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1 Nov 2008

Volume 104, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 104, 091101 (2008); http://dx.doi.org/10.1063/1.3002408 (11 pages)

Wen Lei and Chennupati Jagadish
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Lasers and photodetectors for mid-infrared 2–3 μm applications

Wen Lei and Chennupati Jagadish

J. Appl. Phys. 104, 091101 (2008); http://dx.doi.org/10.1063/1.3002408 (11 pages) | Cited 11 times

Online Publication Date: 5 November 2008

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This paper presents an overview of the recent developments in III–V semiconductor lasers and detectors operating in the 2–3 μm wavelength range, which are highly desirable for various important applications, such as military, communications, molecular spectroscopy, biomedical surgery, and environmental protection. The lasers and detectors with different structure designs are discussed and compared. Advantages and disadvantages of each design are also discussed. Promising materials and structures to obtain high performance lasers and detectors operating in the 2–3 μm region are also suggested.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
85.60.Gz Photodetectors (including infrared and CCD detectors)
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
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A plasma model combined with an improved two-temperature equation for ultrafast laser ablation of dielectrics

Lan Jiang and Hai-Lung Tsai

J. Appl. Phys. 104, 093101 (2008); http://dx.doi.org/10.1063/1.3006129 (8 pages) | Cited 3 times

Online Publication Date: 4 November 2008

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It remains a big challenge to theoretically predict the material removal mechanism in femtosecond laser ablation. To bypass this unresolved problem, many calculations of femtosecond laser ablation of nonmetals have been based on the free electron density distribution without the actual consideration of the phase change mechanism. However, this widely used key assumption needs further theoretical and experimental confirmation. By combining the plasma model and improved two-temperature model developed by the authors, this study focuses on investigating ablation threshold fluence, depth, and shape during femtosecond laser ablation of dielectrics through nonthermal processes (the Coulomb explosion and electrostatic ablation). The predicted ablation depths and shapes in fused silica, by using (1) the plasma model only and (2) the plasma model plus the two-temperature equation, are both in agreement with published experimental data. The widely used assumptions for threshold fluence, ablation depth, and shape in the plasma model based on free electron density are validated by the comparison study and experimental data.
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52.38.Mf Laser ablation
52.25.Jm Ionization of plasmas

Phase contrast imaging simulation and measurements using polychromatic sources with small source-object distances

Bruno Golosio, Pasquale Delogu, Irene Zanette, Massimo Carpinelli, Giovanni Luca Masala, Piernicola Oliva, Arnaldo Stefanini, and Simone Stumbo

J. Appl. Phys. 104, 093102 (2008); http://dx.doi.org/10.1063/1.3006130 (8 pages) | Cited 5 times

Online Publication Date: 4 November 2008

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Phase contrast imaging is a technique widely used in synchrotron facilities for nondestructive analysis. Such technique can also be implemented through microfocus x-ray tube systems. Recently, a relatively new type of compact, quasimonochromatic x-ray sources based on Compton backscattering has been proposed for phase contrast imaging applications. In order to plan a phase contrast imaging system setup, to evaluate the system performance and to choose the experimental parameters that optimize the image quality, it is important to have reliable software for phase contrast imaging simulation. Several software tools have been developed and tested against experimental measurements at synchrotron facilities devoted to phase contrast imaging. However, many approximations that are valid in such conditions (e.g., large source-object distance, small transverse size of the object, plane wave approximation, monochromatic beam, and Gaussian-shaped source focal spot) are not generally suitable for x-ray tubes and other compact systems. In this work we describe a general method for the simulation of phase contrast imaging using polychromatic sources based on a spherical wave description of the beam and on a double-Gaussian model of the source focal spot, we discuss the validity of some possible approximations, and we test the simulations against experimental measurements using a microfocus x-ray tube on three types of polymers (nylon, poly-ethylene-terephthalate, and poly-methyl-methacrylate) at varying source-object distance. It will be shown that, as long as all experimental conditions are described accurately in the simulations, the described method yields results that are in good agreement with experimental measurements.
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42.79.Pw Imaging detectors and sensors
07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
02.50.-r Probability theory, stochastic processes, and statistics

Large electro-optic effect in single-crystal Pb(Zr,Ti)O3 (001) measured by spectroscopic ellipsometry

Tae Dong Kang, Bo Xiao, Vitaliy Avrutin, Ümit Özgür, Hadis Morkoç, Jun Woo Park, Ho Suk Lee, Hosun Lee, Xiaoyu Wang, and David. J. Smith

J. Appl. Phys. 104, 093103 (2008); http://dx.doi.org/10.1063/1.3009655 (5 pages) | Cited 4 times

Online Publication Date: 4 November 2008

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Using spectroscopic ellipsometry electro-optic effect was studied in lead zirconate titanate (PZT) thin films grown epitaxially on Nb-doped SrTiO3(001) substrates by RF magnetron sputtering. A uniaxial multilayer model analysis was applied to extract the linear and quadratic electro-optic coefficients from the shifts in the ordinary and extraordinary refractive indices with electric field applied along the (001) direction. The effective linear and quadratic coefficients were measured as −134.6×10−12 m/V and 8.5×10−18 m2/V2, respectively, at a wavelength of 632.8 nm, while the individual linear electro-optic coefficients r33 and r13 were −157.1 and 22 pm/V, respectively. The existence of the linear electro-optic effect in unpoled PZT films was attributed to the presence of a built-in polarization and simultaneous poling during measurements.
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78.20.Jq Electro-optical effects
78.66.Nk Insulators
42.70.-a Optical materials
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

High power light-emitting diode junction temperature determination from current-voltage characteristics

A. Keppens, W. R. Ryckaert, G. Deconinck, and P. Hanselaer

J. Appl. Phys. 104, 093104 (2008); http://dx.doi.org/10.1063/1.3009966 (8 pages) | Cited 7 times

Online Publication Date: 4 November 2008

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Optical and electrical characteristics of power light-emitting diodes (LEDs) are strongly dependent on the diode junction temperature. However, direct junction temperature determination is not possible and alternative methods must be developed. Current-voltage characteristics of commercial high power LEDs have been measured at six different temperatures ranging between 295 and 400 K. Modeling these characteristics, including variation in the bandgap with temperature, revealed a linear temperature dependence of the forward voltage if the drive current is chosen within a rather limited current range. Theoretically, the voltage intercept can be deduced from the bulk semiconductor bandgap. However, accurate junction temperature determination is only possible if at least two calibration measurements at a particular drive current are performed. The method described in this paper can be applied to calculate the thermal resistance from the junction to any other reference point for any particular LED configuration.
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85.60.Jb Light-emitting devices
85.30.Kk Junction diodes

Efficient Cherenkov emission of broadband terahertz radiation from an ultrashort laser pulse in a sandwich structure with nonlinear core

S. B. Bodrov, M. I. Bakunov, and M. Hangyo

J. Appl. Phys. 104, 093105 (2008); http://dx.doi.org/10.1063/1.3005987 (11 pages) | Cited 8 times

Online Publication Date: 5 November 2008

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A scheme for efficient generation of broadband terahertz radiation by a femtosecond laser pulse propagating in a planar sandwichlike structure is proposed. The structure consists of a thin nonlinear core cladded with prisms made of a material with low terahertz absorption. The focused into a line laser pulse propagates in the core as a leaky or waveguide mode and emits Cherenkov wedge of terahertz waves in the cladding. We developed a theory that describes terahertz generation in such a structure and calculated spatial distribution of the generated terahertz field, its energy spectrum and optical-to-terahertz conversion efficiency. The developed theory predicts the conversion efficiency of up to several percent in a 1 cm long and 1 cm wide Si–LiNbO3–Si sandwich structure with a 20 μm thick nonlinear layer pumped by 8.5 μJ Ti:sapphire laser with pulse duration of 100 fs.
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42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.Wi Nonlinear waveguides
42.79.Gn Optical waveguides and couplers
41.60.Bq Cherenkov radiation

Optical study of a-plane InGaN/GaN multiple quantum wells with different well widths grown by metal-organic chemical vapor deposition

T. S. Ko, T. C. Lu, T. C. Wang, J. R. Chen, R. C. Gao, M. H. Lo, H. C. Kuo, S. C. Wang, and J. L. Shen

J. Appl. Phys. 104, 093106 (2008); http://dx.doi.org/10.1063/1.3013435 (8 pages) | Cited 6 times

Online Publication Date: 7 November 2008

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a-plane InGaN/GaN multiple quantum wells of different widths ranging from 3 to 12 nm grown on r-plane sapphire by metal-organic chemical vapor deposition were investigated. The peak emission intensity of the photoluminescence (PL) reveals a decreasing trend as the well width increases from 3 to 12 nm. Low temperature (9 K) time-resolved PL (TRPL) study shows that the sample with 3-nm-thick wells has the best optical property with a fastest exciton decay time of 0.57 ns. The results of cathodoluminescence and micro-PL scanning images for samples of different well widths further verify that the more uniform and stronger luminescence intensity distribution are observed for the samples of thinner quantum wells. In addition, more effective capturing of excitons due to larger localization energy Eloc and shorter radiative lifetime of localized excitons are observed in thinner well width samples in the temperature dependent TRPL.
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78.55.Cr III-V semiconductors
78.67.De Quantum wells
78.60.Hk Cathodoluminescence, ionoluminescence
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.65.Fg Quantum wells
78.47.jd Time resolved luminescence

Hybrid CdZnO/GaN quantum-well light emitting diodes

J. W. Mares, M. Falanga, A. V. Thompson, A. Osinsky, J. Q. Xie, B. Hertog, A. Dabiran, P. P. Chow, S. Karpov, and W. V. Schoenfeld

J. Appl. Phys. 104, 093107 (2008); http://dx.doi.org/10.1063/1.3013446 (5 pages) | Cited 12 times

Online Publication Date: 10 November 2008

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We report on the demonstration of light emission from hybrid CdZnO quantum-well light emitting diodes. A one-dimensional drift-diffusion method was used to model the expected band structure and carrier injection in the device, demonstrating the potential for 90% internal quantum efficiency when a CdZnO quantum well is used. Fabricated devices produced visible electroluminescence that was found to redshift from 3.32 to 3.15 eV as the forward current was increased from 20 to 40 mA. A further increase in the forward current to 50 mA resulted in a saturation of the redshift.
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85.60.Jb Light-emitting devices
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

On the origin of efficiency roll-off in InGaN-based light-emitting diodes

X. A. Cao, Y. Yang, and H. Guo

J. Appl. Phys. 104, 093108 (2008); http://dx.doi.org/10.1063/1.3009967 (4 pages) | Cited 16 times

Online Publication Date: 10 November 2008

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The external quantum efficiency (EQE) of AlInGaN-based light-emitting diodes (LEDs) on sapphire and bulk GaN substrates was measured over a wide range of pulsed currents with small duty cycles. The current dependence of the EQE appeared to be a strong function of the In content but nearly independent of the dislocation density in the active region. The EQE of the InGaN LEDs peaked at very low currents and decreased dramatically at high currents, whereas the AlGaN UV LED attained a saturated EQE as current increases. In contrast to minimal peak shift in the UV LED, a monotonic current-induced blueshift of the peak energy was seen up to 1 kA/cm2 for the InGaN blue and green LEDs. These results suggest that the capture of delocalized carriers by nonradiative recombination centers such as misfit defects is the major nonthermal mechanism of efficiency roll-off in InGaN LEDs.
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85.60.Jb Light-emitting devices
85.60.Bt Optoelectronic device characterization, design, and modeling

A Monte Carlo simulation of nanoscale magnetic particle morphology and magnetization

Ziyun Di, Dongchen Zhang, and Xianfeng Chen

J. Appl. Phys. 104, 093109 (2008); http://dx.doi.org/10.1063/1.3009975 (4 pages) | Cited 3 times

Online Publication Date: 11 November 2008

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A model based on Monte Carlo technique is applied to investigating the superparamagnetic magnetite (Fe3O4) colloidal nanocrystal clusters (CNCs) proposed by Ge et al. [ Nano Lett. 7, 3203 (2007) ]. In other words, the model investigates the following three aspects of CNCS: the morphology of magnetic particles, the formation of field-induced chainlike patterns, and the induced evolution of the magnetization processes. It is shown that the parameters such as diameter, surfactant molecules per unit, and volume concentration of the magnetic fluid are significant factors that enable one to efficiently manipulate the morphology and magnetization process, which eventually leads to the efficient control of the fabrication and multiple applications. The experiment results also evidenced the presence of this self-assembled chain structures.
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75.50.Tt Fine-particle systems; nanocrystalline materials
75.60.Ej Magnetization curves, hysteresis, Barkhausen and related effects
75.20.-g Diamagnetism, paramagnetism, and superparamagnetism
61.66.-f Structure of specific crystalline solids
82.70.Dd Colloids
02.50.Ng Distribution theory and Monte Carlo studies
81.16.Dn Self-assembly

Fabrication of terahertz hollow-glass metallic waveguides with inner dielectric coatings

Bradley Bowden, James A. Harrington, and Oleg Mitrofanov

J. Appl. Phys. 104, 093110 (2008); http://dx.doi.org/10.1063/1.3013445 (5 pages) | Cited 12 times

Online Publication Date: 12 November 2008

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Hollow-core glass waveguides (HCWs) with inner coatings of silver and polystyrene (PS) have been fabricated for transmission of terahertz radiation. A liquid-phase chemical deposition process was used to deposit silver and PS thin film coatings inside glass tubing. The PS dielectric layer can substantially lower the loss of the HCW compared to the metal-only waveguide. A polymer coating was chosen because it is possible to deposit the dielectric film thickness required for operation in the terahertz regime. Specifically, the dielectric film thickness is proportional to the wavelength so the PS coatings need to be on the order of 10 to 15 μm to minimize transmission losses at terahertz frequencies. This is much thicker than the submicron thick dielectric coatings needed in the IR region. The thickness of the PS film depends on the concentration of PS in the coating solution and the coating rate. Both of these parameters are studied and related to the losses in the waveguides. The lowest loss of 0.95 dB/m at 119 μm (2.5 THz) was obtained for hybrid HE mode propagation in a waveguide with a 8.2 μm thick PS film deposited inside a 2.2 mm bore, 90 cm long glass tube.
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42.79.Gn Optical waveguides and couplers
42.82.Cr Fabrication techniques; lithography, pattern transfer
42.82.Et Waveguides, couplers, and arrays
77.55.-g Dielectric thin films

Large-scale patterning of indium tin oxide electrodes for guided mode extraction from organic light-emitting diodes

Ulf Geyer, Julian Hauss, Boris Riedel, Sebastian Gleiss, Uli Lemmer, and Martina Gerken

J. Appl. Phys. 104, 093111 (2008); http://dx.doi.org/10.1063/1.3014034 (5 pages) | Cited 21 times

Online Publication Date: 12 November 2008

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We describe a cost-efficient and large area scalable production process of organic light-emitting diodes (OLEDs) with photonic crystals (PCs) as extraction elements for guided modes. Using laser interference lithography and physical plasma etching, we texture the indium tin oxide (ITO) electrode layer of an OLED with one- and two-dimensional PC gratings. By optical transmission measurements, the resonant mode of the grating is shown to have a drift of only 0.4% over the 5 mm length of the ITO grating. By changing the lattice constant between 300 and 600 nm, the OLED emission angle of enhanced light outcoupling is tailored from −24.25° to 37°. At these angles, the TE emission is enhanced up to a factor of 2.14.
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85.60.Jb Light-emitting devices
42.70.Qs Photonic bandgap materials
52.77.Bn Etching and cleaning
81.65.Cf Surface cleaning, etching, patterning
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

Epitaxial lateral overgrowth of InP on Si from nano-openings: Theoretical and experimental indication for defect filtering throughout the grown layer

F. Olsson, M. Xie, S. Lourdudoss, I. Prieto, and P. A. Postigo

J. Appl. Phys. 104, 093112 (2008); http://dx.doi.org/10.1063/1.2977754 (6 pages) | Cited 8 times

Online Publication Date: 12 November 2008

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We present a model for the filtration of dislocations inside the seed window in epitaxial lateral overgrowth (ELO). We found that, when the additive effects of image and gliding forces exceed the defect line tension force, filtering can occur even in the openings. The model is applied to ELO of InP on Si where the opening size and the thermal stress arising due to the mask and the grown material are taken into account and analyzed. Further, we have also designed the mask patterns in net structures, where the tilting angles of the openings in the nets are chosen in order to take advantage of the filtering in the openings more effectively, and to minimize new defects due to coalescence in the ELO. Photoluminescence intensities of ELO InP on Si and on InP are compared and found to be in qualitative agreement with the model.
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68.55.ag Semiconductors
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
81.05.Ea III-V semiconductors
78.66.Fd III-V semiconductors
78.55.Cr III-V semiconductors

All-optical diode action with quasiperiodic photonic crystals

Fabio Biancalana

J. Appl. Phys. 104, 093113 (2008); http://dx.doi.org/10.1063/1.3010299 (5 pages) | Cited 16 times

Online Publication Date: 12 November 2008

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We theoretically investigate the possibility of realizing a nonlinear all-optical diode by using the unique features of quasiperiodic one-dimensional photonic crystals. The interplay between the intrinsic spatial asymmetry in odd-order Thue–Morse lattices and Kerr nonlinearity, combined with the unconventional field localization properties of this class of quasiperiodic sequences, gives rise to sharp resonances that can be used to give a polarization-insensitive nonreciprocal propagation with a contrast close to unity for low optical intensities.
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85.60.Dw Photodiodes; phototransistors; photoresistors
42.65.Jx Beam trapping, self-focusing and defocusing; self-phase modulation

The response of semiconductor optical amplifiers containing lateral composition modulation in the quantum wells

J. A. Czaban and D. A. Thompson

J. Appl. Phys. 104, 093114 (2008); http://dx.doi.org/10.1063/1.3010864 (6 pages) | Cited 1 time

Online Publication Date: 12 November 2008

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Tensile strained quantum wells are used to change the polarization sensitivity of semiconductor optical amplifiers. However, lateral composition modulation (LCM), which is present in the quantum wells of the device, is shown to impact the device performance. A broadening of the spectral gain is seen as recombinations from the conduction band to the heavy hole band in the In-rich regions of the LCM. This additional component to the gain from the quantum well will reduce the device gain efficiency at a specific wavelength for a given current, or it may be used to broaden the gain spectrum of an individual quantum well.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.60.Fc Modulation, tuning, and mode locking
42.70.Hj Laser materials

Trace gas measurements using optically resonant cavities and quantum cascade lasers operating at room temperature

S. Welzel, G. Lombardi, P. B. Davies, R. Engeln, D. C. Schram, and J. Röpcke

J. Appl. Phys. 104, 093115 (2008); http://dx.doi.org/10.1063/1.3008014 (15 pages) | Cited 10 times

Online Publication Date: 13 November 2008

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Achieving the high sensitivity necessary for trace gas detection in the midinfrared molecular fingerprint region generally requires long absorption path lengths. In addition, for wider application, especially for field measurements, compact and cryogen free spectrometers are definitely preferable. An alternative approach to conventional linear absorption spectroscopy employing multiple pass cells for achieving high sensitivity is to combine a high finesse cavity with thermoelectrically (TE) cooled quantum cascade lasers (QCLs) and detectors. We have investigated the sensitivity limits of an entirely TE cooled system equipped with an ∼ 0.5 m long cavity having a small sample volume of 0.3 l. With this spectrometer cavity enhanced absorption spectroscopy employing a continuous wave QCL emitting at 7.66 μm yielded path lengths of 1080 m and a noise equivalent absorption of 2×10−7 cm−1 Hz−1/2. The molecular concentration detection limit with a 20 s integration time was found to be 6×108 molecules/cm3 for N2O and 2×109 molecules/cm3 for CH4, which is good enough for the selective measurement of trace atmospheric constituents at 2.2 mbar. The main limiting factor for achieving even higher sensitivity, such as that found for larger volume multi pass cell spectrometers, is the residual mode noise of the cavity. On the other hand the application of TE cooled pulsed QCLs for integrated cavity output spectroscopy and cavity ring-down spectroscopy (CRDS) was found to be limited by the intrinsic frequency chirp of the laser. Consequently the accuracy and advantage of an absolute internal absorption calibration, in theory inherent for CRDS experiments, are not achievable.
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42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy
42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
93.85.-q Instruments and techniques for geophysical research: Exploration geophysics
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Calculation of gas heating in a dc sputter magnetron

I. Kolev and A. Bogaerts

J. Appl. Phys. 104, 093301 (2008); http://dx.doi.org/10.1063/1.2970166 (8 pages) | Cited 13 times

Online Publication Date: 6 November 2008

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The effect of gas heating in laboratory sputter magnetrons is investigated by means of numerical modeling. The model is two-dimensional in the coordinate space and three-dimensional in the velocity space based on the particle-in-cell–Monte Carlo collisions technique. It is expanded in a way that allows the inclusion of the neutral plasma particles (fast gas atoms and sputtered atoms), which makes it possible to calculate the gas temperature and its influence on the discharge behavior in a completely self-consistent way. The results of the model are compared to experimental measurements and to other existing simulation results. The results show that gas heating is pressure dependent (rising with the increase in the gas pressure) and should be taken into consideration at pressures above 10 mTorr.
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52.50.-b Plasma production and heating
79.20.Rf Atomic, molecular, and ion beam impact and interactions with surfaces
84.40.Fe Microwave tubes (e.g., klystrons, magnetrons, traveling-wave, backward-wave tubes, etc.)

Comparative study of laser produced Li plasma plumes from thin film and solid target

Ajai Kumar, R. K. Singh, V. Prahlad, and H. C. Joshi

J. Appl. Phys. 104, 093302 (2008); http://dx.doi.org/10.1063/1.3014031 (8 pages) | Cited 8 times

Online Publication Date: 14 November 2008

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The evolution features of lithium ions and neutrals generated by two different schemes viz. the laser-blow-off (LBO) of multicomponent LiF–C thin film and conventional laser ablation (referred here as LPP) from solid lithium have been studied using optical emission spectroscopic technique. The optical signal emitted by Li I (at 670.8 nm) and Li II (548.4 nm) was monitored as a function of laser fluence, ambient gas pressure, and distance “z” from the target. Apart from their similarities, some interesting differences were noticed in temporal profiles of the plumes generated by LPP and LBO both in vacuum as well as in the presence of the ambient gas. A comparative analysis of experimental results indicates that the ablation mechanism and subsequent laser-plume interaction were responsible for the observed differences in LPP and LBO plumes. Expansion features of the plume are discussed in the light of two different models.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.38.Mf Laser ablation
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
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GaN/AlN short-period superlattices for intersubband optoelectronics: A systematic study of their epitaxial growth, design, and performance

P. K. Kandaswamy, F. Guillot, E. Bellet-Amalric, E. Monroy, L. Nevou, M. Tchernycheva, A. Michon, F. H. Julien, E. Baumann, F. R. Giorgetta, D. Hofstetter, T. Remmele, M. Albrecht, S. Birner, and Le Si Dang

J. Appl. Phys. 104, 093501 (2008); http://dx.doi.org/10.1063/1.3003507 (16 pages) | Cited 44 times

Online Publication Date: 4 November 2008

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We have studied the effect of growth and design parameters on the performance of Si-doped GaN/AlN multiquantum-well (MQW) structures for intersubband optoelectronics in the near infrared. The samples under study display infrared absorption in the 1.3–1.9 μm wavelength range, originating from the photoexcitation of electrons from the first to the second electronic level in the QWs. A commonly observed feature is the presence of multiple peaks in both intersubband absorption and interband emission spectra, which are attributed to monolayer thickness fluctuations in the quantum wells. These thickness fluctuations are induced by dislocations and eventually by cracks or metal accumulation during growth. The best optical performance is attained in samples synthesized with a moderate Ga excess during the growth of both the GaN QWs and the AlN barriers without growth interruptions. The optical properties are degraded at high growth temperatures (>720 °C) due to the thermal activation of the AlN etching of GaN. From the point of view of strain, GaN/AlN MQWs evolve rapidly to an equilibrium average lattice parameter, which is independent of the substrate. As a result, we do not observe any significant effect of the underlayers on the optical performance of the MQW structure. The average lattice parameter is different from the expected value from elastic energy minimization, which points out the presence of periodic misfit dislocations in the structure. The structural quality of the samples is independent of Si doping up to 1020 cm−3. By contrast, the intersubband absorption spectrum broadens and blueshifts with doping as a result of electron-electron interactions. This behavior is independent of the Si doping location in the structure, either in the QWs or in the barriers. It is found that the magnitude of the intersubband absorption is not directly determined by the Si concentration in the wells. Instead, depending on the Al mole fraction of the cap layer, the internal electric field due to piezoelectric and spontaneous polarization can deplete or induce charge accumulation in the QWs. In fact, this polarization-induced doping can result in a significant and even dominant contribution to the infrared absorption in GaN/AlN MQW structures.
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78.67.De Quantum wells
81.07.St Quantum wells
73.21.Fg Quantum wells

Internal stresses in TiN/Ti multilayer coatings deposited by large area filtered arc deposition

Y. H. Cheng, T. Browne, B. Heckerman, J. C. Jiang, E. I. Meletis, C. Bowman, and V. Gorokhovsky

J. Appl. Phys. 104, 093502 (2008); http://dx.doi.org/10.1063/1.3006136 (7 pages) | Cited 9 times

Online Publication Date: 4 November 2008

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A series of TiN/Ti multilayer coatings with fixed TiN layer thickness and different Ti layer thicknesses were deposited using a large area filtered arc deposition technique. X-ray diffraction was used to investigate the crystalline structure, lattice strain, and crystallinity of the deposited coatings. A substrate curvature method was used to measure the internal stress in the multilayer coatings. The influence of the Ti interlayer thickness on the crystalline structure and internal stress in the coatings was systematically studied. It was found that a cubic TiN phase and hexagonal Ti phase exist in all the multilayer coatings. The TiN and Ti layers in the multilayer coatings exhibit a strong (111) and (002) preferred orientation, respectively. With the increase in the Ti layer thickness, the d-spacing decreases and the peak width increases for both TiN (111) and Ti (002) peaks, indicating a decrease in the lattice strain and an increase in the crystallinity of both TiN and Ti phases. It is suggested, that the reduction in the defect density in both TiN and Ti layers and the relaxation of the stain by the diffusion of the Ti atoms in the underneath Ti layer contribute to the decrease in the total internal stress with increasing Ti layer thickness.
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81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.65.Ac Multilayers
68.60.Bs Mechanical and acoustical properties
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization

On designing dielectric elastomer actuators

Mickael Moscardo, Xuanhe Zhao, Zhigang Suo, and Yuri Lapusta

J. Appl. Phys. 104, 093503 (2008); http://dx.doi.org/10.1063/1.3000440 (7 pages) | Cited 19 times

Online Publication Date: 4 November 2008

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Subject to a voltage, a dielectric elastomer can deform substantially, making it a desirable material for actuators. Designing such an actuator, however, has been challenging due to nonlinear equations of state, as well as multiple modes of failure, parameters of design, and measures of performance. This paper explores these issues, using a spring-roll actuator as an example. We formulate the equations of state with two degrees of freedom and describe the constraints due to several modes of failure of the elastomer, including electrical breakdown, electromechanical instability, loss of tension, and tensile rupture. Also included is the compressive limit of the spring. We show that, for the spring-roll actuator, loss of tension in the axial direction will always precede electromechanical instability. We then describe a procedure to maximize the range of actuation by choosing the parameters of design, such as the prestretch of the elastomer and the stiffness of the spring.
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07.07.Tw Servo and control equipment; robots

Picosecond laser structuration under high pressures: Observation of boron nitride nanorods

Luc Museur, Jean-Pierre Petitet, Jean-Pierre Michel, Wladimir Marine, Demetrios Anglos, Costas Fotakis, and Andrei V. Kanaev

J. Appl. Phys. 104, 093504 (2008); http://dx.doi.org/10.1063/1.3000457 (9 pages) | Cited 1 time

Online Publication Date: 4 November 2008

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We report on picosecond UV-laser processing of hexagonal boron nitride (BN) at moderately high pressures above 500 bar. The main effect is specific to the ambient gas and laser pulse duration in the ablation regime: when samples are irradiated by 5 or 0.45 ps laser pulses in nitrogen gas environment, multiple nucleation of a new crystalline product-BN nanorods-takes place. This process is triggered on structural defects, which number density strongly decreases upon recrystallization. Nonlinear photon absorption by adsorbed nitrogen molecules is suggested to mediate the nucleation growth. High pressure is responsible for the confinement and strong backscattering of ablation products. A strong surface structuring also appears at longer 150 ps laser irradiation in similar experimental conditions. However, the transformed product in this case is amorphous strongly contaminated by boron suboxides BxOy.
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81.07.Bc Nanocrystalline materials
79.20.Ds Laser-beam impact phenomena
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
61.46.Df Structure of nanocrystals and nanoparticles ("colloidal" quantum dots but not gate-isolated embedded quantum dots)
81.05.Ea III-V semiconductors
61.82.Rx Nanocrystalline materials

Size-dependent oxidation in ZnO nanoparticles embedded in ion-implanted silica

M. A. Tagliente, M. Massaro, G. Mattei, P. Mazzoldi, V. Bello, and G. Pellegrini

J. Appl. Phys. 104, 093505 (2008); http://dx.doi.org/10.1063/1.3005887 (8 pages) | Cited 3 times

Online Publication Date: 5 November 2008

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ZnO–SiO2 nanocomposites were synthesized by ion implanting a Zn+ beam in a silica slide and by annealing in oxidizing atmosphere at 800 °C. A detailed structural and optical characterization was performed by using glancing incidence x-ray diffraction, transmission electron microscopy combined with selected area electron diffraction and energy dispersive spectrometry, optical absorption, and photoluminescence spectroscopies. Samples obtained with three different Zn+ fluences in the range 1–2×1017 ions/cm2 have been investigated. According to the results, Zn crystalline nanoparticles were found in the as-implanted Zn–SiO2 samples. The size of the Zn nanoparticles was proportional to the implantation fluence. The annealing in oxidizing atmosphere promotes the total oxidation of the Zn nanoparticles with a preferential migration of the nanoparticles toward the surface of the sample along with an opposite and less pronounced diffusion toward the bulk of the matrix. A relatively strong excitonic peak from the ZnO nanoparticles was observed both in the optical absorption and photoluminescence spectra. We found that the oxidation of the Zn nanoparticles is size-dependent because the time necessary for the total oxidation of the nanoparticles increases with the decreasing in the size of the nanoparticles. This size-oxidation correlation has been explained in terms of arguments related to the stress of the Zn nanoparticles.
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81.07.-b Nanoscale materials and structures: fabrication and characterization
81.16.-c Methods of micro- and nanofabrication and processing
61.46.-w Structure of nanoscale materials
61.72.Cc Kinetics of defect formation and annealing
78.55.-m Photoluminescence, properties and materials
71.35.-y Excitons and related phenomena

Nanomechanical behavior of single crystalline SiC nanotubes revealed by molecular dynamics simulations

Zhiguo Wang, Xiaotao Zu, Fei Gao, and William J. Weber

J. Appl. Phys. 104, 093506 (2008); http://dx.doi.org/10.1063/1.3005979 (8 pages) | Cited 1 time

Online Publication Date: 5 November 2008

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Molecular dynamics simulations with Tersoff potentials were used to study the response of single crystalline SiC nanotubes under tensile, compressive, torsional, combined tension-torsional, and combined compression-torsional strains. The simulation results reveal that the nanotubes deform through bond-stretching and breaking and exhibit brittle properties under uniaxial tensile strain, except for the thinnest nanotube at high temperatures, which fails in a ductile manner. Under uniaxial compressive strain, the SiC nanotubes buckle with two modes, i.e., shell buckling and column buckling, depending on the length of the nanotubes. Under torsional strain, the nanotubes buckle either collapse in the middle region into a dumbbell-like structure for thinner wall thicknesses or fail by bond breakage for the largest wall thickness. Both the tensile failure stress and buckling stress decrease under combined tension-torsional and combined compression-torsional strain, and they decrease with increasing torsional rate under combined loading.
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81.07.De Nanotubes
81.40.Np Fatigue, corrosion fatigue, embrittlement, cracking, fracture, and failure
81.40.Lm Deformation, plasticity, and creep
62.20.mj Brittleness
62.25.Mn Fracture/brittleness
62.20.mq Buckling

Analysis of the optical properties of Cu(In1−xGax)3Se5 crystals

M. León, R. Serna, S. Levcenko, G. Gurieva, J. M. Merino, E. J. Friedrich, and E. Arushanov

J. Appl. Phys. 104, 093507 (2008); http://dx.doi.org/10.1063/1.2986159 (5 pages) | Cited 2 times

Online Publication Date: 5 November 2008

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Analysis of the optical properties of bulk Cu(In1−xGax)3Se5 mixed crystals synthesized from the elements as a function of the Ga content is presented. Measurements of the complex dielectric function ε(ω) = ε1(ω)+iε2(ω) were performed at room temperature in the photon energy range of 0.8–4.7 eV using a variable angle of incidence ellipsometer. The spectral dependence of the complex refractive index, the absorption coefficient, and the normal-incidence reflectivity were also derived. The structure observed in the dielectric functions attributed to the interband transitions E0, E1A, and E1B has been modeled using a modification of the Adachi’s model. The results are in excellent agreement with the experimental data over the entire range of photon energies. The model parameters, including the energies corresponding to the lowest direct gap and higher critical points, have been determined using the simulated annealing algorithm. The values of E0 and E1A are found to increase linearly with the increasing Ga content.
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78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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