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1 Nov 2007

Volume 102, Issue 9, Articles (09xxxx)

Issue Cover Spotlight Figure

J. Appl. Phys. 102, 091101 (2007); http://dx.doi.org/10.1063/1.2804122 (21 pages)

Franky So, Benjamin Krummacher, Mathew K. Mathai, Dmitry Poplavskyy, Stelios A. Choulis, and Vi-En Choong
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Recent progress in solution processable organic light emitting devices

Franky So, Benjamin Krummacher, Mathew K. Mathai, Dmitry Poplavskyy, Stelios A. Choulis, and Vi-En Choong

J. Appl. Phys. 102, 091101 (2007); http://dx.doi.org/10.1063/1.2804122 (21 pages) | Cited 84 times

Online Publication Date: 13 November 2007

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Organic light emitting devices (OLEDs) have been the subject of intense research because of their potential for flat panel display and solid state lighting applications. While small molecule OLEDs with very high efficiencies have been demonstrated, solution processable devices are more desirable for large size flat panel display and solid state applications because they are compatible with low cost, large area roll-to-roll manufacturing process. In this review paper, we will present the recent progress made in solution processable OLEDs. The paper will be divided into three parts. In the first part of the paper, we will focus on the recent development of fluorescent polymer OLEDs based on conjugated polyfluorene copolymers. Specifically, we will present results of carrier transport and injection measurements, and discuss how the charge transport and injection properties affect the device performance. In the second part of the paper, we will focus on the recent progress on phosphorescent dye-dispersed nonconjugated polymer OLEDs. Specifically, we will present our recent results on high efficiency green and blue emitting devices based on the dye-dispersed polymer approach. Similar to fluorescent conjugated polymer OLEDs, charge transport and injection properties in dye-dispersed polymer OLEDs also play an important role in the device performance. In the third part of this paper, we will present our results on white emitting phosphorescent OLEDs. Two approaches have been used to demonstrate white emitting OLEDs. First, white emitting OLEDs were made using blue emitting OLEDs with downconversion phosphors. Second, white emitting OLEDs were made by dispersing red, green, and blue phosphorescent dyes into the light emitting layer. High efficiency devices have been demonstrated with both approaches.
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85.60.Jb Light-emitting devices
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Effects of shapes and orientations of reversed domains on the conversion efficiency of second harmonic wave in two-dimensional nonlinear photonic crystals

Jingjuan Li, Zhiyuan Li, and Daozhong Zhang

J. Appl. Phys. 102, 093101 (2007); http://dx.doi.org/10.1063/1.2803740 (5 pages) | Cited 1 time

Online Publication Date: 2 November 2007

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The conversion efficiency of second harmonic wave in two-dimensional nonlinear photonic crystals consisting of square lattice and reversed domains of various shapes and orientations is studied numerically. Different shapes of reversed domains (circle, ellipse, rectangle, and square) are considered. We present numerical results and theoretical validations. The results indicate that for a given lattice symmetry and a fixed filling factor, the conversion efficiency of different order quasiphase matched process can be maximized by adjusting the shape and orientation of the reversed domains.
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42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
42.70.Mp Nonlinear optical crystals

High resolution selective multilayer laser processing by nanosecond laser ablation of metal nanoparticle films

Seung H. Ko, Heng Pan, David J. Hwang, Jaewon Chung, Sangil Ryu, Costas P. Grigoropoulos, and Dimos Poulikakos

J. Appl. Phys. 102, 093102 (2007); http://dx.doi.org/10.1063/1.2802302 (9 pages) | Cited 10 times

Online Publication Date: 5 November 2007

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Ablation of gold nanoparticle films on polymer was explored using a nanosecond pulsed laser, with the goal to achieve feature size reduction and functionality not amenable with inkjet printing. The ablation threshold fluence for the unsintered nanoparticle deposit was at least ten times lower than the reported threshold for the bulk film. This could be explained by the combined effects of melting temperature depression, lower conductive heat transfer loss, strong absorption of the incident laser beam, and the relatively weak bonding between nanoparticles. The ablation physics were verified by the nanoparticle sintering characterization, ablation threshold measurement, time resolved ablation plume shadowgraphs, analysis of ablation ejecta, and the measurement and calculation of optical properties. High resolution and clean feature fabrication with small energy and selective multilayer processing are demonstrated.
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81.07.Bc Nanocrystalline materials
81.05.Bx Metals, semimetals, and alloys
81.16.Mk Laser-assisted deposition
42.62.-b Laser applications
81.65.-b Surface treatments
78.66.Bz Metals and metallic alloys

Design of n-type silicon-based quantum cascade lasers for terahertz light emission

Kristina Driscoll and Roberto Paiella

J. Appl. Phys. 102, 093103 (2007); http://dx.doi.org/10.1063/1.2803896 (8 pages) | Cited 5 times

Online Publication Date: 6 November 2007

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The design of terahertz quantum cascade lasers based on electronic intersubband transitions in Ge/SiGe quantum wells is investigated. A detailed theoretical model of the conduction-band lineup of these heterostructures is first presented and used to show that large quantum confinement in the L valleys can be obtained with properly selected layer compositions and thicknesses. Computation of the key laser design parameters is then discussed, including the important role played by the L-valley ellipsoidal constant-energy surfaces. Finally, the main design issues specific to this material system and its potential for high-performance operation are illustrated by means of two exemplary structures, designed for emission near 50 and 25 μm.
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42.60.By Design of specific laser systems
42.55.Px Semiconductor lasers; laser diodes

Terahertz gain on shallow donor transitions in silicon

R. Kh. Zhukavin, V. N. Shastin, S. G. Pavlov, H.-W. Hübers, J. N. Hovenier, T. O. Klaassen, and A. F. G. van der Meer

J. Appl. Phys. 102, 093104 (2007); http://dx.doi.org/10.1063/1.2804756 (5 pages) | Cited 4 times

Online Publication Date: 7 November 2007

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Small signal gain measurements of optically excited terahertz silicon lasers are reported. Two types of lasers, Si:P and Si:Bi, were investigated. They were optically excited with radiation from a free electron laser or a CO2 laser. The experiments were performed with an oscillator-amplifier scheme where one sample serves as a laser while the other one is an amplifier. In case of the free electron laser the pump frequency corresponds to intracenter excitation of the 2p0 or 2p± states of the P and Bi Coulomb centers, and the gain was determined for the 2p0→1s(E), 2p0→1s(T2) transitions in Si:P and the 2p±→1s(E) transition in Si:Bi. Pumping with a CO2 laser leads to photoexcitation of the Coulomb centers. In this case the gain was determined for the 2p0→1s(T2) of Si:P transition. The gain for intracenter pumping is in the range 5−10 cm−1 while for photoexcitation the gain is considerably less, namely ∼ 0.5 cm−1. The experimental results are analyzed and found to be in good agreement with theoretical calculations based on balance equations.
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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
81.05.Cy Elemental semiconductors

Electro-optical sampling of ultrashort THz pulses by fs-laser pulses at 530 nm with BaTiO3

B. Pradarutti, G. Matthäus, S. Riehemann, G. Notni, J. Limpert, S. Nolte, and A. Tünnermann

J. Appl. Phys. 102, 093105 (2007); http://dx.doi.org/10.1063/1.2809270 (3 pages) | Cited 2 times

Online Publication Date: 14 November 2007

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Electro-optical sampling (EOS) of ultrabroadband terahertz (THz) radiation with the help of ultrashort 530 nm fs-laser pulses is reported. The THz pulses are generated by exciting a surface emitter (InAs) with a parabolic fiber laser amplifier delivering 100 fs pulses at a repetition rate of 75 MHz and an average power of 10 W at 1060 nm. The pulses are frequency doubled to 530 nm for gating and BaTiO3 is used as the detector crystal.
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42.55.Wd Fiber lasers
42.65.Re Ultrafast processes; optical pulse generation and pulse compression
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation

Growth and optical properties of KTa1−xNbxO3 thin films grown by pulsed laser deposition on MgO substrates

A. Rousseau, M. Guilloux-Viry, E. Dogheche, M. Bensalah, and D. Remiens

J. Appl. Phys. 102, 093106 (2007); http://dx.doi.org/10.1063/1.2809400 (4 pages) | Cited 6 times

Online Publication Date: 15 November 2007

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Perovskitelike KTa1−xNbxO3 (KTN, x ranging from 0 to 1) thin films were grown on (100)MgO substrates using pulsed laser deposition. X-ray diffraction analyses evidenced the epitaxial growth of the films along the (100) orientation. The optical properties of KTN planar waveguides were characterized by prism coupling for determination of ordinary and extraordinary refractive indices nTE and nTM (transverse electric and transverse magnetic). The influence of Nb content was also investigated relative to the films’ indices. The film behavior and the substrate-to-layer interface were directly qualified from the measured optical data using the experimental and theoretical approach (iWKB) (inverse Wentzel-Kramers-Brillouin). The results showed a change in the refractive index profile at the interface, which may be related to the existence of structural defects.
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77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
77.55.-g Dielectric thin films
81.05.-t Specific materials: fabrication, treatment, testing, and analysis
81.15.Fg Pulsed laser ablation deposition
78.66.Nk Insulators

Properties of omnidirectional gap and defect mode of one-dimensional photonic crystal containing indefinite metamaterials with a hyperbolic dispersion

Yuanjiang Xiang, Xiaoyu Dai, Shuangchun Wen, and Dianyuan Fan

J. Appl. Phys. 102, 093107 (2007); http://dx.doi.org/10.1063/1.2809446 (5 pages) | Cited 3 times

Online Publication Date: 15 November 2007

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The structure of one-dimensional photonic crystal (1DPC) containing indefinite metamaterials with a hyperbolic dispersion relation is proposed to realize the omnidirectional gap for the design of omnidirectional reflectors and filters. It is demonstrated that this 1DPC can result in a similar zero average index (zero-math) gap usually appearing in the 1DPC containing isotropic left-handed materials. The properties of zero-math gap and defect modes of this photonic crystal are studied in detail. The edge of this zero-math gap is insensitive to incident angle and polarization of light in contrast to the Bragg gap. When a defect layer is introduced into the 1DPC, a defect mode appears inside the omnidirectional gap. The spectral position of defect mode is found to be weakly dependent on the incident angle of light and nearly invariant to the scale length of photonic crystal.
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42.70.Qs Photonic bandgap materials
61.72.-y Defects and impurities in crystals; microstructure

Dynamic control of visible radiation by a liquid crystal filled Fabry-Pérot etalon

S. A. Jewell, S. L. Cornford, and J. R. Sambles

J. Appl. Phys. 102, 093108 (2007); http://dx.doi.org/10.1063/1.2809448 (4 pages)

Online Publication Date: 15 November 2007

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A liquid crystal filled Fabry-Pérot etalon has been constructed to control the resonant transmission of electromagnetic radiation over the visible range of the spectrum. This has been achieved through the use of a 1.5 μm thick homogeneously aligned liquid crystal layer in the core of a silver-clad etalon structure. Applying an electric field across the core reorientates the liquid crystal director and changes the refractive index for incident light polarized parallel to the rubbing direction. By measuring the transmitted intensity as a function of wavelength for a variety of applied voltages shifts in the positions of the resonant transmission modes of up to 80 nm have been observed. In addition, these results have been compared to model data generated using a multilayer optics model to obtain the dispersion of the liquid crystal over the visible range of the electromagnetic spectrum.
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07.60.Ly Interferometers
42.70.Df Liquid crystals
61.30.Gd Orientational order of liquid crystals; electric and magnetic field effects on order
78.40.Dw Liquids
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
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Spectroscopic study of laser-induced Al plasmas with cylindrical confinement

X. K. Shen, J. Sun, H. Ling, and Y. F. Lu

J. Appl. Phys. 102, 093301 (2007); http://dx.doi.org/10.1063/1.2801405 (5 pages) | Cited 8 times

Online Publication Date: 1 November 2007

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The cylindrical confinement of laser-induced plasmas in round pipes has been investigated by optical emission spectra and fast imaging. An obvious enhancement in the emission intensity of Al atomic lines was observed when a round pipe was placed to confine the laser-induced Al plasmas. The enhancement factor for the emission intensities of the Al atomic lines was measured to be around 9 at a time delay of 12 μs when the pipe diameter is 10.8 mm. Assuming local thermodynamic equilibrium conditions, the plasma temperatures are estimated to be in the range from 4000 to 5800 K. It shows that the plasma temperature increased by around 1000 K when the cylindrical confinement was applied. Images of the laser-induced Al plasmas show that the plasmas were compressed into a smaller volume with a pipe presented. The spatial-confinement effects are attributed to the reflection and compression of the shock wave.
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52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.Kn Thermodynamics of plasmas

Langmuir probe data analysis for a magnetized inductive radio-frequency discharge

S. Popescu, Y. Ohtsu, and H. Fujita

J. Appl. Phys. 102, 093302 (2007); http://dx.doi.org/10.1063/1.2802293 (7 pages) | Cited 7 times

Online Publication Date: 2 November 2007

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The properties of an inductively coupled argon plasma in a uniform magnetic field, at pressures of the order of 0.1 Pa, in the presence of a weak current-free electric double layer, are measured with a plane Langmuir probe. The static current-voltage probe characteristics, recorded on the symmetry axis of the experimental device, both in the plasma source and in the main chamber of the setup, showed the existence of two electron populations with different temperatures. The axial profiles of plasma parameters, such as the plasma potential and the cold and hot electron temperatures, offered a new insight into such a complex system. However, both the external magnetic field and the two electron populations contribute to the difficulty of calculation of the local plasma density. A method for solving this problem, involving the definition of an effective electron temperature, is also presented.
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52.70.Ds Electric and magnetic measurements
52.80.Pi High-frequency and RF discharges
52.25.Xz Magnetized plasmas
52.80.Yr Discharges for spectral sources (including inductively coupled plasma)
52.50.Dg Plasma sources

Beyond the step model: Approximate expressions for the field in the plasma boundary sheath

Ralf Peter Brinkmann

J. Appl. Phys. 102, 093303 (2007); http://dx.doi.org/10.1063/1.2772499 (14 pages) | Cited 18 times

Online Publication Date: 6 November 2007

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The transition from quasineutrality to charge depletion is one of the characteristic features of the plasma boundary sheath. For modeling purposes, this transition is often described in terms of the so-called step model which assumes a sharp transition point s (electron step) where the electron density ne drops from a value equal to the ion density ni (in the bulk, x>s) to a value of zero (in the sheath, x<s). Inserted into Poisson’s equation, the step model yields an expression for the field which is realistic deep in the sheath (for x≪s) but fails to merge correctly into the ambipolar field of the bulk. This work considers the transition from quasineutrality to charge depletion more rigorously. Within the framework of asymptotic scale analysis, a family of field approximations is derived which, in the limit of weak spatial ion density variation ∂ni/∂x≪ni/λD, exhibit convergence to the exact solution of the Boltzmann–Poisson equation. The first of the approximations recovers only the step model. Higher order expressions, however, also include the ambipolar field and are valid for all values of x. The simplest of them is proposed as being the best suited for a self-consistent sheath analysis. This statement is backed by numerical experiments which show good results for the ion density distribution provided by the collision dominated sheath model.
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52.40.Kh Plasma sheaths
52.40.Hf Plasma-material interactions; boundary layer effects
52.25.Fi Transport properties
52.25.Dg Plasma kinetic equations
52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions

Influence of the reactor wall composition on radicals' densities and total pressure in Cl2 inductively coupled plasmas: I. Without silicon etching

G. Cunge, N. Sadeghi, and R. Ramos

J. Appl. Phys. 102, 093304 (2007); http://dx.doi.org/10.1063/1.2803880 (8 pages) | Cited 15 times

Online Publication Date: 6 November 2007

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Laser absorption at 355 nm is used to monitor the time variations of the Cl2 density in high-density industrial inductively coupled plasma. This technique is combined with the measurement of the gas temperature from the Doppler width of the 811.5 nm line of argon, added as a trace gas and with the measurement of the total gas pressure with a Baratron gauge. These measurements permit to estimate the mole fractions of Cl2 and Cl species in Cl2 inductively coupled plasmas in a waferless reactor. The impact of the chemical nature of the reactor wall coatings on the Cl and Cl2 mole fractions is studied systematically. We show that under otherwise identical plasma conditions, the Cl mole fraction is completely different when the plasma is operated in SiOCl, AlF, CCl, or TiOCl coated reactors, because the homogeneous recombination probability of Cl atoms is strongly surface dependant. The Cl atom mole fraction reached at 100 W radiofrequency power in SiOCl coated reactor (80%) is much higher than that obtained at 900 W in a “clean” AlF reactor (40%). A simple zero-dimensional model permits to provide the recombination coefficient of Cl atoms, γrec: 0.005 on SiOCl film and about 0.3 on the other three coatings. It is proposed to get benefit of this very high sensitivity of Cl2 dissociation rate to the wall coating for the control of the chamber wall status from the Cl2 density measurements in standard conditions.
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52.25.-b Plasma properties
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
42.62.Eh Metrological applications; optical frequency synthesizers for precision spectroscopy

Influence of the reactor wall composition on radicals’ densities and total pressure in Cl2 inductively coupled plasmas: II. During silicon etching

G. Cunge, N. Sadeghi, and R. Ramos

J. Appl. Phys. 102, 093305 (2007); http://dx.doi.org/10.1063/1.2803881 (11 pages) | Cited 13 times

Online Publication Date: 6 November 2007

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In an industrial inductively coupled plasma reactor dedicated to silicon etching in chlorine-based chemistry, the density of Cl2 molecules and the gas temperature are measured by means of laser absorption techniques, the density of SiClx (x ⩽ 2) radicals by broadband absorption spectroscopy, the density of SiCl4 and ions by mass spectrometry, and the total gas pressure with a capacitance gauge. These measurements permit us to estimate the mole fractions of Cl, SiCl4, and etch product radicals when etching a 200 mm diameter silicon wafer. The pure Cl2 plasma is operated in well prepared chamber wall coating with a thin film of SiOCl, AlF, CCl, or TiOCl. The impact of the chemical nature of the reactor wall’s coatings on these mole fractions is studied systematically. We show that the reactor wall coatings have a huge influence on the radicals densities, but this is not only from the difference on Cl–Cl recombination coefficient on different surfaces. During silicon etching, SiClx radicals sticking on the reactor walls are etched by Cl atoms and recycled into the plasma by forming volatile SiCl4. Hence, the loss of Cl atoms in etching the wall deposited silicon is at least as important as their wall recombination in controlling the Cl atoms density. Furthermore, because SiCl4 is produced at high rate by both the wafer and reactor walls, it is the predominant etching product in the gas phase. However, the percentage of redeposited silicon that can be recycled into the plasma depends on the amount of oxygen present in the plasma: O atoms produced by etching the quartz roof window fix Si on the reactor walls by forming a SiOCl deposit. Hence, the higher the O density is, the lower the SiCl4 density will be, because silicon is pumped by the reactor walls and the SiOCl layer formed is not isotropically etched by chlorine. As a result, in the same pure Cl2 plasma at 20 mTorr, the SiClx mole fraction can vary from 18% in a SiOCl-coated reactor, where the O density is the highest, to 62% in a carbon-coated reactor, where there is no O. In the latter case, most of the Cl mass injected in the reactor is stored in SiCl4 molecules, which results in a low silicon etch rate. In this condition, the Cl mass balance is verified within 10%, and from the silicon mass balance we concluded that SiClx radicals have a high surface loss probability. The impact of the reactor wall coating on the etching process is thus important, but the mechanisms by which the walls control the plasma chemistry is much more complicated than a simple control through recombination reaction of halogen atoms on these surfaces.
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52.77.Bn Etching and cleaning
68.47.Fg Semiconductor surfaces
81.65.Cf Surface cleaning, etching, patterning
81.05.Cy Elemental semiconductors
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)

Characterization of high-pressure capacitively coupled hydrogen plasmas

S. Nunomura and M. Kondo

J. Appl. Phys. 102, 093306 (2007); http://dx.doi.org/10.1063/1.2809345 (7 pages) | Cited 18 times

Online Publication Date: 13 November 2007

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Capacitively coupled very-high-frequency hydrogen plasmas have been systematically diagnosed in a wide range of a gas pressure from 5 mTorr to 10 Torr. The plasma parameters, ion species, and ion energy distributions (IEDs) are measured using a Langmuir probe, optical emission spectroscopy, and energy filtered mass spectrometer. The measurement results show that the ion species in a hydrogen plasma is determined from ionization channels and subsequent ion-molecule reactions. The ions are dominated by H2+ at a less-collisional condition of ≲20 mTorr, whereas those are dominated by H3+ at a collisional condition of ≳20 mTorr. The IED is determined by both the sheath potential drop and ion-neutral collisions in the plasma sheath. The IED is broadened for a collisional sheath at ≳0.3 Torr and the ion bombardment energy is lowered. For high-pressure discharge operated at ≈ 10 Torr, plasmas are characterized by a low electron temperature of ≈ 0.8 eV and a low ion bombardment energy of ≲15 eV.
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52.70.Ds Electric and magnetic measurements
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.40.Kh Plasma sheaths
52.80.-s Electric discharges
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Electron source concept for single-shot sub-100 fs electron diffraction in the 100 keV range

T. van Oudheusden, E. F. de Jong, S. B. van der Geer, W. P. E. M. Op ’t Root, O. J. Luiten, and B. J. Siwick

J. Appl. Phys. 102, 093501 (2007); http://dx.doi.org/10.1063/1.2801027 (8 pages) | Cited 45 times

Online Publication Date: 1 November 2007

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We present a method for producing sub-100 fs electron bunches that are suitable for single-shot ultrafast electron diffraction experiments in the 100 keV energy range. A combination of analytical estimates and state-of-the-art particle tracking simulations show that it is possible to create 100 keV, 0.1 pC, 30 fs electron bunches with a spot size smaller than 500 μm and a transverse coherence length of 3 nm, using established technologies in a table-top setup. The system operates in the space-charge dominated regime to produce energy-correlated bunches that are recompressed by radio-frequency techniques. With this approach we overcome the Coulomb expansion of the bunch, providing a single-shot, ultrafast electron diffraction source concept.
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07.77.Ka Charged-particle beam sources and detectors
07.78.+s Electron, positron, and ion microscopes; electron diffractometers
29.25.Bx Electron sources

Correlating exciton localization with compositional fluctuations in InGaN/GaN quantum wells grown on GaN planar surfaces and facets of GaN triangular prisms

S. Khatsevich, D. H. Rich, X. Zhang, and P. D. Dapkus

J. Appl. Phys. 102, 093502 (2007); http://dx.doi.org/10.1063/1.2802291 (8 pages) | Cited 2 times

Online Publication Date: 2 November 2007

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We have used spatially and temporally resolved cathodoluminescence (CL) to study the carrier recombination dynamics of InGaN quantum wells (QWs) grown on (0001)-oriented planar GaN and {1math01}-oriented facets of GaN triangular prisms prepared by lateral epitaxial overgrowth in a metal-organic chemical vapor deposition system. The effects of In migration during growth on the resulting QW thickness and composition were examined. We employed a variable temperature time-resolved CL imaging approach that enables a spatial correlation between regions of enhanced exciton localization, luminescence efficiency, and radiative lifetime with the aim of distinguishing between excitons localized in In-rich quantum dots and those in the surrounding Ga-rich QW regions.
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73.21.Fg Quantum wells
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
78.60.Hk Cathodoluminescence, ionoluminescence
78.67.De Quantum wells
68.47.Fg Semiconductor surfaces
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy

Crystallization kinetics and recording mechanism of a-Si/Ni bilayer for write-once blue-ray recording

Yung-Chiun Her, Sen-Tsun Jean, and Jyun-Lin Wu

J. Appl. Phys. 102, 093503 (2007); http://dx.doi.org/10.1063/1.2802992 (7 pages) | Cited 6 times

Online Publication Date: 2 November 2007

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For the a-Si/Ni bilayer with a thickness ratio of 4:1, the formation of NiSi and NiSi2 phases took place in the temperature range between 200 and 350 °C and only ∼ 9% of unreacted a-Si would crystallize to c-Si, which will not cause appreciable reflectivity change. As the thickness ratio of a-Si to Ni was increased to 20:1, the formation of NiSi2 phase and subsequent crystallization of a-Si mediated by NiSi2 precipitates were clearly observed. The crystallization temperature of a-Si in the a-Si/Ni bilayer with a thickness ratio of 20:1 was significantly reduced to around 350 °C, which was 130 °C lower than that in the a-Si/Cu bilayer. The activation energies for NiSi2 phase formation and crystallizations of a-Si for the a-Si/Ni bilayer with a thickness ratio of 20:1 were determined to be 1.12±0.09 and 2.19±0.08 eV, respectively. The crystallization behavior of the a-Si(20 nm)/Ni(1 nm) bilayer recording film under pulsed laser irradiation is similar to that under thermal annealing. During the recording process, the NiSi2 phase will precipitate first and serve as the nucleation sites for the following crystallization of the remaining amorphous Si. The maximum value of carrier to noise ratio for 3 T could reach 43 dB for the write-once blue-ray disk with layer structure of a-Si(20 nm)/Ni(1 nm), demonstrating a high potentiality for practical use.
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64.70.K- Solid-solid transitions
64.60.Q- Nucleation
64.75.-g Phase equilibria
81.30.Mh Solid-phase precipitation
81.15.Fg Pulsed laser ablation deposition

Recombination centers in as-grown and electron-irradiated ZnO substrates

N. T. Son, I. G. Ivanov, A. Kuznetsov, B. G. Svensson, Q. X. Zhao, M. Willander, N. Morishita, T. Ohshima, H. Itoh, J. Isoya, E. Janzén, and R. Yakimova

J. Appl. Phys. 102, 093504 (2007); http://dx.doi.org/10.1063/1.2802186 (5 pages) | Cited 7 times

Online Publication Date: 2 November 2007

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Optical detection of magnetic resonance (ODMR) was used to study defects in ZnO substrates irradiated with 3 MeV electrons at room temperature. The Zn vacancy and some other ODMR centers were detected. Among these, the Zn vacancy and two other centers, labeled as LU3 and LU4, were also commonly observed in different types of as-grown ZnO substrates. The LU3 and LU4 are related to intrinsic defects and act as dominating recombination centers in irradiated and as-grown ZnO.
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61.72.J- Point defects and defect clusters
61.80.Fe Electron and positron radiation effects
76.70.-r Magnetic double resonances and cross effects

Size-dependent excitation spectra and energy transfer in Tb3+-doped Y2O3 nanocrystalline

Qingyu Meng, Baojiu Chen, Wu Xu, Yanmin Yang, Xiaoxia Zhao, Weihua Di, Shaozhe Lu, Xiaojun Wang, Jiashi Sun, Lihong Cheng, Tao Yu, and Yong Peng

J. Appl. Phys. 102, 093505 (2007); http://dx.doi.org/10.1063/1.2803502 (6 pages) | Cited 18 times

Online Publication Date: 2 November 2007

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Nanocrystal Y2O3 powders with different grain sizes and various doping concentrations of Tb3+ were prepared by an autocombustion reaction. The size and surface effects on the 4f-5d transitions and energy transfers between Tb3+ ions were studied by using x-ray diffraction, transmission electron microscopy, fluorescent spectra, and luminescent decay. It was found that the excitation spectra are composed of two parts: one is the contribution from the Tb3+ at/near the nanoparticle surfaces; another is from the Tb3+ inside the nanoparticles. The study on the concentration quenching and luminescent decay indicated that the energy transfers depopulating the 5D3 and 5D4 level were assigned to the mechanisms of electric dipole-dipole and exchange interaction, respectively. The size confinement greatly affects the energy transfer quenching the emission from the 5D3 level, but slightly affects the energy transfer quenching the emission from the 5D4 level.
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32.50.+d Fluorescence, phosphorescence (including quenching)
33.15.Hp Barrier heights (internal rotation, inversion, rotational isomerism, conformational dynamics)

Crystallization kinetics and spectroscopic investigations on Tb3+ and Yb3+ codoped glass ceramics containing CaF2 nanocrystals

Lihui Huang, Guanshi Qin, Yusuke Arai, Rajan Jose, Takenobu Suzuki, Yasutake Ohishi, Tatsuya Yamashita, and Yusuke Akimoto

J. Appl. Phys. 102, 093506 (2007); http://dx.doi.org/10.1063/1.2802289 (8 pages) | Cited 17 times

Online Publication Date: 5 November 2007

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Transparent Tb3+ and Yb3+ codoped oxyfluoride glass ceramics containing CaF2 nanocrystals were prepared by melt quenching and subsequent heat treatment. Crystallization kinetics of CaF2 nanocrystals was investigated by differential scanning calorimetric method. The average apparent activation energy Ea of the crystallization was ∼ 498 kJ/mol. Moreover, the value of the Avrami exponent n was 1.01. These results suggest that the crystallization mechanism of CaF2 is a diffusion controlled growth process of needles and plates of finite long dimensions. X-ray diffraction patterns and transmission electron microscopy image confirmed the CaF2 nanocrystals in the glass ceramic. Ultraviolet (UV) and visible emission spectra of the as-made glass and the glass ceramic with an excitation of a 974 nm laser diode were recorded at room temperature. An intense UV emission at 381 nm was observed in the glass ceramic. The origin of the enhancement of the emission at 381 nm was investigated using spectroscopic technique and Judd-Ofelt analysis. The enhancement of the emission at 381 nm could be attributed to the change of the ligand field of Tb3+ ions due to the incorporation of some Tb3+ and Yb3+ ions into CaF2 nanocrystals in the glass ceramic.
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64.70.K- Solid-solid transitions
61.46.Hk Nanocrystals
81.16.-c Methods of micro- and nanofabrication and processing
81.05.Pj Glass-based composites, vitroceramics
66.30.H- Self-diffusion and ionic conduction in nonmetals
78.55.Hx Other solid inorganic materials

Photoluminescence-free photoreflectance spectra using dual frequency modulation

J. Plaza, D. Ghita, J. L. Castaño, and B. J. Garcia

J. Appl. Phys. 102, 093507 (2007); http://dx.doi.org/10.1063/1.2802991 (5 pages)

Online Publication Date: 5 November 2007

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Photoreflectance (PR) spectra are usually obtained by measuring the relative change on the reflectivity of a semiconducting sample induced by a chopped laser beam. The laser beam can also produce photoluminescence (PL) emission at the sample surface which, detected at its same frequency, could appear as an offset distorting the PR spectrum. This parasitic and intrinsically noisy PL signal, not easily discriminated electronically nor optically filtered, can become the dominant part of the PR spectrum at low sample temperatures, hiding spectrum features under its associated noise, or even avoiding data acquisition. An alternative method for PL signal discrimination is proposed in this work, using a different chopping frequency for each light beam: PL and reflected signals will appear each one at its own chopping frequency, while PR signal will be detected at its frequency sum, allowing signal separation by frequency. Both experimental setups are compared using a highly luminescent quantum well structure at low temperatures. While the standard setup suffers the PL limitation, the proposed method overcomes this constraint, allowing good quality spectra to be measured at temperatures as low as 12 K.
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07.60.Rd Visible and ultraviolet spectrometers
42.60.Fc Modulation, tuning, and mode locking
78.67.De Quantum wells

Grain size effect on the switching current in soft ferroelectric lead zirconate titanate

Talal M. Kamel and Gijsbertus de With

J. Appl. Phys. 102, 093508 (2007); http://dx.doi.org/10.1063/1.2803878 (3 pages)

Online Publication Date: 5 November 2007

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Recently, we reported on the appearance of a double peak in the switching current during the reverse poling. In the present paper, the switching current measurements have been carried out on a soft lead zirconate titanate as a function of grain size. While in small grains only a small single switching peak is observed, large grains, however, showed double peak switching, as commonly observed in this material. The pyroelectric coefficient curves show a consistent trend with the switching curves. This behavior is attributed to non-180° domain switching during the reversed poling case as a result of residual stresses developed during forward poling.
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77.84.-s Dielectric, piezoelectric, ferroelectric, and antiferroelectric materials
77.70.+a Pyroelectric and electrocaloric effects
77.80.Fm Switching phenomena
77.80.Dj Domain structure; hysteresis
77.22.Ej Polarization and depolarization

Micro-photoluminescence characterizations of GaInAsP/InP single quantum wires fabricated by dry etching and regrowth

Hirotake Itoh, Masahiro Yoshita, Hidefumi Akiyama, Dhanorm Plumwongrot, Takeo Maruyama, and Shigehisa Arai

J. Appl. Phys. 102, 093509 (2007); http://dx.doi.org/10.1063/1.2804107 (5 pages) | Cited 2 times

Online Publication Date: 5 November 2007

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We characterized the micro-photoluminescence (micro-PL) of GaInAsP/InP single quantum wires (Q-wires) at 6 K. The Q-wires had 6–39-nm lateral widths and a 6-nm vertical thickness and were fabricated by dry etching and regrowth. Micro-PL spectra were clearly observed for all single Q-wires. The spectra revealed a high degree of uniformity along the Q-wires. The PL peak energy showed a systematic blueshift with the reduction of the Q-wire lateral width. The blueshift was attributed to the lateral quantum confinement and strain from a lateral direction, and was 100 meV in a 6-nm-wide Q-wire. Systematic analysis on the PL widths of the Q-wires showed that the PL spectra were broadened by fluctuations of 3 nm in the lateral width of the Q-wire after the dry etching and the regrowth in addition to the broadening caused by the fluctuation in the vertical thickness during the initial film growth and that caused by ion bombardment through a Ti metal mask in the dry etching process. The decreased PL intensities in narrow Q-wires were also analyzed, and they were attributed to decreased PL quantum yield because of damage to the etched sidewalls of the Q-wires during dry etching, and to the absorption cross-sections of the excitation light.
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78.67.Lt Quantum wires
81.07.Vb Quantum wires
78.55.Cr III-V semiconductors
81.65.Cf Surface cleaning, etching, patterning

Defect behavior induced by helium cluster growth in titanium crystals

Jun Wang, Qing Hou, Tieying Sun, Xinggui Long, Xingchun Wu, and Shunzhong Luo

J. Appl. Phys. 102, 093510 (2007); http://dx.doi.org/10.1063/1.2804110 (4 pages) | Cited 6 times

Online Publication Date: 5 November 2007

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The growth of helium cluster in titanium crystals is simulated in great detailed approach using molecular dynamics. We observe that, as the helium cluster grows, defects around the cluster are formed and the local pressure increases. However, at certain point in the growth process, the defects are found to rapidly escape as a whole from the helium cluster, accompanied by the relief of local high pressure and the recovery of Ti crystal structure around the helium cluster.
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61.46.Bc Structure of clusters (e.g., metcars; not fragments of crystals; free or loosely aggregated or loosely attached to a substrate)
61.50.Ah Theory of crystal structure, crystal symmetry; calculations and modeling
62.50.-p High-pressure effects in solids and liquids
64.70.K- Solid-solid transitions
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