bluuebunny/crossref_metadata_2025 huggingface dataset:
Anisotropy of optical absorption in GaSe studied by midinfrared spectroscopy
Authors: Allakhverdiev K. , Fernelius N. , Gashimzade F. , Goldstein J. , Salaev E. , Salaeva Z.
Published: March 2003

Slab samples of ε-GaSe have been prepared with the optical axis (the c axis) in the plane of the face. Previously, most optical studies were made on slabs cleaved from crystals, which have the c axis perpendicular to the face. The infrared absorption spectra of ε-GaSe were measured at room temperature in the spectral range of 4000–450 cm−1 for the e-ray (polarized parallel to the optical axis) and the o-ray (polarized perpendicular to the optical axis). The maximum value of the optical absorption coefficient, α, for both geometries, does not exceed 2.5 cm−1 (E∥c). We find anisotropy in the transmission in the spectral range below ∼2250 cm−1, due to the free carrier absorption, only in the e ray. At T=300 K, acoustic lattice vibrations are the dominant scattering mechanism for carriers moving parallel to the c axis. Absorption bands were observed in e-ray spectra at 891, 945, 1093, and 1270 cm−1. These bands may be due to multiphonon absorption, or to localized vibrational modes stemming from unintentional impurities. It is suggested that the sharp absorption at 891 cm−1 may be due to hole transitions from the lowest-lying valence band in the center of the Brillouin zone to the top of the valence band (Γ6+→Γ4− transition).
Designing strain-balanced GaN/AlGaN quantum well structures: Application to intersubband devices at 1.3 and 1.55 μm wavelengths
Authors: Jovanović V. D. , Ikonić Z. , Indjin D. , Harrison P. , Milanović V. , Soref R. A.
Published: March 2003

A criterion for strain balancing of wurtzite group-III nitride-based multilayer heterostructures is presented. Single and double strain-balanced GaN/AlGaN quantum well structures are considered with regard to their potential application in optoelectronic devices working at communication wavelengths. The results for realizable, strain-balanced structures are presented in the form of design diagrams that give both the intersubband transition energies and the dipole matrix elements in terms of the structural parameters. The optimal parameters for structures operating at λ∼1.3 and 1.55 μm were extracted and a basic proposal is given for a three level intersubband laser system emitting at 1.55μm and depopulating via resonant longitudinal optical (LO) phonons (ℏωLO≈90 meV).
Comparison of the agglomeration behavior of Au and Cu films sputter deposited on silicon dioxide
Authors: Kwon Jang-Yeon , Yoon Tae-Sik , Kim Ki-Bum , Min Seok-Hong
Published: March 2003

The agglomeration behavior of Cu and Au films each with a thickness of 5 and 50 nm, deposited on thermally grown SiO2 by dc magnetron sputtering, was investigated with scanning electron microscopy. The size of Cu islands formed by agglomeration increased with increasing annealing temperature. Also, the agglomeration of Cu films seem to follow the grain boundary grooving process. On the other hand, Au islands have an identical size at different annealing temperatures. Au films were observed to agglomerate via nucleation of voids followed by the fractal growth of voids. The fractal dimension was determined to be 1.7 indicating that the fractal growth of voids can be described with a diffusion limited aggregation model. Finally, the kinetics of agglomeration of the Au films was described with an Avrami-type equation.
Liquid metals as electrodes in polymer light emitting diodes
Authors: Andersson G. G. , Gommans H. H. P. , Denier van der Gon A. W. , Brongersma H. H.
Published: March 2003

We demonstrate that liquid metals can be used as cathodes in light emitting diodes (pLEDs). The main difference between the use of liquid cathodes and evaporated cathodes is the sharpness of the metal–polymer interface. Liquid metal cathodes result in significantly sharper metal–organic interfaces than vapor deposited cathodes, due to the high surface energy of the metals. The sharper interface in pLEDs with liquid metal cathodes is observed by neutral impact collision ion scattering spectroscopy and low energy ion scattering spectroscopy measurements. The influence of interface sharpness on device performance was studied by comparing current–voltage-light characteristics of devices with OC1C10 paraphenylenevinylene (PPV) as electroluminescent polymer and indium tin oxide (ITO) as hole injection electrode, and different cathodes. Comparison of devices using a liquid Ga cathode and an evaporated Al cathode showed that light emission for the liquid Ga cathode is two orders of magnitude larger than for the evaporated Al cathode, and that the external light efficiency is increased by an order of magnitude. Since the work function of Ga and Al is nearly the same, the poor performance for evaporated Al LEDs is attributed to the formation of an interfacial layer where Al has diffused into, and reacted with, the PPV. This interfacial layer has poor electrical conduction compared to pure PPV, and contains quenching sites which reduce light emission. Low work function liquid metal cathodes were studied by using liquid Ca and Ba amalgams. The improved performance of liquid amalgam pLEDs is attributed to the different structure of the metal–polymer interface. The enormous increase in light and current through the amalgam devices compared to those using pure Hg demonstrate that less than 1 ML of a metal with a low work function at the polymer-cathode interface can have a dramatic effect on the performance of the devices. Devices with a liquid Ca amalgam cathode showed an increase of the current (by 50%) and brightness (80%) compared to devices with an evaporated Ca cathode, which is ascribed to reduced diffusion of Ca into the emissive PPV layer.
Comparative study of metal or oxide capped indium–tin oxide anodes for organic light-emitting diodes
Authors: Qiu Chengfeng , Xie Zhilang , Chen Haiying , Wong Man , Kwok Hoi Sing
Published: March 2003

Indium–tin oxide capped with a variety of nanometer-thick metal or oxide buffer layers has been investigated as anodes in organic light-emitting diodes based on N,N′-diphenyl-N,N′ bis(3-methyl-phenyl-1,1′-biphenyl-4,4′-diamine/tris-8-hydroxyquinoline aluminum. Although high work-function metal buffer layers led to enhancement in hole-injection efficiency, none of the metals investigated gave rise to improvement in current or power efficiency. On the other hand, diodes with some of the oxide buffer layers exhibited improvement not only in hole injection but also in power efficiency. In particular, when 1 nm thick praseodymium oxide was used as the cap layer, more than double the power efficiency was obtained.
Spectra and energy levels of Nd3+ in LaSc3(BO3)4
Authors: Gruber John B. , Sardar Dhiraj K. , Zandi Bahram , Reynolds Thomas A. , Alekel Theodore , Keszler Douglas A.
Published: March 2003

We report a detailed spectroscopic analysis of Nd3+(4f3) as a dopant in single crystals of LaSc3(BO3)4 (LSB). Absorption spectra were obtained between 570 and 1750 nm at temperatures as low as 8 K. Fluorescence spectra and lifetimes were obtained at similar temperatures from F3/24 to I9/24, I11/24, and I13/24 multiplet manifolds. The observed spectra are similar to spectra reported earlier for Nd3+ as a dopant in the alpha phase of LSB. The observed crystal-field splitting of the LJ2S+1 multiplet manifolds of Nd3+ was modeled using a Hamiltonian, which includes atomic and crystal field terms. A least-squares fitting between 57 calculated-to-observed energy (Stark) levels gave a root mean square deviation of 8 cm−1 for the 13 lowest-energy multiplet manifolds. With wave functions obtained from the modeling studies, multiplet-to-multiplet and line-to-line branching ratios were calculated as well as the radiative lifetime for emission from F3/24. The results are compared with experimental values obtained in the present study and also with results reported for Nd3+ in the beta phase of LSB.
Enhancing current-perpendicular-to-plane magnetoresistance by adding interfaces within ferromagnetic layers
Authors: Eid K. , Pratt W. P. , Bass J.
Published: March 2003

We show that adding “internal interfaces” within each Co layer can enhance both the total current-perpendicular-to-plane specific resistance, AR, and the change in specific resistance with magnetic field, AΔR, of [Py/Cu/Co/Cu]3 hybrid multilayers, AΔR by over 100%, with only a small increase in the total sample thickness. However, the growth of both AR and AΔR with the number of inserted interfaces is slower than predicted by a simple two-current series-resistor model. We provide evidence that this less rapid growth is due to spin-flipping at the Co/Cu interfaces, plus the formation of “incomplete interfaces” when the Cu layers are very thin.
Effect of the disorder/order phase transition on the electrical and photoelectrical properties of C60 thin films
Authors: Katz E. A. , Faiman D. , Iakoubovskii K. , Isakina A. , Yagotintsev K. A. , Strzhemechny M. A. , Balberg I.
Published: March 2003

This article reports the temperature dependence of the structural and photoelectrical properties of C60 thin films with varying crystal structure and oxygen content near the orientational disorder/order phase transition at about 260 K. X-ray diffraction data demonstrate that highly crystalline oxygen-free C60 films undergo the first-order phase transition at 252 K with a lattice parameter discontinuity Δa/a of 0.22%; no discontinuity was observed in C60 films with coexisting amorphous and nanocrystalline phases. A strong effect of the phase transition on the dark- and photoconductivity in those films is presented: In highly crystalline C60 films both dark- and photoconductivity increase as the temperature is lowered through the transition region. However, opposite temperature changes of the conductivity are observed in nanocrystalline/amorphous C60 films. In situ exposure of samples to oxygen gas suppresses any critical behavior. The experimental results are interpreted in terms of trap-controlled mobility of the dominant charge carriers such that the temperature changes of the conductivity are attributed to variations in the concentration, energy distribution, and population of the carrier traps at the phase transition. Differences in the parameters of the traps, which are associated with the freeze-out of the molecular orientations, may also explain the strong variations in the temperature dependence of the conductivity observed in our C60 films and reported by other authors.
Measurement and control of the electronic temperature in small thin-film structures
Authors: Leoni R. , Buonomo B. , Castellano M. G. , Mattioli F. , Simeone D. , Torrioli G. , Carelli P.
Published: March 2003

We discuss the thermal properties of small metal and insulating structures, interesting for bolometer applications. We report on the characterization of thin-film thermometers based on normal metal–insulator–superconductor tunnel junctions, operating at sub-Kelvin temperature and fabricated by using either niobium or aluminum as superconducting electrodes. The effect of different bias methods on the thermometer current–voltage characteristics is measured as well as the response of small metal structures to absorbed/removed power. The characterization is performed with a helium-3 refrigerator, compatible with the instrumentation generally available in ground or space-borne experiments.
Photonic band structures for three-dimensionally periodic arrays of coated spheres
Authors: Takeda Hiroyuki , Yoshino Katsumi
Published: March 2003

Photonic band structures of three-dimensional periodic structures of opals composed of nanoscale spheres coated with various materials have been studied theoretically. Opals composed of silica spheres coated with different materials can have smaller and larger band gaps at the L and X points, respectively, in the Brillouin zone of the face-centered cubic structure, although the magnitude of the gap at the X point is generally much smaller than that at the L point in conventional opals. Moreover, such a behavior can be explained by applying the same method as the free electron approximation in solid-state physics to photonic crystals. These phenomena may be suitable for practical application of photonic crystals even when the refractive indices of the materials are low.
Time-resolved x-ray diffraction measurements and analysis to investigate shocked lithium fluoride crystals
Authors: Rigg P. A. , Gupta Y. M.
Published: March 2003

Experimental methods to permit continuous, time-resolved x-ray diffraction measurements in plate impact experiments were developed and used to examine lattice deformation in shock compressed LiF single crystals. Using an x-ray streak camera diffraction data with 2–4 ns resolution were obtained from crystals subjected to both shock and ramp wave loading along the [111] direction. Because of the penetration depth of x rays into the sample, interpretation of the ramp wave loading data required an analytic model to simulate the results. The penetration depth used in the model was determined experimentally from the time-resolved shock wave loading data. Good agreement between the simulations and experimental data was found for both loading conditions, suggesting that the analytic model has broad applicability.
All-optical multiwavelength technique for the simultaneous measurement of bulk recombination lifetimes and front/rear surface recombination velocity in single crystal silicon samples
Authors: Sirleto Luigi , Irace Andrea , Vitale Gianfranco , Zeni Luigi , Cutolo Antonello
Published: March 2003

In this article, a contactless, all-optical, nondestructive method for separating the minority carrier recombination lifetime and surface recombination velocities in silicon samples at a low injection level is presented. The technique can be described as a pump–probe method in which the excess carrier density is probed by analyzing the free carrier absorption transient following laser pulse excitation that have several wavelengths. An attractive characteristic of the proposed technique is its capability to measure different values of surface recombination velocity on front and back surfaces of the silicon sample. The theoretical foundation of the method is also analyzed. Moreover, numerical simulations which validate the proposed methodology and preliminary experimental results that prove the applicability of scheme are presented.
Ferroelectric cathodes in transverse magnetic fields
Authors: Dunaevsky A. , Raitses Y. , Fisch N. J.
Published: March 2003

Experimental investigations of a planar ferroelectric cathode in a transverse magnetic field up to 3 kG are presented. It is shown that the transverse magnetic field differently affects the operation of ferroelectric plasma cathodes in “bright” and “dark” emission modes in vacuum. In the bright mode, when the surface plasma is formed, the application of the transverse magnetic field leads to an increase of the surface plasma density. In the dark mode, the magnetic field impedes the electron emission. This result indicates the similarity of the dark emission mode to the surface preflashover, where the transverse magnetic field inhibits the development of secondary electron avalanches along the surface.
Effective microwave surface impedance of a thin type-II superconducting film in the parallel magnetic field
Authors: Wu Chien-Jang
Published: March 2003

The effective microwave surface impedance for a layered structure made of a thin type-II superconducting film as well as a dielectric substrate is theoretically investigated. We consider the superconducting film with thickness comparable or smaller than the penetration depth in the parallel magnetic field, in which a vortex chain is present in the central plane of film. The total effective microwave surface impedance is shown to consist of two parts, one represents the Meissner state response which can be calculated by the usual impedance transform technique, and the other not obtainable from the impedance transform is the vortex response. The effective surface resistance and reactance due to the vortex chain are specifically numerically investigated as functions of the thicknesses of film and dielectric, static magnetic field and microwave frequency as well. For the frequency-dependent surface impedance, numerical results reveal that there exists a crossover frequency such that the surface resistance is equal to the surface reactance. This crossover frequency is further proven to be equal to the depinning frequency for a vortex system.
Modeling the hyperelasticity of magnetic field sensitive gels
Authors: Voltairas P. A. , Fotiadis D. I. , Massalas C. V.
Published: March 2003

The continuum theory, previously developed to quantitatively account for the large deformations observed in gels endowed with electric properties, is extended to magnetic field sensitive gels (ferromagnetic or diamagnetic in origin). The derived analytical formula for the dependence of the gel displacement on the magnetic field can be applied either to control recently developed biomimetic valves and possible artificial muscles constructions or to interpret similar phenomena in biophysics.
Voltage drop at interfaces in multilayer ferroelectrics
Authors: Li Xingjiao , Wang Ningzhang , Bao Junbo , Chen Tao , Xu Jingping , Feng Hanhua , Li Shaoping
Published: March 2003

Based upon a defined surface barrier in ferroelectric multilayers deposited on (100) p-type silicon, ξVa, which bears a portion of the external electrical voltage, a modified empirical power law I=A(ξV)n is established for quantitatively describing detailed I–V dependence in ferroelectric multilayers. The voltage drop at the interface, Vi, which directly affects electrical characteristics of ferroelectric multiplayer system, is studied thoroughly. The voltage drop obtained from the modified empirical power law of the I–V dependence is consistent with that obtained from the C–V dependence model.
Visible photoluminescence from nanostructured Si-based layers produced by air optical breakdown on silicon
Authors: Kabashin A. V. , Meunier M.
Published: March 2003

Pulsed radiation of CO2 laser has been used to produce an optical breakdown on a silicon target in atmospheric air. After several breakdown initiations near the threshold of plasma production, a highly porous layer was formed under the radiation spot on the silicon surface. The fabricated layers presented the porosity of 75%–80% and were formed of silicon nanocrystals imbedded in SiO2 matrix. They exhibited strong photoluminescence (PL) around 2.0 eV, which was stable to a prolonged continuous illumination of samples. Possible mechanisms of nanostructure formation and PL origin are discussed.
Self-assembled vertical GaN nanorods grown by molecular-beam epitaxy
Authors: Tu L. W. , Hsiao C. L. , Chi T. W. , Lo I. , Hsieh K. Y.
Published: March 2003

Dislocation-free vertical GaN pillars in nanoscale were grown on Si (111) surface through self-assembly by molecular-beam epitaxy. No extra catalytic or nanostructural assistance has been employed. These nanorods have a lateral dimension from ≲10 nm to ∼800 nm and a height of ≲50 nm to ≳3 μm protruding above the film, depending on the growth parameters. The top view of the nanorods has a hexagonal shape from scanning electron microscopy. Transmission electron microscopy shows that the nanorods are hexagonal, single crystal GaN along the c-axis. An extra peak at 363 nm originated from nanorods was observed in photoluminescence spectra at 66 K, which is ascribed to the surface states according to the results of surface passivation. Micro-Raman spectroscopy on a single nanorod reveals E1 and E2 modes at 559.0 and 567.4 cm−1, respectively. Large strain was observed in both the transmission electron micrograph and the Raman shift. A possible growth mechanism is discussed.
The role of hole leakage in 1300-nm InGaAsN quantum-well lasers
Authors: Tansu Nelson , Mawst Luke J.
Published: March 2003

We calculate the thermionic escape times of electrons and holes in InGaAsN and InGaAs quantum wells using the most recent input data. The short thermionic escape time of holes from the InGaAsN quantum well indicates that hole leakage may be a significant factor in the poorer temperature characteristics of InGaAsN quantum-well lasers compared to those of InGaAs devices. We suggest a structure that results in an increased escape time, which will allow the reduction of hole leakage in these devices.
Molecular alignment enhancement phenomenon of polymer formed from a liquid crystal monomer in a liquid crystal solvent
Authors: Fujikake Hideo , Murashige Takeshi , Sato Hiroto , Kawakita Masahiro , Kikuchi Hiroshi
Published: March 2003

We report an abnormal alignment enhancement phenomenon of polymer molecules. The alignment order of a rigid-skeleton polymer made from a liquid crystalline monomer in a low-molecular-weight liquid crystal solvent was drastically enhanced with increasing temperature, even though the alignment order of the solution of the liquid crystal and monomer decreased. From polymer molecular alignment observations using polarizing Raman scattering microscopy, it was found that the polymer alignment order was three times greater than that of the original aligned monomer and polymer. This super alignment technique of polymer using a molecular-scaled self-assembly mechanism is applicable to the formation of electrically and/or optically functional nanopolymer wires.
A slit-type atom deflector with near-field light
Authors: Totsuka Kouki , Ito Haruhiko , Suzuki Kiichi , Yamamoto Kazuhiro , Ohtsu Motoichi , Yatsui Takashi
Published: March 2003

We developed a near-field optical deflector for precise direction control of atomic motion using a dipole force. The blue-detuned, near-field light used to deflect atoms was generated near the edge of a 100-nm-wide slit and had a spatial distribution of 126 nm at a distance of 10 nm from the top edge. The deflection angle for a Rb atom was a function of light intensity, frequency detuning, and atomic velocity.
Size dependence of lifetime and absorption cross section of Si nanocrystals embedded in SiO2
Authors: Garcia C. , Garrido B. , Pellegrino P. , Ferre R. , Moreno J. A. , Morante J. R. , Pavesi L. , Cazzanelli M.
Published: March 2003

Photoluminescence lifetimes and optical absorption cross sections of Si nanocrystals embedded in SiO2 have been studied as a function of their average size and emission energy. The lifetimes span from 20 μs for the smallest sizes (2.5 nm) to more than 200 μs for the largest ones (7 nm). The passivation of nonradiative interface states by hydrogenation increases the lifetime for a given size. In contrast with porous Si, the cross section per nanocrystal shows a nonmonotonic behavior with emission energy. In fact, although the density of states above the gap increases for larger nanocrystals, this trend is compensated by a stronger reduction of the oscillator strength, providing an overall reduction of the absorption cross section per nanocrystal for increasing size.
Photonic properties of an inverted face centered cubic opal under stretch and shear
Authors: Babin V. , Garstecki P. , Hołyst R.
Published: March 2003

We present the results of numerical calculations of the dispersion relations for an inverted fcc opal structure subjected to a stretch and shear. We find that shearing of the crystal only lowers the gap width and slightly changes the midgap frequency. Interestingly, that in a large range of stretch amplitudes (up to 10%) the gap width is preserved and even slightly enhanced. The midgap frequency depends almost linearly on the stretch amplitude allowing for tuning an inverted fcc opal structure to a desired operational frequency. This last property may be important for the manufacturing process.
Generating visible single photons on demand with single InP quantum dots
Authors: Zwiller V. , Aichele T. , Seifert W. , Persson J. , Benson O.
Published: March 2003

We present photon correlation measurements performed on a device based on single InP quantum dots. The device consists of a 400 nm thick membrane containing a low density of quantum dots on a metal mirror. Measurements done under continuous excitation reveal a very pronounced antibunching dip while measurements done under pulsed excitation enable the generation of single photons on demand at the optimum wavelength for silicon-based single-photon detectors.
Shot noise in negative-differential-conductance devices
Authors: Song W. , Mendez E. E. , Kuznetsov V. , Nielsen B.
Published: March 2003

We have compared the shot-noise properties at T=4.2 K of a double-barrier resonant-tunneling diode and a superlattice tunnel diode, both of which exhibit negative differential-conductance (NDC) in their current–voltage characteristics. While the noise spectral density of the former device was greatly enhanced over the Poissonian value of 2eI in the NDC region, that of the latter device remained 2eI. This result implies that charge accumulation, not system instability, is responsible for shot-noise enhancement in NDC devices.
Faraday effect of photonic crystals
Authors: Koerdt C. , Rikken G. L. J. A. , Petrov E. P.
Published: March 2003

We measured Faraday rotation in three-dimensional photonic colloidal crystals impregnated with a Faraday active, transparent liquid. The Faraday effect was found to strongly increase inside the stop band, whereas outside it follows the normal spectral behavior of a paramagnetic dielectric with an effective Verdet constant equal to the product of the liquid’s Verdet constant and its filling fraction.
Coalescence of nanometer silver islands on oxides grown by filtered cathodic arc deposition
Authors: Byon Eungsun , Oates Thomas W. H. , Anders André
Published: March 2003

Ultrathin silver films have been deposited on glass and oxide-coated glass using filtered cathodic arc deposition and, for comparison, magnetron sputtering. The energetic differences between these deposition methods lead to initially different film properties. Silver films made by cathodic arc deposition show an earlier onset of island coalescence, indicating a lower aspect ratio than islands produced by evaporation and sputtering. However, the as-deposited films are thermodynamically unstable, exhibiting changes on a timescale of minutes. While films of islands tend to increase their sheet resistance with time, the sheet resistance of contiguous films shows a decrease. Both effects can be explained by silver mobility driven to minimize film and interfacial energy.
Delayed fracture of lead zirconate titanate ferroelectric ceramics under sustained electric field
Authors: Wang Y. , Chu W. Y. , Gao K. W. , Su Y. J. , Qiao L. J.
Published: March 2003

In this letter, we report on delayed fracture of lead zirconate titanate ceramics with a Zr/Ti ratio of 52/48(PZT-5) ferroelectric ceramics in silicon oil under sustained electric field, and that in silicon oil or moist atmosphere under sustained mechanical load. The experimental results show that sustained electric fields may cause delayed fracture of PZT-5 ceramics and there is a threshold field for the delayed fracture. The threshold electric field is less than one third of the critical electric field to cause instant fracture and is also half of the coercive field of the PZT-5 ceramics.
Point x-ray source using graphite nanofibers and its application to x-ray radiography
Authors: Matsumoto Takahiro , Mimura Hidenori
Published: March 2003

A point x-ray emission was obtained from a diode configuration composed of a graphite-nanofiber cold cathode and a conical-shaped copper metal anode. When combined with a highly sensitive charge coupled device (CCD) camera with a scintillator, this x-ray source was then used to obtain x-ray transmission images of a tungsten mesh and an x-ray test chart. The spatial resolution of this x-ray radiography system was on the order of 10 μm and the acquisition time for these images was less than 10 s. This combination of the point x-ray source and the sensitive detection system offers a high-resolution x-ray radiography system.
Recovery process of degraded ferroelectric properties in the forming-gas-annealed Pt/Bi4−xLaxTi3O12/Pt capacitor
Authors: Chon Uong , Jang Hyun M.
Published: March 2003

The recovery of ferroelectric properties in the forming-gas-annealed Pt/Bi4−xLaxTi3O12/Pt (Pt/BLT/Pt) capacitor was studied by examining changes in ferroelectric responses, phase evolution, and spatial distributions of relevant species during the recovery annealing. The degraded ferroelectric properties were practically restored to their original values after the recovery annealing at 600 °C for 10 min in an O2 atmosphere. The following recovery process has been delineated from the present study: (i) the removal of impregnated protons from the degraded capacitor due to the chemical potential difference of protons between the forming-gas-annealed capacitor and the contacting atmosphere, and (ii) the restoration of perovskite BLT phase with the help of replenishment of the Bi and oxygen losses via diffusion from the neighboring intact region to the Bi-depleted columnar region located beneath the top Pt electrode.
Structural evolution of crystalline polymer latex films: Propagating and confined acoustic modes
Authors: Pierno Matteo , Casari Carlo S. , Piazza Roberto , Bottani Carlo E.
Published: March 2003

Structural evolution of polytetrafluoroethylene latex polymer films has been studied by Brillouin light scattering and atomic force microscopy (AFM). Using a tunable sintering process, we have controlled the degree of particle connectivity yielding a crossover in the behavior of acoustic excitations ranging from confined modes to propagating phonons. This transition has been correlated also to the surface morphology by AFM. Fully sintered films are characterized by extended “fibrillar” crystalline regions embedded in a disordered matrix, which still retain features of the original particulate structure.
Removal of dangling bonds and surface states on silicon (001) with a monolayer of selenium
Authors: Tao Meng , Udeshi Darshak , Basit Nasir , Maldonado Eduardo , Kirk Wiley P.
Published: March 2003

Dangling bonds and surface states are inherent to semiconductor surfaces. By passivating dangling bonds on the silicon (001) surface with a monolayer of selenium, surface states are removed from the band gap. Magnesium contacts on selenium-passivated silicon (001) behave ohmically, as expected from the work function of magnesium and the electron affinity of silicon. After rapid thermal annealing and hot-plate annealing, magnesium contacts on selenium-passivated silicon (001) show better thermal stability than on hydrogen-passivated silicon (001), which is attributed to the suppression of silicide formation by selenium passivation.
High-performance continuous-wave operation of superlattice terahertz quantum-cascade lasers
Authors: Köhler Rüdeger , Tredicucci Alessandro , Beltram Fabio , Beere Harvey E. , Linfield Edmund H. , Davies A. Giles , Ritchie David A. , Dhillon Sukhdeep S. , Sirtori Carlo
Published: March 2003

The cw operation of chirped-superlattice quantum-cascade lasers emitting at λ∼67 μm (4.4 THz) is analyzed. Collected (min. 33% efficiency) output powers of 4 mW per facet are measured at liquid helium temperatures and a maximum operating temperature of 48 K is reached. Under pulsed excitation at duty cycles of 0.5%–1%, slightly higher (10%) peak powers are reached, and the device can be operated up to 67 K. Low threshold current densities of 165 and 185 A cm−2 are observed in pulsed and cw operation, respectively. The operation of the laser is examined using the Hakki–Paoli technique to estimate the net gain of the structure.
Tailoring the photonic band gap of a porous silicon dielectric mirror
Authors: Agarwal V. , del Rı́o J. A.
Published: March 2003

A systematic method to fabricate porous-silicon one-dimensional photonic crystals has been engineered to have a photonic bandwidth up to 2000 nm. The observation of the tailorability of the photonic band gap (PBG) underscores the requirement of the large refractive index contrast for making broad PBG structures. In this letter, we present the fabrication and characteristics of such structures that may be promising structures for a large variety of applications.
In
2
O
3
nanowires as chemical sensors
Authors: Li Chao , Zhang Daihua , Liu Xiaolei , Han Song , Tang Tao , Han Jie , Zhou Chongwu
Published: March 2003

We present an approach to use individual In2O3 nanowire transistors as chemical sensors working at room temperature. Upon exposure to a small amount of NO2 or NH3, the nanowire transistors showed a decrease in conductance up to six or five orders of magnitude and also substantial shifts in the threshold gate voltage. These devices exhibited significantly improved chemical sensing performance compared to existing solid-state sensors in many aspects, such as the sensitivity, the selectivity, the response time, and the lowest detectable concentrations. Furthermore, the recovery time of our devices can be shortened to just 30 s by illuminating the devices with UV light in vacuum.
Nanoscale molecular-switch devices fabricated by imprint lithography
Authors: Chen Yong , Ohlberg Douglas A. A. , Li Xuema , Stewart Duncan R. , Stanley Williams R. , Jeppesen Jan O. , Nielsen Kent A. , Stoddart J. Fraser , Olynick Deirdre L. , Anderson Erik
Published: March 2003

Nanoscale molecular-electronic devices comprising a single molecular monolayer of bistable [2]rotaxanes sandwiched between two 40-nm metal electrodes were fabricated using imprint lithography. Bistable current–voltage characteristics with high on–off ratios and reversible switching properties were observed. Such devices may function as basic elements for future ultradense electronic circuitry.
Fabrication of spin-frustrated Sm2Mo2O7 epitaxial films: High throughput optimization using a temperature gradient method
Authors: Nishimura J. , Fukumura T. , Ohtani M. , Taguchi Y. , Kawasaki M. , Ohkubo I. , Koinuma H. , Ohguchi H. , Ono K. , Oshima M. , Tokura Y.
Published: March 2003

Epitaxial thin films of pyrochlore-type ferromagnetic molybdates Sm2Mo2O7, as a geometrically spin-frustrated system, were fabricated by a pulsed-laser deposition. The temperature gradient method combined with the concurrent x-ray diffraction method was exploited for high throughput optimization of the film quality. The excess supply of Mo for compensating the volatile Mo-related species improved the crystallinity. The resistivity and magnetization of the optimized film were 3 mΩ cm and 0.8 μB/Mo at 10 K, respectively, being similar to those of a single crystal. The observed finite anomalous Hall term that persists down to the lowest temperature reflects an appearance of spin chirality in this spin-frustrated system.
The use of capillary force for fabricating probe tips for scattering-type near-field scanning optical microscopes
Authors: Kawata Yoshimasa , Urahama Seiji , Murakami Manambu , Iwata Futoshi
Published: March 2003

We present a fabrication technique of a probe tip for scattering-type near-field microscopes. In the technique, capillary force is used to attach a metal particle at the apex of micropipettes. We can attach various kinds of particles, such as fluorescent particles, semiconductor particles, nonlinear materials, as well as metal particles. We have attached particles smaller than 180 nm in diameter.
Vertical and lateral mobilities in n-(Ga, Mn)N
Authors: Kim Jihyun , Ren F. , Thaler G. T. , Frazier R. , Abernathy C. R. , Pearton S. J. , Zavada J. M. , Wilson R. G.
Published: March 2003

Lateral electron mobilities in 0.2-μm-thick n-(Ga, Mn)N films were obtained from Hall measurements, producing values of 116∼102 cm2/V s in the temperature range from 298 to 373 K. These values are comparable to, but slightly lower than, electron mobilities in n-GaN of the same electron concentration. By sharp contrast, analysis of the reverse saturation current in mesa Schottky diodes fabricated in the n-(Ga, Mn)N show vertical electron mobilities of 840∼336 cm2/V s in the temperature range from 298 to 373 K. This is consistent with a reduction in electron scattering by charged dislocations for vertical transport geometries [M. Misra, A. V. Sampath, and T. D. Moustakas, Appl. Phys. Lett. 76, 1045 (2000)].
Integrated optical distributed feedback laser with Ti:Fe:Er:LiNbO3 waveguide
Authors: Das B. K. , Ricken R. , Sohler W.
Published: March 2003

A distributed feedback (DFB) laser in LiNbO3 is demonstrated using a Ti:Fe:Er:LiNbO3 waveguide with a holographically written photorefractive grating. The DFB laser was combined with a waveguide amplifier on the same substrate. Up to 1.12 mW of output power at λ=1531.35 nm was emitted by the laser/amplifier combination at a pump power level of 240 mW (λP=1480 nm). The emission spectrum consists of the two lowest-order DFB modes of about 3.9 GHz frequency spacing. Whereas the measured threshold gain of ∼3.3 dB/cm approximately agreed with the modeling results, the observed mode spacing was clearly smaller than calculated.
Nanomechanics of membrane tubulation and DNA assembly
Authors: Roopa T. , Shivashankar G. V.
Published: March 2003

We report an interesting regime of tubule formation in multilamellar membrane vesicles. An optically trapped bead is used to apply a localized subpicoNewton force on a cationic vesicle to form a membrane tubule. The force extension curves reveal a saturation phase, with the tubule length extending up to tens of microns, beyond a threshold force 0.6±0.2 pN. We then use the tubule as a sensor for monitoring the dynamics of charge induced DNA integration on cationic membrane vesicles. Our results may also have applications in the development of nanowires and nanofluidic devices.
Influence of intrinsic strain on the surface acoustic wave-induced birefringence in InGaAs–GaAs and InGaAsP–InP multiple-quantum-well optical modulators
Authors: Wang X. T. , Jain F. C.
Published: March 2003

The effect of intrinsic strain on the surface acoustic wave (SAW)-induced birefringence in InGaAs–GaAs and InGaAsP–InP multiple-quantum-well (MQW) optical modulators is investigated. We solve the exciton equation in momentum space using a two-dimensional quadrature method to obtain the birefringence Δn by the SAW induced strain. Our calculations are in good agreement with the experimental results in AlGaAs–GaAs and InGaAs–GaAs MQWs. Intrinsic strain influences the band mixing of valance subbands in quantum wells in the presence of asymmetric strain. We have shown that birefringence (Δn∼0.06) is significantly higher in tensile strained (ε∼0.7%) InGaAsP–InP MQWs than in compressively strained In0.21Ga0.79As–GaAs MQW devices.
Selective desorption of interfacial SiO2
Authors: Copel M.
Published: March 2003

In this letter, we report the selective thermal decomposition of SiO2 from a silicate/Si(001) interface, without silicidation of the dielectric. We observe kinetics that are similar to SiO2 desorption from Si(001), which takes place by SiO volatilization, suggesting that a similar mechanism is responsible at a buried interface. Interface desorption is a route to direct dielectric/silicon structures without prior removal of SiO2.
Oxygen induced p-doping of α-nickel phthalocyanine vacuum sublimed films: Implication for its use in organic photovoltaics
Authors: Anthopoulos Thomas D. , Shafai Torfeh S.
Published: March 2003

The effects of oxygen doping on the charge transport and photovoltaic properties of α-nickel phthalocyanine (α-NiPc) based devices are investigated using in situ and ex situ I–V measurements. I–V characteristics for devices employing gold contacts indicate ohmic conduction at low voltages, followed by space-charge-limited conduction in higher fields. Upon exposure of NiPc to dry air an increase in the hole concentration (p0) from 8.5×1010 to 2.6×1015 m−3 is observed. When the top gold ohmic cathode is replaced by lead, Schottky type behavior is evident with the junction exhibiting photovoltaic effect. The energy conversion efficiency of the cell increases following exposure to oxygen. These results suggest that fabrication of air stable electronic devices based on NiPc is feasible.
Identification of implantation-induced defects in GaN: A near-edge x-ray absorption fine structure study
Authors: Katsikini M. , Pinakidou F. , Paloura E. C. , Wesch W.
Published: March 2003

We apply near-edge x-ray absorption fine structure spectroscopy, at the N K edge, in order to identify the signature of implantation-induced defects in the partial density of empty states in GaN implanted with O, Mg, and Si ions. The dose range was 1014–1018 cm−2. It is found that two of the implantation-induced defects introduce characteristic resonances (hereafter called RL1 and RL2) in the near-edge x-ray absorption fine structure spectra. RL1 appears 1.7 eV below the absorption edge, its formation is independent of the projectile and the implantation dose, and is attributed to nitrogen interstitials. RL2, which appears at about 1.0 eV above the absorption edge, is generated when the dose exceeds 1016 cm−2 and is attributed to nitrogen dangling bonds.
Theory of polarization enhancement in epitaxial BaTiO3/SrTiO3 superlattices
Authors: Neaton J. B. , Rabe K. M.
Published: March 2003

The spontaneous polarization of epitaxial BaTiO3/SrTiO3 superlattices is studied as a function of composition using first-principles density-functional theory within the local density approximation. With the in-plane lattice parameter fixed to that of bulk SrTiO3, the computed superlattice polarization is enhanced above that of bulk BaTiO3 for superlattices with BaTiO3 fraction larger than 40%. In contrast to their bulk paraelectric character, the SrTiO3 layers are found to be tetragonal and polar, possessing nearly the same polarization as the BaTiO3 layers. General electrostatic arguments elucidate the origin of the polarization in the SrTiO3 layers, with important implications for other nanostructured ferroelectrics.
Combined scanning electrochemical atomic force microscopy for tapping mode imaging
Authors: Kueng A. , Kranz C. , Mizaikoff B. , Lugstein A. , Bertagnolli E.
Published: March 2003

With the integration of submicro- and nanoelectrodes into atomic force microscopy (AFM) tips using microfabrication techniques, an elegant approach combining scanning electrochemical microscopy (SECM) with atomic force microscopy has recently been demonstrated. Simultaneous imaging of topography and electrochemistry at a sample surface in AFM tapping mode with integrated SECM–AFM cantilevers oscillated at or near their resonance frequency is shown. In contrast to contact mode AFM imaging frictional forces at the sample surface are minimized. Hence, topographical and electrochemical information of soft surfaces (e.g., biological species) can be obtained.
Fiber-taper-coupled L-band Er3+-doped tellurite glass microsphere laser
Authors: Peng Xiang , Song Feng , Jiang Shibin , Peyghambarian N. , Kuwata-Gonokami Makoto , Xu Lei
Published: March 2003

Er 3+ -doped tellurite glass microspheres were fabricated. The microspheres were made by a spin method. A tapered fiber was used to couple the pump into the microsphere and couple out the laser. L-band microsphere laser is demonstrated. This miniature Er3+-doped tellurite glass microsphere laser can be used in photonic microdevices.
Ultraviolet laser treatment of multiwall carbon nanotubes grown at low temperature
Authors: Kim J. S. , Ahn K. S. , Kim C. O. , Hong J. P.
Published: March 2003

Simple laser irradiation of well-aligned multiwall carbon nanotubes (MWCNTs) was performed to intentionally modify structural defects and to ablate possible contamination of the MWCNTs. Scanning electron microscopy and transmission electron microscopy confirmed the clear presence of the MWCNTs with open tips. A Raman spectra exhibited a decrease in an intensity ratio (ID/IG) of 1352 cm−1 (D band) over 1583 cm−1 (G band) peaks by significantly reducing the amorphous carbon phases of D band peaks. The structural improvement in the MWCNTs after optimum laser exposure resulted in a reduction of the turn-on voltage from 1.0 to 0.6 V/μm and an increase in the emission current.
Polycrystalline silicon layer transfer by ion-cut
Authors: Yun C. H. , Quitoriano N. , Cheung N. W.
Published: March 2003

Ion-cut polycrystalline silicon (polysilicon) layer transfer by thermal separation was demonstrated after observing hydrogen-induced surface blistering and direct bonding of a chemomechanically polished surface. After hydrogen implantation into a chemical-vapor deposited polysilicon wafer (the donor wafer), the wafer surface was polished for wafer bonding. The hydrogen-implanted and polished wafer was then bonded to a thermally oxidized silicon wafer (the handle wafer) by low-temperature wafer direct bonding. The bonded pair was then heated until hydrogen-induced silicon layer cleavage occurred along the hydrogen-implanted layer, resulting in the transfer of the polysilicon layer to the handle substrate. The transferred polysilicon surface had the same roughness as ion-cut single-crystal silicon layers. The layer transfer time of polysilicon was significantly reduced from that of single-crystal silicon, and had an activation energy nearly ten times lower than that of its single-crystal counterpart.
Vacancies and voids in hydrogenated amorphous silicon
Authors: Smets A. H. M. , Kessels W. M. M. , van de Sanden M. C. M.
Published: March 2003

The hydride configurations in the hydrogenated amorphous silicon (a-Si:H) network have been studied by means of infrared absorption spectroscopy. The results on the film mass density of a-Si:H deposited by means of an expanding thermal plasma reveal the presence of two distinct regions in terms of hydrogen content and microstructure: below approximately 14 at. % H a-Si:H contains predominantly divacancies decorated by hydrogen, above 14 at. % H a-Si:H contains microscopic voids. These two distinct regions provide additional information on the origin of the low and high hydride stretching modes at 1980–2010 and 2070–2100 cm−1, respectively.
Effect of tensile strain/well-width combination on the measured gain-radiative current characteristics of 635 nm laser diodes
Authors: Lewis G. M. , Smowton P. M. , Blood P. , Chow W. W.
Published: March 2003

Polarization sensitive measurements, in real units, of gain and spontaneous emission of GaInP lasers with three strain and well-width combinations allow us to isolate the intrinsic effects of strain when the well width is also adjusted to maintain a fixed output wavelength. Varying tensile strain and well width, for 635 nm operation, have no effect on transverse magnetic polarized recombination at fixed gain, which is consistent with a constant effective mass in the uppermost valence band, but the total transparency current decreases from 116 to 83 A cm−2 due to increasing separation of light and heavy hole bands.
Determination of built-in electric fields in quaternary InAlGaN heterostructures
Authors: Teisseyre H. , Suski T. , Łepkowski S. P. , Anceau S. , Perlin P. , Lefebvre P. , Kończewicz L. , Hirayama H. , Aoyagi Y.
Published: March 2003

A study of internal electric field contribution to the light emission mechanism of InAlGaN based multiquantum wells was performed. To determine the magnitude of the built-in electric field we employed: (i) theoretical estimation of the piezoelectric and spontaneous polarizations, (ii) analysis of the emission energy as a function of the quantum well width, (iii) hydrostatic pressure experiments, and finally (iv) measurements of photoluminescence decay. Performed calculations gave high magnitude of the built-in electric field. On the contrary, independently of the quantum well width the pressure shift of the light emission energy and the photoluminescence decay time showed almost constant values. These observations are interpreted as evidence of a lack of the built-in electric field in the used quaternary quantum wells. Possible reasons for the controversies between theory and experiment are suggested.