bluuebunny/crossref_metadata_2025 huggingface dataset:
Excitonic properties of isolated nanometer-sized InAs islands in a GaAs matrix
Authors: Alonso M. I. , Ilg Matthias , Ploog K. , Trampert A.
Published: July 2002

Isolated nanometer-sized InAs islands formed in a GaAs matrix by direct epitaxial growth are studied by optical spectroscopy to get information about the energy relaxation processes of the photoexcited system. Two different relaxation mechanisms that depend upon excitation density are identified: At higher density photoluminescence excitation exhibits a strong oscillatory behavior due to separate carrier relaxation, whereas at lower density direct photogeneration of excitons is dominant. Both results ensue from the strong electron–LO-phonon interaction in the system. The study of these well-characterized samples with isolated InAs islands gives insight into the optical properties of quantum dot systems made of this materials system.
Ion beam vapor deposition for Si epitaxy at low substrate temperatures
Authors: Mohajerzadeh S. , Selvakumar C. R. , Brodie D. E. , Robertson M. D. , Corbett J. M.
Published: July 2002

We report the results of an investigation to grow thin Si films on Si substrates at low substrate temperatures using ionized SiH4 gas generated with a Kaufman type ion gun. This investigation shows island-growth at higher substrate temperatures (500–700 °C) in the form of square-based pyramids. By lowering the substrate temperature to 300 °C, we were able to achieve a planar growth with a growth rate of 8 Å per minute. Transmission electron microscopy (TEM) is used to study the crystalline quality of the samples prepared at different temperatures. The study by high resolution TEM of the sample grown at 300 °C indicates the presence of microtwins in the Si epi-layer.
Reassessment of the assignment of the InM2+3+V
M2+ defect in CdTe and ternary II-VI compounds
Authors: Austin J. C. , Patnaik B. K. , Price K. J. , Hughes Wm. C.
Published: July 2002

Previous perturbed angular correlation (PAC) spectroscopy measurements on the donor indium in CdTe and its alloys have revealed several defect complexes. One defect characterized by two sets of quadrupole interaction parameters, νQ=83 MHz, η=0.08 and νQ=92 MHz, η=0.08, was observed in Hg0.8Cd0.2Te (x=0.2 MCT) and attributed to the substitutional indium–metal vacancy complex InM2+3+VM2+. A defect characterized by νQ=61±1 MHz and asymmetry parameter η between 0 and 0.19 was seen in CdTe and widely attributed to the same complex. Both of these assignments were based mainly on an observed relationship between complex formation and the loss of metal ions. In this article we present PAC measurements on 111In-doped x=0.45 MCT (Hg0.55Cd0.45Te). These measurements reveal defects having quadrupole interactions very similar to those seen previously in CdTe and in x=0.2 MCT. Two unique defect fractions f1 and f2, characterized by νQ1=60±3 MHz, η1≊0–0.2, and νQ2=87±4 MHz, η2≊0–0.15, were seen in x=0.45 MCT, in some cases simultaneously. The observation of both of these interactions in the same material—if they correspond to the defects seen in CdTe and x=0.21 MCT—precludes the possibility that they both correspond to precisely the same defect. We also observed a change in the relative fractions of these two defects with time at room temperature; the fraction f2 vanished over a period of a day, while f1 and f0 (the fraction of indium atoms in sites having cubic or higher symmetry) increased. While we cannot rule out the possibility of a slow electronic transition, at present we favor a model in which one of the interactions (probably the one near 60 MHz) corresponds to a complex in which indium is paired to a fast-diffusing monovalent metal ion like Ag+, Cu+, or Li+.
Arsenic-related defects in SiO2
Authors: Alexandrova S.
Published: July 2002

The location of implanted arsenic (As) atoms in silicon dioxide (SiO2) matrix is discussed based on avalanche injection experimental results. A model is proposed in which As atoms substitute for Si at two different sites in an oxygen-deficient SiO2 network. With an additional supply of oxygen, but one insufficient to achieve a fully oxidized system, As tends again to occupy Si sites but interstitial As atoms can also be present. In a fully oxidized network As is also oxidized, i.e., As is again incorporated on Si sites. In the frame of this model contradictory results of As diffusion in SiO2 obtained by other authors can be understood in a consistent way.
Annealing-environment effects in the epitaxial regrowth of ion-beam-amorphized layers on CaTiO3
Authors: Rankin J. , McCallum J. C. , Boatner L. A.
Published: July 2002

The effects of water-vapor ambients on the solid-state epitaxial regrowth of ion-beam-amorphized, near-surface layers on single-crystal CaTiO3 have been investigated using Rutherford backscattering-channeling spectroscopy, time-resolved reflectivity, and cross-sectional transmission electron microscopy (TEM). The presence of water vapor in the annealing atmosphere increases the thermally induced epitaxial regrowth rate and, within the temperature range studied (400–550 °C), decreases the activation energy for this process. TEM micrographs from samples which were partially regrown in high-H2O-concentration atmospheres revealed uneven amorphous/crystalline interfaces with fluctuations on the order of 5–10 nm. Samples annealed in water-vapor-deficient atmospheres exhibited very flat interfaces after partial epitaxial regrowth. The morphologies of these interfaces are explained in terms of a segregation of hydrogen ahead of the regrowth interface. Additionally, it has been determined that the absence of oxygen does not affect the regrowth rate. Samples annealed in oxygen concentrations as low as 10−21 atm exhibit growth rates that are identical to those measured for air-annealed CaTiO3 samples.
Calculation of thermally induced strains in thin films of any crystal class
Authors: Wieder T.
Published: July 2002

Strains in thin films cause reflection shifts in an x-ray diffraction measurement. These shifts can in turn be used to determine the strains. A model is presented to calculate the reflection shifts caused by a biaxial strain state in a thin film composed of a material of any crystal class. The model takes into account the elastic anisotropy of the film material.
Solid-liquid and solid-vapor equilibrium of Ga-In-As-Sb system and artificial neural network prediction
Authors: Wei Wu , Luming Yan , Guangyu Wei , Ruiwu Peng
Published: July 2002

The GaInAsSb solid-liquid and solid-vapor equilibrium are studied by experimental measurement and computer prediction. The compositions of Ga1−XInXAs1−YSbY epilayers are determined by electroprobe microanalysis. Artificial neural networks trained using experimental data have been used to predict the composition of GaInAsSb epilayers grown by liquid-phase epitaxy. The artificial neural networks are also very useful for predicting the composition range of GaInAsSb grown by metalorganic chemical vapor deposition.
The birefringence of the optically nonlinear crystal N-methyl urea
Authors: Bailey R. T. , Cruickshank F. R. , Pugh D. , Sherwood J. N. , Simpson G. S. , Wilkie S.
Published: July 2002

The linear refractive indices of N-methyl urea have been measured and fitted to a three parameter Sellmeier equation with an accuracy of ±6×10−4–±2×10−4. The values are accurately verified by observation of the external angle of incidence to an (011) plane, required to observe type II phase matching with a fundamental wavelength of 871.6 nm. The phase matching is established as Hobden class nine for this wavelength. The refractive index values are consistent with the C=O direction being considered as the most polar axis.
Electrostatically controlled double-quantum-wire electron interferometers
Authors: Okuda Masahiro
Published: July 2002

Complete descriptions of the calculations and experiments for electrostatically controlled double-quantum-wire electron interferometers are presented. The interferometers are constituted by straight, asymmetric double-quantum-wire structures and triple Schottky gates. The electron wave function in the double-quantum-wire structure is electrostatically controlled by the Schottky gates so that an Aharonov–Bohm-type or a directional-coupler-type interferometer is effectively formed. To show that the interferometers really work, and to obtain structural parameters for a large amplitude of oscillations, numerical calculations of the electron wave propagation were performed assuming two-dimensional electron gas, by use of a scattering matrix formalism obtained by the mode-matching method. The calculated source-drain transmission of the electron wave shows oscillations with almost 100% modulation depth if the potential barrier between both quantum wires and the length between the main (central) gate and the sub- (side) gates are properly designed. On the basis of these calculations, Al0.3Ga0.7As/GaAs modulation-doped double-quantum-well structures were fabricated by standard molecular-beam epitaxy. The ballistic lengths of the two-dimensional electron gas at 4.2 K, determined by Hall measurement, were about 5 μm for the upper well and about 0.6 μm for the lower well.

The interferometers with lengths of about 0.9 μm were fabricated from these quantum-well structures using electron-beam lithography, reactive-ion-beam etching, and liftoff techniques. The fabricated interferometers were refrigerated at 1.8 K, and source-drain resistance was measured at various subgate bias conditions. When the subgate voltages were controlled such that the interferometers are formed, the oscillations of the resistance were observed as a function of the main-gate voltage. Also, the oscillations in the magnetoresistance were observed with magnetic flux penetrating the area surrounded by the split wave functions. The oscillation amplitudes were about 4% for both cases. Finally, to show that the proposed structure is certainly easy to fabricate, the interferometer was fabricated without electron-beam lithographic techniques, in which the oscillations in the resistance were also observed at 1.8 K.
Importance of controlling the Tl-oxide partial pressure throughout the processing of TlBa2CaCu2O7 thin films
Authors: Siegal M. P. , Venturini E. L. , Newcomer P. P. , Overmyer D. L. , Dominguez F. , Dunn R.
Published: July 2002

TlBa2CaCu2O7 (Tl-1212) superconducting films 5000–6000 Å thick have been grown on LaAlO3 (100) substrates using oxide precursors in a closed two-zone thallination furnace. Tl-1212 films can be grown with transition temperatures ∼100 K, and critical current densities measured by magnetization of Jcm(5 K)≳107 A/cm2 and Jcm(77 K)≳105 A/cm2. Processing conditions, substrate temperatures and Tl-oxide source temperatures are found which result in smooth, nearly phase-pure Tl-1212 films. Variations in the respective temperature ramps of the Tl-oxide zone and the substrate zone can greatly influence resulting film properties such as microstructure, morphology, superconducting transition temperature, and critical current density.
Bounds on the complex permittivity of matrix–particle composites
Authors: Sawicz Romuald , Golden Kenneth
Published: July 2002

The complex effective dielectric constant ε* of matrix–particle composites is considered. Such composites consist of separated inclusions of material of type one embedded in a matrix of material of type two. The analytic continuation method is used to derive a series of bounds which incorporate a nonpercolation assumption about the inclusions. The key step in obtaining these improved bounds is to observe that the nonpercolation assumption restricts the support of the measure in the integral representation for ε* (O. Bruno, Proc. R. Soc. London A 433, 353 (1991)). The further the separation of the inclusions, the tighter the restriction on the support. The new bounds are applied to sea ice, which is assumed to consist of a pure ice matrix with random brine inclusions. Using experimental measurements of the average size and separation of the brine pockets, end points of the support of the measure can be determined, and subsequently used to find the allowed range of values of the effective dielectric constant of sea ice. The new bounds are compared with experimental data taken at 4.75 GHz, and exhibit significant improvement over previous fixed volume fraction and Hashin–Shtrikman bounds.
Hysteresis of the phase transformation detected by galvanomagnetic measurements of Ag2Se layers
Authors: Somogyi K. , Sáfrán G.
Published: July 2002

Temperature dependence of the resistivity, carrier concentration, and carrier mobility have been investigated in vacuum deposited Ag2Se thin layers. Properties of silver selenide layers developed by the reaction of poly- and monocrystalline parent Ag films and selenium on NaCl substrates in identical circumstances have been described and compared with one another. Experimental evidences of the hysteresis of the electrical properties due to the reversible first order transformation have been found during heating and cooling cycles. Irreversible changes of these properties in the semiconductor phase were detected.
Laser-generated shock waves in thin films of energetic materials
Authors: Ling Ping , Wight Charles A.
Published: July 2002

Shock waves are generated in thin films of glycidyl azide polymer (GAP) by pulsed laser vaporization of a thin aluminum film. The rapidly expanding aluminum plasma launches a shock wave into the adjacent layer of GAP, initiating chemical reactions. The shock velocity has been measured by use of a velocity interferometer as a function of the thickness of the GAP layer and the fluence of the Nd:YAG laser pulse. Shock pressures as high as 8 GPa have been generated in this manner. Detonation of thick GAP samples has been observed, providing important information about the chemical reaction rates and the thickness of the reaction zone.
Phase stabilities and surface morphologies of (Ti1−xZrx)Si2 thin films on Si(100)
Authors: Dao Y. , Sayers D. E. , Nemanich R. J.
Published: July 2002

The effects of zirconium content on the phase stabilities and the surface morphologies of Ti–Zr alloy disilicide thin films were examined. The (Ti1−xZrx)Si2 thin films were grown by codeposition of Ti and Zr alloys (200 Å) onto Si(100) surfaces in ultrahigh vacuum and followed by in situ thermal annealing at temperatures between 650 and 1000 °C. The structural properties of films were studied by in situ Raman scattering experiments and ex situ x-ray diffraction experiments. The (Ti1−xZrx)Si2 films with low Zr contents (x<0.1) initially form a base-centered orthorhombic structure (C49 phase) and transform to a face-centered orthorhombic structure (C54 phase) at higher temperatures. The C49-to-C54 phase transition temperature increases with increasing Zr content. When the Zr content, x, is higher than 0.1, the C49 phase is stable in the temperature range of 700–1000 °C, and the C49-to-C54 phase transition is not observed. We propose that increasing the Zr content in the alloy silicide films results in a decreased enthalpy difference between these two crystalline structures. An analysis based on the classical theory of nucleation indicates that the nucleation energy barrier for the formation of a C54 cluster is increased. The surface morphologies of the alloy disilicide films were studied using scanning electron microscopy, and the surface roughnesses were quantified using atomic force microscopy. Substrate coverages of the C49 (Ti1−xZrx)Si2 thin films on Si are larger compared to the C54 TiSi2 thin films with the same thicknesses. The agglomeration of the C49 alloy silicide thin films is less severe than for the C54 TiSi2 thin films, because the C49 (Ti1−xZrx)Si2 silicide thin films have lower surface and interface free energies than C54 TiSi2.
Studies and implications of the Hall effect in superconducting and semiconducting YBa2Cu3O7−δ thin films
Authors: Jahanzeb A. , Çelik-Butler Z. , Shan P. C. , Butler D. P.
Published: July 2002

We report Hall effect measurements on three different types of YBa2Cu3O7−δ samples. Two of them were well-oxygenated superconducting films while one of them was an oxygen-depleted semiconducting film. One of the superconducting samples was c-axis oriented while the other had a mixture of c- and off-c-axis orientations as evident from resistivity and Raman spectroscopy analysis. According to Anderson, cuprate superconductors in normal state have separate spin and charge excitations called spinons and holons, respectively. The spinon bandwidth Ws defines the energy scale that determines the temperature dependence of the Hall coefficient. It was found that, for the same experimental conditions, Ws decreased for the sample with an off-c-axis growth compared to the perfectly c-axis oriented sample. The cotangent of Hall angle θH showed a reasonably good quadratic dependence on temperature for both superconducting samples in normal state. In contrast, the semiconducting film, due to an inadequate stoichiometry for superconducting behavior, had a high resistivity and its cotangent of the Hall angle decreased with temperature.
Laser direct writing of micron-size silicon lines from trisilane
Authors: Boughaba S. , Auvert G.
Published: July 2002

Micron-size silicon lines have been deposited from the Ar+ laser-induced pyrolytic decomposition of trisilane (Si3H8) using a direct writing technique. The substrates used were polysilicon/silicon dioxide/monosilicon multilayered structures. The deposited material was determined by electron microdiffraction to be polycrystalline silicon. The morphology and growth kinetics of the silicon lines were investigated at various laser spot scanning speeds, laser beam powers, and reactant gas pressures. Both optical and atomic force microscopes were used to investigate the morphology and roughness of the deposits. The effect of the laser process on the quality of the interfaces underlying the silicon lines was analyzed by means of a transmission electron microscope. From the growth kinetics, the vertical silicon deposition rate was deduced as a function of the laser-induced temperature and gas pressure. The rate laws were then compared to those obtained previously using silane (SiH4) as reactant gas. An unexpected similarity in behavior between the rate laws deduced from the two gases is observed throughout different reactant gas pressure ranges: 0.1–30 mbar for Si3H8 and 5–250 mbar for SiH4. Furthermore, the silicon deposition rate achieved using trisilane is much higher than that obtained with silane (50 times up at pressure of 10 mbar and a surface temperature of 1300 °C) making Si3H8 the prime reactant gas for the laser direct writing of silicon lines. The decomposition mechanism of trisilane molecules will be discussed on the basis of that proposed for silane.
The effect of polar additives on charge carrier transport in polysilylenes
Authors: Valerián H. , Brynda E. , Nes̆půrek S. , Schnabel W.
Published: July 2002

The effect of polar additives is introduced into the theoretical model of charge carrier transport by hopping across disordered rigid polymer matrices. The electrostatic charge carrier–dipole interaction causes a broadening of the energy distribution of transport states which results in a decrease in the charge carrier mobility. The mobility depends on the dipole moment, the molecular dimensions of the additive, and the additive concentration. For geometrically similar additives, the mobility decreases with increasing dipole moment. The theory explains well the observed decrease in the mobility of holes in poly(methyl-phenylsilylene) with increasing concentration of 1-(benzamido)-4-nitronaphthalene.
Electron-phonon interaction and tunneling escape process in GaAs/AlAs quantum wells
Authors: Hernández-Cabrera A. , Aceituno P. , Cruz H.
Published: July 2002

In this work, we have numerically integrated in space and time the effective mass Schrödinger equation for an electron in a GaAs/AlAs quantum well. Considering the electron–phonon interaction and an external electric field, we have studied the electronic tunneling escape process from semiconductor quantum wells. In this way, electronic lifetimes have been obtained at different well widths and applied electric fields.
Modeling temperature-dependent photoreflectance spectra of single AlGaAs/GaAs modulation-doped heterojunctions
Authors: Lu N. H. , Hsu T. M.
Published: July 2002

We attempt to deal with the physical processes involved in the temperature-dependent photoreflectance of single AlGaAs/GaAs modulation-doped heterojunction structures. Building on the assumption that photomodulation mechanism is due chiefly to modification of the band bending in the buffer layer, we apply the Franz–Keldysh theory to simulate the temperature-dependent photoreflectance spectra of single AlGaAs/GaAs modulation-doped heterojunction structures. In view of the nonuniformity of the electric field within the buffer layer, the field profile of which is calculated through the application of self-consistent variational approach, the WKB method is used to approximate the effective change in the dielectric function. A comparison between the experimental and the simulated results attests the validity of our assumption. The effects of temperature on the electric-field strengths, estimated from the extrema of the Franz–Keldysh oscillations, can be accounted for by considering the temperature dependence of the Fermi level.
Growth of epitaxial β-FeSi2 on (100) silicon using Fe–Ti–Si diffusion couples
Authors: Larsen K. Kyllesbech , Tavares J. , Bender H. , Donaton R. A. , Lauwers A. , Maex K.
Published: July 2002

Epitaxial semiconducting iron–disilicide grains have been formed using an Fe–Ti bilayer on (100) silicon. The reaction kinetics of the Fe–Ti–Si diffusion couple have been studied in detail as a function of annealing conditions, and the intermediate titanium thickness. Both rapid thermal annealing and long-time furnace annealing have been used in this investigation. After a two-stage annealing sequence, with a selective wet etch removal of the unreacted top metal stack in between, a distinct trilayered structure was formed. This trilayer was found to consist of a top layer of smaller β-FeSi2 grains of random orientation, an amorphous Ti–O–Fe–Si layer with nanocrystallites embedded, and a bottom layer of uniformly large coherently epitaxial β-FeSi2 grains.
Plasma oxidation mechanisms in tungsten silicide thin films
Authors: Pérez-Casero R. , Perrière J. , Enard J. P. , Martínez-Duart J. M.
Published: July 2002

The oxidation of tungsten disilicide in a rf oxygen plasma at floating potential in the 300–900 °C temperature range has been investigated. The oxidation kinetics and the elemental depth distribution in the films have been analyzed by the complementary use of Rutherford backscattering spectrometry and nuclear reaction analysis. It has been found that the nature of the oxide largely depends on temperature. A mixture of WO3 and SiO2 is grown in the 300–650 °C range. The W:Si ratio in these oxides decreases monotonically with temperature reaching a minimum value at 650 °C. As a matter of fact, a SiO2 surface layer is formed when oxidation is carried out in the 650–900 °C range. Oxygen diffusion through the growing oxide seems to be the dominant rate controlling process. Despite the fact that oxygen diffusion is a process activated by temperature, oxygen diffusion through the mixture of WO3 and SiO2 proceeds more rapidly than through SiO2. This leads to an enhancement of the growth rate at temperatures near 450 °C. Finally, a mechanism of plasma oxidation of tungsten silicide films has been proposed.
In situ Raman characterization of a diamond film during its growth process in a plasma jet chemical vapor deposition reactor
Authors: Rosman N. , Abello L. , Chabert J. P. , Verven G. , Lucazeau G.
Published: July 2002

A setup designed for characterization of a diamond film during its growth in a dc plasma jet chemical vapor deposition reactor is described. It is composed of a pulsed laser and the detector is gated in order to synchronize the Raman detection with the laser pulses. The optical components are designed for working in the visible and near UV. The installation is specially designed for remote detection and can be used in industrial reactors as well as in laboratory experiments. The detectivity of the setup is analyzed through some typical diamond spectra and it is shown that it is of the same order as that of a micro-Raman multichannel spectrometer. Some results are reported on how the temperature and the quality of the film can be controlled during the deposition process or during its etching by H2 reactive plasma. These results are discussed and confirmed by ex situ measurements including Raman, infrared, and scanning electron micrographs obtained on the same samples or in the same conditions as for in situ experiments. The secondary nucleation is responsible for the loss of Raman intensity and it could have some cyclic character.
Dynamic sheath model of collisionless multispecies plasma immersion ion implantation
Authors: Qin Shu , Jin Zhijiang , Chan Chung
Published: July 2002

A collisionless dynamic sheath model for multispecies plasmas has been derived for the application of plasma immersion ion implantation (PIII) processes with multispecies dopant gases. In this dynamic sheath model, a general form of an effective mass M, which includes species compositions and ion masses, is used to determine the sheath expansion during the pulse. The total ion current, total implant dose, and the corresponding dose components for different species can also be determined with this effective mass. This offers a method for easily evaluating PIII processing and obtaining the correct implant doses for different species. A plasma simulation is used to compare this model.
Theory for the rate of crack closure
Authors: Yu C. C. , Lin C. B. , Lee Sanboh
Published: July 2002

The theory for the constant rate of crack closure was investigated. The aim of this theory is to explain why the rate of crack closure is constant during the wetting stage of healing. Assuming that a surface crack in a material is treated by compression or with solvent, its crack closes up at a constant rate and its shape is independent of time. This approach was based on the Gibbs energy of an atom near the boundary between healed and unhealed zones. The results are in satisfactory agreement with experimental data in the literature.
A study of 2H trapping and release in 2H+-irradiated YBa2Cu3O7−δ /MgO〈100〉
Authors: Li Yupu , Kilner J. A. , Tate T. J. , Lee M. J. , Li Y. H. , Somekh R. E. , Przyslupski P.
Published: July 2002

A thin-film sample of YBa2Cu3O7−δ on MgO was irradiated at room temperature with 50-keV 2H+ (deuterium) to a dose of 1×1016 ions cm−2. The film was mainly c-axis textured film, ∼360–420 nm thick, deposited by sputtering on 〈100〉 MgO substrate. The as-implanted sample was divided into several pieces and annealed in a flowing oxygen ambient using (i) a rapid thermal annealing oven, at various temperatures between 450 and 940 °C, and (ii) a conventional annealing furnace, at various temperatures between 100 and 350 °C. Analysis by secondary-ion mass spectroscopy shows that the implanted 2H is a fast diffuser in the 123 phase. The apparent activation temperature (energy) for 2H release from the initial traps within the YBCO film during the anneal is estimated to be ∼175 °C (∼0.97 eV), which is obviously lower than the apparent activation temperature (energy) for 2H release from the initial traps within the MgO substrate (∼550 °C, i.e., ∼1.78 eV). At 200 °C the diffusivity of 2H in the YBCO film is estimated to be ∼1.4×10−13 cm2/s. In the irradiated MgO, during the anneal the migration and release of 2H is thought to be radiation enhanced around the higher damage region. No diffusional broadening or diffusion tail toward the deeply undamaged region was observed.
Raman scattering study of the longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices
Authors: Guan Z. P. , Fan X. W. , Xia H. , Jiang S. S.
Published: July 2002

Raman scattering studies were performed on ZnSe–ZnS strained-layer superlattices with different strains. In the optical phonon regime, the Raman spectra of the ZnSe–ZnS superlattices with a superlattice axis along [001] show optical phonons confined in the ZnSe well and ZnS barrier layers, and display a two-mode behavior corresponding to ZnSe-like and ZnS-like modes. We report the experimental results, by means of Raman scattering of confined longitudinal optical phonons in ZnSe–ZnS strained-layer superlattices. The Raman frequency shift dependence on the layer thickness, superlattice period, and interface strain is presented and discussed.
Negative differential coupling in complex-coupled distributed feedback semiconductor lasers: A longitudinal analysis
Authors: Kruschwitz Brian E. , Brown T. G.
Published: July 2002

A complex-coupled distributed feedback laser using a novel mechanism for creating a periodic gain is investigated. A one-dimensional longitudinal analysis is developed to predict the change in the coupling strength when the carrier density is perturbed. Earlier reports have shown that this differential coupling affects the linewidth, with negative differential coupling offering the possibility for the most significant reductions in the linewidth enhancement factor. The analysis predicts that the linewidth enhancement factor can be reduced by about a factor of 2 because of the negative differential coupling for the particular design studied.
Near-infrared response of photorefractive crystals (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6:Cu and LiNbO3:Fe
Authors: Guo Bo , Yan Li , Zhang Heyi , He Xuehua , Shih Yanhua
Published: July 2002

Material response relevant to volume holographic storage for photorefractive crystals (K0.5Na0.5)0.2(Sr0.75Ba0.25)0.9Nb2O6:Cu (KNSBN:Cu) and LiNbO3:Fe was studied in the near-infrared region. The dark decay time for KNSBN:Cu was measured to be on the order of 106 s. The light-induced erasure time at the 1 mW/cm2 read beam intensity can be ≳1000 h for both KNSBN:Cu and LiNbO3:Fe. Read beam diffraction efficiencies of 28.7% for KNSBN:Cu and 50% for LiNbO3:Fe were obtained at 706 nm when the refractive index grating was written at 706 nm. The decay rate of the refractive index grating with an extraordinary-wave read beam is about 3.6 times smaller than that with an ordinary-wave read beam for KNSBN:Cu. The results show that near-infrared light can be used to read holograms in the photorefractive volume holographic storage application.
Interfacially ordered C60 films on Cu(111) substrates
Authors: Fartash A.
Published: July 2002

C60(111) films are grown on Cu(111)/mica and Cu(111)/Ag(111)/mica substrates, and their structures are studied by x-ray diffraction. For growth temperatures ranging from 130 to 240 °C, well-ordered C60(111) overlayers form in doubling-positioning (twinned) in-plane structures. These in-plane structures are oriented such that their [22̄0] directions are parallel (in-phase), and rotated 30.0° relative to close-packed directions of their Cu(111) substrates. The rotated structures grow depending on the kinetics of the growth process, as well as the surface structural properties of the copper substrates.
The magnetization processes at low field in nearly-zero magnetostrictive amorphous ribbons
Authors: González J.
Published: July 2002

Magnetization processes at low field in nonmagnetostrictive amorphous ribbons treated by means of current annealing take place by a large Barkhausen jump (LBJ) for a value of applied magnetic field which we call critical field H*. This current annealing induces an inhomogeneous transverse magnetic anisotropy in the samples associated with a peculiar domain structure which, in the cross section of the ribbon, consists mainly of two regions: an inner core with the magnetization axially oriented and an external shell with the magnetization being circularly oriented in cross section. The critical field has been determined as a function of the annealing parameters. Its behavior is similar to that reported for induced anisotropy by current annealing although the occurrence of such a Barkhausen jump is not observed at lower intensity annealing and short times. The influence of the applied tensile stress on the critical field is also reported. The critical field increases (negative magnetostriction) or decreases (positive magnetostriction) with the applied stress. It is concluded that the complex distribution of internal stresses and external applied stress play a decisive role in determining the LBJ process.
Energy relaxation processes of hot quasi-two-dimensional excitons in very thin GaAs/AlGaAs quantum wells by exciton–acoustic-phonon interaction
Authors: Yuan Z. L. , Xu Z. Y. , Ge Weikun , Xu J. Z. , Zheng B. Z.
Published: July 2002

The rising time of the excitonic luminescence in GaAs/AlGaAs quantum wells is studied as a function of the well width. For well thickness below approximately 20 Å, we find an increase of rising time with decreasing well width. We explain the dependence of the rising time on well width in very thin quantum wells by the slow-down energy relaxation and/or exciton migration processes due to the decrease of the scattering rate of the exciton–acoustic-phonon interaction.
Chemical-beam-epitaxy growth of indium-containing III–V compounds using triisopropylindium
Authors: Freer R. W. , Lane P. A. , Martin T. , Whitehouse C. R. , Whitaker T. J. , Williams G. M. , Cullis A. G. , Calcott P. D. J. , Nash K. D. , Buchannan H. , Jones A. C. , Rushworth S.
Published: July 2002

Triisopropyl indium (TIPIn) has been investigated as an alternative to trimethyl indium for use in chemical-beam epitaxy (CBE). In previous CBE studies of GaAs/AlGaAs growth, the replacement of methyl-containing precursors with ethyl- and isopropyl-containing precursors has been shown both to widen the substrate temperature window available for growth, and also to reduce unintentional carbon incorporation in the grown layers. In the present study of (100)InxGa1−xAs (0≤x≤0.1) growth using the new TIPIn source, in situ modulated-beam mass spectrometry studies have demonstrated a similar, and technologically very important, widening of the substrate temperature window. Furthermore, use of the new precursor combination, TIPIn and triisopropyl gallium, is also shown to generate state-of-the-art InGaAs material with electrical and optical properties directly comparable to corresponding material grown using molecular-beam epitaxy.
The deposition of large antimony clusters onto silicon and carbon substrates
Authors: Leclerc J. L. , Balmer R. , Taylor S.
Published: July 2002

The deposition of antimony (Sb) clusters onto silicon substrates and on a carbon film is reported. The experimental apparatus allows evaporation of molten Sb from a heated crucible through a small nozzle orifice into a vacuum. TEM micrographs show Sb clusters with sizes between 1.2×104 and 1.3×105 atoms. This is over 1–2 orders of magnitude larger than previously reported. Film growth kinetics were studied during longer evaporation tests on Si substrates. A theory explaining the Sb loss from the crucible is presented which agrees well with experimental data. The results support cluster formation by heterogeneous nucleation on the nozzle orifice walls followed by subsequent entrainment into the vapor stream.
Experimental study of a mercury high-pressure discharge in the course of its start-up phase
Authors: Stambouli M. , Asselman A. , Charrada K. , Zissis G.
Published: July 2002

This work deals with an experimental study of the start-up phase of a mercury high-pressure discharge. We demonstrate, by using electrical and spectroscopic measurements, that a steady-state discharge can be said to be equivalent to a specific stage of the warm-up phase if, for a given current, arc voltage together with several spectral line intensities are the same in both states. Subsequently, using these measurements we were able to: (i) determine the evolution of the electric field and the voltage drop at the electrodes in the course of a start-up phase; and (ii) establish that the determination of the excitation temperature of Hg and Ar excited levels gives us a first indication concerning discharge thermalization and local thermodynamic equilibrium hypothesis limits.
Electronic and physical properties of Al3VxTi1−x (x=1, 0.875, and 0) alloys
Authors: Liu Senying , Hu Rongze , Wang Chongyu
Published: July 2002

In this article, the electronic structure of Al3VxTi1−x (x=1, 0.875, and 0) alloys with and without planar faults has been investigated. The results for the bond order integral and interaction energy between atoms, the local density of states, and the energies of the planar faults (i.e., antiphase boundary and stacking fault) are presented for the alloys. The results show that the directional bonding of the p orbital of Al in the Al3V alloy leads to the brittleness of the alloy, whereas, in the Al3V0.875Ti0.125 alloy, the p orbital bonding characteristic of Al is partly weakened and the s orbital bonding characteristic of Al is partly increased. This may change the structure of the dislocation core. The results of the local density of states (LDOS) of the atoms on the planar faults in the alloys demonstrate that the planar faults make the peaks of the LDOS of Ti and Al on the planar faults shift toward higher energy regions, and thus may increase the structural energies of Ti and Al on the planar faults. With Ti partially replacing V in Al3V, the effect of the planar faults on the LDOS of Ti is weakened, which may decrease the energy of the planar faults. This is confirmed by the result of the energy of the planar faults for Al3VxTi1−x (x=1, 0.875, and 0) alloys. The result of the energies of the planar faults for Al3VxTi1−x (x=1, 0.875, and 0) also shows that the 〈111〉 directional stacking fault is stable in Al3Ti and that the 〈001〉 directional antiphase boundary is stable in Al3V. With Ti partially replacing V in Al3V, the energies of the planar faults decrease and the activity of deformation twining and type 〈110〉 superdislocation increase. This may provide more than five independent slip systems for the Al3V0.875Ti0.125 alloy and may improve the ductility of the Al3V alloy.
Strain relaxation of Si/Ge multilayers: Coherent islands formation and their evolution as a function of the strain
Authors: Carlino E. , Giannini C. , Gerardi C. , Tapfer L. , Mäder K. A. , von Känel H.
Published: July 2002

We report on a structural study of Si/Ge multilayers grown by molecular-beam epitaxy on (100)-Si substrates. The analyses have been performed by using transmission electron microscopy, high-resolution x-ray diffraction, and secondary-ion-mass spectrometry. The investigated specimens differ in number of periods, period thickness, and in the Si/Ge periods thickness ratio. In particular, we investigate the interdiffusion of the Ge atoms in each superlattice period of the epilayer and in the epilayer as whole. The interdiffusion causes a broadening of the nominal thickness of the Ge layer producing a SixGe1−x alloy. Furthermore, the Ge content in the multilayer periods increases as a function of the growth time, i.e., the superlattice periods close to the sample surface contain more Ge atoms if compared to the periods close to the substrate/superlattice interface. We find two steps in the strain relaxation: (i) In each period the strain energy density is partially reduced by the formation of coherent islands; (ii) at a certain value of the strain energy density the shape of the islands changes and the structures relax partially or completely the accumulated strain energy by nucleation of extended defects.
A modified method of side data analysis of deep level transient spectroscopy spectra
Authors: Dmowski K. , Vuillaume D. , Lepley B.
Published: July 2002

A modified method of side data analysis of deep level transient spectroscopy (DLTS) spectra obtained by Lang’s method [J. Appl. Phys. 45, 3023 (1974)] is proposed. It uses two DLTS spectra determined for the same or different ratio of the sampling times. Simple analytical formulas are given to utilize their low- and high-temperature side data in the Arrhenius analysis. The proposed method eliminates the temperature dependence of the spectrum amplitude and does not require the accurate determination of the temperatures of DLTS spectra in their maxima.
Magnetic resonance in films and photodiodes based on poly-(phenyl-phenylene-vinylene)
Authors: Dyakonov V. , Rösler G. , Schwoerer M. , Blumstengel S. , Lüders K.
Published: July 2002

Films of poly-(2-phenyl-1,4-phenylene-vinylene) (PPPV) and photodiodes with PPPV as an active layer were studied by optically (ODMR) and electrically (EDMR) detected electron-spin resonance (ESR). Two different signals were observed in ODMR: enhancement of the photoluminescence (PL) at g=2.01 due to recombination of the photogenerated polarons (s=1/2), and a half-field enhancement signal, attributed to the fusion of triplet excitons. Both processes lead to the formation of singlet excitons. The spectral dependence of the s=1/2 signal follows the low energy part of the PL spectrum, indicating that delayed recombination of distant polarons is influenced by ESR, whereas the cw PL contains both prompt and delayed contributions. The linewidth and the intensity of the ODMR signal depend on the PL excitation intensity. Both effects are due to a decrease of the recombination lifetime of the polaron pairs at higher intensities. The relative decrease of the short-circuit photocurrent ISC through a PPPV photodiode by ESR saturation is due to recombination of nonthermalized, nongeminate excess charge polarons in the active layer of the device. This effect is at least two orders of magnitude stronger than the enhancement of total PL at the same temperature. This feature is found to be common for conjugated polymers investigated so far, and reflects the fact that the total photogenerated ISC is spin dependent, whereas ODMR selects only the nongeminate portion of recombining species in the sample.
Bias voltage propagation in a semi-insulator: Effect on the impulse response of an InGaAs:Fe planar photodetector
Authors: Hurd C. M. , McKinnon W. R.
Published: July 2002

A semiconductor with a high resistivity due to a deep trap is called a semi-insulator. A semi-insulator does not always behave like a normal insulator because of so-called bias voltage propagation, which arises from space charge in the deep trap. This effect is a factor in the operation of various devices based on semi-insulators. We investigate by numerical simulation its role in the transient photoresponse of a planar metal–semiconductor–metal photodetector. We simulate a realistic case where the active layer is InGaAs made semi-insulating by addition of Fe. The simulation uses a two-dimensional, drift/diffusion calculation with realistic conditions where the semi-insulating material is represented by a two-level compensation model with Fe as a deep acceptor that compensates shallow n-type impurities. The results give a microscopic picture on a picosecond time scale of factors that affect the transient response: trapping, space charge, propagation of the bias field, and the spatial distribution of the carriers.
Differential gain of strained InGaAs/InGaAsP quantum-well lasers lattice matched to GaAs
Authors: Park Seoung-Hwan , Kim Hwa-Min , Jeong Weon-Guk , Choe Byung-Doo
Published: July 2002

The theoretical study on the differential gain of InxGa1−xAs/InGaAsP quantum-well (QW) lasers lattice matched to GaAs is presented. These results were also compared with those of InxGa1−xAs/GaAs QW lasers. The differential gain of InGaAs/InGaAsP lasers continuously increases with the In composition. On the other hand, InGaAs/GaAs lasers show the decrease of the differential gain when the In composition exceeds about 0.22. The increase of the differential gain observed in InGaAs/InGaAsP lasers is mainly due to the increase of the subband energy spacing in the conduction band. The decrease of the differential gain observed in InGaAs/GaAs lasers is related to the decrease of the energy spacing in the conduction band and the decrease of the optical matrix element.
Sputter-deposited yttrium–barium–copper–oxide multilayer structures incorporating a thick interlayer dielectric material
Authors: Florence R. G. , Ang S. S. , Brown W. D. , Salamo G. , Schaper L. W. , Ulrich R. K.
Published: July 2002

Physical vapor deposition technologies have been developed which allow the fabrication of multilayer structures consisting of two yttrium–barium–copper–oxide (YBa2Cu3O7−x or YBCO) layers separated by a thick (∼4 μm), low dielectric constant material. The YBCO is buffered from both the substrate and the other films with ion-beam assisted deposited (IBAD) films of yttria-stabilized zirconia (YSZ). The YSZ layer provides both the texture necessary to deposit high-quality YBCO films and protection from the insulating layer material. Using these deposition processes, a variety of materials, such as Pyrex and Haynes alloy, may be used for the substrate. The critical temperature and current values obtained for the two YBCO layers of the completed structure were on the order of 85 K and 2×105 A/cm2, respectively.
Depth profiles of spatially-resolved Raman spectra of a CuInSe2-based thin-film solar cell
Authors: Takei R. , Tanino H. , Chichibu S. , Nakanishi H.
Published: July 2002

A CuInSe2-based thin film solar cell fabricated by the selenization method was investigated by micro-Raman-spectroscopy. By measuring the depth-profile of spatially-resolved Raman spectra taken from the cross section of the cell, the formation of MoSe2 at the Mo/CuInSe2 interface was confirmed.
Hot hole relaxation in the SiGe system
Authors: Yeom K. , Hinckley J. M. , Singh J.
Published: July 2002

A general formalism is presented to study hot carrier relaxation in the valence band of strained and unstrained semiconductors. The approach is based on a six-band k⋅p description of the valence band and an anisotropic Monte Carlo method. We show the results of our studies for the Si–Ge system. Carriers are initially injected in the split-off band and the carrier distribution is followed in time. Results are presented for energy-dependent energy relaxation time. The relaxation times for Si are about 10−13 s while those for Ge are an order of magnitude higher. To study the effect of biaxial strain produced through epitaxy on energy relaxation times we present results for Si0.8Ge0.2 on a {100} silicon substrate. This work is of relevance to the interpretation of pump-probe experiments, which are primarily dependent on the relaxation of the average of the carrier energy.
Electro-optic and photorefractive properties of long-period Fibonacci superlattices
Authors: Dinu Mihaela , Melloch M. R. , Nolte D. D.
Published: July 2002

We have investigated the transverse-field electroabsorption of long-period GaAs/AlGaAs Fibonacci superlattices for three different realizations of the Fibonacci sequence and assessed their bandwidth in photorefractive four-wave mixing experiments. The one-electron density of states exhibits a wide fractal distribution of quasibands, suggesting the ability to tailor bandwidths for optical applications. Many-electron effects and the inter-well coupling shift the excitonic oscillator strength to the low-energy edge of the spectrum in all cases, producing a diffraction bandwidth that is relatively independent of coupling in both weak- and strong-coupling Fibonacci superlattices.
Nanoscale deposition from a metal-covered scanning tunneling microscope tip
Authors: Andoh Hiroki , Yokoi Naoki , Takai Mikio
Published: July 2002

A fabrication method using a metal-covered scanning tunneling microscope tip has been developed to provide a nanoscale deposition method with various metal species. Nanoscale deposition has been carried out on highly oriented pyrolytic graphite with a core tungsten tip, a gold-covered tungsten tip, and an aluminum-covered tungsten tip, by applying tip positive voltage pulses. The threshold voltage, above which deposition occurred, showed a linear dependence on the tip-sample separation and differed for tip material. This suggests that the fabrication mechanism is field-induced and the deposition takes place by the transfer of tip material to the sample.