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12 May 2003

Volume 82, Issue 19, pp. 3147-3362

Issue Cover Spotlight Figure

Appl. Phys. Lett. 82, 3266 (2003); http://dx.doi.org/10.1063/1.1572970 (3 pages)

Michael Mück, Christian Welzel, and John Clarke
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InAs/GaAs(001) quantum dots close to thermodynamic equilibrium

G. Costantini, C. Manzano, R. Songmuang, O. G. Schmidt, and K. Kern

Appl. Phys. Lett. 82, 3194 (2003); http://dx.doi.org/10.1063/1.1572534 (3 pages) | Cited 29 times

Online Publication Date: 6 May 2003

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InAs/GaAs(001) quantum dots are grown at high temperature and extremely low flux and analyzed by in situ scanning tunneling microscopy. A bimodal distribution of dots is observed, composed of “small” and “large” islands. While the former show a broad distribution of sizes and shapes, the latter appear to be highly uniform and have a truncated pyramid shape with irregular octagonal base. (110) and (111) facets are identified and atomically resolved showing (1×1) and (2×2) surface reconstructions, respectively. The shape of the large quantum dots is in excellent agreement with recent theoretical predictions, proving that the chosen deposition conditions are close to thermodynamic equilibrium. © 2003 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
68.60.Dv Thermal stability; thermal effects
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)

Intensity and spatial modulation of spontaneous emission in GaAs by field aperture selecting transport

Thomas D. Boone, Hironori Tsukamoto, and Jerry M. Woodall

Appl. Phys. Lett. 82, 3197 (2003); http://dx.doi.org/10.1063/1.1572467 (3 pages) | Cited 4 times

Online Publication Date: 6 May 2003

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A potential technique for modulating the light emission resulting from excess minority carrier recombination in a semiconductor device is introduced. This process utilizes an electric field to transport a packet of minority carriers past an optical output aperture defined on the surface of the semiconductor. A short burst of light is allowed to escape through the surface of the device as the packet drifts past the opening in the aperture. To first order, the temporal length of the optical pulse will be a function of the width of the excess minority carrier packet, the width of the aperture, and the drift velocity of the excess minority carriers. In p-type gallium arsenide, geometric scales of 5 μm should make possible pulse widths near 100 ps. Initial experimental results are presented confirming the spatial displacement and the attenuation of the external luminescence intensity as a function of applied bias voltage. © 2003 American Institute of Physics.
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78.55.Cr III-V semiconductors
78.20.Jq Electro-optical effects
78.47.-p Spectroscopy of solid state dynamics

Dynamic modeling and scaling of nanostructure formation in the lithographically induced self-assembly and self-construction

Lin Wu and Stephen Y. Chou

Appl. Phys. Lett. 82, 3200 (2003); http://dx.doi.org/10.1063/1.1572963 (3 pages) | Cited 25 times

Online Publication Date: 6 May 2003

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We numerically studied the dynamical formation process and the scaling of the nanostructures in the lithographically induced self-assembly and self-construction of thin polymer films. Our studies show that the period of the self-assembled pillars depends on the ratio between the surface tension force and the electrostatic force. The viscosity of the polymer has no effect on the final pillar shape. When the feature width of the mold is comparable to or smaller than the most unstable disturbance wavelength of the system, the initially self-assembled pillars will merge to form a self-constructed mesa. © 2003 American Institute of Physics.
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85.40.Hp Lithography, masks and pattern transfer
81.16.Nd Micro- and nanolithography

Parametric modulation mechanism of surface acoustic wave on a partially closed crack

J.-Y. Kim, V. A. Yakovlev, and S. I. Rokhlin

Appl. Phys. Lett. 82, 3203 (2003); http://dx.doi.org/10.1063/1.1572552 (3 pages) | Cited 4 times

Online Publication Date: 6 May 2003

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Low-frequency parametric modulation of a pulsed surface acoustic wave by a partially closed fatigue crack is described. The crack is initiated in the plastic-yielding zone induced by a surface cavity and clamped due to the constraint of the surrounding elastic medium. Small periodic loading, superimposed on a static crack opening load, changes periodically the crack closure state and produces a nonlinear modulation of the reflected ultrasonic pulses. We describe quantitative experimental and modeling results relating the reflected modulation spectrum to crack length, closure, and modulation load. Surface acoustic wave measurements, which were performed in situ during a fatigue test, show that the modulation response varies significantly depending on the ratio of the fatigue crack length to the size of the plastic-yielding zone. © 2003 American Institute of Physics.
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68.35.Iv Acoustical properties
62.20.M- Structural failure of materials
68.35.Gy Mechanical properties; surface strains
81.40.Lm Deformation, plasticity, and creep

A high-temperature shape-memory alloy Ni54Mn25Ga21

Huibin Xu, Yunqing Ma, and Chengbao Jiang

Appl. Phys. Lett. 82, 3206 (2003); http://dx.doi.org/10.1063/1.1572540 (3 pages) | Cited 41 times

Online Publication Date: 6 May 2003

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A high-temperature shape-memory alloy, Ni54Mn25Ga21, was developed with a shape-memory effect of 6.1% and a martensitic transformation temperature higher than 250 °C for single crystals. The measured compressive strength and strain were 845 MPa and 20.5%, respectively, with a compressive axis along the growth direction of the rods at room temperature. One thousand thermal cycles were performed on the Ni54Mn25Ga21 without obvious changes of the martensitic structure, transformation behavior, and shape-memory effect, indicating an excellent thermal stability for the present alloy. © 2003 American Institute of Physics.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
64.70.K- Solid-solid transitions
81.30.Kf Martensitic transformations

Reduced critical thickness for relaxing heteroepitaxial films on compliant substrates

G. Kästner and U. Gösele

Appl. Phys. Lett. 82, 3209 (2003); http://dx.doi.org/10.1063/1.1573355 (3 pages) | Cited 5 times

Online Publication Date: 6 May 2003

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It is argued that heteroepitaxial thin films (layers) grown on a compliant substrate are not able to relax their strain elastically by large-area slip across a “weak” layer. Instead, the Matthews model of plastic relaxation is modified by supposing that the interfacial misfit dislocations relax their strain field or even disappear into the weak layer. Consequently, the moving film-threading dislocations experience a reduced drag force. Therefore, the critical film thickness is lowered, in contrast to the enhanced thickness predicted by current theories. A quantitative estimate is given which depends on the nature of the weak layer. Implications include a larger free slip path and potentially a lower density of film-threading dislocations. © 2003 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties
61.72.Bb Theories and models of crystal defects
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
62.20.D- Elasticity

High-quality InAsyP1−y step-graded buffer by molecular-beam epitaxy

M. K. Hudait, Y. Lin, D. M. Wilt, J. S. Speck, C. A. Tivarus, E. R. Heller, J. P. Pelz, and S. A. Ringel

Appl. Phys. Lett. 82, 3212 (2003); http://dx.doi.org/10.1063/1.1572476 (3 pages) | Cited 15 times

Online Publication Date: 6 May 2003

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Relaxed, high-quality, compositionally step-graded InAsyP1−y layers with an As composition of y = 0.4, corresponding to a lattice mismatch of ∼1.3% were grown on InP substrates using solid-source molecular-beam epitaxy. Each layer was found to be nearly fully relaxed observed by triple axis x-ray diffraction, and plan-view transmission electron microscopy revealed an average threading dislocations of 4×106 cm−2 within the InAs0.4P0.6 cap layer. Extremely ordered crosshatch morphology was observed with very low surface roughness (3.16 nm) compared to cation-based In0.7Al0.3As/InxAl1−xAs/InP graded buffers (10.53 nm) with similar mismatch and span of lattice constants on InP. The results show that InAsyP1−y graded buffers on InP are promising candidates as virtual substrates for infrared and high-speed metamorphic III–V devices. © 2003 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.60.Bs Mechanical and acoustical properties
68.37.Lp Transmission electron microscopy (TEM)
61.72.Lk Linear defects: dislocations, disclinations
68.35.B- Structure of clean surfaces (and surface reconstruction)

Structure and phase stability of the CdTiO3–PbTiO3 system

D. Y. Suárez-Sandoval and P. K. Davies

Appl. Phys. Lett. 82, 3215 (2003); http://dx.doi.org/10.1063/1.1573362 (3 pages) | Cited 10 times

Online Publication Date: 6 May 2003

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The formation of solid solutions in the CdTiO3–PbTiO3 system has been examined using x-ray diffraction (XRD) and differential scanning calorimetry. The XRD data showed two solid-solution ranges for the (Pb1−xCdx)TiO3 system at 1170 °C; orthorhombic on the cadmium-rich side and tetragonal on the lead-rich side, separated by a two-phase region for 0.08<x<0.85. Despite the limited range of solubility the tetragonal (Pb1−xCdx)TiO3 solid solutions show a >10% increase in the c/a ratio, demonstrating that the role of Cd in this structure is quite unique. The enthalpy of the tetragonal-to-cubic phase transformation also increases with increasing Cd substitution, although the temperature of the transition is ∼20° lower than that found in pure PbTiO3. © 2003 American Institute of Physics.
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64.70.K- Solid-solid transitions
61.66.Fn Inorganic compounds
81.30.Dz Phase diagrams of other materials
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
65.40.G- Other thermodynamical quantities

Vibrational study of tris-(8-hydroxyquinoline) aluminum/LiF/Al interfaces

P. He, S. D. Wang, S. T. Lee, and L. S. Hung

Appl. Phys. Lett. 82, 3218 (2003); http://dx.doi.org/10.1063/1.1573353 (3 pages) | Cited 9 times

Online Publication Date: 6 May 2003

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Interface studies of tris-(8-hydroxyquinoline) aluminum (Alq3)/LiF/Al were carried out with high-resolution electron-energy-loss spectroscopy. With LiF on Alq3 or Al on LiF, no significant changes in intensity were observed for the loss peak related to the Li–F stretch mode. However, the deposition of less than one monolayer of Al on Alq3/LiF resulted in a strong attenuation of the LiF-loss peak, providing a direct evidence of LiF dissociation in the coexistence of Alq3 and Al with LiF. On the other hand, the LiF-loss peak remained almost unchanged when Al was deposited on poly (9,9-dioctyl-fluorene)/LiF. © 2003 American Institute of Physics.
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68.35.Ja Surface and interface dynamics and vibrations
79.20.Uv Electron energy loss spectroscopy
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Two-photon-induced polymerization in a laser gain medium for optical microstructure

Shiyoshi Yokoyama, Tatsuo Nakahama, Hideki Miki, and Shinro Mashiko

Appl. Phys. Lett. 82, 3221 (2003); http://dx.doi.org/10.1063/1.1573350 (3 pages) | Cited 9 times

Online Publication Date: 6 May 2003

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We have fabricated a polymeric solid-state microcavity in the laser gain media by a two-photon-induced polymerization technique. The photopolymerization resin contains conventional laser dye and dendrimer. A dendrimer can encapsulate the laser dye, and therefore increase its concentration up to 4 wt % with limited energy transfer during the photopolymerization process. The microcavity consisted of <0.4 μm linewidth polymerized strips arranged in layer-by-layer structure. The periodic variation in the refractive index gave rise to Bragg reflection. A laser emission was measured in the microcavity under optical excitation. The spectral linewidth was about 0.15 nm above the lasing threshold. The laser emission is attributed both to the distributed Bragg reflection and to the high gain of a polymeric medium containing laser dye with a high concentration. © 2003 American Institute of Physics.
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42.55.Mv Dye lasers
82.35.Ej Nonlinear optics with polymers
82.50.Pt Multiphoton processes
42.70.Hj Laser materials
42.70.Jk Polymers and organics
42.55.Sa Microcavity and microdisk lasers

Synthesis and optical properties of semiconducting beta-FeSi2 nanocrystals

Q. Wan, T. H. Wang, and C. L. Lin

Appl. Phys. Lett. 82, 3224 (2003); http://dx.doi.org/10.1063/1.1574845 (3 pages) | Cited 11 times

Online Publication Date: 6 May 2003

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Vacuum electron-beam coevaporation of Fe and Si followed by annealing in N2 ambient is used to synthesize beta iron disilicide (β-FeSi2) nanocrystals. The reason for β-FeSi2 nanocrystal formation is discussed based on the principle of minimization of the interface energy. X-ray diffraction studies suggest that β-phase FeSi2 nanostructures turn into the α phase when the annealing temperature is 1000 °C. The optical transmission measurements indicate that the β-FeSi2 nanocrystals have a direct-band structure with the energy gap in the range of 0.84–0.88 eV. A sharp photoluminescence peak of the β-FeSi2 nanocrystals located at 1.5 μm is measured at 77 K. © 2003 American Institute of Physics.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.55.Hx Other solid inorganic materials
61.46.-w Structure of nanoscale materials
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
73.22.Dj Single particle states
61.72.Cc Kinetics of defect formation and annealing

All-silicon omnidirectional mirrors based on one-dimensional photonic crystals

A. Bruyant, G. Lérondel, P. J. Reece, and M. Gal

Appl. Phys. Lett. 82, 3227 (2003); http://dx.doi.org/10.1063/1.1574403 (3 pages) | Cited 48 times

Online Publication Date: 6 May 2003

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We report on the fabrication of monolithic omnidirectional mirrors based on one-dimensional photonic crystals. The mirrors are comprised of chirped and unchirped multiple layers of microporous silicon. Porosities have been chosen to achieve an optimal low refractive index nL ∼ 1.5 and a high refractive index nH ∼ 2.55. Unchirped structures, centered in the near-infrared, exhibit an omnidirectional reflection band of 100 nm, in agreement with the calculated photonic band structure. Chirped structures exhibit an enlarged omnidirectional stop band (340 nm). Given the possibility of easily tailoring the optical thickness of porous silicon, this material is shown to be very practical for engineering omnidirectional mirrors. © 2003 American Institute of Physics.
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42.70.Qs Photonic bandgap materials
42.79.Bh Lenses, prisms and mirrors
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Optical defects produced in fused silica during laser-induced breakdown

S. O. Kucheyev and S. G. Demos

Appl. Phys. Lett. 82, 3230 (2003); http://dx.doi.org/10.1063/1.1573364 (3 pages) | Cited 36 times

Online Publication Date: 6 May 2003

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Fused silica irradiated with ∼ 3-ns 1064-, 355-, and 266-nm laser pulses as well as with ∼ 120-fs 825-nm pulses is studied by a combination of photoluminescence (PL) and Raman scattering spectroscopies. Results show that, for laser fluences above the laser-induced breakdown threshold, in all the cases studied, irradiation results in the formation of four defect-related PL bands centered on ∼ 1.9 (655), 2.2 (565), 2.7 (460), and 4.3 eV (290 nm). Bands centered on 1.9, 2.7, and 4.3 eV are attributed to nonbridging oxygen hole centers (1.9 eV) and oxygen-deficiency defects (2.7 and 4.3 eV). However, defects giving rise to a broad band at ∼ 2.2 eV are unknown. For all the laser-modified samples studied, Raman spectroscopy reveals a dramatic increase in the intensity of D1 and D2 lines, associated with in-phase breathing motions of oxygen atoms in puckered four- and planar three-membered ring structures, respectively. This indicates laser-induced material densification. Based on these results, we discuss physical processes occurring during the catastrophic laser-induced material breakdown, leading to material densification and the formation of point defects. © 2003 American Institute of Physics.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
78.55.Hx Other solid inorganic materials
78.30.Hv Other nonmetallic inorganics
61.72.-y Defects and impurities in crystals; microstructure

Energy of free excitons in CuInSe2 single crystals

M. V. Yakushev, A. V. Mudryi, and R. D. Tomlinson

Appl. Phys. Lett. 82, 3233 (2003); http://dx.doi.org/10.1063/1.1572477 (3 pages) | Cited 3 times

Online Publication Date: 6 May 2003

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CuInSe2 single crystals grown by the vertical Bridgman method were studied using photoluminescence (PL) and optical reflection (OR) at temperatures 4.2 and 78 K. A and B free- exciton peaks were well resolved in both the PL and OR spectra measured at a low temperature from a number of samples. The energy, spectral separation, and the width of the excitonic lines varied considerably for samples with different elemental compositions. The closer to the ideal stoichiometry, the higher the energies and the smaller the A–B separation. Similar changes in the excitonic spectra were revealed for a temperature rise from 4.2 up to 78 K. These measurements explain the scatter in the excitonic parameters and the band-gap values measured in the present work and reported in the literature. © 2003 American Institute of Physics.
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71.35.Cc Intrinsic properties of excitons; optical absorption spectra
81.05.Hd Other semiconductors
78.55.Hx Other solid inorganic materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.66.Bi Elemental solids
61.66.Dk Alloys
71.20.Nr Semiconductor compounds

Electroluminescence of self-assembled Ge hut clusters

M. Stoffel, U. Denker, and O. G. Schmidt

Appl. Phys. Lett. 82, 3236 (2003); http://dx.doi.org/10.1063/1.1572479 (3 pages) | Cited 19 times

Online Publication Date: 6 May 2003

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We have fabricated Si-based light-emitting diodes operating in the near infrared. The active layers of the devices consist of either one or ten layers of Ge/Si self-assembled hut clusters grown by molecular-beam epitaxy. Luminescence is observed in the spectral range between 1.4 and 1.5 μm. For the ten layer stack of Ge islands, electroluminescence is observed up to room temperature. A direct comparison with a pure Si reference p-i-n diode allows us to attribute the luminescence to radiative recombinations between holes localized in the Ge islands and electrons localized in the strained Si above and below the islands. © 2003 American Institute of Physics.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
78.60.Fi Electroluminescence
85.60.Dw Photodiodes; phototransistors; photoresistors

Structural basis for the yield stress in plastic disperse systems

Alejandro G. Marangoni and Michael A. Rogers

Appl. Phys. Lett. 82, 3239 (2003); http://dx.doi.org/10.1063/1.1576502 (3 pages) | Cited 23 times

Online Publication Date: 6 May 2003

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The apparent yield stress is a fundamental rheological characteristic of a plastic material. In this work, we propose a model that relates the structure of a fractal particle network to its yield stress. The model predicts the yield stress to be a function of particle size, solid-liquid surface free energy, the amount of network material, and the fractal dimension of the network. The predicted yield stress corresponded to the stress at the limit of linearity in fat crystal networks. Moreover, a structural definition for the yield stress is proposed. © 2003 American Institute of Physics.
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46.35.+z Viscoelasticity, plasticity, viscoplasticity
83.60.La Viscoplasticity; yield stress
82.70.-y Disperse systems; complex fluids
83.80.Hj Suspensions, dispersions, pastes, slurries, colloids
05.45.Df Fractals
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