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31 May 2004

Volume 84, Issue 22, pp. 4361-4576

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 4409 (2004); http://dx.doi.org/10.1063/1.1757648 (3 pages)

Azita Soleymani, Piroz Zamankhan, and William Polashenski
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X-ray absorption study of the electronic structure of Li-excess spinel Li1+xTi2−xO4 (0 ⩽ x ⩽ 0.33)

Wonkyung Ra, Masanobu Nakayama, Hiromasa Ikuta, Yoshiharu Uchimoto, and Masataka Wakihara

Appl. Phys. Lett. 84, 4364 (2004); http://dx.doi.org/10.1063/1.1755418 (3 pages) | Cited 5 times

Online Publication Date: 12 May 2004

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Li-excess spinel oxide, Li1+xTi2−xO4 (0 ⩽ x ⩽ 0.33), was investigated by x-ray absorption spectra (XAS) technique. XAS results revealed that both oxygen and titanium ions affect electronic structural changes in the system. Early transition metal compounds are considered to have smaller d–d Coulomb correlation energy (Udd) than p–d Charge-transfer energy (Δ), indicating that electronic transfer for charge compensation occurs mainly on transition metal. However, present XAS results show that considerable charge-transfer character due to strong hybridization between titanium 3d and oxygen 2p orbitals exists in this early transition metal oxide system. © 2004 American Institute of Physics.
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78.70.Dm X-ray absorption spectra
71.20.-b Electron density of states and band structure of crystalline solids

Correlated stacks of CdSe/ZnSSe quantum dots

Th. Schmidt, T. Clausen, J. Falta, G. Alexe, T. Passow, D. Hommel, and S. Bernstorff

Appl. Phys. Lett. 84, 4367 (2004); http://dx.doi.org/10.1063/1.1756195 (3 pages) | Cited 6 times

Online Publication Date: 12 May 2004

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The observation and quantitative investigation of the spatial correlation of CdSe quantum dots in stacked CdSe/ZnSSe quantum-dot layers is reported. Using grazing incidence x-ray small angle scattering, the influence of the ZnSSe spacer layer thickness ranging from 20 to 80 Å, as well as the influence of the stacking number (3–10) has been analyzed. Satellite spots, indicative of quantum-dot ordering, have been observed for a spacer thickness of up to 45 Å, and for a stacking number of at least 5. This finding can be explained by a self-organized ordering process driven by the lattice mismatch induced strain. A mean lateral quantum-dot distance ranging from 116 to 145 Å, depending on the spacer thickness, has been found. In addition, an anisotropy of twofold symmetry has been observed, with the strongest correlation signal along 〈1math0〉. © 2004 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
61.05.cf X-ray scattering (including small-angle scattering)

Interaction of Fe+ with the C60 surface: A study about the feasibility of endohedral doping

P. Reinke, S. Eyhusen, M. Büttner, and P. Oelhafen

Appl. Phys. Lett. 84, 4373 (2004); http://dx.doi.org/10.1063/1.1756204 (3 pages) | Cited 1 time

Online Publication Date: 12 May 2004

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The feasibility of endohedral iron doping of C60 is studied using a mass selected ion beam to create an Fe+ beam with variable energy (60–380 eV, 1.2×10−3 C) which is directed at a C60 thin film. The surface is characterized by x-ray photoelectron spectroscopy, and oxidation of the samples is used to discriminate between FeC60 and carbides. The fullerene cages are damaged during the irradiation, and participate in direct Fe–C bond formation. The reaction with oxygen indicates the absence of sizable amount of FeC60. Comparison with reference samples composed of iron and damaged fullerene layers (by Ar+ irradiation) supports this interpretation. The experiment serves as a model study for the investigation of different metal-fullerene combinations. © 2004 American Institute of Physics.
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61.72.up Other materials
61.48.-c Structure of fullerenes and related hollow and planar molecular structures

Enhancement of the green, visible Tb3+ luminescence from Tb-doped silicon-rich silicon oxide by C co-doping

Se-Young Seo and Jung H. Shin

Appl. Phys. Lett. 84, 4379 (2004); http://dx.doi.org/10.1063/1.1757015 (3 pages) | Cited 7 times

Online Publication Date: 12 May 2004

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The effect of C co-doping on the Tb3+ luminescence from Tb-doped silicon-rich silicon oxide (SRSO) films is investigated. Tb-doped SRSO films co-doped with C (SRSO:C) were deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition. The Tb3+ photoluminescence intensity is enhanced by the presence of nanocluster Si (nc-Si), and C co-doping further increases the Tb3+ photoluminescence intensity by more than an order of magnitude. The maximum enhancement is observed at the C content of ∼5 at. %, at which the Tb3+ luminescence is bright enough to be observed by the naked eye under ambient conditions. The 543 nm Tb3+ lifetimes were in the range of 0.5–1.2 ms, comparable to those from Tb-doped silica. Based on the results, we conclude that nanometer-sized nc-Si can excite Tb3+ ions via an Auger-type energy transfer, and that C co-doping greatly increases the efficiency of such exciton-mediated excitation of Tb3+. © 2004 American Institute of Physics.
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78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
71.35.-y Excitons and related phenomena

Guiding and bending of acoustic waves in highly confined phononic crystal waveguides

A. Khelif, A. Choujaa, S. Benchabane, B. Djafari-Rouhani, and V. Laude

Appl. Phys. Lett. 84, 4400 (2004); http://dx.doi.org/10.1063/1.1757642 (3 pages) | Cited 63 times

Online Publication Date: 12 May 2004

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We demonstrate experimentally the guiding and the bending of acoustic waves in highly confined waveguides formed by removing rods from a periodic two-dimensional lattice of steel cylinders immersed in water. Full transmission is observed for a one-period-wide straight waveguide within the full band gap of the perfect phononic crystal. However, when the waveguide width is doubled, destructive interference causes the transmission to vanish in the center of the passband. Waveguiding over a wide frequency range is obtained for a one-period-wide waveguide with two sharp 90° bends. Finite-difference time-domain computations are found to be in good agreement with the measurements. © 2004 American Institute of Physics.
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43.20.Mv Waveguides, wave propagation in tubes and ducts
62.30.+d Mechanical and elastic waves; vibrations
46.40.Cd Mechanical wave propagation (including diffraction, scattering, and dispersion)
63.20.-e Phonons in crystal lattices

Resonant photoemission in Cr silicide at the absorption energy Cr 2p

L. Galán, M. García, J. M. Ripalda, I. Montero, E. Román, D. R. Batchelor, and P. R. Bressler

Appl. Phys. Lett. 84, 4433 (2004); http://dx.doi.org/10.1063/1.1758779 (3 pages)

Online Publication Date: 12 May 2004

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Valence band photoemission has been measured in chromium silicide as a function of the photon energy near the Cr 2p3/2 absorption threshold. Evidence of resonant photoemission is observed for the 3d valence band and the two-hole satellite. The threshold for normal Auger regime is 2.8 eV below the absorption peak and 0.8 eV below the Cr 2p3/2 binding energy, even lower than in pure Cr metal where it is already at extreme levels. The requirement for good resolution in photon energy relative to absorption width for the resonant Raman Auger to be observed is found to be less restrictive than expected. © 2004 American Institute of Physics.
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79.60.Bm Clean metal, semiconductor, and insulator surfaces
71.20.Nr Semiconductor compounds

Structure of grain boundaries in nanostructured ZnO

Zhihao Wu, Yingxue Zhou, Xinyi Zhang, Shiqiang Wei, and Dongliang Chen

Appl. Phys. Lett. 84, 4442 (2004); http://dx.doi.org/10.1063/1.1759060 (3 pages) | Cited 9 times

Online Publication Date: 12 May 2004

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The grain boundary (GB) of nanocrystalline ZnO films is investigated using the x-ray absorption fine structure technique. With the advantage of the dominant GB volume fraction in our samples, the GB structure is found to be neither simply “gas-like” nor “similar to that of coarse-grained phase,” but experiences a transition from the modestly ordered innermost coordination shell around centered atoms to partly disordered second coordination shell and then to completely disordered higher coordination shells. © 2004 American Institute of Physics.
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61.72.Mm Grain and twin boundaries
61.46.-w Structure of nanoscale materials
78.70.Dm X-ray absorption spectra
68.55.-a Thin film structure and morphology

Influence of pulse duration on the amorphization of GeSb thin films under ultrashort laser pulses

S. M. Wiggins, J. Solis, and C. N. Afonso

Appl. Phys. Lett. 84, 4445 (2004); http://dx.doi.org/10.1063/1.1759062 (3 pages) | Cited 9 times

Online Publication Date: 12 May 2004

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Laser-induced amorphization of crystalline, 25-nm-thick, Sb-rich GeSb films has been studied for pump pulse durations in the range from 100 fs up to 6 ns. The dynamics of the phase change has been investigated using real-time reflectivity measurements with picosecond time resolution performed with a streak camera. For pulses in the femtosecond regime, the time required to complete the transformation is of the order of ∼10–15 ns. When the pulse duration is increased to the 1.5–20 ps range, the transformation time decreases to values as short as 400 ps, while for nanosecond laser pulses amorphization is not achievable. This behavior is discussed in terms of the heat flow dynamics of the system and the influence of pulse duration on the initial supercooling and nucleation rate. © 2004 American Institute of Physics.
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64.70.K- Solid-solid transitions
81.40.Wx Radiation treatment (particle and electromagnetic)

Crystal nucleation and growth processes in Ge2Sb2Te5

S. Privitera, C. Bongiorno, E. Rimini, and R. Zonca

Appl. Phys. Lett. 84, 4448 (2004); http://dx.doi.org/10.1063/1.1759063 (3 pages) | Cited 28 times

Online Publication Date: 12 May 2004

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The kinetics of the amorphous-to-crystal transition in Ge2Sb2Te5 thin films have been studied through in situ transmission electron microscopy analyses. By following the time evolution of the grain density and size, the growth velocity and the nucleation rate have been separately measured at different annealing temperatures. Activation energies of 2.9±0.5 eV and 2.3±0.4 eV have been obtained for the nucleation rate and the growth velocity, respectively. The barrier energy for the nucleation of a critical nucleus ΔG∗ has been evaluated, and the scalability of phase change nonvolatile memories has been estimated. © 2004 American Institute of Physics.
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68.55.A- Nucleation and growth
68.60.Wm Other nonelectronic physical properties
64.70.K- Solid-solid transitions
68.37.Lp Transmission electron microscopy (TEM)

Stable extremely-high-damping discrete viscoelastic systems due to negative stiffness elements

Yun-Che Wang and Roderic S. Lakes

Appl. Phys. Lett. 84, 4451 (2004); http://dx.doi.org/10.1063/1.1759064 (3 pages) | Cited 1 time

Online Publication Date: 12 May 2004

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Systems with negative stiffness constituents can have extreme material properties greatly exceeding those of either constituent. We show that a discrete system with a viscoelastic damping element and a negative stiffness element can be made with overall viscoelastic damping orders of magnitude higher than that of any constituent, or of the system with all elements of positive stiffness. The product of stiffness and damping, important for vibration damping, is also enhanced by orders of magnitude. We show this system is unconditionally stable in the high damping regime. The singularity in damping can be made arbitrarily close to the stability boundary. © 2004 American Institute of Physics.
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46.35.+z Viscoelasticity, plasticity, viscoplasticity
46.40.Ff Resonance, damping, and dynamic stability

Random laser action in self-organized para-sexiphenyl nanofibers grown by hot-wall epitaxy

F. Quochi, F. Cordella, R. Orrù, J. E. Communal, P. Verzeroli, A. Mura, G. Bongiovanni, A. Andreev, H. Sitter, and N. S. Sariciftci

Appl. Phys. Lett. 84, 4454 (2004); http://dx.doi.org/10.1063/1.1759384 (3 pages) | Cited 42 times

Online Publication Date: 12 May 2004

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We report on the observation of amplified spontaneous emission and random lasing in self-organized crystalline para-sexiphenyl nanofibers. Using subpicosecond excitation, a lasing threshold is observed on the 0–1 emission band near 425 nm at excitation fluences as low as 0.5 μJ/cm2 (6×1016 cm−3 equivalent density), near the onset of density-dependent recombination processes. The dependence of the nonlinear emission spectrum on both the pump intensity and position of the excitation area are attributed to the interplay between random lasing and amplified spontaneous emission occurring along the nanofibers. © 2004 American Institute of Physics.
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42.55.Wd Fiber lasers
42.55.Zz Random lasers
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures

Blue-light emission from molecular-beam-epitaxially grown GaN/Al0.5Ga0.5N multiple quantum wells with a perturbating layer of Al0.5Ga0.5N monolayers

Y. S. Park, Seung-Ho Lee, Jae-Eung Oh, Chang-Mo Park, and Tae-Won Kang

Appl. Phys. Lett. 84, 4478 (2004); http://dx.doi.org/10.1063/1.1755836 (3 pages) | Cited 8 times

Online Publication Date: 14 May 2004

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We have studied the influence of AlGaN inserting layer into GaN well region on the light emission from a strained GaN/AlGaN multiple-quantum-well system. We have found that, by simply inserting thin AlGaN layer, the luminescence is dramatically redshifted with respect to that of the normal GaN/AlGaN quantum well, which is centered at 2.96 eV, nearly 0.52 eV below the bulk GaN band gap. We attribute this enormous redshift to an additional 0.7 MV/cm field present in the well due to the perturbation of the well region by inserting AlGaN layer. The result is revealed to be of great importance in the design and analysis of nitride heterostructure devices which can be exploited to advantage in nitride materials and device engineering. © 2004 American Institute of Physics.
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78.67.De Quantum wells
81.05.Ea III-V semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.65.Fg Quantum wells
78.55.Cr III-V semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence

Infrared-laser-induced periodic surface structure in azo-dye polymer

Mario Ivanov and Paul Rochon

Appl. Phys. Lett. 84, 4511 (2004); http://dx.doi.org/10.1063/1.1757640 (3 pages) | Cited 11 times

Online Publication Date: 14 May 2004

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Second-harmonic and third-harmonic waves that are generated in an azo-polymer form diffraction patterns without any material deformation. The source is a single IR laser beam at λ = 1064 nm that is weakly absorbed by the polymer. If the IR power is sufficiently high, laser-induced periodic (700 nm) structures grow on the polymer surface and give rise to distinctive diffraction patterns in the visible and ultraviolet. If the IR power is low, there are no surface changes but diffraction patterns that have the same shape still appear. © 2004 American Institute of Physics.
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42.70.Jk Polymers and organics
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.66.Qn Polymers; organic compounds
78.35.+c Brillouin and Rayleigh scattering; other light scattering
68.47.Mn Polymer surfaces
68.35.B- Structure of clean surfaces (and surface reconstruction)

Doping-level-dependent optical properties of GaN:Mn

O. Gelhausen, E. Malguth, M. R. Phillips, E. M. Goldys, M. Strassburg, A. Hoffmann, T. Graf, M. Gjukic, and M. Stutzmann

Appl. Phys. Lett. 84, 4514 (2004); http://dx.doi.org/10.1063/1.1757641 (3 pages) | Cited 11 times

Online Publication Date: 14 May 2004

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The optical properties of molecular-beam-epitaxy-grown GaN with different Mn-doping levels (5–23×1019 cm−3) were studied by cathodoluminescence (CL) and optical transmission spectroscopy. Transmission measurements at 2 K revealed an absorption peak at 1.414±0.002 eV, which was attributed to an internal 5T2→5E transition of the neutral Mn3+ state. The intensity of this Mn-related transmission peak was found to scale with the Mn3+ concentration. The CL measurements showed that Mn-doping concentrations around 1020 cm−3 reduced the near band edge emission intensity by around one order of magnitude. A complete quenching of the donor–acceptor-pair band at 3.27 eV and strong decrease of the yellow luminescence centered at 2.2 eV were attributed to a reduced concentration of VGa. In the infrared spectral range of 0.8–1.4 eV three broad, Mn-doping related CL emission bands centered at 1.01±0.02, 1.09±0.02, and 1.25±0.03 eV were observed. Their origin is attributed to deep donor complexes, which are generated as a result of the heavy Mn-doping. © 2004 American Institute of Physics.
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75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
78.66.Fd III-V semiconductors
78.60.Hk Cathodoluminescence, ionoluminescence
78.40.Fy Semiconductors
61.72.uj III-V and II-VI semiconductors
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
81.40.Gh Other heat and thermomechanical treatments

Microstructures formed in recrystallized Si

K. Watanabe, Y. Anzai, N. Nakanishi, T. Yamazaki, K. Kuramochi, K. Mitsuishi, K. Furuya, and I. Hashimoto

Appl. Phys. Lett. 84, 4520 (2004); http://dx.doi.org/10.1063/1.1758293 (3 pages) | Cited 4 times

Online Publication Date: 14 May 2004

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Our study using systematic transmission electron microscopy observation and simulation shows that microstructures formed in recrystallized Si are characterized as microtwin or lamellar microtwin. Detailed analysis leads to their atomic structures. The discovery of exceptional diffraction spots offers direct evidence of long-periodic-order structures and antiphase boundaries, due to the ordering of projected lamellar microtwins. © 2004 American Institute of Physics.
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81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.Mm Grain and twin boundaries

Determination of the optical energy gap of Ge1−xSnx alloys with 0<x<0.14

H. Pérez Ladrón de Guevara, A. G. Rodríguez, H. Navarro-Contreras, and M. A. Vidal

Appl. Phys. Lett. 84, 4532 (2004); http://dx.doi.org/10.1063/1.1758772 (3 pages) | Cited 21 times

Online Publication Date: 14 May 2004

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The optical energy gap of Ge1−xSnx alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (xc) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration xc, which is theoretically expected to be 0.09, actually it is observed to lie between 0.10<xc<0.13. © 2004 American Institute of Physics.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.30.Hv Other nonmetallic inorganics

A route to diamond wafers by epitaxial deposition on silicon via iridium/yttria-stabilized zirconia buffer layers

S. Gsell, T. Bauer, J. Goldfuß, M. Schreck, and B. Stritzker

Appl. Phys. Lett. 84, 4541 (2004); http://dx.doi.org/10.1063/1.1758780 (3 pages) | Cited 27 times

Online Publication Date: 14 May 2004

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A multilayer structure is presented which allows the deposition of high-quality heteroepitaxial diamond films on silicon. After pulsed-laser deposition of a thin yttria-stabilized zirconia (YSZ) layer on silicon, iridium was deposited by e-beam evaporation. Subsequently, diamond nucleation and growth was performed in a chemical vapor deposition setup. The epitaxial orientation relationship measured by x-ray diffraction is diamond(001)[110]∥Ir(001)[110]∥YSZ(001) [110]∥Si(001)[110]. The mosaicity of the diamond films is about an order of magnitude lower than for deposition directly on silicon without buffer layers and nearly reaches the values reported for single-crystal diamond on Ir/SrTiO3. In the effort towards single-crystal diamond wafers, the present solution offers advantages over alternative growth substrates like large-area oxide single crystals due to the low thermal expansion mismatch. © 2004 American Institute of Physics.
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68.65.Ac Multilayers
81.05.U- Carbon/carbon-based materials
81.15.Jj Ion and electron beam-assisted deposition; ion plating

Optical properties of heterodiamond B2CN using first-principles calculations

Jian Sun, Hui-Tian Wang, Nai-Ben Ming, Julong He, and Yongjun Tian

Appl. Phys. Lett. 84, 4544 (2004); http://dx.doi.org/10.1063/1.1758781 (3 pages) | Cited 22 times

Online Publication Date: 14 May 2004

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We report detailed ab initio calculations of the optical properties of a heterodiamond B2CN compound using the pseudopotential density functional method, including the dielectric function, the refractive index, the absorption, the electron energy-loss spectrum, and the reflectivity. The polarization dependencies of those optical properties are investigated. The results indicate that the electron-deficiency characteristic of B2CN gives rise to some features. © 2004 American Institute of Physics.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Zx New materials: theory, design, and fabrication

Electronic transport through individual ZnO nanowires

Q. H. Li, Q. Wan, Y. X. Liang, and T. H. Wang

Appl. Phys. Lett. 84, 4556 (2004); http://dx.doi.org/10.1063/1.1759071 (3 pages) | Cited 136 times

Online Publication Date: 14 May 2004

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Electronic transport through individual ZnO nanowires has been investigated. The current increases linearly with the bias and the conductance jumps upon ultraviolet illumination. The increase rate upon the illumination is much faster than the decrease rate as the light is off. The decrease rate under vacuum is slower than that in air. These phenomena are related to the surface oxygen species and further confirmed by in situ current–voltage measurements as a function of oxygen pressure at room temperature. Also, the conductance increases greatly as the temperature is raised. These results demonstrate that the surface oxygen species dominate the transport process through individual ZnO nanowires, which indicates their potential application to room temperature gas sensors. © 2004 American Institute of Physics.
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73.63.Nm Quantum wires

Polarity control of ZnO on sapphire by varying the MgO buffer layer thickness

Hiroyuki Kato, Kazuhiro Miyamoto, Michihiro Sano, and Takafumi Yao

Appl. Phys. Lett. 84, 4562 (2004); http://dx.doi.org/10.1063/1.1759377 (3 pages) | Cited 44 times

Online Publication Date: 14 May 2004

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Polarity-controlled ZnO films with an MgO buffer layer were grown on c-plane sapphire by plasma-assisted molecular-beam epitaxy. Convergent beam electron diffraction results showed that Zn-polarity (+c) growth occurred when the MgO layer was thicker than 3 nm, whereas O-polarity (−c) growth occurred when the layer was less than 2 nm. Reflection high-energy electron diffraction results revealed that MgO growth was Stranski–Krastanov mode, and that the growth mode transition from two- to three-dimensional occurred when the layer was thicker than 1 nm. In conclusion, polarity conversion apparently occurs due to the different atomic structure between the wetting layer and islands of MgO. © 2004 American Institute of Physics.
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.55.-a Thin film structure and morphology

Jamming phase diagram of colloidal dispersions by molecular dynamics simulations

Anil Kumar and Jianzhong Wu

Appl. Phys. Lett. 84, 4565 (2004); http://dx.doi.org/10.1063/1.1759767 (3 pages) | Cited 2 times

Online Publication Date: 14 May 2004

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We report a three-dimensional jamming phase diagram of a model colloidal system obtained from molecular dynamics simulations where the inter-colloidal forces are represented by the Derjaguin–Landau–Verwey–Overbeek potential. The jamming threshold is uniquely defined in terms of the critical volume fraction, the critical temperature, and the critical yield stress. The simulation results indicate that near the jamming transition the shear viscosity diverges following a critical-like scaling law as observed for realistic colloids. These results offer a convincing proof for unifying different nonequilibrium transitions in colloids under the concepts of jamming. © 2004 American Institute of Physics.
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64.70.K- Solid-solid transitions
82.70.Dd Colloids
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