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9 Feb 2004

Volume 84, Issue 6, pp. 837-1024

Issue Cover Spotlight Figure

Appl. Phys. Lett. 84, 975 (2004); http://dx.doi.org/10.1063/1.1646453 (3 pages)

Xiang Yang Kong and Zhong Lin Wang
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Scaling law for the compositional dependence of Raman frequencies in SnGe and GeSi alloys

S. F. Li, M. R. Bauer, J. Menéndez, and J. Kouvetakis

Appl. Phys. Lett. 84, 867 (2004); http://dx.doi.org/10.1063/1.1645667 (3 pages) | Cited 10 times

Online Publication Date: 4 February 2004

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The compositional dependence of the Ge–Ge Raman mode in SnGe alloys has been measured in samples grown on Si substrates using a chemical vapor deposition technique. The experimental result, Δω(s) = (−68±5)s (where s is the Sn concentration), is in very good agreement with a theoretical prediction from a simple model with parameters adjusted to the compositional dependence of Raman frequencies in GeSi alloys. © 2004 American Institute of Physics.
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78.30.Am Elemental semiconductors and insulators
78.66.Li Other semiconductors

Transmission electron microscopy and Raman measurements of the misfit stress in a Si tensile strained layer

M. Cabié, A. Ponchet, A. Rocher, V. Paillard, and L. Vincent

Appl. Phys. Lett. 84, 870 (2004); http://dx.doi.org/10.1063/1.1644639 (3 pages) | Cited 4 times

Online Publication Date: 4 February 2004

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A tensile-strained Si layer grown on a Si0.8Ge0.2 pseudo substrate with a nominal lattice mismatch of −0.76% has been studied by transmission electron microscopy using a curvature method and Raman scattering in order to determine experimentally the in-plane component of the epitaxial stress. The stress is obtained by measuring the curvature and the thickness on different areas of a thinned sample. Experimental values of the stress given by the two methods are in good agreement and are close to the nominal one. © 2004 American Institute of Physics.
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78.30.Am Elemental semiconductors and insulators
68.60.Bs Mechanical and acoustical properties

Parametric study on optical properties of digital-alloy In(Ga1−zAlz)As/InP grown by molecular-beam epitaxy

J. D. Song, D. C. Heo, I. K. Han, J. M. Kim, Y. T. Lee, and S.-H. Park

Appl. Phys. Lett. 84, 873 (2004); http://dx.doi.org/10.1063/1.1645666 (3 pages) | Cited 1 time

Online Publication Date: 4 February 2004

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Optical properties of digital-alloy InGaAlAs grown by molecular-beam epitaxy were parametrically investigated by 10-K-photoluminescence (PL) study on (In0.53Ga0.47As)n/(In0.52Al0.48As)n short-period superlattices (SPSs) in the range of n = 1–5 monolayers. Two different peaks are resolved in PL spectra, and the higher energy peak (H) results from an excitonic transition while the lower energy peak (L) is related to a phonon-assisted transition. The H peak energies decrease monotonously as n increases, and it is in good agreement with the band-gap calculation with transfer matrix methods. It is found that two monolayer-period length (n = 2) is the optimum one, where the PL intensity is largest and the ratio of L peak to H peak intensity is lowest. The various compositions (z) of digital-alloy In(Ga1−zAlz)As are prepared and their optical properties are investigated for z = 0.2, 0.4, 0.6, and 0.8. The linewidths of 9-K-PL spectra for various z values are within the range of 10–15 meV, which are comparable to the best analog-alloy InGaAlAs. © 2004 American Institute of Physics.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
73.21.Cd Superlattices
78.55.Cr III-V semiconductors
71.35.-y Excitons and related phenomena
78.66.Fd III-V semiconductors
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
63.20.D- Phonon states and bands, normal modes, and phonon dispersion

Modeling of Si self-diffusion in SiO2: Effect of the Si/SiO2 interface including time-dependent diffusivity

Masashi Uematsu, Hiroyuki Kageshima, Yasuo Takahashi, Shigeto Fukatsu, Kohei M. Itoh, Kenji Shiraishi, and Ulrich Gösele

Appl. Phys. Lett. 84, 876 (2004); http://dx.doi.org/10.1063/1.1644623 (3 pages) | Cited 22 times

Online Publication Date: 4 February 2004

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Self-diffusion of Si in thermally grown SiO2 is modeled taking into account the effect of SiO molecules generated at the Si/SiO2 interface and diffusing into SiO2 to enhance the self-diffusion. Based on the model, a recent self-diffusion experiment of ion-implanted 30Si in SiO2, which showed increasing self-diffusivity with decreasing distance between the 30Si diffusers and Si/SiO2 interface [Fukatsu et al., Appl. Phys. Lett. 83, 3897 (2003)], was simulated, and the simulated results fit the experimental profiles. Furthermore, the simulation predicts that the self-diffusivity would increase for a longer annealing time because more SiO molecules should be arriving from the interface. Such time-dependent diffusivity was indeed found in our follow-up experiments, and the profiles were also fitted by the simulation using a single set of parameters. © 2004 American Institute of Physics.
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66.30.Dn Theory of diffusion and ionic conduction in solids
66.30.H- Self-diffusion and ionic conduction in nonmetals
68.35.Ct Interface structure and roughness
61.72.Cc Kinetics of defect formation and annealing
61.72.up Other materials

Third-order optical nonlinearity originating from room-temperature exciton in layered compounds LaCuOS and LaCuOSe

Hayato Kamioka, Hidenori Hiramatsu, Hiromichi Ohta, Masahiro Hirano, Kazushige Ueda, Toshio Kamiya, and Hideo Hosono

Appl. Phys. Lett. 84, 879 (2004); http://dx.doi.org/10.1063/1.1646221 (3 pages) | Cited 24 times

Online Publication Date: 4 February 2004

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We have studied the third-order optical nonlinearity (χ(3)) of epitaxial thin films of layered compounds LaCuOS and LaCuOSe at room temperature by a spectrally resolved degenerative four-wave mixing technique with femtosecond time resolution. The χ(3) values in both films are sharply resonant to optical absorption bands in the ultraviolet (UV) light region due to room-temperature exciton. The peak values are evaluated to be as large as 2–4×10−9 esu with a fast time response of 250–300 fs. These findings indicate that LaCuOS and LaCuOSe are promising materials for emerging optical nonlinear devices that operate in the UV light region compatible for GaN-based lasers. © 2004 American Institute of Physics.
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42.70.Qs Photonic bandgap materials
71.35.Cc Intrinsic properties of excitons; optical absorption spectra
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
68.55.-a Thin film structure and morphology
42.70.Mp Nonlinear optical crystals
78.40.Ha Other nonmetallic inorganics

Very fast YbxY1−xAlO3 single-crystal scintillators

M. Nikl, N. Solovieva, J. Pejchal, J. B. Shim, A. Yoshikawa, T. Fukuda, A. Vedda, M. Martini, and D. H. Yoon

Appl. Phys. Lett. 84, 882 (2004); http://dx.doi.org/10.1063/1.1645987 (3 pages) | Cited 13 times

Online Publication Date: 4 February 2004

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Scintillation, radioluminescence, photoluminescence, and thermoluminescence characteristics of Czochralski-grown YbxY1−xAlO3 single crystals for x = 0.02–0.45 are described. Under x-ray excitation, the intensity of Yb3+ charge-transfer luminescence is increasing with x and reaches at room temperature more than 11% of Bi4Ge3O12 (BGO) for x = 0.45, while at 80 K it is over 170% of BGO for x ≥ 0.3. At room temperature, scintillation decay shows a dominant decay time of 0.8–0.9 ns for x ≥ 0.3 and no significant slow components were detected. The photoluminescence decay time is tuned by temperature and Yb concentration from tens of nanoseconds at 80 K down to about 0.8 ns at 295 K. Thermoluminescence glow curve below room temperature evidences very low concentration of shallow traps compared to the Ce-doped YAlO3 scintillator. © 2004 American Institute of Physics.
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78.55.Hx Other solid inorganic materials
78.60.Kn Thermoluminescence
78.47.-p Spectroscopy of solid state dynamics
71.70.Gm Exchange interactions

Ordering parameters of self-organized three-dimensional quantum-dot lattices determined from anomalous x-ray diffraction

R. T. Lechner, T. U. Schülli, V. Holý, G. Springholz, J. Stangl, A. Raab, G. Bauer, and T. H. Metzger

Appl. Phys. Lett. 84, 885 (2004); http://dx.doi.org/10.1063/1.1644627 (3 pages) | Cited 7 times

Online Publication Date: 4 February 2004

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Anomalous x-ray diffraction is used to investigate self-organized ordering of PbSe/PbEuTe quantum dot superlattices in which different ordered structures form for different PbSe dot layer spacings. Using a theoretical dot ordering model, the different ordering parameters are determined from the x-ray spectra. © 2004 American Institute of Physics.
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68.65.Hb Quantum dots (patterned in quantum wells)
81.07.Ta Quantum dots
68.65.Cd Superlattices
61.72.Nn Stacking faults and other planar or extended defects

Optical and structural properties of SixSnyGe1−x−y alloys

P. Aella, C. Cook, J. Tolle, S. Zollner, A. V. G. Chizmeshya, and J. Kouvetakis

Appl. Phys. Lett. 84, 888 (2004); http://dx.doi.org/10.1063/1.1645324 (3 pages) | Cited 18 times

Online Publication Date: 4 February 2004

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Single-phase SixSnyGe1−x−y alloys (x ⩽ 0.25,y ⩽ 0.11) were grown on Si using chemical vapor deposition. First principles simulations predict that these materials are thermodynamically accessible and yield lattice constants as a function of Si/Sn concentrations in good agreement with experiment. An empirical model derived from experimental SixGe1−x and SnyGe1−y binary data also provides a quantitative description of the composition dependence of the lattice parameters. Spectroscopic ellipsometry of selected samples yields dielectric functions indicating a band structure consistent with highly crystalline semiconductor materials of diamond symmetry. Incorporation of Si into SnyGe1−y leads to an additional reduction of the E2 critical point, as expected based on the E2 values of Si and Ge. © 2004 American Institute of Physics.
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78.66.Li Other semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
71.15.Pd Molecular dynamics calculations (Car-Parrinello) and other numerical simulations
77.22.Ch Permittivity (dielectric function)
73.20.At Surface states, band structure, electron density of states
81.05.Hd Other semiconductors
68.55.-a Thin film structure and morphology

Ultrafast conductivity dynamics in pentacene probed using terahertz spectroscopy

V. K. Thorsmølle, R. D. Averitt, X. Chi, D. J. Hilton, D. L. Smith, A. P. Ramirez, and A. J. Taylor

Appl. Phys. Lett. 84, 891 (2004); http://dx.doi.org/10.1063/1.1644046 (3 pages) | Cited 22 times

Online Publication Date: 4 February 2004

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We present measurements of the transient photoconductivity in pentacene single crystals using optical-pump terahertz-probe spectroscopy. We have measured the temperature and fluence dependence of the mobility of the photoexcited charge carriers with picosecond resolution. The pentacene crystals were excited at 3.0 eV, which is above the bandgap of ∼ 2.2 eV, and the induced change in the far-infrared transmission was measured. At 30 K, the carrier mobility is μ ≈ 0.4 cm2/V s and decreases to μ ≈ 0.2 cm2/V s at room temperature. The transient terahertz signal reveals the presence of free carriers that are trapped on the time scale of a few picoseconds or less, possibly through the formation of excitons, small polarons, or trapping by impurities. © 2004 American Institute of Physics.
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73.61.Ph Polymers; organic compounds
78.66.Qn Polymers; organic compounds
72.40.+w Photoconduction and photovoltaic effects
71.20.Rv Polymers and organic compounds
71.35.-y Excitons and related phenomena
61.72.S- Impurities in crystals

Ion irradiation effects on the mechanical stability of compressed metallic thin films

P. Goudeau, M. George, and C. Coupeau

Appl. Phys. Lett. 84, 894 (2004); http://dx.doi.org/10.1063/1.1645981 (3 pages) | Cited 1 time

Online Publication Date: 4 February 2004

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Residual stresses in materials and components such as thin films in microelectromechanical systems are developed during the production process and may drastically limit their functionality and lifetime; their minimization is thus of utmost technical importance. Ion beam irradiation in polycrystalline Mo thin films sputter-deposited on (100) silicon wafer at room temperature has been used to reduce the residual stresses present in the layer and thus to improve the mechanical stability of the film/substrate set. Although a strong reduction of residual stresses in the whole film is evidenced, irradiation affects the film-substrate bond strength; defect creation at the film-substrate interface due to ion implantation induces thin-film delamination. © 2004 American Institute of Physics.
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61.80.Jh Ion radiation effects
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
61.82.Bg Metals and alloys
68.60.Bs Mechanical and acoustical properties
61.72.J- Point defects and defect clusters
68.35.Ct Interface structure and roughness

Local vibrational modes of H complexes in Mg-doped GaN grown by molecular beam epitaxy

R. Cuscó, L. Artús, D. Pastor, F. B. Naranjo, and E. Calleja

Appl. Phys. Lett. 84, 897 (2004); http://dx.doi.org/10.1063/1.1645668 (3 pages) | Cited 5 times

Online Publication Date: 4 February 2004

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Local vibrational modes (LVM’s) in Mg-doped GaN grown by molecular beam epitaxy have been studied by Raman scattering. Besides Mg local vibrational modes, several local mode peaks associated with hydrogen complexes are observed in the region around 2200 cm−1 and peaks assigned to carbon-hydrogen complexes are also detected in the region around 2900 cm−1. These modes arise from the presence of unintentional carbon and hydrogen impurities in the sample, which was corroborated by secondary ion mass spectroscopy measurements. Raman scattering evidence of local vibrational modes of the C-H complex in GaN is reported. The behavior of the LVM’s associated with the hydrogen and carbon impurity complexes with annealing temperature is also reported. © 2004 American Institute of Physics.
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78.66.Fd III-V semiconductors
78.30.Fs III-V and II-VI semiconductors
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
71.55.Eq III-V semiconductors

Nonlinear propagation of ultrafast 1.5 μm pulses in high-index-contrast silicon-on-insulator waveguides

Georg W. Rieger, Kuljit S. Virk, and Jeff F. Young

Appl. Phys. Lett. 84, 900 (2004); http://dx.doi.org/10.1063/1.1645991 (3 pages) | Cited 54 times

Online Publication Date: 4 February 2004

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Propagation through silicon-on-insulator (SOI) waveguide structures of 1.53 μm, 100 fs laser pulses with peak powers up to 400 W is studied experimentally and theoretically. The dominant nonlinear effects are two-photon absorption and self-phase modulation. The two-photon absorption coefficient and the nonlinear refractive index of Si obtained in this work are β2 = 0.9 cm/GW and n2 = 0.7×10−13 cm2/W, respectively. At high intensities, free carriers generated by two-photon absorption are demonstrated to have a significant influence on pulse spectra and transmitted power. The figure of merit for all-optical switching obtained in this work (T = 1.8) indicates that a switch based on a SOI waveguide structure might be possible at 1.55 μm. © 2004 American Institute of Physics.
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42.25.Bs Wave propagation, transmission and absorption
42.79.Gn Optical waveguides and couplers
42.65.Wi Nonlinear waveguides
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Enhancement of 1.54 μm photoluminescence observed in Al-doped β-FeSi2

Yoshikazu Terai and Yoshihito Maeda

Appl. Phys. Lett. 84, 903 (2004); http://dx.doi.org/10.1063/1.1646215 (3 pages) | Cited 13 times

Online Publication Date: 4 February 2004

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We present photoluminescence (PL) properties of impurity (Al, Mn, Co)-doped β-FeSi2 produced by ion-beam synthesis. The β-Fe(Si1−xAlx)2 sample showed an increase of both the PL intensity and activation energy (Ea) for a nonradiative recombination path. On the contrary, the β-(Fe1−xMnx)Si2 and the β-(Fe1−xCox)Si2 samples reduced the PL intensity and Ea. We found that the doping of Al atoms occupying the Si sites in β-FeSi2 is effective to enhance the 1.54 μm photoluminescence. © 2004 American Institute of Physics.
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61.72.S- Impurities in crystals
78.55.Hx Other solid inorganic materials
61.80.Jh Ion radiation effects

Tensile strained epitaxial Ge films on Si(100) substrates with potential application in L-band telecommunications

Douglas D. Cannon, Jifeng Liu, Yasuhiko Ishikawa, Kazumi Wada, David T. Danielson, Samerkhae Jongthammanurak, Jurgen Michel, and Lionel C. Kimerling

Appl. Phys. Lett. 84, 906 (2004); http://dx.doi.org/10.1063/1.1645677 (3 pages) | Cited 34 times

Online Publication Date: 4 February 2004

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Tensile strained epitaxial Ge films were grown on Si(100) substrates by ultra-high vacuum chemical vapor deposition. The tensile strain was induced by the thermal expansion coefficient mismatch between Si and Ge during the cooling process from elevated growth temperatures, which induces narrowing of the Ge direct band gap, EgΓ, and pushes the absorption spectrum of Ge toward longer wavelengths. The EgΓ versus strain relation was measured experimentally by photoreflectance and x-ray diffraction, and the result agrees well with calculations by deformation potential theory. With an in-plane tensile strain of 0.21%, the EgΓ of the Ge film grown at 800 °C decreased from 32 meV to 0.768 eV compared with 0.80 for bulk Ge, and corresponded to an absorption edge at 1610 nm. The broadened absorption spectrum of tensile strained Ge makes it promising as a Si-compatible photodector material for L-band (1560–1620 nm) optical communications. © 2004 American Institute of Physics.
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78.66.Db Elemental semiconductors and insulators
65.40.De Thermal expansion; thermomechanical effects
78.40.Fy Semiconductors
68.60.Bs Mechanical and acoustical properties
71.20.Mq Elemental semiconductors
68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.05.Cy Elemental semiconductors

Reduced SnO2 surfaces by first-principles calculations

Wolfgang Bergermayer and Isao Tanaka

Appl. Phys. Lett. 84, 909 (2004); http://dx.doi.org/10.1063/1.1646460 (3 pages) | Cited 28 times

Online Publication Date: 4 February 2004

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SnO2(110) and (101) surfaces with eleven different kinds of terminations for each are systematically investigated by a first-principles projector augmented wave method. Surface energies are discussed as a function of temperature and oxygen partial pressure. Atomic relaxations of the surfaces are then compared. In agreement with previous calculations, the stoichiometric (110) surface is the most stable surface at high oxygen chemical potentials (i.e., low temperature or high pressure). At lower oxygen chemical potentials, however, one of the reduced (101) surface terminations becomes energetically preferred. The other surface terminations are found to be less stable. This is consistent with recent thin-film experimental results. © 2004 American Institute of Physics.
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61.66.Bi Elemental solids
61.66.Dk Alloys
68.35.Md Surface thermodynamics, surface energies
65.40.G- Other thermodynamical quantities

Effect of nitridation on polarity, microstructure, and morphology of AlN films

Y. Wu, A. Hanlon, J. F. Kaeding, R. Sharma, P. T. Fini, S. Nakamura, and J. S. Speck

Appl. Phys. Lett. 84, 912 (2004); http://dx.doi.org/10.1063/1.1646222 (3 pages) | Cited 28 times

Online Publication Date: 4 February 2004

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The polarity of AlN epitaxial layers grown on (0001) sapphire, SiC, and nitrided sapphire substrates was examined by convergent beam electron diffraction, and the morphology and microstructure were characterized by atomic force microscopy and scattering contrast transmission electron microscopy (TEM). The AlN films grown on sapphire and SiC without a buffer layer or nitridation of the substrate were flat and had Al polarity. From TEM studies in both cross section and plan view, it was found that the threading dislocation (TD) density was ∼ 1×108 and 2×1010 cm−2 for screw-component and pure edge component dislocations, respectively. N-face AlN was realized by pregrowth nitridation of sapphire substrates, but these films contained a small volume fraction of Al-face inversion domains which were related to hexagonal pyramids defined by {1math02} facets. The density of screw-component TDs was significantly reduced due to nitridation. Cross-sectional TEM showed that the film grown on nitrided sapphire was almost free of screw-component TDs and that the density of pure edge TDs was ∼ 4×109 cm−2. © 2004 American Institute of Physics.
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81.65.Lp Surface hardening: nitridation, carburization, carbonitridation
68.55.-a Thin film structure and morphology
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
61.72.Ff Direct observation of dislocations and other defects (etch pits, decoration, electron microscopy, x-ray topography, etc.)
68.35.B- Structure of clean surfaces (and surface reconstruction)
68.37.Ps Atomic force microscopy (AFM)

Study on the strength and elongation of free-standing Al beams for microelectromechanical systems applications

Hoo-Jeong Lee, Ping Zhang, and John C. Bravman

Appl. Phys. Lett. 84, 915 (2004); http://dx.doi.org/10.1063/1.1646765 (3 pages) | Cited 4 times

Online Publication Date: 4 February 2004

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This letter reports a study on the strength and elongation of Al in the form of micrometer-scale, free-standing beams. We fabricated microtensile specimens of Al beams by using a simple micromachining process and carried out uniaxial tensile tests in a mechanical test system with a piezoelectric actuator that controls the displacement as small as a few nanometers. The results of the tests suggest that, if the grain size remains the same, the yield strength does not change with the thickness whereas the elongation varies with it very sensitively. In addition, the yield strength of the Al beams is found to be much smaller than that of Al films bonded onto Si substrates but somewhat larger than that of the bulk Al estimated for a comparable grain size from the Hall–Petch relation. © 2004 American Institute of Physics.
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85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.70.Bt Mechanical testing, impact tests, static and dynamic loads
07.10.Cm Micromechanical devices and systems
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

High pressure saturation of hydrogen stored by single-wall carbon nanotubes

Jeremy Lawrence and Gu Xu

Appl. Phys. Lett. 84, 918 (2004); http://dx.doi.org/10.1063/1.1646728 (3 pages) | Cited 25 times

Online Publication Date: 4 February 2004

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Since the early report of adsorption on single-walled carbon nanotubes in 1997, a number of controversial publications have claimed the hydrogen capacity of these materials to be between 0.1 and 10.0 wt %. However, no study has yet demonstrated a plateau of adsorption with pressure that is consistent with monolayer saturation. Others have suggested that the tube bundle structure expand under high pressure to enable higher adsorption on newly uncovered surfaces, but received no confirmation. Using a high-pressure instrument with in situ electrical probes, we found that a plateau is nearly achieved at about 300 atm in the room temperature isotherm. And that nanotube bundles do not expand or swell, shown by the in situ electrical measurements of purified single-walled nanotube (SWNT) bundles. The monolayer saturation plateau was found on bundled SWNT at room temperature corresponding to an adsorption of 0.9 wt %. © 2004 American Institute of Physics.
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84.60.-h Direct energy conversion and storage
68.43.Mn Adsorption kinetics
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