bluuebunny/crossref_metadata_2025 huggingface dataset:
Stopping of 10–50 keV positrons in aluminum
Authors: Baker J. A. , Chilton N. B. , Coleman P. G.
Published: July 2002

We report on new observations of positron stopping in a series of Al foils mounted on glass backplates for incident energies E in the range 10–50 keV. The measured median penetration depths for E≳30 keV are found to differ significantly from values derived from the empirical power law model of A. P. Mills, Jr. and R. J. Wilson [Phys. Rev. A 26, 490 (1982)]. The associated disparity between the measured implantation profiles and their analytic representations, and the consequent implications for defect profiling analysis, are discussed.
Electrical properties of low-temperature-grown CaF2 on Si(111)
Authors: Cho C.-C. , Kim T. S. , Gnade B. E. , Liu H. Y. , Nishioka Y.
Published: July 2002

While epitaxial CaF2 films grown on Si(111) at temperatures above 550 °C exhibited flat capacitance-voltage (C-V) curves, suggesting a pinned CaF2/Si(111) interface, we have observed unpinned C-V curves from as-deposited epitaxial CaF2 grown at 300 °C. Our results demonstrate that C-V characteristics of CaF2/Si(111) are determined by the thermal history, rather than the crystalline quality, of the CaF2 film. Correlations among CaF2/Si interface state density, thermal stress, and atomic bonding at the interface are discussed.
Nearly ideal InP/In0.53Ga0.47As heterojunction regrowth on chemically prepared In0.53Ga0.47As surfaces
Authors: Yablonovitch E. , Bhat R. , Zah C. E. , Gmitter T. J. , Koza M. A.
Published: July 2002

When III-V growth is interrupted for processing, in the ambient laboratory environment for example, regrown heterojunction quality has been rather disappointing in comparison to uninterrupted epitaxial growth. We have conducted a search for surface chemical preparations on In0.53Ga0.47As which would produce the highest-quality InP/In0.53Ga0.47As regrown heterojunction interface, as measured by surface recombination velocity (SRV). After an extensive survey, we have found that dilute bromine-based etching solutions are best for preparing a free In0.53Ga0.47As surface for subsequent InP regrowth. The resulting InP/In0.53Ga0.47As interfacial SRV is ≲20 cm/s, comparable to heterojunctions grown without any interruption at all.
Far-infrared capture of electrons by DX centers
Authors: Plombon J. J. , Bewley W. W. , Felix C. L. , Sherwin M. S. , Hopkins P. , Sundaram M. , Gossard A. C.
Published: July 2002

Intense radiation with photon energy of a few meV can induce the capture of electrons by DX centers in AlxGa1−xAs:Si.
Optical transitions and chemistry at the In0.52Al0.48As/InP interface
Authors: Brasil M. J. S. P. , Nahory R. E. , Quinn W. E. , Tamargo M. C. , Farrell H. H.
Published: July 2002

We report properties of the InAlAs/InP interface and its formation during growth by organometallic molecular beam epitaxy. Taking advantage of the photoluminescence emission occurring at this type II interface, we were able to directly investigate the interface characteristics for different growth conditions. A shift is observed in the energy of the interface recombination transition which we interpret as evidence of a P-As exchange effect dependent on the specific growth sequence. This effect was further investigated by growing interfaces with thin layers (InAs, AlAs, AlP) between the InP and InAlAs. The results can be understood in terms of a model based on bond strength considerations. We predicted and demonstrated that the most stable interface is obtained with incorporation of a thin AlP interfacial layer.
Size dependence of the thermal broadening of the exciton linewidth in GaAs/Ga0.7Al0.3As single quantum wells
Authors: Qiang H. , Pollak Fred H. , Sotomayor Torres C. M. , Leitch W. , Kean A. H. , Stroscio Michael A. , Iafrate Gerald J. , Kim K. W.
Published: July 2002

We have studied the temperature dependence of the linewidth, Γ(T), of the fundamental absorption edge in bulk GaAs and four GaAs/Ga0.7Al0.3As single quantum wells of different well width using photoreflectance. As a result of the size dependence of the exciton-longitudinal optical phonon interaction, the thermal broadening of the linewidth diminishes as the dimensionality and size of the system are reduced.
On the negative differential resistance effect in high-field semiconductor-dielectric systems
Authors: Gradinaru G. , Madangarli V. P. , Sudarshan T. S.
Published: July 2002

The active role of the dielectric in high-field semiconductor-dielectric systems (HFSDS) and the experimental results concerning partial and total surface flashover in HFSDS are presented. The system negative differential resistance effect by partial surface flashover is explained on the basis of a two-channel model with an appropriate equivalent circuit correlated with the potential distribution along the semiconductor.
Cavity formation in semiconductor lasers
Authors: O’Gorman J. , Levi A. F. J. , Coblentz D. , Tanbun-Ek T. , Logan R. A.
Published: July 2002

The temporal development of both lasing light intensity and spectral content is influenced by the number of round-trips photons make inside a Fabry–Perot laser. A surprisingly large number of cavity round trips (n≳100) are required for laser emission intensity and spectral content to approach dc values. With decreasing n the laser increasingly takes on the character of a light emitting diode.
Long range gallium segregation in the AlAs layers of GaAs/AlAs superlattices
Authors: Jusserand Bernard , Mollot Francis
Published: July 2002

We demonstrate from Raman scattering on the AlAs-type optical vibrations in GaAs/AlAs superlattices that small but significant amount of gallium atoms segregate in the AlAs layers over more than 10 monolayers from the AlAs on GaAs interface. We discuss the growth temperature dependence of this effect and its consequences for a global description of the interface roughness in this system.
Nonlinear optical properties of quantum-confined GaAs nanocrystals in Vycor glass
Authors: Justus B. L. , Tonucci R. J. , Berry A. D.
Published: July 2002

Quantum-confined nanocrystallites of GaAs are fabricated in porous Vycor glass and the bound electronic nonlinear refractive index, the two-photon absorption coefficient, and the refraction from carriers generated by two-photon absorption are simultaneously determined using the Z-scan method and compared to those of bulk GaAs. The measured nonlinear refractive index is an order of magnitude larger than that of bulk GaAs at 1060 nm.
Electrical properties of p-type ZnSe:N thin films
Authors: Yang Z. , Bowers K. A. , Ren J. , Lansari Y. , Cook J. W. , Schetzina J. F.
Published: July 2002

The van der Pauw Hall effect measurements from 77–350 K are reported for a series of p-type nitrogen-doped ZnSe thin films. Epitaxial HgSe electrodes were used as ohmic contacts in these experiments.
Calibration of the multiple quantum well probe technique for dry-etch-induced damage analysis
Authors: Green D. L. , Skidmore J. A. , Lishan D. G. , Hu E. L. , Petroff P. M.
Published: July 2002

The multiple quantum well (MQW) probe technique has provided information of the depth distribution of dry-etch-induced damage. The observed changes in the quantum well cathodoluminescence intensities or peak positions have been uniquely attributed to the dry etching process. There are indications, however, that this premise may not be justified for some applications. Two studies are described, which we have carried out to establish critical calibrations of the MQW probe technique. A slow, carefully controlled wet etch of the starting MQW probe structure allowed us to observe a reduction in luminescence intensity of quantum wells located less than 20 nm from the surface. The other study reveals that the thicknesses and relative ordering of the quantum wells may influence the interpretation of the MQW data.
Positive charge and interface state creation at the Si-SiO2 interface during low-fluence and high-field electron injections
Authors: Mir Abdellah , Vuillaume Dominique
Published: July 2002

Positive charge and interface state creation at the Si-SiO2 interface has been studied when low-fluence electron injections (lower than 10−3 C/cm2) are performed at high-field (higher than 7 MV/cm) across the oxide. Temperature dependencies for the formation of both types of interface defects are examined in the range 90–300 K. It is found that positive charge formation is temperature independent, while interface state creation is thermally activated. The former result (no temperature dependence) is consistent with impact ionization in SiO2, while the latter (temperature dependence) is explained by a trap creation due to hydrogen-related species diffusion. These results show that both mechanisms can occur together, and that they trigger two different kinds of damage at the Si-SiO2 interface.
Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase mask
Authors: Hill K. O. , Malo B. , Bilodeau F. , Johnson D. C. , Albert J.
Published: July 2002

A photolithographic method is described for fabricating refractive index Bragg gratings in photosensitive optical fiber by using a special phase mask grating made of silica glass. A KrF excimer laser beam (249 nm) at normal incidence is modulated spatially by the phase mask grating. The diffracted light, which forms a periodic, high-contrast intensity pattern with half the phase mask grating pitch, photoimprints a refractive index modulation into the core of photosensitive fiber placed behind, in proximity, and parallel, to the mask; the phase mask grating striations are oriented normal to the fiber axis. This method of fabricating in-fiber Bragg gratings is flexible, simple to use, results in reduced mechanical sensitivity of the grating writing apparatus and is functional even with low spatial and temporal coherence laser sources.
Effect of surface tension on the growth mode of highly strained InGaAs on GaAs(100)
Authors: Snyder C. W. , Orr B. G. , Munekata H.
Published: July 2002

We have investigated the molecular beam epitaxy growth of highly strained InGaAs on GaAs(100) as a function of the anion to cation flux ratio. Using reflection high energy electron diffraction the evolution of the film morphology is monitored and the surface lattice constant is measured. It is found that the cation to anion flux ratio dramatically affects the growth mode. Under arsenic-rich conditions, growth is characterized by a two-dimensional (2D) to three-dimensional (3D) morphological transformation. However, for cation-stabilized conditions, 3D islanding is completely suppressed, and 2D planar growth is observed. We associate these differences in the growth mode with corresponding changes in the surface tension of the overlayer. A high surface tension stabilizes 2D growth. An analysis which relates surface tension to a critical thickness for the onset of coherent island formation supports this view.
Polarity inversion of CdTe(111) orientation grown on Bi (00.1) by molecular beam epitaxy
Authors: DiVenere A. , Yi X. J. , Hou C. L. , Wang H. C. , Ketterson J. B. , Wong G. K. , Sou I. K.
Published: July 2002

Using in situ reflection high-energy electron diffraction analysis and chemical etching it is shown that CdTe grown on Bi layers deposited on CdTe (1̄1̄1̄)B terminated surfaces result in (111)A terminated surfaces. The Bi layers exhibit streaked diffraction patterns with clear Kikuchi lines; this is the first direct evidence for the layer by layer two-dimensional growth of Bi on CdTe by molecular beam epitaxy.
Bias assisted etching of diamond in a conventional chemical vapor deposition reactor
Authors: Stoner B. R. , Tessmer G. J. , Dreifus D. L.
Published: July 2002

A novel technique for selectively etching diamond films is presented. This letter describes a technique by which diamond may be etched in a conventional plasma assisted chemical vapor deposition (CVD) reactor at rates comparable to those reported for both electron cyclotron resonance and reactive ion etching techniques. This technique involves negatively biasing the diamond film, while it is immersed in a mostly hydrogen containing plasma. Negative dc bias assisted etching of CVD diamond films is performed in both microwave and dc plasma reactors over a wide range of temperatures and pressures. Speculation on the etching mechanisms is also included.
Electro-optic sampling using an aluminum gallium arsenide probe
Authors: Dykaar D. R. , Kopf R. F. , Keil U. D. , Laskowski E. J. , Zydzik G. J.
Published: July 2002

We demonstrate a new electro-optic sampling probe with femtosecond resolution. The probe is a 7-μm-thick film of 80% AlGaAs used in both total internal reflecting and free-standing geometries. A thin-film probe eliminates many of the problems associated with the use of bulk crystals as electro-optic sensors.
Controlled passivation of GaAs by Se treatment
Authors: Scimeca T. , Watanabe Y. , Maeda F. , Berrigan R. , Oshima M.
Published: July 2002

The passivation of GaAs(100) by Se has been achieved in a controlled manner. Results provided by synchrotron radiation photoelectron spectroscopy show that the extent of Se bonding to As and degree of band bending can be systematically controlled by varying the GaAs substrate temperature during Se exposure. Furthermore, the formation of Ga vacancies is also found to depend on substrate temperature.
Fabrication of 5–7 nm wide etched lines in silicon using 100 keV electron-beam lithography and polymethylmethacrylate resist
Authors: Chen Wei , Ahmed Haroon
Published: July 2002

The present limit of around 10 nm for the width of lines fabricated by e-beam lithography using polymethylmethacrylate (PMMA) resist on silicon substrates has been overcome. 5–7 nm wide etched lines in bulk Si substrates have been produced. A 65 nm thick layer of PMMA was exposed with an 80 kV electron beam of diameter smaller than 5 nm. After exposure the resist was developed in 3:7 cellosolve:methanol with ultrasonic agitation. The pattern in resist was transferred to the Si substrate with reactive ion etching. Lines of width varying between 5 and 7 nm were recorded using an S-900 scanning electron microscope which has a resolution of 0.7 nm.
Composition dependence of the in-plane effective mass in lattice-mismatched, strained Ga1−xInxAs/InP single quantum wells
Authors: Meyer B. K. , Drechsler M. , Wetzel C. , Härle V. , Scholz F. , Linke H. , Omling P. , Sobkowicz P.
Published: July 2002

The composition dependence of the in-plane conduction band effective mass in strained 15-nm-thick lattice-mismatched Ga1−xInxAs/InP single quantum wells was determined by conventional cyclotron and optically detected cyclotron resonance techniques. Our results are in agreement with a self-consistent calculation taking into account effects due to nonparabolicity, confinement, strain, and finite two-dimensional carrier densities.
Deep hole traps in p-type nitrogen-doped ZnSe grown by molecular beam epitaxy
Authors: Hu B. , Karczewski G. , Luo H. , Samarth N. , Furdyna J. K.
Published: July 2002

P-type nitrogen-doped ZnSe grown on n+-GaAs by molecular beam epitaxy has been studied by deep-level transient spectroscopy (DLTS) and double correlation DLTS. To achieve p-type doping of ZnSe, we employed an active nitrogen beam produced by a free-radical plasma source. Four hole traps—with activation energies of 0.22, 0.51, 0.63, and 0.70 eV—were detected by DLTS. Two of these—those at 0.51 and 0.63 eV—have never been observed before in ZnSe. They are probably introduced to the material by nitrogen doping. The properties of the other two traps—at 0.22 and 0.70 eV—support the hypothesis that both of them are associated with native defects, in agreement with earlier reports. To our knowledge this is the first report about direct experimental investigation of deep states in p-type ZnSe.
Diffusion of silver in C60 thin films
Authors: Sarkar Dipankar , Halas N. J.
Published: July 2002

The diffusion of silver atoms into C60 solid thin films is observed. The diffusion process is significant at temperatures just above room temperature, with an activation energy of 2.5±0.5 eV (60 kcal/mol). Temperature dependent conductivity measurements indicate that silver atoms form an impurity band in solid C60 with an activation energy of 0.26 eV. The ready interdiffusion of silver and C60 may have important implications in the formation of C60/metal contacts or C60/metal layered structures or materials.
Dependence of conduction-band effective mass on quaternary alloy composition of (In0.52Al0.48As)z(In0.53Ga0.47As)1−z lattice matched to InP
Authors: Cury L. A. , Beerens J. , Praseuth J. P.
Published: July 2002

Cyclotron resonance measurements were carried out on high quality (In0.52Al0.48As)z(In0.53Ga0.47As)1−z thick layers grown on InP substrates by molecular beam epitaxy. The measurements were performed at 60 K and we were able to obtain the electron effective mass dependence with z in the whole range of composition 0≤z≤1. Using the band-gap values as obtained from photoluminescence measurements on the same samples at 60 K, nonparabolicity corrections were taken into account to obtain the effective mass m0* at the conduction band edge. A nonlinear variation m0* with z could be inferred from our experimental data. The expression m0*(z)/me=0.043+0.042z−0.016z2, which includes a quadratic dependence in z (or a so-called bowing parameter), gives a very good fit to our experimental data.
Strained InGaAs/AlGaAs quantum well infrared detectors at 4.5 μm
Authors: Fiore A. , Rosencher E. , Bois P. , Nagle J. , Laurent N.
Published: July 2002

We demonstrate midinfrared photodetection at λ=4.5 μm in a multi-quantum well detector using a strained InGaAs/AlGaAs alloy grown on a GaAs substrate. The detector shows very low dark current of a few pA, a peak unpolarized light responsivity R=12 mA/W for an external 45° angle of incidence, and a background-limited detectivity D*BL=4×1010 cm Hz1/2/W at temperatures up to 95 K in the same conditions. This opens the way to high performance 3–5 and 8–12 μm GaAs-based multispectral detectors
Blue light emission from silicon surfaces prepared by spark-erosion and related techniques
Authors: Rüter D. , Kunze T. , Bauhofer W.
Published: July 2002

We have investigated the photoluminescence (PL) of silicon surfaces treated by spark-erosion and related techniques as a function of gas composition and pressure. PL emission in the wavelength range of 400–550 nm and with decay times ≤5 ns has been observed. PL quantum efficiencies comparable to that of standard wet chemically prepared porous silicon are obtained. We have never found significant PL emission in the red spectral range. Our dry plasma techniques for the preparation of luminescent silicon surfaces are a promising alternative to the common anodizing process in hydrofluoric solutions.
Transverse magnetoresistance: A novel two-terminal method for measuring the carrier density and mobility of a semiconductor layer
Authors: Lowney Jeremiah R. , Thurber W. Robert , Seiler David G.
Published: July 2002

The magnetic-field dependence of the two-terminal magnetoresistance that occurs in rectangularly shaped samples can be used to determine both the free-carrier density and the mobility of a semiconductor layer. An approximate equation for the magnetoresistance was derived for variable length-to-width ratio. This technique was used to determine the electron density and mobility of accumulation layers in n-type Hg0.8Cd0.2Te photoconductive infrared detectors at 6 and 77 K. It should be applicable to a wide variety of fabricated devices and allow significant improvements in processing methods and quality control.
New material with strong electro-optic effect: Rubidium hydrogen selenate (RbHSeO4)
Authors: Salvestrini J. P. , Fontana M. D. , Aillerie M. , Czapla Z.
Published: July 2002

A very large electro-optic Pockels coefficient is reported in a crystal of rubidium hydrogen selenate. This new material is found to be promising for modulation applications because of its very low half-wave voltage (∼27 V) which is determined.
Magneto-impedance effect in amorphous wires
Authors: Panina L. V. , Mohri K.
Published: July 2002

Recent experiments have discovered a giant magneto-impedance (MI) effect in FeCoSiB amorphous wires. This effect includes a sensitive change (as much as 60%) in a high frequency wire voltage by an applied dc magnetic field and is thus a high frequency analog of giant magnetoresistance. We consider this phenomenon in terms of ac complex resistance or impedance. The giant MI effect is demonstrated to arise from a combination of a skin effect and a strong field dependence of the circumferential magnetic permeability associated with circular domain wall movements. The theoretical results agree satisfactorily with the existing experimental data.
Dynamic holography in a reflection/transmission photorefractive quantum-well asymmetric Fabry–Perot
Authors: Kwolek K. M. , Melloch M. R. , Nolte D. D.
Published: July 2002

A multiple-quantum-well film bonded to glass has high transmittance while performing simultaneously as a reflection asymmetric Fabry–Perot electro-optic modulator. When the multiple-quantum-well structure is made semi-insulating through proton implantation, the device becomes photorefractive and sensitive to dynamic space-charge grating formation. We demonstrate a photorefractive Al0.1Ga0.9As/GaAs multiple-quantum-well asymmetric Fabry–Perot device that has diffraction orders in both transmission and reflection. The reflection diffraction efficiency is enhanced by a favorable front/back reflectivity ratio for the incident glass-first interface geometry. We obtain output diffraction efficiencies of 1.3% in reflection and 0.7% in transmission, corresponding to input diffraction efficiencies of 0.15% in reflection and 0.22% in transmission.
Optical transitions in strained Si1−yCy layers on Si(001)
Authors: Kissinger W. , Weidner M. , Osten H. J. , Eichler M.
Published: July 2002

The effect of the carbon content on the optical transitions of Si1−yCy layers grown pseudomorphically on Si(001) substrates was investigated by spectroscopic ellipsometry and electroreflectance spectroscopy for 0≤y≤0.012 in the energy range between 3 and 5 eV. The ellipsometry data show a decrease of the slope of the dielectric function near the critical points and a tendency of a critical point shift with increasing carbon content. This shift was analyzed by measuring and fitting electroreflectance spectra at 80 K, resulting in a weak and linear dependence on the carbon content at all transitions. The E1 critical point energy increases with an increasing carbon content while the E2 energy decreases, both at a rate of about 30 meV/%[C]. The E′0 transition decreases at a smaller rate of about 20 meV/%[C]. The results are discussed in the light of previous discussions and a simple estimate based on silicon deformation potentials.
Determination of excess phosphorus in low-temperature GaP grown by gas source molecular beam epitaxy
Authors: He Y. , El-Masry N. A. , Ramdani J. , Bedair S. M. , McCormick T. L. , Nemanich R. J. , Weber E. R.
Published: July 2002

GaP films, epitaxially grown at a low temperature (LT) of ∼200 °C by gas source molecular beam epitaxy, were reported recently to have excess phosphorus. In this letter, we report on the quantitative determination of the excess phosphorus in the LT films, using various approaches. Analytical scanning transmission electron microscopy, double-crystal x-ray diffraction, and particle-induced x-ray emission showed that the LT GaP films incorporated excess phosphorus of ∼0.6–2 at. %. The amount of excess phosphorus estimated from Raman scattering measurements, using the LO-TO phonon frequency splitting data of the as-grown LT GaP and bulk GaP, was in general agreement with those obtained from other techniques.
Nucleation and coalescence in hydrogenated amorphous silicon studied by scanning tunneling microscopy
Authors: Ikuta Kazuyuki , Tanaka Kazunobu , Yamasaki Satoshi , Miki Kazushi , Matsuda Akihisa
Published: July 2002

Direct subnanometer-scale observation was made on an ultrathin film of hydrogenated amorphous silicon deposited on a highly oriented pyrolytic graphite substrate, using a ultrahigh vacuum scanning tunneling microscopy. Subnanostructures with a size of 5–10 Å were observed on the top surface independent of the film thickness below 400 Å, which are speculated to be SiH3. It is demonstrated that coalescence between nuclei (clusters) is enhanced by a surface diffusion of SiH3 precursors.
Enhanced compositional disordering of quantum wells in GaAs/AlGaAs and InGaAs/GaAs using focused Ga+ ion beams
Authors: Piva P. G. , Poole P. J. , Buchanan M. , Champion G. , Templeton I. , Aers G. C. , Williams R. , Wasilewski Z. R. , Koteles E. S. , Charbonneau S.
Published: July 2002

Spatially selective compositional disordering induced by focused Ga+ ion beam implantation in GaAs/AlGaAs and strained InGaAs/GaAs quantum well structures has been studied using photoluminescence. We find that beyond a certain implantation dosage, the degree of intermixing imparted to a given quantum well saturates and may eventually decline as a result of damage to the semiconductor surface. We overcome this limitation by thermally annealing the sample after implantation to repair the crystalline surface. We show that multiple successive implants interspersed with rapid thermal anneals (RTAs) are successful in locally shifting the optical band gap of quantum wells by many times that attributed to a single implant and RTA.
Observation of ZnSe/GaAs interface states by reflectance difference spectroscopy
Authors: Yang Z. , Wong G. K. , Sou I. K. , Yeung Y. H.
Published: July 2002

Small angle reflectance difference spectroscopy (RDS) has been employed to study the interface states of ZnSe/GaAs heterojunctions. A sharp resonance of ΔR/R near 2.70 eV is observed. The resonance is due to the crossover electronic transition from the valence band of ZnSe to the quantum well states of the heterojunction, as is confirmed by two-photon second harmonic generation spectra. The resonance persists even when the thickness of the ZnSe layer is beyond the critical thickness, suggesting that the interface remains largely intact after strain relaxation. The advantages of small angle RDS in studying buried heterojunctions over other techniques are discussed.
Effect of SiH4/CH4 flow ratio on the growth of β-SiC on Si by electron cyclotron resonance chemical vapor deposition at 500 °C
Authors: Liu Chih-Chien , Lee Chiapyng , Cheng Kuan-Lun , Cheng Huang-Chung , Yew Tri-Rung
Published: July 2002

β-SiC (3C–SiC) films were deposited on silicon substrates by electron cyclotron resonance chemical vapor deposition from SiH4/CH4/H2 mixtures at 500 °C. The crystalline structure and chemical composition of the deposited film were found to depend on the SiH4/CH4 flow ratio. With a sufficient energy supply from microwave power and a SiH4/CH4 flow ratio of 0.5 and lower, stoichiometric SiC could be deposited on Si substrates. Microcrystalline β-SiC was grown at a SiH4/CH4 flow ratio of 0.5, whereas amorphous SiC was obtained at the SiH4/CH4 flow ratios lower than 0.5. When the SiH4/CH4 flow ratio was above 0.5, only polycrystalline Si could be deposited.
Fluxon interaction in an overdamped Josephson transmission line
Authors: Kaplunenko V. K.
Published: July 2002

We have simulated fluxon propagation within a Josephson transmission line that is one of the elements of a rapid single flux quantum (RSFQ) circuit. This line is a parallel connection of damped Josephson junctions coupled by superconducting inductances. Geometries with and without mutual inductances between neighboring cells are considered. If the time interval between two fluxons is less than 0.3 φ0Vc−1 (Vc characteristic voltage) a repulsion between them was observed. The repulsion sets a specific time delay between the fluxons that impose a rigid limitation for high frequency performance of RSFQ logic. The delay depends on the shunting parameter, bias current, and values of interconnecting inductances.
Solid neon sources for plasmas and x-ray lasers
Authors: Dixon R. H. , Ford J. L. , Lee T. N. , Elton R. C.
Published: July 2002

A thin layer of frozen neon is condensed onto a cryogenically cooled substrate for use in generating a neon plasma for x-ray laser research. The freezing process and quasiequilibrium state of the layer are discussed. Spectral purity of a neon plasma formed by laser irradiation has been demonstrated. Control of the layer thinness is possible.
Optical gain due to excitonic transitions in ZnCdSe/ZnMgSSe strained layer quantum well blue–green lasers: Prediction of low threshold under tensile strain
Authors: Huang W. , Jain F. C.
Published: July 2002

Gain coefficient of excitonic transitions is calculated for unstrained as well as strained ZnCdSe/ZnMgSSe multiple quantum well (MQW) lasers as a function of operating wavelength and injection current. Unlike the III–V strained layer quantum well lasers, the gain coefficient due to excitonic transitions is significantly higher in II–VI systems as the exciton binding energy is more than 5 times larger. This results in a primary role for excitons in lasing which has been verified experimentally. Strain induced changes in energy band gaps and effective masses of light and heavy holes are included in the gain and threshold current density calculations, which are in agreement with experimental data available for compressive strained layers grown on GaAs substrates. Significantly low threshold current density of about 180 A/cm2 are predicted for the tensile strained ZnCdSe–ZnMgSSe and ZnCdSe–ZnSeTe quantum well lasers using InP substrates.
Band bleaching and growth dynamics in 45%GeO2–55%SiO2 films
Authors: Simmons-Potter Kelly , Simmons Joseph H.
Published: July 2002

Photoinduced changes in the ultraviolet absorption bands of GeO2–SiO2 glasses have a clear significance in photosensitive processes. In this letter, the dynamics of bleaching and growth of absorption bands in highly photosensitive germanosilicate thin films following exposure to 248 nm excimer laser radiation are discussed. Strong evidence is found in support of a single-photon process for the bleaching of a band at 238 nm and growth of a band at 202.5 nm as a source of photosensitivity. Calculated changes in refractive index are eight times greater than those found in two-photon-induced grating experiments. This suggests avenues for enhancement of photosensitive effects through proper materials conditioning.
Inexpensive and high-precision digital power supply and counting interface for UPS, XPS, and Auger spectrometers
Authors: Lichtenberger Dennis L. , Kellogg Glen Eugene , Kristofzski John G. , Page Dennis , Turner Steve , Klinger Gerry , Lorenzen Jim
Published: July 2002

A digital data-acquisition system has been built to interface two photoelectron spectrometers to a DEC LSI-11/23 processor.
Se chemical passivation and annealing treatment for GaAs Schottky diode
Authors: Xu Huaiqi , Belkouch Saïd , Aktik Cetin , Rasmussen Wolfgang
Published: July 2002

A Se chemical passivation [(NH4)2S+Se] for GaAs Schottky diodes is presented. We have found that our (NH4)2S+Se passivated Schottky diodes have more than one order of magnitude higher forward current density than the (NH4)2Sx passivated ones. In rapid thermal annealing treatment, an initial decrease and then increase of forward current density for (NH4)2Sx passivated diodes is observed. For the (NH4)2S+Se and (NH4)OH treated diodes, a steady decrease of Schottky barrier height with increased annealing temperature is observed. With or without annealing treatment, the (NH4)2S+Se passivated diodes have the lowest barrier height.
Hg resonance lamp with controlled 254-nm line profiles for transient temperature measurements
Authors: Braun W. , Price I. , Scheer M. D.
Published: July 2002

A new method has been described elsewhere for measuring transient temperature changes using the absorption of the 254-nm resonance radiation from an optically thick Hg lamp by traces of Hg vapor. The method is based upon the fact that the Hg resonance lines broaden in a known way with increasing temperature and pressure. The sensitivity and dynamic range of the temperature measurement depends upon the extent of line reversal which varies with the optical depth of the Hg lamp. It is shown here that this can be controlled by means of a proper lamp design. The mounting assembly for the lamp and the circuitry required for this purpose are described. It was demonstrated that the output of such a lamp is stable and reproducible with line profiles (optical densities) that could be varied over about a 20-fold range. Within such a range of operating parameters, the measurement of transient temperature to 1500 K in gases at pressures below 100 Torr could be made.
Emission characteristics of zeolite A ion source
Authors: Satoh Y. , Takebe M. , Iinuma K.
Published: July 2002

An easy method to prepare alkali ion emitters has been realized by using the ion-exchange technique. The thermionic emission characteristics of zeolite A alkali ion sources have been studied. Experimental results indicate that ion transport process within zeolite plays an important role in the emission mechanism.
Initial stage of aluminum nitride film growth on 6H-silicon carbide by plasma-assisted, gas-source molecular beam epitaxy
Authors: Tanaka Satoru , Kern R. Scott , Davis Robert F.
Published: July 2002

The initial stage of AlN film growth on 6H-SiC(0001) substrates by plasma-assisted, gas source molecular beam epitaxy (PAGSMBE) has been investigated in terms of growth mode and interface defects. Cross-sectional high resolution transmission electron microscopy (HRTEM) was used to observe the microstructure of the deposited films and the AlN/SiC interfaces. Surface morphologies and interface atomic structures were compared between films grown on vicinal and on-axis surfaces. Essentially atomically flat AlN surfaces were obtained using on-axis substrates. This is indicative of two-dimensional growth to a thickness of ∼15 Å. Islandlike features were observed on the vicinal surface. The coalescence of these features at steps gave rise to double positioning boundaries (DPBs) as a result of the misalignment of the Si/C bilayer steps with the Al/N bilayers in the growing film. The quality of thicker AlN films is strongly influenced by the concentration of DPBs formed at the outset of growth.
Macintosh/LabVIEW based control and data acquisition system for a single photon counting fluorometer
Authors: Stryjewski Wieslaw J.
Published: July 2002

A flexible software system has been developed for controlling fluorescence decay measurements using the virtual instrument approach offered by LabVIEW. The time-correlated single photon counting instrument operates under computer control in both manual and automatic mode. Implementation time was short and the equipment is now easier to use, reducing the training time required for new investigators. It is not difficult to customize the front panel or adapt the program to a different instrument. We found LabVIEW much more convenient to use for this application than traditional, textual computer languages.
Direct current conduction properties of sputtered Pt/(Ba0.7Sr0.3)TiO3/Pt thin films capacitors
Authors: Hsu W. Y. , Luttmer J. D. , Tsu R. , Summerfelt S. , Bedekar M. , Tokumoto T. , Nulman J.
Published: July 2002

Current flow through Pt/(Ba0.7Sr0.3)TiO3/Pt stack consists of both polarization current and electronic leakage current, which were separated by monitoring the discharging current when applied voltage was turned off. Electronic current comes from electrical field enhanced Schottky emission at the electrode–dielectric interface, and dominates the current flow at high electric field. At low electric field, polarization current prevails. The voltage and time dependence of the polarization current can be modeled by a distribution of Debye-type relaxations. The relaxation time and capacitance derived from current–time measurements were applied to simulate the current–voltage behavior, where good fitting to experimental result was obtained.
57Fe Mössbauer spectroscopy in a diamond-anvil cell at variable high pressures and cryogenic temperatures
Authors: Hearne G. R. , Pasternak M. P. , Taylor R. D.
Published: July 2002

A detailed description is given of a new facility for transmission 57Fe Mössbauer spectroscopy (MS) studies on absorbers in a miniature diamond-anvil cell in the temperature range 10–300 K and pressure range 0–45 GPa. Spectra with an adequate signal-to-noise ratio for a reliable analysis to be made have each been measured in 20–30 h of data accumulation time. This has been accomplished in spite of the severe constraints imposed by both the minute sample area and the high attenuation of the 14.4 keV probing radiation by the diamond anvils compared with the experimental conditions of conventional MS experiments. A custom-made Rh(57Co) ‘‘point’’ source with a high specific activity has been used in conjunction with absorbers that have an optimal enrichment (10%–20%) of 57Fe. The successful implementation of 57Fe Mössbauer spectroscopy at very high pressures and variable cryogenic temperatures has been accomplished by making relatively simple refinements to existing elements of high-pressure MS methodology.
X-ray photoelectron diffraction of (100)-oriented chemical vapor deposited diamond films on silicon (100)
Authors: Schaller E. , Küttel O. M. , Aebi P. , Schlapbach L.
Published: July 2002

(100)-oriented diamond films have been grown on silicon (100) in a microwave plasma assisted chemical vapor deposition (CVD) tubular system. X-ray photoelectron diffraction (XPD) has been used to study such oriented polycrystalline films. Comparing the diffractograms of a natural diamond (100) surface and of polycrystalline (100)-oriented CVD diamond films quite similar features are observed. XPD measurements after 8 min of bias treatment show that the tiny crystals are already preferentially oriented at deposition parameters required for (100)-oriented film growth. Our measurements indicate a strong need to control the growth parameters very carefully during the first minutes of growth to get an orientation.
One-dimensional exciton diffusion in GaAs quantum wires
Authors: Nagamune Y. , Watabe H. , Sogawa F. , Arakawa Y.
Published: July 2002

One-dimensional diffusion of excitons in GaAs quantum wires was observed by using microphotoluminescence measurements at low temperature. The observed diffusion length increased with decreasing wire width from 30 to 15 nm, and decreased from 15 to 7 nm, where maximum diffusion length was about 4 μm for the 15 nm quantum wire, which is the largest value so far reported. It is considered that the change of diffusion length versus wire width is caused by the competition between one-dimensional character and the interface fluctuation.
Optical monitoring of protein crystals in time-resolved x-ray experiments: Microspectrophotometer design and performance
Authors: Chen Ying , Šrajer Vukica , Ng Kingman , LeGrand Alan , Moffat Keith
Published: July 2002

We describe a single crystal microspectrophotometer for optical monitoring of protein crystals during time-resolved x-ray diffraction experiments. It is a single beam device with a diode array detector for simultaneous multiwavelength measurement of optical spectra. The time resolution of the microspectrophotometer is 2.5 ms, which matches the ms time resolution of the x-ray experiments obtained at existing synchrotron sources. Optical densities of single crystals can be reliably measured up to 2.5 OD, with the linear range extending to 2 OD. The tight focusing of the light beam allows monitoring of crystals as small as 30–50 μm, an important feature since successful reaction initiation and optical monitoring of crystals in time-resolved x-ray diffraction experiments often require very small crystals. The compact design of the microspectrophotometer and the long objective-to-crystal distance make it compatible with the crowded environment of time-resolved x-ray diffraction experiments.
Retarding field energy analyzer for the characterization of negative glow sheet plasmas in a magnetic field
Authors: Mathew J. , Meger R. A. , Fernsler R. F. , Gregor J. A.
Published: July 2002

A retarding field energy analyzer has been developed for diagnosing 300 μs duration, 60 cm×60 cm negative glow, sheet plasmas immersed in a 150–250 G axial magnetic field. The electron density in these 4.5 kV, 13 A, 120 mTorr discharges in air and other gases, is high enough to reflect X-band microwaves. The presence of the magnetic field makes the suppression of secondary electrons from the Faraday collector surface more difficult. The approach taken here is to bias the entire collection circuit and the amplifiers 90 V positive with respect to the data acquisition room. The differentially pumped analyzer is designed to accept electrons with a large range of perpendicular velocities, and it measures the parallel velocity distribution function of the discharge electrons entering a 0.64-mm-diam hole in the anode plate. It gives valuable information about the energy spectrum of the energetic beam electrons emitted from the cathode, and the effect of energy loss and scattering processes on this propagating beam component. Additionally, since the analyzer sampling hole is offset from the anode-cathode axis, the current density profile can be measured for different bias voltages on the retarding grid, by rotating the linear cathode about the vertical anode-cathode axis. These profiles give the sheet thickness for the beam and plasma components of the negative glow discharge. It also gives useful information about the scattering induced beam spreading and its effects on the plasma sheet thickness and electron density.
Rate of consumption of Cu in soldering accompanied by ripening
Authors: Kim H. K. , Tu K. N.
Published: July 2002

What is the rate of consumption of Cu in soldering reactions has been a critical question in electronic packaging technology. The Cu films are consumed by Cu–Sn compound formation. Because the Cu thickness is limited and the rework of a solder joint requires a layer of unreacted Cu, the loss of Cu in soldering must be under control. At the solder interface, Cu–Sn intermetallic compounds do not form layered structures. Rather, the Cu6Sn5 phase grows as scalloplike grains into the molten solder and ripening occurs between the grains. Therefore, it has been difficult to determine the compound growth rate, and in turn the Cu consumption rate. Using cross-sectional and top-polished samples, we have measured the total volume of Cu–Sn intermetallic compounds formed between eutectic SnPb alloy and Cu substrate as a function of reflow time and temperature. We have deduced that after 1 min reflow, for example, the thickness of Cu consumed was about 0.36, 0.47, and 0.69 μm at 200, 220, and 240 °C, respectively.
Gas condensation source for production and deposition of size-selected metal clusters
Authors: Goldby I. M. , von Issendorff B. , Kuipers L. , Palmer R. E.
Published: July 2002

We describe the construction and performance of a gas condensation cluster source. The source was designed for deposition of mass-selected metal clusters with controlled landing energy. We have produced clusters of Pbn (n=2–∼300) and Agn (n=∼20–∼300) with sufficient intensity to deposit size-selected clusters to a density of 1012 clusters/cm2 in 10 min. The landing energy of the clusters can be controlled from ∼25 to 800 eV.
Polarization switching in AlGaAs/GaAs distributed feedback lasers between the stable single longitudinal modes
Authors: Ouchi Toshihiko , Majima Masao , Ikeda Sotomitsu , Ono Takeo , Uchida Mamoru , Handa Yuichi
Published: July 2002

TE/TM polarization mode switching is demonstrated in an AlGaAs/GaAs distributed feedback laser diode. The device has two current injecting electrodes and as an active layer a conventional multiquantum well (MQW) structure. A small amplitude modulation of the current into one of the electrodes switches the polarization mode. Each mode corresponds to a stable single longitudinal mode of a distributed feedback laser. The lasing wavelength can be tuned continuously, without mode hopping, over 0.7 nm by controlling the total injection current. This new kind of polarization switching is promising for optical switching systems and optical frequency division multiplexing devices.
Critical assessment of thermal models for laser sputtering at high fluences
Authors: Miotello Antonio , Kelly Roger
Published: July 2002

A critical assessment of thermal models for laser sputtering at high fluences is presented. It is argued that the model explaining such sputtering by involving a subsurface superheating effect misinterprets the meaning of ‘‘vaporization’’ and ‘‘boiling’’. As a result inappropriate boundary conditions are used, including those for both the surface temperature and for the surface temperature gradient. On the contrary, it is shown that explosive boiling (also termed phase explosion) in the sense pioneered by Martynyuk and by Fucke and Seydel remains the only thermal mechanism able to explain laser sputtering at high fluences.
Stress induced Li-Li pairs reorientation in Al-Li alloys
Authors: Pérez-Landazábal J. I. , San Juan J. , Nó M. L.
Published: July 2002

Internal friction measurements have been performed in order to study the Li mobility in Al-Li alloys. Coherent δ′ precipitation (nonequilibrium diffusion process) promotes a low temperature background increase according to Schoeck’s theory. A stable Pz relaxation has been observed at around 460 K (∼1 Hz). The analysis of the behaviour of this relaxation allows us to identify the Pz peak with the Zener relaxation associated to the stress induced Li-Li pairs reorientation. The relaxation parameters lead us to determine the activation energy and the diffusion coefficient of Li. Zener relaxation has been observed in Al-Li alloys.
Noise properties of direct current SQUIDs with quasiplanar YBa2Cu3O7 Josephson junctions
Authors: Faley M. I. , Poppe U. , Urban K. , Hilgenkamp H. , Hemmes H. , Aarnink W. , Flokstra J. , Rogalla H.
Published: July 2002

We describe the noise performance of dc SQUIDs fabricated with quasiplanar ramp-type Josephson junctions on the basis of c-axis-oriented YBa2Cu3O7/PrBa2Cu3O7 thin-film heterostructures. The noise spectrum of the dc SQUIDs was measured with dc- and ac-bias schemes at different temperatures and showed values below 10−5 Φ0/Hz1/2 down to frequencies of about 1 Hz at 70 K. Up to now for the magnetic flux noise and the energy resolution obtained at 1 kHz and 77 K the best values were 2.5×10−6, Φ0/Hz1/2 and 3×10−31 J/Hz, respectively. A study of the white and 1/f noises of the SQUIDs was performed. The influence of magnetic flux, bias current, high static magnetic fields, and aging on the SQUID noise were investigated. The junctions and devices do not degrade due to aging in air or thermal cycling.
Photorefractive asymmetric Fabry–Pérot quantum wells: Transverse-field geometry
Authors: Kwolek K. M. , Melloch M. R. , Nolte D. D. , Brost G. A.
Published: July 2002

Photorefractive asymmetric Fabry–Pérot quantum-well structures yield significantly enhanced diffraction during four-wave mixing by employing the sensitive amplitude and phase control of multiple-beam interference within the device. We present an Al0.1Ga0.9As/GaAs photorefractive quantum-well device with a near optimal input diffraction efficiency of 0.36% and an AlAs/GaAs quantum-well device with an output diffraction efficiency of 200%.
Broadly tunable sub-500 fs pulses from an additive-pulse mode-locked thulium-doped fiber ring laser
Authors: Nelson L. E. , Ippen E. P. , Haus H. A.
Published: July 2002

We report an additive-pulse mode-locked (APM) thulium-doped fiber ring laser producing 350–500 fs pulses tunable from 1798 to 1902 nm. The laser operates in the soliton regime, where periodic perturbations cause predictable sidebands and modulation in the optical spectrum.
C60 thin film transistors
Authors: Haddon R. C. , Perel A. S. , Morris R. C. , Palstra T. T. M. , Hebard A. F. , Fleming R. M.
Published: July 2002

N-channel field effect transistors with excellent device characteristics have been fabricated by utilizing C60 as the active element. Measurements on C60 thin films in ultrahigh vacuum show on-off ratios as high as 106 and field effect mobilities up to 0.08 cm2/V s.