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15 Sep 2007

Volume 102, Issue 6, Articles (06xxxx)

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Amplification of terahertz radiation in quantum cascade structures

Cosimo Mauro, Richard P. Green, Alessandro Tredicucci, Fabio Beltram, Harvey E. Beere, and David A. Ritchie

J. Appl. Phys. 102, 063101 (2007); http://dx.doi.org/10.1063/1.2779265 (3 pages) | Cited 5 times

Online Publication Date: 17 September 2007

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We explore the use of quantum cascade structures as amplifiers for terahertz radiation. For this purpose, two section devices were fabricated, in which the output from a ν ∼ 2.3 THz laser section is amplified in a subsequent stage. For relatively short devices we obtain an amplification of 1.5×, in good agreement with the calculated level. Based on these results we predict that amplifications of up to 13 dB could be obtained for such structures using increased amplifier lengths of a few millimeters.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Simultaneous sudden changes of electrical behavior at the threshold in laser diodes

L. F. Feng, D. Li, C. Y. Zhu, C. D. Wang, H. X. Cong, X. S. Xie, and C. Z. Lu

J. Appl. Phys. 102, 063102 (2007); http://dx.doi.org/10.1063/1.2779278 (5 pages) | Cited 7 times

Online Publication Date: 17 September 2007

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Precise ac electrical measurements as well as dc I‐V plots at forward bias have been used to characterize multi-quantum-well (MQW) laser diodes. Step offsets of apparent conductance, apparent capacitance, junction voltage, series resistance, and ideality factor at lasing threshold were observed. To compare the electrical characteristics of different diodes numerical simulations of the IdV/dI‐I curve of the double-heterostructure (DH) laser were performed. We find that the conventional model used to explain this curve is not very satisfactory. Our simulations demonstrate that the junction voltage, series resistance, and ideality factor for both DH and MQW lasers exhibit the same sudden changes at threshold. Perhaps the electrical behavior near threshold for all laser diodes is very similar.
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42.55.Px Semiconductor lasers; laser diodes
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)

Noise characterization of light-emitting devices based on conjugated copolymers of fluorene and thiophene moieties

Lin Ke, Weihua Tang, Yang Song, Zhi Kuan Chen, and Soo Jin Chua

J. Appl. Phys. 102, 063103 (2007); http://dx.doi.org/10.1063/1.2778739 (4 pages) | Cited 2 times

Online Publication Date: 18 September 2007

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Degradation induced changes in the structural and optical properties of polyfluorene-based light-emitting diodes are examined by using electroluminescence and low frequency noise (LFN) spectroscopic techniques. The materials studied are poly[2,7-(9,9′-dihexylfluorene)-alt-bithiophene] (P1) and poly[2,7-(9,9′-dihexylfluorene)-alt-thieno[3,2-b]thiophene] (P2). Improved emission spectra for a light-emitting device based on polymer P2 in terms of current efficiency, spectra stability, and lifetime are observed. A polymer P2-based device also presents long lifetime predicted by the smaller slope in the initial LFN spectra. Correlation of device LFN spectra with polymer structure change and lifetime is established. The increase in noise level predicts the undergoing degradation in bulk material and the increase in the noise slope predicts the fluctuation of carrier number and change in polymer structure. The redshift in emission spectrum for P2 after long-time driving is also picked up by the LFN spectrum.
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85.60.Jb Light-emitting devices
78.60.Fi Electroluminescence
61.41.+e Polymers, elastomers, and plastics

Polarized spectral characterization and laser demonstration of Nd3+:Li2Gd4(MoO4)7 crystal

Haomiao Zhu, Yanfu Lin, Yujin Chen, Xinghong Gong, Qiguang Tan, Zundu Luo, and Yidong Huang

J. Appl. Phys. 102, 063104 (2007); http://dx.doi.org/10.1063/1.2779286 (8 pages) | Cited 6 times

Online Publication Date: 18 September 2007

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Detailed polarized spectral properties of Nd3+:Li2Gd4(MoO4)7 crystal grown by the Czochralski method have been investigated. The Judd-Ofelt intense parameters Ω2,4,6, the absorption and emission cross sections, and the branching ratios were calculated for each polarization of the main transitions. The large absorption cross section (16.6×10−20 cm2 of π polarization) and broad absorption band (7 nm of π polarization) around 804 nm indicate that this crystal can be pumped efficiently by diode lasers. The broad emission bands from the 4F3/2 multiplet show that the crystal is a promising medium for tunable and short pulse lasers. Pumped by a Ti:sapphire laser, the maximum 514 mW quasi-cw laser output around 1060 nm has been obtained with a 1.5% output couple, the threshold is 35 mW, and the slope efficiency is 32%.
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42.55.Rz Doped-insulator lasers and other solid state lasers
78.40.Ha Other nonmetallic inorganics
78.55.Hx Other solid inorganic materials

Effects of oxygen partial pressure on packing density and laser damage threshold of TiO2 thin films

Jianke Yao, Zhengxiu Fan, Yunxia Jin, Yuanan Zhao, Hongbo He, and Jianda Shao

J. Appl. Phys. 102, 063105 (2007); http://dx.doi.org/10.1063/1.2779243 (4 pages) | Cited 11 times

Online Publication Date: 18 September 2007

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TiO2 films are deposited by electron beam evaporation as a function of oxygen partial pressure. The packing density, refractive index, and extinction coefficient all decrease with the increase of pressure, which also induces the change of the film’s microstructure, such as the increase of voids and H2O concentration in the film. The laser-induced damage threshold (LIDT) of the film increases monotonically with the rise of pressure in this experiment. The porous structure and low nonstoichiometric defects absorption contribute to the film’s high LIDT. The films prepared at the lowest and the highest pressure show nonstoichiometric and surface-defects-induced damage features, respectively.
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78.66.-w Optical properties of specific thin films
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
61.80.Ba Ultraviolet, visible, and infrared radiation effects (including laser radiation)
68.55.-a Thin film structure and morphology
61.43.Gt Powders, porous materials
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Broadband near-infrared emission from transparent Ni2+-doped silicate glass ceramics

Shifeng Zhou, Huafang Dong, Heping Zeng, Botao Wu, Bin Zhu, Hucheng Yang, Shiqing Xu, Zhiyu Wan, and Jianrong Qiu

J. Appl. Phys. 102, 063106 (2007); http://dx.doi.org/10.1063/1.2775310 (4 pages) | Cited 13 times

Online Publication Date: 20 September 2007

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Transparent Ni2+-doped MgO–Al2O3–TiO2–SiO2 glass ceramics were prepared, and the optical properties of Ni2+-doped glass ceramics were investigated. Broadband emission centered at 1320 nm was observed by 980 nm excitation. The longer wavelength luminescence compared with Ni2+-doped Li2O–Ga2O3–SiO2 glass ceramics is ascribed to the low crystal field hold by Ni2+ in MgO–Al2O3–TiO2–SiO2 glass ceramics. The change in optical signals at the telecommunication bands with or without 980 nm excitation was also measured when the seed beam passes through the bulk gain host.
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78.55.Qr Amorphous materials; glasses and other disordered solids
61.43.Fs Glasses
81.05.Je Ceramics and refractories (including borides, carbides, hydrides, nitrides, oxides, and silicides)

Analysis of photon recycling using metallic photonic crystal

Yong-Sung Kim, Shawn-Yu Lin, Allan S. P. Chang, Jae-Hwang Lee, and Kai-Ming Ho

J. Appl. Phys. 102, 063107 (2007); http://dx.doi.org/10.1063/1.2779271 (6 pages) | Cited 1 time

Online Publication Date: 20 September 2007

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We investigate a photon recycling scheme using two-dimensional metallic photonic crystals made of silver to improve the energy efficiency of an incandescent light source. A theoretical framework is presented to analyze the resultant photon-recycled lighting system. Calculation results show that the system can reach a maximum luminous efficiency of 125 lm/W, which is 8 times higher than that of a bare blackbody radiation at 2800 K. The color temperature of the system is calculated to be around 3500 K or below, while the color rendering index is between 68 and 90. These results suggest that a photon-recycled incandescent light source using metallic photonic crystals can be a viable alternative future lighting solution.
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42.70.Qs Photonic bandgap materials
42.72.-g Optical sources and standards
07.60.Dq Photometers, radiometers, and colorimeters

Optical method for measuring the azimuthal anchoring strength of liquid crystals using pitch values determined in imperfect samples

Tsung-Ta Tang, Hsin-Ying Wu, Chia-Jen Lin, and Ru-Pin Pan

J. Appl. Phys. 102, 063108 (2007); http://dx.doi.org/10.1063/1.2781320 (4 pages)

Online Publication Date: 20 September 2007

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An improved Grandjean–Cano wedge method for measuring the pitch of a chiral nematic liquid crystal is demonstrated. This method is easy to implement and can yield results of high accuracy when it is used for measuring the liquid crystal surface anchoring strength.
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07.60.Fs Polarimeters and ellipsometers
06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
42.70.Df Liquid crystals
61.30.-v Liquid crystals

Pump-probe imaging of femtosecond pulsed laser ablation of silicon with thermally grown oxide films

Joel P. McDonald, John A. Nees, and Steve M. Yalisove

J. Appl. Phys. 102, 063109 (2007); http://dx.doi.org/10.1063/1.2778740 (8 pages) | Cited 8 times

Online Publication Date: 21 September 2007

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Femtosecond pulsed laser ablation of silicon substrates with thin thermally grown oxide films (20–1200 nm) was studied using pump-probe microscopy techniques. Images from both the front and side of the ablation event produced at a laser fluence of 1.3 J/cm2 were obtained, and results from the two imaging geometries were compared yielding the optical properties of the ablated material. Ablation dynamics were studied over the time scale from 0 to 10.35 ns following the onset of ablation, and ablated material velocities ranging from 200±20 to −3010±360 m/s were determined depending on the thermal oxide film thickness.
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81.65.-b Surface treatments
42.62.-b Laser applications
78.47.-p Spectroscopy of solid state dynamics

Characteristic functions for uniformly twisted birefringent media

Iryna Valyukh, Sergiy Valyukh, Hans Arwin, and Vladimir Chigrinov

J. Appl. Phys. 102, 063110 (2007); http://dx.doi.org/10.1063/1.2781300 (7 pages) | Cited 2 times

Online Publication Date: 21 September 2007

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We present two functions for characterization of birefringent media with application to twisted nematic liquid crystal (LC) cells in the field off state. The first function is for reflective LC cells and the second is for transmissive LC cells. Based on the Stockes formalism, it is shown that these functions describe the ability of a layer of the twisted birefringent medium to change polarization of the output light and are invariant to the layer orientation. The characteristic functions are found in simple procedures and can be used for simultaneous determination of retardation, its wavelength dispersion, and twist angle, as well as for solving optimization problems.
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42.70.Df Liquid crystals
61.30.-v Liquid crystals
78.20.Fm Birefringence

Porous silicon omnidirectional mirrors and distributed Bragg reflectors for planar waveguide applications

E. Xifré-Pérez, L. F. Marsal, J. Ferré-Borrull, and J. Pallarès

J. Appl. Phys. 102, 063111 (2007); http://dx.doi.org/10.1063/1.2784019 (5 pages) | Cited 4 times

Online Publication Date: 28 September 2007

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The use of omnidirectional mirrors (an special case of distributed Bragg reflectors) as cladding for planar waveguides is proposed and analyzed. The proposed structure is an all-porous silicon multilayer consisting of a core layer inserted between two omnidirectional mirrors. The transfer matrix method is applied for the modal analysis. The influence of the parameters of the waveguide structure on the guided modes is analyzed. These parameters are the layer thickness and number of periods of the omnidirectional mirror, and the refractive index and thickness of the core layer. Finally, the confinement of the omnidirectional mirror cladding is analyzed with respect to two other different distributed Bragg reflector claddings.
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42.25.Gy Edge and boundary effects; reflection and refraction
42.82.Et Waveguides, couplers, and arrays
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Measurement of energy transfer at an isolated substrate in a pulsed dc magnetron discharge

M. Čada, J. W. Bradley, G. C. B. Clarke, and P. J. Kelly

J. Appl. Phys. 102, 063301 (2007); http://dx.doi.org/10.1063/1.2779287 (9 pages) | Cited 20 times

Online Publication Date: 18 September 2007

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The power density delivered by particles to an electrically isolated substrate in an asymmetric bipolar pulsed dc unbalanced magnetron has been quantified. The plasma source was operated in argon with a titanium target, and measurements were made using both a calorimeter probe and time-resolved Langmuir probe incorporated into a specially made substrate holder. The main results from the calorimeter probe show clearly that with increased pulse frequency (from dc to 350 kHz) and reduced duty cycle (90%–50%), the particle power density (from ions, electrons, sputtered Ti, and backscattered Ar) at the substrate increases significantly. For instance, at 350 kHz and 60% duty cycle, the total power density is 83 mW/cm2, about 60% higher than in dc mode for the same time-average discharge power. However, from an inventory of the individual particle contributions to the total power density derived from time-resolved Langmuir measurements and a simple model of the substrate sheath and plasma internal processes, we predict values of power density much lower than those measured. The measured and calculated values are in close agreement for the results obtained in dc mode but diverge at high frequencies. It is believed that this is due to the Langmuir probe measurements being unable to observe the presence of high-energy ions, created during the transient peaks in the electron temperature at the transitions from on off and off on [ J. W. Bradley et al., Plasma Sources Sci. Technol. 11, 165 (2002) ] which subsequently bombard the substrate. This paper shows conclusively the benefit of pulsing the magnetron over and above dc operation for enhancing the ion power per depositing neutral in the ion assisted deposition process.
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52.70.-m Plasma diagnostic techniques and instrumentation
52.70.Nc Particle measurements
52.77.Dq Plasma-based ion implantation and deposition
52.80.-s Electric discharges
81.15.Jj Ion and electron beam-assisted deposition; ion plating

One-dimensional bubble model of pulsed discharge in water

XinPei Lu

J. Appl. Phys. 102, 063302 (2007); http://dx.doi.org/10.1063/1.2783848 (4 pages) | Cited 10 times

Online Publication Date: 19 September 2007

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In this paper, a one-dimensional bubble model of pulsed discharge in water is presented. With a total input energy of 0.63 J, the simulation results show that when the bubble collapses at the center of the bubble, the plasma pressure oscillates strongly. It oscillates between 800 and 1150 atm with an oscillation frequency of about 6.9 MHz, while at r = R/2 (R: bubble radius), the gas velocity oscillates intensely at the same frequency. It oscillates between −235 and 229 m/s when the bubble radius reaches its minimum. But it does not oscillate at r = R because of the inertia of the surrounding water. The bubble collapses and reexpands with almost the same speed as that of the zero-dimensional (0D) model. This further confirms why the shock wave pressure from the 0D mode has a good agreement with the experimental results since the shock wave pressure is only determined by the bubble wall velocity v(R).
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52.80.Wq Discharge in liquids and solids
52.65.Kj Magnetohydrodynamic and fluid equation
52.25.-b Plasma properties
52.35.Fp Electrostatic waves and oscillations (e.g., ion-acoustic waves)
52.35.Tc Shock waves and discontinuities

Extracting ion emission lines from femtosecond-laser plasma x-ray spectra heavily contaminated by spikes

S. V. Gasilov, A. Ya. Faenov, T. A. Pikuz, P. Villoresi, L. Poletto, S. Stagira, F. Calegari, C. Vozzi, and M. Nisoli

J. Appl. Phys. 102, 063303 (2007); http://dx.doi.org/10.1063/1.2780872 (7 pages) | Cited 2 times

Online Publication Date: 20 September 2007

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Nowadays charged-coupled device (CCD) detectors are widely used for the registration of multicharged ions x-ray spectra. These spectra are generated in a plasma during interaction of ultrashort, ultraintense laser pulses with solid targets. Strong parasitic radiation from the plasma affects CCD detectors and contaminates resulting spectra, so that spectral features can be completely covered by noise even during measurements with a very short accumulation time. In this work we propose a “mean to median” (M2M) algorithm for noise suppression in femtosecond laser plasma x-ray spectra. Series of spectra is necessary for the identification of corrupted data points by the developed method. The algorithm was tested with model spectra which reflect main features of experimental data. In practice we used it for extracting information about spectral lines of Ne-like Fe ions and He-like Al ions which allowed us to calculate plasma parameters. It is demonstrated that M2M method is able to clean spectra with more than 10% of corrupted pixels. Fluctuations in intensity of spectral lines induced by laser instability do not affect validity of the proposed method.
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52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.70.La X-ray and γ-ray measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)
32.30.Rj X-ray spectra

Guided modes and loss in a plasma-filled Bragg waveguide

J. D. Chatterton and J. L. Shohet

J. Appl. Phys. 102, 063304 (2007); http://dx.doi.org/10.1063/1.2776374 (7 pages) | Cited 2 times

Online Publication Date: 24 September 2007

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The dispersion and radiation loss of propagating modes in a plasma-filled Bragg waveguide are investigated. The Bragg waveguide at a center frequency of 10 GHz is modeled with the transfer-matrix method, which has been used to analyze optical Bragg fibers. We calculate the dispersion and radiation loss of the TE01, TM01, and HE11 modes and show how they vary as a function of plasma density. As the plasma density inside the waveguide increases, the cutoff frequency of each mode increases. An increase in plasma density increases the radiation loss in the TE01 mode while it decreases the radiation loss in the TM01 mode; the effect on the HE11 mode is to increase the radiation loss for frequencies above 10.2 GHz and decrease the radiation loss for frequencies below 10.2 GHz.
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52.35.Hr Electromagnetic waves (e.g., electron-cyclotron, Whistler, Bernstein, upper hybrid, lower hybrid)
52.40.Fd Plasma interactions with antennas; plasma-filled waveguides

Capacitively coupled radio-frequency hydrogen discharges: The role of kinetics

L. Marques, J. Jolly, and L. L. Alves

J. Appl. Phys. 102, 063305 (2007); http://dx.doi.org/10.1063/1.2779268 (14 pages) | Cited 10 times

Online Publication Date: 24 September 2007

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This paper presents a systematic characterization of capacitively coupled radio-frequency hydrogen discharges, produced within a parallel plate cylindrical setup at different rf applied voltages (Vrf = 50–600 V), frequencies (f = 13.56–40.68 MHz), and pressures (p = 0.2–1 torr). A two-dimensional, time-dependent fluid model for charged particle transport is self-consistently solved coupled to a homogeneous kinetic model for hydrogen, including vibrationally excited molecular species and electronically excited atomic species. Numerical simulations are compared with experimental measurements of various plasma parameters. A good quantitative agreement is found between simulations and experiment for the coupled electrical power and the plasma potential. The model underestimates the values of the electron density, the self-bias potential, and the H(n = 1) atom density with respect to measurements, but agrees with experiment when predicting that all these parameters increase with either Vrf, f, or p. The dissociation degree is about 10−3 for the work conditions considered. Simulations adopt a wall recombination probability for H atoms that was experimentally measured, thus accounting for surface modification with discharge operating conditions. Results show the key role played by the atomic wall recombination mechanism in plasma description.
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52.80.Pi High-frequency and RF discharges
52.25.Fi Transport properties
52.65.-y Plasma simulation
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

A model for Si, SiCH, SiO2, SiOCH, and porous SiOCH etch rate calculation in inductively coupled fluorocarbon plasma with a pulsed bias: Importance of the fluorocarbon layer

V. Raballand, G. Cartry, and C. Cardinaud

J. Appl. Phys. 102, 063306 (2007); http://dx.doi.org/10.1063/1.2784016 (8 pages) | Cited 3 times

Online Publication Date: 25 September 2007

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In a previous paper we showed that selective etching of porous SiOCH with respect to SiO2 and SiCH is clearly enhanced when using a pulsed bias in inductively coupled fluorocarbon plasma. To understand this pulsed process, a model for etch rate calculation is developed in the present paper. This model explains the etching/deposition threshold shift toward higher bias voltage in pulsed conditions. Rather good confidence is obtained with experimental SiO2, Si, SiOCH and SiCH etch rates. Porous SiOCH etching is found to behave slightly differently compared to SiO2 or SiCH; its chemical etching is assumed to occur even during the beginning of off period. This point could explain why good selectivities between porous SiOCH and SiO2 and SiCH are obtained.
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81.65.Cf Surface cleaning, etching, patterning
52.77.Bn Etching and cleaning
68.47.Fg Semiconductor surfaces
68.47.Gh Oxide surfaces
68.35.Dv Composition, segregation; defects and impurities
79.60.Bm Clean metal, semiconductor, and insulator surfaces

Angular distribution and dose measurements of hard x-ray emission from intense laser-plasma interaction

B. S. Rao, P. A. Naik, V. Arora, R. A. Khan, and P. D. Gupta

J. Appl. Phys. 102, 063307 (2007); http://dx.doi.org/10.1063/1.2781383 (4 pages) | Cited 3 times

Online Publication Date: 26 September 2007

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An experimental study on hard x-ray Bremsstrahlung radiation due to fast electron generation from a planar solid copper target irradiated by 150 mJ, 45 fs Ti:sapphire laser pulses at I ∼ 1018 W/cm2 is reported. Angular distribution of the hard x-ray dose rate (hν ≥ 40 keV) is observed to be strongly forward peaked in the direction of target normal with a measured peak value of 40 μSv/h, at a distance of 500 mm from the target. Two sources of this radiation, one at the laser irradiated target and the other at the chamber wall facing the target, are inferred from the measurements of angular distribution and distance dependence of the x-ray dose rate.
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52.59.Px Hard X-ray sources
52.50.Jm Plasma production and heating by laser beams (laser-foil, laser-cluster, etc.)
52.38.Ph X-ray, γ-ray, and particle generation

Electron density modulation in an asymmetric bipolar pulsed dc magnetron discharge

S. K. Karkari, A. R. Ellingboe, C. Gaman, I. Swindells, and J. W. Bradley

J. Appl. Phys. 102, 063308 (2007); http://dx.doi.org/10.1063/1.2783980 (8 pages) | Cited 5 times

Online Publication Date: 26 September 2007

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This paper investigates the spatial and temporal variation in plasma electron density over a region between 5 and 10 cm above the race-track region of a pulsed magnetron sputtering target. The pulse operation is performed using an asymmetric bipolar pulsed dc power supply, which provides a sequence of large negative “on-phase” voltage (−350 V) and a small positive “reverse-phase” voltage (+10 V) for 55% of the pulse duration (10 μs). The electron density is measured using a floating microwave hairpin resonance probe. The results show electron expulsion from the target in the initial on phase, which propagates with a characteristic speed exceeding the ion thermal speed. In the steady state on phase, a consistent higher density is observed. A quantitative model has been developed to explain the resultant density drops in the initial on phase. While in the reverse phase, we observed an anomalous growth in density at a specific location from the target (d>7 cm). The mechanism behind the increase in electron density has been attributed to the modulation in spatial plasma potential, which was measured earlier in the same apparatus using a floating emissive probe [ J. W. Bradley et al., Plasma Sources Sci. Technol. 13, 189 (2004) ].
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52.80.Pi High-frequency and RF discharges
52.50.Dg Plasma sources
52.70.Gw Radio-frequency and microwave measurements
52.25.Fi Transport properties

Measurement and modeling of Ar/H2/CH4 arc jet discharge chemical vapor deposition reactors. I. Intercomparison of derived spatial variations of H atom, C2, and CH radical densities

C. J. Rennick, J. Ma, J. J. Henney, J. B. Wills, M. N. R. Ashfold, A. J. Orr-Ewing, and Yu. A. Mankelevich

J. Appl. Phys. 102, 063309 (2007); http://dx.doi.org/10.1063/1.2783890 (13 pages) | Cited 8 times

Online Publication Date: 28 September 2007

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Comparisons are drawn between spatially resolved absorption spectroscopy data obtained for a 6.4 kW dc arc jet reactor, operating with Ar/H2/CH4 gas mixtures, used for deposition of thin, polycrystalline diamond films, and the results of a two-dimensional (r,z) computer model incorporating gas activation, expansion into the low pressure reactor, and the chemistry of the neutral and charged species. The experimental measurements, using either cavity ring-down spectroscopy or diode laser absorption spectroscopy, determined absolute number densities of H(n = 2) atoms, and column densities of C2(a 3Πu), C2(X 1Σg+), and CH(X 2Π) radicals, with vibrational and rotational quantum state resolutions, and their variation with height through the horizontally propagating arc jet plume. Spectra were also analyzed to obtain temperatures and local electron densities [from Stark broadening of H(n = 2) absorption lines]. The experimental data are directly compared with the output data of the model that returns spatially inhomogeneous temperature, flow velocities, and number densities of 25 neutral and 14 charged species. Under the base operating conditions of the reactor [11.4 SLM (standard liters per minute) of Ar and 1.8 SLM of H2 entering the primary torch, with addition of 80 SCCM (SCCM denotes cubic centimeter per minute at STP) of CH4 downstream; 6.4 kW input power; reactor pressure of 50 Torr], the calculated and measured column and number densities agree to within factors of 2–3, the model reproduces the spatial dependence of column densities, and the mean temperatures of C2(a) and CH(X) radicals derived from spectra and model results are in good agreement. The model also captures the variation of these parameters with changes to operating conditions of the reactor such as flows of H2 and CH4, and input power. Further details of the model and the insights it provides are the subject of the accompanying paper [ Mankelevich et al., J. Appl. Phys. 102, 063310 (2007) ].
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84.70.+p High-current and high-voltage technology: power systems; power transmission lines and cables
82.33.Ya Chemistry of MOCVD and other vapor deposition methods
82.20.Wt Computational modeling; simulation

Measurement and modeling of Ar/H2/CH4 arc jet discharge chemical vapor deposition reactors II: Modeling of the spatial dependence of expanded plasma parameters and species number densities

Yu. A. Mankelevich, M. N. R. Ashfold, and A. J. Orr-Ewing

J. Appl. Phys. 102, 063310 (2007); http://dx.doi.org/10.1063/1.2783891 (11 pages) | Cited 12 times

Online Publication Date: 28 September 2007

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Detailed methodology and results are presented for a two-dimensional (r,z) computer model applicable to dc arc jet reactors operating on argon/hydrogen/hydrocarbon gas mixtures and used for chemical vapor deposition of micro- and nanocrystalline diamond and diamondlike carbon films. The model incorporates gas activation, expansion into the low pressure reactor chamber, and the chemistry of the neutral and charged species. It predicts the spatial variation of temperature, flow velocities and number densities of 25 neutral and 14 charged species, and the dependence of these parameters on the operating conditions of the reactor such as flows of H2 and CH4 and input power. Selected outcomes of the model are compared with experimental data in the accompanying paper [ C. J. Rennick et al., J. Appl. Phys. 102, 063309 (2007) ]. Two-dimensional spatial maps of the number densities of key radical and molecular species in the reactor, derived from the model, provide a summary of the complicated chemical processing that occurs. In the vortex region beyond the plume, the key transformations are CH4→CH3↔C2H2↔large hydrocarbons; in the plume or the transition zone to the cooler regions, the chemical processing involves C2Hx↔(CHy and CHz), C3Hx↔(CHy and C2Hz), (C2Hy and C2Hz)↔C4Hx↔(CHy and C3Hz). Depending on the local gas temperature Tg and the H/H2 ratio, the equilibria of H-shifting reactions favor C, CH, and C2 species (in the hot, H-rich axial region of the plume) or CH2, C2H, and C2H2 species (at the outer boundary of the transition zone). Deductions are drawn about the most abundant C-containing radical species incident on the growing diamond surface (C atoms and CH radicals) within this reactor, and the importance of chemistry involving charged species is discussed. Modifications to the boundary conditions and model reactor geometry allow its application to a lower power arc jet reactor operated and extensively studied by Jeffries and co-workers at SRI International, and comparisons are drawn with the reported laser induced fluorescence data from these studies.
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81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.16.Be Chemical synthesis methods
68.55.A- Nucleation and growth
52.77.-j Plasma applications
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
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Advanced nanoimprint lithography using a graded functional imprinting material tailored for liquid crystal alignment

Jin Seog Gwag, Masahito Oh-e, Makoto Yoneya, Hiroshi Yokoyama, H. Satou, and S. Itami

J. Appl. Phys. 102, 063501 (2007); http://dx.doi.org/10.1063/1.2778742 (5 pages) | Cited 14 times

Online Publication Date: 17 September 2007

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Nanoimprint lithography technology, which is able to easily create nanometer-resolution two-dimensional surface grooves on substrates over a large area by a step and stamp process, opens up an intriguing opportunity for functional liquid crystal (LC) alignment. We describe here a hybrid polymer consisting of two distinct moieties with largely different thermomechanical properties and surface activity. We present the results of nanoimprint lithography experiment using the hybrid polymer to demonstrate the practical applicability as an LC alignment layer. This material shows excellent capability both as a nanoimprinting material, requiring softness at moderate temperatures, and as a LC alignment layer, requiring sufficient rigidity. LC devices using the alignment layer show fairly stable electro-optic characteristics even after thermal aging, due to its high thermal stability. The soft component of the hybrid polymer helps it provide the multiple imprinting capabilities at higher temperatures virtually without any mold damages.
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61.30.-v Liquid crystals
42.79.Kr Display devices, liquid-crystal devices
81.16.Nd Micro- and nanolithography
81.16.Rf Micro- and nanoscale pattern formation

Lattice thermal conductivity of group-IV and III–V semiconductor alloys

Sadao Adachi

J. Appl. Phys. 102, 063502 (2007); http://dx.doi.org/10.1063/1.2779259 (7 pages) | Cited 11 times

Online Publication Date: 17 September 2007

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The room-temperature thermal conductivity of semiconductor alloys is analyzed using a simplified model of the alloy-disorder scattering. Good agreement is achieved between the present model and published experimental data on various group-IV and III−V semiconductor alloys. A complete set of alloy-disorder parameters are estimated, which makes it possible to calculate the lattice thermal conductivity for optional composition of III−V semiconductor alloys, including III−N alloys. An ordering effect is also examined for the explanation of some intermetallic and semiconductor compounds like CuAu and SiC.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves

Thermal conductance across grain boundaries in diamond from molecular dynamics simulation

Taku Watanabe, Boris Ni, Simon R. Phillpot, Patrick K. Schelling, and Pawel Keblinski

J. Appl. Phys. 102, 063503 (2007); http://dx.doi.org/10.1063/1.2779289 (7 pages) | Cited 7 times

Online Publication Date: 17 September 2007

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We determine the dependence of the interfacial conductance on twist angle for (001) symmetric twist grain boundaries (GBs) in diamond. We find that the conductances are extremely large, ranging from 7.7 to 17.6 GW/m2 K. Nevertheless, when normalized to the single-crystal conductivity, the resulting Kapitza lengths are actually longer in diamond than in Si, indicating that the diamond GBs are relatively worse conductors of heat. This result is consistent with the poorer bonding across the diamond grain boundaries. We find that the interfacial conductance and Kapitza length can be well fitted by an extended Read-Shockley model.
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66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves
61.72.Mm Grain and twin boundaries

Conformational degree and molecular orientation in rubrene film by in situ x-ray absorption spectroscopy

Li Wang, Shi Chen, Lei Liu, Dongchen Qi, Xingyu Gao, Jegadesan Subbiah, Sindhu Swaminathan, and Andrew TS Wee

J. Appl. Phys. 102, 063504 (2007); http://dx.doi.org/10.1063/1.2778636 (6 pages) | Cited 3 times

Online Publication Date: 17 September 2007

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The conformation degree and molecular orientation during the growth of rubrene films on Si(111) and Au(111) have been studied by in situ x-ray absorption spectroscopy. The backbones of rubrene molecules on Au(111) are twisted at the first few layers; in contrast, no appreciable twisting is observed on Si(111) even at a thickness of approximately 1.5 nm. The planarization of the backbone in the first few layers is due to strong molecule–substrate interactions between rubrene and Si(111). The rubrene molecules on Au(111) have a backbone tilt angle of 41° and a phenyl side group tilt of 64° with respect to the substrate surface, suggesting the crystalline nature of the films. Ex situ atomic force microscopy measurements confirm that the rubrene film grown on Au(111) is crystalline and the growth direction is along its crystallographic c axis.
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78.70.Dm X-ray absorption spectra
68.55.-a Thin film structure and morphology
68.37.Ps Atomic force microscopy (AFM)
68.35.B- Structure of clean surfaces (and surface reconstruction)
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