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6 Apr 2009

Volume 94, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 94, 141901 (2009); http://dx.doi.org/10.1063/1.3111813 (3 pages)

G. S. Huang (黄高山), S. Kiravittaya, V. A. Bolaños Quiñones, F. Ding (丁飞), M. Benyoucef, A. Rastelli, Y. F. Mei (梅永丰), and O. G. Schmidt
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Optical properties of rolled-up tubular microcavities from shaped nanomembranes

G. S. Huang (黄高山), S. Kiravittaya, V. A. Bolaños Quiñones, F. Ding (丁飞), M. Benyoucef, A. Rastelli, Y. F. Mei (梅永丰), and O. G. Schmidt

Appl. Phys. Lett. 94, 141901 (2009); http://dx.doi.org/10.1063/1.3111813 (3 pages) | Cited 21 times

Online Publication Date: 6 April 2009

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Tubular optical microcavities have been fabricated by releasing prestressed SiO/SiO2 bilayer nanomembranes from polymer sacrificial layers, and their geometrical structure is well controlled by defining the shape of nanomembranes via photolithography. Optical measurements at room temperature demonstrate that resonant modes of microtubular cavities rolled up from circular shapes can be tuned in peak energy and relative intensity along the tube axes compared to those from square patterns. The resonant modes shift to higher energy with decreasing number of tube wall rotations and thickness, which fits well to finite-difference time-domain simulations. Polarization resolved measurements of the resonant modes indicate that their polarization axes are parallel to the tube axis, independent of the polarization of the excitation laser.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials

Room temperature photoluminescence of tensile-strained Ge/Si0.13Ge0.87 quantum wells grown on silicon-based germanium virtual substrate

Yanghua Chen, Cheng Li, Zhiwen Zhou, Hongkai Lai, Songyan Chen, Wuchang Ding, Buwen Cheng, and Yude Yu

Appl. Phys. Lett. 94, 141902 (2009); http://dx.doi.org/10.1063/1.3114408 (3 pages) | Cited 8 times

Online Publication Date: 6 April 2009

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We report a room temperature study of the direct band gap photoluminescence of tensile-strained Ge/Si0.13Ge0.87 multiple quantum wells grown on Si-based germanium virtual substrates by ultrahigh vacuum chemical vapor deposition. Blueshifts of the luminescence peak energy from the Ge quantum wells in comparison with the Ge virtual substrate are in good agreement with the theoretical prediction when we attribute the luminescence from the quantum well to the cΓ1-HH1 direct band transition. The reduction in direct band gap in the tensile strained Ge epilayer and the quantum confinement effect in the Ge/Si0.13Ge0.87 quantum wells are directly demonstrated by room temperature photoluminescence.
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78.67.De Quantum wells
71.20.Mq Elemental semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
73.21.Fg Quantum wells
81.07.St Quantum wells
78.55.Ap Elemental semiconductors
81.05.Cy Elemental semiconductors

Ab initio molecular dynamics study of the hydrogen-deuterium exchange in bulk lithiumborohydride (LiBH4)

M. Ramzan and R. Ahuja

Appl. Phys. Lett. 94, 141903 (2009); http://dx.doi.org/10.1063/1.3115032 (2 pages) | Cited 6 times

Online Publication Date: 6 April 2009

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The hydrogen storage is still a challenge for mobile applications. The diffusion of hydrogen in solids is considered as a general model system but there is limited knowledge available for the dynamics of hydrogen in complex hydrides. In this letter, we present a systematic approach to study the hydrogen-deuterium exchange in bulk LiBH4 based on ab initio molecular dynamics. Our results predict the single hydrogen (deuterium) exchange in LiBH4, which supports the experimental results. The self-diffusion constant 〈D〉 of deuterium in LiBH4 is around 1.42×10−8 m2 s−1.
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84.60.-h Direct energy conversion and storage
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
66.30.H- Self-diffusion and ionic conduction in nonmetals

Structural and optical investigations of periodically polarity inverted ZnO heterostructures on (0001) Al2O3

J. S. Park, T. Goto, S. K. Hong, S. H. Lee, J. W. Lee, T. Minegishi, S. H. Park, J. H. Chang, D. C. Oh, J. Y. Lee, and T. Yao

Appl. Phys. Lett. 94, 141904 (2009); http://dx.doi.org/10.1063/1.3114989 (3 pages) | Cited 4 times

Online Publication Date: 7 April 2009

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We report the structural and optical properties of one-dimensional grating of ZnO consisting of periodically polarity inverted structures on (0001) Al2O3 substrates. The inversion domain boundaries (IDBs) between the Zn- and the O-polar ZnO regions were clearly observed by transmission electronic microscopy. The investigation of spatially resolved local photoluminescence (PL) revealed strong excitonic emission at the interfacial region including the IDBs. The possible mechanism of strong PL has been discussed by the consideration of atomic configuration and carrier collection including its lifetime and diffusion process in Zn- and O-polar regions. Therefore the authors conclude that the IDBs can be active for the strong emission not a nonradiative center.
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73.40.Lq Other semiconductor-to-semiconductor contacts, p-n junctions, and heterojunctions
66.30.H- Self-diffusion and ionic conduction in nonmetals
78.55.Et II-VI semiconductors
78.66.Hf II-VI semiconductors
73.61.Ga II-VI semiconductors
68.55.-a Thin film structure and morphology

Direct observation of ion exchange in mechanically activated LiH+MgB2 system using ultrahigh field nuclear magnetic resonance spectroscopy

Jian Zhi Hu, Ja Hun Kwak, Zhenguo Yang, Xuefei Wan, and Leon L. Shaw

Appl. Phys. Lett. 94, 141905 (2009); http://dx.doi.org/10.1063/1.3110966 (3 pages) | Cited 6 times

Online Publication Date: 8 April 2009

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The LiBH4+MgH2 system has great potential for hydrogen vehicle applications. In this study, the reported solid-state hydrogenation system made of LiH+1/2MgB2 has been investigated using ultrahigh field nuclear magnetic resonance spectroscopy. It is found that Mg–Li ion exchange occurs within MgB2 during ball milling to form a compound of (Mg1−xLi2x)B2, which facilitates the formation of LiBH4 in the subsequent hydriding reaction. This discovery offers a scientific foundation for investigating the detailed mechanisms of solid-state hydrogenation and dehydrogenation of the LiBH4+MgH2 system in the future.
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82.30.Hk Chemical exchanges (substitution, atom transfer, abstraction, disproportionation, and group exchange)
84.60.-h Direct energy conversion and storage
76.60.-k Nuclear magnetic resonance and relaxation

The influence of nanoscale atomic-layer-deposited alumina coating on the fatigue behavior of polycrystalline silicon thin films

M. Budnitzki and O. Pierron

Appl. Phys. Lett. 94, 141906 (2009); http://dx.doi.org/10.1063/1.3112565 (3 pages) | Cited 6 times

Online Publication Date: 8 April 2009

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The influence of atomic-layer-deposited alumina surface coatings on the fatigue of polycrystalline silicon thin films was investigated. Tests were performed on 2-μm-thick notched cantilever-beam structures actuated at resonance ( ∼ 40 kHz) that were coated with ∼ 20 nm of alumina deposited at 100 °C. The coated devices show a drastically different frequency evolution behavior as compared to uncoated specimens, while no surface oxide thickening during cycling is observed in bright-field transmission electron microscopy. Both results are consistent with a surface degradation mechanism for polycrystalline silicon fatigue such as the reaction-layer mechanism. The improved frequency evolution has the potential of increasing the performance of resonator-based microelectromechanical system sensors.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.me Fatigue
62.25.-g Mechanical properties of nanoscale systems

Optical and electronic properties of one-dimensional Ca3Co2O6 thin films: Influence of the oxygen pressure

R. Moubah, S. Colis, G. Schmerber, J. Petersen, and A. Dinia

Appl. Phys. Lett. 94, 141907 (2009); http://dx.doi.org/10.1063/1.3114373 (3 pages) | Cited 7 times

Online Publication Date: 8 April 2009

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Thin films of Ca3Co2O6 were fabricated on c-cut sapphire substrates by pulsed laser deposition. The films are polycrystalline and can be grown in a large interval of oxygen pressure with significant influence on the film texture. X-ray photoelectron spectroscopy analyses indicated that only Co3+ ions are present in our samples for both octahedral and trigonal prismatic sites. The band gap measured by ultraviolet-visible light absorption experiments was found about 1.35 eV and slightly larger when the oxygen partial pressure during the deposition is increased from 10−3 to 7×10−2 mbar. Temperature dependent transport measurements showed a semiconducting behavior of the films and the associated band gap was found similar to the one evaluated by optical measurements and reported by theoretical studies.
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78.66.Nk Insulators
78.40.Ha Other nonmetallic inorganics
71.20.-b Electron density of states and band structure of crystalline solids
79.60.Dp Adsorbed layers and thin films
81.15.Fg Pulsed laser ablation deposition
68.55.jm Texture

Electronic and structural properties of molybdenum thin films as determined by real-time spectroscopic ellipsometry

J. D. Walker, H. Khatri, V. Ranjan, Jian Li, R. W. Collins, and S. Marsillac

Appl. Phys. Lett. 94, 141908 (2009); http://dx.doi.org/10.1063/1.3117222 (3 pages) | Cited 6 times

Online Publication Date: 8 April 2009

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Real-time spectroscopic ellipsometry (RTSE) is shown to be an effective contactless probe of radio frequency magnetron sputtered molybdenum thin films used as the back electrode in chalcopyrite [Cu(In,Ga)Se2] solar cells. A series of Mo thin films was sputtered onto soda-lime glass substrates at Ar pressures ranging from 4 to 20 mTorr. RTSE measurements reveal how Ar pressure affects the nucleation and growth mechanisms that influence the films’ ultimate grain structure and properties. Determinations of the free electron relaxation times at optical frequencies reveal that higher pressures lead to a smaller average grain size and increased void volume fraction.
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68.55.A- Nucleation and growth
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
72.15.Lh Relaxation times and mean free paths
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
73.61.At Metal and metallic alloys
81.15.Cd Deposition by sputtering

SiGe nanorings by ultrahigh vacuum chemical vapor deposition

C.-H. Lee, Y.-Y. Shen, C. W. Liu, S. W. Lee, B.-H. Lin, and C.-H. Hsu

Appl. Phys. Lett. 94, 141909 (2009); http://dx.doi.org/10.1063/1.3116619 (3 pages) | Cited 8 times

Online Publication Date: 9 April 2009

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Formation of SiGe nanorings from Si capped Si0.1Ge0.9 quantum dots (QDs) grown at 500 °C by ultrahigh vacuum chemical vapor deposition was investigated. SiGe nanorings have average diameter, width, and depth of 185, 30, and 9 nm, respectively. Based on both Raman and x-ray diffraction results, the formation of SiGe nanorings can be attributed to Ge outdiffusion from central SiGe QDs during in situ annealing. Moreover, the depth of SiGe nanorings can be controlled by Si cap thickness. The Si cap is essential for nanorings formation.
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81.16.-c Methods of micro- and nanofabrication and processing
61.05.C- X-ray diffraction and scattering
61.72.Cc Kinetics of defect formation and annealing
66.30.H- Self-diffusion and ionic conduction in nonmetals
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.65.Hb Quantum dots (patterned in quantum wells)

On the influence of elastic strain on the accommodation of carbon atoms into substitutional sites in strained Si:C layers grown on Si substrates

N. Cherkashin, M. J. Hÿtch, F. Houdellier, F. Hüe, V Paillard, A. Claverie, A. Gouyé, O. Kermarrec, D. Rouchon, M. Burdin, and P. Holliger

Appl. Phys. Lett. 94, 141910 (2009); http://dx.doi.org/10.1063/1.3116648 (3 pages) | Cited 4 times

Online Publication Date: 9 April 2009

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Measurements of strain and composition are reported in tensile strained 10- and 30-nm-thick Si:C layers grown by chemical vapor deposition on a Si (001) substrate. Total carbon concentration varies from 0.62% to 1.97%. Strain measurements were realized by high-resolution x-ray diffraction, convergent-beam electron diffraction, and geometric phase analysis of high-resolution transmission electron microscopy cross-sectional images. Raman spectroscopy was used for the deduction of the substitutional concentration. We demonstrate that in addition to the growth conditions, strain accumulating during deposition, thus depending on a layer thickness, has an influence on the final substitutional carbon composition within a strained Si:C layer.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.30.Hv Other nonmetallic inorganics
73.61.Le Other inorganic semiconductors
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
62.20.F- Deformation and plasticity
81.40.Lm Deformation, plasticity, and creep

Band offsets at the Ge/GeO2 interface through hybrid density functionals

Peter Broqvist, Jan Felix Binder, and Alfredo Pasquarello

Appl. Phys. Lett. 94, 141911 (2009); http://dx.doi.org/10.1063/1.3116612 (3 pages) | Cited 20 times

Online Publication Date: 10 April 2009

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Band offsets of the Ge/GeO2 interface are calculated through a hybrid density functional scheme. We first generate a model of disordered GeO2 through ab initio molecular dynamics to describe the oxide component. For addressing the interface, we then consider an atomistic model in which amorphous GeO2 is connected to crystalline Ge through a suboxide transition region showing regular structural parameters. The band offsets are obtained through the application of an alignment scheme, which reproduces the experimental band gaps of the interface components. The calculated valence band offset of 3.7 eV favors the low-energy side of the range of measured offsets.
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73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
71.20.Nr Semiconductor compounds
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations

Experimental and computational studies on structural transitions in the LiBH4–LiI pseudobinary system

H. Oguchi, M. Matsuo, J. S. Hummelshøj, T. Vegge, J. K. Nørskov, T. Sato, Y. Miura, H. Takamura, H. Maekawa, and S. Orimo

Appl. Phys. Lett. 94, 141912 (2009); http://dx.doi.org/10.1063/1.3117227 (3 pages) | Cited 21 times

Online Publication Date: 10 April 2009

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Structural transition properties of the LiBH4+xLiI (x = 0–1.00) pseudobinary system were examined by powder x-ray diffraction and differential scanning calorimetry combined with periodic density functional theory calculations. We experimentally and computationally confirmed the stabilization of the high-temperature [hexagonal, lithium super(fast-)ionic conduction] phase of LiBH4 with x = 0.33 and 1.00, and the results also imply the existence of intermediate phases with x = 0.07–0.20. The studies are of importance for further development of LiBH4 and the derived hydrides as solid-state electrolytes.
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64.70.K- Solid-solid transitions
81.70.Pg Thermal analysis, differential thermal analysis (DTA), differential thermogravimetric analysis
82.45.Gj Electrolytes
66.30.H- Self-diffusion and ionic conduction in nonmetals
65.40.gk Electrochemical properties

Surface effects on buckling of nanowires under uniaxial compression

Gang-Feng Wang and Xi-Qiao Feng

Appl. Phys. Lett. 94, 141913 (2009); http://dx.doi.org/10.1063/1.3117505 (3 pages) | Cited 54 times

Online Publication Date: 10 April 2009

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Based on the conventional Euler buckling model, uniaxial compression tests have been utilized recently to measure the mechanical properties of nanowires. However, owing to the increasing ratio of surface area to bulk at nanoscale, the influence of surface energy becomes prominent and should be taken into consideration. In this letter, an analytical relation is given for the critical force of axial buckling of a nanowire by accounting for both the effects of surface elasticity and residual surface tension. This study might be helpful to characterize the mechanical properties of nanowires or design nanobeam-based devices in a wide range of applications.
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62.23.Hj Nanowires
62.20.mq Buckling
81.40.Lm Deformation, plasticity, and creep
68.35.Md Surface thermodynamics, surface energies
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.35.Gy Mechanical properties; surface strains

Flow serration and shear-band propagation in bulk metallic glasses

H. M. Chen, J. C. Huang, S. X. Song, T. G. Nieh, and J. S. C. Jang

Appl. Phys. Lett. 94, 141914 (2009); http://dx.doi.org/10.1063/1.3117508 (3 pages) | Cited 31 times

Online Publication Date: 10 April 2009

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Flow serration in bulk metallic glasses (BMGs) was analyzed using high-sensitivity strain gauges. Based on the displacement-time profile for one serration, shear-band propagating speed was determined and found to be insensitive to the applied strain rates. The disappearance of serration at high strain rates is a result that the signal of displacement burst was overwhelmed by the applied strain rate. In comparison with the ductile Pd-based and brittle Mg-based BMGs, the ductility of BMGs appears to be closely related to the dynamics during shear-band propagation.
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81.05.Kf Glasses (including metallic glasses)
61.43.Fs Glasses
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.mj Brittleness
81.40.Lm Deformation, plasticity, and creep
62.20.fk Ductility, malleability
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