Volume 101, Issue 9, 01 May 2007
- lasers, optics, and optoelectronics
- plasmas and electrical discharges
- structural, mechanical, thermodynamic, and optical properties of condensed matter
- electronic structure and transport
- magnetism and superconductivity
- dielectrics and ferroelectricity
- nanoscale science and design
- device physics
- applied biophysics
- interdisciplinary and general physics
- proceedings of the tenth joint mmm/intermag conference
Index of content:
- LASERS, OPTICS, AND OPTOELECTRONICS
Triarylamine siloxane anode functionalization/hole injection layers in high efficiency/high luminance small-molecule green- and blue-emitting organic light-emitting diodes101(2007); http://dx.doi.org/10.1063/1.2719276View Description Hide Description
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 -based OLEDs. Green-emitting OLEDs with maximum luminance , power and forward external quantum efficiencies as high as and , respectively, and turn-on voltages are achieved in devices of the structure, -diphenyl--bis(-trichlorosilylpropylphenyl)(1,-biphenyl)-4,-diamine /1,4-bis(1-naphthylphenylamino)biphenyl (NPB)/Alq doped with -di(3-heptyl)quinacridone (DIQA)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline . Also, bright and efficient blue-emitting OLEDs with turn-on voltages , maximum luminance , and and power and external forward quantum efficiencies, respectively, are achieved in devices of the structure, . interlayers are fabricated by spin casting -diphenyl--bis(-trichlorosilylpropylphenyl)(1,-biphenyl)-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 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.
Mid-infrared high finesse microcavities and vertical-cavity lasers based on IV–VI semiconductor/ broadband Bragg mirrors101(2007); http://dx.doi.org/10.1063/1.2720096View Description Hide Description
We report on molecular beam epitaxially grown high-reflectivity broadband Bragg mirrors for mid-infrared devices using IV–VI semiconductors and . 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 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 mirrorreflectivity 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 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.
Optical response of a strongly anisotropic thin film as a nonmagnetic negative phase velocity material101(2007); http://dx.doi.org/10.1063/1.2717124View Description Hide Description
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 and medium 3 of refraction index. As a second case, the anisotropicthin film and medium 3 are interchanged. These geometries correspond to the Kretchmann [E. Kretschmann and H. Raether, Z. Naturforsch. A23, 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 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 anisotropicmaterial and the medium with refraction index. In the second region, the structure of the reflectivity is due to the propagation of light in the highly anisotropicmaterial, with negative phase velocity.
Optical gain, loss, and transparency current in high performance mid-infrared interband cascade lasers101(2007); http://dx.doi.org/10.1063/1.2723188View Description Hide Description
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 to 270 K. In this temperature range, the optical loss of IC lasers increases from at to at , the transparency current density rises from to , and the differential gain decreases from to with a characteristictemperature of . The implications of these observed characteristics for IC lasers are discussed.
101(2007); http://dx.doi.org/10.1063/1.2734885View Description Hide Description
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 silvernanoparticles 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 and up to 16-fold enhancement at for 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.
Effect of elastic strain redistribution on electronic band structures of compressively strained GaInAsP/InP membrane quantum wires101(2007); http://dx.doi.org/10.1063/1.2723868View Description Hide Description
The effect of redistribution of elasticstrain 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 epitaxialgrowth is theoretically studied using an 8-band 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 wirestructures 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.
101(2007); http://dx.doi.org/10.1063/1.2728764View Description Hide Description
A fiber-optic confocal sensor for noncontactballisticmeasurements is described. Determination of motion at velocities of 1.7 km/s with an uncertainty as small as 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.
Optical properties and ferroelectric engineering of vapor-transport-equilibrated, near-stoichiometric lithium tantalate for frequency conversion101(2007); http://dx.doi.org/10.1063/1.2723867View Description Hide Description
Near-stoichiometric lithiumtantalate (SLT) crystals were produced from congruent lithiumtantalate 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.
101(2007); http://dx.doi.org/10.1063/1.2730566View Description Hide Description
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.
GaN hybrid microcavities in the strong coupling regime grown by metal-organic chemical vapor deposition on sapphire substrates101(2007); http://dx.doi.org/10.1063/1.2728744View Description Hide Description
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 distributed Bragg reflectors(DBRs) on the lower side of the cavity and 9 pairs of as the upper mirror, to obtain cavity 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 reflectivityspectra of the Bragg mirrors are not significantly limited by absorption in the layers.
On the performance of different bimetallic combinations in surface plasmon resonance based fiber optic sensors101(2007); http://dx.doi.org/10.1063/1.2721779View Description Hide Description
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.
- PLASMAS AND ELECTRICAL DISCHARGES
101(2007); http://dx.doi.org/10.1063/1.2720256View Description Hide Description
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, dielectricheating, 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.
101(2007); http://dx.doi.org/10.1063/1.2724796View Description Hide Description
A series of time resolved microwave attenuation measurements are performed of the electron number density of an electron beam generated, CO laser excited nonequilibrium 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 ( vs ). 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 scatteringmeasurements are also presented, which exhibit minimal temperature change , 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 detachment by four to five orders of magnitude, compared to the equilibrium value.
101(2007); http://dx.doi.org/10.1063/1.2724240View Description Hide Description
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 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 dioxideplasma. 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 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 .
- STRUCTURAL, MECHANICAL, THERMODYNAMIC, AND OPTICAL PROPERTIES OF CONDENSED MATTER
Fourier analysis applied on in situ laser reflectometry during III-nitride metal organic chemical vapor deposition growth101(2007); http://dx.doi.org/10.1063/1.2722247View Description Hide Description
In this article we present the Fourier analysis of kinetic reflectometryspectra 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 -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.
101(2007); http://dx.doi.org/10.1063/1.2724793View Description Hide Description
Periodic interlayers were employed during the metal-organic chemical vapor deposition of epitaxialGaN 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 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 interlayer characterized using cross-sectional transmission electron microscopy, we observed a different behavior induced by the interlayers: Si-rich inverted hexagonal pyramids with their base on the (0001) plane and six sidewalls on the plane were found near the top surface of the GaNfilm at the location of insertion layer characterized using electron energy loss spectroscopy. The preferential deposition of the 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 .
101(2007); http://dx.doi.org/10.1063/1.2721784View Description Hide Description
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.
101(2007); http://dx.doi.org/10.1063/1.2717550View Description Hide Description
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.
101(2007); http://dx.doi.org/10.1063/1.2718872View Description Hide Description
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 siliconpowders with a size distribution of were ejected to the molten silicondroplets at a selected part of the free-fall tube using argon carrier gas. It was considered that the attached siliconpowders 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 silicondroplets at solidification was speculated to decrease from to below 50 °C by seeding power ejection. This resulted in an increase of average minority carrier lifetime from to .
101(2007); http://dx.doi.org/10.1063/1.2722251View Description Hide Description
Presented here is an explanation for the use of a commercial optical scanner for the mapping of doping density within SiC substrates and as a local probe for variations. This method provides a fast and cost effective method for determining 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 luminance values with . It is shown that features presented in the calculated maps strongly correlate to those observed in Lehighton resistivity maps.