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15 Jun 2006

Volume 99, Issue 12, Articles (12xxxx)

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Epitaxial growth and optical properties of semiconductor quantum wires

Xue-Lun Wang and Valia Voliotis

J. Appl. Phys. 99, 121301 (2006); http://dx.doi.org/10.1063/1.2212056 (38 pages) | Cited 23 times

Online Publication Date: 28 June 2006

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In this paper we present a review on major advances achieved over the past ten years in the field of fabrication of semiconductor quantum wires (QWRs) using epitaxial growth techniques and investigation of their optical properties. We begin the review with a brief summary on typical epitaxial QWRs developed so far. We next describe the state-of-the-art structural qualities of epitaxial QWRs in terms of (i) size uniformity between wires, (ii) heterointerface uniformity, (iii) crystal purity, and (iv) strength of lateral quantum confinement. Several prominent breakthroughs have been accomplished concerning the improvements of wire qualities, including (i) realization of V-shaped GaAs/AlGaAs QWRs in the “real one-dimensional” (1D) regime in which exciton states can extend coherently over distances exceeding 1 μm, (ii) reduction of residual impurity concentrations in V-shaped GaAs/AlGaAs QWRs to a level comparable to that in an equivalent quantum well (QWL), which resulted in the semiconductor QWR with room-temperature photoluminescence efficiency exceeding that of a QWL, and (iii) reduction of the multimonolayer (ML) interface fluctuations on the second-grown arm QWL surface, in old-generation T-shaped GaAs/AlGaAs QWRs, to the single-ML level. The second part of this article is devoted to the discussion of optical properties of epitaxial QWRs, such as exciton dynamics, fine structure of exciton levels, and nonlinear effects, studied by means of high-spatial resolution spectroscopy, i.e., microphotoluminescence experiments. We will concentrate our discussions on V-shaped GaAs/AlGaAs QWRs and put an emphasis on demonstrating how the interface quality influences wire’s optical properties. The properties of QWRs in the “zero-dimensional quantum box regime” and QWRs in the real 1D regime will be presented in separate sections. We will show that the realization of QWRs in the real 1D regime makes possible the investigation of intrinsic 1D effects by focusing on a single perfect 1D wire region using microscopic techniques. This has led to important results, for instance, (i) the demonstration of the square-root dependence of 1D exciton radiative recombination lifetimes down to a temperature as low as 10 K (limited by the experimental setup) and (ii) the clear demonstration of the existence of Mott transition in a 1D exciton system which is a fundamental problem under long debate.
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68.65.La Quantum wires (patterned in quantum wells)
81.10.-h Methods of crystal growth; physics and chemistry of crystal growth, crystal morphology, and orientation
78.67.Lt Quantum wires
71.35.-y Excitons and related phenomena
78.55.Cr III-V semiconductors
71.30.+h Metal-insulator transitions and other electronic transitions
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Interpretation of polarization pinning due to scattering loss differentiation in asymmetric vertical-cavity surface-emitting laser cavities

Elbert Nhan and Spilios Riyopoulos

J. Appl. Phys. 99, 123101 (2006); http://dx.doi.org/10.1063/1.2193058 (17 pages)

Online Publication Date: 16 June 2006

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The asymmetric aperture geometry in vertical-cavity surface-emitting laser cavities has long been known through experiments to effectively pin the polarization state of the output laser emission. In this article, we present a theoretical explanation for the observed effects due to noncircular aperture shape. The crux of our theory is that the unequal scattering losses of the laser beam between orthogonal polarizations of the same cavity eigenmodes lead to polarization pinning. Polarization-dependent calculations for elliptic apertures show that the scattering losses are lower when the polarization mode is aligned with the longest aperture axis, in agreement with experimental observations. This imbalance in scattering losses is higher than any intrinsic gain-related polarization dependence, and we suggest that it is the determining factor for polarization selection above a few-percent asymmetry in cavity geometry.
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42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation

Detecting clandestine material with nuclear resonance fluorescence

J. Pruet, D. P. McNabb, C. A. Hagmann, F. V. Hartemann, and C. P. J. Barty

J. Appl. Phys. 99, 123102 (2006); http://dx.doi.org/10.1063/1.2202005 (11 pages) | Cited 20 times

Online Publication Date: 19 June 2006

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We study the performance of a class of interrogation systems that exploit nuclear resonance fluorescence (NRF) to detect specific isotopes. In these systems the presence of a particular nuclide is inferred by observing the preferential attenuation of photons that strongly excite an electromagnetic transition in that nuclide. Estimates for the false positive/negative error rates, radiological dose, and detection sensitivity associated with discovering clandestine material embedded in cargo are presented. The relation between performance of the detection system and properties of the beam of interrogating photons is also considered. Bright gamma-ray sources with fine energy and angular resolution, such as those based on Thomson upscattering of laser light, are found to be associated with uniquely low radiological dose, scan times, and error rates. For this reason a consideration of NRF-based interrogation systems may provide impetus for efforts in light source development for applications related to national security and industry.
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07.85.Fv X- and γ-ray sources, mirrors, gratings, and detectors
29.25.-t Particle sources and targets

Singlet excited state absorption of porphyrin molecules for pico- and femtosecond optical limiting application

N. M. Barbosa Neto, S. L. Oliveira, L. Misoguti, C. R. Mendonça, P. J. Gonçalves, I. E. Borissevitch, L. R. Dinelli, L. L. Romualdo, A. A. Batista, and S. C. Zilio

J. Appl. Phys. 99, 123103 (2006); http://dx.doi.org/10.1063/1.2204350 (4 pages) | Cited 7 times

Online Publication Date: 20 June 2006

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This work employs the Z-scan technique with 120 fs pulses to investigate the singlet excited state absorption spectra of tetrapyridyl and tetrasulfonatophenyl porphyrins. We have used a three-energy-level model to adjust Z-scan curves in order to obtain the singlet excited absorption cross section from 460 to 800 nm. Starting from these values, we determine the spectra of the ratio between excited and ground singlet state absorption cross sections, whose values are as good as the best found in the literature for reverse saturable absorbers. The results obtained point these porphyrins as good candidates for applications in optical limiting of ultrashort pulses.
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42.70.Jk Polymers and organics
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
78.40.Me Organic compounds and polymers
78.30.Jw Organic compounds, polymers
33.20.Ea Infrared spectra
33.20.Kf Visible spectra
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)
31.50.Bc Potential energy surfaces for ground electronic states

Coherent diffraction and random scattering in thiol-ene–based holographic polymer-dispersed liquid crystal reflection gratings

R. L. Sutherland, V. P. Tondiglia, L. V. Natarajan, P. F. Lloyd, and T. J. Bunning

J. Appl. Phys. 99, 123104 (2006); http://dx.doi.org/10.1063/1.2206876 (12 pages) | Cited 12 times

Online Publication Date: 21 June 2006

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Bragg diffraction and random scattering in reflective holographic polymer-dispersed liquid crystal gratings are modeled using a matrix approach for a stack of low-high index layer pairs and an effective medium theory. Scattering is due to both random roughness of layer interfaces and random index variations within the layers. These are related to random liquid crystal droplet size and location as well as random orientation of the symmetry axes of bipolar droplets. Characteristic parameters governing coherent diffraction efficiency and random scattering are obtained partly from experiments, where possible, and partly from calculations based on a model of an effective medium applied to the grating. Calculations of grating transmittance are then compared to experimental transmittance spectra. Effects of scattering, primarily a decrease in baseline transmittance with wavelength and a small reduction in diffraction efficiency at the Bragg wavelength, are found to be due primarily to index inhomogeneities within the liquid-crystal-rich layers of the grating.
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42.40.Eq Holographic optical elements; holographic gratings
42.79.Kr Display devices, liquid-crystal devices
42.70.Df Liquid crystals

Synthesis and characterization of ZnO thin film grown by electron beam evaporation

D. C. Agarwal, R. S. Chauhan, Amit Kumar, D. Kabiraj, F. Singh, S. A. Khan, D. K. Avasthi, J. C. Pivin, M. Kumar, J. Ghatak, and P. V. Satyam

J. Appl. Phys. 99, 123105 (2006); http://dx.doi.org/10.1063/1.2204333 (6 pages) | Cited 19 times

Online Publication Date: 22 June 2006

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Highly transparent, conducting, highly oriented, and almost single phase ZnO films have been deposited by simple e-beam evaporation method, and the deposition parameters were optimized. The films were prepared by (a) evaporation of ZnO at different substrate temperatures and (b) evaporation of ZnO at room temperature and subsequent annealing of the films in oxygen ambient at different temperatures. The characterizations of the film were performed by optical absorption spectroscopy (UV-visible), Fourier transform infrared spectroscopy, resistivity measurement, transmission electron microscopy (TEM), photoluminescence, and x-ray diffraction measurement. Absorption spectra revealed that the films were highly transparent and the band gap of the pre- and postannealed films was in good agreement with the reported values. The band gap of the films increases on increasing the substrate temperature as well as annealing temperature, whereas the resistivity of the film decreases with substrate temperature and increases with annealing temperature. Fourier transform infrared spectroscopy of ZnO films confirms the presence of Zn–O bonding. X-ray diffraction, electron diffraction, and TEM images with high resolution and Raman spectra of the films showed the formation of crystalline ZnO having wurtzite structure.
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81.05.Dz II-VI semiconductors
81.15.-z Methods of deposition of films and coatings; film growth and epitaxy
68.55.A- Nucleation and growth
68.55.-a Thin film structure and morphology
78.66.Hf II-VI semiconductors
73.61.Ga II-VI semiconductors

Optical spectroscopic analysis of selected area epitaxially regrown n+ gallium nitride

D. Wang, M. Park, Y. N. Saripalli, M. A. L. Johnson, C. Zeng, D. W. Barlage, and J. P. Long

J. Appl. Phys. 99, 123106 (2006); http://dx.doi.org/10.1063/1.2204755 (6 pages) | Cited 1 time

Online Publication Date: 23 June 2006

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Gallium nitride (GaN) metal-insulator-semiconductor field-effect transistor with regrown by selected area metal organic vapor-phase-epitaxy n+ layer has been analyzed by micro-Raman and microphotoluminescence (micro-PL) spectroscopy. The material properties of the regrown n+ layer and the intrinsic layer in the gate region were extracted by using both spectroscopies. The free-carrier concentration of the regrown GaN layer and the intrinsic layer were determined by line shape analysis of the coupled plasmon-phonon mode to be 4.7×1017 and <3×1016 cm−3, respectively. The inefficient substitutions of Ga vacancy (VGa) by Si result in relatively low carrier concentration in the regrown GaN layer. From the shift of E2(2) Raman peak and the near-band-edge (NBE) PL peak, the biaxial compressive stress in the intrinsic layer was found to be 0.4 GPa. The residual stress was found to be fully relaxed in the regrown layer. The Si doping concentration in the regrown layer was determined to be 2×1019 cm−3 based on the potential fluctuations introduced redshift of its NBE PL peak.
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78.30.Fs III-V and II-VI semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
68.60.Bs Mechanical and acoustical properties
61.72.S- Impurities in crystals
71.45.Gm Exchange, correlation, dielectric and magnetic response functions, plasmons

Optical limiting in defective quadratic nonlinear photonic crystals

Yong Zeng, Xiaoshuang Chen, and Wei Lu

J. Appl. Phys. 99, 123107 (2006); http://dx.doi.org/10.1063/1.2206408 (4 pages) | Cited 6 times

Online Publication Date: 23 June 2006

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By using a nonlinear transfer matrix method and an iteration method, we investigated an optical limiting in a defective quadratic nonlinear photonic crystal under strong pump-depletion conditions. Due to a giant enhancement of the second harmonic (SH) generation induced by the strong localization of light near the defective band, the progress of the energy exchange from the forward fundamental frequency (FF) channel to the SH channels (both forward and backward) and the backward FF channel is speeded up. The structure can therefore be used as a high-efficient compact optical power limiter. We have also studied the effects of two important parameters of the defective nonlinear periodic structure, the number of the periods and the nonlinear efficiency of the composite material, on the optical limiting.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Qs Photonic bandgap materials
42.70.Mp Nonlinear optical crystals

Analysis of limitations to wallplug efficiency and output power for quantum cascade lasers

I. Vurgaftman and J. R. Meyer

J. Appl. Phys. 99, 123108 (2006); http://dx.doi.org/10.1063/1.2206847 (7 pages) | Cited 5 times

Online Publication Date: 26 June 2006

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We have carried out detailed optical and thermal simulations of quantum cascade laser (QCL) operation as a function of material and device parameters. The main objective of the study is to evaluate the potential for improvements in the cw wallplug efficiencies and output powers beyond the current state of the art, when such parameters as the internal loss, series resistance, doping level in the active region, number of QCL stages, ridge width, cavity length, current density, and operating temperature are varied within reasonable ranges. For the test case of a narrow-ridge λ = 4.8 μm QCL mounted epitaxial side down, we project that a maximum wallplug efficiency of >10% and cw output power of >1 W may be feasible at room temperature, if the net internal loss can be reduced by 30% from its best current value.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Influence of defects in the output power of c- and a-axis growth Nd:YVO4 single laser crystals

Charles R. Shawley, Cai-Lin Wang, and Kelvin G. Lynn

J. Appl. Phys. 99, 123109 (2006); http://dx.doi.org/10.1063/1.2207722 (4 pages) | Cited 1 time

Online Publication Date: 26 June 2006

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The differences in the output powers of a- and c-axis growth neodymium-doped yttrium orthovanadate (Nd:YVO4) crystals grown by the Czochralski method were investigated. It was found that the differences in the output powers are due to crystal defects, such as veils and grain boundaries, which lead to changes in the thermal and optical properties of Nd:YVO4 and reduce the quality of the laser beam. To relieve internal stresses introduced by the crystal growth process, boules form grain boundaries and other thermodynamically favorable defects in the boule. Because c-axis grown Nd:YVO4 crystals do not contain a slip system, no grain boundaries or ghost veils can form. As a result, c-axis grown boules are highly vulnerable to thermal and stress fractures and require careful growth parameter selection to overcome these defects. However, a-axis grown crystal contain a slip system and can form thermodynamically stable defects including grain boundaries, scatter centers, and ghost veils. In this paper, data are presented showing that properly grown c-axis growth crystal, having no grain boundaries or veils, is more suited for high output power applications than a-axis grown one.
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42.70.-a Optical materials
81.10.Fq Growth from melts; zone melting and refining
61.72.Mm Grain and twin boundaries
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)
42.55.Rz Doped-insulator lasers and other solid state lasers

Separate-confinement-oxidation vertical-cavity surface-emitting laser structure

Robert P. Sarzała and Włodzimierz Nakwaski

J. Appl. Phys. 99, 123110 (2006); http://dx.doi.org/10.1063/1.2206129 (9 pages) | Cited 6 times

Online Publication Date: 27 June 2006

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In the present paper, a comprehensive self-consistent three-dimensional model is used to analyze physical aspects of the operation of oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) and to optimize their structures. The impact of the built-in radial confining mechanisms created by oxide apertures, i.e., the influence of their diameters and localizations on radial confinements of both the current injection into VCSEL active regions and electromagnetic fields of successive cavity modes, has been investigated. Basically, there are two extreme cases: the index-guided (IG) VCSELs with the aperture localized at the antinode position of the optical standing wave, characterized by a very low lasing threshold but exhibiting rather poor mode selectivity, and the gain-guided (GG) VCSELs with the aperture shifted to the node position, ensuring usually the single-fundamental-mode operation, but at the expense of much higher lasing threshold. In the present paper, the separate-confinement-oxidation VCSEL structure has been proposed exhibiting simultaneously advantages of both the IG VCSELs (the low lasing threshold) and the GG ones (the single-fundamental-mode operation).
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
85.30.De Semiconductor-device characterization, design, and modeling
81.65.Mq Oxidation

All-optical diodes based on photonic crystal molecules consisting of nonlinear defect pairs

Hui Zhou, Kai-Feng Zhou, Wei Hu, Qi Guo, Sheng Lan, Xu-Sheng Lin, and Achanta Venu Gopal

J. Appl. Phys. 99, 123111 (2006); http://dx.doi.org/10.1063/1.2207726 (9 pages) | Cited 8 times

Online Publication Date: 28 June 2006

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We investigate the unidirectional transmission behavior of photonic crystal (PC) molecules consisting of nonlinear defect pairs by use of the coupled mode theory and the finite-difference time-domain technique. As compared with the all-optical diodes based on single asymmetrically confined PC defects (or PC atoms), a significant enhancement in the transmission contrast is achieved. A qualitative comparison of the dynamical response to external excitation is carried out for nonlinear PC atoms and molecules and the physical origins responsible for the enhancement of transmission contrast are clarified. In addition, the tolerance of the resulting optical diodes against the fabrication error is discussed. Furthermore, it is revealed that the figure of merit, which is defined as the product of the threshold transmission and the transmission contrast, can be greatly improved by dropping the use of air defects. Some abnormal and intriguing transmission behaviors are observed in the PC molecules without using air defects.
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42.70.Qs Photonic bandgap materials
42.79.-e Optical elements, devices, and systems

A spectroscopic comparison of femtosecond-laser-modified fused silica using kilohertz and megahertz laser systems

Wilbur J. Reichman, Denise M. Krol, Lawrence Shah, Fumiyo Yoshino, Alan Arai, Shane M. Eaton, and Peter R. Herman

J. Appl. Phys. 99, 123112 (2006); http://dx.doi.org/10.1063/1.2207556 (5 pages) | Cited 16 times

Online Publication Date: 29 June 2006

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Waveguides were written in fused silica using both a femtosecond fiber laser with a 1 MHz pulse repetition rate and a femtosecond amplified Ti:sapphire laser with a 1 kHz repetition rate. Confocal Raman and fluorescence microscopies were used to study structural changes in the waveguides written with both systems. A broad fluorescence band, centered at 650 nm, associated with nonbridging oxygen hole center (NBOHC) defects was observed after waveguide fabrication with the megahertz laser. With the kilohertz laser system these defects were only observed for pulse energies above 1 μJ. Far fewer NBOHC defects were formed with the megahertz laser than with kilohertz writing, possibly due to thermal annealing driven by heat accumulation effects at 1 MHz. When the kilohertz laser was used with pulse energies below 1 μJ, the predominant fluorescence was centered at 550 nm, a band assigned to the presence of silicon clusters (Eδ′). We also observed an increase in the intensity of the 605 cm−1 Raman peak relative to the total Raman intensity, corresponding to an increase in the concentration of three-membered rings in the lines fabricated with both laser systems.
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42.86.+b Optical workshop techniques
42.62.-b Laser applications
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.79.Gn Optical waveguides and couplers
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Reverse-bias emission sheds light on the failure mechanism of degraded vertical-cavity surface-emitting lasers

D. K. McElfresh, L. D. Lopez, and D. Vacar

J. Appl. Phys. 99, 123113 (2006); http://dx.doi.org/10.1063/1.2206852 (8 pages)

Online Publication Date: 29 June 2006

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The phenomenon of reverse-bias electroluminescence (EL) has been exploited extensively to elucidate the physics of failure of Si-based semiconductor devices. In this article, we study the reverse-bias emission of vertical-cavity surface-emitting lasers (VCSELs) that have been stressed under accelerated temperature, humidity, and bias. Using quantitative emission microscopy in conjunction with other electrical and optical measurements, we show that the dark line defect dislocation network responsible for VCSEL degradation can be imaged directly as reverse-bias EL and that the spectra of the reverse-bias EL provide further insight into the failure mechanism.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.60.Da Resonators, cavities, amplifiers, arrays, and rings
85.30.De Semiconductor-device characterization, design, and modeling

Measurement of negative permittivity and permeability from experimental transmission and reflection with effects of cell misalignment

Dongxing Wang, Jiangtao Huangfu, Lixin Ran, Hongsheng Chen, Tomasz M. Grzegorczyk, and Jin Au Kong

J. Appl. Phys. 99, 123114 (2006); http://dx.doi.org/10.1063/1.2209030 (5 pages) | Cited 3 times

Online Publication Date: 30 June 2006

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The measurement of the negative effective permittivity and permeability of a material is a direct way to verify its left-handed properties. In this paper, a transmission experiment for a left-handed material (LHM) sample is performed in a parallel plate waveguide. The effective frequency dispersive permittivity and permeability are retrieved from the experimental scattering data and found to be simultaneously negative in a specific frequency range. The simulation results also show that the misalignments of the metallic inclusions of the LHM introduce some dips to the retrieved curves, which prevent us from obtaining ideal curves following the Lorentz or Drude model.
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42.70.-a Optical materials
77.22.Ch Permittivity (dielectric function)
61.72.Qq Microscopic defects (voids, inclusions, etc.)
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Toluene oxidation in a plasma-catalytic system

M. Magureanu, N. B. Mandache, E. Gaigneaux, C. Paun, and V. I. Parvulescu

J. Appl. Phys. 99, 123301 (2006); http://dx.doi.org/10.1063/1.2204353 (7 pages) | Cited 7 times

Online Publication Date: 20 June 2006

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Oxidative removal of toluene in a dielectric barrier discharge reactor combined with manganese catalysts downstream was investigated. Toluene input concentration was varied in the range of 415–2227 ppm. The discharge was operated in pulsed mode, with short pulses of 23–35 kV peak voltage. At 7 W average power, toluene conversion was 60%–70%, independent on the toluene input concentration and on the total gas flow rate in the range of 110–330 SCCM (SCCM denotes cubic centimeter per minute at STP). Toluene total oxidation was favored at high residence time of the gas in the discharge zone and low toluene concentration, when the main reaction product was CO2 with selectivities of 80%–85%. The addition of the catalysts led to a 15%–20% increase in toluene conversion with respect to the values obtained in the plasma, due to oxidation with ozone on the catalyst surface.
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82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
52.77.-j Plasma applications
52.80.-s Electric discharges
82.20.Hf Product distribution

Theoretical investigation of a photoconductively switched high-voltage spark gap

B. H. P. Broks, J. Hendriks, W. J. M. Brok, G. J. H. Brussaard, and J. J. A. M. van der Mullen

J. Appl. Phys. 99, 123302 (2006); http://dx.doi.org/10.1063/1.2204756 (9 pages) | Cited 1 time

Online Publication Date: 22 June 2006

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In this contribution, a photoconductively switched high-voltage spark gap with an emphasis on the switching behavior is modeled. It is known experimentally that not all of the voltage that is present at the input of the spark gap is switched, but rather a fraction of it drops across the spark gap. This voltage drop depends on the voltage that is present at the input of the spark gap with higher voltages resulting in a smaller drop. We have investigated two possible causes of this: the cathode fall and the resistance of the plasma arc. Using an analytical model of the cathode fall, we have established that the cathode fall can be excluded as the cause of the observed voltage drop. A one-dimensional, time-dependent non-local thermal equilibrium fluid model of the arc plasma has been made. Using this model, the plasma properties have been analyzed for various values of the switched current with emphasis on the conductivity. A good qualitative match between the observed and the simulated dissipation in the gap was found. This indicates that the finite arc resistance is the cause of the observed voltage drop.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.75.Kq Plasma switches (e.g., spark gaps)
52.25.Fi Transport properties
52.65.Kj Magnetohydrodynamic and fluid equation

Influence of argon and oxygen on charge-state-resolved ion energy distributions of filtered aluminum arcs

Johanna Rosén, André Anders, Stanislav Mráz, Adil Atiser, and Jochen M. Schneider

J. Appl. Phys. 99, 123303 (2006); http://dx.doi.org/10.1063/1.2206413 (5 pages) | Cited 10 times

Online Publication Date: 26 June 2006

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The charge-state-resolved ion energy distributions (IEDs) in filtered aluminum vacuum arc plasmas were measured and analyzed at different oxygen and argon pressures in the range of 0.5–8.0 mTorr. A significant reduction of the ion energy was detected as the pressure was increased, most pronounced in an argon environment and for the higher charge states. The corresponding average charge state decreased from 1.87 to 1.0 with increasing pressure. The IEDs of all metal ions in oxygen were fitted with shifted Maxwellian distributions. The results show that it is possible to obtain a plasma composition with a narrow charge-state distribution as well as a narrow IED. These data may enable tailoring thin film properties through selecting growth conditions that are characterized by predefined charge state and energy distributions.
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52.80.Mg Arcs; sparks; lightning; atmospheric electricity
52.80.Vp Discharge in vacuum
52.50.Dg Plasma sources
52.70.Nc Particle measurements

Study of kink instabilities in 1 m long, free-standing plasma channels used for ion beam transport

S. Neff, R. Knobloch-Maas, A. Tauschwitz, D. H. H. Hoffmann, and S. S. Yu

J. Appl. Phys. 99, 123304 (2006); http://dx.doi.org/10.1063/1.2206694 (4 pages)

Online Publication Date: 28 June 2006

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Experiments at the Gesellschaft für Schwerionenforschung study the transport of heavy-ion beams in plasma channels in collaboration with the Advanced Reactor Innovative Engineering Study for Inertial Fusion Energy. One important aspect of channel transport is the suppression of instabilities. The susceptibility of discharge channels for magnetohydrodynamic instabilities is studied and their growth rate is measured with a fast framing camera. The images are analyzed with a computer code, determining the growth rates of the instability during later stages of the discharge. Magnetohydrodynamical (MHD) instabilities were observed only for high pressure and even then only at late stages of the discharge. The only observed MHD modes were kink instabilities and our measurements indicate that they pose no problem for transport channels with reactor parameters.
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52.35.Py Macroinstabilities (hydromagnetic, e.g., kink, fire-hose, mirror, ballooning, tearing, trapped-particle, flute, Rayleigh-Taylor, etc.)
52.25.Fi Transport properties
52.40.Mj Particle beam interactions in plasmas
52.58.Qv Electrostatic and high-frequency confinement
28.52.Av Theory, design, and computerized simulation
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Measurement of the ion feature of collective Thomson scattering in collisionless plasmas

I. E. Olivares and H.-J. Kunze

J. Appl. Phys. 99, 123305 (2006); http://dx.doi.org/10.1063/1.2207830 (3 pages) | Cited 1 time

Online Publication Date: 29 June 2006

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It is demonstrated that the collisionless theory of Salpeter [Phys. Rev. 120, 1528 (1960)] can be fitted rather well to the ion feature of collective Thomson scattering on dense plasmas in equilibrium for elements up to argon.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Tx Emission, absorption, and scattering of particles
52.35.Dm Sound waves
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Thermal annealing study of swift heavy-ion irradiated zirconia

Jean-Marc Costantini, Andrée Kahn-Harari, François Beuneu, and François Couvreur

J. Appl. Phys. 99, 123501 (2006); http://dx.doi.org/10.1063/1.2198927 (7 pages) | Cited 6 times

Online Publication Date: 16 June 2006

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Sintered samples of monoclinic zirconia (α-ZrO2) have been irradiated at room temperature with 6.0 GeV Pb ions in the electronic slowing down regime. X-ray diffraction (XRD) and micro-Raman spectroscopy measurements showed unambiguously that a transition to the “metastable” tetragonal phase (β-ZrO2) occurred at a fluence of 6.5×1012 cm−2 for a large electronic stopping power value ( ≈ 32.5 MeV μm−1). At a lower fluence of 1.0×1012 cm−2, no such phase transformation was detected. The back-transformation from β- to α-ZrO2 induced by isothermal or isochronal thermal annealing was followed by XRD analysis. The back-transformation started at an onset temperature around 500 K and was completed by 973 K. Plots of the residual tetragonal phase fraction deduced from XRD measurements versus annealing temperature or time are analyzed with first- or second-order kinetic models. An activation energy close to 1 eV for the back-transformation process is derived either from isothermal annealing curves, using the so-called crosscut method, or from the isochronal annealing curve, using a second-order kinetic law. Correlation with the thermal recovery of ion-induced paramagnetic centers monitored by electron paramagnetic resonance spectroscopy is discussed. Effects of crystallite size evolution and oxygen migration upon annealing are also addressed.
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81.40.Gh Other heat and thermomechanical treatments
61.80.Jh Ion radiation effects
78.30.Hv Other nonmetallic inorganics
61.85.+p Channeling phenomena (blocking, energy loss, etc.)
64.70.K- Solid-solid transitions
76.30.Lh Other ions and impurities

Physical properties of lanthanum monosulfide thin films grown on (100) silicon substrates

M. Cahay, K. Garre, X. Wu, D. Poitras, D. J. Lockwood, and S. Fairchild

J. Appl. Phys. 99, 123502 (2006); http://dx.doi.org/10.1063/1.2201998 (6 pages) | Cited 11 times

Online Publication Date: 16 June 2006

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Thin films of lanthanum monosulfide (LaS) have been deposited on Si (100) substrates by pulsed laser deposition. The films are golden yellow in appearance with a mirrorlike surface morphology and a sheet resistance around 0.1 Ω/◻, as measured using a four-probe measurement technique. The thin films are characterized by atomic force microscopy (AFM), x-ray diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM), ellipsometry, and Raman spectroscopy. The root-mean-square variation of (1 μm thick) film surface roughness measured over a 1 μm2 area by AFM was found to be 1.74 nm. XRD analysis of fairly thick films (micrometer size) reveals the growth of the cubic rocksalt structure with a lattice constant of 5.863(7) Å, which is close to the bulk LaS value. HRTEM images reveal that the films are comprised of nanocrystals separated by regions of amorphous material. Two beam bright field TEM images show that there is a strain contrast in the Si substrate right under the interface with the LaS film and penetrating into the Si substrate. This suggests that there is an initial epitaxial-like growth of the LaS film on the Si substrate that introduces a strain as a result of the 8% lattice mismatch between the film and substrate. Ellipsometry measurements of the LaS films are well characterized by a Drude-Lorentz model from which an electron concentration of about 2.52×1022 cm−3 and a mobility around 8.5 cm2/V s are derived. Typical crystalline LaS features were evident in Raman spectra of the films, but the spectra also revealed their disordered (polycrystalline) nature.
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68.55.A- Nucleation and growth
81.15.Fg Pulsed laser ablation deposition
81.16.Mk Laser-assisted deposition
68.55.-a Thin film structure and morphology
73.61.Ng Insulators
78.30.Am Elemental semiconductors and insulators

Formation of a ZnS/Zn(S,O) bilayer buffer on CuInS2 thin film solar cell absorbers by chemical bath deposition

M. Bär, A. Ennaoui, J. Klaer, T. Kropp, R. Sáez-Araoz, N. Allsop, I. Lauermann, H.-W. Schock, and M. C. Lux-Steiner

J. Appl. Phys. 99, 123503 (2006); http://dx.doi.org/10.1063/1.2202694 (9 pages) | Cited 14 times

Online Publication Date: 16 June 2006

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The application of Zn compounds as buffer layers was recently extended to wide-gap CuInS2 (CIS) based thin film solar cells. Using an alternative chemical deposition route for the buffer preparation aiming at the deposition of a single-layer, nominal ZnS buffer without the need for any toxic reactants such as hydrazine has helped us to achieve a similar efficiency as respective CdS-buffered reference devices. In order to shed light on the differences of other Zn-compound buffers deposited in conventional chemical baths [chemical bath deposition (CBD)] compared to the buffer layers deposited by this alternative CBD process, the composition of the deposited buffers was investigated by x-ray excited Auger electron and x-ray photoelectron spectroscopy to potentially clarify their superiority in terms of device performance. We have found that in the early stages of this alternative CBD process a thin ZnS layer is formed on the CIS, whereas in the second half of the CBD the growth rate is greatly increased and Zn(S,O) with a ZnS/(ZnS+ZnO) ratio of ∼ 80% is deposited. Thus, a ZnS/Zn(S,O) bilayer buffer is deposited on the CIS thin film solar cell absorbers by the alternative chemical deposition route used in this investigation. No major changes of these findings after a postannealing of the buffer/CIS sample series and recharacterization could be identified.
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81.15.Lm Liquid phase epitaxy; deposition from liquid phases (melts, solutions, and surface layers on liquids)
84.60.Jt Photoelectric conversion
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Gold nanorod extinction spectra

Stuart W. Prescott and Paul Mulvaney

J. Appl. Phys. 99, 123504 (2006); http://dx.doi.org/10.1063/1.2203212 (7 pages) | Cited 61 times

Online Publication Date: 16 June 2006

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Geometrical factors suitable for use in Mie–Gans theory are derived from discrete dipole aproximation generated spectra for a number of pseudonanorods permitting the rapid calculation of extinction spectra. It is shown that the rod width, rod end-cap geometry (flat, oblate spheroid, and sphere) and the rod size distribution all have a significant effect on the position of the peak absorbance. Moreover, it is shown that spectrometric characterization of nanorods is possible given an independent measure of rod width and suitable assumptions regarding the end-cap geometry and the form of the rod-length distribution; under such conditions the full width half maximum and the extinction peak uniquely determine the average rod length and the breadth of the distribution.
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78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.70.-a Optical materials
61.46.Hk Nanocrystals

Assessment of microscopic lattice structures in dilute (AlGaIn)NAs laser materials by local mode spectroscopy and numerical simulations

D. N. Talwar

J. Appl. Phys. 99, 123505 (2006); http://dx.doi.org/10.1063/1.2205353 (10 pages) | Cited 1 time

Online Publication Date: 19 June 2006

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In the framework of a rigid-ion model we report the results of a comprehensive Green’s function analyses of the Fourier transform infrared (FTIR) absorption and Raman scattering data on localized vibrational modes (LVMs) to investigate the microscopic lattice structures related with the nitrogen incorporation in (AlGaIn)NAs alloys. Contrary to the outcome of a recent FTIR report of LVMs on GaInNAs multiple-quantum well structures, our group-theoretical analyses of impurity vibrations support the earlier IR and Raman scattering experiments providing corroboration to the fact that upon annealing and/or by increasing In(Al) composition in GaIn(Al)NAs there occur structural changes causing N environment to transform from NAsGa4 to NAsIn(Al)Ga3 and/or NAsIn2(Al2)Ga2 microstructures. From the force variation correlation with bond covalency for the closest mass acceptor CAs(a−) and isoelectronic NAs(i) defects in GaAs we have obtained the corrected value of u for Al–N bond predicting NAs local mode in AlAs ( ∼ 510 cm−1) at a higher frequency than that of GaAs:N (471 cm−1). Theoretical results of impurity modes for Al–N complexes in GaAlNAs are compared and discussed with the existing Raman data.
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78.30.Fs III-V and II-VI semiconductors
78.67.De Quantum wells
42.55.Px Semiconductor lasers; laser diodes
71.55.Eq III-V semiconductors
63.20.Pw Localized modes
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