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26 Jun 2006

Volume 88, Issue 26, Articles (26xxxx)

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Appl. Phys. Lett. 88, 261107 (2006); http://dx.doi.org/10.1063/1.2213912 (3 pages)

Stanley S. Hong, Berthold K. P. Horn, Dennis M. Freeman, and Michael S. Mermelstein
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Injection dependence of spontaneous radiative recombination in crystalline silicon: Experimental verification and theoretical analysis

P. P. Altermatt, F. Geelhaar, T. Trupke, X. Dai, A. Neisser, and E. Daub

Appl. Phys. Lett. 88, 261901 (2006); http://dx.doi.org/10.1063/1.2218041 (3 pages) | Cited 16 times

Online Publication Date: 26 June 2006

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The radiative recombination coefficient B in crystalline bulk silicon is enhanced by the Coulomb attraction between electrons and holes. This effect is weakened at high carrier densities due to screening. We measure the resulting dependence of B on the free-carrier density (i) by reinterpreting published data and (ii) with photoluminescence and photovoltaic measurements. We calculate the Coulomb enhancement by determining the electron-hole pair correlation function at zero interparticle distance, assuming a Debye interaction potential. Both bound and scattering state contributions are fully taken into account. Due to screening, B decreases with increasing free-carrier density.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.55.Ap Elemental semiconductors
72.40.+w Photoconduction and photovoltaic effects

Evidence for adiabatic heating during fracture of W-reinforced metallic glass composites

Min Ha Lee and Daniel J. Sordelet

Appl. Phys. Lett. 88, 261902 (2006); http://dx.doi.org/10.1063/1.2208269 (3 pages) | Cited 8 times

Online Publication Date: 27 June 2006

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At appropriate strain rates below their glass transition temperature, amorphous alloys deform inhomogeneously by strain localization within narrow shear planes. In the current study, we present experimental evidence of viscous flow by adiabatic heating at the fracture surface of W-reinforced Hf-based metallic glass composites under quasistatic uniaxial compressive deformation. Also, based on microstructural analysis, estimation of the temperature rise in a shear band shows that when fracture occurs, it releases a high level of strain energy, which when converted to heat, reaches temperatures sufficiently high enough to induce melting or softening of the W in the composite, producing vein patterns.
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81.40.Lm Deformation, plasticity, and creep
81.40.Gh Other heat and thermomechanical treatments
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials
62.20.F- Deformation and plasticity
64.70.P- Glass transitions of specific systems
64.70.Q- Theory and modeling of the glass transition

In situ neutron diffraction measurement of transient temperature and stress fields in a thin plate

Wanchuck Woo, Zhili Feng, Xun-Li Wang, Ke An, Camden R. Hubbard, Stan A. David, and Hahn Choo

Appl. Phys. Lett. 88, 261903 (2006); http://dx.doi.org/10.1063/1.2209888 (3 pages) | Cited 7 times

Online Publication Date: 27 June 2006

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Separating the elastic and thermal strains has been a long standing problem for in situ neutron diffraction measurement of transient temperature and stress fields. Using the plane stress condition, we demonstrate a method to decompose the thermal and elastic strains from the measured lattice spacing changes. The method was validated using a thin plate subjected to a local moving heat source. The methodology developed herein has practical applications for a variety of materials processing technologies such as welding, forming, and heat treatment.
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81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Electronic and thermodynamic properties of β-Ga2O3

Haiying He, Miguel A. Blanco, and Ravindra Pandey

Appl. Phys. Lett. 88, 261904 (2006); http://dx.doi.org/10.1063/1.2218046 (3 pages) | Cited 8 times

Online Publication Date: 27 June 2006

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Electronic and thermodynamic properties of β-Ga2O3 are investigated in the framework of density functional theory. The equilibrium structural parameters and Debye temperature are obtained through fitting of the energy surface to the equation of state. Analysis of the band structure shows the valence band maximum to be degenerate at Γ and M, whereas the conduction band minimum is predicted to be at Γ. The valence band is almost flat, indicating a rather large effective mass for holes, whereas the calculated electron effective mass comes out to be about 0.12, expressed in units of the free electron mass.
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71.20.Nr Semiconductor compounds
64.30.-t Equations of state of specific substances
71.15.Mb Density functional theory, local density approximation, gradient and other corrections
63.70.+h Statistical mechanics of lattice vibrations and displacive phase transitions
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor

Role of alloy fluctuations in photoluminescence dynamics of AlGaN epilayers

E. Kuokstis, W. H. Sun, M. Shatalov, J. W. Yang, and M. Asif Khan

Appl. Phys. Lett. 88, 261905 (2006); http://dx.doi.org/10.1063/1.2218275 (3 pages) | Cited 11 times

Online Publication Date: 27 June 2006

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The near-band-edge (NBE) photoluminescence (PL) of AlGaN layers with different Al content was analyzed in a wide range of excitation intensities and temperatures. The PL peculiarities indicated that tails of density of states are formed in AlGaN alloys due to the fluctuation of the alloy composition. The model involving recombination through one type of nonradiative center is proposed. The dependence of NBE PL integrated intensity on excitation power for AlGaN is weaker than that for GaN, which is attributed to carrier localization in alloys with compositional fluctuations and, thus, reduction of probability of nonradiative recombination.
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78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors
82.80.-d Chemical analysis and related physical methods of analysis
71.20.Nr Semiconductor compounds
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
73.50.Gr Charge carriers: generation, recombination, lifetime, trapping, mean free paths

Relationship between binding site and pressure dependence for defect-hydrogen complexes in ZnO

M. G. Wardle, J. P. Goss, and P. R. Briddon

Appl. Phys. Lett. 88, 261906 (2006); http://dx.doi.org/10.1063/1.2218303 (3 pages) | Cited 8 times

Online Publication Date: 27 June 2006

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The effect of hydrostatic pressure on local vibrational modes of hydrogen defects in ZnO has been studied by first-principles methods. We find that the sign and magnitude of the frequency shift rate are strongly dependent on the local environment. In the case of isolated hydrogen, the bond centered (BC) and antibonding (AB) configurations lead to positive and negative pressure shifts, in agreement with previous work. However, this result cannot be extended to defect-hydrogen complexes. In general, the sign of the shift does not indicate whether the hydrogen atom is located at a BC or AB site.
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63.20.Pw Localized modes
61.72.-y Defects and impurities in crystals; microstructure

Long dephasing time in self-assembled InAs quantum dots at over 1.3 μm wavelength

J. Ishi-Hayase, K. Akahane, N. Yamamoto, M. Sasaki, M. Kujiraoka, and K. Ema

Appl. Phys. Lett. 88, 261907 (2006); http://dx.doi.org/10.1063/1.2217156 (3 pages) | Cited 16 times

Online Publication Date: 28 June 2006

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Excitonic dephasing is investigated in InAs self-assembled quantum dots fabricated by the strain-compensation technique. The exciton ground-state emission is centered at the wavelength of 1420 nm at 5 K. Transient four-wave mixing measurements under resonant excitation clearly demonstrate a long dephasing time of 1.09 ns at 5 K, corresponding to the homogeneous broadening of 1.2 μeV. The extrapolated zero-temperature homogeneous broadening is limited only by the population lifetime of the exciton ground state. At slightly increased temperatures, the acoustic-phonon broadening becomes dominant on dephasing.
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78.67.Hc Quantum dots
73.21.La Quantum dots

Modeling heterostructures of nanophononic crystals by continuum model with microstructures

G. L. Huang and C. T. Sun

Appl. Phys. Lett. 88, 261908 (2006); http://dx.doi.org/10.1063/1.2217189 (3 pages) | Cited 1 time

Online Publication Date: 28 June 2006

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The application of nanophononic heterogeneous materials requires efficient and accurate modeling and analysis tools. For nanostructured materials, the use of atomistic simulation may be a potential solution. However, it is well known that this approach is beset with prohibitive computing time and an astronomical amount of data generated. In this letter, a microstructure continuum theory is employed to describe wave propagation in these materials. The key step in this approach is the establishment of a relationship between the local kinematics and the global continuum variables so that the characteristics of the nanostructure are captured. Propagation of longitudinal harmonic waves in nanophononic media is considered and the resulting dispersion curves are used to evaluate the accuracy of the microstructure continuum model.
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63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
61.46.-w Structure of nanoscale materials
61.72.-y Defects and impurities in crystals; microstructure

Size dependence of photoluminescence and resonant Raman scattering from ZnO quantum dots

Hsin-Ming Cheng, Kuo-Feng Lin, Hsu-Cheng Hsu, and Wen-Feng Hsieh

Appl. Phys. Lett. 88, 261909 (2006); http://dx.doi.org/10.1063/1.2217925 (3 pages) | Cited 49 times

Online Publication Date: 28 June 2006

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ZnO quantum dots (QDs) of controlled sizes have been fabricated by a simple sol-gel method. The blueshift of room-temperature photoluminescence measurement from free exciton transition are observed decreasing with the QD size that is ascribed to the quantum confinement effect. From the resonant Raman scattering, the coupling strength between electron and longitudinal optical phonon, deduced from the ratio of the second- to the first-order Raman scattering intensity, diminishes with reducing the ZnO QD diameter. The size dependence of electron-phonon coupling is principally a result of the Fröhlich interaction.
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81.07.Ta Quantum dots
81.05.Dz II-VI semiconductors
78.55.Et II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
73.21.La Quantum dots
78.67.Hc Quantum dots

Epitaxial growth of GaN on copper substrates

S. Inoue, K. Okamoto, N. Matsuki, Tae-Won Kim, and H. Fujioka

Appl. Phys. Lett. 88, 261910 (2006); http://dx.doi.org/10.1063/1.2213178 (3 pages) | Cited 12 times

Online Publication Date: 28 June 2006

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We have grown GaN films on Cu(111) substrates using pulsed laser deposition. We have found that GaN(0001) grows epitaxially on Cu(111) when employing low temperature grown AlN buffer layers with an in-plane epitaxial relationship of AlN[11math0]‖Cu[1math0] Reflection high-energy electron diffraction images have exhibited sharp streaky patterns, indicating that GaN grows with a flat surface. Electron backscattering diffraction observations have revealed that neither 30° rotational domains nor cubic phase domains exist in the GaN films. Spectroscopic ellipsometry measurements have shown that the heterointerfaces in the GaN/AlN/Cu structure are abrupt. The epitaxial growth of GaN on Cu substrates is likely to raise the power limit for future light emitting and electron devices due to the high thermal conductivity of Cu.
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81.05.Ea III-V semiconductors
81.15.Fg Pulsed laser ablation deposition
68.55.A- Nucleation and growth
72.15.Eb Electrical and thermal conduction in crystalline metals and alloys
79.20.Kz Other electron-impact emission phenomena

Study of pore structure and stability in porous low-k interconnects using electrolyte voltammetry

D. M. Meng, N. L. Michael, Choong-Un Kim, and Young-Joon Park

Appl. Phys. Lett. 88, 261911 (2006); http://dx.doi.org/10.1063/1.2218060 (3 pages) | Cited 2 times

Online Publication Date: 28 June 2006

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This letter presents a step-mode voltammetry method which uses ion diffusivity to characterize pore structure in both dense and porous low dielectric constant materials (low k) in patterned interconnect structures. Findings reveal that the intramolecular space in dense low k acts like a small physical pore network. It is determined that electrolyte ions can migrate through such space in dense low k, but with higher activation energy than in porous low k or the bulk solution, 0.31 eV vs 0.18–0.19 eV. Also, this study finds that the pores in ultralow k are not stable but can either coalesce or collapse depending on stress conditions.
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61.43.Gt Powders, porous materials
85.40.Ls Metallization, contacts, interconnects; device isolation
77.55.-g Dielectric thin films
82.45.Rr Electroanalytical chemistry
66.30.H- Self-diffusion and ionic conduction in nonmetals
77.22.Ch Permittivity (dielectric function)

Facets evolution and surface electrical properties of nonpolar m-plane ZnO thin films

J. Zúñiga-Pérez, V. Muñoz-Sanjosé, E. Palacios-Lidón, and J. Colchero

Appl. Phys. Lett. 88, 261912 (2006); http://dx.doi.org/10.1063/1.2218320 (3 pages) | Cited 20 times

Online Publication Date: 28 June 2006

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ZnO thin films have been grown along the nonpolar [10math0] direction by metal organic vapor phase epitaxy. The ZnO (10math0) surface develops well defined facets. The orientation of the topographic normals reveals that the inclination angle of the facets increases as thicker films are considered, attaining a maximum value of about 28.4°. This angle corresponds to {10math1}- and {10mathmath}-type facets. The origin of this faceting is discussed in terms of thermodynamic stability and kinetics arguments. The surface electrical properties of the facets have been studied by Kelvin probe microscopy, showing that the surface has different contact potential domains that alternate along the polar [0001] direction.
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81.05.Dz II-VI semiconductors
73.25.+i Surface conductivity and carrier phenomena
68.47.Fg Semiconductor surfaces
73.61.Ga II-VI semiconductors
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology

Thermal stability and its prediction of bulk metallic glass systems

Mingxu Xia, Shuguang Zhang, Jianguo Li, and Chaoli Ma

Appl. Phys. Lett. 88, 261913 (2006); http://dx.doi.org/10.1063/1.2218324 (3 pages) | Cited 9 times

Online Publication Date: 28 June 2006

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We report a linear relationship between the thermal stability (ΔTx) and the amorphous formation enthalpy (ΔHam) of typical bulk metallic glasses (BMGs). The value of the slope of ΔTx vs ΔHam reflects the sensitivity of thermal stability to composition and the sign of the slope implies whether the local atomic structure of amorphous phase is similar to that of primary crystalline phase. A positive slope suggests a similarity between them and vice versa. We also demonstrate that the thermal stability can be fastly assessed and predicted by ΔHam at less cost using Ti53Cu15Ni18.5Al7M3Si3B0.5 (M = Hf,Sc,Ta,Nb) BMGs.
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61.43.Fs Glasses
82.60.Cx Enthalpies of combustion, reaction, and formation

Thermal conductivity enhancement in water-in-FC72 nanoemulsion fluids

B. Yang and Z. H. Han

Appl. Phys. Lett. 88, 261914 (2006); http://dx.doi.org/10.1063/1.2218325 (3 pages) | Cited 19 times

Online Publication Date: 28 June 2006

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The strategy of adding solid particles to fluids for improving thermal conductivity has been pursued for nearly a century. In this work, a concept of using liquid nanodroplets for enhancing thermal conductivity has been developed and was demonstrated in water-in-FC72 suspensions, called “nanoemulsion fluids.” The thermal conductivity of FC72 is found to be increased by up to 52% for a nanoemulsion fluid containing 12 vol % water nanodroplets of radius 9.8 nm. Such types of nanoemulsion fluids possess long-term stability and can be mass produced because of no needs for solid nanoparticles. The development of nanoemulsion fluids would open a direction for thermal fluids studies.
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66.25.+g Thermal conduction in nonmetallic liquids
82.70.Kj Emulsions and suspensions

Real time multicomponent echo particle image velocimetry technique for opaque flow imaging

Hairong Zheng, Lingli Liu, Logan Williams, Jean R. Hertzberg, Craig Lanning, and Robin Shandas

Appl. Phys. Lett. 88, 261915 (2006); http://dx.doi.org/10.1063/1.2216875 (3 pages) | Cited 13 times

Online Publication Date: 29 June 2006

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This letter reports on a contrast-based ultrasonic particle imaging technique (echo PIV) for measuring multicomponent velocity vectors in opaque flows with excellent temporal (up to 0.5 ms) and spatial (up to 0.4 mm) resolution. Ultrasound contrast microbubbles are used as flow tracers, and digitally acquired rf data are converted into B-mode images for PIV analysis. Here, velocity fields from various flow patterns (including rotating and transient vortex flows) that are difficult to measure using other opaque flow methods such as ultrasound Doppler or magnetic resonance imaging are measured using echo PIV. This nonintrusive technique should be a promising addition to opaque flow diagnostics.
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47.80.Jk Flow visualization and imaging
43.58.-e
43.60.Lq
47.32.Ef Rotating and swirling flows

Stress control structures for microelectromechanical systems using structural mechanics approach

T. Krupenkin, V. A. Lifton, A. Tran, J. Vuillemin, and D. W. Carr

Appl. Phys. Lett. 88, 261916 (2006); http://dx.doi.org/10.1063/1.2218771 (3 pages) | Cited 1 time

Online Publication Date: 30 June 2006

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A method of stress control in microelectromechanical systems (MEMS) devices is presented that consists of creating counterbalancing structures to position stressed layers at the neutral plane of the device, eliminating the bending momentum acting on the device. Upon metallization, many MEMS elements such as silicon membranes show substantial bow under the stress developed as a result of the difference in the thermal expansion coefficients of a metal and silicon. The proposed membranes with the counterbalancing structures remain flat in the entire test temperature range (25–150 °C). The method gives material-independent solution to stress-induced curvature problems in a variety of ultrathin devices.
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07.10.Cm Micromechanical devices and systems
85.85.+j Micro- and nano-electromechanical systems (MEMS/NEMS) and devices
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity
65.40.De Thermal expansion; thermomechanical effects
85.40.Ls Metallization, contacts, interconnects; device isolation

Optical properties of inorganic AgSb recording thin film

Y. H. Fang, P. C. Kuo, P. W. Chen, Wei-Chih Hsu, C. Y. Chou, and T. H. Wu

Appl. Phys. Lett. 88, 261917 (2006); http://dx.doi.org/10.1063/1.2218774 (3 pages) | Cited 1 time

Online Publication Date: 30 June 2006

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50 nm Ag1−xSbx (x = 10.8–25.5) thin films were prepared by magnetron sputtering. Thermal analysis shows that the phase change occurs around 250 °C. The optical property analysis show that the as-deposited Ag80.9Sb19.1 films have high reflectivity of about 62%–73%. After heat treatment at 300 °C, the contrast of Ag80.9Sb19.1 film is 12.5%–17% for wavelengths between 400 and 800 nm. Dynamic test shows that using the Ag80.9Sb19.1 film as the memory layer of write once optical disk, a carrier-to-noise ratio of about 45 dB can be achieved at λ = 657 nm, numerical aperture of 0.65, and a linear velocity of 3.5 m/s.
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42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
81.40.Gh Other heat and thermomechanical treatments
42.79.Vb Optical storage systems, optical disks

Characteristic fragment size distributions in dynamic fragmentation

Fenghua Zhou, Jean-François Molinari, and K. T. Ramesh

Appl. Phys. Lett. 88, 261918 (2006); http://dx.doi.org/10.1063/1.2216892 (3 pages) | Cited 6 times

Online Publication Date: 30 June 2006

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The one-dimensional fragmentation of a dynamically expanding ring (Mott’s problem) is studied numerically to obtain the fragment signatures under different strain rates. An empirical formula is proposed to calculate an average fragment size. Rayleigh distribution is found to describe the statistical properties of the fragment populations.
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81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
62.20.M- Structural failure of materials

Higher lying conduction band in GaN and AlN probed by photoluminescence spectroscopy

N. Nepal, K. B. Nam, J. Li, M. L. Nakarmi, J. Y. Lin, and H. X. Jiang

Appl. Phys. Lett. 88, 261919 (2006); http://dx.doi.org/10.1063/1.2217160 (3 pages) | Cited 2 times

Online Publication Date: 30 June 2006

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Photoluminescence spectroscopy has been employed to study the band edge emissions in GaN and AlN epilayers up to 800 K. Two distinctive activation processes have been observed in both GaN and AlN. The first process occurring below Tt = 325 K (Tt = 500 K) for GaN (AlN) is due to the activation of free excitons to free carriers, whereas the second occurring above Tt with an activation energy of 0.29 eV (0.3 eV) for GaN (AlN) is believed to be associated with a higher lying conduction band (Γ3) at about 0.3 eV above the conduction band minimum (Γ1). An emission line at about 0.29 eV above the dominant transition in GaN was also observed at 700 K, corroborating the assignment of Γ3. The values of Tt are a direct measure of the onset temperature at which free excitons dissociate into free carriers.
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71.20.Nr Semiconductor compounds
78.55.Cr III-V semiconductors
71.35.-y Excitons and related phenomena

Tailored emission color synthesis using microfacet quantum wells consisting of nitride semiconductors without phosphors

M. Funato, T. Kotani, T. Kondou, Y. Kawakami, Y. Narukawa, and T. Mukai

Appl. Phys. Lett. 88, 261920 (2006); http://dx.doi.org/10.1063/1.2217259 (3 pages) | Cited 28 times

Online Publication Date: 30 June 2006

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A color synthesis based on InGaN/GaN quantum wells (QWs) grown on GaN microfacets formed by regrowth on SiO2 mask stripes is demonstrated. The microfacet structure is composed of (0001), {11math2}, and {11math0} planes, and the InGaN well thickness and composition are spatially inhomogeneous due to the diffusion of the adatoms among the facets. These properties allow microfacet QWs, which, for example, emit yellow from the (0001) facet and blue from the {11math2} facet, to be fabricated, of which the luminescence appears white due to the additive color mixing. Using a mask pattern that consists of regions with and without stripes, the emissions from the microfacet QWs and from planar QWs are synthesized to produce the desired apparent output colors.
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81.07.St Quantum wells
81.05.Ea III-V semiconductors
78.67.De Quantum wells
78.55.Cr III-V semiconductors
68.65.Fg Quantum wells
68.43.Jk Diffusion of adsorbates, kinetics of coarsening and aggregation
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