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2 Apr 2007

Volume 90, Issue 14, Articles (14xxxx)

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Appl. Phys. Lett. 90, 142110 (2007); http://dx.doi.org/10.1063/1.2720640 (3 pages)

M. T. Björk, O. Hayden, H. Schmid, H. Riel, and W. Riess
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Nd3+ ion shift under domain inversion by electron beam writing in LiNbO3

P. Molina, D. Sarkar, M. O. Ramírez, J. García Solé, L. E. Bausá, B. J. García, and J. E. Muñoz Santiuste

Appl. Phys. Lett. 90, 141901 (2007); http://dx.doi.org/10.1063/1.2719036 (3 pages) | Cited 8 times

Online Publication Date: 4 April 2007

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Ferroelectric domain inversion has been obtained in Nd3+ doped lithium niobate by means of direct electron beam writing. The local effects of the polarization inversion on the optical transitions of Nd3+ ions have been studied by low temperature high resolution site selective spectroscopy. Inverted regions present different axial crystal field acting over Nd3+ ions compared with noninverted (original) regions. The results can be interpreted in terms of slight shifts of Nd3+ ions along the ferroelectric c axis within the Li+ octahedrons, as a result of the lattice rearrangement after the domain inversion processes.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization
61.80.Fe Electron and positron radiation effects
78.30.Hv Other nonmetallic inorganics

Evidence for segregation of Te in Ge2Sb2Te5 films: Effect on the “phase-change” stress

L. Krusin-Elbaum, C. Cabral, Jr., K. N. Chen, M. Copel, D. W. Abraham, K. B. Reuter, S. M. Rossnagel, J. Bruley, and V. R. Deline

Appl. Phys. Lett. 90, 141902 (2007); http://dx.doi.org/10.1063/1.2719148 (3 pages) | Cited 40 times

Online Publication Date: 4 April 2007

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The authors present direct evidence for Te segregation to the grain boundaries in chalcogenide Ge2Sb2Te5 films by using transmission electron microscopy scans with a 0.5 nm diameter focused probe. This finding is consistent with the observed impeded grain growth and with the post-transition relief of a “spikelike” stress, fully to the pretransition level. Te motion shows up in void formation below 200 °C, a pileup of Te at the surface and its loss at higher (above 400 °C) temperatures. Tuning the driving force for this segregation may be key for the optimal phase-change material design.
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64.75.-g Phase equilibria
68.35.Dv Composition, segregation; defects and impurities
81.30.Mh Solid-phase precipitation
61.72.Mm Grain and twin boundaries
68.60.Bs Mechanical and acoustical properties
68.35.Gy Mechanical properties; surface strains

Recombination dynamics of excitons in Mg0.11Zn0.89O alloy films grown using the high-temperature-annealed self-buffer layer by laser-assisted molecular-beam epitaxy

Masashi Kubota, Takeyoshi Onuma, Atsushi Tsukazaki, Akira Ohtomo, Masashi Kawasaki, Takayuki Sota, and Shigefusa F. Chichibu

Appl. Phys. Lett. 90, 141903 (2007); http://dx.doi.org/10.1063/1.2719168 (3 pages) | Cited 9 times

Online Publication Date: 4 April 2007

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Recombination dynamics of excitons in Mg0.11Zn0.89O epilayers grown by laser-assisted molecular-beam epitaxy on a ScAlMgO4 substrate were investigated. By using the MgZnO high-temperature-annealed self-buffer layer (HITAB), the value of full width at half maximum of the near-band-edge (NBE) photoluminescence (PL) peak at 3.6 eV was decreased from 133 to 94 meV at 293 K, and the intensity ratio of the NBE emission to the deep emission band centered around 2.2 eV was increased by a factor of 3. Also, the PL lifetime of the NBE peak at 293 K under the excitation density of 1 μJ/cm2 was increased from 49 to 60 ps. These results suggest that HITAB gave rise to improved alloy compositional homogeneity and reduced concentration of point defects.
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81.05.Dz II-VI semiconductors
71.35.-y Excitons and related phenomena
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths
73.61.Ga II-VI semiconductors
78.55.Et II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Dehydrogenation from 3d-transition-metal-doped NaAlH4: Prediction of catalysts

A. Blomqvist, C. Moysés Araújo, P. Jena, and R. Ahuja

Appl. Phys. Lett. 90, 141904 (2007); http://dx.doi.org/10.1063/1.2719244 (3 pages) | Cited 10 times

Online Publication Date: 4 April 2007

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A fundamental understanding of the role of catalysts in improving the kinetics and thermodynamics of hydrogen sorption in NaAlH4 is the key for using this material in hydrogen storage. The authors present a systematic theoretical study of energies needed to desorb hydrogen in 3d transition metal (Sc–Cu)-doped NaAlH4. They show that Cr and Fe atoms can be far more effective catalysts than Ti in desorbing hydrogen. The role of the 3d metal atoms in improving the thermodynamics of dehydrogenation is attributed to a significant shortening of the bond length with neighboring Al atoms.
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68.43.Mn Adsorption kinetics
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
65.40.G- Other thermodynamical quantities
84.60.-h Direct energy conversion and storage

Vapor pressure critical amplitudes from the normal boiling point

S. Velasco, F. L. Román, J. A. White, and A. Mulero

Appl. Phys. Lett. 90, 141905 (2007); http://dx.doi.org/10.1063/1.2719642 (3 pages) | Cited 3 times

Online Publication Date: 4 April 2007

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The authors propose a method to estimate the two first critical amplitudes for the vapor pressure of a fluid in terms only of the reduced pressure, Pbr = Pb/Pc, and temperature, Tbr = Tb/Tc, of the normal boiling point. The method is based on the fact that the product (1−Tr)Pr presents a maximum near the critical region. Based on a study of 43 fluids, the authors found that the reduced pressure and temperature of that maximum can be obtained from simple relations in terms of the parameter h ≡ Tbr ln Pbr/(Tbr−1). These relations are checked against additional data for 1608 fluids.
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64.70.F- Liquid-vapor transitions

Photoluminescence study of semipolar {10math1} InGaN/GaN multiple quantum wells grown by selective area epitaxy

Hongbo Yu, L. K. Lee, Taeil Jung, and P. C. Ku

Appl. Phys. Lett. 90, 141906 (2007); http://dx.doi.org/10.1063/1.2720302 (3 pages) | Cited 14 times

Online Publication Date: 5 April 2007

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Semipolar InGaN/GaN multiple quantum wells (MQWs) were fabricated on the {10math1} facets of GaN pyramidal structures by selective area epitaxy. Optical properties of the MQWs were investigated by photoluminescence (PL) in comparison with (0001) MQWs. Compared with (0001) MQWs, the internal electric field in the {10math1} MQWs was remarkably reduced, the PL peak redshifted monotonically with the increasing temperature, and the internal quantum efficiency was estimated to be improved by a factor of 3. These results suggest that the {10math1} planes are promising for improving the performance of III-nitride light emitters owing to their surface stability and suppression of polarization effects.
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78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.65.Fg Quantum wells
81.15.Kk Vapor phase epitaxy; growth from vapor phase
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)

How surface stresses lead to size-dependent mechanics of tensile deformation in nanowires

M. Ravi Shankar and Alexander H. King

Appl. Phys. Lett. 90, 141907 (2007); http://dx.doi.org/10.1063/1.2718487 (3 pages) | Cited 8 times

Online Publication Date: 6 April 2007

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It has been proposed that surface and interface stresses can modify the elastic behavior in nanomaterials such as nanowires. The authors show that surface stresses modify the tensile response of nanowires only when nonlinear elastic effects become important leading to cross terms between the applied stress and the surface stress. These effects are only significant when the radius of the nanowire is of the order of a few nanometers. The resulting alteration of tensile stiffness, though effected in part by the nonlinear elastic modulus, is particularly wrought by a modification of the stress state in the deformed nanowire.
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68.65.La Quantum wires (patterned in quantum wells)
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
62.20.D- Elasticity

Growth temperature dependence of structural properties of AlN films on ZnO (000math) substrates

Kohei Ueno, Atsushi Kobayashi, Jitsuo Ohta, and Hiroshi Fujioka

Appl. Phys. Lett. 90, 141908 (2007); http://dx.doi.org/10.1063/1.2719167 (3 pages) | Cited 12 times

Online Publication Date: 6 April 2007

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The authors have investigated substrate temperature dependence of the structural properties of AlN grown on atomically flat ZnO (000math) substrates using pulsed laser deposition. The growth of AlN on ZnO at above 500 °C proceeds in a three-dimensional mode and results in the formation of films with poor crystallinity and rough surfaces. However, reduction in the growth temperature leads to a remarkable improvement in crystalline quality and surface morphology. The authors have found that AlN films with stepped and terraced surfaces grow in the layer-by-layer mode at substrate temperatures below 300 °C. X-ray reflectivity measurements have revealed that these improvements in structural properties can be attributed to the suppression of the interfacial reactions between AlN and ZnO.
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68.55.-a Thin film structure and morphology
68.47.Fg Semiconductor surfaces
68.55.A- Nucleation and growth
81.15.Fg Pulsed laser ablation deposition
68.35.Ct Interface structure and roughness

Monodisperse star-shaped compound and its blend in uncapped polyfluorene matrices as the active materials for high-performance pure blue light-emitting devices

Rui Zhu, Wen-Yong Lai, Hong-Yu Wang, Nan Yu, Wei Wei, Bo Peng, Wei Huang, Lin-Tao Hou, Jun-Biao Peng, and Yong Cao

Appl. Phys. Lett. 90, 141909 (2007); http://dx.doi.org/10.1063/1.2718272 (3 pages) | Cited 7 times

Online Publication Date: 6 April 2007

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The authors have demonstrated high-performance blue light-emitting device based on a monodisperse star-shaped compound. The single-layer device fabricated with solution procedure exhibited a turn-on voltage of 3.5 V, maximum luminance of 5545 cd m−2, luminance efficiency (LE) of 1.09 cd A−1, and external quantum efficiency (QEext) of 1.51% with the CIE coordinate of (0.16, 0.12). The authors also blended this highly efficient monodisperse compound in uncapped polyfluorene matrices. Enhanced performances of the uncapped polyfluorenes were obtained with improved electroluminescent stability. A maximum LE of 2.33 cd A−1 and QEext of 2.95% were achieved.
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85.60.Jb Light-emitting devices
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