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1 May 2007

Volume 101, Issue 9, Articles (09xxxx)

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Triarylamine siloxane anode functionalization/hole injection layers in high efficiency/high luminance small-molecule green- and blue-emitting organic light-emitting diodes

Qinglan Huang, Jianfeng Li, Tobin J. Marks, Guennadi A. Evmenenko, and Pulak Dutta

J. Appl. Phys. 101, 093101 (2007); http://dx.doi.org/10.1063/1.2719276 (13 pages) | Cited 5 times

Online Publication Date: 2 May 2007

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High efficiency/high luminance small-molecule organic light-emitting diodes (OLEDs) are fabricated by combining thin, covalently bound triarylamine hole injection/adhesion interlayers with hole- and exciton-blocking/electron transport interlayers in tris(8-hydroxyquinolato)aluminum(III) (Alq) and tetrakis(2-methyl-8-hydroxyquinolinato)borate (BQ4−)-based OLEDs. Green-emitting OLEDs with maximum luminance ∼ 85 000 cd/m2, power and forward external quantum efficiencies as high as 15.2 lm/W and 4.4±0.5%, respectively, and turn-on voltages ∼ 4.5 V are achieved in devices of the structure, ITO/N,N′-diphenyl-N,N′-bis(p-trichlorosilylpropylphenyl)(1,1′-biphenyl)-4,4′-diamine (TPD-Si2)/1,4-bis(1-naphthylphenylamino)biphenyl (NPB)/Alq doped with N,N′-di(3-heptyl)quinacridone (DIQA)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/Li/AgMg. Also, bright and efficient blue-emitting OLEDs with turn-on voltages ∼ 5.0 V, maximum luminance ∼ 30 000 cd/m2, and ∼ 5.0 lm/W and 1.6±0.2% power and external forward quantum efficiencies, respectively, are achieved in devices of the structure, ITO/TPD-Si2/NPB/BQ4−/BCP/Li/Al. TPD-Si2 interlayers are fabricated by spin casting N,N′-diphenyl-N,N′-bis(p-trichlorosilylpropylphenyl)(1,1′-biphenyl)-4,4′-diamine onto the ITO surface, while BCP interlayers are introduced by thermal evaporation. The excellent OLED performance is attributed to the differing functions of the above two interlayers: (1) The TPD-Si2 layer has a direct impact on hole injection by reducing the injection barrier and improving interfacial cohesion, and an indirect but strong effect on electron injection by altering internal electric fields. (2) The BCP layer, doped with lithium, directly reduces the electron injection barrier. Incorporation of both interlayers in OLED structures affords synergistically enhanced hole/electron injection and recombination efficiency. The results demonstrate a strategy to enhance OLED performance and an alternative strategy to increase electron density in electron-limited devices.
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85.60.Jb Light-emitting devices

Mid-infrared high finesse microcavities and vertical-cavity lasers based on IV–VI semiconductor/BaF2 broadband Bragg mirrors

T. Schwarzl, M. Eibelhuber, W. Heiss, E. Kaufmann, G. Springholz, A. Winter, and H. Pascher

J. Appl. Phys. 101, 093102 (2007); http://dx.doi.org/10.1063/1.2720096 (5 pages) | Cited 5 times

Online Publication Date: 2 May 2007

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We report on molecular beam epitaxially grown high-reflectivity broadband Bragg mirrors for mid-infrared devices using IV–VI semiconductors and BaF2. This material combination exhibits a high ratio between the refractive indices of up to 3.5, leading to a broad mirror stop band with a relative width of 75%. To verify the high quality of the PbEuTe/BaF2 Bragg mirrors, we study a half-wavelength microcavity formed by mirrors with only three periods. The resonance of the microcavity has a narrow linewidth of 5.2 nm corresponding to a very high finesse of 750. From this, a mirror reflectivity higher than 99.7% is deduced, in good agreement to transfer matrix simulations. Furthermore, we demonstrate mid-infrared continuous-wave vertical-cavity surface-emitting lasers based on these mirrors. Optical excitation of laser structures with a PbSe active region results in stimulated emission at various cavity modes between 7.3 and 5.9 μm at temperatures between 54 and 135 K. Laser emission is evidenced by a strong linewidth narrowing with respect to the linewidth of the cavity mode and a clear laser threshold at a pump power of 130 mW at 95 K.
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42.79.Bh Lenses, prisms and mirrors
42.55.Px Semiconductor lasers; laser diodes
42.60.Da Resonators, cavities, amplifiers, arrays, and rings

Optical response of a strongly anisotropic thin film as a nonmagnetic negative phase velocity material

P. H. Hernández, G. Martínez, Gregorio H. Cocoletzi, H. Azucena-Coyotecatl, and J. Díaz-Hernández

J. Appl. Phys. 101, 093103 (2007); http://dx.doi.org/10.1063/1.2717124 (4 pages)

Online Publication Date: 4 May 2007

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We study the optical response of a strongly anisotropic nonmagnetic material. As a first case, it is considered an anisotropic nonmagnetic thin film (medium 2) sandwiched between medium 1 of refraction index n1 and medium 3 of refraction index n3. As a second case, the anisotropic thin film and medium 3 are interchanged. These geometries correspond to the Kretchmann [ E. Kretschmann and H. Raether, Z. Naturforsch. A 23, 2135 (1968) ; E. Kretschmann, Z. Phys. 241, 313 (1971) ] and Otto [ A. Otto, Z. Phys. 216, 398 (1968) ] attenuated total reflectivity arrays, respectively. Our main goal is to obtain a negative phase velocity propagation. To achieve this we choose an index of refraction n1>1 and show how the appropriate angle of incidence allows us to induce a negative phase velocity nonmagnetic material. We focused our attention in two distinct regions that emerge in the reflectivity curves when varying the angle of incidence. In the first region, the minima of the reflectivity are interpreted in terms of the coupling of light with the modes at the interface between the anisotropic material and the medium with refraction index n3. In the second region, the structure of the reflectivity is due to the propagation of light in the highly anisotropic material, with negative phase velocity.
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42.70.-a Optical materials
42.79.Wc Optical coatings

Optical gain, loss, and transparency current in high performance mid-infrared interband cascade lasers

Alex Soibel, Kamjou Mansour, Yueming Qiu, Cory J. Hill, and Rui Q. Yang

J. Appl. Phys. 101, 093104 (2007); http://dx.doi.org/10.1063/1.2723188 (4 pages) | Cited 11 times

Online Publication Date: 4 May 2007

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The net modal gain, optical loss, and transparency current of high-performance, narrow ridge waveguide interband cascade (IC) lasers have been measured using the Hakki–Paoli technique in the temperature range from T = 78 to 270 K. In this temperature range, the optical loss of IC lasers increases from αw ≈ 17 cm−1 at T = 78 K to αw ≈ 35 at T = 270 K, the transparency current density rises from Itr = 10 to 330 A/cm2, and the differential gain decreases from gd ≈ 2.2 cm/A to gd ≈ 0.06 cm/A with a characteristic temperature of T0 = 130 K. The implications of these observed characteristics for IC lasers are discussed.
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42.55.Px Semiconductor lasers; laser diodes
42.60.By Design of specific laser systems

Surface plasmon enhanced silicon solar cells

S. Pillai, K. R. Catchpole, T. Trupke, and M. A. Green

J. Appl. Phys. 101, 093105 (2007); http://dx.doi.org/10.1063/1.2734885 (8 pages) | Cited 283 times

Online Publication Date: 7 May 2007

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Thin-film solar cells have the potential to significantly decrease the cost of photovoltaics. Light trapping is particularly critical in such thin-film crystalline silicon solar cells in order to increase light absorption and hence cell efficiency. In this article we investigate the suitability of localized surface plasmons on silver nanoparticles for enhancing the absorbance of silicon solar cells. We find that surface plasmons can increase the spectral response of thin-film cells over almost the entire solar spectrum. At wavelengths close to the band gap of Si we observe a significant enhancement of the absorption for both thin-film and wafer-based structures. We report a sevenfold enhancement for wafer-based cells at λ = 1200 nm and up to 16-fold enhancement at λ = 1050 nm for 1.25 μm thin silicon-on-insulator (SOI) cells, and compare the results with a theoretical dipole-waveguide model. We also report a close to 12-fold enhancement in the electroluminescence from ultrathin SOI light-emitting diodes and investigate the effect of varying the particle size on that enhancement.
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84.60.Jt Photoelectric conversion
85.60.Jb Light-emitting devices

Effect of elastic strain redistribution on electronic band structures of compressively strained GaInAsP/InP membrane quantum wires

F. Ferdous and A. Haque

J. Appl. Phys. 101, 093106 (2007); http://dx.doi.org/10.1063/1.2723868 (5 pages)

Online Publication Date: 8 May 2007

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The effect of redistribution of elastic strain relaxation on the energy band structures of GaInAsP/InP compressively strained membrane quantum wires fabricated by electron-beam lithography, reactive-ion etching and two-step epitaxial growth is theoretically studied using an 8-band k⋅p method. Anisotropic strain analysis by the finite element method shows that due to etching away the top and the bottom InP clad layers in membrane structures, redistribution of strain occurs. It is found that strain redistribution increases the effective bandgap of membrane quantum wire structures causing a blueshift of the emission frequency. Comparison with effective bandgap calculations neglecting confinement and band mixing demonstrates that neglect of these effects leads to an overestimation of the change in the bandgap. We have also investigated the effect of variation of wire width, barrier strain compensation, number of stacked quantum wire layers, and thickness of the top and the bottom residual InP layers in membrane structures on the change in the effective bandgap of membrane structures.
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73.21.Hb Quantum wires
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.D- Elasticity
78.67.Lt Quantum wires
62.40.+i Anelasticity, internal friction, stress relaxation, and mechanical resonances
68.65.La Quantum wires (patterned in quantum wells)

Noncontact ballistic motion measurement using a fiber-optic confocal sensor

E. Shafir, G. Berkovic, Y. Horovitz, G. Appelbaum, E. Moshe, E. Horovitz, A. Skutelski, M. Werdiger, L. Perelmutter, and M. Sudai

J. Appl. Phys. 101, 093107 (2007); http://dx.doi.org/10.1063/1.2728764 (7 pages) | Cited 3 times

Online Publication Date: 8 May 2007

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A fiber-optic confocal sensor for noncontact ballistic measurements is described. Determination of motion at velocities of 1.7 km/s with an uncertainty as small as ±0.3% is demonstrated for both a projectile and a free-surface target. The fibers detect the passage of the object at their conjugate image points created by low F/# optics. This results in an output signal comprising a train of sharp pulses each precisely identifying when the ballistic object traverses an image point. Since the ballistic object does not contact the sensor at the time of imaging, the measurements do not perturb the motion, enabling multi-fragment measurement, as well as repetitive measurements of the same object point.
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06.30.Gv Velocity, acceleration, and rotation
42.81.Pa Sensors, gyros
06.30.Bp Spatial dimensions (e.g., position, lengths, volume, angles, and displacements)
07.60.Pb Conventional optical microscopes

Optical properties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium tantalate for frequency conversion

D. S. Hum, R. K. Route, G. D. Miller, V. Kondilenko, A. Alexandrovski, J. Huang, K. Urbanek, R. L. Byer, and M. M. Fejer

J. Appl. Phys. 101, 093108 (2007); http://dx.doi.org/10.1063/1.2723867 (12 pages) | Cited 16 times

Online Publication Date: 9 May 2007

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Near-stoichiometric lithium tantalate (SLT) crystals were produced from congruent lithium tantalate by vapor transport equilibration, and several important optical and ferroelectric properties were measured. The effect of vapor transport conditions and surface preparation on reproducible ferroelectric engineering of SLT has been studied. Control of these effects along with dramatic decreases in the sensitivity to photorefractive damage and 532 nm absorption has allowed near-room-temperature generation of 10 W of continuous wave 532 nm radiation by second harmonic generation from 29 W of 1064 nm radiation in a 4 cm long device.
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42.70.Gi Light-sensitive materials
42.65.Ky Frequency conversion; harmonic generation, including higher-order harmonic generation
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
77.80.-e Ferroelectricity and antiferroelectricity
61.66.Bi Elemental solids
61.66.Dk Alloys

Stimulated Raman scattering in tellurite glasses as a potential system for slow light generation

Guanshi Qin, Rajan Jose, and Yasutake Ohishi

J. Appl. Phys. 101, 093109 (2007); http://dx.doi.org/10.1063/1.2730566 (5 pages) | Cited 16 times

Online Publication Date: 10 May 2007

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In this article, slow light generation via stimulated Raman scattering (SRS) in tellurite-based glasses is investigated based on characterization of Raman gain coefficients and an evaluation method of slow light generation we developed. The effects of different heavy metal oxides additions on slow light generation via SRS in tellurite-based glasses are also discussed. Our results show that designed tellurite-based glass is a promising candidate for slow light generation via SRS.
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42.65.Dr Stimulated Raman scattering; CARS
42.65.Es Stimulated Brillouin and Rayleigh scattering
78.35.+c Brillouin and Rayleigh scattering; other light scattering
42.70.Ce Glasses, quartz

GaN hybrid microcavities in the strong coupling regime grown by metal-organic chemical vapor deposition on sapphire substrates

A. Alyamani, D. Sanvitto, A. A. Khalifa, M. S. Skolnick, T. Wang, F. Ranalli, P. J. Parbrook, A. Tahraoui, and R. Airey

J. Appl. Phys. 101, 093110 (2007); http://dx.doi.org/10.1063/1.2728744 (3 pages) | Cited 7 times

Online Publication Date: 11 May 2007

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We observe exciton-photon strong coupling at low and room temperature in the ultraviolet spectral region in a GaN-based one wavelength bulk microcavity. The hybrid cavity is composed of 25 pairs of epitaxially grown Al0.2Ga0.8N/Al0.6Ga0.4N distributed Bragg reflectors (DBRs) on the lower side of the cavity and 9 pairs of Si3N4/SiO2 as the upper mirror, to obtain cavity Q values up to 160. Anticrossing is observed between the cavity mode and the bulk GaN exciton, showing the formation of polariton modes with normal mode splitting of 43 meV. The lower polariton dispersion is observed in both reflectivity and photoluminescence, with good agreement between the two obtained over a large tuning range of 40 meV. Good fits are obtained to the spectra of the Bragg mirrors. From simulations it is found that the reflectivity spectra of the Bragg mirrors are not significantly limited by absorption in the Al0.2Ga0.8N layers.
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78.66.Fd III-V semiconductors
78.40.Fy Semiconductors
78.55.Cr III-V semiconductors
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
71.35.-y Excitons and related phenomena
71.36.+c Polaritons (including photon-phonon and photon-magnon interactions)

On the performance of different bimetallic combinations in surface plasmon resonance based fiber optic sensors

Anuj K. Sharma and B. D. Gupta

J. Appl. Phys. 101, 093111 (2007); http://dx.doi.org/10.1063/1.2721779 (6 pages) | Cited 19 times

Online Publication Date: 15 May 2007

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In the present work, we have investigated the capability of different bimetallic combinations to be used in a fiber optic sensor based on the technique of surface plasmon resonance. The metals considered for the present analysis are silver, gold, copper, and aluminum. The performance of the sensor with different bimetallic combinations is evaluated and compared numerically. The performance is analyzed in terms of three parameters: sensitivity, signal-to-noise ratio (SNR), and operating range. On the basis of the comparison and some logical criteria, the best possible bimetallic combination along with requisite thickness distribution is predicted. The bimetallic combination is capable of simultaneously providing the larger values of sensitivity, SNR, and operating range, which is not possible with any single metallic layer.
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42.81.Pa Sensors, gyros
73.20.Mf Collective excitations (including excitons, polarons, plasmons and other charge-density excitations)
07.60.Vg Fiber-optic instruments
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Vortical flow control on a conical fore body cross section using an array of pulsed dc actuators

Kunwar Pal Singh and Subrata Roy

J. Appl. Phys. 101, 093301 (2007); http://dx.doi.org/10.1063/1.2720256 (7 pages) | Cited 2 times

Online Publication Date: 1 May 2007

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Flow control on a conical fore body cross section of an aircraft is studied using plasma discharge by considering the neutral gas flow at 17.5 deg angle of attack. The equations governing the motion of electrons, ions as well as Poisson’s equation are solved together with Navier-Stokes and energy equation for neutrals to study flow control. A single barrier discharge actuator is not sufficient to control the flow on the entire length of the fore body. An arrangement of multiple electrodes powered with pulsed dc voltage has been suggested for controlling such flows. The effects of joule heating of plasma, dielectric heating, and electrodynamic force have been investigated, separately and then combined on flow control. It is found that joule heating results in high temperature of the dielectric surface, however; electrodynamic force contributes prominently to flow control. A three-dimensional analysis is necessary to validate results with experiments.
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47.85.ld Boundary layer control
47.32.cb Vortex interactions
47.85.Gj Aerodynamics
52.75.-d Plasma devices
52.80.-s Electric discharges
52.50.Nr Plasma heating by DC fields; ohmic heating, arcs

Mitigation of electron attachment to oxygen in high pressure air plasmas by vibrational excitation

K. Frederickson, W. Lee, P. Palm, I. V. Adamovich, J. W. Rich, and W. R. Lempert

J. Appl. Phys. 101, 093302 (2007); http://dx.doi.org/10.1063/1.2724796 (7 pages) | Cited 3 times

Online Publication Date: 3 May 2007

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A series of time resolved microwave attenuation measurements are performed of the electron number density of an electron beam generated, CO laser excited nonequilibrium O2/N2 plasma. Resonant absorption of infrared radiation from the CO laser produces the nonequilibrium state, in which the heavy species vibrational modes are disproportionately excited, compared to the rotational and translational modes (Tvib ≈ 2000–3000 K vs TR/T ≈ 300 K). It is shown that this results in an increase in the plasma free electron lifetime by two orders of magnitude compared to the unexcited cold gas, an effect which is ascribed to complete mitigation of rapid three-body electron attachment to molecular oxygen. A series of heavy species filtered pure rotational Raman scattering measurements are also presented, which exhibit minimal temperature change (+50 K), indicating that the observed lifetime increase cannot be due to heavy-species thermal effects. Finally, computational modeling results infer an increase in the rate of O2− detachment by four to five orders of magnitude, compared to the equilibrium value.
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52.20.Fs Electron collisions
52.20.Hv Atomic, molecular, ion, and heavy-particle collisions
52.70.Gw Radio-frequency and microwave measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.40.Mj Particle beam interactions in plasmas
52.38.Dx Laser light absorption in plasmas (collisional, parametric, etc.)

27.12 MHz plasma generation in supercritical carbon dioxide

Ayato Kawashima, Hiromichi Toyota, Shinfuku Nomura, Toshihiko Takemori, Shinobu Mukasa, Tsunehiro Maehara, and Hiroshi Yamashita

J. Appl. Phys. 101, 093303 (2007); http://dx.doi.org/10.1063/1.2724240 (4 pages) | Cited 9 times

Online Publication Date: 8 May 2007

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An experiment was conducted for generating high-frequency plasma in supercritical carbon dioxide; it is expected to have the potential for applications in various types of practical processes. It was successfully generated at 6−20 MPa using electrodes mounted in a supercritical cell with a gap of 1 mm. Emission spectra were then measured to investigate the physical properties of supercritical carbon dioxide plasma. The results indicated that while the emission spectra for carbon dioxide and carbon monoxide could be mainly obtained at a low pressure, the emission spectra for atomic oxygen could be obtained in the supercritical state, which increased with the pressure. The temperature of the plasma in supercritical state was estimated to be approximately 6000−7000 K on the assumption of local thermodynamic equilibrium and the calculation results of thermal equilibrium composition in this state showed the increase of atomic oxygen by the decomposition of CO2.
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52.50.Dg Plasma sources
52.70.Kz Optical (ultraviolet, visible, infrared) measurements
52.25.Os Emission, absorption, and scattering of electromagnetic radiation
52.25.Kn Thermodynamics of plasmas
82.33.Xj Plasma reactions (including flowing afterglow and electric discharges)
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
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Fourier analysis applied on in situ laser reflectometry during III-nitride metal organic chemical vapor deposition growth

C. Simbrunner, H. Sitter, and A. Bonanni

J. Appl. Phys. 101, 093501 (2007); http://dx.doi.org/10.1063/1.2722247 (7 pages)

Online Publication Date: 1 May 2007

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In this article we present the Fourier analysis of kinetic reflectometry spectra acquired during metalorganic chemical vapor deposition. We can show that offset errors due to background radiation can be completely removed by the method itself without using filters or lock-in amplifiers. Additionally, calibration of the reflected intensity is needed as long as the response of the detector is linear to the reflected intensity of the sample. By analyzing the time dependent part of the signal growth rate, layer thickness and the refractive index of the growing layer can be deduced. We demonstrate that, by applying the method to the GaN:Mg δ-doping process, it is possible to obtain accurate information about the time, optical properties, and thickness of the grown multilayers with a resolution down to the monolayer range.
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78.67.Pt Multilayers; superlattices; photonic structures; metamaterials
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
81.05.Ea III-V semiconductors
61.72.uj III-V and II-VI semiconductors
68.65.Ac Multilayers

Defect reduction by periodic SiNx interlayers in gallium nitride grown on Si (111)

K. Y. Zang, Y. D. Wang, L. S. Wang, S. Y. Chow, and S. J. Chua

J. Appl. Phys. 101, 093502 (2007); http://dx.doi.org/10.1063/1.2724793 (4 pages) | Cited 3 times

Online Publication Date: 1 May 2007

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Periodic SiNx interlayers were employed during the metal-organic chemical vapor deposition of epitaxial GaN on AlN buffer layers grown on Si (111) substrates. The growth and the evolution of defects were studied in this paper. A reduction of the threading dislocation density to ∼ 109 cm−2 was observed on the surface of GaN by counting the surface pit density from the atomic force microscopy results. Besides the observation of the continuous bending and subsequent recombination of the threading dislocations related to the periodic conduction of the SiNx interlayer characterized using cross-sectional transmission electron microscopy, we observed a different behavior induced by the SiNx interlayers: Si-rich inverted hexagonal pyramids with their base on the (0001) plane and six sidewalls on the (10math1) plane were found near the top surface of the GaN film at the location of SiNx insertion layer characterized using electron energy loss spectroscopy. The preferential deposition of the SiNx on the sidewalls of the pit defects leads to the subsequently selective growth of the GaN beyond the pit defects, which leads to the burying of the pits and the reduction of the pit defects within the film due to the micromasking effect of the SiNx.
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68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
68.55.A- Nucleation and growth
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.40.Lm Deformation, plasticity, and creep
62.20.F- Deformation and plasticity

Photoluminescence and self-interference in germanium-doped silica films

Y. M. Yang, L. W. Yang, M. Q. Cai, and Paul K. Chu

J. Appl. Phys. 101, 093503 (2007); http://dx.doi.org/10.1063/1.2721784 (4 pages) | Cited 3 times

Online Publication Date: 2 May 2007

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Germanium-doped silica films were prepared by magnetron cosputtering and postannealing. The photoluminescence properties and their dependence on the Ge contents and annealing temperature were investigated. Our experiments indicate that the observed light emission originates from the neutral oxygen vacancy defects. The substructures in the luminescence bands of the films were found to result from multiple-beam interferences of the emission in the optical cavity formed by the transparent films.
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78.66.Nk Insulators
78.55.Hx Other solid inorganic materials
68.55.Ln Defects and impurities: doping, implantation, distribution, concentration, etc.
68.55.-a Thin film structure and morphology
81.40.Gh Other heat and thermomechanical treatments

Scaling of fracture energy in tensile debonding of viscoelastic films

Muralidhar Seshadri, Sunil Saigal, Anand Jagota, and Stephen J. Bennison

J. Appl. Phys. 101, 093504 (2007); http://dx.doi.org/10.1063/1.2717550 (8 pages) | Cited 2 times

Online Publication Date: 3 May 2007

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The work done to separate viscoelastic adherends is often dominated by energy dissipation due to the bulk deformation that accompanies the intrinsic processes of interfacial separation. The inter-relationship between bulk and interfacial deformation processes is studied here by analyzing a one-dimensional model for steady-state crack propagation between a rigid substrate and a thin viscoelastic film when the latter is subjected to tensile loading and the former is fixed. The viscoelastic layer is represented by a standard linear solid and is connected to the rigid substrate via a Dugdale cohesive zone model. The principal result of the analysis is a prediction for the dependence of the total work of fracture on the rate of loading. A threshold crack-tip velocity that governs the onset of dissipation is determined as a function of the film thickness and the interfacial and viscoelastic parameters of the film. Based on the ratio of the crack-tip velocity to the threshold velocity, three velocity regimes are identified where the energy dissipation is low, high, or intermediate. These correspond, respectively, to the overlap of the cohesive zone with the film material that is completely relaxed, is completely unrelaxed, or is in the process of relaxation. An approximate solution for the scaling of fracture energy in these three regimes has been presented. Finally, the relevance of these results to a two-dimensional problem is discussed.
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46.50.+a Fracture mechanics, fatigue and cracks
46.35.+z Viscoelasticity, plasticity, viscoplasticity
46.55.+d Tribology and mechanical contacts

Seeding method with silicon powder for the formation of silicon spheres in the drop method

Zhengxin Liu, Takehiko Nagai, Atsushi Masuda, Michio Kondo, Kazutoshi Sakai, and Koichi Asai

J. Appl. Phys. 101, 093505 (2007); http://dx.doi.org/10.1063/1.2718872 (5 pages) | Cited 6 times

Online Publication Date: 3 May 2007

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Silicon spheres with a size distribution around 1.0 mm diameter, which are applicable to spherical solar cells, were formed by dropping molten silicon through a nozzle in a free-fall tube, namely, the drop method. Here we show a seeding technique for the formation of silicon spheres. In this technique, pure silicon powders with a size distribution of 1−75 μm were ejected to the molten silicon droplets at a selected part of the free-fall tube using argon carrier gas. It was considered that the attached silicon powders on the droplets worked as nuclei and stimulated the solidification to occur at low undercooling from one place. Characterizations with scanning electron microscope, carrier lifetime, and photoluminescence measurements demonstrated that the crystallinity of silicon spheres were significant improved by the seeding method. The undercooling of molten silicon droplets at solidification was speculated to decrease from ∼ 250 °C to below 50 °C by seeding power ejection. This resulted in an increase of average minority carrier lifetime from <0.1 μs  to >1.0 μs.
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81.10.Fq Growth from melts; zone melting and refining
81.30.Fb Solidification
64.70.D- Solid-liquid transitions
78.55.Ap Elemental semiconductors
72.20.Jv Charge carriers: generation, recombination, lifetime, and trapping
61.66.Bi Elemental solids

Free carrier distribution profiling of 4H-SiC substrates using a commercial optical scanner

Joshua D. Caldwell, Orest J. Glembocki, Sharka M. Prokes, Evan R. Glaser, Karl D. Hobart, Darren M. Hansen, Gilyong Chung, Alexander V. Bolotnikov, and Tangali S. Sudarshan

J. Appl. Phys. 101, 093506 (2007); http://dx.doi.org/10.1063/1.2722251 (7 pages) | Cited 2 times

Online Publication Date: 3 May 2007

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Presented here is an explanation for the use of a commercial optical scanner for the mapping of doping density (ND) within SiC substrates and as a local probe for ND variations. This method provides a fast and cost effective method for determining ND homogeneity, examining local electrical characteristics, and recognizing defect sites including areas of different polytypes or polycrystallinity. Hall effect and micro-Raman spectroscopy were used to calibrate the transmission amplitude, integrated area and scanner red, green, blue (RGB) luminance values with ND. It is shown that features presented in the calculated ND maps strongly correlate to those observed in Lehighton resistivity maps.
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61.72.S- Impurities in crystals
72.20.My Galvanomagnetic and other magnetotransport effects
78.30.Hv Other nonmetallic inorganics

On the influence of loading profile upon the tensile failure of stainless steel

G. T. (Rusty) Gray, III, N. K. Bourne, and B. L. Henrie

J. Appl. Phys. 101, 093507 (2007); http://dx.doi.org/10.1063/1.2720099 (9 pages) | Cited 2 times

Online Publication Date: 3 May 2007

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A material placed in direct contact with a high explosive experiences a Taylor wave (triangular-shaped) shock loading profile. While a large number of studies have probed the structure, properties, and tensile response of materials subjected to square-topped shock loading pulses histories, few studies have systematically quantified the influence of shock-wave profile shape on material response. Samples of 316L stainless steel were shock loaded to peak stresses of 6.6, 10.2, and 14.5 GPa to examine the influence of square-topped and triangular (Taylor wave)-shaped pulse loading on the dynamic tensile behavior (spallation). The 316L SS samples were loaded with a square-topped pulse to each peak shock stress, using a pulse duration of 0.9 μs. They displayed increasing incipient spallation damage with increasing peak stress. Samples loaded to the peak shock stresses of 6.6 and 10.2 GPa with a Taylor-wave loading pulse (which immediately unloads the sample after the peak Hugoniot stress is achieved) exhibited no damage. Only the 14.5 GPa Taylor pulse shocked sample exhibited both a pull-back signal and incipient damage following tensile loading. The damage evolution in the square-topped shocked samples was found to be a mixture of void and strain localization damage, the void fraction increasing with peak shock amplitude. With the Taylor-wave loading profile of amplitude 14.5 GPa, a high incidence of shear localization and low incidence of void formation was observed. Detailed analysis of the damage evolution as a function of shock pulse shape revealed that a nominally equivalent level of incipient damage was obtained using a Taylor-wave or square-topped loading pulse when a similar rear sample surface stress-time total impulse was applied. In order to induce equivalent damage with the two pulse shapes, the impulse applied needed to be nominally matched. For this to occur, the Taylor-wave profile required twice the amplitude of the square one and the durations of each pulse needed to be appropriately scaled. Detailed metallographic, microtextural, and void shape and size analyses of the damage evolution are presented as a function of the inferred loading pulse shape and the peak Hugoniot stress.
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62.50.-p High-pressure effects in solids and liquids
81.40.Lm Deformation, plasticity, and creep
81.40.Jj Elasticity and anelasticity, stress-strain relations
62.20.F- Deformation and plasticity
61.72.Qq Microscopic defects (voids, inclusions, etc.)

Experimental determination of the thermal conductivity of liquid UO2 near the melting point

M. Sheindlin, D. Staicu, C. Ronchi, L. Game-Arnaud, B. Remy, and A. Degiovanni

J. Appl. Phys. 101, 093508 (2007); http://dx.doi.org/10.1063/1.2721091 (9 pages) | Cited 2 times

Online Publication Date: 3 May 2007

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The article gives an account of measurements of the thermal conductivity of liquid UO2. The sample was heated up to above the melting point by a laser pulse of a controlled shape, and the produced thermogram of temperature history was measured by a fast and accurate pyrometer with a time resolution of 10 μs. The experiment shows that the rate of temperature increase during the ascending part of the pulse changes moderately across the melting point. Due to the high power input, this effect cannot be explained in terms of the sole intake of latent heat of fusion. By solving the related heat transfer equation with a 2D-axisymmetric numerical model, it is demonstrated that this feature depends principally on heat conduction in the sample, and proves that the thermal conductivities of solid and liquid are not very different. A theoretical sensitivity study assessing the influence of the liquid thermal conductivity on the pulse temperature evolution showed that the conductivity of the liquid can be deduced from the fitting of the thermograms with a numerical precision of the order of 1%. The analysis reveals that the thermal conductivity is weakly correlated with the effective heat losses during the pulse and to the melting enthalpy, so that the uncertainty in its evaluation by fitting the experimental thermograms with model predictions is satisfactory. The value of the thermal conductivity of liquid UO2 near the melting point resulted to be 2.6±0.35 W m−1 K−1, where the magnitude of the uncertainty is much lower than the scatter of the previously published, discordant measurements.
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66.25.+g Thermal conduction in nonmetallic liquids
64.70.D- Solid-liquid transitions
82.60.Cx Enthalpies of combustion, reaction, and formation

Co-doped anatase TiO2 nanofibers fabricated by electrospinning

C. W. Jia, E. Q. Xie, J. G. Zhao, and H. G. Duan

J. Appl. Phys. 101, 093509 (2007); http://dx.doi.org/10.1063/1.2724535 (4 pages) | Cited 5 times

Online Publication Date: 4 May 2007

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Co-doped TiO2 nanofibers with an average diameter of ∼ 70 nm were fabricated with electrospinning method. X-ray diffraction measurements show that the nanofibers possess pure anatase structure. The obtained nanofibers exhibit evident room temperature ferromagnetism through magnetic measurement. The photoluminescence of the Co-doped TiO2 nanofibers is composed of two emission bands attributed to self-trapped excitons and oxygen vacancies, different from that of pure TiO2 nanofibers which only includes emission due to self-trapped excitons. Photoluminescence intensity due to oxygen vacancies vary with annealing atmosphere and Co concentration, and the ferromagnetic moment increases with the increment of oxygen vacancies.
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61.46.-w Structure of nanoscale materials
75.50.Pp Magnetic semiconductors
75.75.-c Magnetic properties of nanostructures
75.50.Dd Nonmetallic ferromagnetic materials
78.67.-n Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures
78.55.Hx Other solid inorganic materials

Compressional wave generation in droplets of water deposited on a quartz crystal: Experimental results and numerical calculations

G. Couturier, R. Boisgard, C. Jai, and J. P. Aimé

J. Appl. Phys. 101, 093510 (2007); http://dx.doi.org/10.1063/1.2730556 (5 pages) | Cited 2 times

Online Publication Date: 7 May 2007

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The generation of shear and compressional waves in liquids by AT-cut quartz crystals has received much attention in the past; however, this is not the case for droplets deposited onto quartz crystals. In this paper, we investigate the compressional wave generation in droplets and use different techniques to correlate the compressional wave generation to the shape of the droplets using frequency and dissipation measurements in an oscillator circuit while using a video camera to record the shape during evaporation. A good correlation between eigenmodes predicted by a finite element method and those obtained experimentally are observed.
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47.35.De Shear waves
47.55.dr Interactions with surfaces
47.80.Jk Flow visualization and imaging
47.11.Fg Finite element methods
64.70.F- Liquid-vapor transitions
02.70.Dh Finite-element and Galerkin methods

Determination of stacking fault probability in fcc Fe–Mn–Si–Al alloy by electron diffraction

X. D. Wang, B. X. Huang, Y. H. Rong, and L. Wang

J. Appl. Phys. 101, 093511 (2007); http://dx.doi.org/10.1063/1.2655682 (5 pages) | Cited 1 time

Online Publication Date: 8 May 2007

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The stacking disorder in the fcc structure of Fe–23Mn–2Si–2Al alloy after tensile testing at −75 °C was investigated by electron diffraction, using a relationship between the stacking fault probability and the shift of diffraction spots deduced in the present work (and based on Kakinoki’s theory [ Acta Crystallogr. 23, 875 (1967) ] and Kajiwara’s previous work [ Jpn. J. Appl. Phys. 9, 385 (1970); J. Phys. Soc. Jpn. 22, 795 (1967) ]). Shifts of diffraction spots along the [111] direction in the fcc structure were observed, and the stacking fault probabilities in two selected areas with different densities of stacking faults were determined as α = 0.15 and α = 0.35. The stacking fault probabilities measured by electron diffraction are much larger than average values determined by x-ray diffraction, indicating that the distribution of stacking faults is localized. A mechanism for the γfcc→εhcp transformation is suggested, whereby during deformation the following sequences take place: dissociation of perfect dislocation→localization of stacking faults→evolution from stacking disorder to stacking order→the formation of the perfect hcp martensite (α = 1) or the hcp martensite with stacking faults (α→1). This proposed mechanism for strain-induced hcp martensite formation should be also applicable to the thermally induced hcp martensite.
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61.72.Nn Stacking faults and other planar or extended defects
64.70.K- Solid-solid transitions
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