-Ming Liu, Shih-Ze Sun, Chieh-Feng Chang, Chang-Chi Pan, Guan-Ting Chen, Jen-Inn Chyi, Vitalyi Gusev, and Chi-Kuang Sun

Appl. Phys. Lett. 90, 041902 (2007); http://dx.doi.org/10.1063/1.2433755 (3 pages) | Cited 5 times

Online Publication Date: 22 January 2007

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This letter examines the anharmonic decay in GaN of subthermal (ℏω⪡kBT) coherent longitudinal acoustic (LA) phonons with a frequency in the subterahertz range. In a collisionless regime (ωτ>1), the anharmonic decay rate of subterahertz coherent LA phonons in GaN shows square dependence on the phonon frequency and cubic dependence on the crystal temperature. As confirmed by the authors experiments, this behavior agrees with a model based on Herring scattering, indicating its importance for the estimation of acoustic attenuation in the subterahertz range.
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63.20.Ry Anharmonic lattice modes
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Magnetic resonances in metallic double split rings: Lower frequency limit and bianisotropy

Lei Zhou and S. T. Chui

Appl. Phys. Lett. 90, 041903 (2007); http://dx.doi.org/10.1063/1.2431776 (3 pages) | Cited 7 times

Online Publication Date: 22 January 2007

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The authors employ a rigorous theory to study the electromagnetic resonances in double split rings of circular cross sections. The inter-ring interactions split each single-ring mode to two modes with different symmetries, and the bianisotropy of each mode is suppressed as two rings approach. They obtain analytical expressions to estimate the frequency of the fundamental (magnetic) mode, which facilitate the design of structures with the lowest possible resonance frequencies. Numerical calculations based on the present theory are supported by finite-difference-time-domain simulations on realistic structures and experimental data published previously.
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75.30.Gw Magnetic anisotropy

Fluorescent polymer-porous silicon microcavity devices for explosive detection

I. A. Levitsky, W. B. Euler, N. Tokranova, and A. Rose

Appl. Phys. Lett. 90, 041904 (2007); http://dx.doi.org/10.1063/1.2432247 (3 pages) | Cited 17 times

Online Publication Date: 22 January 2007

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Conjugated polymers entrapped in porous silicon microcavity have been studied as optical sensors for low volatility explosives such as trinitrotoluene. The fluorescence spectra of entrapped polymers were modulated by the microcavity via a spectral “hole” that matches the resonance peak of the microcavity reflectance. Exposure of the porous silicon microcavity containing entrapped polymer to explosives vapor results in a red shift of the resonance peak and the spectral hole, accompanied by the quenching of the fluorescence. This multiplexed response provides multiple monitoring parameters, enabling the development of an optical sensor array for the detection of target explosive vapor.
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07.07.Df Sensors (chemical, optical, electrical, movement, gas, etc.); remote sensing
42.70.Jk Polymers and organics

Nonlinear electrical conducting behavior of carbon nanotube networks in silicone elastomer

C. H. Liu and S. S. Fan

Appl. Phys. Lett. 90, 041905 (2007); http://dx.doi.org/10.1063/1.2432283 (3 pages) | Cited 14 times

Online Publication Date: 22 January 2007

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In this work the authors investigated the electrical conducting properties of multiwalled carbon nanotube (MWNT) networks in the flexible polydimethylsiloxane rubber as a function of applied voltages. The results indicated that the I-V curves showed nonlinear relationships, which can be fitted to quadratic functions. The electrical resistance of the samples varies with the voltages more sharply in lower range. The nonlinear mechanism was related to the materials system including both the MWNTs and the matrix material. Furthermore, the I-V characteristic of the MWNT network/ZnO nanocomposite layer junction has shown a well rectified behavior at low voltages.
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73.63.Rt Nanoscale contacts
73.40.Ei Rectification

Yellow-emitting Sr3SiO5:Ce3+,Li+ phosphor for white-light-emitting diodes and yellow-light-emitting diodes

Ho Seong Jang and Duk Young Jeon

Appl. Phys. Lett. 90, 041906 (2007); http://dx.doi.org/10.1063/1.2432947 (3 pages) | Cited 64 times

Online Publication Date: 23 January 2007

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In this letter, a yellow-emitting Sr3SiO5:Ce3+,Li+ phosphor is reported. Through transitions of 5d→4f (2F7/2 and 2F5/2) in Ce3+, the phosphor showed a very broad and strong yellow emission under near ultraviolet (UV) or blue light excitation. The energy levels of Ce3+ in Sr3SiO5 were suggested from its absorption and excitation spectra. White light could be obtained by combining this phosphor with 460 or 405 nm light-emitting diodes (LEDs) [(x,y) = (0.3086,0.3167) or (0.3173, 0.3103)]. Additionally, a yellow LED was fabricated using a near-UV LED (380 nm chip) with Sr3SiO5:Ce3+,Li+.
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78.40.Ha Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials
85.60.Jb Light-emitting devices

Raman scattering spectra of coupled LO-phonon-plasmon modes in N-In codoped p-type ZnO thin films

J. F. Kong, H. Chen, H. B. Ye, W. Z. Shen, J. L. Zhao, and X. M. Li

Appl. Phys. Lett. 90, 041907 (2007); http://dx.doi.org/10.1063/1.2432955 (3 pages) | Cited 6 times

Online Publication Date: 23 January 2007

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The authors report on a detailed Raman study of N–In codoped p-type ZnO thin films with the hole density ranging from 7.30×1016 to 2.30×1018 cm−3. In addition to the identification of E2(high) modes of ZnO and InN at ∼ 434 and ∼ 488 cm−1, respectively, clear coupled longitudinal-optical phonon-hole-plasmon modes have been observed. A theoretical analysis combining the deformation potential and electro-optic mechanisms can well reproduce the line shapes of the coupled modes, where the yielded hole densities and mobilities are found to be in good agreement with the data from Hall measurements.
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78.30.Fs III-V and II-VI semiconductors
78.66.Hf II-VI semiconductors
78.20.Jq Electro-optical effects
73.50.Dn Low-field transport and mobility; piezoresistance
63.20.kk Phonon interactions with other quasiparticles

Room temperature epitaxial growth of m-plane GaN on lattice-matched ZnO substrates

Atsushi Kobayashi, Satoshi Kawano, Yuji Kawaguchi, Jitsuo Ohta, and Hiroshi Fujioka

Appl. Phys. Lett. 90, 041908 (2007); http://dx.doi.org/10.1063/1.2433758 (3 pages) | Cited 37 times

Online Publication Date: 23 January 2007

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The authors have grown high-quality m-plane GaN (1math00) films on ZnO (1math00) substrates using pulsed laser deposition. They have found that annealing in a box made of ceramic ZnO improves the surface morphology of m-plane ZnO substrates and have succeeded in the layer-by-layer growth of m-plane GaN on the annealed ZnO substrates even at room temperature (RT). X-ray diffraction reveals that high crystalline quality m-plane GaN grows on the RT buffer layer at 700 °C. The 300-nm-thick m-plane GaN film grown on the ZnO substrate contains the residual strains because the lattice mismatches between them are quite small. The relationship of in-plane and out-of-plane strains is consistent with the calculation using the elastic constants.
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81.05.Ea III-V semiconductors
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
61.72.Cc Kinetics of defect formation and annealing
68.35.B- Structure of clean surfaces (and surface reconstruction)

Photogalvanic effects for interband absorption in AlGaN/GaN superlattices

K. S. Cho, Y. F. Chen, Y. Q. Tang, and B. Shen

Appl. Phys. Lett. 90, 041909 (2007); http://dx.doi.org/10.1063/1.2435591 (3 pages) | Cited 15 times

Online Publication Date: 23 January 2007

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The linear and circular photogalvanic effects (CPGEs), induced by ultraviolet (325 nm) radiation, have been observed in the (0001)-oriented Al0.15Ga0.85N/GaN superlattices. The CPGE current changes sign upon reversing the radiation helicity, and it is up to two orders of magnitude larger than that obtained by far-infrared radiation. This result suggests the existence of a sizeable Rashba spin splitting in AlGaN/GaN superlattices. It also provides a possibility for the generation of spin orientation-induced current at room temperature.
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72.40.+w Photoconduction and photovoltaic effects
73.63.-b Electronic transport in nanoscale materials and structures

Evolution of W optical center in Si-implanted epitaxial SiGe at low temperature annealing

J. Tan, G. Davies, S. Hayama, and A. Nylandsted Larsen

Appl. Phys. Lett. 90, 041910 (2007); http://dx.doi.org/10.1063/1.2435976 (3 pages)

Online Publication Date: 24 January 2007

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The authors have investigated the effect of Ge concentration on the evolution of W optical center (W center) in Si-implanted epitaxial Si1−xGex at low temperature annealing. From the results of photoluminescence, the annealing behavior of W center can be separated into two regimes, i.e., W centers in Si1−xGex alloy with Ge <5% are stable, otherwise not stable. The annealing behavior of W center is similar to {311} defect in Si1−xGex. It is suggested that the dissipation of excess interstitials by outdiffusion at low temperature is one of the candidate mechanisms for the retardation of transient enhanced diffusion of boron in Si1−xGex.
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61.72.J- Point defects and defect clusters
61.72.Cc Kinetics of defect formation and annealing
78.55.Hx Other solid inorganic materials
66.30.J- Diffusion of impurities
71.55.Ht Other nonmetals

Multidirectional observation of an embedded quantum dot

Takashi Kita, Tomoya Inoue, Osamu Wada, Mitsuru Konno, Toshie Yaguchi, and Takeo Kamino

Appl. Phys. Lett. 90, 041911 (2007); http://dx.doi.org/10.1063/1.2436633 (3 pages) | Cited 3 times

Online Publication Date: 24 January 2007

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The authors succeeded in observing atomic scale images of undamaged single InAs quantum dots (QDs) embedded in the GaAs matrix using high resolution transmission electron microscope equipped with focused ion beam system. The QD can be viewed from multidirections, and a conclusive and comprehensible determination of the size and the shape anisotropy has been realized. Asymmetry of the structural properties has been confirmed between the [110] and [−110] crystal directions. The embedded QD is elongated along the [−110] axis. The strain-field pattern is also asymmetric according to the shape anisotropy. The results will enable the investigation of the exact structure anisotropy influencing the atomlike properties of QDs.
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68.65.Hb Quantum dots (patterned in quantum wells)
68.37.Lp Transmission electron microscopy (TEM)

Grain boundary impurity and porosity effects on the yield strength of nanocrystalline materials

James C. M. Li

Appl. Phys. Lett. 90, 041912 (2007); http://dx.doi.org/10.1063/1.2430491 (3 pages) | Cited 5 times

Online Publication Date: 25 January 2007

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The effect of impurities on the equilibrium and nonequilibrium boundaries is analyzed for nanocrystalline materials. Due to the purity of equilibrium grain boundaries, the strength levels off to a limiting value in the range of nano grain sizes. This limiting value increases with the total impurity content. Due to nonequilibrium grain boundaries and their effect on impurity segregation, the strength may decrease with decreasing grain size in the nanosize regime. These considerations can explain all the observed effects of grain size on strength without invoking other mechanisms for the nanosize grains. By controlling the impurity content and porosity of the grain boundaries, it is possible to synthesize ultrastrong and ductile materials.
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81.07.Bc Nanocrystalline materials
62.25.-g Mechanical properties of nanoscale systems
61.46.Hk Nanocrystals
61.72.Mm Grain and twin boundaries
64.75.-g Phase equilibria
81.40.Lm Deformation, plasticity, and creep

Self-organized nanocrystalline stripe patterns generated during early crystallization of a nonequilibrium metallic glass

Jian-Min Li

Appl. Phys. Lett. 90, 041913 (2007); http://dx.doi.org/10.1063/1.2436707 (3 pages) | Cited 2 times

Online Publication Date: 25 January 2007

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The author reports a unique early-stage (the crystallized volume fraction ∼ 0.20) self-organized nanocrystalline stripe pattern phenomenon that nanoscale phase separation and nucleation simultaneously work together, representing a type that had never been previously observed in a metastable alloy solid. The first observation of mesoscopic self-organized crystallization showed that self-organization is not limited to the case of organic matter, thus potentially offering an insight into the mechanism of the nonequilibrium (nano)crystallization of metallic glasses.
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81.05.Kf Glasses (including metallic glasses)
81.16.Dn Self-assembly
61.43.Fs Glasses
64.60.Q- Nucleation
64.75.-g Phase equilibria

Random surface roughness influence on gas damped nanoresonators

G. Palasantzas

Appl. Phys. Lett. 90, 041914 (2007); http://dx.doi.org/10.1063/1.2435328 (3 pages) | Cited 7 times

Online Publication Date: 25 January 2007

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The author investigates quantitatively the influence of random surface roughness on the quality factor Q of nanoresonators due to noise by impinging gas molecules. The roughness is characterized by the amplitude w, the correlation length ξ, and the roughness exponent H that describes fine roughness details at short wavelengths. Surface roughening (decreasing H and increasing ratio w/ξ) leads to lower Q, which translates to lower sensitivity to external perturbations, and a higher limit to mass sensitivity. The influence of the exponent H is shown to be important as that of w/ξ, indicating the necessity for precise control of the surface morphology.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
68.35.B- Structure of clean surfaces (and surface reconstruction)

Domain wall broadening mechanism for domain size effect of enhanced piezoelectricity in crystallographically engineered ferroelectric single crystals

Wei-Feng Rao and Yu U. Wang

Appl. Phys. Lett. 90, 041915 (2007); http://dx.doi.org/10.1063/1.2435584 (3 pages) | Cited 16 times

Online Publication Date: 25 January 2007

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Computer modeling and simulation reveal a domain wall broadening mechanism that explains the domain size effect of enhanced piezoelectric properties in domain engineered ferroelectric single crystals. The simulation shows that, under electric field applied along the nonpolar axis of single crystal without domain wall motion, the domain wall broadens and serves as embryo of field-induced new phase, producing large reversible strain free from hysteresis. This mechanism plays a significant role in the vicinity of interferroelectric transition temperature and morphotropic phase boundary, where energy difference between stable and metastable phases is small. Engineered domain configuration fully exploits this domain wall broadening mechanism.
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77.80.Dj Domain structure; hysteresis
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.B- Phase transitions and Curie point
77.65.-j Piezoelectricity and electromechanical effects

Ground and excited state transitions in as-grown Ga0.64In0.36N0.046As0.954 quantum wells studied by contactless electroreflectance

Robert Kudrawiec, Marta Gladysiewicz, Jan Misiewicz, Fumitaro Ishikawa, and Klaus H. Ploog

Appl. Phys. Lett. 90, 041916 (2007); http://dx.doi.org/10.1063/1.2432230 (3 pages) | Cited 9 times

Online Publication Date: 26 January 2007

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The optical transitions of as-grown Ga0.64In0.36N0.046As0.954 multiple quantum wells grown at the low temperature of 375 °C were studied by contactless electroreflectance (CER). The investigation was carried out at room temperature for a set of samples having quantum well (QW) widths ranging from 3.9 to 8.1 nm. The ground and the excited state transitions were clearly observed in CER spectra (the ground state transition was observed at the wavelength of 1.9 μm for the 8.1 nm wide QW). The experimental QW transition energies were compared with theoretical predictions based on an effective mass formalism model. Good agreement between experimental data and theoretical calculations has been obtained assuming that the conduction band offset for GaInNAs/GaAs interface is 80% and the electron effective mass is 0.09m0.
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78.67.De Quantum wells
78.20.Jq Electro-optical effects
73.63.Hs Quantum wells
73.21.Fg Quantum wells
73.20.At Surface states, band structure, electron density of states
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Complex excitonic recombination kinetics in ZnO: Capture, relaxation, and recombination from steady state

F. Bertram, J. Christen, A. Dadgar, and A. Krost

Appl. Phys. Lett. 90, 041917 (2007); http://dx.doi.org/10.1063/1.2432259 (3 pages) | Cited 15 times

Online Publication Date: 26 January 2007

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The kinetics of relaxation and recombination processes of excitons in an epitaxial-grown thick ZnO layer has been examined using time-resolved cathodoluminescence. The unique feature of this technique allows the full analysis of excitation from thermal equilibrium into true steady state and the relaxation back into thermal equilibrium. The luminescence at 5 K is characterized by a rich structure of excitonic lines: XA, I1, I2, I6, I8, and I9 as well as the excited states I6*, I8*, and I9* are clearly resolved. The efficient capture of the free excitons by impurities is directly visualized during the onset as well as the decay. This capture feeds the neutral impurity bound excitons I8 and I9, the initial decay of which becomes delayed. The ionized impurity bound excitons I1 and I2 exhibit a very fast initial decay due to the carrier capture by the impurities followed by a persistent, significantly slower nonexponential component.
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73.61.Ga II-VI semiconductors
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
71.35.-y Excitons and related phenomena
78.66.Hf II-VI semiconductors
78.47.-p Spectroscopy of solid state dynamics
78.60.Hk Cathodoluminescence, ionoluminescence

In situ growth regime characterization of cubic GaN using reflection high energy electron diffraction

J. Schörmann, S. Potthast, D. J. As, and K. Lischka

Appl. Phys. Lett. 90, 041918 (2007); http://dx.doi.org/10.1063/1.2432293 (3 pages) | Cited 17 times

Online Publication Date: 26 January 2007

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Cubic GaN layers were grown by plasma-assisted molecular beam epitaxy on 3C-SiC (001) substrates. In situ reflection high energy electron diffraction was used to quantitatively determine the Ga coverage of the GaN surface during growth. Using the intensity of the electron beam as a probe, optimum growth conditions of c-GaN were found when a 1 ML Ga coverage is formed at the surface. 1 μm thick c-GaN layers had a minimum surface roughness of 2.5 nm when a Ga coverage of 1 ML was established during growth. These samples revealed also a minimum full width at half maximum of the (002) rocking curve.
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81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
52.77.-j Plasma applications
68.55.A- Nucleation and growth
68.35.B- Structure of clean surfaces (and surface reconstruction)

Raman study of thermochromic phase transition in tungsten trioxide nanowires

Dong Yu Lu, Jian Chen, Huan Jun Chen, Li Gong, Shao Zhi Deng, Ning Sheng Xu, and Yu Long Liu

Appl. Phys. Lett. 90, 041919 (2007); http://dx.doi.org/10.1063/1.2435616 (3 pages) | Cited 17 times

Online Publication Date: 26 January 2007

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Tungsten trioxide (WO3) nanowires were synthesized by thermal evaporation of tungsten powder in two steps: tungsten suboxide (WO3−x) nanowires were synthesized, and then oxidized in O2 ambient and transformed into WO3 nanowires. Raman spectroscopy was applied to study the thermochromic phase transition of one-dimensional WO3 nanowires. From the temperature dependence of the characteristic mode at 33 cm−1 in WO3, the phase transition temperature was determined. It was found that the phase transition of WO3 nanowires was reversible and the phase transition temperatures were even lower than that of WO3 nanopowder.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
78.30.Hv Other nonmetallic inorganics

Metallic phase transition investigated via terahertz time-domain spectroscopy

K. J. Chau, K. M. Rieckmann, and A. Y. Elezzabi

Appl. Phys. Lett. 90, 041920 (2007); http://dx.doi.org/10.1063/1.2432299 (3 pages)

Online Publication Date: 26 January 2007

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Terahertz time-domain spectroscopy is applied to noninvasively investigate the solid-liquid phase transition of metallic gallium (Ga) microparticles over the temperature range from 21.2 to 38.2 °C. Fourier and correlation analyses of the transmitted radiation reveal solid-liquid phase transformation at the melting transition temperature Tm and particle conglomeration at a temperature Tc>Tm.
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64.70.D- Solid-liquid transitions
78.70.Gq Microwave and radio-frequency interactions
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