bluuebunny/crossref_metadata_2025 huggingface dataset:
Highly uniform and high-density GaAs/(GaAs)4(AlAs)2 quantum wires grown on (775)B-oriented GaAs substrates by molecular beam epitaxy
Authors: Higashiwaki M. , Yamamoto M. , Shimomura S. , Hiyamizu S.
Published: July 2002

Extremely uniform and high-density GaAs/(GaAs)4(AlAs)2 quantum wires (QWRs) were self-organized in a thin GaAs/(GaAs)4(AlAs)2 quantum well grown on a (775)B GaAs substrate with a regularly corrugated AlAs–on–GaAs interface and a flat GaAs–on–AlAs interface by molecular beam epitaxy. A strong photoluminescence (PL) peak at λ=692 nm from the GaAs/ (GaAs)4(AlAs)2 QWRs showed large polarization anisotropy [P=(I∥−I⊥)/(I∥+I⊥)=0.19]. A full width at half-maximum of the PL peak from the QWRs was as small as 15 meV at 14 K, which is smaller than those of any self-organized GaAs/AlGaAs QWRs reported so far. Density of the QWRs ( 8×105 QWRs/cm) is the same as the highest ever reported. These results indicate that the QWRs grown on the (775)B GaAs substrate meet requirements for applications to QWR lasers (high uniformity, high density, high optical quality, and simple fabrication process).
Positron and gamma-photon production and nuclear reactions in cascade processes initiated by a sub-terawatt femtosecond laser
Authors: Shkolnikov P. L. , Kaplan A. E. , Pukhov A. , Meyer-ter-Vehn J.
Published: July 2002

We demonstrate theoretically that sub-terawatt lasers are capable of producing, through specially arranged cascade processes in optimal targets, substantial amounts of nuclear radiation (positrons, gamma-photons, neutrons, and fission fragments).
Chemical ordering in wurtzite InxGa1−xN layers grown on (0001) sapphire by metalorganic vapor phase epitaxy
Authors: Ruterana P. , Nouet G. , Van der Stricht W. , Moerman I. , Considine L.
Published: July 2002

A diffraction analysis in the transmission electron microscope was carried out on InxGa1−xN layers grown on (0001) sapphire by metalorganic vapor phase epitaxy on top of thick GaN buffer layers. It is found that the ternary InxGa1−xN layers can be chemically ordered. The In and Ga atoms occupy, respectively, the two simple hexagonal sublattice sites related by the glide mirrors and helicoidal axes of the P63 mc symmetry group of the wurtzite GaN. The symmetry of the ordered ternary is subsequently lowered by the disappearance of these operations, and it is shown to agree with the P3ml space group.
Self-regulated growth of tilted superlattices by atomic layer epitaxy
Authors: Hartmann J. M. , Charleux M. , Cibert J. , Mariette H.
Published: July 2002

We report on a self-regulated method for the growth of tilted superlattices. It relies on the reconstructed surfaces alternatively stabilized during the atomic layer epitaxy (ALE) of compound semiconductors. The c(2×2)+(2×1) Cd-stabilized and the (2×1) Te-stabilized surfaces alternatively formed during the ALE of CdTe and CdMn(Mg)Te ensure a self-regulation of the growth at 0.5 monolayer deposited per ALE cycle for both CdTe and CdMn(Mg)Te. We are thus able to overcome the problem of precise flux control inherent to tilted superlattices.
In-plane gates and nanostructures fabricated by direct oxidation of semiconductor heterostructures with an atomic force microscope
Authors: Held R. , Vancura T. , Heinzel T. , Ensslin K. , Holland M. , Wegscheider W.
Published: July 2002

The surface of shallow Ga[Al]As heterostructures is locally oxidized with an atomic force microscope. The electron gas underneath the oxide is depleted. We demonstrate experimentally that these depleted regions of the two-dimensional electron gas can be made highly resistive at liquid nitrogen temperatures. Thus, local anodic oxidation of high electron mobility transistors with an atomic force microscope provides a novel method to define nanostructures and in-plane gates. Two examples, namely antidots and quantum point contacts as in-plane gate transistors have been fabricated and their performance at low temperatures is discussed.
Superconducting and normal-state properties of YBa2Cu3O7−δ -bicrystal grain boundary junctions in thin films
Authors: Hilgenkamp H. , Mannhart J.
Published: July 2002

A model, based on universal properties of high-Tc superconductors only, is presented for the transport characteristics of grain boundaries in these materials. The model accounts for the superconducting as well as for the normal-state transport properties observed in a comprehensive study of YBa2Cu3O7−δ [001]-tilt bicrystalline grain boundaries with varying misorientation angles and of two different symmetries.
SiO
2
-enhanced synthesis of Si nanowires by laser ablation
Authors: Wang N. , Zhang Y. F. , Tang Y. H. , Lee C. S. , Lee S. T.
Published: July 2002

Si nanowires with uniform size have been synthesized by laser ablation of highly pure Si powder targets mixed with SiO2. A bulk quantity of Si nanowires was successfully obtained by mixing 30%–70% of SiO2 into the Si powder target. SiO2 played a crucial role in enhancing the formation and growth of the Si nanowires. The morphology and microstructure of the Si nanowire tips have been systematically characterized by means of high-resolution transmission electron microscopy. No evidence of metal was found at the tips. The results suggest that Si oxide is more important than metal in catalyzing the formation of Si nanowires.
High-strength materials produced by precipitation of icosahedral quasicrystals in bulk Zr–Ti–Cu–Ni–Al amorphous alloys
Authors: Xing L. Q. , Eckert J. , Löser W. , Schultz L.
Published: July 2002

Zr 62−x Ti x Cu 20 Ni 8 Al 10  (3⩽x⩽5) amorphous alloys crystallize via precipitation of icosahedral quasicrystals in the primary crystallization step, leading to nano-sized quasicrystals embedded in an amorphous matrix. Ti is the decisive component favoring the precipitation of quasicrystals. The mechanical properties of the crystallized alloys with different amounts of quasicrystalline phase were measured by compression and bending tests. If the volume fraction of quasicrystalline precipitates is below about 50%, the strength increases with an increasing amount of quasicrystalline precipitates, but the ductility does not decrease significantly in comparison with the amorphous counterpart. The fracture stress reaches 1835 MPa for 50 vol % of quasicrystals. Quasicrystalline precipitates of more than 60 vol % lead to reduction of ductility and strength. This shows a way of producing bulk quasicrystalline materials of high strength by crystallization of bulk amorphous alloys.
“Compliant” twist-bonded GaAs substrates: The potential role of pinholes
Authors: Kopperschmidt P. , Senz St. , Scholz R. , Gösele U.
Published: July 2002

By twist wafer bonding, thin (100) GaAs layers were transferred onto (100) GaAs handling wafers in order to fabricate structures like those suggested in the literature as “compliant universal substrates.” Heteroepitaxial InP and InGaAs films were grown on the GaAs twist-bonded layers. Twisted and untwisted grains of the epitaxial film with diameters from 0.1 to several μm without threading dislocations were observed by transmission electron microscopy. Twisted grains grew on the twist-bonded layer, while the untwisted grains grew directly on the GaAs handling wafer and were caused by pinholes in the twist-bonded GaAs layer. It is suggested that the lateral limitation of the epitaxial growth of grains on the thin twisted GaAs layer caused by the presence of pinholes reduces the density of threading dislocations in the strain-relaxed film and might be a mechanism for the observed low density of threading dislocations in lattice-mismatched epitaxial films grown on twist-bonded “compliant universal substrates.”
Thickness dependence of second-harmonic generation in thin films fabricated from ionically self-assembled monolayers
Authors: Heflin J. R. , Figura C. , Marciu D. , Liu Y. , Claus R. O.
Published: July 2002

An ionically self-assembled monolayer (ISAM) technique for thin-film deposition has been employed to fabricate materials possessing the noncentrosymmetry that is requisite for a second-order, χ(2), nonlinear optical response. As a result of the ionic attraction between successive layers, the ISAM χ(2) films self-assemble into a noncentrosymmetric structure that has exhibited no measurable decay of χ(2) at room temperature over a period of more than one year. The second-harmonic intensity of the films exhibits the expected quadratic dependence on film thickness up to at least 100 bilayers, corresponding to a film thickness of 120 nm. The polarization dependence of the second-harmonic generation yields a value of 35° for the average tilt angle of the nonlinear optical chromophores away from the surface normal.
Lateral-junction vertical-cavity surface-emitting laser grown by molecular-beam epitaxy on a GaAs (311) A-oriented substrate
Authors: Vaccaro Pablo O. , Ohnishi Hajime , Fujita Kazuhisa
Published: July 2002

A vertical-cavity surface-emitting laser was fabricated using a lateral p-n junction to inject carriers in the InGaAs active layer. The lateral p-n junction is formed in GaAs epilayers doped only with silicon and grown by molecular-beam epitaxy on a patterned GaAs (311) A-oriented substrate. This design allows the use of electrically insulating distributed Bragg reflectors and coplanar contacts while simplifying device process. Pulsed-mode operation at room temperature was obtained with a threshold current of 2.3 mA. Light emission spectrum has a single peak at 942 nm with a full width at half maximum of 0.15 nm.
Transmission electron microscopy investigation of Co thin films on GaAs(001)
Authors: Mangan M. A. , Spanos G. , Ambrose T. , Prinz G. A.
Published: July 2002

Transmission electron microscopy (TEM) observations of the microstructure of epitaxial Co thin films on GaAs(001) are reported. Cross-sectional TEM confirmed both bcc-Co and hcp-Co exist in a single 345 Å film. During film growth by molecular beam epitaxy, the epitaxial bcc-Co layer forms first at the GaAs interface, and hcp-Co islands subsequently form at the free surface of the bcc-Co layer when it reaches a thickness of about 145 Å. The bcc-Co film is single crystal at early stages of growth, but later may develop into multiple bcc crystals. The final hcp-Co grain size is roughly 15–20 nm, and selected area electron diffraction showed these grains are strongly textured. Four previously unreported variants of the hcp-Co/GaAs orientation relationship were observed in which the c axis of the hcp unit cell lies out of the plane of the film.
Dynamics of photoexcited carriers in ZnO epitaxial thin films
Authors: Yamamoto Aishi , Kido Takeo , Goto Takenari , Chen Yefan , Yao Takafumi , Kasuya Atsuo
Published: July 2002

Optical gain spectra of ZnO epitaxial thin films have been measured by using a pump–probe technique. The optical gain is thought to be due to electron–hole plasma. In the differential absorption spectra, we observed saturation of the exciton absorption, band-gap renormalization, as well as the optical gain. From the temporal changes of these structures, the dynamical properties of the photoexcited carriers are discussed.
Raman phonon modes of zinc blende InxGa1−xN alloy epitaxial layers
Authors: Tabata A. , Leite J. R. , Lima A. P. , Silveira E. , Lemos V. , Frey T. , As D. J. , Schikora D. , Lischka K.
Published: July 2002

Transverse-optical (TO) and longitudinal-optical (LO) phonons of zinc blende InxGa1−xN (0⩽x⩽0.31) layers are observed through first-order micro-Raman scattering experiments. The samples are grown by molecular-beam epitaxy on GaAs (001) substrates, and x-ray diffraction measurements are performed to determine the epilayer alloy composition. Both the TO and LO phonons exhibit a one-mode-type behavior, and their frequencies display a linear dependence on the composition. The Raman data reported here are used to predict the A1 (TO) and E1 (TO) phonon frequencies of the hexagonal InxGa1−xN alloy.
Fabricating tunable semiconductor devices with an atomic force microscope
Authors: Held R. , Lüscher S. , Heinzel T. , Ensslin K. , Wegscheider W.
Published: July 2002

We fabricated quantum wires of different geometries in Ga[Al]As heterostructures by local oxidation of the semiconductor surface with an atomic force microscope. By magnetotransport measurements at low temperatures on these wires the electronic width is determined and compared to the geometrical width. An extremely small lateral depletion length of the order of 15 nm and a high specularity of the scattering at the confining walls is found. Furthermore, we demonstrate experimentally that these quantum wires can be tuned by a combination of in-plane gates and top gates.
Electrode versus space-charge-limited conduction in organic light-emitting diodes
Authors: Wolf U. , Barth S. , Bässler H.
Published: July 2002

A criterion is established to distinguish electrode limited charge injection from a metal into a random organic dielectric from space charge limited current flow. The crucial material parameters are the charge carrier mobility and the zero field injection barrier. It rests upon parametrizing the injection current as a function of electric field, temperature, and injection barrier by mutual adjustment of the electron current injected across a Mg:Ag|Alq3 electrode on the one hand and Monte Carlo simulation of the injection yield for a hopping system on the other hand.
Strain relaxation of faceted Ge islands on Si(113)
Authors: Zhu Jian-hong , Miesner C. , Brunner K. , Abstreiter G.
Published: July 2002

We studied the formation and strain relaxation of Ge islands on Si(113) grown at 700 °C by molecular beam epitaxy. Atomic force microscopy reveals that they are mainly (113) top faceted and show a mesa-like shape. Initially formed Ge islands tend to elongate along [332̄] due to their anisotropic shear strain. With increasing Ge coverage, they grow mainly laterally towards particular directions, forming large V-shaped Ge clusters. This is analyzed to be caused by the formation of {111} slide plane-associated dislocations. The dislocations are shown to be localized in the islands. Considerable Si material has diffused into the Ge islands. Strain and Ge content in the islands are quantitatively analyzed by micro-Raman spectroscopy.
Nonvolatile two-color holographic recording in Tb-doped LiNbO3
Authors: Lee Myeongkyu , Takekawa Shunji , Furukawa Yasunori , Kitamura Kenji , Hatano Hideki , Tanaka Satoru
Published: July 2002

We have found that Tb-doped near-stoichiometric LiNbO3 crystals have three different types of energy levels: ultraviolet (UV) absorption centers just above the valence band, shallow electron traps slightly below the conduction band, and deep traps located about 1.9 eV (λ≈650 nm) below it. Using this shallow trap as an intermediate energy state, two-color holographic recording has been carried out at λ=852 nm with a UV gating light at 313 nm. A few % of diffraction efficiency was achieved, and the measured two-color sensitivity was 0.01–0.02 cm/J. No reduction of diffraction efficiency was observed during 4 h continuous readout, and the bit-error-rate of constructed digital hologram was less than 10−5.
Temperature dependence of electron concentration in type-converted silicon by 1×1017 cm−2 fluence irradiation of 1 MeV electrons
Authors: Matsuura Hideharu , Uchida Yoshitsugu , Nagai Naoto , Hisamatsu Tadashi , Aburaya Takashi , Matsuda Sumio
Published: July 2002

The conduction type of boron (B)-doped silicon (Si) changes from p type into n type by the 1×1017 cm−2 fluence irradiation (high-fluence irradiation) of 1 MeV electrons. The temperature dependence of the electron concentration n(T) obtained from Hall-effect measurements is reported. From the analysis of n(T), the density and energy level of the defects created by the high-fluence irradiation are determined to be 1.5×1014 cm−3 and EC−0.30 eV, respectively, where EC is the energy level at the bottom of the conduction band. Moreover, the compensated density is 9.5×1013 cm−3, which is in agreement with the density of B that acts as an acceptor, determined by Fourier-transform infrared spectroscopy.
Pyroelectric or piezoelectric compensated ferroelectric composites
Authors: Ploss Beatrix , Ploss Bernd , Shin F. G. , Chan H. L. W. , Choy C. L.
Published: July 2002

The matrix and inclusions of 0–3 composites of lead titanate (PT) in polyvinylidenefluoride trifluoroethylene (P(VDF–TrFE)) have been independently poled by a special poling method. The polarization states of both constituents are investigated by the measurement of the dynamic pyroelectric coefficients of the composites in the temperature range of 20–90 °C, within which the copolymer matrix undergoes a ferroelectric-paraelectric phase change. The pyroelectric coefficients of PT and P(VDF–TrFE) have the same sign, while their piezoelectric coefficients have opposite signs. This allows the preparation of composites with enhanced pyroelectric but reduced piezoelectric activity when the matrix and inclusions are polarized in the same direction, or vice versa if the constituents are oppositely polarized. For a PT volume fraction of 27% it was possible to prepare a pyroelectric composite with vanishing piezoelectric activity or a piezoelectric composite with vanishing pyroelectric activity by poling the matrix and inclusions in parallel or antiparallel directions.
The origin of optical gain in cubic InGaN grown by molecular beam epitaxy
Authors: Holst J.-Chr. , Hoffmann A. , Rudloff D. , Bertram F. , Riemann T. , Christen J. , Frey T. , As D. J. , Schikora D. , Lischka K.
Published: July 2002

The optical properties of cubic InGaN samples with varying In content are investigated to provide insight into the processes responsible for optical amplification. The samples were grown by molecular beam epitaxy on GaAs substrates. The structural and optical properties were studied by means of time-resolved and time-integrated photoluminescence spectroscopy and cathodoluminescence microscopy, as well as gain measurements at various temperatures. From these measurements, localized states are proposed to be responsible as recombination mechanism. The cathodoluminescence measurements evidence a direct correlation of the degree of In fluctuation and the efficiency of optical amplification of the samples.
Selective growth of InGaN quantum dot structures and their microphotoluminescence at room temperature
Authors: Tachibana Koichi , Someya Takao , Ishida Satomi , Arakawa Yasuhiko
Published: July 2002

We have fabricated InGaN quantum dot (QD) structures on hexagonal pyramids of GaN, using metalorganic chemical vapor deposition with selective growth. Intense photoluminescence was observed from the sample at room temperature. To directly observe the emitting areas, microphotoluminescence intensity images with a spatial resolution of a few hundred nanometers were used. The images show the emission was only from the tops of the hexagonal pyramids. The width of the emitting areas is about 300 nm, which is comparable to the spatial resolution of the images. Such a narrow width of emission areas indicates that InGaN QDs are formed on the tops of pyramids.
Submillimeter spectroscopy of tilted Nd1.85Ce0.15CuO4−δ films: Observation of a mixed ac-plane excitation
Authors: Pimenov A. , Pronin A. V. , Loidl A. , Kampf A. P. , Krasnosvobodtsev S. I. , Nozdrin V. S.
Published: July 2002

The anisotropic conductivity of a series of tilted Nd1.85Ce0.15CuO4−δ thin films was measured by quasioptical spectroscopy in the frequency range 6 cm−1<ν<40 cm−1. Two characteristic features have been observed in the low-temperature transmission spectra. The first one at ν=12 cm−1 was shown to reflect the c-axis plasma frequency of Nd1.85Ce0.15CuO4−δ. The second feature represents a mixed ab-plane/c-axis excitation. The frequency of this resonance may be changed in a controllable way by rotating the polarization of the incident radiation.
Measurement of plasma electron temperature and effective charge during tokamak disruptions
Authors: Whyte D. G. , Humphreys D. A. , Taylor P. L.
Published: July 2002

The fundamental plasma parameters of electron temperature (Te), and effective charge (Zeff) are measured simultaneously for the first time during a tokamak major disruption. The measurement of Te and Zeff is critical in understanding disruption behavior, since together they define both the plasma’s resistivity and ionization/energy balance. The measurement technique, which uses extreme ultraviolet (XUV) helium recombination radiation, is described and validated self-consistently by spectroscopy and other diagnostics on the DIII-D tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159].
Transition probability of Cu I, Ag I, and Au I from weakest bound electron potential model theory
Authors: Zheng Nengwu , Wang Tao , Yang Ruyi
Published: July 2002

In this paper, the weakest bound electron potential model (WBEPM) theory is employed to study the transition probability of many-valence-electron systems Cu I, Ag I, and Au I. According to this theory, a coupled equation is used to obtain the parameters Z*, n*, and l*, which are needed in the calculation of transition probability. In the calculation, spectral fine structure is taken into consideration, and node numbers of silver and gold are adjusted. Compared with the accepted values, the results reported here are fairly good, and agreement is within the uncertainty of the accepted values. In our discussion, we give the explanation of node number adjustment from the view of WBEPM theory.
Modeling tunneling and generation mechanisms governing the nonequilibrium transient in pulsed metal–oxide–semiconductor diodes
Authors: Vercik Andrés , Faigon Adrián Nestor
Published: July 2002

The transient behavior of tunnel metal–oxide–semiconductor structures, pulsed into inversion, is quantitatively described. A simple model for the measured transient currents is proposed, based on the integral form of the continuity equation, leading to an uncoupled solution of the Continuity and Poisson equations. Experimental results for structures with p-type or n-type substrates and different oxide thicknesses are fitted. A map showing the different behavior patterns in terms of surface generation velocity and oxide thickness is given.
Nucleation and growth of C54 grains into C49 TiSi2 thin films monitored by micro-Raman imaging
Authors: Privitera S. , La Via F. , Spinella C. , Quilici S. , Borghesi A. , Meinardi F. , Grimaldi M. G. , Rimini E.
Published: July 2002

The first-order C49–C54 allotropic-phase transition has been studied in TiSi2 thin films by electrical measurements and micro-Raman spectroscopy. To evaluate the parameters describing the kinetics of the transition and the barrier energy for the nucleation, micro-Raman spectroscopy has been used as a microscopy technique: spectra have been acquired scanning large silicide areas (100×50 μm2) and have been processed to obtain images which show the morphological evolution of the C54 grains during the transition. For temperatures between 680 and 720 °C, the converted area fraction has been determined at different annealing times and compared with electrical measurements. The two methods agree quite well. Both density and size of the C54 grains have been measured, thus allowing to separately determine the nucleation and growth parameters as a function of temperature. A nucleation and growth model has been fitted to data obtaining an activation energy of 4.9±0.7 eV for the nucleation rate and 4.5±0.9 eV for the growth velocity. Such values, coincident within experimental errors, indicate that the nucleation barrier energy for the transition to the C54 phase is very low.
Epitaxial growth of germanium dots on Si(001) surface covered by a very thin silicon oxide layer
Authors: Barski A. , Derivaz M. , Rouvière J. L. , Buttard D.
Published: July 2002

We show that germanium dots can be directly grown by molecular beam epitaxy (MBE) on a silicon (001) surface covered by a thin (1.2-nm-thick) thermal silicon oxide layer. We describe the experimental procedure, which induces the growth of a high density (1011/cm2) of nanometric germanium dots on silicon oxide. Germanium dots grown by MBE exhibit an epitaxial relationship with the underlying silicon substrate. We show that despite the presence of the thin silicon oxide layer, the silicon grown to embed the germanium dots is also single crystal in epitaxy with the silicon substrate. Nanometric size high-density crystalline inclusions in silicon oxide, seen in high-resolution transmission electron microscopy observations, are tentatively proposed as nucleation seeds for germanium dots and silicon lateral overgrowth on silicon oxide.
Influence of deep level impurities on modulation response of InGaP light emitting diodes
Authors: Guina M. , Dekker J. , Tukiainen A. , Orsila S. , Saarinen M. , Dumitrescu M. , Sipilä P. , Savolainen P. , Pessa M.
Published: July 2002

The effect of deep level impurities on static and dynamic properties of InGaP-based light emitters grown by all-solid-source molecular-beam epitaxy is analyzed. The improvement of the output power and the decrease in modulation bandwidth induced by the burn-in process are explained by the recombination enhanced annealing of one deep level trap. This assumption is experimentally proven through comparison of small-signal analysis for resonant cavity light-emitting diodes operating at 650 nm and deep level transient spectroscopy results. Finally, the concentration of the midgap recombination center N3 in the active region is shown to play an important role in the performance of the InGaP devices.
Interdiffusion of In and Ga in InGaN/GaN multiple quantum wells
Authors: Chuo Chang-Cheng , Lee Chia-Ming , Chyi Jen-Inn
Published: July 2002

Thermal stability of InxGa1-xN/GaN multiple quantum wells with InN mole fraction of ∼0.23 and ∼0.30 was investigated by postgrowth thermal annealing. Low temperature photoluminescence spectroscopy was employed to determine the temperature dependence of the interdiffusion coefficient of In and Ga in InGaN/GaN quantum wells. The interdiffusion process is characterized by a single activation energy of about 3.4±0.5 eV and governed by vacancy-controlled second-nearest-neighbor hopping. Due to composition inhomogeneity, lower diffusivity is observed at the early stage of thermal annealing.
Thick polymer-stabilized liquid crystal films for microwave phase control
Authors: Fujikake Hideo , Kuki Takao , Nomoto Toshihiro , Tsuchiya Yuzuru , Utsumi Yozo
Published: July 2002

This article describes the use of thick polymer-stabilized liquid crystal films in a new design for microwave variable phase shifters. A fine μm-order sized polymer network was formed in a 100-μm-thick liquid crystal film, using a photopolymerization-induced phase-separation method to stabilize the molecular alignment of the liquid crystal. Measurement of the electro-optic properties of the liquid crystal film revealed that the relaxation response time of the liquid crystal alignment was drastically decreased by doping the polymer at a concentration of several wt %. A new variable phase shifter composed of a microstrip transmission line (length: 193 mm, width: 200 μm) was also fabricated by using the liquid crystal film as the dielectric material. This device exhibited a microwave phase shift of −80° at a frequency of 20 GHz, when a drive voltage of 70 Vrms was applied vertically to the liquid crystal film.
Ultraviolet photoconductive detector based on epitaxial Mg0.34Zn0.66O thin films
Authors: Yang W. , Vispute R. D. , Choopun S. , Sharma R. P. , Venkatesan T. , Shen H.
Published: July 2002

We report on the fabrication and characterization of visible blind ultraviolet photodetectors based on MgxZn1−xO. Using pulsed laser deposition technique, Mg0.34Zn0.66O thin films with a bandgap of 4.05 eV were epitaxially grown on c-plane sapphire substrates. The structural, electrical, and optical properties of epilayers were characterized using various techniques. Based on the Mg0.34Zn0.66O films, planar geometry photconductive type metal–semiconductor–metal photodetectors were fabricated. At a 5 V bias, a high responsivity of 1200 A/W was achieved at 308 nm, and the visible rejection (R308 nm/R400 nm) was more than four orders of magnitude. The 10%–90% rise and fall time were 8 ns and 1.4 μs, respectively.
Self-similarity of strongly stratified inviscid flows
Authors: Billant Paul , Chomaz Jean-Marc
Published: July 2002

It is well-known that strongly stratified flows are organized into a layered pancake structure in which motions are mostly horizontal but highly variable in the vertical direction. However, what determines the vertical scale of the motion remains an open question. In this paper, we propose a scaling law for this vertical scale Lv when no vertical lengthscales are imposed by initial or boundary conditions and when the fluid is strongly stratified, i.e., when the horizontal Froude number is small: Fh=U/NLh≪1, where U is the magnitude of the horizontal velocity, N the Brunt–Väisälä frequency and Lh the horizontal lengthscale. Specifically, we show that the vertical scale of the motion is Lv=U/N by demonstrating that the inviscid governing equations in the limit Fh→0, without any a priori assumption on the magnitude of Lv, are self-similar with respect to the variable zN/U, where z is the vertical coordinate. This self-similarity fully accounts for the layer characteristics observed in recent studies reporting spontaneous layering from an initially vertically uniform flow. For such a fine vertical scale, vertical gradients are large, O(1/FhLh). Therefore, even if the magnitude of the vertical velocity is small and scales like FhU, the leading order governing equations of these strongly stratified flows are not two-dimensional in contradiction with a previous conjecture. The self-similarity further suggests that the vertical spectrum of horizontal kinetic energy of pancake turbulence should be of the form E(kz)∝N2kz−3, giving an alternative explanation for the observed vertical spectra in the atmosphere and oceans.
Intersubband Raman Laser
Authors: Liu H. C. , Cheung Iva W. , SpringThorpe A. J. , Dharma-wardana C. , Wasilewski Z. R. , Lockwood D. J. , Aers G. C.
Published: July 2002

An intersubband Raman laser has been realized in an artificial GaAs/AlGaAs three-level quantum-well structure. A CO2 laser in resonance with the one-to-three level transition is used as the pump, while the lasing emission occurs via the three-to-two level transition. The one-to-two level spacing is designed to be in resonance with the AlAs-like longitudinal optical phonon mode, favoring the Raman process. This work presents an alternative mechanism for realizing intersubband lasers and opens up new possibilities in reaching the far infrared region and achieving room-temperature operation.
Elastic relaxation of truncated pyramidal quantum dots and quantum wires in a half space: An analytical calculation
Authors: Glas Frank
Published: July 2002

We give a fully analytical solution for the elastic displacement and strain fields of arbitrarily shaped truncated pyramidal quantum dots (QDs) and trapezoidal quantum wires buried in a half space, assuming linear isotropic elasticity. The half-space geometry pertains in particular to QD semiconductor structures both during and after growth. The calculations are illustrated by examples showing quantitatively that with respect to the case of the infinite matrix and depending on the depth of the QD under the free surface the strain relaxation afforded by this surface may significantly affect the magnitude and the distribution of the various strain components inside the QD as well as in the matrix.
Structural and magnetic properties of Fe–Ge layer produced by Fe ion-implantation into germanium
Authors: Venugopal R. , Sundaravel B. , Wilson I. H. , Wang F. W. , Zhang X. X.
Published: July 2002

Fe ions were implanted into Ge (110) using a metal vapor vacuum arc ion source. The samples were characterized by Rutherford backscattering spectrometry (RBS), Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and superconducting quantum interference device magnetometry. The crystalline quality of the implanted layer was identified by RBS random/channeling measurements. The depth profile of the implanted Fe ions was obtained by AES. Low dose implantation causes formation of Fe–Ge precipitates whereas high dose implantation causes formation of Fe precipitates in the implanted layer as confirmed by TEM and XPS measurements. Magnetic measurements show the superparamagnetism of the Fe and Fe–Ge clusters at high temperatures.
Optical properties of self-assembled InAs quantum islands grown on InP(001) vicinal substrates
Authors: Salem B. , Olivares J. , Guillot G. , Bremond G. , Brault J. , Monat C. , Gendry M. , Hollinger G. , Hassen F. , Maaref H.
Published: July 2002

We have investigated the effect of misorientated InP(001) substrates on the optical properties of InAs quantum islands (QIs) grown by molecular-beam epitaxy in the Stranski–Krastanow regime. Detailed temperature-dependent photoluminescence (PL), excitation density PL, and polarization of photoluminescence (PPL) are studied. PPL shows a high degree of linear polarization (near 40%) for the nominally oriented substrate n and for the substrate with 2° off miscut angle toward the [110] direction (2° F), while it is near 15% for the substrate with 2° off miscut angle towards [010] direction (2° B), indicating the growth of InAs quantum wires on nominal and 2° F substrates and of InAs quantum dots on 2° B substrate. These island shapes are confirmed by morphological investigations performed by atomic force microscopy. The integrated PL intensity remains very strong at room temperature, as much as 36% of that at 8 K, indicating a strong spatial localization of the carriers in the InAs QIs grown on InP(001).
Time-resolved and time-integrated photoluminescence in ZnO epilayers grown on Al2O3(0001) by metalorganic vapor phase epitaxy
Authors: Jung S. W. , Park W. I. , Cheong H. D. , Yi Gyu-Chul , Jang Hyun M. , Hong S. , Joo T.
Published: July 2002

We report on photoluminescence (PL) spectra of ZnO films grown by low pressure metalorganic vapor phase epitaxy. For PL measurements, high quality ZnO thin films were epitaxially grown on Al2O3(0001) substrates. Time-integrated PL spectra of the films at 10 K clearly exhibited free A and B excitons at 3.376 and 3.382 eV and bound exciton peaks at 3.360, 3.364, and 3.367 eV. With increasing temperature, intensities of the bound exciton peaks drastically decreased and a free exciton peak was dominant above 40 K. Furthermore, time-resolved PL measurements at the free exciton peak were carried out at room temperature. The decay profiles were of double-exponential form, and the decay time constants of 180 ps and 1.0 ns were obtained using a least-square fit of the data.
Self-consistent-field theory for interacting polymeric assemblies. I. Formulation, implementation, and benchmark tests
Authors: Roan Jiunn-Ren , Kawakatsu Toshihiro
Published: July 2002

A self-consistent-field theory for interacting polymeric assemblies is formulated, implemented, and tested using bispherical coordinates. The theory can treat interacting spherical assemblies of same size and of different sizes. Possible generalization of the theory to ellipsoidal and cylindrical assemblies is discussed.
Domain wall motion effect on the anelastic behavior in lead zirconate titanate piezoelectric ceramics
Authors: Bourim El Mostafa , Tanaka Hidehiko , Gabbay Maurice , Fantozzi Gilbert , Cheng Bo Lin
Published: July 2002

Three undoped lead zirconate titanate (PZT) ceramics were prepared with compositions close to the morphotropic phase boundary: Pb(Zr0.50Ti0.50)O3, Pb(Zr0.52Ti0.48)O3, and Pb(Zr0.54Ti0.46)O3. Internal friction Q−1 and shear modulus G were measured versus temperature from 20 °C to 500 °C. Experiments were performed on an inverted torsional pendulum at low frequencies (0.1, 0.3, and 1 Hz). The ferroelectric–paraelectric phase transition results in a peak (P1) of Q−1 correlated with a sharp minimum M1 of G. Moreover the Q−1(T) curves show two relaxation peaks called R1 and R2 respectively, correlated with two shear modulus anomalies called A1 and A2 on the G(T) curves. The main features of the transition P1 peak are studied, they suggest that its behavior is similar to the internal friction peaks associated with martensitic transformation. The relaxation peak, R1 and R2 are both attributed to motion of domain walls (DWs), and can be analyzed by thermal activated process described by Arrhenius law. The R2 peak is demonstrated to be due to the interaction of domain walls and oxygen vacancies because it depends on oxygen vacancy concentration and electrical polarization. However, the R1 peak is more complex; its height is found to be increased as stress amplitude and heating rate increase. It seems that the R1 peak is influenced by three mechanisms: (i) relaxation due to DW–point defects interaction, (ii) variation of domain wall density, and (iii) domain wall depinning from point defect clusters.
Excitonic luminescence linewidths in AlGaN alloys with high aluminum concentrations
Authors: Coli Giuliano , Bajaj K. K. , Li J. , Lin J. Y. , Jiang H. X.
Published: July 2002

In this work, we report a study of the behavior of linewidths of excitonic photoluminescence transitions measured at 10 K in AlGaN alloys for high Al concentrations of 0.5 and 0.7. Our samples were grown by low-pressure metalorganic chemical vapor deposition on (0001) oriented sapphire substrates. We find that the values of the excitonic linewidths we measure agree very well with those calculated using a model in which the broadening effect is assumed to be due to compositional disorder in completely random semiconductor alloys thus attesting to an excellent quality of our samples even with high Al concentrations.
Excitons in the layered insulators ZnI2 and CdI2:Zn
Authors: Yunakova O. N. , Miloslavsky V. K. , Kovalenko E. N.
Published: July 2002

The fundamental electronic absorption spectra in the layered compounds ZnI2 and Zn-doped CdI2 are investigated in the spectral interval 3–5.9 eV. The samples are thin grain-oriented films deposited on quartz substrates. It is found that ZnI2, unlike CdI2, is a direct-gap insulator, despite the similarity of the crystal structures of these compounds. The introduction of Zn atoms into the cation sublattice of CdI2 at concentrations x⩾1% leads to the vanishing of the absorption from indirect transitions and to the appearance of a strong exciton band at the fundamental absorption band edge. The parameters of the exciton bands (spectral position, half-width Γ, oscillator strength f) are measured in the temperature interval 80–330 K. The measured temperature dependence of Γ in both compounds is typical for three-dimensional excitons. In ZnI2 the oscillator strength decreases with increasing T because of the Debye–Waller factor, while in CdI2:Zn it increases. This last result is evidence of the forbidden character of the direct optical transition at the interband absorption edge in CdI2, which is partially allowed because of the exciton–phonon interaction.
Chaotic domains: A numerical investigation
Authors: Cross M. C. , Meiron D. , Tu Yuhai
Published: July 2002

We study the chaotic domain state in rotating convection using a model equation that allows for a continuous range of roll orientations as in the experimental system. Methods are developed for extracting the domain configuration from the resulting patterns that should be applicable to a wide range of domain states. Comparison with the truncated three mode amplitude equation description is made.
Photoluminescent determination of Mn concentration and its diffusion in semi-insulating GaAs
Authors: Sasaki Yasuhiro , Sato Tomoru , Matsushita Koichi , Hariu Takashi , Shibata Yukio
Published: July 2002

A method of quantitative analysis of Mn impurity in GaAs by photoluminescence intensity ratio between shallow donor–shallow acceptor and shallow donor–Mn acceptor pair emissions is proposed and experimentally verified by the diffusion of Mn into liquid-encapsulated Czochralski (LEC) and horizontal Bridgman (HB) GaAs. The concentration of Mn piled up near the surface by heat treatment in hydrogen without intentional doping was determined by this method. Very different behaviors of Mn diffusion were observed in one of the HB wafers and suggested that the replacement of Si from Ga to As sites enhances it. It was shown by the simultaneous observation of deep level photoluminescence that VGa and AsGa can also be responsible for the enhanced diffusion of Mn.
Electrical properties of silicon nitride films plasma-deposited from SiF4, N2, and H2 source gases
Authors: Fujita Shizuo , Ohishi Toshiyuki , Toyoshima Hideo , Sasaki Akio
Published: July 2002

The electrical properties of the fluorinated silicon nitride films deposited from reactive plasma of SiF4-N2-H2 or SiF2-N2-H2 gas mixtures were investigated by changing the source SiF4 flow rate. A resistivity and breakdown strength as high as 1014–1016 Ω cm and 5–10 MV/cm, respectively, were reproducibly observed over a wide range of the N/Si ratio. The deep-trap densities and energy levels are not very dependent on the film composition. Their slight variation in the present deposition conditions can be understood by the slight variation of the spin density and the optical gap. The dielectric constant and the interface state density between silicon and silicon nitride are 4.8–6.0 and (1–5)×1012 cm−2 eV−1, respectively.
A new plasma source based on contact ionization
Authors: Schrittwieser R. , Koslover R. , Karim R. , Rynn N.
Published: July 2002

A new type of plasma source is presented: A collisionless plasma is formed by producing ions on one end and electrons on the other of a cylindrical vacuum chamber in a solenoidal magnetic field. The ions are produced by contact ionization of potassium on tungsten. The source of electrons is a LaB6 plate. In the usual single-ended Q machine the elements rhenium, iridium, and platinum are tested as ionizing metals for potassium and barium.
Inductively coupled fluxon oscillators
Authors: Cirillo M.
Published: July 2002

We study numerically the propagation of fluxons along two Josephson transmission lines which are coupled by an inductive element. This structure, for particular choices of the coupling parameters, reveals an interesting variety of nonlinear phenomena. The numerical simulations show good qualitative agreement with analytical calculations regarding the most significant events (reflection and transmission of fluxons). Experimental results obtained from real Josephson devices are also reported and the possible application of these devices is discussed in detail.
Characterization of Si-SiO2 interface states: Comparison between transient capacitance and conductance techniques
Authors: Vuillaume D. , Bourgoin J. C.
Published: July 2002

The conductance versus frequency technique [G(ω)] and deep level transient spectroscopy (DLTS) are both used to characterize Si-SiO2 interface states. However, no direct comparison between the two methods has yet been performed. We have performed a systematic study, with these two techniques of the same metal oxide semiconductor structures (Si-SiO2 on n- and p-type materials) made with various technologies and we have compared the performances of each technique: energy range accessible, energy resolution, sensitivity. We have shown that the conductance technique provides a density of states larger than the one obtained by DLTS, that its sensitivity and energy range are smaller. Its energy resolution is also smaller as illustrated by the fact that localized levels are detected by DLTS which are not with the conductance technique.
Geometric statistics and dynamic fragmentation
Authors: Grady D. E. , Kipp M. E.
Published: July 2002

The present study is focused on the distributions in particle size produced in dynamic fragmentation processes. Previous work on this subject is reviewed. We then examine the one-dimensional fragmentation problem as a random Poisson process and provide comparisons with expanding ring fragmentation data. Next we explore the two-dimensional (area) and, less extensively, the three-dimensional (volume) fragmentation problem. Mott’s theory of random area fragmentation is developed, and we propose an alternative application of Poisson statistics which leads to an exponential distribution in fragment size. Both theoretical distributions are compared with analytic and computer studies of random area geometric fragmentation problems, including those suggested by Mott, the Voronoi construction, a variation of the Johnson–Mehl construction, and several methods of our own. We find that size distributions from random geometric fragmentation are construction dependent, and that a conclusive choice between the two distributions cannot be made. A tentative application of the maximum entropy principle to fragmentation is discussed. The statistical theory is extended to include a concept of statistical heterogeneity in the fragmentation process. Finally, comparisons are made with various, dynamic fragmentation data.
Photothermal investigation of transport in semiconductors: Theory and experiment
Authors: Fournier Daniele , Boccara Claude , Skumanich Andrew , Amer Nabil M.
Published: July 2002

The photothermal deflection technique has been extended as a contactless, in situ method to investigate transport in solids with an emphasis on semiconductors. A theoretical model is developed which quantitatively describes the transport behavior, and is shown to be in excellent agreement with experimental results. For semiconductors, this approach yields the thermal diffusivity, the electronic diffusivity, the minority-carrier lifetime and the surface recombination velocity.
Interactions between Au and Cu across a Ni barrier layer
Authors: Chang Chin-An
Published: July 2002

The interactions between Au and Cu across a Ni barrier layer have been studied using a wide range of Ni thickness. Above 300 °C, extensive interdiffusion between Au and Cu has been observed, with little involvement of the Ni layer. The interdiffusion between Au and Cu results in a nearly continuous change and uniform distribution in composition for both the parent Au and Cu layers, different from those of the binary Au/Cu system. Reaction mechanisms involved are suggested, and the impact of such interactions on the packaging metallurgy is discussed.
Thermal properties of pure and varistor ZnO at low temperatures
Authors: Lawless W. N. , Gupta T. K.
Published: July 2002

Specific heat data are reported on pure and varistor ZnO, 1.7–25 K, and thermal conductivity data are reported on varistor ZnO, 1.7–15 K. In pure ZnO interstitials give rise to an Einstein contribution (ωE=84 cm−1) to the specific heat above 10 K and to a Schottky contribution (δ≂10−5 eV) below 4 K due possibly to ordering. The calorimetric Debye temperature is 399.5 K. In varistor ZnO the specific heat is dominated below 20 K by compensating charge densities (∼1018 cm−3) on the interfacial barriers which give rise to two Schottky terms (δ=7×10−4 and 3×10−3 eV). All of these non-Debye excitations are localized (do not carry heat). No evidence is seen in the thermal conductivity for Kapitza-resistance effects at the heavily doped grain boundaries.