bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantum-well optical modulator at terahertz frequencies
Authors: Maslov A. V. , Citrin D. S.
Published: June 2003

We predict that strong modulation of the optical properties of quantum wells at terahertz frequencies is possible. In this regime, the coherent nonlinear mixing of the optical and the terahertz-modulation fields leads to strong modification of the frequency dependence of the modulation depth.
Improved brightness and efficiency in Ce codoped ZnS:TbOF electroluminescent thin films
Authors: Kim J. P. , Davidson M. R. , Holloway P. H.
Published: June 2003

ZnS:TbOF alternating current thin film electroluminescent devices codoped with Ce by rf magnetron sputtering were shown to be brighter than Tb singly doped films. This brightness improvement is shown to results predominantly from increased radiative efficiency, with a secondary improvement in excitation efficiency. At 20 V above threshold for electroluminescence, V20, the radiative efficiency of Ce codoped films is improved ≈130%. The nonlinear exponential decay and improved efficiency of electroluminescent emission for Ce codoped films is attributed to energy transfer from Ce+3 to Tb+3 luminescent centers. The improved excitation efficiency is attributed to reduced space charge.
Size dependence of luminescent properties for hexagonal YBO3:Eu nanocrystals in the vacuum ultraviolet region
Authors: Wei Zheng-Gui , Sun Ling-Dong , Liao Chun-Sheng , Jiang Xiao-Cheng , Yan Chun-Hua , Tao Ye , Hou Xue-Ying , Ju Xin
Published: June 2003

The vacuum ultraviolet (VUV) spectra of the nanosized YBO3:Eu crystals, which have already been prepared by a more facile sol–gel pyrolysis process at a relatively low annealing temperature in our previous work, were investigated and compared to the bulk synthesized by conventional solid-state reactions. In the emission spectra, the size dependence of color purity in YBO3:Eu under VUV irradiation is in good agreement with our previous results obtained under ultraviolet excitation, and a relatively better color purity could be obtained in the YBO3:Eu phosphors with a smaller particle size. Moreover, the VUV excitation spectra of nanocrystals are also distinctly different from those of the bulk and submicrosized samples. The size dependence of the Eu–O charge transfer process, the B3O99− ion absorption, and the Y–O excitonic transition, as well as the quenching concentration, were reported and discussed.
Effect of liner thickness on electromigration lifetime
Authors: Liniger E. G. , Hu C. -K. , Gignac L. M. , Simon A.
Published: June 2003

Electromigration lifetime was measured as a function of liner thickness for Cu/SiO2 interconnect structures. A significant increase in mean lifetime was observed for structures in which the liner thickness at the base of the test via was less than approximately 6 nm, with a current density <5 mA/μm2 in the power line connected to the test via. This is attributed to the continuous flow of Cu across the thin and possibly discontinuous liner at the base of the via. For extremely thin liner coverage, <1.4 nm at the base of the via and 2.5 at the bottom of the test line, the mean lifetime was observed to decrease as a rapid diffusion path was created which partially offset the beneficial effects of continuous flow. Failure distributions appeared to be trimodal and this was confirmed through failure analysis. In the case of thin liner coverage (<6 nm), early fails, which are typically characterized by slitlike voids at the via/line interface, were not observed.
Electronic thermal pressure and equation of state of gold at high temperature and high pressure
Authors: Matsui Masanori , Shima Nobuyuki
Published: June 2003

We calculate the electronic thermal pressure of Au at temperatures up to 20 000 K for four compressions η=1−V/V0=0.0, 0.12, 0.24, and 0.36 (where V is the volume and V0 the ambient volume), using the uniform electron gas model with the random-phase approximation for the exchange-correlation potential. We show the isochoric electronic thermal pressure increases essentially in proportion to the square of temperature T2, which is in contrast to the result from the usual Mie–Grüneisen–Debye-type treatment where the isochoric thermal pressure increases linearly with T above the Debye temperature. We find the isochoric electronic thermal pressure difference from 300 K is relatively insensitive to compression η, and is insignificant at temperatures less than a few thousand K, having the values of 0.087, 0.549, and 2.065 GPa at T=2000, 5000, and 10 000 K, respectively, at η=0.0. The accuracy of previously proposed T−P−V equations of state of Au is discussed in relation to their use as pressure calibration standards at high temperature and high pressure.
Mechanisms of electrostriction and giant piezoelectric effect in relaxor ferroelectrics
Authors: Vikhnin V. S. , Blinc R. , Pirc R.
Published: June 2003

Two mechanisms of electrostriction in relaxor ferroelectrics of lead magnesium niobate (PMN) type are proposed. The first is based on the interaction of the soft-lattice polarization and lattice distortions with the charge transfer fluctuations and with the active ion displacements for the localized Nb–O–Nb and O–Nb–O triads in the disordered relaxor state. The second mechanism is due to the effect of simultaneous interaction of quasilocal vibrations with the soft-lattice polarization and lattice distortions. In the latter case, the quasilocal Ti-O stretching mode in PMN and lead zinc niobate (PZN) doped with lead titanate (PT) is considered. While the first mechanism explains the magnitude of electrostriction coefficients in undoped PMN and PZN, the second one explains the drastic increase of electrostriction upon PT doping. It is also shown that a giant piezoelectric effect appears in both cases under the action of an applied electric field, and the field dependence of the piezoelectric coefficients is calculated.
Kinetics of highly spin-polarized electron photoemission from an InGaAlAs strained layer by energy and spin-resolved measurements
Authors: Mamaev Yu. , Drouhin H.-J. , Lampel G. , Subashiev A. , Yashin Yu. , Rochansky A.
Published: June 2003

High-resolution energy distribution curves and spin polarization versus energy distribution curves from an AlInGaAs layer, capped by a heavily doped thin GaAs quantum well layer has been measured. Polarization P of up to 83% in conjunction with quantum yield Y=0.5% at T=130 K has been obtained. These results are compared to polarization and quantum yield spectra at high excitation power. The narrow-band quantum well is shown to provide large effective negative electron affinity values with no harm to electron polarization. The studies in linear and nonlinear excitation regimes bring insight into the kinetics of photoemission and favor the photoemission model with elastic electron tunneling through the surface barrier.
Observation of population transfer to dark exciton states by using spin-diffracted four-wave mixing
Authors: Ikeuchi O. , Adachi S. , Sasakura H. , Muto S.
Published: June 2003

We observed the exciton spin dynamical processes in GaAs quantum wells at low temperatures by three-pulse spin-diffracted four-wave mixing measurements. After investigating the merits and demerits of this method as compared with the pump–probe method, we discuss the excitation power dependence of exciton spin relaxation and the population transfer to the dark states.
Growth diagram and morphologies of AlN thin films grown by molecular beam epitaxy
Authors: Koblmueller G. , Averbeck R. , Geelhaar L. , Riechert H. , Hösler W. , Pongratz P.
Published: June 2003

A growth diagram for molecular beam epitaxy of AlN on sapphire and 6H–SiC was established using reflection high energy electron diffraction, atomic force microscopy, and Rutherford backscattering spectrometry. In varying the Al/N ratio and growth temperature, distinctive surface morphologies emerge, which are assigned to three regimes of growth, one N-rich (Al/N<1) and two Al-rich (Al/N>1) regimes. Under N-rich conditions, AlN films exhibit rough surface morphologies. In contrast, Al-rich conditions produce excellent smooth surface morphologies, but with the constraint of Al droplet formation at very high Al/N ratios and low temperatures. The differentiation between N-rich and Al-rich regimes is given only by the Al/N ratio, while the two Al-rich regimes (intermediate self-regulated and droplet regime) are separated by the boundary line of Al droplet formation. For this boundary an Arrhenius dependence of growth temperature was found, yielding an activation energy of 3.4±0.1 eV. The observed morphology transitions are attributed to varying surface adatom mobilities present under the different Al/N ratios.
Flux line lattice structure and behavior in antiphase boundary free vicinal YBa2Cu3O7−δ thin films
Authors: Durrell J. H. , Mennema S. H. , Jooss Ch. , Gibson G. , Barber Z. H. , Zandbergen H. W. , Evetts J. E.
Published: June 2003

Field angle dependent critical current, magneto-optical microscopy and high resolution electron microscopy studies have been performed on YBa2Cu3O7−δ thin films grown on miscut substrates. High resolution electron microscopy images show that the films studied exhibited clean epitaxial growth with a low density of antiphase boundaries and stacking faults. Any antiphase boundaries (APBs) formed near the film substrate interface rapidly healed rather than extending through the thickness of the film. Unlike vicinal films grown on annealed substrates, which contain a high density of antiphase boundaries, magneto-optical imaging showed no filamentary flux penetration in the films studied. The flux penetration is, however, asymmetric. This is associated with intrinsic pinning of flux strings by the tilted a–b planes and the dependence of the pinning force on the angle between the local field and the a–b planes. Field angle dependent critical current measurements exhibited the striking vortex channeling effect previously reported in vicinal films. By combining the results of three complementary characterization techniques it is shown that extended APB free films exhibit markedly different critical current behavior compared to APB rich films. This is attributed to the role of APB sites as strong pinning centers for Josephson string vortices between the a–b planes.
Broadband photopyroelectric thermal spectroscopy of a supercooled liquid near the glass transition
Authors: Bentefour E. H. , Glorieux C. , Chirtoc M. , Thoen J.
Published: June 2003

Using different photopyroelectric configurations, we have experimentally studied the frequency dependence of the thermal properties of the glass former glycerol near its glass transition. With our method, an unambiguous and separate determination of the frequency dependence of the specific heat capacity and the thermal conductivity is possible. Within experimental uncertainty, in the frequency range from 0.1 Hz to 1100 Hz, unlike the specific heat capacity, the thermal conductivity of glycerol does not show any frequency dependence through the glass transition region. We have also extended the thermal spectroscopy frequency range to 100 kHz, which is one decade higher than other methods.
Model for high-strain-rate deformation of uranium–niobium alloys
Authors: Addessio F. L. , Zuo Q. H. , Mason T. A. , Brinson L. C.
Published: June 2003

A thermodynamic approach is used to develop a framework for modeling uranium–niobium alloys under the conditions of high-strain rate. Using this framework, a three-dimensional phenomenological model, which includes nonlinear elasticity (equation of state), phase transformation, crystal reorientation, rate-dependent plasticity, and porosity growth, is presented. An implicit numerical technique is used to solve the evolution equations for the material state. Comparisons are made between the model and data for low-strain rate loading and unloading as well as heating and cooling experiments. Comparisons of the model and data also are made for low- and high-strain-rate uniaxial stress and uniaxial strain experiments. A uranium–6 wt % niobium alloy is used in comparisons of the model and experiment.
Field-induced microdomain liquid-crystal display modes
Authors: Ohno Hiroshi , Okada Hiroyuki , Onnagawa Hiroyoshi
Published: June 2003

The electro-optical characteristics of field-induced microdomain liquid-crystal display modes are investigated. One side of the substrate has radial electrodes and the other side has concentric electrodes. Large numbers of multidomain structures are induced in one pixel. The electric field gradually decreases from the center to the outer multidomains due to the radial electrodes. The contrast ratio of the homeotropic alignment cell is 210:1. Symmetric viewing angle characteristics without gray scale inversion are also obtained.
Submicron probes for Hall magnetometry over the extended temperature range from helium to room temperature
Authors: Novoselov K. S. , Morozov S. V. , Dubonos S. V. , Missous M. , Volkov A. O. , Christian D. A. , Geim A. K.
Published: June 2003

We report on mesoscopic Hall sensors made from various materials and their suitability for accurate magnetization studies of submicron samples over a wide temperature range and, especially, at room temperature. Among the studied devices, the best stability and sensitivity have been found for Hall probes made from a high-concentration two-dimensional electron gas (HC-2DEG). Even at 300 K, such submicron probes can reliably resolve local changes in dc magnetic field of ≈1 G, which corresponds to a flux sensitivity of less than 0.1 φ0 (φ0=h/e is the flux quantum). The resolution increases 100 times at temperatures below 80 K. It is also much higher for the detection of ac magnetic fields because resistance fluctuations limiting the low-frequency stability of the studied devices can be eliminated. Our second choice for room-temperature Hall micromagnetometry is gold Hall probes, which can show a sensitivity of the order of 10 G. The capabilities of HC-2DEG and gold micromagnetometers are demonstrated by measuring nm-scale movements of individual domain walls in a ferromagnet.
Quantum-well-width dependencies of postgrowth thermal annealing effects of InGaN/GaN quantum wells
Authors: Chung Yi-Yin , Lin Yen-Sheng , Feng Shih-Wei , Cheng Yung-Chen , Lin En-Chiang , Yang C. C. , Ma Kung-Jen , Hsu Cheng , Chuang Hui-Wen , Kuo Cheng-Ta , Tsang Jian-Shihn
Published: June 2003

Optical measurements of temperature-dependent photoluminescence (PL) spectral peak, integrated PL intensity and PL decay time, and microstructure analyses with high-resolution transmission electron microscopy showed the strong dependencies of thermal annealing effects on quantum well (QW) width in InGaN/GaN QW structures. With different QW widths, different levels of strain energy were built. Upon thermal annealing, energy relaxation resulted in the reshaping of quantum dots and hence the changes of optical properties. Thermal annealing at 800 °C of a narrow QW width (2 nm) structure led to regularly distributed quantum dots (QDs) and improved optical quality. However, thermal annealing at the same temperature of a sample of larger QW width (4 nm) did not show QD formation. In this situation, even higher local strains around QWs were speculated. Also, degraded optical quality was observed.
Multitone quantum simulations of saturating tunnel junction mixers
Authors: Withington S. , Kittara P. , Yassin G.
Published: June 2003

When designing instruments for submillimeter-wave astronomy it is important to understand precisely how saturating superconducting tunnel-junction mixers behave. In this article, a procedure is described for calculating the behavior of tunnel-junction circuits when multitone high-level signals are applied. The procedure is based on a full quantum-mechanical description of photon-assisted tunneling, and uses Harmonic Newton to search for a set of voltages and currents that satisfy the circuit equations at every frequency present. A number of illustrative simulations are presented, and it is shown that Harmonic Newton can be used to model saturating superconductor–insulator–superconductor mixers in a robust and numerically efficient manner.
Radio frequency magnetron sputtering deposition of calcium phosphate coatings: The effect of resputtering on the coating composition
Authors: Feddes B. , Wolke J. G. C. , Jansen J. A. , Vredenberg A. M.
Published: June 2003

Radio frequency magnetron plasma sputtering deposition is a fairly complex deposition method. Deposition of the film is influenced by the energetic bombardment of several species, like energetic electrons, neutrals, ions from the sputtering target and the plasma, and ultraviolet light. In this work we study the origin and the degree of (preferential) resputtering of species from a calcium phosphate ceramic film during the deposition process. We found that negatively charged oxygen is mostly responsible for the resputtering phenomena that occur. The degree of resputtering was determined by comparing the amount of material deposited behind and at the position of an aperture, which was positioned between the sputtering target and the substrate. The degree of resputtering can be derived, because the material behind the aperture is almost completely protected from bombardment by negatively charged oxygen. We found that at low Ar pressure, more than half of the material deposited is resputtered, preferentially P. Resputtering of the coating decreases with an increase in Ar pressure. An explanation for this may be that more negatively charged, energetic oxygen is ejected from the target at low Ar pressure, while at high Ar pressure more neutral, less energetic, oxygen is ejected. The former gives a higher resputtering rate.
Magnetic properties of sputtered barium ferrite thick films
Authors: Capraro S. , Chatelon J. P. , Joisten H. , Le Berre M. , Bayard B. , Barbier D. , Rousseau J. J.
Published: June 2003

The development of devices that combine a magnetic material with a semiconductor chip is a major focus of current research. Barium hexaferrite (BaFe12O19 or BaM) thick films are deposited here using a rf sputtering system. The films are amorphous and nonmagnetic after deposition. Postdeposition thermal annealings are employed to obtain magnetic properties. The effects of the substrate, thermal annealing and thickness of BaM on the magnetic properties are studied using a vibrating sample magnetometer. The initial results show good magnetic properties for the two subtrates studied and after thermal annealing above 800 °C. The magnetic properties of the thick films are close to the bulk (BaM) ones.
Guiding characteristics of surface acoustic waves visualized using photoluminescence quenching
Authors: Takagaki Y. , Ramsteiner M. , H. Ploog K.
Published: June 2003

We restrict the surface acoustic wave (SAW) propagation on a GaAs surface within a waveguide by means of reduction of the SAW velocity realized by metal gratings. The spatial variation of the SAW amplitude is evaluated by mapping the efficiency of the quenching of photoluminescence by the SAWs. Confinement of the SAWs is accomplished at a frequency of 1.4 GHz when the angle of SAW incidence with respect to the waveguide boundary is kept less than ∼10° for SAW velocity reduction of 4%. Mapping also reveals interference stripes associated with the SAWs bounce at the waveguide boundary.
Ultrafast sampling measurements using the photomodulated kinetic inductance in high-Tc superconductors
Authors: Stevens C. J. , Bianchi G. , Ryan J. F.
Published: June 2003

We provide a demonstration of ultrafast sampling measurements using the photokinetic effect in high-Tc superconductors to observe fast electrical signals in and rf resonator structure. We observe direct mixing of an applied optical sampling pulse with an ultrafast electrical signal capacitatively coupled to the sampling microbridge. The structures used here are 10 μm wide bridges lithographically defined in thin Tl2Ba2CaCu2O10 films. The time resolution and sensitivity of the technique are evaluated and the effects of bias currents on signal strengths are explored.
Polarization effects in AlGaN/GaN and GaN/AlGaN/GaN heterostructures
Authors: Heikman Sten , Keller Stacia , Wu Yuan , Speck James S. , DenBaars Steven P. , Mishra Umesh K.
Published: June 2003

The influence of AlGaN and GaN cap layer thickness on Hall sheet carrier density and mobility was investigated for Al0.32Ga0.68N/GaN and GaN/Al0.32Ga0.68N/GaN heterostructures deposited on sapphire substrates. The sheet carrier density was found to increase and saturate with the AlGaN layer thickness, while for the GaN-capped structures it decreased and saturated with the GaN cap layer thickness. A relatively close fit was achieved between the measured data and two-dimensional electron gas densities predicted from simulations of the band diagrams. The simulations also indicated the presence of a two-dimensional hole gas at the upper interface of GaN/AlGaN/GaN structures with sufficiently thick GaN cap layers. A surface Fermi-level pinning position of 1.7 eV for AlGaN and 0.9–1.0 eV for GaN, and an interface polarization charge density of 1.6×1013–1.7×1013 cm−2, were extracted from the simulations.
Perpendicular magnetic anisotropy and stripe domains in ultrathin Co/Au sputtered multilayers
Authors: Donzelli O. , Palmeri D. , Musa L. , Casoli F. , Albertini F. , Pareti L. , Turilli G.
Published: June 2003

Magnetometric and magnetic force microscopy (MFM) measurements were performed on [Co(6 Å)/Au(27 Å)]27 multilayers grown at different sputtering pressures (0.8–9 Pa). All the as-prepared samples display perpendicular magnetic anisotropy that has its maximum value at intermediate pressures, while a monotonic decrease of the saturation magnetization was found with increasing pressure. As found by MFM, a relevant characteristic of these systems is the magnetic stripe domain structure. The application of the stripe domain model of Draaisma and de Jonge [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)] allows us to satisfactorily reproduce the magnetization processes and to determine the anisotropy constants (Keff) starting from the domain width. The obtained Keff values display the same behavior and order of magnitude as those deduced from the area enclosed between parallel and perpendicular magnetization curves. The measurement of the stripe domain size with MFM thus results in an easy and reliable method for the evaluation of the anisotropy constants in multilayers with perpendicular anisotropy.
Three-dimensional self-consistent radiation transport model for the fluid simulation of plasma display panel cell
Authors: Kim H. C. , Yang S. S. , Lee J. K.
Published: June 2003

In plasma display panels (PDPs), the resonance radiation trapping is one of the important processes. In order to incorporate this effect in a PDP cell, a three-dimensional radiation transport model is self-consistently coupled with a fluid simulation. This model is compared with the conventional trapping factor method in gas mixtures of neon and xenon. It shows the differences in the time evolutions of spatial profile and the total number of resonant excited states, especially in the afterglow. The generation rates of UV light are also compared for the two methods. The visible photon flux reaching the output window from the phosphor layers as well as the total UV photon flux arriving at the phosphor layer from the plasma region are calculated for resonant and nonresonant excited species. From these calculations, the time-averaged spatial profiles of the UV flux on the phosphor layers and the visible photon flux through the output window are obtained. Finally, the diagram of the energy efficiency and the contribution of each UV light are shown.
Evaluation of acoustic physical constants of LiNbO3 at hypersonic frequencies
Authors: Lim H. S. , Huang J. D. , Zhang V. L. , Kuok M. H. , Ng S. C.
Published: June 2003

The velocity angular dispersion of bulk acoustic modes propagating parallel to the surfaces of X-, Y- and Z-cut crystal plates of LiNbO3 has been measured by Brillouin scattering. The elastic and piezoelectric stress constants of LiNbO3 at hypersonic frequencies, determined from a simultaneous fit to data from the three crystal cuts, are found to be only marginally more accurate than those based on only the X-cut data. Calculations based on the constants fitted to the X-cut data alone predicted accurately the velocity angular dispersion of the surface Rayleigh wave measured simultaneously. The first leaky surface wave on the Z-cut crystal is also experimentally observed whose velocity angular dispersion agrees with the theoretical dispersion computed from the fitted constants. Evidence of dispersion between ultrasonic and hypersonic velocities is found for both surface and bulk acoustic waves.
Infrared optical absorbance of intersubband transitions in GaN/AlGaN multiple quantum well structures
Authors: Zhou Qiaoying , Chen Jiayu , Pattada B. , Manasreh M. O. , Xiu Faxian , Puntigan Steve , He L. , Ramaiah K. S. , Morkoç Hadis
Published: June 2003

Intersubband transitions in Si-doped molecular beam epitaxy grown GaN/AlGaN multiple quantum wells on c-plane sapphire were investigated using the Fourier-transform infrared optical absorption technique. Several GaN quantum well samples were grown with either AlGaN bulk or GaN/AlGaN short period superlattice barriers. The measurements were made in a waveguide configuration utilizing a facet polished at 45° to the c plane. The integrated area of the intersubband transitions in several waveguides cut from different location of the wafer was measured, from which we estimated the two-dimensional electron gas density (σ). The measured values of σ are about two orders of magnitude larger than the Si doping level of ∼8×1017 cm−3, which is consistent with the polarization effects, particularly considering the large number of GaN/AlGaN interfaces. The internal quantum efficiency of the intersubband transitions was estimated to be on the order of 40% for samples with superlattice barriers.
Growth and characterization of GaN:Mn epitaxial films
Authors: Graf T. , Gjukic M. , Hermann M. , Brandt M. S. , Stutzmann M. , Görgens L. , Philipp J. B. , Ambacher O.
Published: June 2003

The oxidation states of Mn in epitaxial GaN films grown by plasma induced molecular beam epitaxy were investigated by electron spin resonance (ESR), elastic recoil detection, superconducting quantum interference device magnetization, and photothermal deflection spectroscopy measurements. Comparison of the measured Mn2+ spin densities with the total Mn concentrations in GaN of about 1020 cm−3 indicates that the majority of Mn is present in the neutral Mn3+ acceptor state. In samples codoped with Si, electrons are transferred to the Mn acceptors, which is observed by a strong increase of the Mn2+ spin densities in ESR. Simultaneously, this charge transfer reduces the characteristic optical absorption features at 1.5 and above 1.8 eV, suggesting their assignment to a Mn3+-related gap state.
Optical transitions and upconversion properties of Ho3+ doped ZnO−TeO2 glass
Authors: Boyer J. C. , Vetrone F. , Capobianco J. A. , Speghini A. , Bettinelli M.
Published: June 2003

Optical absorption, luminescence spectra and upconversion of both red to green and infrared to green luminescence have been studied for a Ho3+ doped zinc tellurite glass of composition 19ZnO−80TeO2−1Ho2O3. From the absorption data, Judd–Ofelt parameters for the Ho3+ ion are calculated along with the radiative transition rates, branching ratios and radiative lifetimes of the major Ho3+ emitting levels. The upconversion luminescence for both 646 and 754 nm excitation was attributed to a two photon excited state absorption upconversion mechanism that occurs via the I75 level. For an excitation power density of 610 W/cm2 an upconversion efficiency of 1.22% was determined for the case of 646 nm excitation, which is considerably higher than what has been reported in Ho3+ doped fluoride single crystals. Under 754 nm excitation, an upconversion efficiency of 0.27% was observed for a excitation power density of 610 W/cm2, which is reasonably low compared to other Ho3+ doped materials.
Ultraviolet-light-induced near-infrared photorefractivity and two-color holography in highly Mg-doped LiNbO3
Authors: Zhang Guoquan , Tomita Yasuo
Published: June 2003

We investigate the ultraviolet (UV)-light-induced transient photorefractivity in a highly Mg-doped LiNbO3 crystal. The thermal activation energy of the UV-light-induced defect centers responsible for the induced transient photorefractivity was measured to be 0.65 eV and holes were found to be the major charge carriers. These results provide a solid basis for the model that intermediate states O− are the UV-light-induced defect centers in the crystal. We also demonstrate quasinondestructive two-color holography using UV-light-induced transient photorefractivity. The two-color recording sensitivity was measured to be as high as 0.05 cm/J with a recording intensity of 5 W/cm2 at 780 nm and a gating intensity of 0.47 W/cm2 at 365 nm.
Free-standing porous silicon single and multiple optical cavities
Authors: Ghulinyan M. , Oton C. J. , Bonetti G. , Gaburro Z. , Pavesi L.
Published: June 2003

Porous silicon free-standing microcavity structures, with different layer designs, have been fabricated. Single microcavities show transmission resonances in the technologically relevant wavelength region of 1.55 μm with quality factors up to 3380. High-order cavities show sub-nm transmission peaks over the whole stop band. Coupled microcavity structures, where splitting of the degenerate cavity mode occurs, lead to multiple transmission peaks in a limited region of the stop band. We also report incident angle-dependent measurements, where transmission peak blueshift and splitting of transverse electric and transverse magnetic polarized modes due to porous silicon birefringence were observed.
AlGaInP/mirror/Si light-emitting diodes with vertical electrodes by wafer bonding
Authors: Horng R. H. , Huang S. H. , Wuu D. S. , Chiu C. Y.
Published: June 2003

In a previous study, we reported a highly efficient AlGaInP light-emitting diode (LED) with a Au/AuBe/SiO2/Si mirror substrate (MS) fabricated by wafer bonding, where a planar electrode structure is used. In view of the more efficient epilayer area utilized, AlGaInP/mirror/barrier/Si LEDs with vertical electrodes are proposed in this work. A variety of barrier layers (Pt/Ti, TaN/Ta, and TiN/Ti) have been incorporated into the mirror structure. The mirror quality after bonding is a confirmed key issue in obtaining vertical MS–LEDs with high brightness. It is found that AuBe thickness has a large effect on the final MS–LED performance due to the difference in the interdiffusion of Be atoms in each mirror structure. The diffusion of excess Be atoms diffusing to the mirror side results in a rougher surface and inferior reflectivity. The luminance intensity of an AlGaInP LED chip (626 nm) with an optimum AuBe thickness can reach a maximum of ∼165 mcd at 20 mA with a forward voltage of 2.1 V. After encapsulation into lamps, the peak power efficiency can reach 21.7%, which corresponds to a 9 mW output at 20 mA. Therefore, the MS structure can be extended to fabricate high-brightness AlGaInP LEDs on Si with conventional vertical electrodes.
Magnetic characteristics and frequency dependence of permeability in Fe–B–N thin films
Authors: Phan M. H. , Yu S. C. , Chung J. S. , Kim J. S. , Kim Y. M.
Published: June 2003

Magnetic characteristics and the frequency dependence of permeability for Fe–B–N thin films have been studied. Among the compositions investigated, Fe94.9B1.3N3.8 exhibited excellent soft-magnetic properties: High saturation induction, Bs∼ 20.6 kG, and low coercivity, Hc∼ 0.1 Oe. An anomalous change of the incremental permeability ratio (PR) curve as a function of external magnetic field for the films measured above 4 MHz was observed, indicating a considerable variation in the magnetic softness in the high-frequency region. The changes of PR in low external fields seem to be useful for the evaluation of the magnetic softness in thin-film-type materials.
Deep-defect-induced quenching effects in semi-insulating GaN layers detected by photoelectrical spectroscopic techniques
Authors: Witte H. , Krtschil A. , Lisker M. , Schrenk E. , Christen J. , Krost A. , Kuhn B. , Scholz F.
Published: June 2003

Quenching effects induced by additional below-bandgap illumination in undoped semi-insulating GaN were investigated using optical admittance spectroscopy (OAS) and photocurrent (PC) spectroscopy as well as optically excited, thermally stimulated currents (TSC). In OAS and PC, a decrease of defect-related signals due to the quenching light was observed. The thermal quenching of the defect band between 2.7 and 3.3 eV shows a good agreement with thermal emissions as measured by TSC, indicating the same defects cause the optical transitions in OAS/PC and the thermal transitions in TSC. The thermal emission in the temperature region between 250 and 300 K, which is responsible for the thermal quenching of the blue band (BB) in OAS, also shows an optical quenching under below-bandgap excitation.
Si dangling-bond-type defects at the interface of (100)Si with ultrathin HfO2
Authors: Stesmans A. , Afanas’ev V. V.
Published: June 2003

Electron spin resonance analysis of (100)Si/HfO2 interfaces prepared by chemical vapor deposition of the oxide using three chemically different precursors reveals that the trivalent Si defects common for Si/SiO2 interfaces—Pb0 and Pb1 centers—are universally observed upon hydrogen detachment. The density of the Pb0 is higher than in the (100)Si/SiO2 structures and is sensitive to the deposition process. However, the density can be significantly reduced by annealing of the Si/HfO2 structures in O-containing ambient, likely through re-establishing the Si/SiO2 interface. Also, the Pb-type centers can be entirely passivated by hydrogen already at 400 °C. The density of fast interface states closely follows the variations in the Pb0 center density, suggesting it as the dominant contribution to the fast interface states.
Measurement of carbon nanotube–polymer interfacial strength
Authors: Barber Asa H. , Cohen Sidney R. , Wagner H. Daniel
Published: June 2003

The force required to separate a carbon nanotube from a solid polymer matrix has been measured by performing reproducible nanopullout experiments using atomic force microscopy. The separation stress is found to be remarkably high, indicating that carbon nanotubes are effective at reinforcing a polymer. These results imply that the polymer matrix in close vicinity of the carbon nanotube is able to withstand stresses that would otherwise cause considerable yield in a bulk polymer specimen.
Bubble size control and measurement in the generation of ferrofluid foams
Authors: Drenckhan W. , Elias F. , Hutzler S. , Weaire D. , Janiaud E. , Bacri J.-C.
Published: June 2003

Ordered ferrofluid foams in tubes or channels offer promising possibilities for transporting and processing small gas (or liquid) samples. For this purpose, monodisperse bubbles can be produced, with fine control of their volume over at least three orders of magnitude, by the application of a variable magnetic field gradient during bubble generation. Electrical resistance measurements can be used to count bubbles, determine their volume, and identify the foam structure they form in the tube or channel.
In situ control of electron gas dimensionality in freely suspended semiconductor membranes
Authors: Höhberger Eva M. , Krämer Tomas , Wegscheider Werner , Blick Robert H.
Published: June 2003

We present fabrication and measurements of gated suspended low-dimensional electron systems. The core component of the device is a low-dimensional electron gas embedded in a free-standing beam processed from a GaAs/AlGaAs heterostructure. The dimensionality of the electronic system is fully controlled by a number of gating electrodes on the suspended membranes. Operation in the quantum Hall regime, in the one-dimensional case, and as zero-dimensional quantum dots is demonstrated. The resulting devices can be applied as ultrasensitive bolometers and as nanoelectromechanical circuits that reach the ultimate limits of displacement detection.
Coulomb explosion in femtosecond laser ablation of Si(111)
Authors: Roeterdink W. G. , Juurlink L. B. F. , Vaughan O. P. H. , Dura Diez J. , Bonn M. , Kleyn A. W.
Published: June 2003

The ablation of ionized silicon is studied using near-infrared femtosecond laser pulses in the fluence regime from 1 to 9 J/cm2. Two major peaks are observed in the mass spectrum corresponding to Si+ and Si2+. In the time-of-flight transients of Si+, a bimodal structure is observed. The fast Si+ peak corresponds to a velocity half of that observed for Si2+. This momentum scaling is clearly indicative of a Coulomb explosion.
Electrically tunable waveguide laser based on ferroelectric liquid crystal
Authors: Kasano Masahiro , Ozaki Masanori , Yoshino Katsumi , Ganzke Dirk , Haase Wolfgang
Published: June 2003

Laser action in a waveguide configuration has been demonstrated in a planar alignment cell of dye-doped chiral smectic liquid-crystal mixtures with a short pitch helical structure. In this configuration, doped dye can effectively be excited by a pump beam illuminating perpendicularly the helical axis and the laser light emitted along the helical axis propagates in the waveguide. Lasing wavelength can be tuned by adjusting the periodicity of the helical pitch upon applying the electric field.
Optimum channel thickness in pentacene-based thin-film transistors
Authors: Lee Jiyoul , Kim Kibum , Kim Jae Hoon , Im Seongil , Jung Duk-Young
Published: June 2003

We report on the influence of pentacene channel thickness on the field-effect hole mobility in pentacene-based thin-film transistors (TFTs) that employ the top-contact mode for the source/drain electrodes. Our pentacene channel layers were deposited in the thickness range of 16–90 nm by thermal evaporation on 450 nm thick Al2O3+x dielectric films. The TFTs with increasingly thinner pentacene layers displayed correspondingly higher hole mobility, but an optimum thickness was determined to be about 30 nm because the TFTs with pentacene layers thinner than 30 nm exhibited high leakage current in the off-state bias regime. After a proper chemical treatment was performed onto the Al2O3+x gate dielectric, our optimized TFT with a 30 nm thick pentacene channel exhibited high mobility of ∼0.2 cm2/V s with an on/off current ratio of 105.
Self-excited oscillatory dynamics of capillary bridges in electric fields
Authors: Klingner Anke , Herminghaus Stephan , Mugele Frieder
Published: June 2003

We studied the stability of capillary bridges between flat, parallel, and dielectrically coated electrodes as a function of the voltage applied between them. The stability limits of the capillary bridge state and the state consisting of two separated droplets are shifted with respect to ordinary capillary hysteresis at zero voltage. Surprisingly, we found that the system can oscillate periodically between the two states within a certain range of applied voltage and electrode separation. These oscillations could be applied to promote mixing in electrowetting-based microfluidic devices. We present a model based on the balance between interfacial and electrostatic energies, which explains the experimental findings quantitatively.
Parameters of wires during electric explosion
Authors: Tkachenko S. I. , Vorob’ev V. S. , Malyshenko S. P.
Published: June 2003

We use the nucleation theory of electric explosion of wire to estimate wire parameters at the explosion for a wide range of experimental conditions. We analyze a number of the major parameters including the limit value of energy deposited during the initial resistive phase of the explosion and discuss the correspondence of these results to previously published experimental data.
Polycrystalline silicon/CoSi2 Schottky diode with integrated SiO2 antifuse: a nonvolatile memory cell
Authors: Herner S. B. , Mahajani M. , Konevecki M. , Kuang E. , Radigan S. , Dunton S. V.
Published: June 2003

A Schottky diode consisting of doped polycrystalline silicon (polysilicon) and CoSi2 films is described. When an SiO2 antifuse thin film is grown in between the polysilicon and CoSi2, the film stack can function as a nonvolatile one-time programmable memory cell. The cell is programmed when the SiO2 that insulates the doped polysilicon from the CoSi2 is broken down by applying a large biasing field, and unprogrammed when the antifuse is not broken down. By taking advantage of the ability to grow SiO2 directly on CoSi2, the entire device can made with only two masking steps and relatively simple tool set, while achieving high density.
Electrical properties and transport in boron nitride nanotubes
Authors: Radosavljević M. , Appenzeller J. , Derycke V. , Martel R. , Avouris Ph. , Loiseau A. , Cochon J.-L. , Pigache D.
Published: June 2003

We have fabricated electronic devices based on single-walled boron nitride nanotubes (BNNTs). Our measurements indicate that all BNNTs are semiconducting, and p-doped. Temperature dependence of two terminal transport experiments suggests that at low drain fields, transport is dominated by thermionic emission over 250–300 meV Schottky contact barriers. Gate-induced barrier modulation was observed in vertically scaled devices, resulting in field-effect transistor operation.
Improved output coupling efficiency of a ZnS:Mn thin-film electroluminescent device with addition of a two-dimensional SiO2 corrugated substrate
Authors: Do Y. R. , Kim Y.-C. , Cho S.-H. , Ahn J.-H. , Lee J.-G.
Published: June 2003

We present a study of the effects of adding a two-dimensional (2-D) SiO2 corrugated substrate onto a ZnS:Mn thin-film electroluminescence (EL) device on its EL properties. Two-step irradiated hologram lithography and reactive ion etching were used to fabricate a 2-D square-lattice-dotted pattern of SiO2 on the glass substrate. The addition of this corrugation leads to a 220% enhancement in the EL efficiency at 40 V above the threshold voltage under a 500-Hz sinusoidal excitation. This improvement in the output coupling efficiency is due to the diffraction scattering produced by the 2-D SiO2 periodic corrugations.
45° rotational epitaxy of SrTiO3 thin films on sulfide-buffered Si
Authors: Yoo Y.-Z. , Ahmet P. , Jin Zheng-Wu , Nakajima K. , Chikyow T. , Kawasaki M. , Konishi Y. , Yonezawa Y. , Song J. H. , Koinuma H.
Published: June 2003

Sulfide was employed as a buffer layer for the growth of SrTiO3 (STO) thin films on Si. In order to utilize a relationship of a 45° rotational lattice match between Si and STO, ZnS, with almost the same lattice constant as Si, was used as the buffer. The buffer layer showed a partially disordered region at the ZnS/Si interface, owing to steady interdiffusion between ZnS and Si. STO film on ZnS buffered Si showed the rotational epitaxy with respect to Si and sharp STO/ZnS interface. Propagation of stacking faults from the ZnS/Si interface was observed, but those plane defects were terminated at the rotational STO/ZnS interface, resulting in high-quality STO films. The dielectric constant of the STO/ZnS film was 34.
Marangoni effect in nanosphere-enhanced laser nanopatterning of silicon
Authors: Lu Y. , Theppakuttai S. , Chen S. C.
Published: June 2003

We report a Marangoni effect in nanosphere-enhanced laser direct nanopatterning of silicon surface. A monolayer of nanosphere array was formed on the silicon substrate by self-assembly. A 248-nm excimer laser was used to irradiate the sample surface. Due to optical field enhancement between the nanosphere and the substrate, the silicon surface was locally melted. The molten material was redistributed due to surface tension forces, resulting in the formation of a nanodent array. The morphology of the nanodents changed from bowl-type to “Sombrero” with increase of laser intensity as a result of a Marangoni effect that arises due to the competition between a thermocapillary force and a chemicapillary force acting on the molten material.
Orientation control of poly(vinylidenefluoride-trifluoroethylene) crystals and molecules using atomic force microscopy
Authors: Kimura Kuniko , Kobayashi Kei , Yamada Hirofumi , Horiuchi Toshihisa , Ishida Kenji , Matsushige Kazumi
Published: June 2003

We have developed an aligning technique for polymer crystals and molecular chains utilizing contact-mode atomic force microscopy (AFM). We have aligned lamellar crystals and molecular chains of poly(vinylidenefluoride-trifluoroethylene) thin films. By scanning the film surface using an AFM cantilever tip at the temperature range of 70–100 °C, we aligned the crystals to the scan direction. Moreover, we successfully aligned the molecular chains to the scan direction by scanning at a higher temperature (135 °C). The aligned chains subsequently formed large lamellar crystals, which were still ferroelectric.
Spin valves with spin-engineered domain-biasing scheme
Authors: Lu Z. Q. , Pan G.
Published: June 2003

Synthetic spin-filter spin valves with spin-engineered biasing scheme “sub/Ta/NiFe/IrMn/NiFe/NOL/Cu1/CoFe/Cu2/CoFe/Ru/CoFe/IrMn/Ta” were developed. In the structure, the orthogonal magnetic configuration for biasing and pinning field was obtained by one-step magnetic annealing process by means of spin flop, which eliminated the need for two antiferromagnetic materials with distinctively different blocking temperatures and two-step magnetic annealing as in conventional exchange biasing scheme. The longitudinal domain biasing of spin valves was achieved by using interlayer coupling field through Cu1 spacer. By adjusting the thickness of the Cu1 layer, the interlayer coupling biasing field can provide domain stabilization and was sufficiently strong to constrain the magnetization in coherent rotation. This can prevent Barkhausen noises associated with magnetization reversal. We report here a proof of concept study of such a domain-biasing scheme, which has its important technological applications in nanoscale spin valve and magnetic tunneling junction read heads and other spintronic devices.
Extremely low threshold-current-density InGaAs quantum-well lasers with emission wavelength of 1215–1233 nm
Authors: Tansu Nelson , Yeh Jeng-Ya , Mawst Luke J.
Published: June 2003

Extremely low threshold-current-density In0.4Ga0.6As quantum-well (QW) lasers have been realized in the 1215–1233 nm wavelength regime. The measured room-temperature threshold current density of the InGaAs QW lasers with a cavity length of 1000 μm is only 90 A/cm2 at an emission wavelength of 1233 nm.
Atomic hydrogen cleaning of polarized GaAs photocathodes
Authors: Maruyama T. , Luh D.-A. , Brachmann A. , Clendenin J. E. , Garwin E. L. , Harvey S. , Kirby R. E. , Prescott C. Y. , Prepost R.
Published: June 2003

Atomic hydrogen cleaning followed by heat cleaning at 450 °C was used to prepare negative-electron-affinity GaAs photocathodes. When hydrogen ions were eliminated, quantum efficiencies of 15% were obtained for bulk GaAs cathodes, higher than the results obtained using conventional 600 °C heat cleaning. The low-temperature cleaning technique was applied to thin strained GaAs cathodes used for producing highly polarized electrons. No depolarization was observed even when the optimum cleaning time of about 30 s was extended by a factor of 100.
Direct observation of lateral current spreading in ridge waveguide lasers using scanning voltage microscopy
Authors: Ban D. , Sargent E. H. , Hinzer K. , Dixon-Warren St. J. , SpringThorpe A. J. , White J. K.
Published: June 2003

We report results of two-dimensional (2D) local voltage measurement of the transverse cross section of operating multiquantum-well ridge-waveguide (RWG) lasers. We observe lateral nonuniformity of local voltage in the n-cladding layers of the laser and attribute the voltage variation to 2D carrier transport effect within the RWG lasers. The quantitative evaluation of this effect indicates the local vertical current density to be ∼40% smaller at the edge of the ridge than at its center. Our results demonstrate the strength and application of scanning voltage microscopy technique in quantitatively delineating 2D current flow in operating optoelectronic devices.