bluuebunny/crossref_metadata_2025 huggingface dataset:
GaN/Al
x
Ga
1−x
N
quantum wells grown by molecular beam epitaxy with thickness control at the monolayer scale
Authors: Grandjean N. , Massies J.
Published: July 2002

GaN/Al x Ga 1−x N quantum wells (QWs) are grown by molecular beam epitaxy (MBE) on c-plane sapphire substrates. Both the Al composition and the well thickness are determined in situ from reflection high-energy electron diffraction intensity oscillations. It is demonstrated that MBE growth allows controlling the QW width at the monolayer (ML) scale from sample to sample. 9 K photoluminescence (PL) spectra exhibit well-resolved QW-related emission peaks (linewidths of 20–30 meV). The variation of the GaN/Al0.11Ga0.89N QW energy versus the well thickness (from 3 to 15 MLs) shows the presence of a strong built-in electric field in the quantum structure. Room-temperature PL of QWs is also presented.
Band discontinuity fluctuations and local chemistry at the GaSe–Si interface
Authors: Zacchigna M. , Sirigu L. , Almeida J. , Berger H. , Gregoratti L. , Marsi M. , Kiskinova M. , Margaritondo G.
Published: July 2002

Photoemission spectromicroscopy data on Si–GaSe with 0.2 μm lateral resolution revealed laterally inhomogeneous interface Se–Si chemical reactions. The local Si 2p fits agrees with interface Si selenide parameters; the Se 3d peaks indicate a Si–selenide component; the Ga 3d peaks exhibit a metallic component whose intensity increases with Si coverage. This local interface reaction is related to lateral changes in the band lineup.
Dynamics of gain in vertical cavity lasers and amplifiers at 1.53 μm using femtosecond photoexcitation
Authors: Bouché N. , Dupuy C. , Meriadec C. , Streubel K. , Landreau J. , Manin L. , Raj R.
Published: July 2002

Conditions for the optimization of short pulse amplification and gating times in vertical cavity laser amplifiers are elucidated using femtosecond photoexcitation. The experimental results pertaining to the temporal response are analyzed by a theoretical model taking into account the carrier heating and the Fabry–Perot nature of the cavity. A 20 dB amplification gain is obtained in a vertical cavity structure at 1.53 μm with a capacity to function at a rate >25 Gb/s.
Tunneling characteristics of nonuniform ultrathin oxides
Authors: Ting D. Z.-Y.
Published: July 2002

The reverse-bias current–voltage characteristics of metal–oxide–silicon tunnel structures containing nonuniform ultrathin oxide layers are analyzed using a numerical three-dimensional quantum mechanical scattering calculation. We find that, in general, roughness at the Si/SiO2 interface renders the oxide layer more permeable, but does not qualitatively alter I–V characteristics. In the direct tunneling regime interface roughness induces lateral localization of wave functions which leads to preferential current paths, and is characterized by current densities which increase with island size. In the Fowler–Nordheim tunneling regime, however, interface roughness affects transport primarily through scattering, which increases with island size, and results in current densities which decrease with island size. We have also shown that appropriate one-dimensional models may be used to estimate the effect of interface roughness in limiting cases.
Anisotropic dielectric properties of structured electrorheological fluids
Authors: Wen Weijia , Ma Hongru , Yim Tam Wing , Sheng Ping
Published: July 2002

We present an approach to monitor the structure-induced anisotropic dielectric properties of electrorheological fluids. The particles used are made from uniform glass microspheres coated with an inner magnetic layer and an outer dielectric layer. Under an applied magnetic field, the particles are found to form columnar structures with a body-centered-tetragonal crystalline arrangement inside the columns. It is shown that the structure-induced dielectric constant can be consistently explained through first-principles calculations.
Atomic structure and electronic properties of single-wall carbon nanotubes probed by scanning tunneling microscope at room temperature
Authors: Hassanien A. , Tokumoto M. , Kumazawa Y. , Kataura H. , Maniwa Y. , Suzuki S. , Achiba Y.
Published: July 2002

A detailed three-dimensional structural analysis of single-walled carbon nanotubes was carried out using a scanning tunneling microscope (STM) operated at room temperature in ambient conditions. On a microscopic scale, the images show tubes condensed in ropes as well as tubes which are separated from each other. For a single-wall nanotube rope, the outer portion is composed of highly oriented nanotubes with nearly uniform diameter and chirality. On separated nanotubes, atomically resolved images show variable chirality ranges between 0°–30°, and variable diameter (1–3 nm), with no one type dominant. From STM and scanning tunneling spectroscopy measurements we confirmed the correlation between chirality and the electronic properties, namely the tuning from metallic to semiconducting. We also observed a rectifying behavior correlated with the chiral angle of 25°, an important observation for nanodevices application.
Characteristic tunnel-type conductivity and magnetoresistance in a CoO-coated monodispersive Co cluster assembly
Authors: Peng D. L. , Sumiyama K. , Yamamuro S. , Hihara T. , Konno T. J.
Published: July 2002

We have studied electrical conductivity, σ, and magnetoresistance in a CoO-coated monodispersive Co cluster assembly fabricated by a plasma–gas–aggregation-type cluster beam deposition technique. The temperature dependence of σ is described in the form of log σ vs 1/T for 7<T<80 K. The magnetoresistance ratio (ρ0−ρ3T)/ρ0 increases sharply with decreasing temperature below 25 K: from 3.5% at 25 K to 20.5% at 4.2 K. This marked increase (by a factor of 6) is much larger than those observed for conventional metal–insulator granular systems. These results are ascribed to the Coulomb blockade effect in the monodispersed cluster assemblies.
Band-gap narrowing and potential fluctuation in Si-doped GaN
Authors: Lee In-Hwan , Lee J. J. , Kung P. , Sanchez F. J. , Razeghi M.
Published: July 2002

We investigate the optical properties of two sets of Si-doped GaN epitaxial layers with different degree of compensation. The electron concentration dependence of the band-gap energy measured by photoluminescence is interpreted as band-gap narrowing effect and evaluated by a simple relation. The photoluminescence peak positions of heavily compensated samples are shifted downward with respect to those of moderately compensated samples, and the down shift becomes larger at higher electron density. Based on analysis of photoluminescence spectra, these prominent behaviors are accounted for by band-edge potential fluctuation associated with inhomogeneous residual impurities.
Multiwire screw-pinch loads for generation of terawatt x-ray radiation
Authors: Golub T. A. , Volkov N. B. , Spielman R. B. , Gondarenko N. A.
Published: July 2002

A multiwire screw pinch (a variant of a Z pinch) for high-current 100 ns pulsed power generators used for terawatt x-ray radiation is proposed and discussed. Wires, twisted along a curved load surface, are suggested to be an effective way to create the initial axial magnetic field, to begin the rotation of the plasma, and to mitigate the growth of Rayleigh–Taylor instabilities. A screw-pinch design is predicted by a zero-dimensional model to provide more efficient energy transfer from the pulse power generator to the load compared with a straight, cylindrical wire array.
Self-sustained current oscillation above 100 GHz in a GaAs/AlAs superlattice
Authors: Schomburg E. , Henini M. , Chamberlain J. M. , Steenson D. P. , Brandl S. , Hofbeck K. , Renk K. F. , Wegscheider W.
Published: July 2002

A GaAs/AlAs superlattice with a large miniband (120 meV) showed self-sustained current oscillation at a frequency of 103 GHz giving rise to microwave emission (power 0.5 mW). The emission line had a linewidth of about 1 MHz and was tuneable by about 800 MHz. An analysis suggests that the transport in the superlattice was mainly due to electrons in the lowest miniband and that the oscillation was caused by traveling dipole domains. We also observed frequency locking of the current oscillation attributed to a synchronization of domain propagation by the external high-frequency field.
Electrical isolation of GaN by ion implantation damage: Experiment and model
Authors: Uzan-Saguy C. , Salzman J. , Kalish R. , Richter V. , Tish U. , Zamir S. , Prawer S.
Published: July 2002

Electrical and optical isolation of unintentionally doped GaN layers due to the damage created by H+ and He+ ions passing through the layer are demonstrated. As a result of the irradiation, the sample resistance increases by 11 orders of magnitude and the band-to-band photoluminescence (PL) emission is totally quenched. Following annealing (1000 °C, 30 s), the conductivity can be nearly completely recovered, whereas only partial recovery of the PL emission is obtained. A model is proposed which invokes the presence of potential barriers for electronic transport across extended defects as the major factor limiting carrier mobility. Radiation defects increase these barriers, thus affecting the sample resistivity. This model fits the experimental results for both H and He induced damage extremely well and thus proves that defects created by nuclear collisions of the ions traversing the layer are responsible for the observed effects.
Enhancement of the photoluminescence intensity in short-period GaAs/AlAs superlattices with different well and barrier thickness
Authors: Krylyuk S. , Korbutyak D. V. , Litovchenko V. G. , Hey R. , Grahn H. T. , Ploog K. H.
Published: July 2002

The transition from an indirect to a direct energy band structure has been induced in short-period GaAs/AlAs superlattices by going from a symmetric to an asymmetric distribution of the well and the barrier thickness within the unit cell of the superlattice. Reducing the barrier thickness dB to half the well thickness dW moves the lowest state in the conduction band from the X point in the AlAs barrier to the Γ point in the GaAs well. For dW=2dB, the band structure becomes therefore direct for all values of dW. This change in the type of energy gap is accompanied by a significant enhancement of the integrated photoluminescence intensity for asymmetric superlattices.
Interfacial structure of molecular beam epitaxial grown cubic-GaN films on GaAs(001) probed by x-ray gazing-angle specular reflection
Authors: Li J. H. , Chen H. , Cai L. C. , Cui S. F. , Yu W. X. , Zhou J. M. , Huang Q. , Mai Z. H. , Zheng W. L. , Jia Q. J.
Published: July 2002

We report on a study of interfacial structure of GaN films grown on GaAs(001) substrates by plasma-assisted molecular beam epitaxy using x-ray grazing-angle specular reflection. We show that interfacial layers with electron densities differing from those of GaN and GaAs were formed upon deposition of GaN. It is also found that the interfacial structure of our systems depends strongly on the course of the initial layer deposition. The phase purity of the GaN films was examined by x-ray reciprocal space mapping. A simple kinetic growth model suggested by our results has been presented.
Ultrafast carrier dynamics in germanium nanoparticles
Authors: Tognini P. , Stella A. , De Silvestri S. , Nisoli M. , Stagira S. , Cheyssac P. , Kofman R.
Published: July 2002

It is shown that the ultrafast carrier dynamics in Ge nanoparticles involving the electronic density of states well above the gap can be separated in two regimes: a faster one (∼1 ps) characterized by bleaching of the absorption around 490 nm and a slower one (up to a few hundred picoseconds) governed by band gap renormalization. There is also a clear correspondence between the spectral response in both regimes and a bulk-like band structure for sizes below the exciton Bohr radius. Quantum confinement is manifested through the measurable blueshift of E1+Δ1 spectral structure in the ultrafast optical response.
Relations between interaction force and frequency shift in large-amplitude dynamic force microscopy
Authors: Dürig U.
Published: July 2002

Large-amplitude dynamic force microscopy based on measuring shifts of the resonance frequency of the force sensor has proved to be a powerful imaging tool. General expressions relating arbitrary interaction forces to resonance frequency shifts are derived using variational methods and Fourier expansion of the tip motion. For interactions with a range much shorter than the vibration amplitude, the frequency shift can be expressed in terms of a convolution product involving the interaction force and a weakly divergent kernel. The convolution can be inverted, thus enabling one to recover unequivocally interaction potentials and forces from measured frequency shift data.
Observation of ZnSe/GaAs interface states by photomodulation reflectance difference spectroscopy
Authors: Yang Z. , Chen Y. H. , Sou I. K.
Published: July 2002

We show that part of the resonance feature at 2.7 eV in the reflectance difference (RD) spectra of ZnSe thin film grown on (100) GaAs substrate [Z. Yang, I. K. Sou, Y. H. Yeung, G. K. L. Wong, J. Wang, C. Jin, and X. Hou, J. Vac. Sci. Technol. B 14, 2973 (1996)] can be suppressed by a beam of monochromatic light below the ZnSe band gap. This light-sensitive part of the resonance contains contributions from at least two anisotropic interface electron traps near the conduction band edge of ZnSe that have distinctly different photocarrier release characteristics. Each state releases the captured electrons through more than one channel. Our results reconfirm that at least part of the RD resonance feature at 2.7 eV is due to the electronic transitions involving anisotropic interface states.
Effects of O2, H2, and N2 gases on the field emission properties of diamond-coated microtips
Authors: Lim S. C. , Stallcup R. E. , Akwani I. A. , Perez J. M.
Published: July 2002

We report the effects of O2, H2, and N2 residual gases on the field emission properties of uncoated and diamond-coated individual Mo microtips. The microtips are made using electrochemical etching techniques and positioned 5 μm from the anode using a scanning tunneling microscopy system. We observe that the field emission (FE) current and turn-on voltage of diamond-coated microtips are significantly less degraded by O2 exposure than those of uncoated Mo microtips. H2 exposure enhances the FE properties of both uncoated and diamond-coated microtips, while N2 exposure does not have any significant effect.
GaN homoepitaxial layers grown by metalorganic chemical vapor deposition
Authors: Leszczynski M. , Beaumont B. , Frayssinet E. , Knap W. , Prystawko P. , Suski T. , Grzegory I. , Porowski S.
Published: July 2002

GaN homoepitaxial layers were grown using metalorganic chemical vapor deposition on the highly conductive GaN bulk crystals grown at high hydrostatic pressure. The epitaxial growth process was monitored by reflectivity of red laser light. The oscillations of its intensity served for the precise evaluation of the growth rate. The layers were then investigated using far-infrared reflectivity (FIR), x-ray diffraction, and photoluminescence. The FIR spectrum gave small free-electron concentration in the layers in contrast to the substrates which had about 2.5×1019 cm−3 of free electrons. In x-ray diffraction, the peaks of the substrate and of the layer were separated due to the difference between the lattice parameters of the bulk GaN substrate (expanded by free electrons and point defects) and the layer which had a low free electron concentration. Both peaks had rocking curves with half widths of about 20 arcsec. The photoluminescence spectrum contained narrow peaks of less than 1 meV with the excitonic lines well resolved. The spectrum was dominated by the donor-bound exciton peak. All acceptor-related peaks had small intensities that indicated a very low acceptor concentration in the layers.
Mechanical tuning of tunnel gaps for the assembly of single-electron transistors
Authors: Carlsson Sven-Bertil , Junno Tobias , Montelius Lars , Samuelson Lars
Published: July 2002

We have fabricated gold single-electron transistors (SETs), operating up to 25 K, with tunnel gaps that could be individually tuned during fabrication. A combination of atomic-force-microscopy manipulation of nanodiscs and in situ electrical measurements was used to form statically stable tunnel gaps between the discs and lithographically defined electrodes. The gap resistances could be tuned to predetermined values over three orders of magnitude between ∼1 MΩ and ∼2 GΩ, corresponding to gap widths in the range of 3–10 Å. We report on SETs with symmetrically and asymmetrically coupled islands, i.e., with equal or different tunnel resistances. In the asymmetric SET a distinct Coulomb staircase was observed.
Photoluminescence study of the initial stages of island formation for Ge pyramids/domes and hut clusters on Si(001)
Authors: Schmidt O. G. , Lange C. , Eberl K.
Published: July 2002

Stranski–Krastanov growth of Ge on Si(001) can result in various different types of islands such as domes, pyramids, and hut clusters. A systematic study of the impact of the different island phases on photoluminescence (PL) spectra is presented and reveals simultaneous PL from the wetting layer and islands in the pyramids/domes phase and only island related PL in the phase of hut clusters. A rather abrupt transformation from pyramids/domes to hut clusters is reported by changing the growth temperature from 600 to 580 °C. The wetting layer and island related energy transitions as a function of Ge coverage are presented for growth temperatures ranging from 360 to 700 °C. At Ts=360 °C no island formation is present. The influence of Oswald ripening and material interdiffusion on the PL properties is discussed.
Optical investigation of infrared dyes in hybrid thin films
Authors: Casalboni M. , De Matteis F. , Prosposito P. , Pizzoferrato R.
Published: July 2002

We report on steady-state absorption and emission measurements for two infrared (IR) dye molecules, IR5 and IR1051, incorporated in hybrid organic/inorganic zirconia-based thin films synthesized with a sol-gel technique. For comparison, measurements in liquid solutions have also been carried out, and an estimation of the relative quantum efficiency in solid glassy material with respect to liquid solution is reported. Absorption and emission spectra are discussed in terms of physical and chemical interactions with the host matrix. The present work provides the evidence of successful doping of solid films with dyes for planar waveguides emitting in the infrared region up to 1.3 μm.
The effect of laser-produced plasma expansion on the ion population
Authors: Krása J. , Láska L. , Rohlena K. , Pfeifer M. , Skála J. , Králiková B. , Straka P. , Woryna E. , Wolowski J.
Published: July 2002

Changes in the ion population during the expansion of Ag, Cu, Pb, and Ta laser-produced plasmas in vacuum were simultaneously measured with two coaxial ion collectors between 82 and 187 cm from a target. The plasma was produced with an iodine laser delivering a power density up to ∼1015 W/cm2 onto the target. It was experimentally proved that the charge Q of expanding ions and the corresponding ion current density j are really decreasing with the distance L as Q∝L−2 and j∝L−3, respectively. It implies that the “freezing” of charge states dominates in that interval of L.
Thermodynamic theory of stress distribution in epitaxial Pb(Zr, Ti)O3 thin films
Authors: Kim H. Joon , Oh S. Hoon , Jang Hyun M.
Published: July 2002

A phenomenological thermodynamic model has been developed to account for the effects of the film thickness on various properties of ferroelectric thin films. To this end, we have suitably incorporated a position-dependent stress distribution function into the elastic Gibbs function. Various physical properties can be predicted as a function of the film thickness using this modified thermodynamic formalism. A comparison of the theoretical predictions with experimental values of the average strain and the para-ferro transition temperature indicates that the tensile stress caused by the cubic-tetragonal displacive phase transition dominates over the compressive thermal stress in the epitaxially oriented tetragonal Pb(Zr, Ti)O3 thin films.
Nanometer-Au-particle-enhanced photorefractivity in a polymer composite
Authors: Wang Feng , Chen Zhijian , Zhang Bo , Gong Qihuang , Wu Kuiwang , Wang Xuesong , Zhang Baowen , Tang Fangqiong
Published: July 2002

Doped with nanometer Au particles, a low-Tg photorefractive polymer composite, poly(N-vinylcarbazole):2,4,7-trinitro-9-fluorenone:4-(dicyanovinyl-N,N-diethylaniline), exhibited an effective enhancement on its photorefractivity. Measurements on two-beam coupling (TBC) as well as four-wave mixing were carried out at 633 nm wavelength. At an applied electric field of 118 V/μm, TBC coefficient as high as 206 cm−1 was obtained in the sample doped with Au particles, while only 152 cm−1 in the sample without Au particles. The measurements on the effective electro-optic coefficient and the photorefractive phase shift for two samples with and without doping Au particles were performed, respectively. The observations of distinct difference on the photorefractive phase shift and no difference on the effective electro-optic coefficient implied that enhancement on the photorefractivity was due to the increment of the density of the effective trap center by doping Au particles.
Proximity effect thermometer for local electron temperature measurements on mesoscopic samples
Authors: Aumentado J. , Chandrasekhar V. , Eom J. , Baldo P. M. , Rehn L. E.
Published: July 2002

Using the strong temperature-dependent resistance of a normal metal wire in proximity to a superconductor, we have been able to measure the local temperature of electrons heated by flowing a direct-current (dc) in a metallic wire to within a few tens of millikelvin at low temperatures. By placing two such thermometers at different parts of a sample, we have been able to measure the temperature difference induced by a dc flowing in the samples. This technique may provide a flexible means of making quantitative thermal and thermoelectric measurements on mesoscopic metallic samples.
Buried laser waveguides in neodymium-doped BK-7 by K+–Na+ ion-exchange across a direct-bonded interface
Authors: Gawith Corin B. E. , Bhutta Tajamal , Shepherd David P. , Hua Ping , Wang Ji , Ross Graeme W. , Smith Peter G. R.
Published: July 2002

We report a technique for producing single-step buried K+–Na+ ion-exchanged waveguide lasers in neodymium doped BK-7. Direct bonding is the basis for this process, providing atomic contact between two chemically modified BK-7-type substrates followed by a 350 °C treatment suitable for simultaneous annealing and intersubstrate ion exchange. Characterization of a 6 mm long device was performed using a Ti:sapphire laser operating at 808 nm. The resultant laser output exhibited TE polarized single-spatial-mode operation with losses of <0.4 dB cm−1 and a maximum output power of 8.5 mW for 249 mW of absorbed pump power.
Metal–insulator–metal injector for ballistic electron emission spectroscopy
Authors: Heer R. , Rakoczy D. , Ploner G. , Strasser G. , Gornik E. , Smoliner J.
Published: July 2002

We introduce a solid-state version of ballistic electron emission microscopy/spectroscopy (BEEM/BEES) on GaAs–AlGaAs heterostructures using a metal–insulator–metal (MIM) injector structure that replaces the tip of the scanning tunneling microscope (STM). In the present work, the MIM injector is realized by an Al–Al2O3–Al tunnel junction yielding an easy-to-fabricate three-terminal device for ballistic electron spectroscopy. The device principle is applied to several GaAs–AlGaAs structures. The barrier heights obtained from the onsets of the ballistic current spectra are in good agreement with self-consistent calculations as well as earlier experimental results achieved with STM-based BEES.
Strong photoluminescence from monosubstituted polyacetylenes containing biphenylyl chromophores
Authors: Huang Yuan Ming , Ge Weikun , Lam Jacky W. Y. , Zhong Tang Ben
Published: July 2002

Monosubstituted polyacetylenes are generally regarded as unlikely candidates for photoluminescent materials. We have, however, observed intense deep-blue emission in a series of photoexcited poly(1–alkynes) (PAs) containing biphenylyl pendants (–{HC=C[(CH2)m–OCO–biphenyl–OC7H15]}n–where m=2, 3, 4, and 9). The photoluminescence (PL) is readily observable by naked eyes under normal room illumination conditions, whose integrated intensity is threefold higher than that of poly(1–phenyl–1–butyne), a well-known highly luminescent disubstituted polyacetylene. A red PL band has also been detected in the PAs. Using the extended-Hückel-tight-binding method, we have calculated the density of states of the PAs, and it is found that the functional pendants have dramatically modified the band states of polyacetylene. The strong PL of the PAs is satisfactorily explained by the engineered electronic structures.
Detection and analysis of early failures in electromigration
Authors: Gall M. , Capasso C. , Jawarani D. , Hernandez R. , Kawasaki H. , Ho P. S.
Published: July 2002

The early failure issue in electromigration (EM) has been an unresolved subject of study over the last several decades. A satisfying experimental approach for the detection and analysis of early failures has not been established yet. In this study, a technique utilizing large interconnect arrays in conjunction with the well-known Wheatstone Bridge is presented. A total of more than 20 000 interconnects were tested. The results indicate that the EM failure mechanism studied here follows lognormal behavior down to the four sigma level.
Thin-film metrology of silicon-on-insulator materials
Authors: Zollner S. , Lee T.-C. , Noehring K. , Konkar A. , Theodore N. D. , Huang W. M. , Monk D. , Wetteroth T. , Wilson S. R. , Hilfiker J. N.
Published: July 2002

Spectroscopic rotating-analyzer ellipsometry employing a compensator was used to measure the ellipsometric angles and depolarization from 0.73 to 5.4 eV of commercial separation by implantation of oxygen wafers. The data were analyzed to find the thicknesses of the native oxide cap, the top Si layer, and the buried oxide (BOX). From the depolarization in the spectral region of interference fringes, we determine layer thickness nonuniformities. Although a reasonable agreement between the data can be found by describing the BOX with the optical constants of thermal oxide, it can be improved by modeling the BOX as an effective medium consisting of thermal oxide and amorphous Si. The physical justification for this model is the presence of Si islands near the BOX/substrate interface. We compare our ellipsometry results with a destructive analysis using electron microscopy and secondary ion mass spectrometry.
Refractive index and gap energy of cubic InxGa1−xN
Authors: Goldhahn R. , Scheiner J. , Shokhovets S. , Frey T. , Köhler U. , As D. J. , Lischka K.
Published: July 2002

Spectroscopic ellipsometry studies have been carried out in the energy range from 1.5 to 4.0 eV in order to determine the complex refractive indices for cubic InGaN layers with various In contents. The films were grown by molecular-beam epitaxy on GaAs(001) substrates. By studying GaN films, we prove that for the analysis of optical data, a parametric dielectric function model can be used. Its application to the InGaN layers yields, in addition, the composition dependence of the average fundamental absorption edge at room temperature. From the latter, a bowing parameter of 1.4 eV is deduced.
Nonlinear resonance ultrasonic vibrations in Czochralski-silicon wafers
Authors: Ostapenko S. , Tarasov I.
Published: July 2002

A resonance effect of generation of subharmonic acoustic vibrations is observed in as-grown, oxidized, and epitaxial silicon wafers. Ultrasonic vibrations were generated into a standard 200 mm Czochralski-silicon (Cz-Si) wafer using a circular ultrasound transducer with major frequency of the radial vibrations at about 26 kHz. By tuning frequency (f) of the transducer within a resonance curve, we observed a generation of intense f/2 subharmonic acoustic mode assigned as a “whistle.” The whistle mode has a threshold amplitude behavior and narrow frequency band. The whistle is attributed to a nonlinear acoustic vibration of a silicon plate. It is demonstrated that characteristics of the whistle mode are sensitive to internal stress and can be used for quality control and in-line diagnostics of oxidized and epitaxial Cz-Si wafers.
Neutron intrinsic gettering on electrical property of gate oxynitride in metal-oxide-Si capacitor
Authors: Chang-Liao Kuei-Shu , Yi Nan-Kuang , Huang Jenn-Gwo
Published: July 2002

The electrical property of gate oxynitride in metal-oxide-Si capacitor is improved by a neutron-intrinsic-gettering (NIG) treatment. This improvement can be attributed to the reduction of nitrogen concentration in the oxynitride bulk and the decrease of interstitial oxygen defect in the silicon. For the oxynitride formed using NIG-treated Si substrate, the breakdown electric field is increased and the reliability is improved. A significant improvement of electrical property in gate oxynitride is observed by a NIG treatment including a fast neutron dose of 7.2×1016 cm−2 and an anneal at 1100 °C for 6 h. This NIG treatment would be promising for the improvement of electrical properties in gate oxynitrides.
Reactivation of Mg acceptor in Mg-doped GaN by nitrogen plasma treatment
Authors: Kim Sang-Woo , Lee Ji-Myon , Huh Chul , Park Nae-Man , Kim Hyun-Soo , Lee In-Hwan , Park Seong-Ju
Published: July 2002

Mg-doped GaN films, grown by metalorganic chemical vapor deposition, were treated with a nitrogen plasma after a conventional rapid thermal annealing (RTA). The conductivity of the p-type GaN film was greatly enhanced by nitrogen plasma treatment, and exhibited a higher sheet hole concentration as well as lower sheet resistance than the RTA samples. A photoluminescence (PL) band which peaked at 3.27 eV was new, and a band at 2.95 eV was markedly attenuated in the plasma treated samples. PL measurements suggest that self-compensation in a Mg-doped GaN caused by the nitrogen vacancies is effectively reduced by the nitrogen plasma treatment, leading to an enhanced p-type conductivity. In addition, the plasma-treated sample revealed a drastic reduction in specific contact resistance by three orders of magnitude, compared with the RTA samples.
Particle imaging technique for measuring the deformation rate of hydrogel microstructures
Authors: Olsen Michael G. , Bauer Joseph M. , Beebe David J.
Published: July 2002

A technique for measuring the instantaneous deformation rate of hydrogel microstructures is introduced. In developing this technique, we have adapted microscopic particle image velocimetry, a method for measuring velocity fields in microfluidic devices. Small fluorescent seed particles are incorporated into the hydrogel microstructure, and as the structure swells or contracts, the displacement of the seed particles over some small time Δt is measured using a cross-correlation technique. By providing local deformation rate data in hydrogel microstructures, this technique will allow for optimization of device designs as well as providing a means for determining the validity of hydrogel expansion models.
Complete surface exfoliation of 4H–SiC by H+- and Si+-coimplantation
Authors: Bennett J. A. , Holland O. W. , Budde M. , Thomas D. K. , Feldman L. C.
Published: July 2002

Implantation of 4H–SiC with H+1 and Si+28 ions followed by annealing is shown to result in complete ejection or exfoliation of the implanted layer. This is in contrast to H+-only implantation where only partial exfoliation of the surface can be achieved. The mechanisms of this process and its dependence on implantation conditions are discussed. It is shown that amorphization of the surface region during Si+ irradiation is a necessary condition to produce this effect, and that it depends critically upon the thickness of the amorphous layer. Stress, induced by bulk recrystallization of the amorphized layer, acts as an additional driving force for H+ induced exfoliation causing the surface layer to separate completely at a depth near the end-of-range of the H+ ions. The morphologies of the newly exposed surfaces are studied by profilometry measurements and atomic force microscopy.
Modification of the growth mode of Ge on Si by buried Ge islands
Authors: Usami N. , Araki Y. , Ito Y. , Miura M. , Shiraki Y.
Published: July 2002

Photoluminescence experiments on double Ge layers were performed to give deep insights on the growth mode of Ge on Si in the presence of buried 4.5 monolayers of Ge islands. The critical coverage of the island formation and the wetting layer thickness were confirmed to be reduced in the second Ge layer. In addition, a drastic increase of the island density as well as a shape transition were observed by atomic force microscopy. These modifications of the growth mode are explained in terms of the surface strain induced by the buried Ge islands and the reduction of the nucleation barrier due to the alloying.
Nanometer-scale resolution of a chloromethylated calixarene negative resist in electron-beam lithography: Dependence on the number of phenolic residues
Authors: Sakamoto T. , Manako S. , Fujita J. , Ochiai Y. , Baba T. , Yamamoto H. , Teshima T.
Published: July 2002

We have investigated a chloromethylated calixarene, p-chloromethylmethoxcalix[n]arene (CMC[n]AOMe) (n=5,6,7), as a negative resist in electron-beam lithography. Each CMC[n]AOMe resist has a resolution of about 12 nm and a sensitivity of about 0.8 mC/cm2 which varies slightly with n (or molecular weight). A sub-10-nm Si wire has been fabricated by halide plasma etching and a CMC[n]AOMe resist as an etching mask. Because the resist pattern edge is smooth, Si wires with 7-nm width and 10-μm length were performed without any breaking.
Piconewton regime dynamic force microscopy in liquid
Authors: Tamayo Javier , Humphris Andrew D. L. , Miles Mervyn J.
Published: July 2002

In this work, a cantilever in a fluid is driven by a mixed signal composed of a standard driving signal and a feedback signal consisting of the amplified and phase shifted oscillation signal. This mimics the oscillation of a cantilever with a quality factor up to three orders of magnitude higher than its natural Q (∼1). This technique allows the identification of the resonance frequency of the cantilever by mechanical excitation of the fluid. The improved sensitivity has been checked by imaging a very soft sample of 1% agarose gel in the dynamic mode. A force smaller than 50 pN could be applied to the sample, improving the spatial resolution and the phase contrast significantly. This technique provides a major improvement in atomic force microscopy/spectroscopy in liquids.
Microstructure of (Ba, Sr)TiO3 thin films deposited by physical vapor deposition at 480 °C and its influence on the dielectric properties
Authors: Werner Matthew C. , Banerjee Indrajit , McIntyre Paul C. , Tani Noriaki , Tanimura Michio
Published: July 2002

The orientation and microstructure of (Ba, Sr)TiO3 (BST) deposited via physical vapor deposition at 480 °C was studied using x-ray diffraction, atomic force microscopy, and transmission electron microscopy. Annealing Pt/BST (previously annealed at 400 °C) at 800 °C in O2 results in grain growth, enhancement of the {100} texture and a 20% increase in the dielectric constant. The 400 °C annealed films become more textured in the {100} orientation as film thickness is increased. Finally, it appears that an interfacial capacitance, rather than the “bulk” dielectric constant limits the total capacitance density of the films.
Barrier-width dependence of quantum efficiencies of GaN/AlxGa1−xN multiple quantum wells
Authors: Shin Eun-joo , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

We present the results of picosecond time-resolved photoluminescence (PL) measurements for a set of 30 Å well GaN/AlxGa1−xN (x∼0.2) multiple-quantum-well (MQW) structures with varying barrier widths LB from 30 to 100 Å, grown by metalorganic chemical-vapor deposition. The PL quantum efficiency and the recombination lifetime of these MQWs were observed to increase monotonously with an increase of the barrier width up to 80 Å. These behaviors were explained by considering two distinct mechanisms that control the radiative recombination efficiencies in MQWs. When the barrier width is below the critical thickness, the nonradiative recombination rate increases with a decrease of the barrier width due to enhanced probabilities of the electron and hole wave functions at the interfaces as well as in the AlGaN barriers. On the other hand, the misfit dislocation density increases as the barrier width approaches the critical thickness, which can result in an enhanced nonradiative interface recombination rate. Our studies here have shown that the optimal GaN/AlGaN (x∼0.2) MQW structures for UV light-emitter applications are those with barrier widths ranging from 40 to 80 Å.
Influence of threading dislocations on the near-bandedge photoluminescence of wurtzite GaN thin films on SiC substrate
Authors: Qiu X. G. , Segawa Y. , Xue Q. K. , Xue Q. Z. , Sakurai T.
Published: July 2002

Wurtzite GaN thin films have been grown on the 6H–SiC substrates by plasma-assisted molecular-beam epitaxy. The correlation between crystalline structure and photoluminescence (PL) properties is examined by high-resolution x-ray diffraction and PL measured at different temperatures from 5 to 300 K. At 5 K, the PL spectrum is dominated by two near-bandedge emissions at 3.470 and 3.427 eV. At room temperature, a broad yellow luminescence (YL) centered at 2.16 eV is present in the PL spectrum. The intensities of both the 3.427 eV peak and the YL are found to be strongly related to the threading-dislocation density of the GaN epilayers.
Epitaxial growth of laminar crystalline silicon on CaF2
Authors: Schroeder B. R. , Meng Shuang , Bostwick A. , Olmstead Marjorie A. , Rotenberg Eli
Published: July 2002

Si heteroepitaxy on CaF2 was studied with x-ray photoelectron spectroscopy and diffraction and low-energy electron diffraction to determine the interface bonding and silicon overlayer growth mode. The CaF2 surface was prepared by irradiation with low-energy electrons and exposure to arsenic, which replaced surface fluorine atoms with arsenic. Thin Si films (1.3 nm) were subsequently deposited at 550 °C. The Si films completely cover the CaF2 substrate and have a type-B orientation. The resultant interface has Si–Ca bonds, with the As surfactant layer terminating the Si surface in a 1×1 structure.
Passively mode-locked picosecond lasers with up to 59 GHz repetition rate
Authors: Krainer L. , Paschotta R. , Moser M. , Keller U.
Published: July 2002

We present very compact Nd:YVO4 lasers which are passively mode-locked with a semiconductor saturable absorber mirror at very high repetition rates between 39 and 59 GHz. We achieved between 30 and 80 mW of average output power and pulse durations of 4.8–5.5 ps. As the pulse-to-pulse spacing is only 17 ps at 59 GHz, we are approaching the limit in repetition rate which is set by the pulse duration.
Elastic properties of magnetostrictive thin films using bending and torsion resonances of a bimorph
Authors: Sárközi Zs. , Mackay K. , Peuzin J. C.
Published: July 2002

The modification of the elastic properties of giant magnetostriction alloy films due to an applied magnetic field (the ΔE effect), has been studied. Two different types of films were deposited on Si substrates: (i) single layers of TbDyFeCo alloys typically 1000 nm thick and (ii) nanocomposite multilayer films of FeCo/TbFeCo each having a typical thickness of 6 nm. Both types of films were rendered magnetically anisotropic with a well defined in-plane easy axis. Rectangular samples were cut out of these bimorphs and firmly glued at one end to a heavy base to form a simple cantilever structure. The variations of film elastic moduli were deduced from the shifts of the cantilever resonance frequencies as a function of bias field for two basic configurations: (i) field applied along the easy axis and (ii) field applied along the hard axis. In contrast with previous work, both flexural and torsion resonance modes were excited and studied. As a result the field induced variations of both planar traction modulus and the shear modulus were obtained and new interesting features were discovered. In particular strongly negative values of the shear modulus were observed (at least in the nanocomposite films) in the vicinity of the divergence in the transverse magnetic susceptibility at saturation field along the hard axis. A simple but complete theoretical analysis shows that the uniaxial anisotropy model together with the assumption of isotropic magnetoelastic coupling gives a good semiquantitative understanding of all the experimental results.
Phase diagram of epitaxial ferromagnets: Erbium (0001) on sapphire (112̄0)
Authors: Durfee C. S. , Flynn C. P.
Published: July 2002

We have accurately determined the elastic state and the magnetic phase diagram of epitaxially clamped Er on Al2O3 through the range of strong bulk magnetostriction. The elastic free energy is too small to explain the observed change of the Curie point. The additional required energy of ∼5% of the self-energy for full perpendicular magnetization must arise from differences of domain configuration between the magnetized bulk and the film.
A density-functional study of small titanium clusters
Authors: Wei S. H. , Zeng Zhi , You J. Q. , Yan X. H. , Gong X. G.
Published: July 2002

The atomic structures and electronic properties of small TiN (N=2–10) clusters have been studied by using the density-functional theory with a local spin density approximation. We find that the inner-shells (3s3p) of the titanium atom plays an important role in the formation of the small clusters. We have obtained the ground state of titanium clusters, Ti7 is found to be a magic cluster, which is in good agreement with the experimental results. Starting with Ti8 cluster some features of the electronic structure of the titanium bulk have been developed. The ionization potentials and magnetic moments for these small titanium clusters are also presented.
Microplow-row lithography and fabrication of submicrometer magnetic structures
Authors: Li S. P. , Lebib A. , Peyrade D. , Natali M. , Chen Y.
Published: July 2002

This letter describes the use of a microplow-row lithography technique to fabricate wire and dot arrays with submicrometer resolution. Fabrication of patterned magnetic structures with or without resist processing was demonstrated that shows the simplicity and the usefulness of this technique. The performance of the patterned magnetic structures compares favorably with that of similar fabrication by using conventional methods.
Room-temperature intense emission at 1534 nm in Er-doped Cd3Al2Si3O12 glass
Authors: Lihui Huang , Xingren Liu , Hai Lin , Jiuling Lin
Published: July 2002

In this work, a promising solid-state laser material, Er-doped cadmium aluminum silicate glass, has been synthesized by solid-state reaction. Very intense emission at 1534 nm, corresponding to the I413/2→I415/2 transition of Er3+ ions, was observed upon both 488 nm Ar+ laser and 632.8 nm He–Ne laser excitations at room temperature. Possible cross-relaxation processes in the glass are discussed.
Polarized photoluminescence from solid films of nematic and chiral-nematic poly(p-phenylene)s
Authors: Katsis D. , Chen H. P. , Chen S. H. , Rothberg L. J. , Tsutsui T.
Published: July 2002

Thermotropic nematic and chiral-nematic poly(p-phenylene)s were prepared into well-aligned films between fused-silica substrates in which conjugated backbones were uniaxially and helically oriented. With unpolarized photoexcitation at 350 nm, a nematic film produced a degree of linear polarization of 9 near the emission peak at 410 nm with no evidence of excimer formation. With the same photoexcitation of a chiral-nematic film, the degree of circular polarization was found to vary from −1.3 in the 390–430 nm spectral region to between +0.3 and +0.9 beyond the edge of the selective reflection band. The crossover behavior unique to light emission from the selective reflection region remains inexplicable with existing theories on light propagation through periodically structured films.
Magnetic barrier in confined two-dimensional electron gases: Nanomagnetometers and magnetic switches
Authors: Governale Michele , Boese Daniel
Published: July 2002

We investigate the conductance properties of a hybrid ferromagnet-semiconductor structure consisting of a confined two-dimensional electron gas and a transverse ferromagnetic strip on top. Within the framework of the Landauer–Büttiker model, we develop an alternative way to consider magnetic fields. Our method describes devices ranging from a recently realized nanomagnetometer down to quasi-one-dimensional quantum wires. We provide a rigorous way to relate the measured resistance to the actual magnetization of the strip. Regarding the quasi-one-dimensional wires we propose a device application, a tunable magnetic switch.
Optically induced propulsion of small particles in an evenescent field of higher propagation mode in a multimode, channeled waveguide
Authors: Tanaka Takuo , Yamamoto Sadahiko
Published: July 2002

We report experimental results in which Mie particles were optically propelled in an evanescent field generated in a multimode, channeled waveguide. Polystyrene latex spheres 4 μm in diameter were trapped laterally at the high intensity region of the evanescent field and were driven along the waveguide channel in the evanescent field of either fundamental or higher guided modes.
Effect of columnar defects in YBa2Cu3O7−x ramp-edge Josephson junctions
Authors: Kim D. H. , Lee C. W. , Lee T. W. , Hwang H. G. , Sung G. Y. , Choi C. H. , Hahn T. S.
Published: July 2002

We investigated the transport properties in YBa2Cu3O7−x (YBCO) ramp-edge Josephson junctions before and after 1.3 GeV Pb-ion irradiation. The barrier of the junctions was formed by structural modification of the ramp-edge surface of the YBCO base electrode. After a dose of ∼2×1010 Pb ions/cm2, both critical current and normal resistance of the junctions increased by ∼30% and 10%, respectively, except near the transition temperature. We interpret such an enhancement of the critical current as a result of impeding Josephson vortex motion by pinned magnetic flux in the electrodes.
Well-defined nanostructure nucleation sites provided by the faceted TiC(111) surface
Authors: Günster J. , Heinrich J. G. , Dieckhoff S. , van Eys-Schäfer H. , Otani S. , Souda R.
Published: July 2002

Reproducible growth of well-aligned nanostructures with an average nearest-neighbor distance of a few hundred nanometers is achieved on an array of strong nucleation sites provided by the facetted TiC(111) surface. The growth of the complex shaped nanostructures, composed out of carbon encapsulated TiCx crystallites interconnected by nanorods, 25 nm in diameter, is fed by the thermal decomposition of TiC. The length of the nanorods and the shape of the nanocrystallites is determined by the applied annealing program.
Imaging phonon drag in gallium nitride
Authors: Stanton N. M. , Akimov A. V. , Kent A. J. , Cavill S. A. , Cheng T. S. , Foxon C. T.
Published: July 2002

Phonon-drag imaging has been used to study the electron–phonon interaction in a low-mobility n-type gallium nitride (GaN) epilayer grown by molecular beam epitaxy on sapphire. Currents up to j=4.7×103 A/m2 were induced by transverse acoustic phonon drag of electrons in this structure. Such currents are much bigger than predicted by the conventional theory of electron–phonon interaction. We explain this difference by the fact that, in low-mobility GaN, the 2kF cutoff for electron–phonon transitions is relaxed.