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20 Jul 2009

Volume 95, Issue 3, Articles (03xxxx)

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Appl. Phys. Lett. 95, 033502 (2009); http://dx.doi.org/10.1063/1.3178556 (3 pages)

Akihito Ikedo, Takahiro Kawashima, Takeshi Kawano, and Makoto Ishida
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Blueshift in optical band gap in nanocrystalline Zn1−xCaxO films deposited by sol-gel method

Kamakhya Prakash Misra, R. K. Shukla, Atul Srivastava, and Anchal Srivastava

Appl. Phys. Lett. 95, 031901 (2009); http://dx.doi.org/10.1063/1.3184789 (3 pages) | Cited 14 times

Online Publication Date: 20 July 2009

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A blueshift in the optical band gap of nanocrystalline Zn1−xCaxO thin films has been obtained. A 12.72% enhancement in the band gap of ZnO thin films has been obtained using Ca dopant for the first time. The band gap widens from 3.38 to 3.81 eV as the Ca concentration increases from x = 0 to x = 0.15. The films are deposited by sol-gel method and have a hexagonal wurtzite phase with no indication of calcium. Grain size lies in the range of 12–92 nm. Atomic force micrographs indicate much smaller rms surface roughness showing significantly smooth surfaces.
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78.66.Hf II-VI semiconductors
81.16.-c Methods of micro- and nanofabrication and processing
81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
68.35.bg Semiconductors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
71.20.Nr Semiconductor compounds

Lattice polarity detection of InN by circular photogalvanic effect

Q. Zhang, X. Q. Wang, X. W. He, C. M. Yin, F. J. Xu, B. Shen, Y. H. Chen, Z. G. Wang, Y. Ishitani, and A. Yoshikawa

Appl. Phys. Lett. 95, 031902 (2009); http://dx.doi.org/10.1063/1.3186042 (3 pages) | Cited 3 times

Online Publication Date: 20 July 2009

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We report an effective and nondestructive method based on circular photogalvanic effect (CPGE) to detect the lattice polarity of InN. Because of the lattice inversion between In- and N-polar InN, the energy band spin splitting is opposite for InN films with different polarities. Consequently under light irradiation with the same helicity, CPGE photocurrents in In- and N-polar layers will have opposite directions, thus the polarity can be detected. This method is demonstrated by our CPGE measurements in both n- and p-type InN films.
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72.40.+w Photoconduction and photovoltaic effects
68.55.ag Semiconductors
61.80.-x Physical radiation effects, radiation damage
81.70.Cv Nondestructive testing: ultrasonic testing, photoacoustic testing
81.05.Ea III-V semiconductors

Photoluminescence of highly excited AlN: Biexcitons and exciton-exciton scattering

R. A. R. Leute, M. Feneberg, R. Sauer, K. Thonke, S. B. Thapa, F. Scholz, Y. Taniyasu, and M. Kasu

Appl. Phys. Lett. 95, 031903 (2009); http://dx.doi.org/10.1063/1.3186044 (3 pages) | Cited 11 times

Online Publication Date: 20 July 2009

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Low-temperature photoluminescence spectra of nominally undoped high quality AlN layers on SiC and Al2O3 substrates are reported. Under high excitation conditions, we observe several bands that increase superlinearly with the excitation density. Based on temperature and excitation level dependences recorded on different samples, we identify a band 36 meV below the free A-exciton transition as due to exciton-exciton scattering (P2 band) and a second band down-shifted from the A-exciton transition by 27 meV as due to biexciton recombination. The combined data yield an exciton binding energy of 48 meV.
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71.35.Gg Exciton-mediated interactions
78.55.Cr III-V semiconductors

Electromechanical instability in semicrystalline polymers

Xuanhe Zhao and Zhigang Suo

Appl. Phys. Lett. 95, 031904 (2009); http://dx.doi.org/10.1063/1.3186078 (3 pages) | Cited 19 times

Online Publication Date: 20 July 2009

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When a layer of a semicrystalline polymer is subject to a tensile force in its plane and a voltage through its thickness, the deformation of the layer is initially homogeneous, but it then localizes. The electromechanical instability sets in when the force and the voltage reach critical conditions. The critical conditions are determined in this paper and are related to the following two special cases: the Considère condition for the necking instability and the Stark–Garton condition for the pull-in instability. The general critical conditions show that a tensile force can markedly reduce the critical voltage.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.41.+e Polymers, elastomers, and plastics
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Tuneable optical lenses from diamond thin films

A. Kriele, O. A. Williams, M. Wolfer, D. Brink, W. Müller-Sebert, and C. E. Nebel

Appl. Phys. Lett. 95, 031905 (2009); http://dx.doi.org/10.1063/1.3183534 (3 pages) | Cited 16 times

Online Publication Date: 20 July 2009

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Nanocrystalline diamond (NCD) membranes of 150 nm thickness and diameters in the millimeter range grown by microwave-assisted chemical-vapor deposition were bulged to investigate their mechanical properties and their use as tuneable optical lenses. The NCD films were grown at different CH4/H2 gas mixtures to vary the sp2/sp3 ratio and thereby to tune their mechanical, optical, and surface morphology properties. By applying gas over pressure the membrane forms a lens shaped geometry. From deflection data we calculated Young’s moduli which decrease with increasing CH4/H2 ratio from 1160 GPa at 0.5% to 900 GPa at 7%. Optical lens applications show a variation in the focal point from infinity to 3.5 mm. The data indicate that NCD is a promising material for tuneable optical lenses applications.
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42.79.-e Optical elements, devices, and systems

Photoluminescence and photoabsorption blueshift of nanostructured ZnO: Skin-depth quantum trapping and electron-phonon coupling

J. W. Li, X. J. Liu, L. W. Yang, Z. F. Zhou, G. F. Xie, Y. Pan, X. H. Wang, J. Zhou, L. T. Li, Likun Pan, Zhuo Sun, and Chang Q. Sun

Appl. Phys. Lett. 95, 031906 (2009); http://dx.doi.org/10.1063/1.3184566 (3 pages) | Cited 5 times

Online Publication Date: 20 July 2009

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Although the size- and shape-induced blueshift in the photoluminescence and photoabsorption of nanostructured ZnO has been extensively investigated, the underlying mechanism remains yet unclear. Here we show that theoretical reproduction of the observed trends clarifies that the blueshift originates from the Hamiltonian perturbation due to the broken-bond-induced local strain and quantum trapping and electron-phonon coupling in the surface skin up to two atomic layers in depth while bonds in the core interior retain their bulk nature. The extent of the blue shift depends on the tunable fraction of undercoordinated atoms in the surface skin. Therefore, the quantum confinement effect is indeed more “superficial” than first thought [ H. Winn, OE Mag. 8, 10 (2005) ].
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78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
71.38.-k Polarons and electron-phonon interactions
63.22.Kn Clusters and nanocrystals

Mechanism of strain relaxation by twisted nanocolumns revealed in AlGaN/GaN heterostructures

V. P. Kladko, A. V. Kuchuk, N. V. Safryuk, V. F. Machulin, A. E. Belyaev, H. Hardtdegen, and S. A. Vitusevich

Appl. Phys. Lett. 95, 031907 (2009); http://dx.doi.org/10.1063/1.3184569 (3 pages) | Cited 4 times

Online Publication Date: 20 July 2009

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The structural properties of AlGaN/GaN heterostructures grown by metal organic chemical vapor deposition on sapphire substrates with different thicknesses were studied by high-resolution x-ray diffraction. The relation between the deformations and dislocation densities in the layer and substrate was established. The dependence of the system’s curvature on the lattice mismatch, caused by different fractions of nanoblock twists with respect to the c-plane, was determined. A mechanism of elastic strain relaxation was proposed.
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62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
81.40.Jj Elasticity and anelasticity, stress-strain relations
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Lk Linear defects: dislocations, disclinations
62.20.D- Elasticity
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Evidence of Germanium precipitation in phase-change Ge1−xTex thin films by Raman scattering

E. Gourvest, S. Lhostis, J. Kreisel, M. Armand, S. Maitrejean, A. Roule, and C. Vallée

Appl. Phys. Lett. 95, 031908 (2009); http://dx.doi.org/10.1063/1.3186077 (3 pages) | Cited 9 times

Online Publication Date: 21 July 2009

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In situ annealing Raman scattering is used to evidence germanium postcrystallization in the crystalline phase of Ge-rich phase-change GeTe thin films. Both reflectivity and Raman scattering show that the crystallization temperature of the as-deposited amorphous phase increases with increasing Ge content going from Tc = 180 °C (Ge0.5Te0.5) to 360 °C for Ge-rich Ge0.76Te0.24. The crystallized phase adopts the rhombohedral α-GeTe phase structure, and whatever the starting composition. For Ge-rich GeTe we observe a second characteristic temperature around 375 °C, which signs the crystallization of a precipitated cubic Ge phase and thus the presence of two distinct phases.
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81.30.Mh Solid-phase precipitation
64.70.kg Semiconductors
78.30.Hv Other nonmetallic inorganics
78.66.Li Other semiconductors
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Twinning partial multiplication at grain boundary in nanocrystalline fcc metals

Y. T. Zhu, X. L. Wu, X. Z. Liao, J. Narayan, S. N. Mathaudhu, and L. J. Kecskés

Appl. Phys. Lett. 95, 031909 (2009); http://dx.doi.org/10.1063/1.3187539 (3 pages) | Cited 14 times

Online Publication Date: 22 July 2009

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Most deformation twins in nanocrystalline face-centered cubic (fcc) metals have been observed to form from grain boundaries. The growth of such twins requires the emission of Shockley partials from the grain boundary on successive slip planes. However, it is statistically improbable for a partial to exist on every slip plane. Here we propose a dislocation reaction and cross-slip mechanism on the grain boundary that would supply a partial on every successive slip plane for twin growth. This mechanism can also produce a twin with macrostrain smaller than that caused by a conventional twin.
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61.72.Mm Grain and twin boundaries
61.72.Lk Linear defects: dislocations, disclinations

The determination of the bulk residual doping in indium nitride films using photoluminescence

M. Moret, S. Ruffenach, O. Briot, and B. Gil

Appl. Phys. Lett. 95, 031910 (2009); http://dx.doi.org/10.1063/1.3187914 (3 pages) | Cited 9 times

Online Publication Date: 22 July 2009

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We extend to any temperature, the sophisticated calculation of the evolution of the 2 K photoluminescence energy of InN proposed by Arnaudov et al. [Phys. Rev. B 69, 115216 (2004) ], in view of determining the residual doping of thin films. From the detailed line shape modeling, we extract the full width at half maximum of the photoluminescence line which, in the first order, varies like n0.51 at low temperature. This allows us to propose a handy tool for rapid residual doping evaluation. Last, temperature and inhomogeneous broadening effects are analyzed. Ignoring the latter is shown to lead to an overestimation of the residual doping.
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78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.

Enhanced absorption in one-dimensional phononic crystals with interfacial acoustic waves

B. J. Lee and A. C. To

Appl. Phys. Lett. 95, 031911 (2009); http://dx.doi.org/10.1063/1.3182819 (3 pages) | Cited 2 times

Online Publication Date: 22 July 2009

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The present study demonstrates that interfacial acoustic waves can be excited at the interface between two phononic crystals. The interfacial acoustic wave between two phononic crystals is analogous to the surface electromagnetic wave between two photonic crystals. While past works on phononic crystals exploit the unique bandgap phenomenon in periodic structures, the present work employs the Bloch wave in the stop band to excite interfacial acoustic waves that propagate along the interface and decay into both media. As a result, the interfacial wave can be employed in an effective wave filter in which the incident acoustic wave energy is guided by the interfacial wave to the lateral direction; thus, its propagation in the normal direction is inhibited. The advantage of using two phononic crystals is to excite the interfacial wave even at normal incidence.
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68.35.Iv Acoustical properties
71.20.-b Electron density of states and band structure of crystalline solids
63.20.-e Phonons in crystal lattices

Extension of the diffuse mismatch model for thermal boundary conductance between isotropic and anisotropic materials

John C. Duda, Justin L. Smoyer, Pamela M. Norris, and Patrick E. Hopkins

Appl. Phys. Lett. 95, 031912 (2009); http://dx.doi.org/10.1063/1.3189087 (3 pages) | Cited 21 times

Online Publication Date: 22 July 2009

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This model is an extension of the diffuse mismatch model (DMM), tailored to accurately predict thermal boundary conductance (hBD) at interfaces where one material comprising the interface is characterized by high elastic anisotropy. Temperature-dependent specific heat is calculated with this vibrational model and compared to published values. Modifications to the DMM that incorporate the vibrational model are presented with predictions of hBD at a metal-graphite interface. This model slightly underestimates experimental data, as expected, as the large acoustic mismatch between metals and graphite suggests inelastic scattering, something the DMM does not take into account.
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68.35.Ja Surface and interface dynamics and vibrations
73.40.Ns Metal-nonmetal contacts
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
68.35.Gy Mechanical properties; surface strains

Reduction of dislocation density in epitaxial GaN layers by overgrowth of defect-related etch pits

J. L. Weyher, H. Ashraf, and P. R. Hageman

Appl. Phys. Lett. 95, 031913 (2009); http://dx.doi.org/10.1063/1.3171928 (3 pages) | Cited 9 times

Online Publication Date: 22 July 2009

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GaN templates grown by the metal organic chemical vapor deposition method were etched in a defect-selective molten salts eutectic and were subsequently overgrown by a GaN layer using the hydride vapor phase epitaxy (HVPE) method. Optimized conditions of etching and of HVPE growth processes resulted in a significant reduction of the dislocations density (DD). Local areas virtually free of dislocations were obtained on ∼ 50% of the surface, while the average DD was reduced from 3×109 cm−2 in the template to about 2×107 cm−2 in the HVPE-grown GaN layer. A model has been developed to explain the mechanism of reduction of the DD during the overgrowth process. The model was confirmed by the photoetching of cleaved layers.
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68.55.ag Semiconductors
81.05.Ea III-V semiconductors
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.65.Cf Surface cleaning, etching, patterning

Observation of space-separated multiexciton generation in photocurrent of Au/por-Si/p-Si structure

O. M. Sreseli, O. S. El’tsina, L. V. Belyakov, and D. N. Goryachev

Appl. Phys. Lett. 95, 031914 (2009); http://dx.doi.org/10.1063/1.3186785 (3 pages) | Cited 3 times

Online Publication Date: 24 July 2009

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Photocurrent efficiency (ratio of the number of photogenerated carriers to the number of absorbed photons) of Au/por-Si/p-Si structure in a wide spectral range was investigated. The low-energy edge of nanoporous silicon (por-Si) sensibility was determined, and the enhancement in por-Si photocurrent efficiency was observed for incident photon energies above 3.5 eV. This threshold was demonstrated to exceed twice a bandgap of most nanocrystals in por-Si. Carrier tunneling among nanocrystals through thin oxide layers was shown as well. Efficiency enhancement is explained by space-separated double-exciton generation, when a high-energy photon absorbed in a nanocrystal results in two excitons in adjacent nanocrystals.
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73.63.Bd Nanocrystalline materials
73.50.Pz Photoconduction and photovoltaic effects
73.40.Ns Metal-nonmetal contacts
73.40.Gk Tunneling
61.43.Gt Powders, porous materials
71.35.-y Excitons and related phenomena
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