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15 Jan 2007

Volume 101, Issue 2, Articles (02xxxx)

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Two pathways for photon upconversion in model organic compound systems

S. Baluschev, V. Yakutkin, G. Wegner, B. Minch, T. Miteva, G. Nelles, and A. Yasuda

J. Appl. Phys. 101, 023101 (2007); http://dx.doi.org/10.1063/1.2409618 (4 pages) | Cited 22 times

Online Publication Date: 16 January 2007

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We have studied the phenomenon of photon upconversion in systems of two model compounds as highly efficient blue emitters sensitized with metallated macrocycle molecules in thin films. The bimolecular upconversion process in these systems is based on the presence of a metastable triplet excited state of the macrocycles giving rise to dramatically different photophysical characteristics relative to the other known methods for photon upconversion such as two-photon absorption, parametric processes, second harmonic generation, and sequential multiphoton absorption. The chosen blue emitter molecules have suitably positioned triplet levels: in the case of an oligofluorine—essentially higher and in the case of diphenylanthracene—lower than the sensitizer porphyrin platinum triplet level and thus two excitation pathways for photon upconversion were observed and investigated.
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33.70.Jg Line and band widths, shapes, and shifts
33.80.Rv Multiphoton ionization and excitation to highly excited states (e.g., Rydberg states)

Phase motion of accelerated electrons in vacuum laser acceleration

J. F. Hua, Y. Z. Lin, Ch. X. Tang, Y. K. Ho, and Q. Kong

J. Appl. Phys. 101, 023102 (2007); http://dx.doi.org/10.1063/1.2422711 (4 pages) | Cited 4 times

Online Publication Date: 16 January 2007

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The phase stability in the capture and acceleration scenario (CAS) is studied and compared with that of conventional linear electron accelerators (CLEAs). For the CAS case, it has been found that a slow phase slippage occurs due to the difference between the electron velocity and the phase velocity of the longitudinal accelerating electric field. Thus, CAS electrons cannot remain in a fixed small phase region of the accelerating field to obtain a quasimonoenergy gain in contrast to the stability of phase oscillation in CLEAs. Also, the energy spread of the output electron beam for the CAS case cannot be kept as small as the CLEA because there is no good phase bunching phenomenon generated by phase oscillation.
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41.75.Jv Laser-driven acceleration
41.75.Fr Electron and positron beams
29.27.Bd Beam dynamics; collective effects and instabilities

Imaging concentric modulations in transverse modes of a vertical-cavity surface emitting laser using a scanning near-field optical microscope

M. D. Holton, P. Rees, and P. R. Dunstan

J. Appl. Phys. 101, 023103 (2007); http://dx.doi.org/10.1063/1.2423138 (6 pages) | Cited 1 time

Online Publication Date: 16 January 2007

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Concentric standing waves have been spatially imaged in the near-field regime within the optical aperture of a vertical-cavity surface emitting laser (VCSEL) using a scanning near-field optical microscope. Using the microscope’s near-field collection mode and subsequent detection via a spectrometer it has been possible to design an experiment to determine the spatial location of multiple lasing modes in addition to concentric standing waves. At low injection current above threshold the standing waves influence and modulate the optical emission from multiple transverse modes. These results are discussed in relation to cavity and aperture effects, and pattern formation in VCSELs. Surface defects arising in the aperture are also seen to affect the optical output and are briefly discussed.
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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
07.79.Fc Near-field scanning optical microscopes
42.65.Sf Dynamics of nonlinear optical systems; optical instabilities, optical chaos and complexity, and optical spatio-temporal dynamics

Er3+-doped BaY2F8 crystal waveguides for broadband optical amplification at 1.5 μm

V. Toccafondo, A. Cerqueira S., Jr., S. Faralli, E. Sani, A. Toncelli, M. Tonelli, and F. Di Pasquale

J. Appl. Phys. 101, 023104 (2007); http://dx.doi.org/10.1063/1.2404327 (6 pages)

Online Publication Date: 16 January 2007

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Integrated waveguide amplifiers based on high concentration Er3+ doped BaY2F8 crystals are numerically studied by combining a full-vectorial finite element based modal analysis and propagation-rate equations. Using realistic input data, such as the absorption/emission cross sections and Er level lifetimes measured on grown crystal samples, we investigate the amplifier performance by optimizing the total Er concentration. We predict optimum gain coefficient up to 5 dB/cm and broad amplification bandwidth exceeding 80 nm with 1480 nm pumping.
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42.55.Rz Doped-insulator lasers and other solid state lasers
42.60.By Design of specific laser systems
42.82.Gw Other integrated-optical elements and systems

Photothermal, Photocarrier and Raman Characterization of Te-doped GaSb

R. Velázquez-Hernández, J. García-Rivera, M. E. Rodriguez Garcia, S. Jimenez Sandoval, J. G. Mendoza-Alvarez, and J. A. Garcia

J. Appl. Phys. 101, 023105 (2007); http://dx.doi.org/10.1063/1.2409765 (8 pages) | Cited 4 times

Online Publication Date: 17 January 2007

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Noncontact, nonintrusive photocarrier (PCR) and photothermal radiometry (PTR) as well as Raman spectroscopy were used to characterize GaSb and Te-doped GaSb wafers used to grow GaSb-based heterostructures for infrared applications. The results indicated excellent sensitivity of PTR and PCR to the Te distribution on the wafer surface. The results from the three methodologies were consistent and indicated that the Te was segregated toward the edge of the GaSb wafer. The PTR and PCR laser-based techniques show great potential advantages to characterize the homogeneity of the Te doping in the GaSb wafers and for other steps in the fabrication of semiconductor heterostructures.
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81.05.Ea III-V semiconductors
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
61.72.uj III-V and II-VI semiconductors
78.30.Fs III-V and II-VI semiconductors
81.70.Fy Nondestructive testing: optical methods

Coupled dipole method to compute optical torque: Application to a micropropeller

Patrick C. Chaumet and C. Billaudeau

J. Appl. Phys. 101, 023106 (2007); http://dx.doi.org/10.1063/1.2409490 (6 pages) | Cited 20 times

Online Publication Date: 19 January 2007

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The coupled dipole method is a volume integral equation method which allows computation of the scattered field from an arbitrary object (shape and relative permittivity). This method has been extended to the computation of optical forces. In this article we further extend the coupled dipole method to the computation of optical torque. First, we establish the equation to obtain the optical torque using the coupled dipole method, stressing the importance of the radiative reaction term. Second, we compare our theory to existing models for validation. Third, we apply our method to the computation of optical torque, from a plane wave circularly polarized on a micropropeller. The influence of geometry and relative permittivity on the optical torque is studied.
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37.10.Vz Mechanical effects of light on atoms, molecules, and ions

The influence of annihilation processes on the threshold current density of organic laser diodes

Christian Gärtner, Christian Karnutsch, Uli Lemmer, and Christof Pflumm

J. Appl. Phys. 101, 023107 (2007); http://dx.doi.org/10.1063/1.2425003 (9 pages) | Cited 26 times

Online Publication Date: 19 January 2007

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We examine the impact of various annihilation processes on the laser threshold current density of a multilayer organic laser diode by numerical simulation. Our self-consistent numerical model treats the dynamics of electrons, holes, and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into an optical model. In our approach, a three layer waveguide structure is taken into account and the resulting laser rate equations are solved. Various annihilation processes are included as reactions between the different particle species in the device employing typical annihilation rates and material properties of organic semiconductors. By systematically varying the device dimensions and the annihilation rate coefficients, the dominating quenching processes are identified. The threshold current density is found to depend sensitively on the thickness of the emission layer. The influence of annihilation processes on the threshold current density is quantified as a function of the emission layer thickness and various annihilation rate coefficients. Using typical annihilation rate coefficients singlet-polaron annihilation is found to be the dominating quenching process. Maximum annihilation rate coefficients are calculated allowing a threshold current density below 1 kA/cm2. Singlet-triplet annihilation is recognized as another main loss process for singlet excitons. In our model the singlet exciton density is increased by triplet-triplet annihilation whereas it is diminished by singlet-triplet annihilation. The ratio of the rate coefficients for singlet-triplet and triplet-triplet annihilations is identified to be critical for the total number of singlet excitons being quenched by triplet excitons.
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42.55.Px Semiconductor lasers; laser diodes

Analyses of the optical and magneto-optical spectra of Tb3Ga5O12

John B. Gruber, Dhiraj K. Sardar, Raylon M. Yow, U. V. Valiev, A. K. Mukhammadiev, V. Yu. Sokolov, Irina Amin, K. Lengyel, I. S. Kachur, Valeriya G. Piryatinskaya, and Bahram Zandi

J. Appl. Phys. 101, 023108 (2007); http://dx.doi.org/10.1063/1.2408344 (14 pages) | Cited 6 times

Online Publication Date: 19 January 2007

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Absorption spectra of the 7FJ, 5D4,3,2, 5G6,5,4, and 5L10,9 multiplet manifolds of Tb3+ (4f8) in D2 sites in cubic garnet Tb3Ga5O12 (TbGaG) are investigated at sample temperatures between 1.8 K and room temperature. Absorption measurements extend from 5000 to 340 nm. From analyses of temperature-dependent (hot-band) absorption spectra, many of the crystal-field split energy (Stark) levels of the 2S+1LJ multiplet manifolds of Tb3+ are identified and confirmed from analyses of the fluorescence spectra observed between 485 and 680 nm, representing transitions from the 5D4 to the 7FJ manifolds. Each manifold is split by the crystal field into 2J+1 Stark levels. Some of these manifolds, including the ground-state manifold 7F6, consist of Stark levels that are accidentally degenerate, or nearly so, making transitions to or from these levels appear as unresolved spectra, even at the lowest temperature investigated (1.8 K). To resolve these spectra, we have investigated the Zeeman and magneto-optical spectra for representative manifolds 5D4, 7F5, and 7F6 at temperatures of 78 and 85 K and magnetic fields up to 7 kOe. The data are interpreted using the Stark levels and wave functions from a crystal-field splitting calculation that involved 80 individual Stark levels identified from the optical spectra of the 7FJ and quintet states reported in this study. Good agreement is obtained between the calculated and the experimental Stark levels. The calculated energy and symmetry label for each Stark level in the 5D4, 7F5, and 7F6 manifolds suitably interpret the spectral properties observed in the magneto-optical spectra, including the experimental assignment reported in the literature for the ground state as a quasidoublet {Γ1,Γ2}.
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78.30.Hv Other nonmetallic inorganics
78.40.Ha Other nonmetallic inorganics
78.20.Ls Magneto-optical effects
75.10.Dg Crystal-field theory and spin Hamiltonians
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
75.50.Gg Ferrimagnetics

Improved uniformity of target illumination by combining a lens array and the technique of spectral dispersion

Xiujuan Jiang, Shenlei Zhou, and Zunqi Lin

J. Appl. Phys. 101, 023109 (2007); http://dx.doi.org/10.1063/1.2424533 (5 pages) | Cited 4 times

Online Publication Date: 23 January 2007

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A scheme using a lens array and the technique of spectral dispersion is presented to improve target illumination uniformity in laser produced plasmas. Detailed two-dimensional simulation shows that a quasi-near-field target pattern, of steeper edges and without side lobes, is achieved with a lens array, while interference stripes inside the pattern are smoothed out by the use of the spectral dispersion technique. Moving the target slightly from the exact focal plane of the principal focusing lens can eliminate middle-scale-length intensity fluctuation further. Numerical results indicate that a well-irradiated laser spot with small nonuniformity and great energy efficiency can be obtained in this scheme.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
42.60.Jf Beam characteristics: profile, intensity, and power; spatial pattern formation
42.79.Bh Lenses, prisms and mirrors

Gain properties of an Er3+ complex in a poly(methylmethacrylate) matrix for 1540 nm broadband optical amplification

Anh Quoc Le Quang, V. G. Truong, A.-M. Jurdyc, B. Jacquier, J. Zyss, and I. Ledoux

J. Appl. Phys. 101, 023110 (2007); http://dx.doi.org/10.1063/1.2409607 (7 pages) | Cited 6 times

Online Publication Date: 23 January 2007

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Emission properties of an erbium organic complex embedded in a polymer matrix are investigated for laser and optical amplification applications in the 1550 nm telecommunication range. The evaluation of stimulated emission cross sections is inferred from the Judd-Ofelt model [ B. R. Judd et al., Phys. Rev. 169, 130 (1968); G. S. Ofelt, J. Chem. Phys. 37, 511 (1962) ] based on experimental spectroscopic data, resulting in the determination of the three Ω2 = 1.92×10−19, Ω4 = 1.82×10−20, and Ω6 = 1.65×10−20 Judd-Ofelt coefficients. These parameters are then used to calculate the oscillator strengths between states and allow us to evaluate spontaneous emission probabilities, radiative lifetimes, branching ratios, and quantum efficiencies of levels. Stimulated emission cross-sections are determined using the McCumber method [ D. E. Mc Cumber, Phys. Rev. 134, A299 (1964) ]. Net gain cross sections are calculated for different values of the inversion population in Er3+ ions. Results show that organic polymer hosts doped with erbium complexes are very attractive materials for amplification in the telecommunication spectral range, as they display high gain cross section as compared to other erbium-containing materials, coupled with a wide and relatively flat amplification spectrum (up to 70 nm) which covers the whole spectral range currently used for wavelength division multiplexing operation.
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42.70.Jk Polymers and organics
78.45.+h Stimulated emission
78.55.Kz Solid organic materials
42.79.Sz Optical communication systems, multiplexers, and demultiplexers

Radiative and nonradiative recombination processes in ZnCdSe/ZnCdMgSe multi-quantum-wells

S. K. Zhang, H. Lu, W. B. Wang, B. B. Das, N. Okoye, M. Tamargo, and R. R. Alfano

J. Appl. Phys. 101, 023111 (2007); http://dx.doi.org/10.1063/1.2427098 (7 pages) | Cited 1 time

Online Publication Date: 23 January 2007

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Carrier recombination through radiative and nonradiative processes in lattice-matched n-Zn0.5Cd0.5Se/Zn0.21Cd0.19Mg0.6Se multi-quantum-wells (MQWs) was investigated by temperature-dependent time-resolved photoluminescence (PL) spectroscopy. The n-Zn0.5Cd0.5Se/Zn0.21Cd0.19Mg0.6Se MQW samples with different well widths were grown on InP substrates by molecular beam epitaxy. The PL decay times and the PL intensities were measured as functions of temperature. For a doping level of 1×1018 cm−3, the dominant mechanism of the radiative process was found to be free carrier recombination while excitonic recombination was absent due to the effect of strong carrier screening. The nonradiative mechanism was determined to be hole capture through multiphonon emission (MPE). The expressions of the nonradiative MPE recombination lifetime, the PL decay time, and the PL intensity were deduced as functions of temperature and were used to fit the measured temperature dependence of the PL decay times and the PL intensities. The MPE activation energies and relative defect densities for the samples with different well widths were obtained. A simple method is suggested to investigate the interfacial defects of quantum wells.
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72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
78.55.Et II-VI semiconductors
78.67.De Quantum wells
61.72.uj III-V and II-VI semiconductors

Self-formation of quasiperiodic void structure in CaF2 induced by femtosecond laser irradiation

Xiao Hu, Ye Dai, Luyun Yang, Juan Song, Congshan Zhu, and Jianrong Qiu

J. Appl. Phys. 101, 023112 (2007); http://dx.doi.org/10.1063/1.2430911 (3 pages) | Cited 10 times

Online Publication Date: 24 January 2007

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We report the self-formation of quasiperiodic void structure with the length of several hundred micrometers inside the CaF2 crystal. The quasiperiodical voids along the propagation direction of the laser beam were formed spontaneously after the irradiation of a single femtosecond laser beam which was focused at a fixed point inside the crystal sample. The length of the void array varied with the focal depth beneath the sample surface. The possible mechanism of the self-formed void structure was discussed.
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61.72.Qq Microscopic defects (voids, inclusions, etc.)
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)

Thermal lens spectroscopy through phase transition in neodymium doped strontium barium niobate laser crystals

C. Jacinto, T. Catunda, D. Jaque, J. García Solé, and A. A. Kaminskii

J. Appl. Phys. 101, 023113 (2007); http://dx.doi.org/10.1063/1.2430938 (6 pages) | Cited 6 times

Online Publication Date: 24 January 2007

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In this work the anisotropic thermal lens (TL) effect was studied in Nd3+ doped SrxBa1−xNb2O6 crystals with two different stoichiometries (x = 0.47 and 0.61). TL measurements were carried out as a function of temperature from 30 up to 140 °C so that the effect of phase transition was investigated. A strong enhancement in the TL effect during the ferro-to-paraelectric phase transition has been observed. Additionally, we have found an inversion of the TL signal with the crystal stoichiometry. This fact is explained in terms of a different thermal expansion along the c axis for both crystals, as measured by x-ray diffraction experiments. The obtained results open the possibility to tailor a crystal stoichiometry that minimizes the TL effects.
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77.84.Ek Niobates and tantalates
77.84.Cg PZT ceramics and other titanates
77.80.B- Phase transitions and Curie point
61.66.Bi Elemental solids
61.66.Dk Alloys
65.40.De Thermal expansion; thermomechanical effects

Expansion of the laser ablation vapor plume into a background gas. I. Analysis

Sy-Bor Wen, Xianglei Mao, Ralph Greif, and Richard E. Russo

J. Appl. Phys. 101, 023114 (2007); http://dx.doi.org/10.1063/1.2431080 (13 pages) | Cited 12 times

Online Publication Date: 25 January 2007

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A study of the gas dynamics of the vapor plume generated during laser ablation was conducted including a counterpropagating internal shock wave. The density, pressure, and temperature distributions between the external shock wave front and the sample surface were determined by solving the integrated conservation equations of mass, momentum, and energy. The positions of the shock waves and the contact surface (boundary that separates the compressed ambient gas and the vapor plume) were obtained when the incident laser energy that is transferred to the vapor plume and to the background gas, E, and the vaporized sample mass, M, are specified. The values for E and M were obtained from a comparison of the calculated trajectories of the external shock wave and the contact surface with experimental results for a copper sample under different laser fluences. Thus E and M, which are the two dominant parameters for laser ablation and which cannot be measured directly, can be determined. In addition, the internal shock wave propagation within the vapor plume was determined; the interaction of the internal shock wave with the sample may be one of the mechanisms inducing liquid sample ejection during laser ablation.
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52.38.Mf Laser ablation
52.35.Tc Shock waves and discontinuities
52.25.Dg Plasma kinetic equations
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Fi Transport properties

Laser ablation induced vapor plume expansion into a background gas. II. Experimental analysis

Sy-Bor Wen, Xianglei Mao, Ralph Greif, and Richard E. Russo

J. Appl. Phys. 101, 023115 (2007); http://dx.doi.org/10.1063/1.2431085 (14 pages) | Cited 20 times

Online Publication Date: 25 January 2007

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Laser ablation of copper with a 4 ns laser pulse at 1064 nm was studied with a series of synchronized shadowgraph (100 fs laser pulses at 400 nm) and emission images (spectral line at 515 nm). Data were obtained at two laser pulse energies (10 and 30 mJ) and in three background gases (He, Ne, and Ar) at atmospheric pressure. The laser energy conversion ratio and the amount of sample vaporized for ablation in each condition were obtained by the theoretical analysis reported in paper I from trajectories of the external shock wave, internal shock wave, and contact surface between the Cu vapor and the background gas. All three quantities were measured from shadowgraph and emission images. The results showed that E, the amount of energy that is absorbed by the copper vapor, decreases as the atomic mass of the background gas increases; and M, the mass of the sample converted into vapor, is almost independent of the background gas [ Horn et al., Appl. Surf. Sci. 182, 91 (2001) ]. A physical interpretation is given based on the phenomena observed in shadowgraph and emission images during the first tens of nanoseconds after the beginning of the laser pulse for ablation in different background gases. In addition, an internal shock wave was observed in the emission images during the first tens of nanoseconds after the laser pulse, which strikes the surface and should be one of the mechanisms inducing the liquid sample ejection. Also, a significant vortex ring near the target was observed in emission images at longer times after the laser pulse (>100 ns) which distorts the otherwise hemispherical expansion of the vapor plume.
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52.38.Mf Laser ablation
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.35.Tc Shock waves and discontinuities
52.40.Hf Plasma-material interactions; boundary layer effects
52.35.We Plasma vorticity

Order relaxation of a poled azo dye in a high Tg, fully aromatic polyimide

Alessia Quatela, Fabio De Matteis, Mauro Casalboni, Federica Stella, Marco Colombo, and Antonio Zaopo

J. Appl. Phys. 101, 023116 (2007); http://dx.doi.org/10.1063/1.2422712 (6 pages) | Cited 1 time

Online Publication Date: 25 January 2007

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A guest-host polymer system with potential use in electro-optic devices is discussed. The polymer host is a high Tg, fully aromatic polyimide and the guest chromophore is disperse red 19. Relaxation mechanisms of polar order after electric field alignment procedure have been investigated by measuring the isothermal decay of the macroscopic nonlinear optical coefficient d333 at different temperatures below glass transition temperature (Tg), upon the removal of the poling electric field. All the decay curves can be fitted by a double exponential function. Below Tg, the slower relaxation time shows an Arrhenius temperature dependence. An extrapolation to room temperature allows to predict the time stability which results to be longer than 30 years. In addition, absorption spectra measurements of the films were performed before and immediately after poling procedure to estimate independently the polar order of the dipoles through the decrease of the absorption coefficients.
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42.70.Jk Polymers and organics
42.79.Wc Optical coatings
78.66.Qn Polymers; organic compounds
42.70.Nq Other nonlinear optical materials; photorefractive and semiconductor materials
78.20.Jq Electro-optical effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)

Frequency response of solid-state impact ionization multipliers

Joshua L. Beutler, Carleton S. Clauss, Michael S. Johnson, Aaron R. Hawkins, Mike D. Jack, George R. Chapman, and Ken Kosai

J. Appl. Phys. 101, 023117 (2007); http://dx.doi.org/10.1063/1.2426376 (9 pages) | Cited 3 times

Online Publication Date: 26 January 2007

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A study of the frequency response of solid-state impact ionization multipliers (SIMs) is presented that emphasizes the role of resistive and capacitive elements of the device to establish response limitations. SIMs are designed to amplify input currents from an external source through the impact ionization mechanism. An equivalent circuit model for the SIM is developed based on its current versus voltage characteristics, which is used to derive a frequency response model. Theoretical frequency response matches very closely to measured responses for first generation SIM devices constructed on p-type silicon epitaxial layers with nickel silicide Schottky contact injection points. Devices were measured using a photodiode as a current source under light intensities between 74 nA and 7.4 μA. These SIMs were shown to have a low frequency response that follows a KT/I relationship. Using an external photodiode with an effective capacitance of 6.8 pF, frequency response for a 1.8 μA input current was limited to 100 kHz. A large effective barrier resistance due to the Schottky contact and 12 kΩ space charge resistance dependent on device geometry dominate the response. Future SIM designs with higher frequency response will have to significantly lower both the input barrier resistance and space charge resistance.
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85.30.De Semiconductor-device characterization, design, and modeling
73.30.+y Surface double layers, Schottky barriers, and work functions

Effect of injection current on the repeatability of laser diode junction voltage-temperature measurements

L. Kirkup, W. Kalceff, and G. McCredie

J. Appl. Phys. 101, 023118 (2007); http://dx.doi.org/10.1063/1.2427097 (6 pages) | Cited 1 time

Online Publication Date: 26 January 2007

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The junction-voltage temperature relationship of a laser diode is used to determine the temperature of the device in the range −20 to 120 °C. We consider changes that occur to this relationship when the diode is driven at its nominal operating current and above. It is found that estimates of the parameters that appear in a function fitted to the voltage-temperature data are sensitive to driving currents at and above the nominal operating current. Changes in parameter estimates are attributed to degradation due to surface recombination at facets occurring when the laser diode is overdriven. An indication of this degradation is the change in the ideality factor that occurs as the current to the device is raised. This work indicates that the temperature of the junction can be determined over the range −20 to 120 °C with a standard uncertainty of ≈ 0.5 °C.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems
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Control of plasma space potentials and chemical vapor deposition of nanocrystalline diamond films in surface-wave excited low-pressure plasmas

Jaeho Kim and Makoto Katsurai

J. Appl. Phys. 101, 023301 (2007); http://dx.doi.org/10.1063/1.2409761 (8 pages) | Cited 6 times

Online Publication Date: 17 January 2007

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A dc biasing method, developed in this work, has been investigated for the control of plasma space potentials and the chemical vapor deposition of nanocrystalline diamond (NCD) films in a planar surface-wave excited plasma at gas pressures below 100 mTorr. A negative dc voltage was applied to a specially shaped thin metal plate attached below the upper dielectric window with respect to the grounded substrate and discharge chamber, instead of the conventional positive substrate dc biasing method. Plasma parameters were measured using a single-probe and deposited films were evaluated by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. The application of the dc bias voltage (0 to −150 V) enabled the net dc bias current (−0.46–+0.6 A) to be varied and plasma space potentials to be decreased over a wide range (34–7 V) in the bulk region, resulting in the control of the bombarding ion energy on the grounded substrate. The vertical plasma parameter profiles showed the spatial difference in electron temperature between the local surface-wave region ( ∼ 10 eV) near the upper dielectric window and the bulk region (below 3 eV). It was found that the spatial difference in electron temperature permits the control of net currents and plasma space potentials in the dc biasing method. NCD films were deposited with smooth surfaces (rms = 12.4 nm), a deposition rate of about 63 nm/h, and a continuous surface coverage on Si substrates maintained at a temperature of about 650 °C for hydrogen-based CO–H2 plasmas by biasing with −70 V to the metal plate.
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52.77.Dq Plasma-based ion implantation and deposition
52.50.Dg Plasma sources
52.70.Ds Electric and magnetic measurements
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
68.55.A- Nucleation and growth
81.05.Cy Elemental semiconductors

Statistical instability of barrier microdischarges operating in townsend regime

V. P. Nagorny

J. Appl. Phys. 101, 023302 (2007); http://dx.doi.org/10.1063/1.2425196 (9 pages)

Online Publication Date: 17 January 2007

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The dynamics of barrier microdischarges operating in a Townsend regime is studied analytically and via kinetic particle-in-cell/Monte Carlo simulations. It is shown that statistical fluctuations of the number of charged particles in the discharge gap strongly influence the dynamics of natural oscillations of the discharge current and may even lead to a disruption of the discharge. Analysis of the statistical effects based on a simple model is suggested. The role of external sources in stabilizing microdischarges is clarified.
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52.35.Qz Microinstabilities (ion-acoustic, two-stream, loss-cone, beam-plasma, drift, ion- or electron-cyclotron, etc.)
52.80.Dy Low-field and Townsend discharges
52.65.Rr Particle-in-cell method
52.65.Pp Monte Carlo methods
52.25.Gj Fluctuation and chaos phenomena
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)

Effect of dc bias control on the power absorption in low-pressure, radio-frequency capacitive sheaths

D. Gahan and F. Soberón

J. Appl. Phys. 101, 023303 (2007); http://dx.doi.org/10.1063/1.2422748 (9 pages) | Cited 5 times

Online Publication Date: 18 January 2007

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Many of today’s processing plasma tools are operated at low pressures to achieve high etch directivity and reduce side erosion on the wafer. At these pressures electron-neutral collisions are rare and the electrons cannot gain energy through the ohmic heating process. Instead, the heating mechanism is attributed to a stochastic process between the electrons and the sheath electric field. Theoretical models of this stochastic process include the hard wall approximation and the pressure heating effect. The former is inconsistent with electron current conservation at the sheath, while the latter shows a difference in power absorption when electron loss to the electrodes is considered. This article examines the effects of electron current on a capacitive sheath by controlling this current with an additional dc bias on the electrode. Experimental and particle-in-cell model results for a low-pressure argon plasma are compared and presented. Results show that the electron power absorption is more effective when the electron current is removed. The model also shows a high harmonic content on the sheath voltage which is attenuated by removing the electron current. These high-frequency harmonics are measured in the experiment with a floating probe, and their correlation with the electron current is in agreement with the model results.
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52.40.Kh Plasma sheaths
52.77.Bn Etching and cleaning
52.50.Nr Plasma heating by DC fields; ohmic heating, arcs
52.40.Hf Plasma-material interactions; boundary layer effects
52.65.Rr Particle-in-cell method
52.70.Ds Electric and magnetic measurements

Correlation of gas-phase composition with film properties in the plasma-enhanced chemical vapor deposition of hydrogenated amorphous carbon nitride films

Dongping Liu, Jie Zhou, and Ellen R. Fisher

J. Appl. Phys. 101, 023304 (2007); http://dx.doi.org/10.1063/1.2424402 (9 pages) | Cited 7 times

Online Publication Date: 18 January 2007

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Hydrogenated amorphous carbon nitride (a-C:N:H) films were synthesized from CH4/N2, C2H4/N2, and C2H2/N2 gas mixtures using inductively coupled rf plasmas. These deposition systems were characterized by means of optical emission spectroscopy and mass spectrometry (MS). The effects of varying the nitrogen partial pressure on film growth and film properties were investigated, and experimental results indicate that the hydrocarbon species produced in the gas phase contribute directly to film growth. Although the CN radical is formed in the mixed gas systems, it does not appear to be a factor in controlling the rate of film deposition. The nature and energy of the ions in these systems were explored with MS. No clear dependence of ion energy on mass or plasma conditions was observed. Although films formed in the methane and ethylene systems were relatively smooth, a-C:N:H films prepared from acetylene-nitrogen plasmas had comparatively rough surfaces, most likely as a result of the strong gas-phase polymerization process produced by the ion-molecule reactions, CnHy++C2H2→C(n+2)Hy++H2 (n>1, y = 1–3). Correlations between the a-C:N:H growth processes and the gas-phase plasma diagnostic data are discussed.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
52.77.Dq Plasma-based ion implantation and deposition
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
61.43.Er Other amorphous solids
78.66.Nk Insulators

Investigation of the interaction of a laser pulse with a preformed Gaussian Sn plume for an extreme ultraviolet lithography source

Y. Tao, M. S. Tillack, S. S. Harilal, K. L. Sequoia, and F. Najmabadi

J. Appl. Phys. 101, 023305 (2007); http://dx.doi.org/10.1063/1.2426883 (8 pages) | Cited 23 times

Online Publication Date: 22 January 2007

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The interaction of a laser pulse with a Sn preplasma formed by a low energy prepulse was investigated for an extreme ultraviolet (EUV) lithography light source. A much lower ion kinetic energy and nearly the same conversion efficiency from laser to in-band (2% bandwidth) 13.5 nm EUV light were simultaneously observed as compared with those from the direct interaction with a solid surface. The reason comes from the interaction of the laser pulse with a smooth preplume induced by the prepulse. The density profile of the preplume was measured with time-resolved shadowgraphy and could be fitted with a Gaussian function. The energy of the ions located at the flux peak Ep scales with the length of the preplume ls as Ep∝1/ls. Laser absorption in the low-density preplume and ion acceleration during plasma expansion are discussed. This result provides a general way to control particle energy from a laser plasma interaction.
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52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
42.72.Bj Visible and ultraviolet sources
94.20.Fg Plasma temperature and density
52.70.Kz Optical (ultraviolet, visible, infrared) measurements

Calculation of ozone generation by positive dc corona discharge in coaxial wire-cylinder reactors

Ashraf Yehia

J. Appl. Phys. 101, 023306 (2007); http://dx.doi.org/10.1063/1.2426927 (5 pages) | Cited 3 times

Online Publication Date: 22 January 2007

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A theoretical equation has been derived in this paper for calculation of ozone generation by positive dc corona discharge in coaxial wire-cylinder reactors. The derived equation has been based on the theories of the positive dc coronas reported in the literature and extended to account the ozone destruction within the corona discharge plasma generated in the reactor. The equation has been investigated with experimental results for ozone generated in a coaxial wire-cylinder reactor under different discharge conditions. The reactor was stressed with a positive dc voltage and fed by dry air flowing with constant rates at atmospheric pressure and room temperature. The theoretical results calculated by the derived equation have shown good agreement with the experimental results over the whole range of the investigated parameters. Subsequently, the derived equation is valid to predict the ozone concentration generated in the investigated reactor under any discharge conditions.
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52.80.Hc Glow; corona
52.50.Dg Plasma sources

Spectroscopic study on the vibrational populations of N2 C 3Π and B 3Π states in a microwave nitrogen discharge

Takeshi Sakamoto, Haruaki Matsuura, and Hiroshi Akatsuka

J. Appl. Phys. 101, 023307 (2007); http://dx.doi.org/10.1063/1.2426975 (7 pages) | Cited 15 times

Online Publication Date: 23 January 2007

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We measured the band spectra (first and second positive systems) of the nitrogen molecule to examine the vibrational and rotational temperatures of the C 3Π and B 3Π states by optical emission spectroscopy. We compared the experimentally measured and the calculated spectra to determine those temperatures of the generated plasma. We generated a microwave discharge nitrogen plasma in a cylindrical quartz tube (26 mm inside diameter) with a discharge pressure of 0.5–1.0 Torr. The microwave frequency was 2.45 GHz and the output power was set at 600 W. It was found that Tv ≈ 0.5–0.7 eV and Tr ≈ 0.07–0.15 eV at B 3Π (v = 7, 8, and 9), whereas Tv ≈ 0.65–0.9 eV and Tr ≈ 0.06–0.16 eV at C 3Π (v = 0 and 1). Both rotational temperatures obtained from first and second positive systems were in good agreement. We also compared the measured vibrational populations with theoretical calculations, in which vibrational distribution function at N2 X and electron energy distribution function are calculated self-consistently.
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52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.80.Pi High-frequency and RF discharges
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.50.Dg Plasma sources
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