bluuebunny/crossref_metadata_2025 huggingface dataset:
Control of Ge dots in dimension and position by selective epitaxial growth and their optical properties
Authors: Kim E. S. , Usami N. , Shiraki Y.
Published: July 2002

Control of dimension and position of Ge dots on a nanoscale is accomplished by combining Stranski–Krastanov growth mode with selective epitaxial growth technique in windows surrounded by SiO2 films on Si substrates. The dimension and the number of these dots are controlled by the size of the windows, and a single dot is grown in a window with the size of less than 300 nm. Moreover, clear phonon-resolved photoluminescence (PL) is observed from the Ge dots, reflecting the improved uniformity in their dimensions. The PL energy is found to be strongly dependent on the dot’s dimension.
Evidence for compositional inhomogeneity in low In content (InGa)N obtained by resonant Raman scattering
Authors: Behr D. , Wagner J. , Ramakrishnan A. , Obloh H. , Bachem K.-H.
Published: July 2002

Resonant Raman scattering has been used to study hexagonal InxGa1−xN films with x≈0.1, grown by metal–organic chemical vapor deposition on sapphire substrates. To vary the energy difference between the fundamental gap energy of the (InGa)N and the photon energy of the discrete laser emission lines used for recording the spectra, the sample temperature was varied between 300 and 870 K. Raman scattering by the (InGa)N A1(LO) phonon shows a clear resonance profile when the fundamental energy gap approaches the incident photon energy, with a maximum enhancement in scattering efficiency of 10 measured relative to the scattering strength of the E2 phonon mode. The (InGa)N A1(LO) phonon was found to shift to higher frequencies with respect to the E2 mode when the experimental conditions were varied from excitation below the fundamental energy gap of (InGa)N to above-band-gap excitation. This frequency shift is explained by the presence of compositional inhomogeneity, which results in localized regions with higher In content, and thus, lower gap energy and phonon frequency, and regions with lower In content, and consequently, higher gap energy and phonon frequency.
Scanning tunneling microscopy and scanning tunneling spectroscopy of self-assembled InAs quantum dots
Authors: Legrand B. , Grandidier B. , Nys J. P. , Stiévenard D. , Gérard J. M. , Thierry-Mieg V.
Published: July 2002

We present cross-sectional scanning tunneling microscopy images and scanning tunneling spectroscopy results of InAs quantum dots grown on GaAs. The samples contain 12 arrays of quantum dots. The analysis of the scanning tunneling microscope images reveals the self-alignment of the dots as well as the different dot interfaces with the under- and overgrown GaAs layers. We measure the strain distribution along the [001] direction in the (110) plane. The roughness of the dot interfaces along the [1̄10] direction is also estimated and local spectroscopy of the dots evidences the electronic confinement (measured gap of 1.25 eV compared with 0.4 eV for bulk InAs).
Preferential growth of cubic GaN on sapphire (0001) substrates by metal organic molecular beam epitaxy
Authors: Kurobe T. , Sekiguchi Y. , Suda J. , Yoshimoto M. , Matsunami H.
Published: July 2002

Preferential growth of high-quality cubic GaN on sapphire (0001) substrates was realized at 800 °C under a Ga-rich condition by metal organic molecular beam epitaxy. Hexagonal GaN was grown under a N-rich condition. On the contrary, under the Ga-rich condition, the growing layer changed from a hexagonal phase to a cubic phase as the growth proceeds, which was verified by in situ reflection high-energy electron diffraction. The low-temperature photoluminescence (PL) of this layer was dominated by a sharp and intense excitonic emission of cubic GaN, indicating high crystal quality. The results of PL and x-ray diffraction indicate that the polytype of GaN can be intentionally controlled by changing V/III ratios without serious degradation of crystal quality. In addition, enhancement of cubic phase growth by using a low-temperature grown buffer layer is also discussed.
Observation of spin splitting in single InAs self-assembled quantum dots in AlAs
Authors: Thornton A. S. G. , Ihn T. , Main P. C. , Eaves L. , Henini M.
Published: July 2002

Using magneto-tunneling spectroscopy, we observe the Zeeman spin splitting of the ground state of a single InAs quantum dot grown within AlAs. We obtain values for the g factor of different quantum dots between +0.52±0.08 and +1.6±0.2, with magnetic field applied in the plane of the dot. This value for the g factor is considerably different from that of bulk InAs (g=−14.8), and we explain this using a simple three band k⋅p calculation. Using the spin split states of the dot as a probe, we observe the complete spin polarization of the emitter accumulation layer.
Quasiphase matched second harmonic generation in ZnSe waveguide structures modulated by focused ion beam implantation
Authors: Kühnelt M. , Leichtner T. , Kaiser S. , Hahn B. , Wagner H. P. , Eisert D. , Bacher G. , Forchel A.
Published: July 2002

Second harmonic generation is investigated in a ZnSe/ZnS0.22Se0.78 waveguide grown by metal organic vapor phase epitaxy on (001) GaAs substrate. Phase matching is achieved by a first order periodic modulation of the nonlinear susceptibility χ(2) within the waveguide structure. The modulation is generated by a focused ion beam implantation technique, resulting in a well confined damage of the crystalline structure. The observed phase matching wavelengths corresponding to various modulation periods between 1.6 and 1.9 μm agree with those predicted by calculations.
Selective formation and alignment of InAs quantum dots over mesa stripes along the [011] and [001] directions on GaAs (100) substrates
Authors: Zhang R. , Tsui R. , Shiralagi K. , Convey D. , Goronkin H.
Published: July 2002

We have studied the selective formation of InAs self-organized quantum dots on top of [001]- and [011]-oriented mesa stripes on patterned GaAs (100) substrates. The GaAs stripes are also grown by selective area epitaxy. The dot density and spatial distribution depend on both the stripe orientation and the width of the (100) top facet of the stripe. The density is higher for stripes aligned in the [001] direction, and lower for those aligned in the [011] direction, respectively, when compared to that obtained on a planar substrate under the same growth conditions. In addition, the dot uniformity is improved by reducing the top facet width below 200 nm in the growth of the mesa stripes, and well-aligned rows of dots are obtained for sub-100-nm widths.
Soft breakdown in ultrathin gate oxides: Correlation with the percolation theory of nonlinear conductors
Authors: Houssa M. , Nigam T. , Mertens P. W. , Heyns M. M.
Published: July 2002

The dielectric breakdown under constant current stressing of 4.2 nm SiO2 gate oxides is investigated. After soft breakdown, which corresponds to an anomalous increase of the stress-induced leakage current of metal–oxide–semiconductor capacitors, the current behaves like a power law of the applied gate voltage VG. After soft breakdown, charge is further injected into the SiO2 layer in order to extract the effective resistivity ρeff of the system as a function of the density of oxide traps D generated in the layer. It is found that ρeff behaves like a power law of (D−Dc) where Dc is the critical density of traps generated at soft breakdown. These results are in fair agreement with the predictions of the percolation theory of nonlinear conductor networks. Besides, the value of the critical exponent related to the resistivity is close to the one expected in two dimensions.
Light emission from individual self-assembled InAs/GaAs quantum dots excited by tunneling current injection
Authors: Yamanaka K. , Suzuki K. , Ishida S. , Arakawa Y.
Published: July 2002

We report emission from individual quantum dots excited by tunneling current injection using a scanning tunneling microscope (STM). By scanning the STM tip above the self-assembled InAs quantum dots, a spatially resolved scanning tunneling luminescence (STL) image was measured, which contained a fluorescent circular region with a diameter of 50 nm originating from a single InAs quantum dot. It was found that the spatial resolution of the STL system was about 40 nm, which is mainly due to lateral diffusion of holes injected into a GaAs capping layer grown at low temperature (480 °C). We also obtained STL spectra with a sharp single luminescent peak from a single InAs quantum dot.
Highly regular self-organization of step bunches during growth of SiGe on Si(113)
Authors: Darhuber A. A. , Zhu J. , Holý V. , Stangl J. , Mikulı́k P. , Brunner K. , Abstreiter G. , Bauer G.
Published: July 2002

We have studied the structural properties of highly periodic arrays of terrace steps in a Si/SiGe multilayer grown on a miscut Si(113) substrate by atomic force microscopy, x-ray reflection and high resolution x-ray diffraction. The data reveal a regular array of step bunches with vertical correlation within the multilayer and periodic surface steps extending over lengths of several tens of microns. The (113)-faceted terraces have a lateral period of about 360 nm which is locally modulated due to a long-range waviness of the surface.
Pulsed laser deposition of oxides: Why the optimum rate is about 1 Å per pulse
Authors: Strikovski Mikhail , Miller John H.
Published: July 2002

An adiabatic thermalization model is developed to describe a typical pulsed laser deposition (PLD) process. The interaction of atoms in the plume, ejected from the target, with those of the background gas (e.g., oxygen) is specifically considered. The model gives a physical definition of the “plume range,” which depends on the particular PLD system, and calculates the range values. One prediction is that when the target-to-substrate distance is optimized, the deposition rate exhibits an extremely weak dependence on pulse intensity and will always be about 1 Å per pulse.
Single laser beam-induced surface deformation on azobenzene polymer films
Authors: Bian S. , Li L. , Kumar J. , Kim D. Y. , Williams J. , Tripathy S. K.
Published: July 2002

The surface deformation induced by a Gaussian laser beam on azobenzene polymer film reveals strong polarization dependence and establishes that an optical field component in the direction of the intensity gradient is essential for the deformation process. The experimental results allow us to infer the phase relationship between the surface relief gratings and the writing interference pattern.
Single zero-dimensional excitons in CdSe/ZnSe nanostructures
Authors: Kümmell T. , Weigand R. , Bacher G. , Forchel A. , Leonardi K. , Hommel D. , Selke H.
Published: July 2002

Zero-dimensional excitons (0DXs) in CdSe/ZnSe nanostructures have been studied by time- and spatially resolved photoluminescence spectroscopy. The three-dimensional confinement is confirmed by an exciton lifetime up to 550 ps, independent of temperature up to 130 K. By preparing mesa structures with diameters down to 50 nm as local probes, an extremely high spatial resolution is achieved, giving experimental access to single 0DXs. A splitting of the ground state into a linearly polarized doublet with an energy spacing up to 1.5 meV is found, varying from dot to dot in sign and magnitude. This indicates a noncircular shape with no preferential orientation of the dots. The dot density is estimated to increase from 5×1010 to 1.5×1011 cm−2, when changing the nominal CdSe layer thickness from 1 to 3 ML, i.e., close to the critical thickness.
An oxygen sensor based on copper(I)-conducting CuTi2(PO4)3 glass ceramics
Authors: Yamamoto Kousuke , Kasuga Toshihiro , Nogami Masayuki
Published: July 2002

An electrochemical device for sensing oxygen at low temperature, e.g., 100 °C, was developed by using fast Cu+-conducting ceramics. As the solid electrolyte, a dense glass ceramic consisting of crystalline CuTi2(PO4)3 and Cu3(PO4)2 phases, which three-dimensionally interlock with each other, has been successfully prepared by the controlled crystallization of the glass in the Cu–Ti–P–O system. The electromotive force generated between the electrodes showed a good Nernstian response to the partial pressure of oxygen even at a relatively low temperature such as 100 °C. The mechanism for oxygen sensing was discussed with regard to the fast Cu+ ion conductivity and the redox reaction between Cu+ ion in the dense glass ceramic and oxygen gas.
MeV γ-ray yield from solid targets irradiated with fs-laser pulses
Authors: Gahn C. , Pretzler G. , Saemann A. , Tsakiris G. D. , Witte K. J. , Gassmann D. , Schätz T. , Schramm U. , Thirolf P. , Habs D.
Published: July 2002

We have investigated the MeV bremsstrahlung which is emitted when fast electrons generated by the interaction of 200 mJ, 130 fs Ti:sapphire laser pulses with a preformed plasma penetrate into a solid target. Employing different targets the dependence of the γ-ray spectrum on the atomic number was studied. We detected single γ photons with an energy up to 2.5 MeV and found a maximum conversion efficiency of the laser energy into MeV bremsstrahlung of 4×10−6. Data analysis using a Monte Carlo code revealed a fast-electron temperature of 0.9 MeV.
Mechanism for interfacial adhesion strength of an ion beam mixed Cu/polyimide with a thin buffer layer
Authors: Chang G. S. , Chae K. H. , Whang C. N. , Kurmaev E. Z. , Zatsepin D. A. , Winarski R. P. , Ederer D. L. , Moewes A. , Lee Y. P.
Published: July 2002

A Cu (400 Å)/Al (50 Å)/polyimide system showed larger adhesion strength than that of Cu (400 Å)/polyimide after N2+ ion beam mixing. X-ray emission spectroscopy was performed to elucidate the mechanism of adhesion enhancement of the ion beam mixed Cu (400 Å)/polyimide with a thin Al buffer layer. Cu L2,3 x-ray emission spectra showed the formation of a CuAl2O4 layer which is strongly correlated with the large adhesion strength of a Cu/Al/polyimide. A decrease in adhesion strength at an ion dose higher than 5×1015 cm−2 was also explained by the formation of an amorphous carbon. This was understood by investigating C Kα x-ray emission spectra. The overall spectroscopic results were in accordance with the behavior of quantitative adhesion strength.
Observation of supercurrent distribution in YBa2Cu3O7−δ thin films using THz radiation excited with femtosecond laser pulses
Authors: Shikii S. , Kondo T. , Yamashita M. , Tonouchi M. , Hangyo M. , Tani M. , Sakai K.
Published: July 2002

We have demonstrated that the supercurrent distribution in current-biased YBa2Cu3O7−δ thin films can be obtained by measuring the radiation power of THz electromagnetic pulses excited with femtosecond laser pulses. As the radiation power is proportional to the square of the bias current density at the laser spot position, the two-dimensional current distribution can be obtained from the intensity distribution of THz radiation by scanning the laser spot. The characteristic supercurrent distribution is analyzed by using the critical-state model.
Photonic crystals for micro lightwave circuits using wavelength-dependent angular beam steering
Authors: Kosaka Hideo , Kawashima Takayuki , Tomita Akihisa , Notomi Masaya , Tamamura Toshiaki , Sato Takashi , Kawakami Shojiro
Published: July 2002

Light-beam steering that is extremely wavelength dependent has been demonstrated by using photonic crystals fabricated on Si. The scanning span reached 50° with only a 1% shift of incident wavelength at around 1 μm. The resulting angular dispersion is two orders of magnitude larger than that achieved with conventional prisms or gratings. The application of such superprism phenomena promises to enable the fabrication of integrated micro lightwave circuits that will allow more efficient use of wavelength resources when used in wavelength multiplexers/demultiplexers or dispersion compensators by enabling lower loss and broader bandwidth.
Synthesis and properties of Sr2CeO4 blue emission powder phosphor for field emission displays
Authors: Jiang Yong Dong , Zhang Fuli , Summers Christopher J. , Wang Zhong Lin
Published: July 2002

A blue emission powder phosphor Sr2CeO4 for field emission displays was prepared using a chemical coprecipitation technique, which is most suitable for large-scale production. The powders were fired at different temperatures to optimize the properties. Firing the powder at 1200 °C for 2 h gave the highest luminescence efficiency of 5.4 lm/W at 4 kV and 29.0 lm/W at 10 kV. The emission peak of this phosphor is at ∼470 nm and Commission International de l’Eclairage coordinates are x=0.19, y=0.26.
Charge separation in coupled InAs quantum dots and strain-induced quantum dots
Authors: Schoenfeld W. V. , Lundstrom T. , Petroff P. M. , Gershoni D.
Published: July 2002

We present an InAs self-assembled quantum dot structure designed to spatially separate and store photo-generated electron-hole pairs. The structure consists of pairs of strain-coupled quantum dots. Separation of electron-hole pairs into the quantum dots and strain-induced quantum dots has been observed using power-dependant photoluminescence and bias-dependent photoluminescence.
Quantum box size effect on vertical self-alignment studied using cross-sectional scanning tunneling microscopy
Authors: Legrand B. , Nys J. P. , Grandidier B. , Stiévenard D. , Lemaı̂tre A. , Gérard J. M. , Thierry-Mieg V.
Published: July 2002

InAs quantum boxes separated by GaAs spacer layers are known to exhibit a vertical self-organization along the growth direction. The alignment probability between two sets of quantum boxes depends strongly on the spacer layer thickness Zs. In this letter, we study samples containing multiple arrays of quantum boxes separated by GaAs spacer layers of various thicknesses, using cross-sectional scanning tunneling microscopy. This work experimentally evidences that the spacer layer characteristic thickness Zs0 below which a vertical self-alignment occurs, depends on the size of the quantum boxes. These results are interpreted using a theoretical two-dimensional model.
Reactive ion etch-induced effects on the near-band-edge luminescence in GaN
Authors: Cheung R. , Withanage S. , Reeves R. J. , Brown S. A. , Ben-Yaacov I. , Kirchner C. , Kamp M.
Published: July 2002

GaN grown on c-plane sapphire substrates has been reactive ion etched successfully in a SF6 plasma with an etch rate of 29 nm/min. The etch rate does not change with substrate temperatures between 10 and 50 °C. Optical transitions have not been destroyed after etching, instead, two additional lower energy transitions appear close to the band-edge luminescence. The two additional transitions are related to defect states that bind excitons. The defect-bound states exhibit different behavior compared to the free excitonic states in that their normalized intensities decrease more rapidly as temperature increases, the peaks exist only up to 80 K, and their line energies show no temperature dependence.
Ion-channeling study of the SiC/Si/SiO2/Si interface
Authors: Jiang W. , Thevuthasan S. , Weber W. J. , Namavar F.
Published: July 2002

Ion channeling has been used in a detailed study of 3C–SiC films grown by chemical vapor deposition on a Si/SiO2/Si substrate. For a 160-nm-thick 〈100〉-oriented SiC film, the results show a minimum yield (χmin) of ∼28% at the SiC–Si interface, while a SiC film with a thickness of ∼2.4 μm, grown under identical conditions, was almost defect free (χmin=5.3%) in the surface region. Angular scans around the 〈110〉 axis revealed the existence of a superlattice structure at the SiC–Si interface. The strain-induced angular shift was determined to be 0.16°±0.05°, indicating a kink between the SiC and Si layers along the inclined 〈110〉 axis. A modified model is suggested to interpret the experimental observations.
Evidence for a dominant midgap trap in n-ZnSe grown by molecular beam epitaxy
Authors: Hierro A. , Kwon D. , Goss S. H. , Brillson L. J. , Ringel S. A. , Rubini S. , Pelucchi E. , Franciosi A.
Published: July 2002

A midgap deep level in n-type ZnSe grown by molecular beam epitaxy (MBE) on In0.04Ga0.96As/GaAs is detected and investigated by deep level optical spectroscopy and cathodoluminescence spectroscopy. The deep level has an optical threshold energy of 1.46 eV below the conduction band edge, and its concentration strongly depends on the Zn:Se beam pressure ratio during initial nucleation of the ZnSe layer. The concentration of this level decreases by a factor of ∼8 for Se rich vs Zn rich nucleation conditions, correlating with a decrease in the Se vacancy concentration for Se-rich nucleation. The investigation of photocapacitance transients revealed a strong interaction of the 1.46 eV level with both the conduction and the valence bands. Moreover, this level showed the largest optical cross section (emission rate of ∼103 s−1) of all of the levels found in the ZnSe layer. Taken together, these observations suggest this level may be an important recombination-generation center in MBE-grown ZnSe devices on GaAs substrates.
Plasmon-based terahertz emission from quantum well structures
Authors: Bakshi P. , Kempa K. , Scorupsky A. , Du C. G. , Feng G. , Zobl R. , Strasser G. , Rauch C. , Pacher Ch. , Unterrainer K. , Gornik E.
Published: July 2002

Growing coherent plasma oscillations (plasma instability) can be generated in quantum well structures, where a sufficient population inversion is maintained in the carrier distribution by appropriate injection–extraction configurations. The collective response characteristics of such structures have been calculated. Such structures have been grown, and their radiation emission spectra observed. The experimental results are in agreement with the theoretical predictions. The emission maximum is in the terahertz frequency range, and occurs at an intersubband plasmon frequency. This shows that population inversion is achieved in these structures.
Enhancement of thermal stability of NiSi films on (100)Si and (111)Si by Pt addition
Authors: Mangelinck D. , Dai J. Y. , Pan J. S. , Lahiri S. K.
Published: July 2002

The effect of a small amount of Pt (5 at. %) on the thermal stability of NiSi film on (100) and (111) Si substrates has been investigated both by in situ annealing inside an x-ray photoelectron spectroscopy system and by ex situ rapid thermal annealing. The addition of platinum increases the disilicide nucleation temperature to 900 °C leading to a better stability of NiSi at high temperatures. In the presence of Pt, NiSi films on both (111)Si and (100)Si substrates develop a texture with the relationship (100)NiSi∥(111)Si and (010)NiSi∥(100)Si. The increase in thermal stability has been explained in terms of the nucleation concept.
Infrared and photoluminescence spectroscopy of p-doped self-assembled Ge dots on Si
Authors: Rokhinson L. P. , Tsui D. C. , Benton J. L. , Xie Y.-H.
Published: July 2002

We report infrared photocurrent (PC) and photoluminescence (PL) spectroscopy of self-assembled Ge dots grown on Si(100) by molecular beam epitaxy. PL spectra show a transition from two- to three-dimensional growth as the Ge thickness exceeds 7 Å. The sum of the PC peak energy and PL energy from Ge dots is found to be approximately equal to the energy band gap of Si. Boron doping changes the energy spectrum of the dots: PL peaks from both doped Ge dots and from the wetting layer are shifted to higher energy, compared to the undoped samples. Also, the TO phonon energy from the wetting layer is reduced to 38 meV.
Hydrogen stability in diamond-like carbon films during wear tests
Authors: Racine B. , Benlahsen M. , Zellama K. , Zarrabian M. , Villain J. P. , Turban G. , Grosman A.
Published: July 2002

Hydrogen stability has been investigated during wear tests in particular diamond-like carbon films deposited in a dual electron cyclotron resonance radio frequency glow discharge plasma system, at two different substrate bias voltages equal to −30 and −600 V, for series I and series II, respectively. Combined infrared absorption and elastic recoil detection analysis experiments are used to fully characterize the films in their as-deposited state and after wear tests. The results indicate clearly that the total bonded hydrogen content as well as the total hydrogen concentration decrease during wear tests for the two series. Two mechanisms are discussed to explain these results, e.g., the surface contact graphitization due to heating effect and the hydrogen diffusion to the free surface as a result of plastic deformation process.
Magneto-optical properties and the potential application of GaAs with magnetic MnAs nanoclusters
Authors: Akinaga H. , Miyanishi S. , Tanaka K. , Van Roy W. , Onodera K.
Published: July 2002

The giant magneto-optical effect of ferromagnetic MnAs nanoclusters embedded in GaAs is shown. The Faraday rotation angle at 0.98 μm reaches about 0.2°/μm at 2000 Oe. The potential of this magneto-optical material for use as a Faraday rotator operating at 0.98 μm is demonstrated by the Verdet constant of the film which is 16 times larger than that of (Cd,Mn,Hg)Te commercially developed for an optical isolator. The fabrication of this material is compatible with that of semiconductor-based devices.
Comparison of the electrical and thermal stability of stress- or radiation-induced leakage current in thin oxides
Authors: Riess P. , Ceschia M. , Paccagnella A. , Ghibaudo G. , Pananakakis G.
Published: July 2002

In this work, the leakage currents induced by electrical stress and by irradiation are compared. We show that the Ig–Vg characteristics of a 5.4-nm-thick oxide are very similar for both types of stresses. Moreover, we show that the electrical stability and the annealing behavior of the defects at the origin of the leakage current are very similar. We demonstrate that the radiation-induced leakage current can be fully annealed. Comparing the properties of the stress-induced leakage current and the radiation-induced leakage current, we believe that these currents are both due to the same type of defects which are supposed to be related to trapped holes.
Quantum-dot concentrator and thermodynamic model for the global redshift
Authors: Barnham Keith , Marques Jose Luis , Hassard John , O’Brien Paul
Published: July 2002

The use of quantum dots can turn the old concept of a luminescent solar collector into a practical concentrator. The quantum efficiency, tunability of absorption threshold, and size of the redshift make quantum dots an ideal replacement for the organic dyes whose performance limited this inexpensive technology. Progress in photovoltaic cells, in particular, the ability of quantum-well cells to tune the band gap, also suggests high efficiency is possible in solar and thermophotovoltaic applications. A thermodynamic model is used to show quantitatively how the separation of absorption and luminescent peaks under global illumination is related to the spread of quantum-dot sizes. Hence, the redshift can be determined during the growth process. The model can be used to optimize concentrator performance and to study the effect of reabsorption, which is important for high concentration even if the quantum efficiency is unity. This model provides a quantitative explanation for the contribution of the spread of sizes to the redshift, which should help in the extraction of the much smaller, single-dot effects.
Extracting interaction forces and complementary observables in dynamic probe microscopy
Authors: Dürig U.
Published: July 2002

We address the problem of interaction sensing in dynamic probe microscopy. An iterative method is presented for extracting the laws of interaction from dynamically measured observables. The method exploits the fact that in the limit of negligible anharmonicity of the tip motion, the observables are related to corresponding force laws via linear convolution operators involving weakly divergent kernels, which represent weighted monopole or dipole moments of the interaction probed by the tip.
Manganese surface segregation in NiMnSb
Authors: Ristoiu Delia , Nozières J. P. , Borca C. N. , Borca B. , Dowben P. A.
Published: July 2002

A quantitative analysis of the surface composition of the Heusler alloy NiMnSb has been undertaken using angle-resolved x-ray photoemission spectroscopy and the surface composition characterized. With sufficient annealing cycles, the stoichiometric surface evolves to a surface that is manganese rich. This indicates that the surface enthalpy is different from the bulk for the Heusler alloy NiMnSb.
Highly electrically conductive indium–tin–oxide thin films epitaxially grown on yttria-stabilized zirconia (100) by pulsed-laser deposition
Authors: Ohta Hiromichi , Orita Masahiro , Hirano Masahiro , Tanji Hiroaki , Kawazoe Hiroshi , Hosono Hideo
Published: July 2002

Highly electrically conductive indium–tin–oxide thin films were epitaxially grown on an extremely flat (100) surface of yttria-stabilized zirconia single-crystal substrates at a substrate temperature of 600 °C by a pulsed-laser deposition technique. A resistivity down to 7.7×10−5 Ω cm was reproducibly obtained, maintaining optical transmission exceeding 85% at wavelengths from 340 to 780 nm. The carrier densities of the films were enhanced up to 1.9×1021 cm−3, while the Hall mobility showed a slight, almost linear, decrease from 55 to 40 cm2 V−1 s−1 with increasing SnO2 concentration. The low resistivity is most likely the result of the good crystal quality of the films.
Spontaneous emission model of lateral light extraction from heterostructure light-emitting diodes
Authors: Ochoa D. , Houdré R. , Stanley R. P. , Ilegems M. , Benisty H. , Hanke C. , Borchert B.
Published: July 2002

We investigate the extraction of light from semiconductor light-emitting diodes made of dielectric multilayer stacks with quantum-well sources. The model is a combination of a rigorous vertical model of dipole emission and an in-plane ray-tracing model. The vertical model is shown to conveniently provide the relevant horizontal decay length of the various kinds of in-plane propagating modes. The proposed combination of the two models accounts for the lateral extraction as well as light recycling in the active layers.
Ultrasonic measurement of the elastic constants of potassium hydrogen phthalate single crystals
Authors: Alex A. V. , Philip J.
Published: July 2002

Many organic crystals with orthorhombic symmetry exhibit electro-optic, ferroelectric, and triboluminescence properties. The potassium salt of phthalic acid (KAP), with the formula C8H5O4K, finds application in soft x-ray spectroscopy due to its large d spacing and has been reported to be piezoelectric as well. KAP crystallizes into orthorhombic structure with space group Pca21 All nine elastic constants of KAP have been determined by measuring the velocity of longitudinal and transverse ultrasonic waves propagating along different symmetry directions. Velocities have been measured by the ultrasonic pulse echo overlap technique by injecting waves generated by quartz transducers through a suitable bonding medium into the crystal. The McSkimin Δt correction has been applied to determine the velocities accurately. The temperature variation of selected elastic constants over the range 300–420 K has also been measured and reported.
Global analysis of a model pulsing drug delivery oscillator based on chemomechanical feedback with hysteresis
Authors: Li Bingtuan , Siegel Ronald A.
Published: July 2002

A simple model for an autonomous pulsing drug delivery system was previously introduced. This model involves negative feedback action, with hysteresis, of an enzyme on the permeability of a membrane through which substrate, at constant external concentration, must diffuse to reach the enzyme. The qualitative dynamics of this model permit, depending on system parameters and external driving substrate concentration, two separate single steady state, double steady state, and permanently alternating (oscillatory) behaviors. The present contribution is concerned with rigorous proofs regarding the global stability of steady states when permanent alternation is precluded, and the existence and globally asymptotic stability of a limit cycle in the permanently alternating case. Also, we prove that more restrictive but often realistic conditions on the system parameters imply limitations on the number of alternations the system can undergo before reaching steady state.
Band gap engineering in amorphous AlxGa1−xN: Experiment and ab initio calculations
Authors: Chen Hong , Chen Kuiying , Drabold D. A. , Kordesch M. E.
Published: July 2002

Amorphous alloys of aluminum nitride and gallium nitride deposited at 100 K at compositions ranging from pure AlN to pure GaN with optical band gaps which vary linearly with composition from 3.27 eV (a-GaN) to 5.95 eV (a-AlN) have been synthesized. Ab initio molecular dynamics calculations for these alloys reproduce the band gap versus composition data and give specific information on the electronic localization of the band tail states. There are no midgap states in amorphous AlxGa1−xN alloys. The calculated models have mixed four-fold and three-fold coordination and have no wrong (homopolar nuclear) bonds, demonstrating the strong ionicity in amorphous AlxGa1−xN alloys. It has been found that the valence band tail states are mostly localized on the three-fold coordinated N sites while the conduction band tail states are mostly localized on the three-fold coordinated Ga or Al sites.
Determination of the actuator sensitivity of electromechanical polypropylene films by atomic force microscopy
Authors: Peltonen Jouko , Paajanen Mika , Lekkala Jukka
Published: July 2002

The actuator functionality of electromechanical polypropylene films was studied using atomic force microscopy. The film carries a permanent electric charge and includes microbubbles as a result of two-dimensional stretching of the film. The thickness change of various film structures covered with electrodes was measured as a function of external voltage. The dependence was found to be nonlinear, the thickness change in the range 0.001%–0.1% of the total film thickness and affected by the internal charge density of the film. Applying a capacitor model including an air gap within the polymer layer enabled the determination of the Young’s modulus, the interfacial charge density and the actuator sensitivity of the studied structures.
Study of the electronic conduction mechanism in Nb-doped SrTiO3 thin films with Ir and Pt electrodes
Authors: Gopalan Sundararaman , Balu Venkat , Lee Jian-Hung , Hee-Han Jeong , Lee Jack C.
Published: July 2002

Strontium titanate (ST) is an attractive material for dynamic random access memory applications. Doping ST with other elements such as Nb has been shown to reduce problems such as frequency dispersion and resistance degradation associated with this material. In this report, we study the electronic conduction mechanism in Nb-doped ST films [Sr(Ti1−xNbxO3)] with Ir and Pt electrodes. Film thicknesses were of the order of 35–40 nm. It was found that the “true” leakage current satisfied both Schottky and Frenkel–Poole conduction mechanism equations for both positive and negative polarities. The effect of Nb content (x=0, 0.001, 0.01, and 0.05) on the barrier heights using both the mechanisms were determined. It was found that the barrier height decreased with increased Nb content, which was correlated with the increase in leakage current.
Design and characterization of collimated effusive gas beam sources: Effect of source dimensions and backing pressure on total flow and beam profile
Authors: Guevremont Jeffrey M. , Sheldon Stanley , Zaera Francisco
Published: July 2002

The total flux and the flow profile of gas-phase molecular beams generated by a number of assemblies involving different combinations of apertures and capillary arrays were measured for a wide range of backing pressures covering the molecular and viscous flow regimes. Specifically, δ=10 and 50 μm diameter, L=2 mm thick glass capillary arrays were combined with single apertures of φ=0.17, 1.0, and 11.4 mm diameters and L=0.1, 0.5, and 2 mm thickness in order to design high-flux beam dosers with a high degree of collimation. The variations in the total flux and the spatial profile of the beams were tested as a function of the backing pressure, which was varied between 10−4 and 102 Torr, by a sampling movable skimmer. The data obtained under the molecular flow (low backing pressure) regime corroborate some conclusions from previous reports. In particular, it is shown here that the conductance of the dosers (the ratio of the total flux of the beam to the backing pressure) depends only on their geometry in that regime. The beam profile, on the other hand, deteriorates with increasing backing pressure because of the increase in gas–wall and gas–gas collisions in the intermediate “opaque” flow regime where the mean free path of the gas, λ, is larger than the diameter of the capillaries, δ, but smaller than the length of the tubes, L. As λ approaches δ, a transition “slip” regime is reached, and a drop in the conductance of the doser is observed. Finally, by pressures where λ<0.05δ, a viscous laminar flow is established where the conductance of the doser increases with pressure and its directionality improves as well. The implications of our results to the design of molecular beams for specific applications are briefly discussed.
Using convective flow splitting for the direct printing of fine copper lines
Authors: Cuk Tanja , Troian Sandra M. , Hong Cheong Min , Wagner Sigurd
Published: July 2002

Liquid ribbons of solutions of copper hexanoate in a volatile solvent were drawn on a glass slide using either fine glass capillaries or an ink jet printer. After solvent evaporation, the solute was observed to segregate into multiple pairs of stripes much narrower than the initial ribbon diameter. These stripes were then converted to pure copper by annealing. Surface profiles indicate that the thickness, width, and number of lines formed are strongly dependent on the solution viscosity and volume per unit length deposited. From flow visualization studies and surface profiling, we have found that evaporative cooling produces Bénard–Marangoni convection patterns which accrete the solute along two key boundaries of the flow, namely the three phase contact line and the outer edge of a stagnant region about the ribbon apex. These findings suggest that optimization of the deposition and evaporation process can be used to “write” fine metallic lines from a wider liquid precursor.
Advances in the molecular-beam epitaxial growth of artificially layered heteropolytypic structures of SiC
Authors: Fissel Andreas , Schröter Bernd , Kaiser Ute , Richter Wolfgang
Published: July 2002

The controlled growth of SiC heteropolytypic structures consisting of hexagonal and cubic polytypes has been performed by solid-source molecular-beam epitaxy. On on-axis substrates, 4H/3C/4H–SiC(0001) and 6H/3C/6H–SiC(0001) structures were obtained by first growing the 3C–SiC layer some nanometer thick at lower substrate temperatures (T=1550 K) and Si-rich conditions and a subsequent growth of α-SiC on top of the 3C–SiC layer at higher T (1600 K) under more C-rich conditions. On off-axis substrates, multiheterostructures consisting of 4H/3C- or 6H/3C-stacking sequences were also obtained by first nucleating selectively one-dimensional wire-like 3C–SiC on the terraces of well-prepared off-axis α-SiC(0001) substrates at low T(<1500 K). Next, SiC was grown further in a step-flow growth mode at higher T and Si-rich conditions. After the growth, many wire-like regions consisting of 3C–SiC were found also within the hexagonal layer material matrix indicating a simultaneous step-flow growth of both the cubic and the hexagonal SiC material.
Ultralow noise current amplifier based on a cryogenic current comparator
Authors: Gay F. , Piquemal F. , Genevès G.
Published: July 2002

This article describes the development of a current amplifier operating at 4.2 K designed for very low current measurements. A cryogenic current comparator with a winding ratio of 10 000/1, a low noise dc superconducting quantum interference device, and a very efficient magnetic shield are combined to reach an equivalent input noise of 4 fA/√Hz in the signal bandwidth and a flicker noise corner around 0.5 Hz.
Some relationships within the nonlocal (pair–site) chemical reactivity formalism of density functional theory
Authors: Chamorro E. , Contreras R. , Fuentealba P.
Published: July 2002

General and exact relationships within the nonlocal (pair–site) reactivity context are explicitly derived in the framework of density functional theory. The equations are expressed in terms of linear response functions and associated kernels of the Fukui function and its derivatives with respect to the electron number at constant external potential. It is shown that under a local approximation for the general nonlocal pair–site kernels, the nonlocality character of the chemical response must appear only implicitly through the changes in the chemical potential of the system. In the framework of a more general nonlocal reactivity theory, new relations can be written through the incorporation of more spatial coordinates. The case of a three-site equation between Fukui functions has been explored.
Effects of cross-sectional elongation on the resistive edge modes
Authors: Anderson J. , Nordman H. , Weiland J.
Published: July 2002

Resistive edge modes in a shifted noncircular tokamak geometry are investigated in the electrostatic limit. The reduced Braghinskii equations are used as a model for the electrons and an advanced fluid model for the ions. An eigenvalue problem is derived from these equations which is solved numerically. It is found that the resistive ballooning modes are stabilized by plasma elongation for peaked density profiles. In addition, it is found that the resistive ηi-modes (ηi=Ln/LTi) may be either stabilized or destabilized by elongation depending on the collision frequency.
Influence of trapped and interfacial charges in organic multilayer light-emitting devices
Authors: Brütting W. , Riel H. , Beierlein T. , Riess W.
Published: July 2002

The influence of trapped and interfacial charges on the device characteristics of organic multilayer light-emitting devices is investigated. We have studied devices consisting of 20 nm copper phthalocyanine as buffer and hole-injection layer, 50 nm N,N′-di(naphthalene-1-yl)-N,N′ diphenyl-benzidine (NPB) as hole-transport layer, and 65 nm tris(8-hydroxyquinolinato)aluminum (Alq3) as electron transport and emitting layer sandwiched between a high-work-function metal and a semitransparent Ca electrode. Current–voltage measurements show that the device characteristics in negative bias direction and at low positive bias are influenced by charges trapped within the organic layers. This is manifested by a strong dependence of the current on the direction and speed of the voltage sweep in this range. Low-frequency capacitance–voltage and static charge measurements reveal a voltage-independent capacitance in negative bias direction and a significant increase between 0 and 2 V, indicating the presence of negative interfacial charges at the NPB/Alq3 interface. Transient experiments show that the delay time of electroluminescence under forward bias conditions is controlled by the buildup of internal space charges rather than by charge-carrier transport through the organic layers.
Electroreflectance and surface photovoltage spectroscopies of semiconductor structures using an indium–tin–oxide-coated glass electrode in soft contact mode
Authors: Datta Shouvik , Ghosh Sandip , Arora B. M.
Published: July 2002

Measurements of electroreflectance and surface photovoltage spectroscopy of semiconductor structures are described using a transparent indium–tin–oxide-coated glass electrode in soft contact mode on the semiconductor surface. This improvisation (simplification) reduces the magnitude of the ac modulation voltage necessary for the electroreflectance measurement to less than a volt from about a kV (∼103 V) as required in the conventional contactless setup. This soft contact mode also enhances the sensitivity of the surface photovoltage signal by three orders of magnitude. We also formulate an analytical criterion to extract the transition energies of a quantum well from the surface photovoltage spectrum.
Drilled alternating-layer three-dimensional photonic crystals having a full photonic band gap
Authors: Notomi M. , Tamamura T. , Kawashima T. , Kawakami S.
Published: July 2002

We propose a three-dimensional photonic crystal structure having a wide full photonic band gap in the optical regime, which can be fabricated by an alternating-layer deposition and etching (drilling) process. This fabrication process is much simpler than that previously reported. The combination of current lithographic technology and autocloning bias-sputtering deposition is a promising way of realizing these photonic crystals.
Features of nanometer scale islands on CdSe/ZnSe surfaces
Authors: Zhang B. P. , Manh D. D. , Wakatsuki K. , Segawa Y.
Published: July 2002

We examined the early-stage (within a few hours) properties in air of nanometer scale islands formed on a CdSe/ZnSe surface. When a CdSe/ZnSe surface grown in vacuum was exposed to air, some islands were newly formed. The aspect ratio of the island was found to increase first and then decrease. A depletion zone was observed in the perimeter of the island which indicates the intermixing of ZnSe into the island. Surface analyses using Auger electron spectroscopy indicated that the island was composed mainly of oxides and the outer shell was Zn enriched and Cd depleted. This letter shows that surface oxidation plays a key role in the formation of islands on CdSe/ZnSe surfaces in atmosphere.
Atomistic computer simulation of the clay–fluid interface in colloidal laponite
Authors: Leote de Carvalho R. J. F. , Skipper N. T.
Published: July 2002

Monte Carlo and molecular dynamics computer simulations have been used to study the structure and dynamics of the interlayer aqueous solution in a colloidal sodium laponite clay at 277 K. The system studied has a clay–clay spacing of 34.06 Å, and contains 1200 interlayer water molecules and 24 sodium counterions. The density profiles for interlayer species show two distinct layers of surface water as one moves away from the clay particles. The innermost of these layers is strongly oriented to form hydrogen bonds to the surface oxygen atoms. Radially averaged pair distributions have been calculated as a function of distance from the clay surfaces, and show that throughout our system the water structure is significantly perturbed from the bulk. In particular, we observe an increase in the second nearest-neighbor oxygen–oxygen distance, similar to that reported for low-density water at 268 K [A. K. Soper and M. A. Ricci, Phys. Rev. Lett. 84, 2881 (2000)]. The majority of the sodium counterions are fully hydrated by six water molecules. These hydrated ions have a strong tendency to remain close to the solid surfaces, as so-called “outer-sphere” complexes. However, we also observe cations further from the clay sheets, in the diffuse layer. Diffusion of water and cations in the plane of the clay sheets is comparable to that in the bulk, but is significantly reduced normal to the clay sheets.
Topographic organization of Hebbian neural connections by synchronous wave activity
Authors: Mihaliuk Eugene , Wackerbauer Renate , Showalter Kenneth
Published: July 2002

Experimental studies have revealed that the refinement of early, imprecise connections in the developing visual system involves activity in the retina before the onset of vision. We study the evolution of initially random unidirectional connections between two excitable layers of FitzHugh–Nagumo neurons with simulated spontaneous activity in the input layer. Lateral coupling within the layers yields synchronous neural wave activity that serves as a template for the Hebbian learning process, which establishes topographically precise interlayer connections.