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23 Jun 2008

Volume 92, Issue 25, Articles (25xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 92, 254102 (2008); http://dx.doi.org/10.1063/1.2945893 (3 pages)

M. Trinker, S. Groth, S. Haslinger, S. Manz, T. Betz, S. Schneider, I. Bar-Joseph, T. Schumm, and J. Schmiedmayer
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Anisotropic damping of longitudinal optical phonon-plasmon coupling modes of InN films

Yoshihiro Ishitani, Masayuki Fujiwara, Xinqiang Wang, Song-Bek Che, and Akihiko Yoshikawa

Appl. Phys. Lett. 92, 251901 (2008); http://dx.doi.org/10.1063/1.2951614 (3 pages) | Cited 7 times

Online Publication Date: 23 June 2008

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The energy broadenings of the higher energy branch of the longitudinal optical (LO) phonon-plasmon coupling modes for E1(LO) and A1(LO) are analyzed for InN films by infrared reflectance and spectroscopic ellipsometry. Larger broadening for the vibration parallel to c of A1(LO)-plasmon coupling is found with the decrease in electron density. This phenomenon is caused by the plasmon lifetime anisotropy, and is attributed to the reduction in crystal defects causing isotropic carrier scattering and the remaining of defects along the c axis such as threading dislocations and columnar grain boundaries.
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63.20.kk Phonon interactions with other quasiparticles
78.66.Fd III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
61.72.Mm Grain and twin boundaries

Three-dimensional morphology of nanoporous gold

Takeshi Fujita, Li-Hua Qian, Koji Inoke, Jonah Erlebacher, and Ming-Wei Chen

Appl. Phys. Lett. 92, 251902 (2008); http://dx.doi.org/10.1063/1.2948902 (3 pages) | Cited 50 times

Online Publication Date: 23 June 2008

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We report transmission electron tomography of nanoporous gold fabricated by chemically dealloying Au35Ag65 films. A number of algorithms were employed to quantitatively characterize the complex three-dimensional nanoporous structure. It was found that gold ligaments and nanopore channels are topologically and morphologically equivalent, i.e., they are inverses of each other in three-dimensional space. Statistical analysis reveals that this bicontinuous nanostructured material is actually quasiperiodic and has, on average, a near zero surface curvature. These quantitative measurements will help in understanding the structural stability of nanoporous gold and in modeling its physical and chemical performances.
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61.43.Gt Powders, porous materials
61.46.-w Structure of nanoscale materials

Mechanical behavior of ultralow-dielectric-constant mesoporous amorphous silica

M. Rauf Gungor, James J. Watkins, and Dimitrios Maroudas

Appl. Phys. Lett. 92, 251903 (2008); http://dx.doi.org/10.1063/1.2949556 (3 pages) | Cited 2 times

Online Publication Date: 23 June 2008

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Using molecular-dynamics simulations, we examine the mechanical behavior of mesoporous amorphous silicas that are considered as ultralow-dielectric-constant materials in microelectronics. We study structures with a regular array of spherical pores and densities between 88% and 72% of the amorphous silica normal density. We find that the Young modulus depends on density according to a sublinear power-law scaling relationship and decreases with decreasing mesopore size. Upon uniaxial compression, an elastic instability is triggered in structures with less-than-critical density or mesopore size.
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81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.de Elastic moduli
77.22.Ch Permittivity (dielectric function)
61.43.Gt Powders, porous materials

Thermally tunable polymer microlenses

Xian Huang, Chao-Min Cheng, Li Wang, Bin Wang, Chih-Chuan Su, Mon-Shu Ho, Philip R. LeDuc, and Qiao Lin

Appl. Phys. Lett. 92, 251904 (2008); http://dx.doi.org/10.1063/1.2945646 (3 pages) | Cited 9 times

Online Publication Date: 23 June 2008

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Polymer microlenses capable of using heat to control its focal length are presented. The microlenses are created by exposing droplets of the polymer SU-8 to UV light. By altering the temperature of the microlenses via on-chip heating, their curvature and focal length are actively controlled without mechanical movements. By directly and indirectly measuring temperature-dependent changes of the focal length, we test the ability of the microlenses as a tunable imaging component. The microlenses have potential use in applications such as laser systems, functional biomimetics, and endoscopy.
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42.79.Bh Lenses, prisms and mirrors

Carrier lifetimes in type-II InAs quantum dots capped with a GaAsSb strain reducing layer

Y. D. Jang, T. J. Badcock, D. J. Mowbray, M. S. Skolnick, J. Park, D. Lee, H. Y. Liu, M. J. Steer, and M. Hopkinson

Appl. Phys. Lett. 92, 251905 (2008); http://dx.doi.org/10.1063/1.2949741 (3 pages) | Cited 15 times

Online Publication Date: 23 June 2008

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Carrier lifetimes have been measured for long-wavelength emitting InAs quantum dots (QDs) capped with a thin GaAsSb layer. Above a critical Sb composition, a type-II system is formed, resulting in an increase in the carrier lifetime. The carrier lifetime in a strongly type-II structure is increased by a factor ∼ 54 in comparison to the lifetime in a type-I structure. In addition, the type-II carrier lifetime varies across the inhomogeneously broadened ground-state emission, while the type-I QD lifetime is invariant.
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73.63.Kv Quantum dots
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.21.La Quantum dots

Ductile Fe–Nb–B bulk metallic glass with ultrahigh strength

J. H. Yao, J. Q. Wang, and Y. Li

Appl. Phys. Lett. 92, 251906 (2008); http://dx.doi.org/10.1063/1.2949747 (3 pages) | Cited 29 times

Online Publication Date: 23 June 2008

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The authors report a simple Fe-based Fe71Nb6B23 ternary bulk metallic glass with a record high strength of 4.85 GPa as well as an appreciable compressive plastic strain of 1.6%. This finding is associated with the unique attribute of the alloying element Nb, which favors the formation of a networklike structure and holds high Poisson’s ratio. A fracture feature with a combination of vein pattern and nanoscale corrugations under compression is clearly characterized in this glass. The fractographic observations correlate well with the observed improvements in plasticity.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.43.Fs Glasses
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.dj Poisson's ratio

Shaped graded materials with an apparent negative thermal conductivity

C. Z. Fan, Y. Gao, and J. P. Huang

Appl. Phys. Lett. 92, 251907 (2008); http://dx.doi.org/10.1063/1.2951600 (3 pages) | Cited 12 times

Online Publication Date: 24 June 2008

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Based on a first-principles approach, we exploit a class of shaped graded materials in which thermal energy is apparently controlled to transfer from a region of lower temperature to a region of higher temperature. This phenomenon, which is in contrast to our common intuition, is indicative of an apparent negative thermal conductivity (ANTC). Further analysis shows that the ANTC is related to a symmetric oscillation of paired thermal conductivities with specific gradation profiles, which are shown to satisfy a sum rule. Such shaped graded materials can serve as good candidates for thermal rectification.
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44.10.+i Heat conduction

Magneto-optical properties of transparent divalent iron phosphate glasses

Hirofumi Akamatsu, Koji Fujita, Shunsuke Murai, and Katsuhisa Tanaka

Appl. Phys. Lett. 92, 251908 (2008); http://dx.doi.org/10.1063/1.2952460 (3 pages) | Cited 9 times

Online Publication Date: 24 June 2008

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We have prepared glasses having xFeO⋅(100−x)P2O5 (mol %) (x = 50.0,54.0,57.1) compositions by melting under mild reducing condition and found that these glasses exhibit fairly high transmittance in the visible range and large Faraday effect at the wavelength of about 400 nm. 57Fe Mössbauer spectra confirm that almost all the iron ions are present as Fe2+ in the glasses. A spin glass transition is observed at low temperatures in the temperature dependence of magnetic susceptibility. Intense optical absorption in the ultraviolet and infrared wavelength ranges occurs by the charge transfer transition from O2− to Fe2+ and the intra-atomic d-d transition, respectively. The analysis on the wavelength dependence of the Faraday rotation angle using the Van Vleck–Hebb theory has revealed that the charge transfer transition contributes more significantly to the Faraday effect owing to the large effective transition probability, which is comparable to those reported for glasses containing 4f rare-earth ions. The magneto-optical figure of merit shows a maximum at around 380 nm.
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78.20.Ls Magneto-optical effects
76.80.+y Mössbauer effect; other γ-ray spectroscopy
78.40.Ha Other nonmetallic inorganics
78.30.Hv Other nonmetallic inorganics
75.50.Lk Spin glasses and other random magnets
75.30.Cr Saturation moments and magnetic susceptibilities

Activation and carrier mobility in high fluence B implanted germanium

S. Mirabella, G. Impellizzeri, A. M. Piro, E. Bruno, and M. G. Grimaldi

Appl. Phys. Lett. 92, 251909 (2008); http://dx.doi.org/10.1063/1.2949088 (3 pages) | Cited 19 times

Online Publication Date: 24 June 2008

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High doping regimes of B implanted Ge have been accurately characterized combining Hall effect technique and nuclear reaction analysis. Preamorphized Ge was implanted with B at 35 keV (spanning the 0.25–25×1020 B/cm3 concentration range) and recrystallized by solid phase epitaxy at 360 °C. The Hall scattering factor and the maximum concentration of active B resulted rH = 1.21 and ∼ 5.7×1020 B/cm3, respectively. The room-temperature carrier mobility was accurately measured, decreasing from ∼ 300 to 50 cm2/V s in the investigated dopant density, and a fitting empirical law is given. These results allow reliable evaluation for Ge application in future microelectronic devices.
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72.20.Fr Low-field transport and mobility; piezoresistance
72.80.Cw Elemental semiconductors
72.20.My Galvanomagnetic and other magnetotransport effects
61.72.uf Ge and Si

Extraction of ultraviolet emitting silicon species from strongly hydrogenated nanoporous silicon

V. Lysenko, V. Onyskevych, O. Marty, V. A. Skryshevsky, Y. Chevolot, and C. Bru-Chevallier

Appl. Phys. Lett. 92, 251910 (2008); http://dx.doi.org/10.1063/1.2948955 (3 pages) | Cited 2 times

Online Publication Date: 26 June 2008

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Ultraviolet emitting silicon species were extracted from strongly hydrogenated porous silicon nanostructures. Their photoluminescence spectra depend on size distribution of the species and can be tuned by centrifugation. Molecular structure of the extracted Si species is assumed to be very similar to some kinds of polysilanes which were theoretically described earlier by Allan et al. [Phys. Rev. B 48, 7951 (1993)]. Absence of photoluminescence signal coming from the polysilanes in the initial porous nanostructures is supposed to be due to the competitive absorption and to the energy transfer between the polysilanes and Si red emitting porous nanoparticles.
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78.55.Ap Elemental semiconductors
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
61.43.Gt Powders, porous materials

Latent heat investigation by photopyroelectric calorimetry

F. Mercuri, M. Marinelli, S. Paoloni, U. Zammit, and F. Scudieri

Appl. Phys. Lett. 92, 251911 (2008); http://dx.doi.org/10.1063/1.2951620 (3 pages) | Cited 10 times

Online Publication Date: 27 June 2008

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We report on an approach for photopyroelectric calorimetry enabling a frequency dependence detection of the specific heat and of the latent heat exchanged over first order transitions. It has been applied to the nematic-isotropic transition of 4-n-octyl-4′-cyanobiphenyl liquid crystal confined in a silica nanoparticles network, where the specific heat shows a double peak structure. The larger strain involved with the nematic nucleating over the low temperature peak induces a considerable lower latent heat than the one involved over the higher temperature peak. The frequency dependent measurements have shown a different dynamics of the nematic nucleating over the two peaks.
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61.30.-v Liquid crystals
65.20.-w Thermal properties of liquids
64.70.M- Transitions in liquid crystals
64.60.Q- Nucleation

Insight into the Raman shifts and optical absorption changes upon annealing polymer/fullerene solar cells

Je-Jung Yun, Jeffrey Peet, Nam-Sung Cho, Guillermo C. Bazan, Seung Joon Lee, and Martin Moskovits

Appl. Phys. Lett. 92, 251912 (2008); http://dx.doi.org/10.1063/1.2940205 (3 pages) | Cited 14 times

Online Publication Date: 27 June 2008

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Raman shifts and optical absorption spectra of bulk heterojunction films were measured to elucidate the origin of the optimum annealing parameters. A series of device optimization studies revealed 413 K to be the optimum annealing temperature, leading to a power conversion efficiency of 2.95%. The highest power conversion efficiency coincides with the highest peak in the UV-visible absorption and the lowest full width at half maximum of the –C=C– symmetric stretching signal in the Raman spectra. The changes observed in the vibronic shifts could be useful in obtaining information about the optimal performance and processing conditions for polymer optoelectronic devices.
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84.60.Jt Photoelectric conversion
78.40.-q Absorption and reflection spectra: visible and ultraviolet
78.30.-j Infrared and Raman spectra

Improved resonance characteristics of GaAs beam resonators by epitaxially induced strain

H. Yamaguchi, K. Kato, Y. Nakai, K. Onomitsu, S. Warisawa, and S. Ishihara

Appl. Phys. Lett. 92, 251913 (2008); http://dx.doi.org/10.1063/1.2952957 (3 pages) | Cited 10 times

Online Publication Date: 27 June 2008

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Micromechanical-beam resonators were fabricated using a strained GaAs film grown on relaxed In0.1Ga0.9As/In0.1Al0.9As buffer layers. The natural frequency of the fundamental mode was increased 2.5–4 times by applying tensile strain, showing good agreement with the model calculation assuming strain of 0.35% along the beam. In addition, the Q factor of 19 000 was obtained for the best sample, which is one order of magnitude higher than that for the unstrained resonator. This technique can be widely applied for improving the performance of resonator-based micro-/nanoelectromechanical devices.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
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