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12 Apr 2004

Volume 84, Issue 15, pp. 2721-2955

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 2883 (2004); http://dx.doi.org/10.1063/1.1702137 (3 pages)

Pu Xian Gao and Zhong L. Wang
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Strain-rate and temperature dependent plastic yield in carbon nanotubes from ab initio calculations

Traian Dumitrică and Boris I. Yakobson

Appl. Phys. Lett. 84, 2775 (2004); http://dx.doi.org/10.1063/1.1695630 (3 pages) | Cited 27 times

Online Publication Date: 7 April 2004

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Lattice yield to tension within practical time and temperatures is determined by the probability of defect formation. Its rate in nanotubes depends in turn on the transition state and activation barrier for the Stone–Wales bond rotations. Systematic ab initio computations of the barriers for the tubes of various symmetries and radii led to accurate calculation of plastic yield strain as a function of temperature and test duration. © 2004 American Institute of Physics.
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62.25.-g Mechanical properties of nanoscale systems
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Visualization of electrochemical doping and light-emitting junction formation in conjugated polymer films

Jun Gao and Justin Dane

Appl. Phys. Lett. 84, 2778 (2004); http://dx.doi.org/10.1063/1.1702126 (3 pages) | Cited 46 times

Online Publication Date: 7 April 2004

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In situ electrochemical doping and light-emitting junction formation in conjugated polymer films have been visualized by direct imaging of a massive planar device structure under UV illumination. Both p- and n-doping of conjugated polymers have been observed. When the p- and n-doped regions meet, a light-emitting p–n junction forms and significant current flow begins. The reversal of applied bias creates many tiny and transient light-emitting p–n junctions in the previously n-doped region by the uneven growth of new p-doped areas. Eventually a stable emission zone is formed closer to the new cathode, from a polymer film that shows almost no photoluminescence due to doping. © 2004 American Institute of Physics.
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85.60.Jb Light-emitting devices
81.05.Lg Polymers and plastics; rubber; synthetic and natural fibers; organometallic and organic materials
82.45.-h Electrochemistry and electrophoresis
78.66.Qn Polymers; organic compounds
61.72.up Other materials

Strain hardening of an amorphous matrix composite due to deformation- induced nanocrystallization during quasistatic compression

Jae-Chul Lee, Yu Chan Kim, Jae-Pyoung Ahn, Sunghak Lee, and Byeong-Joo Lee

Appl. Phys. Lett. 84, 2781 (2004); http://dx.doi.org/10.1063/1.1697631 (3 pages) | Cited 12 times

Online Publication Date: 7 April 2004

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Some amorphous matrix composites reinforced with micron-sized crystalline phases exhibited strengthening behaviors during quasistatic compression. Homogeneous precipitation of nanocrystallites from the amorphous matrix is believed to be responsible for the observed strengthening phenomenon. According to a molecular dynamics simulation, quasistatic deformation can indeed promote the homogeneous precipitation of nanocrystallites, which, in turn, can serve as a reinforcing phase. Based on findings in this study, a strengthening mechanism operative in the amorphous matrix composite is proposed. © 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
61.46.-w Structure of nanoscale materials
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation
61.43.-j Disordered solids
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
61.43.Bn Structural modeling: serial-addition models, computer simulation
81.05.Ni Dispersion-, fiber-, and platelet-reinforced metal-based composites
81.07.Bc Nanocrystalline materials

Anisotropic spectroscopy of nitrogen K-edge in group-III nitrides

Shang-Peng Gao, Aihua Zhang, Jing Zhu, and Jun Yuan

Appl. Phys. Lett. 84, 2784 (2004); http://dx.doi.org/10.1063/1.1691498 (3 pages) | Cited 9 times

Online Publication Date: 7 April 2004

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Ab initio calculation of nitrogen K-edges for use in the core-level spectroscopy of industrially important group-III nitrides (AlN, GaN, InN) has been carried out systematically including the core-hole effect. The theoretical spectra for transition into final states with px,y and pz symmetries are in good agreement with the available anisotropic electron energy-loss measurements. Our spectra can be used as “fingerprints” to characterize the group-III nitrides, for example, not only to distinguish between different polymorphs of group-III nitrides, but also to identify the presence of surface oxidation. We have also presented our simulated results in terms of an orientation-average spectrum and a sample orientation-dependent dichroic spectrum for future reference. © 2004 American Institute of Physics.
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79.20.Uv Electron energy loss spectroscopy
68.49.Jk Electron scattering from surfaces
81.65.Mq Oxidation
78.20.Fm Birefringence
68.43.Bc Ab initio calculations of adsorbate structure and reactions
73.20.At Surface states, band structure, electron density of states

Thermal hysteresis in the luminescence of Cr3+ ions in Sr0.6Ba0.4 (NbO3)2

M. O. Ramirez, D. Jaque, M. Montes, J. García Solé, L. E. Bausá, and L. Ivleva

Appl. Phys. Lett. 84, 2787 (2004); http://dx.doi.org/10.1063/1.1695631 (3 pages) | Cited 4 times

Online Publication Date: 7 April 2004

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The temperature dependence of the emission spectrum of Cr3+ ions in Sr0.6Ba0.4 (NbO3)2 has been systematically investigated around the ferroelectric phase transition of this crystal ( ≈ 370 K). In spite of the strong thermal quenching of the luminescence occurring at these temperatures, the emission spectrum is still clearly detectable and shows dramatic changes in both line shape and intensity when passing through the phase transition temperature. A redshift of about 6 nm occurs when the crystal is driven from the polar to the nonpolar phase. In addition, this shift displays a thermal hysteresis, then providing the possibility of realizing a bistable spectral system based on this material. © 2004 American Institute of Physics.
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77.80.Dj Domain structure; hysteresis
77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.B- Phase transitions and Curie point
78.55.Hx Other solid inorganic materials

Relaxation of metallic Zr46.75Ti8.25Cu7.5Ni10Be27.5 bulk glass-forming supercooled liquid

Ping Wen, De Qian Zhao, Ming Xiang Pan, Wei Hua Wang, Ya Ping Huang, and Mei Li Guo

Appl. Phys. Lett. 84, 2790 (2004); http://dx.doi.org/10.1063/1.1699467 (3 pages) | Cited 18 times

Online Publication Date: 7 April 2004

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The mechanical relaxation of metallic Zr46.75Ti8.25Cu7.5Ni10Be27.5 supercooled liquid has been measured by dynamic mechanical analysis. The relaxation behaviors are found to fit well the Kohlrausch–Williams–Watts equation in time domain as well as the Havriliak–Negami equation in frequency domain. Characteristic quantities relaxation time and time–temperature superposition are among the properties also exhibited. The metallic supercooled liquid is found to have common relaxation characteristics of the nonmetallic glass-forming supercooled liquids, demonstrating a connection of the underlying physics for quite different classes of glass formers. © 2004 American Institute of Physics.
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62.10.+s Mechanical properties of liquids
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition

Efficient electron spin detection with positively charged quantum dots

K. Gündoğdu, K. C. Hall, Thomas F. Boggess, D. G. Deppe, and O. B. Shchekin

Appl. Phys. Lett. 84, 2793 (2004); http://dx.doi.org/10.1063/1.1695637 (3 pages) | Cited 19 times

Online Publication Date: 7 April 2004

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We report the application of time- and polarization-resolved photoluminescence up-conversion spectroscopy to the study of spin capture and energy relaxation in positively and negatively charged, as well as neutral InAs self-assembled quantum dots. When compared to the neutral dots, we find that carrier capture and relaxation to the ground state is much faster in the highly charged dots, suggesting that electron–hole scattering dominates this process. The long spin lifetime, short capture time, and high radiative efficiency of the positively charged dots, indicates that these structures are superior to both quantum well and neutral quantum dot light-emitting diode spin detectors for spintronics applications. © 2004 American Institute of Physics.
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73.21.La Quantum dots
78.47.-p Spectroscopy of solid state dynamics
78.55.Cr III-V semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping

Directed nanostructural evolution in Ti0.8Ce0.2N layers grown as a function of low-energy, high-flux ion irradiation

T.-Y. Lee, S. Kodambaka, J. G. Wen, R. D. Twesten, J. E. Greene, and I. Petrov

Appl. Phys. Lett. 84, 2796 (2004); http://dx.doi.org/10.1063/1.1699468 (3 pages) | Cited 3 times

Online Publication Date: 7 April 2004

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We use a combination of alloying and low-energy ion irradiation during film growth to controllably manipulate the nanostructure of TiN-based layers. Ti0.8Ce0.2N films are grown on SiO2 at 350 °C using UHV reactive magnetron sputtering in N2. The N2+-to-metal ratio incident at the growing film is maintained constant at 15, while the ion energy EN2+ is varied from 14 to 45 eV. Films grown with EN2+ = 14 eV consist of equiaxed nanograins with an average size of 2.0 nm, while layers deposited with EN2+ = 45 eV exhibit a 2-nm-wide nanocolumnar structure. In both cases, the films are dense, atomically smooth, and have strong 002 texture with low stress. This is in dramatic contrast to TiN films deposited under the same conditions, which are underdense with extremely rough surfaces and consist of broad (≃ 30 nm) columns. We attribute the formation of controlled nanostructures to the interplay between ion-irradiation-induced effects and thermally driven Ce surface segregation, leading to continuous renucleation during film growth. © 2004 American Institute of Physics.
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61.46.-w Structure of nanoscale materials
81.07.Bc Nanocrystalline materials
61.82.Rx Nanocrystalline materials
61.80.Jh Ion radiation effects
81.15.Cd Deposition by sputtering
68.55.-a Thin film structure and morphology
68.35.Dv Composition, segregation; defects and impurities
68.55.A- Nucleation and growth

Compression of Ti3Si0.5Ge0.5C2 to 53 GPa

Bouchaib Manoun, H. P. Liermann, R. P. Gulve, S. K. Saxena, A. Ganguly, M. W. Barsoum, and C. S. Zha

Appl. Phys. Lett. 84, 2799 (2004); http://dx.doi.org/10.1063/1.1699477 (3 pages) | Cited 32 times

Online Publication Date: 7 April 2004

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Using a synchrotron radiation source and a diamond anvil cell, we measured the pressure dependence of the lattice parameters of a polycrystalline Ti3Si0.5Ge0.5C2 sample. Up to a pressure of 53 GPa, no phase transformations were observed. As for the isostructural hexagonal Ti3SiC2, the compressibility along the c axis was greater than along a. The bulk modulus is 183±4 GPa with a pressure derivative of 3.4±0.2. This work shows that the replacement of Si by Ge in Ti3SiC2 results in a systematic decrease in the bulk moduli. © 2004 American Institute of Physics.
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62.20.F- Deformation and plasticity
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
62.50.-p High-pressure effects in solids and liquids
81.40.Lm Deformation, plasticity, and creep
61.66.Fn Inorganic compounds

Effects of spacer thickness on quantum efficiency of the solar cells with embedded Ge islands in the intrinsic layer

Arnold Alguno, Noritaka Usami, Toru Ujihara, Kozo Fujiwara, Gen Sazaki, Kazuo Nakajima, Kentaro Sawano, and Yasuhiro Shiraki

Appl. Phys. Lett. 84, 2802 (2004); http://dx.doi.org/10.1063/1.1697632 (3 pages) | Cited 2 times

Online Publication Date: 7 April 2004

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We report on the effects of spacer thickness on the external quantum efficiency (EQE) of the solar cells with Ge islands embedded into the intrinsic region of the Si-based p-i-n diode. The EQE response of the solar cells in the near-infrared region is dependent on the spacer thickness that separates the layers of self-assembled Ge islands. It was found that the EQE response has an optimum value when the spacer thickness can sustain a good vertical ordering of islands. On the other hand, random nucleation of islands due to a thicker spacer layer exhibits an inferior EQE response. Furthermore, a drastic decrease of the EQE response of the solar cells for a thinner spacer layer was observed. © 2004 American Institute of Physics.
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84.60.Jt Photoelectric conversion
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
78.55.Ap Elemental semiconductors

Effects of electron concentration on the optical absorption edge of InN

J. Wu, W. Walukiewicz, S. X. Li, R. Armitage, J. C. Ho, E. R. Weber, E. E. Haller, Hai Lu, William J. Schaff, A. Barcz, and R. Jakiela

Appl. Phys. Lett. 84, 2805 (2004); http://dx.doi.org/10.1063/1.1704853 (3 pages) | Cited 87 times

Online Publication Date: 7 April 2004

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InN films with free electron concentrations ranging from mid-1017 to mid-1020 cm−3 have been studied using optical absorption, Hall effect, and secondary ion mass spectrometry. The optical absorption edge covers a wide energy range from the intrinsic band gap of InN of about 0.7 to about 1.7 eV which is close to the previously accepted band gap of InN. The electron concentration dependence of the optical absorption edge energy is fully accounted for by the Burstein–Moss shift. Results of secondary ion mass spectrometry measurements indicate that O and H impurities cannot fully account for the free electron concentration in the films. © 2004 American Institute of Physics.
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78.66.Fd III-V semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
72.20.My Galvanomagnetic and other magnetotransport effects
68.55.-a Thin film structure and morphology
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
73.61.Ga II-VI semiconductors

Optical transition in discrete levels of Si quantum dots

X. L. Wu and F. S. Xue

Appl. Phys. Lett. 84, 2808 (2004); http://dx.doi.org/10.1063/1.1704872 (3 pages) | Cited 16 times

Online Publication Date: 7 April 2004

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Photoluminescence (PL) measurements have been carried out on Si quantum dots (QDs) with sizes of 2–4 nm embedded in Si oxide films. Conspicuous multiple PL peaks separated in energy by up to 104 meV are observed and attributed to optical transitions in discrete energy levels of Si QDs. A band-mixing model of direct and indirect gaps in a nanometer environment consisting of nc-Si core and SiOx surface layer has been proposed for calculation of electronic states. Good agreement is achieved between the observed PL peaks and calculated results. This work provides a path for realizing Si QD laser. © 2004 American Institute of Physics.
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78.67.Hc Quantum dots
78.55.Ap Elemental semiconductors
68.65.Hb Quantum dots (patterned in quantum wells)
73.21.La Quantum dots

Fatigue failure mechanisms of single-walled carbon nanotube ropes embedded in epoxy

Y. Ren, Y. Q. Fu, K. Liao, F. Li, and H. M. Cheng

Appl. Phys. Lett. 84, 2811 (2004); http://dx.doi.org/10.1063/1.1703837 (3 pages) | Cited 15 times

Online Publication Date: 7 April 2004

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In this work, fatigue failure mechanisms of single-walled carbon nanotube (SWCNT) bundles embedded in epoxy matrix under repeated tensile load were studied. Observed damage and failure modes include: (1) splitting of SWCNT bundles, (2) kink formation and subsequent failure in SWCNTs, and (3) fracture of SWCNT bundles. Patterns of crack propagation under tension in SWCNTs were studied by molecular mechanics simulations, where defect-free SWCNTs and SWCNTs with two different modes of Stone–Wales defects were studied. It is demonstrated by the results of molecular mechanics simulation that the observed fracture surfaces of SWCNT can be reproduced reasonably well, suggesting possible fatigue failure mechanisms of SWCNT in the composite. © 2004 American Institute of Physics.
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81.05.Ni Dispersion-, fiber-, and platelet-reinforced metal-based composites
61.46.-w Structure of nanoscale materials
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Strain relaxation and its effect in exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates

A. B. M. Almamun Ashrafi, Naguyen Thanh Binh, Bao-ping Zhang, and Yusaburo Segawa

Appl. Phys. Lett. 84, 2814 (2004); http://dx.doi.org/10.1063/1.1705722 (3 pages) | Cited 16 times

Online Publication Date: 7 April 2004

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Thickness-dependent strain relaxation and its role on exciton resonance energies of epitaxial ZnO layers grown on 6H-SiC substrates have been studied. The magnitudes of strain were determined experimentally by x-ray diffraction measurements. The strain ratios under biaxial stresses (Δc/c0)/(Δa/a0) of epitaxial ZnO layers grown on SiC and Al2O3 substrates were estimated to be 0.38 and 0.50, respectively. The strain-induced band shift δEA/δεzz for ZnO/SiC and ZnO/Al2O3 heterostructures was analyzed by photoluminescence with the values of 13.1 and 14.6 eV, respectively. These comparative strain-induced band shifts, as well as Poisson ratios, evidenced the role of lattice deformation kinetics induced by different lattice mismatches in the ZnO/SiC and ZnO/Al2O3 heterostructures. © 2004 American Institute of Physics.
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62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
78.66.Hf II-VI semiconductors
81.40.Jj Elasticity and anelasticity, stress-strain relations
68.35.Gy Mechanical properties; surface strains
78.55.Et II-VI semiconductors
62.20.D- Elasticity

Narrow photoluminescence linewidth (<17 meV) from highly uniform self-assembled InAs/GaAs quantum dots grown by low-pressure metalorganic chemical vapor deposition

Tao Yang, Jun Tatebayashi, Shiro Tsukamoto, Masao Nishioka, and Yasuhiko Arakawa

Appl. Phys. Lett. 84, 2817 (2004); http://dx.doi.org/10.1063/1.1711163 (3 pages) | Cited 19 times

Online Publication Date: 7 April 2004

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We report highly uniform self-assembled InAs quantum dots (QDs) emitting at 1.3 μm, grown on GaAs substrates by low-pressure metalorganic chemical vapor deposition. By optimizing the InAs growth rate and capping the QDs with GaAs using triethylgallium as a gallium source, we have achieved a narrow photoluminescence (PL) inhomogeneous linewidth of 16.5 meV (at 7 K) from QDs with a density of 1.7×1010 cm−2. Furthermore, we show by temperature-dependent PL measurements that the QDs exhibit almost no dependence of linewidth on temperature due to their high uniformity. © 2004 American Institute of Physics.
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78.67.Hc Quantum dots
78.55.Cr III-V semiconductors
81.16.Dn Self-assembly

Domain wall displacement induced by subnanosecond pulsed current

C. K. Lim, T. Devolder, C. Chappert, J. Grollier, V. Cros, A. Vaurès, A. Fert, and G. Faini

Appl. Phys. Lett. 84, 2820 (2004); http://dx.doi.org/10.1063/1.1711168 (3 pages) | Cited 51 times

Online Publication Date: 7 April 2004

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We show that a single current pulse as short as 0.4 ns can trigger domain wall (DW) displacement in spin-valve stripes of 0.3 μm width inserted into a coplanar waveguide. The experiments were carried out with varying current pulse amplitude, duration, polarity, and applied static magnetic field. In zero field, DW displacement occurs in the same direction as the conduction electron current. In finite applied field, the direction of DW displacement is that favored by the field orientation. In both cases, the DW displacement occurs only above a critical current density jc of the order of 106 A/cm2. The distance traveled by the DW along the stripe increases with the current pulse amplitude and applied field strength, but it does not depend on the pulse duration between 0.4 and 2 ns. © 2004 American Institute of Physics.
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75.60.Ch Domain walls and domain structure
85.75.Bb Magnetic memory using giant magnetoresistance
75.47.De Giant magnetoresistance

Fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films

Xidong Chen, J. P. Sullivan, T. A. Friedmann, and J. Murray Gibson

Appl. Phys. Lett. 84, 2823 (2004); http://dx.doi.org/10.1063/1.1713048 (3 pages) | Cited 22 times

Online Publication Date: 7 April 2004

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In this letter, we report fluctuation microscopy studies of medium-range ordering in amorphous diamond-like carbon films and the effect of annealing on this ordering. Annealed and unannealed diamond-like carbon films have almost identical short-range order. Our fluctuation microscopy results, however, indicate the presence of medium range order or clustering in the films on a lateral length scale that exceeds 1 nm. Within the clustered regions, the dominant local ordering appears to be diamond-like, and graphite-like ordering is not observed. Thermal annealing up to 600 °C leads to an increase in diamond-like clustering with no onset of graphite-like clustering. However, after high temperature annealing up to 1000 °C, graphite-like clustering becomes apparent as a result of the conversion of diamond-like carbon to graphite-like carbon. The results on the as-deposited films and films annealed up to 600 °C suggest that a spontaneous medium range ordering process occurs in diamond-like carbon films during and subsequent to film growth, and this may play an important role in stress relaxation. © 2004 American Institute of Physics.
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68.55.-a Thin film structure and morphology
68.37.-d Microscopy of surfaces, interfaces, and thin films
61.43.Er Other amorphous solids
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
81.40.Jj Elasticity and anelasticity, stress-strain relations
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