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8 Sep 2008

Volume 93, Issue 10, Articles (10xxxx)

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Appl. Phys. Lett. 93, 101905 (2008); http://dx.doi.org/10.1063/1.2977760 (3 pages)

Mei Wang, Yinwei Li, Tian Cui, Yanming Ma, and Guangtian Zou
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Optical properties of GaN/AlGaN quantum wells grown on nonpolar substrates

T. J. Badcock, P. Dawson, M. J. Kappers, C. McAleese, J. L. Hollander, C. F. Johnston, D. V. Sridhara Rao, A. M. Sanchez, and C. J. Humphreys

Appl. Phys. Lett. 93, 101901 (2008); http://dx.doi.org/10.1063/1.2971205 (3 pages) | Cited 16 times

Online Publication Date: 8 September 2008

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In this paper we report on the optical properties of a series of GaN/AlGaN multiple quantum well structures grown on a-plane (11math0) GaN, which had been deposited on r-plane (1math02) sapphire substrates, compared to a reference GaN template of the same orientation. The low temperature photoluminescence spectrum of the template layer is dominated by two emission bands, which we attribute to recombination involving excitons in the bulk of the layer and electrons and holes trapped at basal-plane stacking faults, designated X1 and X2, respectively. The photoluminescence spectra from the quantum well structures show similar emission bands except that both X1 and X2 shift to higher energy with decreasing quantum well thickness. The shift to higher energy is due to the effects of quantum confinement on carriers trapped at the stacking faults that intersect the quantum wells, as well as those excitons that are localized within the quantum wells. This assignment is based partly on excitation spectroscopy that reveals exciton transitions associated with electrons from the n = 1 and n = 2 quantum well confined states.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
71.35.-y Excitons and related phenomena

On electromechanical stability of dielectric elastomers

R. Díaz-Calleja, E. Riande, and M. J. Sanchis

Appl. Phys. Lett. 93, 101902 (2008); http://dx.doi.org/10.1063/1.2972124 (3 pages) | Cited 32 times

Online Publication Date: 8 September 2008

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This paper studies the competition between electric and mechanical force fields simultaneously applied to a polar elastomer that can lead to electric breakdown. The analysis of the system, performed assuming that the free energy of the elastomer is simply the addition of polarizing and stretching energies leads to the classical “thermodynamic” (in this case “electromechanical”) stability.
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77.22.Jp Dielectric breakdown and space-charge effects
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Enhanced nonradiative Auger recombination in p-type modulation doped InAs/GaAs quantum dots

Y. D. Jang, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, J. Park, D. Lee, H. Y. Liu, M. Hopkinson, R. A. Hogg, and A. D. Andreev

Appl. Phys. Lett. 93, 101903 (2008); http://dx.doi.org/10.1063/1.2975961 (3 pages) | Cited 5 times

Online Publication Date: 8 September 2008

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The photoluminescence efficiency and carrier recombination time of p-type modulation doped InAs/GaAs quantum dots (QDs) have been measured as a function of doping density. At 10 K the carrier lifetime decreases from 1200 to 350 ps over the doping range of 0 and 30 acceptors/QD. This behavior is attributed to an enhancement of the Auger-type recombination due to the presence of extrinsic holes in the QDs. The hole density dependence of the Auger process is found to be weaker than in bulk semiconductors and quantum wells (QWs).
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73.63.Kv Quantum dots
73.63.Hs Quantum wells
78.55.Cr III-V semiconductors
61.72.uj III-V and II-VI semiconductors
79.20.Fv Electron impact: Auger emission
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Vacancy formation during oxidation of silicon crystal surface

M. Suezawa, Y. Yamamoto, M. Suemitsu, N. Usami, and I. Yonenaga

Appl. Phys. Lett. 93, 101904 (2008); http://dx.doi.org/10.1063/1.2979708 (3 pages)

Online Publication Date: 8 September 2008

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To study the formation of interstitials and vacancies during oxidation of silicon crystals, we applied a quenching method, namely, oxidation at high temperatures in mixed gases of water vapor and H2 followed by the quenching into water. Contrary to our expectation, the vacancy concentration was higher than that of the thermal equilibrium during the short duration of oxidation.
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61.72.jd Vacancies
81.65.Mq Oxidation
81.40.Gh Other heat and thermomechanical treatments

Origin of hardness in WB4 and its implications for ReB4, TaB4, MoB4, TcB4, and OsB4

Mei Wang, Yinwei Li, Tian Cui, Yanming Ma, and Guangtian Zou

Appl. Phys. Lett. 93, 101905 (2008); http://dx.doi.org/10.1063/1.2977760 (3 pages) | Cited 41 times

Online Publication Date: 9 September 2008

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First-principles calculations were performed on the superhard material, WB4 (Vicker hardness exceeding 46 GPa), to reveal the origin of its high hardness. Our simulated lattice parameters, bulk modulus, and hardness are in excellent agreement with the experimental data. A three-dimensional B network with a peculiar B2 dimer along the z-axis and a xy planar honeycomb B sublattice is uncovered to be mainly responsible for the high hardness. We further predicted that five other transition metal B compounds (TMB4, TM = Re, Mo, Ta, Os, and Tc) within the WB4 structure are potential superhard materials.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.Qp Friction, tribology, and hardness
62.20.de Elastic moduli
61.66.Fn Inorganic compounds

Kinetic Monte Carlo study of metal organic chemical vapor deposition growth mechanism of GaSb quantum dots

K. Fu and Y. Fu

Appl. Phys. Lett. 93, 101906 (2008); http://dx.doi.org/10.1063/1.2981515 (3 pages) | Cited 3 times

Online Publication Date: 9 September 2008

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The growth dynamics of self-assembled GaSb quantum dots (QDs) on GaAs substrate in the strain-induced Stranski–Krastanov mode was investigated using kinetic Monte Carlo method. The strain induced by the lattice mismatch between the epitaxial material and the substrate was shown to be directly responsible for the QD formation and the transition of growth mode from two dimensional to three dimensional.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.65.-k Low-dimensional, mesoscopic, nanoscale and other related systems: structure and nonelectronic properties

Bond strength and interface energy between Pd membranes and TiAl supports

H. R. Gong, Y. H. He, and B. Y. Huang

Appl. Phys. Lett. 93, 101907 (2008); http://dx.doi.org/10.1063/1.2970966 (3 pages) | Cited 9 times

Online Publication Date: 10 September 2008

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Intermetallic TiAl alloy is proposed as a promising support for Pd membranes. First principles calculations reveal that coherent Pd/TiAl interfaces possess high values of bond strengths. Calculations also show that Ti-terminated (100) Pd/(100) TiAl and (110) Pd/(110) TiAl interfaces are energetically favorable with negative interface energies of about −3.1 J/m2, and that the bond strengths of Pd–Ti are bigger than those of Pd–Al. In addition, densities of states calculations suggest that a stronger chemical bonding is formed in the Pd/TiAl interface than corresponding Pd or TiAl bulks, which agrees well with similar experimental observations in literature.
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73.20.At Surface states, band structure, electron density of states
71.20.Be Transition metals and alloys
71.15.-m Methods of electronic structure calculations

Site-controlled growth and luminescence of InAs quantum dots using in situ Ga-assisted deoxidation of patterned substrates

P. Atkinson, S. Kiravittaya, M. Benyoucef, A. Rastelli, and O. G. Schmidt

Appl. Phys. Lett. 93, 101908 (2008); http://dx.doi.org/10.1063/1.2980445 (3 pages) | Cited 33 times

Online Publication Date: 10 September 2008

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Site-controlled growth of single and pairs of InAs quantum dots is demonstrated on ex situ electron-beam patterned (001) GaAs substrates using in situ Ga-assisted deoxidation prior to overgrowth. 6–8 ML of gallium deposited at a substrate temperature of 460 °C in the absence of arsenic followed by a brief anneal under arsenic is used to remove the surface oxide without damaging a pattern consisting of ∼ 100 nm wide, ∼ 20 nm deep holes. Single dot luminescence is shown from a dilute array (10 μm spacing) of such site-controlled dots, located only 8 nm from the regrowth interface.
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68.65.Hb Quantum dots (patterned in quantum wells)
78.67.Hc Quantum dots
61.72.Cc Kinetics of defect formation and annealing
78.55.Cr III-V semiconductors

Plasmon resonances and electron phase shifts near Au nanospheres

T. C. Petersen, M. Bosman, V. J. Keast, and G. R. Anstis

Appl. Phys. Lett. 93, 101909 (2008); http://dx.doi.org/10.1063/1.2980505 (3 pages) | Cited 5 times

Online Publication Date: 10 September 2008

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Quantitative phase maps from Au and polystyrene nanospheres were obtained using the transport of intensity equation approach for phase retrieval in the electron microscope. These were compared to surface plasmon maps, as generated by electron energy loss spectroscopy. Large phase excursions were observed in the vacuum regions surrounding the Au nanospheres, yet not near adjacent polystyrene nanospheres. It is proposed that these phase excursions are associated with surface plasmon excitation. These results suggest that phase retrieval offers an alternative means for sensing and interpreting the optical response of nanostructured metals.
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73.22.Lp Collective excitations
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
79.20.Uv Electron energy loss spectroscopy

Germanium substrate loss during low temperature annealing and its influence on ion-implanted phosphorous dose loss

N. Ioannou, D. Skarlatos, C. Tsamis, C. A. Krontiras, S. N. Georga, A. Christofi, and D. S. McPhail

Appl. Phys. Lett. 93, 101910 (2008); http://dx.doi.org/10.1063/1.2981522 (3 pages) | Cited 10 times

Online Publication Date: 10 September 2008

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In this work we demonstrate germanium loss from uncapped substrates during low temperature drive-in annealing in inert ambient. An Arrhenius law with an activation energy of 2.03 eV describes the measured loss rate of germanium as a function of temperature. Accurate simulations of implanted phosphorous profiles in nonpassivated substrates have been performed considering the extracted germanium loss rate. A capping layer on the germanium surface reduces phosphorous dose loss, with Si3N4 being more efficient than SiO2. The capping layer material also affects the extent of dopant concentration-dependent diffusion for high dose phosphorous implantation.
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61.72.sd Impurity concentration
61.72.uf Ge and Si
61.72.Cc Kinetics of defect formation and annealing
77.55.-g Dielectric thin films
66.30.J- Diffusion of impurities

Nature of heterointerfaces in GaAs/InAs and InAs/GaAs axial nanowire heterostructures

Mohanchand Paladugu, Jin Zou, Ya-Nan Guo, Xin Zhang, Yong Kim, Hannah J. Joyce, Qiang Gao, H. Hoe Tan, and C. Jagadish

Appl. Phys. Lett. 93, 101911 (2008); http://dx.doi.org/10.1063/1.2978959 (3 pages) | Cited 20 times

Online Publication Date: 11 September 2008

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The structural and compositional characteristics of heterointerfaces of Au-catalyzed GaAs/InAs and InAs/GaAs axial nanowire heterostructures were comprehensively investigated by transmission electron microscopy. It has been found that the GaAs/InAs interface is not sharp and contains an InGaAs transition segment, and in contrast, the InAs/GaAs interface is atomically sharp. This difference in the nature of heterointerfaces can be attributed to the difference in the affinity of the group III elements with the catalyst material.
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73.40.Kp III-V semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
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)
68.37.Og High-resolution transmission electron microscopy (HRTEM)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
68.65.La Quantum wires (patterned in quantum wells)

Fourth-order shear elastic constant assessment in quasi-incompressible soft solids

Mathieu Rénier, Jean-Luc Gennisson, Christophe Barrière, Daniel Royer, and Mathias Fink.

Appl. Phys. Lett. 93, 101912 (2008); http://dx.doi.org/10.1063/1.2979875 (3 pages) | Cited 10 times

Online Publication Date: 11 September 2008

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In isotropic quasi-incompressible media, an expression of the elastic energy density has been developed as a function of the second-, third-, and fourth-order elastic constants (respectively μ, A, D). Thus the shear nonlinearity parameter βS depends only on these coefficients. In this letter βS is measured using finite amplitude plane shear waves in agar-gelatin based phantoms. Combining the results with recently published measurements of μ and A on the same phantoms, the fourth-order shear elastic constant D is found to be of the order of 10 kPa and thus of the same order of magnitude as μ and A.
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87.63.D- Ultrasonography
87.19.R- Mechanical and electrical properties of tissues and organs

Atomic structure of the interface between SrTiO3 thin films and Si(001) substrates

Shao-Bo Mi, Chun-Lin Jia, Venu Vaithyanathan, Lothar Houben, Jürgen Schubert, Darrell G. Schlom, and Knut Urban

Appl. Phys. Lett. 93, 101913 (2008); http://dx.doi.org/10.1063/1.2981524 (3 pages) | Cited 11 times

Online Publication Date: 11 September 2008

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The structure of the SrTiO3/Si interface is determined by high-angle annular dark field imaging in combination with a recently developed technique based on aberration-corrected high-resolution transmission electron microscopy. At the interface, a monolayer of SrO faces the terminating plane of silicon. In this monolayer, the strontium atoms lie above the face-center of four silicon atoms in the terminating plane, and the oxygen atoms are located directly above the terminating silicon atoms. This structure, which is the dominant type of interface structure observed in this system, agrees with one of the interface structures predicted by first-principles calculations.
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68.35.Ct Interface structure and roughness
68.37.Og High-resolution transmission electron microscopy (HRTEM)

Quantification of porosity and deposition rate of nanoporous films grown by oblique-angle deposition

D. J. Poxson, F. W. Mont, M. F. Schubert, J. K. Kim, and E. F. Schubert

Appl. Phys. Lett. 93, 101914 (2008); http://dx.doi.org/10.1063/1.2981690 (3 pages) | Cited 24 times

Online Publication Date: 11 September 2008

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We propose an analytic model that accurately predicts the porosity and deposition rate of nanoporous films grown by oblique-angle deposition. The model employs a single fitting parameter and takes into account geometrical factors as well as surface diffusion. We have determined the porosity and deposition rate from the measured refractive index and thickness of SiO2 and indium tin oxide nanoporous films deposited at various incident angles. Comparison of experimental data with the model reveals excellent agreement. The theoretical model allows for the predictive control of refractive index, porosity, and deposition rate for a wide range of deposition angles and materials.
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81.05.Rm Porous materials; granular materials
78.66.Nk Insulators
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.aj Insulators
68.35.Fx Diffusion; interface formation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Plastic deformation and optical behavior of high-purity synthetic diamond crystal subjected to high stress load at room temperature

H. Sumiya, Y. Nakamoto, K. Shimizu, and H. Kanda

Appl. Phys. Lett. 93, 101915 (2008); http://dx.doi.org/10.1063/1.2978070 (3 pages) | Cited 2 times

Online Publication Date: 12 September 2008

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The optical behavior around the culet of a diamond anvil made of high-purity and defect-free synthetic diamond crystal, which was plastically deformed at room temperature, was investigated by cathodoluminescence spectroscopy. It was found that the free exciton peaks weaken while the A-band and 2BD bands appear at the culet center where plastic deformation occurred. It was demonstrated that the free exciton peaks near the edge of the culet shift to the long wavelength side, indicating that the band structure of the peripheral areas of the culet changes because of residual strain caused by the plastic deformation in the culet center.
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81.40.Lm Deformation, plasticity, and creep
61.72.Bb Theories and models of crystal defects
78.60.Hk Cathodoluminescence, ionoluminescence
71.35.-y Excitons and related phenomena
78.20.-e Optical properties of bulk materials and thin films
62.20.fq Plasticity and superplasticity

Size-dependent mechanical properties of molybdenum nanopillars

Ju-Young Kim and Julia R. Greer

Appl. Phys. Lett. 93, 101916 (2008); http://dx.doi.org/10.1063/1.2979684 (3 pages) | Cited 26 times

Online Publication Date: 12 September 2008

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We report the deformation behavior of single crystalline molybdenum nanopillars in uniaxial compression, which exhibits a strong size effect called the “smaller is stronger” phenomenon. We show that higher strengths arise from the increase in the yield strength rather than through postyield strain hardening. We find the yield strength at nanoscale to depend strongly on sample size and not on the initial dislocation density, a finding strikingly different from that of the bulk metal.
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62.25.-g Mechanical properties of nanoscale systems
81.07.Bc Nanocrystalline materials
81.40.Lm Deformation, plasticity, and creep
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
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