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24 Oct 2005

Volume 87, Issue 17, Articles (17xxxx)

Issue Cover Spotlight Figure

Appl. Phys. Lett. 87, 172506 (2005); http://dx.doi.org/10.1063/1.2120911 (3 pages)

T. Kimura, Y. Otani, and J. Hamrle
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Decomposition in as-grown (Ga,In)(N,As) quantum wells

X. Kong, A. Trampert, E. Tournié, and K. H. Ploog

Appl. Phys. Lett. 87, 171901 (2005); http://dx.doi.org/10.1063/1.2108108 (3 pages) | Cited 20 times

Online Publication Date: 17 October 2005

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We report on the investigation of the local element distribution in as-grown (Ga,In)(N,As) quantum wells with high In and N contents by using low-loss electron energy-loss spectroscopy combined with dark-field transmission electron microscopy. The (Ga,In)(N,As) quantum wells were grown on GaAs(001) substrates at different growth temperatures by molecular-beam epitaxy. Lateral modulations on the nanometer scale were detected with reversal In and N distributions pointing to the existence of regions with a more favorable Ga–N and In–As bond configurations, respectively. These composition fluctuations are the driving force for the morphological instabilities at the interfaces. Lowering the growth temperature of the quantum well results in a more homogeneous element distribution of the quaternary compound. This result is discussed with regard to the influence of the epitaxial strain and cohesive bond energy on the alloy formation during epitaxial growth.
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68.65.Fg Quantum wells
82.80.-d Chemical analysis and related physical methods of analysis
79.20.Uv Electron energy loss spectroscopy
68.37.Lp Transmission electron microscopy (TEM)
68.55.A- Nucleation and growth
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
68.60.Bs Mechanical and acoustical properties

Patterning of sub-10-nm Ge islands on Si(100) by directed self-assembly

Olivier Guise, John T. Yates, Jeremy Levy, Joachim Ahner, Venugopalan Vaithyanathan, and Darrell G. Schlom

Appl. Phys. Lett. 87, 171902 (2005); http://dx.doi.org/10.1063/1.2112198 (3 pages) | Cited 20 times

Online Publication Date: 17 October 2005

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A process is reported for creating arbitrary patterns of sub-10-nm Ge islands on a Si(100) substrate by directed self-assembly. Carbon-based templates are created on Si substrates by electron-beam-induced deposition using high-resolution electron beam lithography. Ozone etching, followed by annealing in ultra-high vacuum, yields small (<4 nm) SiC nucleation sites for subsequently deposited Ge. Quantitative analysis of atomic force microscope images reveals templated Ge islands with mean diameter d ∼ 8 nm, averaging 2000±500 atoms per island, with controlled spacings as small as 35 nm, and 2 nm absolute positional accuracy. The Ge/Si nanostructures reported here may find use in end-of-scaling classical computing and single-electron devices and spin-based quantum computing architectures.
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81.05.Cy Elemental semiconductors
81.07.Bc Nanocrystalline materials
81.16.Dn Self-assembly
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.16.Nd Micro- and nanolithography
81.65.Cf Surface cleaning, etching, patterning
81.40.Gh Other heat and thermomechanical treatments
68.37.Ps Atomic force microscopy (AFM)

Macroscopic thermoelectric inhomogeneities in (AgSbTe2)x(PbTe)1−x

Nancy Chen, Franck Gascoin, G. Jeffrey Snyder, Eckhard Müller, Gabriele Karpinski, and Christian Stiewe

Appl. Phys. Lett. 87, 171903 (2005); http://dx.doi.org/10.1063/1.2056590 (3 pages) | Cited 32 times

Online Publication Date: 17 October 2005

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Exceptionally high thermoelectric figure of merit (zT>2), has been reported for (Ag1−ySbTe2)0.05(PbTe)0.95, which may involve the nanoscale microstructure. However, conflicting reports on the same materials claim only zT of 1 or less. Here we show that (Ag1−ySbTe2)0.05(PbTe)0.95 materials are multiphase on the scale of millimeters despite appearing homogeneous by x-ray diffraction and routine electron microscopy. Using a scanning Seebeck microprobe, we find significant variation of Seebeck coefficient (including both n-type and p-type behavior in the same sample) that can explain the discrepancy in reported zT. More homogeneous samples can be prepared with faster cooling rates.
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72.15.Jf Thermoelectric and thermomagnetic effects

Dielectric mismatch effects on the electronic and optical properties of GaN/HfO2 quantum wells

T. A. S. Pereira, J. S. de Sousa, G. A. Farias, J. A. K. Freire, M. H. Degani, and V. N. Freire

Appl. Phys. Lett. 87, 171904 (2005); http://dx.doi.org/10.1063/1.2112188 (3 pages) | Cited 3 times

Online Publication Date: 17 October 2005

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In this work, we demonstrate that the image charges effect induced by the dielectric mismatch strongly modifies the electronic structure of GaN/HfO2 quantum wells (QWs) in such a way that the ideal QW confinement model is no longer suitable for the description of these systems. Particularly, two different confinement regimes were observed for narrow and wide QWs. In the former, electrons, light and heavy holes are spatially localized in the same region. In wide QWs, heavy holes are confined in the interfacial regions due to the strong attraction of the image charges, which does not occur for electrons and light holes. As a consequence, optical transitions involving electrons and heavy holes become less efficient in wide QWs.
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73.21.Fg Quantum wells
73.20.At Surface states, band structure, electron density of states
78.67.De Quantum wells

Strong anisotropic spin dynamics in narrow n-InGaAs/AlGaAs (110) quantum wells

K. Morita, H. Sanada, S. Matsuzaka, C. Y. Hu, Y. Ohno, and H. Ohno

Appl. Phys. Lett. 87, 171905 (2005); http://dx.doi.org/10.1063/1.2112193 (3 pages) | Cited 9 times

Online Publication Date: 17 October 2005

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Anisotropic spin dynamics of two-dimensional electrons in strained n-InGaAs/AlGaAs (110) quantum wells (QWs) is investigated by a time-resolved Faraday rotation technique. Strong anisotropy of the relaxation time for the electron spins in parallel (τ‖) and perpendicular (τ⊥) to the QWs is observed (τ⊥/τ‖ ∼ 60) at 150 K as a result of the enhanced D'yakonov–Perel' (DP) spin relaxation mechanism. At 5 K, an anisotropic feature of the spin relaxation time is also observed in the presence of in-plane magnetic field, suggesting that the DP mechanism is effective for low-temperature spin relaxation.
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75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
75.75.-c Magnetic properties of nanostructures
78.20.Ls Magneto-optical effects

Chemical passivity of III-VI bilayer terminated Si(111)

Jonathan A. Adams, Aaron A. Bostwick, Fumio S. Ohuchi, and Marjorie A. Olmstead

Appl. Phys. Lett. 87, 171906 (2005); http://dx.doi.org/10.1063/1.2112200 (3 pages) | Cited 4 times

Online Publication Date: 17 October 2005

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The chemical stability of Si(111), terminated with bilayer AlSe and GaSe, upon exposure to atmosphere, N2 and O2, was investigated with core-level and valence band photoelectron spectroscopy. Si(111):GaSe and Si(111):AlSe both form stable, unreconstructed surfaces with no states in the silicon energy gap; their atomic structures are nearly identical. However, similarities in surface electronic and atomic structure do not imply similar chemical passivity. While Si(111):GaSe is largely unaffected by the exposures, Si(111):AlSe reacts irreversibly with both pure O2 and atmosphere, removing over math of the Se and permanently destroying long-range order.
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81.05.Cy Elemental semiconductors
81.05.Hd Other semiconductors
68.47.Fg Semiconductor surfaces
81.65.Rv Passivation
71.20.Mq Elemental semiconductors
73.20.At Surface states, band structure, electron density of states
71.20.Nr Semiconductor compounds
68.55.-a Thin film structure and morphology
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Thermally induced second-order nonlinearity in silica-based glasses

Michael Fokine, Kazuya Saito, and Akira J. Ikushima

Appl. Phys. Lett. 87, 171907 (2005); http://dx.doi.org/10.1063/1.2112205 (3 pages) | Cited 3 times

Online Publication Date: 17 October 2005

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This letter presents the observation of second-harmonic generation in pure silica and germanium-doped glass samples after heat treatment at elevated temperatures followed by rapid quenching to room temperature. No applied voltage has been used and the samples do not contain crystals. The induced nonlinearity is located near the surfaces of the sample. The possible origin of the second-order nonlinearity is discussed.
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42.65.An Optical susceptibility, hyperpolarizability
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
61.43.Fs Glasses

Work function tuning of nickel silicide by co-sputtering nickel and silicon

Nivedita Biswas, Jason Gurganus, and Veena Misra

Appl. Phys. Lett. 87, 171908 (2005); http://dx.doi.org/10.1063/1.2115072 (3 pages) | Cited 9 times

Online Publication Date: 17 October 2005

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Co-sputtered nickel silicide films were evaluated on thin layers of SiO2 gate dielectrics. Work function values ranging from 4.86 eV for Ni rich films to 4.3 eV were observed at 400 °C and were found to be a strong function of the Ni and Si ratio in the films. Phase analysis indicated the presence of different phases of NixSiy for varying concentrations of Ni and Si. High-temperature characteristics, leakage, and change in equivalent oxide thickness values were also evaluated for selected conditions. Rutherford backscattering, x-ray diffraction, Auger electron spectroscopy and high-resolution transmission electron microscopy were used for material analyses.
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73.30.+y Surface double layers, Schottky barriers, and work functions
73.40.Ns Metal-nonmetal contacts
82.80.Yc Rutherford backscattering (RBS), and other methods of chemical analysis
82.80.Pv Electron spectroscopy (X-ray photoelectron (XPS), Auger electron spectroscopy (AES), etc.)
68.37.Lp Transmission electron microscopy (TEM)

Atomically flat aluminum-oxide barrier layers constituting magnetic tunnel junctions observed by in situ scanning tunneling microscopy

M. Mizuguchi, Y. Suzuki, T. Nagahama, and S. Yuasa

Appl. Phys. Lett. 87, 171909 (2005); http://dx.doi.org/10.1063/1.2108121 (3 pages) | Cited 4 times

Online Publication Date: 18 October 2005

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Observation using in situ scanning tunneling microscopy of the layers constituting a magnetic tunnel junction with a naturally oxidized aluminum barrier layer revealed an extremely flat aluminum-oxide surface. It was clarified from line-scan images that the aluminum-oxide barrier layer has atomic steps. This flatness, which is surprising given that the aluminum-oxide film is amorphous, reduced electron scattering within the barrier, leading to momentum-dependent tunneling, which should enable the fabrication of advanced devices, such as spin-polarized resonant tunneling transistors.
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75.47.Np Metals and alloys
75.50.Bb Fe and its alloys
68.37.Ef Scanning tunneling microscopy (including chemistry induced with STM)
61.43.Er Other amorphous solids

Electroluminescence from two fluorinated organic emitters embedded in polyvinylcarbazole

Umberto Giovanella, Chiara Botta, Antonio Papagni, Riccardo Tubino, and Luciano Miozzo

Appl. Phys. Lett. 87, 171910 (2005); http://dx.doi.org/10.1063/1.2115079 (3 pages) | Cited 6 times

Online Publication Date: 18 October 2005

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Electroluminescence and photoluminescence of two fluorinated dyes emitting in the blue and in the green, blended with polyvinylcarbazole and an oxadiazole compound, are reported. Organic light-emitting diodes realized with about 0.1 wt % dye concentration show interesting performances. Excited state complexes reduce the effectiveness of energy transfer in the blue-emitting device while, for the green-emitting device, the main mechanism of exciton generation is based on charge trapping at the emissive dye.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
78.55.Kz Solid organic materials
71.35.-y Excitons and related phenomena

980 nm laser-diode-excited intense blue upconversion in Tm3+/Yb3+-codoped gallate–bismuth–lead glasses

Q. Y. Zhang, T. Li, Z. H. Jiang, X. H. Ji, and S. Buddhudu

Appl. Phys. Lett. 87, 171911 (2005); http://dx.doi.org/10.1063/1.2115082 (3 pages) | Cited 33 times

Online Publication Date: 18 October 2005

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Intense blue-upconversion in Tm3+/Yb3+-codoped gallate–bismuth–lead glasses has been achieved under an excitation from a commercially available 980 nm laser diode. Energy transfer processes and excited-state absorption account for the population of the math emitting level of the Tm3+. Although the addition of GeO2 has enhanced the glass thermal stability, the phonon mode associated with vibration of GeO2 has almost no influence on the blue-upconversion intensity and the radiative lifetime of math level. The dependence of the phonon energy of the host on contributions from multiphonon decay on the fluorescence has been discussed. Significant enhancement of the blue-upconversion has also been observed in gallate–bismuth–lead glasses with the incorporation of PbF2 content.
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78.55.Qr Amorphous materials; glasses and other disordered solids
42.70.Ce Glasses, quartz

Thermoelectric properties and microstructure of c-axis-oriented Ca3Co4O9 thin films on glass substrates

Y. F. Hu, E. Sutter, W. D. Si, and Qiang Li

Appl. Phys. Lett. 87, 171912 (2005); http://dx.doi.org/10.1063/1.2117615 (3 pages) | Cited 15 times

Online Publication Date: 19 October 2005

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c-axis-oriented Ca3Co4O9 thin films have been grown directly on glass (fused silica) substrate by pulsed laser deposition. Detailed microstructure analysis showed stacking faults abundant throughout the films. However, the Seebeck coefficient ( ∼ 130 μV/K) and resistivity ( ∼ 4.3 mΩ cm) of these films on glass substrate at room temperature were found comparable to those of the single-crystal samples. The presence of these structural defects could reduce thermal conductivity, and thus enhance the overall performance of cobaltate films to be potentially used in the thermoelectric devices.
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72.20.Pa Thermoelectric and thermomagnetic effects
68.55.-a Thin film structure and morphology
73.61.-r Electrical properties of specific thin films
66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Relaxation of biaxial tensile strain in ultrathin metallic films: Ductile void growth versus nanocrystalline domain formation

M. Rauf Gungor and Dimitrios Maroudas

Appl. Phys. Lett. 87, 171913 (2005); http://dx.doi.org/10.1063/1.2108128 (3 pages) | Cited 7 times

Online Publication Date: 19 October 2005

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We report a computational analysis of the atomistic mechanisms of relaxation of biaxially applied tensile strains over a range of strain levels up to 17% in free-standing ultrathin metallic films with the film plane oriented normal to the [111] crystallographic direction. The analysis is based on molecular-dynamics simulations using slab supercells that contain millions of atoms to model single-crystalline thin films without and with cylindrical voids oriented normal to the film plane and penetrating through the film thickness. At high levels of applied strain (>8%), a strain relaxation regime other than the ductile void growth is revealed that gives rise to a practically uniform distribution of dislocations in the film and subsequent formation of nanometer-scale face-centered-cubic crystalline domains, i.e., a single-to-polycrystalline structural transition. It is demonstrated that in this strain relaxation regime, void growth is inhibited as the dislocations emitted from the void surface are pinned by their interaction with the simultaneously generated network of defects in the nanocrystalline material.
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68.60.Bs Mechanical and acoustical properties
61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
64.70.K- Solid-solid transitions
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances

Bulkier glass formability enhanced by minor alloying additions

D. Ma, H. Cao, L. Ding, Y. A. Chang, K. C. Hsieh, and Y. Pan

Appl. Phys. Lett. 87, 171914 (2005); http://dx.doi.org/10.1063/1.2115074 (3 pages) | Cited 6 times

Online Publication Date: 19 October 2005

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In this study, we present a computational thermodynamic strategy to obtain a minor but optimum amount of additional element into a base alloy to improve its glass-forming ability, through thermodynamically calculating the maximum liquidus depressions caused by various alloying addition (or replacement) schemes. We demonstrate the successful use of Zr56.2Cu31.3Ni4.0Al8.5 as the base alloy with the addition of 4.9% Ti, by observing a significant increase in the glass-forming ability of more than 100% in terms of the diameter of the glass formed from the base alloy to the one with the addition of 4.9% Ti. The approach presented here can be considered as a universal method to synthesize novel and bulkier metallic glasses not only of scientific interest but also potential technological applications.
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81.05.Kf Glasses (including metallic glasses)
81.05.Bx Metals, semimetals, and alloys
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition
65.60.+a Thermal properties of amorphous solids and glasses: heat capacity, thermal expansion, etc.

Epitaxial growth of multiferroic YMnO3 on GaN

A. Posadas, J.-B. Yau, C. H. Ahn, J. Han, S. Gariglio, K. Johnston, K. M. Rabe, and J. B. Neaton

Appl. Phys. Lett. 87, 171915 (2005); http://dx.doi.org/10.1063/1.2120903 (3 pages) | Cited 28 times

Online Publication Date: 21 October 2005

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In this work, we report on the epitaxial growth of multiferroic YMnO3 on GaN. Both materials are hexagonal with a nominal lattice mismatch of 4%, yet x-ray diffraction reveals an unexpected 30° rotation between the unit cells of YMnO3 and GaN that results in a much larger lattice mismatch (10%) compared to the unrotated case. Estimates based on first principles calculations show that the bonding energy gained from the rotated atomic arrangement compensates for the increase in strain energy due to the larger lattice mismatch. Understanding the energy competition between chemical bonding energy and strain energy provides insight into the heteroepitaxial growth mechanisms of complex oxide-semiconductor systems.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
68.55.-a Thin film structure and morphology
68.60.Bs Mechanical and acoustical properties
75.70.Ak Magnetic properties of monolayers and thin films
77.55.-g Dielectric thin films
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