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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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Terahertz quantum cascade lasers with double-resonant-phonon depopulation

Benjamin S. Williams, Sushil Kumar, Qi Qin, Qing Hu, and John L. Reno

Appl. Phys. Lett. 88, 261101 (2006); http://dx.doi.org/10.1063/1.2216112 (3 pages) | Cited 25 times

Online Publication Date: 26 June 2006

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We present two different terahertz quantum cascade laser (QCL) designs based on GaAs/Al0.3Ga0.7As heterostructures that feature a depopulation mechanism of two longitudinal-optical phonon scattering events. This scheme is intended to improve high temperature operation by reducing thermal backfilling of the lower radiative state. The better of these two devices displays a threshold current density of 170 A/cm2 at 5 K and lases up to 138 K in pulsed mode and 105 K in continuous-wave mode. However, contrary to expectation, we observed no improvement in temperature performance compared to single-resonant-phonon designs, which suggests that the thermal backfilling is not yet a limiting factor for high temperature terahertz QCL operation.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Light-emitting defects and epitaxy in alkali-ion-implanted α quartz

J. Keinonen, S. Gąsiorek, P. K. Sahoo, S. Dhar, and K. P. Lieb

Appl. Phys. Lett. 88, 261102 (2006); http://dx.doi.org/10.1063/1.2215615 (3 pages) | Cited 2 times

Online Publication Date: 27 June 2006

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Light-emitting centers in alkali-ion-implanted α quartz have been investigated with respect to the solid phase epitaxial growth of the ion irradiation induced amorphous zone. Cathodoluminescence was studied under the conditions of chemical epitaxy in annealing the samples, implanted with 2.5×1016 50 keV Na ions/cm2 or 175 keV Rb ions/cm2, in math atmosphere in the temperature range of 673–1173 K. In addition to the known intrinsic subbands at 2.40, 2.79, and 4.30 eV, which previously were associated with specific defects in the silica matrix, a strong violet band at 3.65 eV and a band at 3.25 eV have been identified. Both are intimately correlated with the presence of the implanted alkali atoms and recrystallization process. With respect to the 3.25 eV band reported in the literature, they are discussed to be correlated with the presence of nanoclusters in Si-enriched, and Ge- and Sn-implanted SiO2 structures.
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78.60.Hk Cathodoluminescence, ionoluminescence
61.80.Jh Ion radiation effects
61.72.up Other materials
81.40.Gh Other heat and thermomechanical treatments
61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)

Terahertz emission profile from laser-induced air plasma

Hua Zhong, Nick Karpowicz, and X.-C. Zhang

Appl. Phys. Lett. 88, 261103 (2006); http://dx.doi.org/10.1063/1.2216025 (3 pages) | Cited 24 times

Online Publication Date: 27 June 2006

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We report the characterization of the emission profile of terahertz waves generated by four-wave mixing in the presence of laser-induced plasma. Highly directional terahertz waves with a divergence angle smaller than 10° are measured in the presence of long plasmas (>10 mm). Frequency-dependent interference structures in the angular distribution of the radiation are observed under tighter focusing conditions and are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz generation from four-wave mixing in air plasmas is a promising source for spectroscopy and imaging in the terahertz range.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
51.70.+f Optical and dielectric properties
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Enhanced second harmonic generation from nanoscale double-hole arrays in a gold film

Antoine Lesuffleur, L. Kiran Swaroop Kumar, and Reuven Gordon

Appl. Phys. Lett. 88, 261104 (2006); http://dx.doi.org/10.1063/1.2218057 (3 pages) | Cited 33 times

Online Publication Date: 27 June 2006

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We present enhanced second harmonic generation (SHG) from arrays with a basis of overlapping double holes. The arrays were created by focused-ion beam milling through a gold film, and the measurements were performed in the transmission geometry. By fixing the array periodicity and varying the spacing between the holes, the SHG was enhanced by an order of magnitude for the single case where the apexes of the double-hole structure were nearly touching. Numerical calculations showed a local electric field enhancement that agrees with the SHG observations. This work shows the potential of double-hole structures for nonlinear optics at the nanoscale.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.66.Bz Metals and metallic alloys

Spin dynamics in a diluted magnetic semiconductor quantum well studied by pump-probe absorption spectroscopy: Magnetic-field-induced suppression of electron-spin relaxation

A. Murayama, K. Seo, K. Nishibayashi, I. Souma, and Y. Oka

Appl. Phys. Lett. 88, 261105 (2006); http://dx.doi.org/10.1063/1.2216111 (3 pages) | Cited 8 times

Online Publication Date: 27 June 2006

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Exciton spin dynamics is studied in a diluted magnetic semiconductor quantum well of Cd0.95Mn0.05Te by pump-probe absorption spectroscopy under magnetic fields. The time dependences of the saturated absorbance for the higher- and lower-energy spin states of heavy-hole (hh) excitons clarify the following exciton-spin relaxation process in magnetic fields: ultrafast hh-spin relaxation with the formation of dark excitons and subsequent electron-spin relaxation. The electron-spin relaxation due to the s-d exchange mechanism involving Mn spins is suppressed in a high magnetic field by field-induced pinning of the Mn spins.
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75.50.Pp Magnetic semiconductors
75.40.Gb Dynamic properties (dynamic susceptibility, spin waves, spin diffusion, dynamic scaling, etc.)
73.21.Fg Quantum wells
71.35.-y Excitons and related phenomena
75.30.Et Exchange and superexchange interactions

Reliability of strain-balanced Ga0.331In0.669As/Al0.659In0.341As/InP quantum-cascade lasers under continuous-wave room-temperature operation

A. Evans and M. Razeghi

Appl. Phys. Lett. 88, 261106 (2006); http://dx.doi.org/10.1063/1.2217144 (3 pages) | Cited 5 times

Online Publication Date: 27 June 2006

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The reliability of high power strain-balanced Ga0.331In0.669As/Al0.659In0.341As/InP intersubband quantum cascade lasers (QCLs) emitting at a wavelength of 4.6 μm has been studied. Constant current aging is reported for two randomly selected high-reflectivity-coated QCLs with an output power over 100 mW. QCLs are tested under continuous-wave operation at a heat sink temperature of 298 K(25 °C) corresponding to an internal temperature of 378 K (105 °C). Over 4000 h of continuous testing is reported without any decrease in output power.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Lensless focusing with subwavelength resolution by direct synthesis of the angular spectrum

Stanley S. Hong, Berthold K. P. Horn, Dennis M. Freeman, and Michael S. Mermelstein

Appl. Phys. Lett. 88, 261107 (2006); http://dx.doi.org/10.1063/1.2213912 (3 pages) | Cited 7 times

Online Publication Date: 29 June 2006

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We consider the coherent superposition of unfocused wave fronts for lensless focusing of electromagnetic waves with subwavelength resolution. Near the focal point, intensity distributions generated using the approach approximate those generated using lenses. Far from the focal point, discretization of spatial frequencies results in a trade-off between the number of wave fronts and the accuracy of the approximation. We experimentally demonstrate the feasibility of the approach by generating an approximation of an azimuthally polarized Bessel beam with a focal spot diameter (full width at half maximum intensity) of 0.37λ.
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.25.Kb Coherence
42.25.Ja Polarization

Observation of optical gain in solutions of CdS quantum dots at room temperature in the blue region

Qusai Darugar, Wei Qian, and Mostafa A. El-Sayed

Appl. Phys. Lett. 88, 261108 (2006); http://dx.doi.org/10.1063/1.2217138 (3 pages) | Cited 11 times

Online Publication Date: 29 June 2006

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The optical gain dynamics has been studied for two CdS quantum dot samples dispersed in toluene at room temperature. This was carried out by using femtosecond transient absorption technique with an excitation at 400 nm and gain measurement was studied at the fluorescence maxima (440 and 460 nm). The optical gain lifetime was found to be as long as 20 ps under pump fluence as low as 0.77 mJ/cm2. The low threshold is the result of long lifetime of electrons and holes and narrow emission bandwidth. These results suggest that CdS quantum dots in solution are excellent gain media for optically pumped high power blue lasers.
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78.67.Hc Quantum dots
78.66.Hf II-VI semiconductors
78.55.Et II-VI semiconductors
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
78.47.-p Spectroscopy of solid state dynamics

Plasma dynamics of water breakdown at a water surface induced by femtosecond laser pulses

C. Sarpe-Tudoran, A. Assion, M. Wollenhaupt, M. Winter, and T. Baumert

Appl. Phys. Lett. 88, 261109 (2006); http://dx.doi.org/10.1063/1.2217158 (3 pages) | Cited 11 times

Online Publication Date: 29 June 2006

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Femtosecond laser pulse induced ultrafast plasma dynamics studies of water breakdown in the range up to 250 ps are reported. We combine transient imaging techniques together with spectrally resolved reflection spectroscopy to monitor the early breakdown dynamics at the water surface with a laser intensity being 1.5 above threshold. We observe a 20 ps delay before the plasma expands with an initial velocity of 5900 m/s. The transient electron density after formation of the plasma is 1.2×1021/cm3. A recombination on a picosecond time scale with a rate of (1.6×10−9±0.3×10−9) cm3/s is found.
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52.80.Wq Discharge in liquids and solids
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.30.-q Plasma dynamics and flow
52.25.Os Emission, absorption, and scattering of electromagnetic radiation

Control of subband energy levels of quantum dots using InGaAs gradient composition strain-reducing layer

T. Amano, S. Yamauchi, T. Sugaya, and K. Komori

Appl. Phys. Lett. 88, 261110 (2006); http://dx.doi.org/10.1063/1.2218825 (3 pages) | Cited 1 time

Online Publication Date: 30 June 2006

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We propose a method to control the subband energy levels of quantum dots (QDs) using an InGaAs gradient composition strain-reducing layer (GC-SRL). A large band shift of 70 meV was realized using a GC-SRL at the fourth-order energy level in both the samples. In addition, the QDs with and without a GC-SRL exhibited an exponential and constant increase in the subband space, respectively. These results indicate square-well-shaped and crucible-shaped potential band structures. The GC-SRL enabled the control of not only the subband energy but also the confinement energy of these potential structures.
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73.21.La Quantum dots

Color-center waveguides in low-energy electron-bombarded lithium fluoride

Alessio Rocchetti, Gaetano Assanto, Rosa Maria Montereali, Enrico Nichelatti, and Fabrizia Somma

Appl. Phys. Lett. 88, 261111 (2006); http://dx.doi.org/10.1063/1.2218039 (3 pages) | Cited 4 times

Online Publication Date: 30 June 2006

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We employ a differential version of m-line spectroscopy through grating coupling in order to measure the refractive index of low-energy electron irradiated lithium fluoride crystal channel waveguides for broadband emission. Using photoresist films and a holographic setup for grating fabrication, we perform an accurate characterization of bidimensional structures and a direct quantitative comparison between treated and blank materials.
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42.79.Gn Optical waveguides and couplers
42.70.-a Optical materials
42.79.Dj Gratings
61.72.J- Point defects and defect clusters
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Frequency mixing of off-axis focused Gaussian beams: An approach to measure the phase velocity distribution

Daquan Lu, Liejia Qian, Yongzhong Li, and Dianyuan Fan

Appl. Phys. Lett. 88, 261112 (2006); http://dx.doi.org/10.1063/1.2218066 (3 pages) | Cited 2 times

Online Publication Date: 30 June 2006

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In this letter we theoretically study the frequency mixing of two off-axis focused Gaussian beams. The optimal phase mismatch corresponding to maximized conversion efficiency depends on the separation of off-axis beams and can be expressed in terms of the phase velocities analytically, which provides an effective approach to measure the phase velocity distribution of a Gaussian beam.
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42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

Fiber-coupled single-photon detectors based on NbN superconducting nanostructures for practical quantum cryptography and photon-correlation studies

W. Słysz, M. Węgrzecki, J. Bar, P. Grabiec, M. Górska, V. Zwiller, C. Latta, P. Bohi, I. Milostnaya, O. Minaeva, A. Antipov, O. Okunev, A. Korneev, K. Smirnov, B. Voronov, et al.

Appl. Phys. Lett. 88, 261113 (2006); http://dx.doi.org/10.1063/1.2218105 (3 pages) | Cited 23 times

Online Publication Date: 30 June 2006

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We have fabricated and tested a two-channel single-photon detector system based on two fiber-coupled superconducting single-photon detectors (SSPDs). Our best device reached the system quantum efficiency of 0.3% in the 1540-nm telecommunication wavelength with a fiber-to-detector coupling factor of about 30%. The photoresponse consisted of 2.5-ns-wide voltage pulses with a rise time of 250 ps and timing jitter below 40 ps. The overall system response time, measured as a second-order, photon cross-correlation function, was below 400 ps. Our SSPDs operate at 4.2 K inside a liquid-helium Dewar, but their optical fiber inputs and electrical outputs are at room temperature. Our two-channel detector system should find applications in practical quantum cryptography and in antibunching-type quantum correlation measurements.
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85.25.Pb Superconducting infrared, submillimeter and millimeter wave detectors
42.81.Pa Sensors, gyros
03.67.Dd Quantum cryptography and communication security
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Effect of wire number on x-pinch discharges

J. S. Green, S. N. Bland, M. Collett, A. E. Dangor, K. Krushelnick, F. N. Beg, and I. Ross

Appl. Phys. Lett. 88, 261501 (2006); http://dx.doi.org/10.1063/1.2216110 (3 pages) | Cited 4 times

Online Publication Date: 26 June 2006

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An investigation has been carried out into the x-ray emission and plasma dynamics of a “tabletop” x-pinch plasma generated by passing a 40 ns, 35 kA current pulse through two or four 5 μm thick tungsten wires. It was found that a four-wire x pinch consistently resulted in an order of magnitude higher energy hard x-ray yield (>3.5 keV) than a two-wire pinch. The total x-ray emission from a four-wire pinch also had a shorter pulse length such that the total x-ray power was increased by more than a factor of 2. Observations of x-pinch discharges driven by a higher energy 160 kA, 80 ns generator are also presented. In this case x pinches with two, four, and six wires were made using thicker aluminum and molybdenum wires and the number of wires is shown to play a significant role in the plasma dynamics. Increasing the number of wires enhanced the reproducibility of the discharge and the x-ray pulse.
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52.58.Lq Z-pinches, plasma focus, and other pinch devices
52.59.Qy Wire array Z-pinches
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.59.Px Hard X-ray sources
52.50.Dg Plasma sources
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Effect of collisions in the stratified presheath in electronegative plasmas

J. I. Fernández Palop, J. Ballesteros, R. Morales Crespo, and M. A. Hernández

Appl. Phys. Lett. 88, 261502 (2006); http://dx.doi.org/10.1063/1.2218319 (3 pages) | Cited 7 times

Online Publication Date: 28 June 2006

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The effect of collisions in the formation of a stratified presheath in electronegative plasmas is analyzed. Collisions do not affect to the sheath-presheath structure in pure electropositive plasmas; however, in the case of electronegative plasmas, they can produce a large effect, changing the electric potential distribution in the presheath from monotonic to oscillatory, and viceversa. The influence of the collision rate on the plasma parameter space region in which the presheath becomes stratified is analyzed.
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52.40.Kh Plasma sheaths
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.20.Fs Electron collisions
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
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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
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