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1 May 2009

Volume 105, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 105, 091101 (2009); http://dx.doi.org/10.1063/1.3099572 (44 pages)

A. Rogalski, J. Antoszewski, and L. Faraone
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Third-generation infrared photodetector arrays

A. Rogalski, J. Antoszewski, and L. Faraone

J. Appl. Phys. 105, 091101 (2009); http://dx.doi.org/10.1063/1.3099572 (44 pages) | Cited 90 times

Online Publication Date: 11 May 2009

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Hitherto, two distinct families of multielement detector arrays have been used for infrared (IR) imaging system applications: linear arrays for scanning systems (first generation) and two-dimensional arrays for staring systems (second generation). Nowadays, third-generation IR systems are being developed which, in the common understanding, provide enhanced capabilities such as larger numbers of pixels, higher frame rates, better thermal resolution, multicolor functionality, and/or other on-chip signal-processing functions. In this paper, fundamental and technological issues associated with the development and exploitation of third-generation IR photon detectors are discussed. In this class of detectors the two main competitors, HgCdTe photodiodes and quantum-well photoconductors, are considered. This is followed by discussions focused on the most recently developed focal plane arrays based on type-II strained-layer superlattices and quantum dot IR photodetectors. The main challenges facing multicolor devices are concerned with complicated device structures, thicker and multilayer material growth, and more difficult device fabrication, especially for large array sizes and/or small pixel dimensions. This paper also presents and discusses the ongoing detector technology challenges that are being addressed in order to develop third-generation infrared photodetector arrays.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.60.Dw Photodiodes; phototransistors; photoresistors
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
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Coherent transition radiation from a helically microbunched electron beam

E. Hemsing and J. B. Rosenzweig

J. Appl. Phys. 105, 093101 (2009); http://dx.doi.org/10.1063/1.3121207 (6 pages) | Cited 5 times

Online Publication Date: 1 May 2009

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The coherent transition radiation emitted from an electron beam with higher-order spatial microbunching is analyzed. The characteristic angular and phase dependence can be used to identify the dominant bunching structure of such beams, which can be generated during the harmonic interaction in optical klystron modulators and free-electron lasers, and used as tunable sources of coherent light with orbital angular momentum.
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41.60.Dk Transition radiation
41.85.Ct Particle beam shaping, beam splitting
41.75.Ht Relativistic electron and positron beams
29.25.-t Particle sources and targets

Optimal photonic-crystal parameters assuring single-mode operation of 1300 nm AlInGaAs vertical-cavity surface-emitting laser

Tomasz Czyszanowski, Robert P. Sarzała, Maciej Dems, Włodzimierz Nakwaski, Hugo Thienpont, and Krassimir Panajotov

J. Appl. Phys. 105, 093102 (2009); http://dx.doi.org/10.1063/1.3115449 (10 pages) | Cited 4 times

Online Publication Date: 4 May 2009

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We present a self-consistent analysis of InP-based 1300 nm AlInGaAs photonic-crystal vertical-cavity surface-emitting lasers (PhC VCSELs) and tunnel-junction PhC VCSELs, and analyze the influence of the electrical confinement, the PhC hole diameter and etching depth, and the size of the single defect optical aperture on the threshold current and the transverse mode discrimination. We also investigate the thermal performance of the two VCSEL configurations. As a result we determine the optimal PhC parameters assuring stable, single-mode operation in a broad range of driving currents.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
42.70.Qs Photonic bandgap materials
42.55.Tv Photonic crystal lasers and coherent effects
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Femtosecond laser-induced shockwave formation on ablated silicon surface

Senthilnathan Panchatsharam, Bo Tan, and Krishnan Venkatakrishnan

J. Appl. Phys. 105, 093103 (2009); http://dx.doi.org/10.1063/1.3122047 (4 pages) | Cited 5 times

Online Publication Date: 4 May 2009

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This paper reports an experimental study of the vapor plume and shockwave propagation generated during femtosecond laser ablation. An Yb-doped fiber amplified/oscillator laser with 1030 nm wavelength and pulse repetition rate up to 26 MHz was used to ablate a blank silicon substrate in an ambient air environment. The interaction of internal and external shockwaves with the sample surface was found to cause raised spherical rims around the central ablation regime within the crater. Rims formed by primary and secondary shockwaves can be clearly observed. Analysis of the rim formation is studied in detail.
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62.50.Ef Shock wave effects in solids and liquids
78.47.-p Spectroscopy of solid state dynamics
79.20.Ds Laser-beam impact phenomena

High performance laser linewidth broadening for stimulated Brillouin suppression with zero parasitic amplitude modulation

P. Mitchell, A. Janssen, and J. K. Luo

J. Appl. Phys. 105, 093104 (2009); http://dx.doi.org/10.1063/1.3116149 (6 pages) | Cited 2 times

Online Publication Date: 4 May 2009

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To enable optimum network transmission, the ideal is to launch as high laser power as possible into the optical fiber, to overcome the effects of fiber attenuation and maintain an acceptable signal to noise ratio. Launching high powers into a fiber however, results in unwanted nonlinear effects. Stimulated Brillouin scattering (SBS) is one of the nonlinear effects which reflects a significant proportion of the transmitted optical power back to the transmitter, degrading the system severely. This paper reports the development of a digitally selected supermode distributed Bragg reflector monolithic laser chip which can provide significant linewidth broadening using a pure frequency modulation technique by application of a dither current. By modifying a small segment of the laser chip material refractive index, it produces a modulation of the longitudinal mode and hence laser frequency; the monolithic laser chip has reduced the effects of SBS significantly with very little parasitic amplitude modulation.
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42.60.Fc Modulation, tuning, and mode locking
42.55.Px Semiconductor lasers; laser diodes
42.65.Es Stimulated Brillouin and Rayleigh scattering

Preparation and spectroscopic characterization of Nd3+:Y2O3 nanocrystals suspended in polymethyl methacrylate

Dhiraj K. Sardar, Sreerenjini Chandra, John B. Gruber, Waldemar Gorski, Maogen Zhang, and Jun Ho Shim

J. Appl. Phys. 105, 093105 (2009); http://dx.doi.org/10.1063/1.3122300 (8 pages) | Cited 4 times

Online Publication Date: 4 May 2009

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We describe a method to fabricate polymethyl methacrylate (PMMA), a polymeric host, in which nanocrystals of Nd3+:Y2O3 are suspended. The spectroscopic properties of this material are analyzed using the standard Judd–Ofelt technique. The phenomenological Judd–Ofelt intensity parameters are used to calculate the radiative decay rates and the branching ratios of the 4F3/2→4IJ (J = 9/2, 11/2, 13/2, and 15/2) intermanifold transitions. The room temperature fluorescence lifetime has been measured for the most intense 4F3/2→4I11/2 emission transition. Emission cross sections for the intense intermanifold transitions and peak emission cross sections for the intense inter-Stark transitions are also reported. Assignments to individual Stark levels of the 4IJ manifolds have been made and compared with the calculated splittings reported earlier. Finally, the spectroscopic properties of the Nd3+:Y2O3 nanocrystals suspended in PMMA are compared with those of Nd3+ doped in various host materials. Detailed optical analysis led to favorable values of fluorescence lifetime and emission cross section for the 4F3/2→4I11/2 transition, which suggest that the Nd3+:Y2O3 nanocrystals embedded in PMMA would have potential for various photonic applications including laser systems and optical communication devices.
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81.16.-c Methods of micro- and nanofabrication and processing
81.07.Bc Nanocrystalline materials
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
78.55.-m Photoluminescence, properties and materials
78.67.Bf Nanocrystals, nanoparticles, and nanoclusters
42.70.Hj Laser materials
42.55.Rz Doped-insulator lasers and other solid state lasers

Raman and electron microscopy analysis of carbon nanotubes exposed to high power laser irradiance

Krishna Ramadurai, Christopher L. Cromer, Anne C. Dillon, Roop L. Mahajan, and John H. Lehman

J. Appl. Phys. 105, 093106 (2009); http://dx.doi.org/10.1063/1.3116165 (6 pages) | Cited 4 times

Online Publication Date: 5 May 2009

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High power laser radiometry requires efficient and damage-resistant detectors. The current study explores the evolving nature of carbon nanotube coatings for such detectors upon their exposure to incrementally increasing laser power levels. Electron microscopy images along with the D-band to G-band intensity ratios from the Raman spectra from eight irradiance levels are used to evaluate changes before and after the exposure. Electron microscopy images of the exposed multiwalled carbon nanotubes revealed the formation of intermittent pockets of moundlike structures at high power densities exceeding 11 kW/cm2. Raman spectroscopy measurements also demonstrated higher values for the ratio of the D-band intensity to that of the G-band, suggesting the possible transformation of nanotubes into structurally different forms of carbon. Exposure to a sample of single-walled nanotubes did not demonstrate the evolution of structural changes, which could be due in part to the higher irradiance levels relative to the damage threshold, employed in the experiment.
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61.46.Fg Nanotubes
64.70.Nd Structural transitions in nanoscale materials
61.82.Rx Nanocrystalline materials
78.67.Ch Nanotubes
61.48.De Structure of carbon nanotubes, boron nanotubes, and other related systems
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
81.07.De Nanotubes
78.30.Na Fullerenes and related materials

Electroluminescence from p-i-n structure fabricated using crystalline silicon on glass technology

T. Mchedlidze, T. Arguirov, M. Holla, and M. Kittler

J. Appl. Phys. 105, 093107 (2009); http://dx.doi.org/10.1063/1.3124358 (5 pages) | Cited 6 times

Online Publication Date: 6 May 2009

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Strong electroluminescence was detected at room temperature from a p-i-n structure fabricated using crystalline silicon on glass technology. The luminescence spectra at small to moderate carrier injection levels contains strong peak with maximum at energy position Eph ∼ 0.8 eV. Additionally, a broad emission band in the range of energies 1 eV<Eph<1.16 eV appears at high injection levels. Obtained results suggest that the low energy peak can be attributed to dislocation related luminescence (DRL), while at least part of the high-energy emission band should be attributed to band-to-band transitions. A shift in the DRL peak position by the electric field present in the structure was observed. The shift is related to strong Stark effect. The relatively high efficiency of room temperature luminescence suggests the possibility for application of the structure for all-silicon light emitter.
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78.60.Fi Electroluminescence
78.20.Jq Electro-optical effects
61.72.Hh Indirect evidence of dislocations and other defects (resistivity, slip, creep, strains, internal friction, EPR, NMR, etc.)

The influence of fabrication deviations on the photonic band gap of three-dimensional inverse woodpile nanostructures

Léon A. Woldering, Allard P. Mosk, R. Willem Tjerkstra, and Willem L. Vos

J. Appl. Phys. 105, 093108 (2009); http://dx.doi.org/10.1063/1.3103777 (10 pages) | Cited 6 times

Online Publication Date: 6 May 2009

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The effects of unintended deviations from ideal inverse woodpile photonic crystals on the photonic band gap are discussed. Such deviations occur during the nanofabrication of the crystal. By computational analyses it is shown that the band gap of this type of crystal is robust to most types of deviations that relate to the radii, position, and angular alignment of the pores. However, the photonic band gap is very sensitive to tapering of the pores, i.e., conically shaped pores instead of cylindrical pores. To obtain three-dimensional inverse woodpile photonic crystals with a large volume, our work shows that with modern fabrication performances, reduction in tapering contributes most significantly to a high photonic strength.
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81.16.-c Methods of micro- and nanofabrication and processing
61.43.Gt Powders, porous materials
42.70.Qs Photonic bandgap materials
73.22.-f Electronic structure of nanoscale materials and related systems

Influence of polarization induced electric fields on the wavelength and the refractive index of intersubband transitions in AlN/GaN coupled double quantum wells

L. B. Cen, B. Shen, Z. X. Qin, and G. Y. Zhang

J. Appl. Phys. 105, 093109 (2009); http://dx.doi.org/10.1063/1.3124373 (5 pages) | Cited 4 times

Online Publication Date: 7 May 2009

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The influence of polarization induced electric fields on the wavelength and the refractive index of intersubband transitions (ISBTs) in AlN/GaN coupled double quantum wells (CDQWs) has been performed by solving Schrödinger and Poisson equations self-consistently. It is found that the intersubband refractive index change of the ISBT between the ground state and the second excited state (1odd-2odd ISBT) increases, while that of the ISBT between the ground state and the third excited state (1odd-2even ISBT) decreases with the increase of the polarization induced electric fields. The maximal intersubband refractive index change of the 1odd-2odd ISBT can be up to 0.142. The results are finally used to discuss the prospects of nitride quantum wells for electric-optical modulation via cross-phase modulation operating within optical communication wavelength range.
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73.21.Fg Quantum wells
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
78.20.Jq Electro-optical effects

Time-resolved lasing action from single and coupled photonic crystal nanocavity array lasers emitting in the telecom band

Dirk Englund, Hatice Altug, and Jelena Vučković

J. Appl. Phys. 105, 093110 (2009); http://dx.doi.org/10.1063/1.3116563 (4 pages) | Cited 1 time

Online Publication Date: 7 May 2009

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We measure the lasing dynamics of single and coupled photonic crystal nanocavity array lasers fabricated in the indium gallium arsenide phosphide material system. Under short optical excitation, single cavity lasers produce pulses as fast as 11 ps (full width at half maximum), while coupled cavity lasers show significantly longer lasing duration which is not explained by a simple rate equation model. A finite difference time domain simulation including carrier gain and diffusion suggests that asynchronous lasing across the nanocavity array extends the laser’s pulse duration.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Tv Photonic crystal lasers and coherent effects
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.70.Qs Photonic bandgap materials

Droplet formation in matrix-assisted pulsed-laser evaporation direct writing of glycerol-water solution

Yafu Lin, Yong Huang, and Douglas B. Chrisey

J. Appl. Phys. 105, 093111 (2009); http://dx.doi.org/10.1063/1.3116724 (6 pages) | Cited 13 times

Online Publication Date: 8 May 2009

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Matrix-assisted pulsed-laser evaporation direct-write (MAPLE DW) is emerging as a promising technique for printing microelectronics as well as fabricating biological constructs. For disparate MAPLE DW-based microfabrication applications, the droplet formation during MAPLE DW should be first carefully understood. Toward this goal, this study aims to study the effects of laser fluence and material properties of material to be transferred on the formed droplet in direct writing glycerol-water droplets using MAPLE DW. It was found that (1) at a given glycerol concentration ratio, the droplet diameter was linearly dependent on the laser fluence, and the slope of this relationship was dependent on the glycerol concentration, and (2) the droplet diameter had no systematic relationship with the glycerol concentration ratio. This study reveals important phenomena for droplet formation in MAPLE DW; further theoretical modeling is expected to further explain these observations.
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81.20.Ev Powder processing: powder metallurgy, compaction, sintering, mechanical alloying, and granulation
42.62.-b Laser applications

Bragg diffraction of guided optical waves by spin dipole waves in a ferrimagnetic heterostructure

O. V. Kolokoltsev, A. M. Grishin, Oliver Cortés Pérez, and C. L. Ordóñez-Romero

J. Appl. Phys. 105, 093112 (2009); http://dx.doi.org/10.1063/1.3116733 (5 pages)

Online Publication Date: 8 May 2009

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The peculiarities of the Bragg diffraction of guided optical waves (GOWs) by spin-dipole waves (SDWs) in a ferrite heterostructure, based on Bi3Fe5O12 (BIG) thin film deposited on a standard YIG/GGG sample, is presented. It is shown that the efficiency of waveguide magneto-optic interaction between GOWs and SDWs in BIG/YIG/GGG can be 4–14 times large compared to the standard YIG/GGG waveguide, even in the case when the interacting waves are localized in YIG waveguide layer.
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42.79.Gn Optical waveguides and couplers
75.30.Ds Spin waves
78.20.Ls Magneto-optical effects
78.66.-w Optical properties of specific thin films
75.70.Cn Magnetic properties of interfaces (multilayers, superlattices, heterostructures)
75.50.Gg Ferrimagnetics

Photoinduced current transient spectroscopy technique applied to the study of point defects in polycrystalline CdS thin films

Fikry El Akkad and Habib Ashour

J. Appl. Phys. 105, 093113 (2009); http://dx.doi.org/10.1063/1.3117510 (6 pages)

Online Publication Date: 8 May 2009

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CdS thin films of variable thickness (between 160 and 1200 nm) were prepared using rf magnetron sputtering. X-ray diffraction measurements showed that the films have hexagonal structure and that the crystallites are preferentially oriented with the 〈002〉 axis perpendicular to the substrate surface. The results of electrical conductivity measurements as a function of film thickness and of temperature provide evidence that the conductivity is controlled by a thermally activated mobility in the presence of an intergrain barrier. The room temperature barrier height ϕ decreases with the increase in film thickness. Values of ϕ between 0 and 0.25 eV were determined. Photoinduced current transient spectroscopy performed on five samples having different thicknesses showed the presence of 11 traps with activation energies in the range 0.08–1.06 eV; deeper traps being observed on thinner films. By comparison with literature results, seven traps are attributed to native defects and foreign impurities (mainly Cu, Au, and Ag). Four other traps, not previously observed, are attributed to residual defects. The observation that deeper traps are detected in samples with larger barrier heights has been discussed and interpreted in terms of the energy band profile near the grain boundary.
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61.72.J- Point defects and defect clusters
71.20.Nr Semiconductor compounds
72.40.+w Photoconduction and photovoltaic effects
81.15.Cd Deposition by sputtering
61.72.Mm Grain and twin boundaries
73.61.Ga II-VI semiconductors
68.55.ag Semiconductors

Microstructure and texture developments in multiple pulses excimer laser crystallization of GaAs thin films

Daniel Pirzada and Gary J. Cheng

J. Appl. Phys. 105, 093114 (2009); http://dx.doi.org/10.1063/1.3124601 (7 pages)

Online Publication Date: 11 May 2009

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In this paper, we observed and characterized changes in the microstructure and texture during recrystallization and grain growth in polycrystalline GaAs thin films using multiple pulses crystallization by a KrF excimer laser. Films of various thicknesses were studied to assess film thickness and laser energy density effects. In the low temperature domain corresponding to the partial melting regime, normal grain growth was observed. In the superlateral grain growth regime the increase in grain size was notable with grain sizes much greater than the film thickness. A bimodal grain size distribution emerged implying the onset of secondary grain growth. The change in grain size distribution, texture, and grain boundary texture were analyzed using scanning electron microscopy and electron backscatter diffraction. It was found that grain growth is accompanied by a strengthening in {001} texture, indicating that the grain growth phenomenon is strain energy driven. The experimental results are explained with theory of secondary grain growth in thin films.
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68.55.ag Semiconductors
68.55.jm Texture
81.40.Ef Cold working, work hardening; annealing, post-deformation annealing, quenching, tempering recovery, and crystallization
61.72.Mm Grain and twin boundaries
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
79.20.Ds Laser-beam impact phenomena

Experimental study of surface-wave-assisted microwave transmission through a single subwavelength slit

Liang Wang, Jin-Xiang Cao, You Lv, Lei Liu, Tian-Ye Niu, and Yin-Chang Du

J. Appl. Phys. 105, 093115 (2009); http://dx.doi.org/10.1063/1.3125330 (6 pages) | Cited 1 time

Online Publication Date: 12 May 2009

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In this study, we made a systematic investigation in the enhanced transmission of X-band microwaves through a single subwavelength slit aperture surrounded by periodic grooves in metallic aluminum plates. The influence of the grooves’ number, width, and depth on the transmission spectra was examined. By tuning these geometrical parameters, the transmission enhancement could be maximally optimized. The transmission spectra at s-polarization was also measured and compared to those at p-polarization. It is shown that the extraordinarily enhanced transmission at p-polarization is associated with the surface waves of Brewster–Zenneck modes on the metal-air interface. Our experimental results will be helpful to the control of the propagation of electromagnetic waves in the microwave regime.
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78.70.Gq Microwave and radio-frequency interactions
41.20.Jb Electromagnetic wave propagation; radiowave propagation
68.47.De Metallic surfaces

Dynamic modeling of a midinfrared quantum cascade laser

A. Hamadou, S. Lamari, and J.-L. Thobel

J. Appl. Phys. 105, 093116 (2009); http://dx.doi.org/10.1063/1.3124379 (6 pages) | Cited 4 times

Online Publication Date: 13 May 2009

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Based on a three-level rate equations model, we analyze through numerical simulations the population and photon number dynamics present within the cavity of a midinfrared quantum cascade laser. We find in particular that the injection current influences significantly the electron number dynamics trajectory. In addition, the equations that allow for the determination of the turn-on delay (tth) and buildup (Δt) times are derived within the premises of our model in the most general case. The effects of the spontaneous emission factor β on Δt are also explored.
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42.55.Px Semiconductor lasers; laser diodes
42.55.Ah General laser theory
42.50.Md Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency

Radiation damage to amorphous carbon thin films irradiated by multiple 46.9 nm laser shots below the single-shot damage threshold

L. Juha, V. Hájková, J. Chalupský, V. Vorlíček, A. Ritucci, A. Reale, P. Zuppella, and M. Störmer

J. Appl. Phys. 105, 093117 (2009); http://dx.doi.org/10.1063/1.3117515 (6 pages) | Cited 4 times

Online Publication Date: 14 May 2009

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High-surface-quality amorphous carbon (a-C) optical coatings with a thickness of 45 nm, deposited by magnetron sputtering on a silicon substrate, were irradiated by the focused beam of capillary-discharge Ne-like Ar extreme ultraviolet laser (CDL = capillary-discharge laser; XUV = extreme ultraviolet, i.e., wavelengths below 100 nm). The laser wavelength and pulse duration were 46.9 nm and 1.7 ns, respectively. The laser beam was focused onto the sample surface by a spherical Sc/Si multilayer mirror with a total reflectivity of about 30%. The laser pulse energy was varied from 0.4 to 40 μJ on the sample surface. The irradiation was carried out at five fluence levels between 0.1 and 10 J/cm2, accumulating five different series of shots, i.e., 1, 5, 10, 20, and 40. The damage to the a-C thin layer was investigated by atomic force microscopy (AFM) and Nomarski differential interference contrast (DIC) optical microscopy. The dependence of the single-shot-damaged area on pulse energy makes it possible to determine a beam spot diameter in the focus. Its value was found to be equal to 23.3±3.0 μm using AFM data, assuming the beam to have a Gaussian profile. Such a plot can also be used for a determination of single-shot damage threshold in a-C. A single-shot threshold value of 1.1 J/cm2 was found. Investigating the consequences of the multiple-shot exposure, it has been found that an accumulation of 10, 20, and 40 shots at a fluence of 0.5 J/cm2, i.e., below the single-shot damage threshold, causes irreversible changes of thin a-C layers, which can be registered by both the AFM and the DIC microscopy. In the center of the damaged area, AFM shows a-C removal to a maximum depth of 0.3, 1.2, and 1.5 nm for 10-, 20- and 40-shot exposure, respectively. Raman microprobe analysis does not indicate any change in the structure of the remaining a-C material. The erosive behavior reported here contrasts with the material expansion observed earlier [ L. Juha et al., Proc. SPIE 5917, 91 (2005) ] on an a-C sample irradiated by a large number of femtosecond pulses of XUV high-order harmonics.
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61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
42.79.Wc Optical coatings
81.15.Cd Deposition by sputtering
68.55.at Other materials
78.30.Hv Other nonmetallic inorganics
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Calculating the responsivity of a resonant-cavity-enhanced Si1−xGex/Si multiple quantum well photodetector

Mukul K. Das and N. R. Das

J. Appl. Phys. 105, 093118 (2009); http://dx.doi.org/10.1063/1.3117519 (8 pages) | Cited 1 time

Online Publication Date: 14 May 2009

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In this paper, the responsivity of a resonant-cavity-enhanced SiGe/Si multiple quantum well photodetector has been theoretically investigated. The present model considers the effects of material parameters of Si1−xGex and recombination of carriers confined by the potential barriers at the heterointerfaces of Si1−xGex/Si quantum wells on the detector responsivity for different Ge-contents (x). The effect of electric field due to applied bias on the emission of confined carriers from quantized subbands has also been included in the analysis. Results show that though high Ge-content is required to enhance the responsivity of the SiGe/Si photodetector for long-haul communication (at 1.3 and 1.55 μm), the confinement of holes at the heterointerfaces may be significant to affect the responsivity adversely. Studies at 1.3 and 1.55 μm also show that the responsivity of the photodetector for a particular Ge-content in the active layer can be increased by suitable choice of well parameters and applied bias.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
73.21.Fg Quantum wells

Ultimate limit and temperature dependency of light-emitting diode efficiency

Oskari Heikkilä, Jani Oksanen, and Jukka Tulkki

J. Appl. Phys. 105, 093119 (2009); http://dx.doi.org/10.1063/1.3125514 (9 pages) | Cited 5 times

Online Publication Date: 15 May 2009

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We discuss the ultimate limit of performance of semiconductor light-emitting diodes (LEDs) and its dependence on temperature. It is known that in high quality semiconductor materials it is, in principle, possible to reach wall plug efficiencies exceeding unity, which allows electroluminescent cooling in addition of very high efficiency light emission. Our simulation results suggest a few fairly simple measures that may further improve the external quantum efficiency (EQE) of LEDs toward the electroluminescent cooling limit. These include reducing the current density, modifying the LED structure by making thicker active regions and barrier layers, and doping of the active material. Our calculations also indicate that, contrary to the present understanding, operating LEDs at relatively high temperatures of 400–600 K may, in fact, improve the performance.
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85.60.Jb Light-emitting devices
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Short pulse, high power microwave radiation source with a laser-induced sheet plasma mirror

Takeshi Higashiguchi and Noboru Yugami

J. Appl. Phys. 105, 093301 (2009); http://dx.doi.org/10.1063/1.3117522 (4 pages) | Cited 1 time

Online Publication Date: 8 May 2009

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We have demonstrated the short pulse, high power microwave radiation source using an ultraviolet laser-induced sheet plasma mirror in a gas-filled x-band rectangular waveguide from the conventional microwave sources and components. A laser-induced sheet plasma with an overdense plasma acts as a plasma mirror. The long pulse propagating in the gas-filled waveguide was sliced by the sheet plasma mirror at two different points along the waveguide. We observed about twice the power of the pulse by adding the two sliced microwave pulses produced by this scheme. A maximum peak power of 200 kW with a pulse duration of 10 ns (full width at half maximum) from the long microwave pulse source with a pulse duration of 0.8 μs was observed.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.-m Plasma diagnostic techniques and instrumentation

Increase of the deposition rate in reactive sputtering of metal oxides using a ceramic nitride target

D. Severin, O. Kappertz, T. Nyberg, S. Berg, A. Pflug, and M. Wuttig

J. Appl. Phys. 105, 093302 (2009); http://dx.doi.org/10.1063/1.3124380 (4 pages) | Cited 4 times

Online Publication Date: 11 May 2009

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We present a method to eliminate hysteresis effects and to increase the deposition rate for the reactive sputtering of metal oxides. This is achieved by using a ceramic nitride target in an argon-oxygen atmosphere. Although the use of a ceramic nitride target leads to pronounced changes of the processing characteristics, incorporation of nitrogen into the growing film is very small. These observations can be theoretically predicted using an extension of Berg’s model [ S. Berg and T. Nyberg, Thin Solid Films 476, 215 (2005) ] to two different reactive gases and a compound target.
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81.15.Cd Deposition by sputtering
81.05.Bx Metals, semimetals, and alloys
68.55.at Other materials

Magnetic confinement in a ring-cusp ion thruster discharge plasma

Anita Sengupta

J. Appl. Phys. 105, 093303 (2009); http://dx.doi.org/10.1063/1.3106087 (10 pages) | Cited 2 times

Online Publication Date: 12 May 2009

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An experimental investigation, in conjunction with a volume averaged analytical model, has been developed to improve the confinement and production of the discharge plasma for plasma thrusters and ion sources. The research conducted explores the discharge performance of a ring-cusp ion source based on the magnetic field configuration, geometry, and power level. Analytical formulations for electron and ion confinement are developed to predict the ionization efficiency for a given discharge chamber design. Explicit determination of discharge loss and volume averaged plasma parameters are obtained via a series of experimental measurements on a ring-cusp NASA Solar Technology Application Readiness (NSTAR) ion thruster to assess the validity of the analytical model. Measurements of the discharge loss with multiple magnetic field configurations compare well with plasma parameter predictions for propellant utilizations between 80% and 95%. The results indicate that increasing the magnetic strength of the first closed magnetic contour line reduces Maxwellian electron diffusion and electrostatically confines the ion population and subsequent loss to the anode wall. The results also indicate that increasing the strength and minimizing the area of the magnetic cusps improves primary electron confinement, increasing the probability of an ionization collision prior to loss at the cusp.
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52.55.-s Magnetic confinement and equilibrium
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.50.Dg Plasma sources
52.80.-s Electric discharges

Evolution of the plasma composition of a high power impulse magnetron sputtering system studied with a time-of-flight spectrometer

Efim Oks and André Anders

J. Appl. Phys. 105, 093304 (2009); http://dx.doi.org/10.1063/1.3125443 (9 pages) | Cited 10 times

Online Publication Date: 12 May 2009

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The plasma of a high power impulse magnetron sputtering system has been investigated using a time-of-flight spectrometer. The target materials included high sputter yield materials (Cu, Ag), transition metals (Nb, Cr, Ti), and carbon (graphite); the sputtering gases were argon, krypton, and nitrogen, and two different target thicknesses were selected to consider the role of the magnetic field strength. Measurements for selected combinations of those parameters give quantitative information on the transition from gas-dominated to metal-dominated (self-sputtering) plasma, on the fractions of ion charge states, and in the case of molecular gases, on the fraction of atomic and molecular ions.
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52.70.-m Plasma diagnostic techniques and instrumentation
52.77.Dq Plasma-based ion implantation and deposition
81.15.Cd Deposition by sputtering

Particle model analyses of N2O dilution with He on electrical characteristics of radio-frequency discharges

G. Younis, M. Yousfi, and B. Despax

J. Appl. Phys. 105, 093305 (2009); http://dx.doi.org/10.1063/1.3125442 (12 pages) | Cited 1 time

Online Publication Date: 14 May 2009

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The electrical characteristics (voltage, electric field, charged particle densities, dissipated power, particle energy, etc.) are analyzed in the case of low pressure (0.5 and 1 Torr) radio-frequency (rf) discharges in nitrous oxide (N2O)/Helium (He) mixtures. An optimized and validated particle model has been used for these analyses in the case of gradual dilutions of N2O with He buffer gas. A specific care is carried on the power density evolution and variation which show a complex behavior as a function of He proportion (up to 85%). These analyses are based on a microscopic approach enabling one to show the contribution of the different inelastic processes mainly between electrons and respectively N2O and He gases. This approach enables also one to show the discharge region (the positive column or the plasma region) where the power is preferentially dissipated. The power density variation is found to be mainly proportional to the electron density variation. The latter is dependent on the different processes occurring between the charged particles [i.e., electrons, negative ions (O− and NO−), and positive ions (N2O+ and He+)] and the neutral gas mixture (N2O and He). Furthermore, the particle model shows the role of the electron-He collisions on the variation in the electron energy and distribution. This allows more particularly explaining the effects of N2O dilution with He on the dissipated power variation in terms of creation and loss of electrons through collision processes.
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52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.80.Pi High-frequency and RF discharges
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