bluuebunny/crossref_metadata_2025 huggingface dataset:
370 °C clean for Si molecular beam epitaxy using a HF dip
Authors: Eaglesham D. J. , Higashi G. S. , Cerullo M.
Published: July 2002

We describe a new low-temperature clean for Si molecular beam epitaxy. Growth is carried out on Si wafers subjected to an ≊10–60 s clean in a buffered HF solution prior to insertion in the growth chamber. We demonstrate low defect densities (<105 cm−2) at 380 °C without either the conventional high-temperature cleaning step to desorb a chemical oxide or the use of a glovebox for chemical cleaning and transfer to the vacuum chamber. Wafers are given an ≊200 °C prebake in situ to remove hydrocarbons, and then raised to the growth temperature prior to deposition. For (100) substrates the transition from amorphous deposition to crystalline growth occurs at ≊370 °C, below the temperature at which hydrogen should desorb. On (111) the minimum temperature for epitaxy is ≊500 °C, as expected. We attribute this difference to the large number of undercoordinated Si atoms present on (100), which allows growth to take place even on the hydrogen-terminated surface. Secondary-ion mass spectrometry suggests that contamination levels at the substrate-epilayer interface using this HF dip are comparable to those for conventional (‘‘Shiraki’’) chemical oxide desorption techniques. Omission of the prebake step leads to a highly carbon-contaminated surface: this surface has the remarkable property that Si will grow epitaxially on it, but does not wet it (so that at high temperature Si/Si:C grows as islands).
Double dc SQUID for flux-locked-loop operation
Authors: Foglietti V.
Published: July 2002

A novel double dc superconducting quantum interference device (SQUID) is proposed. It consists of two SQUIDs with different geometries in cascade configuration. The structure makes the device particularly suitable for flux-locked-loop operation without need of a room-temperature heterodyne modulation circuitry and a matching network between the SQUID and the preamplifier, thus simplifying the readout electronic system. The noise performance is calculated as a function of the electrical parameters of the device. The noise contribution of the room-temperature preamplifier can be made negligible using appropriate SQUID parameters while the slew rate of the system can approach 106Φ0/s.
Textured (100) yttria-stabilized zirconia thin films deposited by plasma-enhanced chemical vapor deposition
Authors: Holzschuh H. , Suhr H.
Published: July 2002

Thin films of yttria-stabilized zirconia were deposited by plasma-enhanced chemical vapor deposition on quartz Si(100), Si(111), Ni, and the steels V2A and hastelloy at substrate temperatures (Ts): 673–873 K. The metal β-diketonates Y(thd)3 and Zr(thd)4 were used as precursors. The fully stabilized fluorite-type cubic structure was obtained over a wide range of yttria contents from 3.5 to 80 mol % (Ts=773 K). The quality of the films depended on the match of the thermal expansion coefficients of substrate and deposit.
Photovoltaic transistors based on a steady-state internal polarization effect in asymmetric semiconductor superlattices
Authors: Liu Chun-Ting , Luryi Serge
Published: July 2002

In semiconductor superlattices lacking the reflection symmetry, transient internal polarization fields have previously been reported. We show that a modified structure can generate a steady-state photovoltage. We then propose a new class of photovoltaic transistors in which this voltage directly controls the conductivity of the transistor channel.
Observation of inverse U-shaped profiles after platinum diffusion in silicon
Authors: Zimmermann H. , Ryssel H.
Published: July 2002

Inverse U-shaped profiles of platinum in silicon were obtained after diffusion at 700 °C. Deep level transient spectroscopy was used to determine the platinum concentrations in various depths of the samples. In the bulk, a higher concentration of platinum was found than at the surface. Therefore, these platinum profiles are called ‘‘inverse U-shaped profiles.’’ These diffusion profiles can be described by the Frank–Turnbull mechanism, assuming that the initial concentration of vacancies is higher than their equilibrium concentration. Therefore, the observation of inverse U-shaped profiles after platinum diffusion may serve as the evidence that diffusion processes, which involve vacancies, are determined in the bulk by the initial vacancy concentration and not by the vacancy equilibrium concentration at temperatures of ∼850 °C and below.
Submicron YBa2Cu3O7−δ-Ag-YBa2Cu3O7−δ superconducting proximity junctions
Authors: Gijs M. A. M. , Giesbers J. B. , van Delft F. C. M. J. M. , Timmering C. E. , Gerrits A. M. , Slob A.
Published: July 2002

Using a deep submicron structuring process for oxide superconducting films, we have fabricated planar YBa2Cu3O7−δ-Ag-YBa2Cu3O7–δ proximity junctions operating in the liquid nitrogen temperature region. Under microwave radiation Shapiro steps are generated in the current-voltage characteristic, in accordance with the resistively shunted junction model.
New nonlinear optical crystal thienylchalcone and its harmonic generation properties
Authors: Kitaoka Yasuo , Sasaki Takatomo , Nakai Sadao , Goto Yoshitaka
Published: July 2002

Large single crystals (60×20×8 mm3) of a new nonlinear crystal thienylchalcone (T-17) were grown from acetone by the falling temperature method and the nonlinear optic properties were determined. The effective nonlinear optical coefficient, the angular acceptance (Δθ,Δφ), and the walk-off angle were 7.1 pm/V, 0.053° cm, 1.2° cm1/2, and 3.6°, respectively. Thienylchalcone could be type-I phase matched for the light of wavelengths longer than 910 nm. It is an attractive nonlinear crystal for generating green and blue lasers.
Internal piezoelectric fields in GaInSb/InAs strained-layer superlattices probed by optically induced microwave radiation
Authors: Xu L. , Zhang X. -C. , Auston D. H. , Wang W. I.
Published: July 2002

Using a newly developed femtosecond electro-optic technique, a built-in electric field near a semiconductor surface can be measured by a contactless approach. We have observed for the first time equal strength but opposite direction of strain-induced piezoelectric fields from lattice-mismatched A face and B face of GaInSb/InAs strained-layer superlattices by optically induced microwave radiation.
Growth and electrical properties of ultrafine fibers
Authors: Webb G. W.
Published: July 2002

We report on free-standing ultrafine fibers formed using the gas-evaporation method in the presence of electric fields. Materials up to 10 cm in length have been prepared in this way. Electrical measurements on Pd fibers 3 mm in length and about 200–300 Å in diameter are given.
Current gain reduction by 0.45 eV oxygen related level in graded band-gap AlGaAs base-emitter of heterojunction bipolar transistors
Authors: Watanabe Kazuo , Yamazaki Hajime , Wada Kazumi
Published: July 2002

There are good indications that the deep level around 0.45 eV induced by oxygen acts as an effective electrical recombination center in heterojunction bipolar transistors with a graded band-gap AlGaAs emitter-base junction. The transistor, with a concentration of the 0.45 eV level on the order of about 5×1015 cm−3 in the junction region, shows a very small current gain of 3–4, down from more than 30 at a collector current density of 1×103 A/cm3. This reduction is greater than the previously reported one by the level approximately 0.6 eV in heterojunction bipolar transistors. The level about 0.45 eV is detected from the junction region that has an oxygen atom concentration of more than 1018 cm−3. This level may also act as an optically nonradiative recombination center.
Introduction of pinning centers into Tl-(1223) phase of Tl–Sr–Ca–Cu–O systems
Authors: Tomoichi Kamo , Doi Toshiya , Soeta Atsuko , Yuasa Toyotaka , Inoue Naomi , Aihara Katsuzo , Shin-Pei Matsuda
Published: July 2002

Pinning centers were successfully introduced into the (1223) phase of Tl–Sr–Ca–Cu–O system using its derivative (Tl/Pb)–(Sr/Ba)–Ca–Cu–O with the nominal composition of (0.5/0.5)−(1.6/0.4)–2–3. The bulk sample showed a strong flux pinning characteristics. The critical current density, Jc, was obtained from the magnetization measurement. The intragrain Jc was ∼43,000 A/cm2 at 77 K and 1 T. The transport Jc of a tape with the (1223) material in core was ∼500 A/cm2 at 77 K and 5 T.
Flux creep in the Josephson mixed state of granular-oriented YBa2Cu3O7−x thin thin films
Authors: Jones E. C. , Christen D. K. , Klabunde C. E. , Thompson J. R. , Norton D. P. , Feenstra R. , Lowndes D. H. , Budai J. D.
Published: July 2002

A self-consistent critical current model in the Josephson mixed state is proposed for a series of c-oriented, polycrystalline and for a series of epitaxial triaxially oriented YBCO thin films. The flux pinning activation energies were experimentally determined from electrical transport measurements over a wide range of temperatures and were found to behave quite differently for the two types of granular film. With these activation energies applied to a superconductor-normal metal-superconductor weak-link system, thermally activated flux motion is shown to reproduce the experimentally measured critical current densities.
New insights on the effect of hydrogen to tungsten hexafluoride partial pressure ratio on plasma deposited tungsten thin films
Authors: Kim Yong Tae , Hong Jong Sung , Min Suk-Ki
Published: July 2002

Resistivities of tungsten thin films deposited by plasma enhanced chemical vapor deposition are very sensitive to the H2/WF6 partial pressure ratio, while the resistivities of tungsten films deposited by low pressure chemical vapor deposition are insensitive to the H2/WF6 ratio. The reason is investigated with x-ray diffraction, transmission electron microscopy, Auger electron spectroscopy and optical emission spectroscopy. As a result, when the H2/WF6 partial pressure ratio is higher than 15, plasma deposited tungsten has a low resistive (11 μΩ cm) bcc structure without F impurities. However, if the H2/WF6 ratios are decreased, porous and β-phase W films are formed due to the incomplete reduction of F concentrations.
Imaging of the microwave field distribution in Josephson tunnel junctions
Authors: Krulle C. A. , Doderer T. , Quenter D. , Huebener R. P. , Pöpel R. , Niemeyer J.
Published: July 2002

A new method is introduced for imaging of the standing-wave patterns inside superconducting tunnel junctions coupled to an external microwave source. The method is based on electron-beam scanning of the tunnel junctions using the concept of low-temperature scanning electron microscopy. For two different junction geometries, results are shown which confirm the established models for the propagation of microwaves inside superconducting tunnel junctions.
Comment on ‘‘Observation of two in-plane epitaxial states in YBa2Cu3O7−δ films on yttria-stabilized ZrO2’’
Authors: Kwok Hoi S.
Published: July 2002

The changes in the superconducting properties of YBa2Cu3O7−δ films are due to different growth conditions. They are affected by both the in-grain properties and overall texture. (AIP)
Vertically coupled InGaAsP/InP buried rib waveguide filter
Authors: Alferness R. C. , Buhl L. L. , Koren U. , Miller B. I. , Young M. , Koch T. L.
Published: July 2002

We report the first demonstration of narrowband (<17 Å full width at half maximum) wavelength selective grating-assisted coupling between vertically stacked buried rib InGaAsP/InP channel waveguides. These vertically coupled channel waveguides form the basis both of integrable filters essential to photonic circuits such as amplifier/filter and demultiplexer/detector circuits as well as a general approach to vertical integration of photonic circuits.
Electro-optic sampling of high-speed silicon integrated circuits using a GaAs probe tip
Authors: Heutmaker Michael S. , Harvey George T. , Bechtold Philip F.
Published: July 2002

Electro-optic sampling is performed on a high-speed silicon multiplexer integrated circuit using a gain-switched semiconductor laser and an external probe tip fabricated from GaAs. An approximate electrostatic model is used to calculate the dependence of the electro-optic modulation on the height of the probe tip above the circuit, the geometry of the circuit, and the dielectric constants of the probe tip material and the passivation layer (if any) on the circuit. The measured variation of the electro-optic modulation with probe tip height is in good agreement with the prediction of the model.
High-speed magnetization reversal near the compensation temperature of amorphous GdTbFe
Authors: Aeschlimann M. , Vaterlaus A. , Lutz M. , Stampanoni M. , Meier F. , Siegmann H. C. , Klahn S. , Hansen P.
Published: July 2002

Using spin-polarized photoemission with a pulsed laser as light source, it is shown that the time for a thermally induced magnetization reversal depends critically on the temperature of the sample. For amorphous GdTbFe the time is shorter (longer) than the duration of the 16 ns laser pulses if the initial temperature is below (above) the compensation temperature.
High power, high efficiency antiguide laser arrays
Authors: Major J. S. , Mehuys D. , Welch D. F. , Scifres D. R.
Published: July 2002

An antiguide laser diode array has been fabricated which radiates a predominantly single lobe far field with a full-width at half-maximum of 1.5 times the diffraction limit at a continuous wave output power of 500 mW with a differential quantum efficiency of 50%. By spectrally resolving the far field of the device at high operating powers it is shown that the two longitudinal modes correspond to adjacent lateral modes of the array. The operation of the second lateral mode is the result of spatial hole-burning of the first optical mode.
Spectrum of hot-electron luminescence from high electron mobility transistors
Authors: Zappe Hans P. , As D. J.
Published: July 2002

GaAs and InGaAs high electron mobility transistors (HEMTs) have been found to emit visible and infrared radiation when subject to high drain/source biases. This luminescence has been studied and analyzed spectrally; there appear to be components due to electron indirect intraband transitions, in addition to recombination in various layers of the HEMT. The distinct recombination peaks indicate that carriers are distributed vertically in the transistor; the relative strength of luminescence at particular energies may provide indications of the relative carrier densities. This electroluminescence spectrum strongly suggests that real-space transfer may be significant in HEMTs.
Impact-ionization-induced transfer of recombination in single- and multiple-monolayer quantum wells of GaInAs/InP
Authors: Omling P.
Published: July 2002

In an optically detected impact-ionization (ODII) investigation of single- and multiple-monolayer quantum wells of GaInAs in InP the behavior of the radiative recombination during microwave irradiation is studied. The transfer of impact-ionized carriers from the InP layers to the thin GaInAs quantum wells results in a decrease of the bound exciton recombination in InP and a corresponding increase in the exciton recombination in the quantum wells. The recombination in the thickest quantum well (≊20 monolayers) shows a more complicated behavior, and an explanation based on defect-related recombination, including bound exciton and free-to-bound recombinations, is proposed. It is also shown that the ODII technique is able to resolve emission bands from free excitons, bound excitons, and free-to-acceptor recombinations in thick GaInAs layers.
Optical glow spectra arising from low-energy N2, N+2 and electron bombardment of MgF2 surfaces
Authors: Qi J. , Barnes A. V. , Espy S. L. , Riehl-Chudoba M. , Sun C-N. , Albridge R. G. , Tolk N. H.
Published: July 2002

Photon emission spectra resulting from the impact of N2, N+2, and electron beams on magnesium fluoride in an ultrahigh vacuum environment were measured and compared for beam energies in the range of 200–2000 eV. Unexpectedly, only the ion- and electron-induced spectra exhibited broad fluorescence. The observed data suggest that the broad fluorescence arising from low-energy ion bombardment is due primarily to the transfer of electronic energy to the surface by resonance or Auger neutralization. Since molecular nitrogen is a major constituent of the atmosphere at orbital altitudes, these measurements bear directly on radiation-induced glow and erosion processes on surfaces of spacecraft in low-earth orbit.
Nonlinear optical effects on the surface of acridine yellow-doped lead-tin fluorophosphate glass
Authors: He K. X. , Bryant William , Venkateswarlu Putcha
Published: July 2002

The second- and third-order nonlinear optical properties of acridine yellow-doped lead-tin fluorophosphate (LTF) glass have been directly studied by measurement of surface enhanced second harmonic generation and third harmonic generation. The three photon excitation fluorescence is also observed. Based on these results, the large nonlinearities of the acridine LTF system which is a new nonlinear optical material are experimentally demonstrated.
Blue Stark shifts in (InAs)1−x(GaSb)x-AlSb superlattices
Authors: Wong K. B. , Jaros M.
Published: July 2002

We demonstrate that the electronic structure of (InAs)1−x(GaSb)x-AlSb superlattices can be engineered so that the application of an external electric field increases the superlattice gap. We find that such a blue Stark shift can only be achieved in thin layer superlattices.
Columnar growth in combustion deposited diamond thin films
Authors: Hoff H. A. , Craigie C. J. , Dantsker E. , Pande C. S.
Published: July 2002

The morphology of the nucleation and growth surfaces of oxygen-acetylene combustion deposited diamond thin films has been studied using scanning electron microscopy. The diameters of both nucleation cells and growth surface grains have been measured and analyzed statistically. The general shape of the distributions is found to change from nucleation to growth surface for each film. The frequency distribution of cells is generally normal, whereas, the distribution for the growth surface is found to be always lognormal. The change in shape of the distributions from normal for the nucleation surface to lognormal for the growth surface can be explained from a stochastic theory of growth.
Nitrogen implantation for molecular beam deposited CuInSe2 thin films
Authors: Kohiki Shigemi , Nishitani Mikihiko , Negami Takayuki , Nishikura Kumiko , Hirao Takashi
Published: July 2002

We have implanted N+ ions at an accelerating voltage of 10 kV and a dose of 5×1017 ions cm−2 in molecular beam deposited stoichiometric CuInSe2 thin films at various substrate temperatures (380, 515, 600, and 720 K). The conduction type of the films changed from n to p type when the substrate temperature was below 515 K during implantation. The conduction type was n type when the film temperature during implantation was above 600 K. The resistivity measured at 300 K was 1.07×104, 0.417, 0.653, 160, and 121 Ω cm for the films before implantation, implanted at 380, 515, 600, and 720 K, respectively. Both the conduction type and the resistivity of the films could be changed by N+ doping. Fabrication of homojunction diodes may be achieved in n-type CuInSe2 films by using the low energy nitrogen ion implantation.
Off-axis sputter deposition of YBa2Cu3O7 thin films for microwave applications
Authors: Greene L. H. , Bagley B. G. , Feldmann W. L. , Barner J. B. , Shokoohi F. , Miceli P. , Wilkens B. J. , Fathy A. , Kalokitis D. , Pendrick V.
Published: July 2002

Thin films of superconducting YBa2Cu3O7 were grown in situ by off-axis sputter deposition for microwave device fabrication. These ∼1 cm2 films, which are reproducible, exhibit midpoint Tc’s of 89–90.5 K as measured by ac susceptibility, ion channeling yields of 4.7%–6%, and c-axis rocking-curve half-widths of 0.5°, even with a rich microstructure as seen by scanning electron microscopy. Two films were photodefined into miniature X-band microwave bandpass filters. These narrow-band filters (0.5% bandwidth) exhibited 4.4-and 4.5-dB insertion losses at 77 K and 9.25 GHz, with little temperature dependence below 80 K.
Leaky-wave interarray coupling for coherent-power scaling of phase-locked diode-laser arrays of antiguides
Authors: Mawst L. J. , Botez D. , Jansen M. , Sergant M. , Peterson G. , Roth T. J.
Published: July 2002

Long-range coupling and phase locking of several 10-element resonant antiguided arrays via (lateral) radiation leakage is demonstrated. Coupling occurs for interarray separations as large as 90 μm, as evidenced from far-field patterns and spectrally resolved near-field patterns. Diffraction-limited-beam operation is achieved from a 244-μm aperture (four coupled 10-element arrays) device. Intermodal discrimination between the in-phase mode and adjacent higher-order modes is shown to be maximized for a resonant ensemble.
Generation of blue coherent light from a continuous-wave semiconductor laser using an organic crystal-cored fiber
Authors: Harada Akinori , Okazaki Yoji , Kamiyama Koji , Umegaki Shinsuke
Published: July 2002

Efficient frequency doubling of a continuous-wave semiconductor laser was attained by using a nonlinear optical fiber with a single crystal core of an organic 3,5-dimethyl-1- (4-nitrophenyl)pyrazole. The second-harmonic power generated with a 15 mm length of the fiber reached 0.16 mW from the incident 16.6-mW fundamental laser power at 884 nm. Nonlinear optical properties of the crystal and a fabrication technique of the fiber are also reported.
All-gaseous doping during chemical-beam epitaxial growth of InGaAs/InGaAsP multi-quantum-well lasers
Authors: Tsang W. T. , Choa F. S. , Logan R. A. , Tanbun-Ek T. , Sergent A. M.
Published: July 2002

The use of all-gaseous doping during chemical-beam epitaxy (CBE) offers several important advantages. Injection of dopants, together with the metalorganic group-III beam, automatically leads to uniform dopant distribution across the entire wafer. Vapor dopant sources also ensure long-term stability and reproducibility. It also allows for instant doping profile control. Most important of all, an all-gaseous doping CBE system is particularly attractive for system manufacture and device production applications. We investigated n- and p-type dopings in InP and InGaAs during CBE using tetraethyltin (TESn) and diethylzinc (DEZn), respectively, and confirmed their suitability for use in growing high-quality long-wavelength InGaAs(P)/InGaAsP semiconductor injection lasers. Buried heterostructure lasers fabricated from CBE-grown 1.5-μm six quantum-well base wafers and metalorganic vapor-phase-epitaxy regrown iron-doped InP have excellent current-voltage characteristics and threshold currents as low as 8 mA. cw operation with threshold current as low as 23 mA at 80 °C and output power of ∼10 mW was achieved for diodes having one facet ∼85% reflective coated.
Optical investigations of GaAs-GaInP quantum wells and superlattices grown by metalorganic chemical vapor deposition
Authors: Omnes F. , Razeghi M.
Published: July 2002

Recent experimental results on the photoluminescence and photoluminescence excitation of GaAs-Ga0.51In0.49P lattice-matched quantum wells and superlattices are discussed. The full width at half maximum of a 10-period GaAs-GaInP superlattice with Lz=90 Å and LB=100 Å is 4 meV at 4 K. The photoluminescence excitation exhibits very sharp peaks attributed to the electron to light-hole and electron to heavy-hole transitions. The GaInP-GaAs interface suffers from memory effect of In, rather than P or As elements.
A δ-doped GaAs/graded InxGa1−xAs/GaAs pseudomorphic structure grown by low-pressure metal organic chemical vapor deposition
Authors: Hsu W. C. , Chen C. M. , Hsu R. T.
Published: July 2002

A δ-doped GaAs/graded InxGa1−xAs/GaAs pseudomorphic structure grown by low-pressure metal-organic chemical vapor deposition was demonstrated for the first time. The graded InxGa1−x As layer in which the composition x ranged from 0.25 to 0.20 was strained to obtain a pseudomorphic structure. Furthermore, a δ-doped high-electron mobility transistor( δ-HEMT) employing a graded InxGa1−x As layer was successfully fabricated. Due to better electron confinement and lower interface roughness scattering, the present structure reveals higher saturation current density, higher transconductance, and higher product of mobility and two-dimensional sheet density as compared to those of conventional HEMTs which were also fabricated by the same system and procedure.
High-power quantum-well modulators exploiting resonant tunneling
Authors: Morgan Robert A. , Chirovsky Leo M. F. , Focht Marlin W. , Leibenguth Ronald E.
Published: July 2002

We propose and demonstrate novel surface-normal multiple-quantum-well reflection modulators which are operable at high intensities while maintaining high contrast (greater than 10) by exploiting resonant tunneling between adjacent wells. At the resonant tunneling field, the lifetime of the excited (highly absorbing) state is dramatically shortened, thus minimizing carrier-density-dependent saturation effects. Moreover, we show that even in a Fabry–Perot cavity, device deterioration due to saturation is greatly reduced.
Electrical behavior of Yb ion in p- and n-type InP
Authors: Seghier D. , Benyattou T. , Bremond G. , Ducroquet F. , Gregoire J. , Guillot G. , Lhomer C. , Lambert B. , Toudic Y. , Le Corre A.
Published: July 2002

In this letter are reported admittance spectroscopy experiments on Yb-doped p- and n-type InP grown by the synthesis method. The purpose is to give a clear understanding of the rare-earth ion electrical behavior. In p-type material, the results indicate the presence of two peaks in the conductance spectra at low temperature. The activation energies of these levels have been found to be 42±5 and 50±5 meV above the valence band. We attribute them to Mg and Yb, respectively. In n-type material, the conductance spectra present a peak at low temperature with an activation energy of 29±3 meV below the conduction band and we attribute it to Yb ion. To explain the origin of these Yb-related traps, we propose that this ion acts as an isoelectronic trap in InP.
Carrier diffusion effects in time-resolved photoluminescence
Authors: Bailey D. W. , Stanton C. J.
Published: July 2002

We model time-resolved photoluminescence in GaAs using an ensemble Monte Carlo method coupled with a k⋅p calculation of the band structure. We show that on a picosecond scale, carrier diffusion perpendicular to the layer significantly reduces the density at the surface and consequently has a first-order effect on luminescence measurements. To illustrate this we compare the calculated luminescence, with and without diffusion, to experimental data.
Li-doped Bi three-layered superconducting whiskers
Authors: Matsubara Ichiro , Ogura Toru , Yamashita Hiroshi , Kinoshita Makoto , Kawai Tomoji
Published: July 2002

Li-doped Bi2Sr2Ca2Cu3O10 (2223) whiskers of single crystals with several millimeters length have been successfully prepared and it is revealed that Li-doping is effective for increasing critical temperature (Tc) of the 2223 phase. By annealing Bi2Sr2CaCu2O8 (2212) whiskers in a calcined powder with a composition of Bi2Sr2Ca4Cu6Pb0.5LixOy (x=0.2, 0.3) only within the very narrow temperature region, the conversion from the 2212 phase to the 2223 phase and Li-doping are achieved at the same time. The Tc value of the Li-doped 2223 whiskers (108.1 K) is reproducibly 1.1 K higher than that of undoped 2223 whiskers prepared by the same method using a calcined powder without Li.
Organometallic vapor phase epitaxial growth and characterization of GaInP/AlInP quantum wells on 10°-off GaAs substrates using ethyldimethylindium
Authors: Wang T. Y. , Welch D. F. , Scifres D. R. , Treat D. W. , Bringans R. D. , Street R. A. , Anderson G. B.
Published: July 2002

GaInP/AlInP quantum wells (QWs) have been grown on (100) and 10°-off (100) GaAs substrates in an organometallic vapor phase epitaxy (OMVPE) reactor at reduced pressure. Photoluminescence (PL) studies revealed an increase in peak energy and narrowing in linewidth for the 10°-off QWs, due to the suppression of the formation of ordered microstructures during the OMVPE process. Low temperature PL results of GaInP/AlInP QWs as thin as 13 Å are presented for the first time. The 10°-off QW showed a dominant peak at 551 nm, the shortest wavelength ever reported in GaInP/AlInP QWs. The origin of PL for the 13-Å QWs is, however, different from that for thicker QWs as a result of the transfer of electrons to localized states in the barrier layer.
Fabrication of TlCaBaCuO step-edge Josephson junctions with hysteretic behavior
Authors: Martens J. S. , Hietala V. M. , Zipperian T. E. , Vawter G. A. , Ginley D. S. , Tigges C. P. , Plut T. A. , Hohenwarter G. K. G.
Published: July 2002

One way to create hysteretic Josephson junctions from the currently available nonhysteretic high temperature superconducting junctions is to artificially add capacitance. We have adapted a multilayer technique for artificial capacitance addition/grain boundary modification to produce TlCaBaCuO step-edge junctions exhibiting large amounts of hysteresis at 77 K. A gaplike structure is present at 32–36 mV that has a temperature dependence that does not contradict that predicted by the Bardeen-Cooper-Schrieffer theory. In addition, the junctions show fast switching times, less than a fixture limited 20 ps, and Fraunhofer-like dependence of critical current on magnetic field.
Lateral band gap modulation by buried stressor structures in III–V compounds semiconductor quantum-well structures
Authors: Xu Z. , Wassermeier M. , Li Y. J. , Petroff P. M.
Published: July 2002

A buried stressor structure was fabricated using in situ focused ion beam sputter etching and molecular beam epitaxy to produce a lateral band gap modulation in GaAs/AlGaAs quantum-well structures. Strain induced band gap modulation and the consequent carrier confinement were demonstrated by low temperature cathodoluminescence. A 12 meV deep potential well was demonstrated. The observed band gap shrinkage as a function of distance from the stressor was obtained and is found in good agreement with the theory.
Transmission electron microscopy study of Ag contacts to a- and c-axis oriented YBa2Cu3O7 thin films
Authors: Gong Z. H. , Fagerberg R. , Vassenden F. , Grepstad J. K. , Ho/ier R.
Published: July 2002

The interface atomic structure of in situ sputter deposited silver contacts to a- and c-axis oriented thin films of YBa2Cu3O7 (YBCO) was investigated with cross-section transmission electron microscopy (TEM). A structurally disordered interface layer, approximately 25 Å thick, was found for Ag contacts to c-axis oriented films. Electron diffraction analysis provides clear evidence for loss of the 11.7 Å lattice periodicity along the YBCO c-axis in this zone. No such disordered interface layer could be identified in TEM images of Ag contacts to a-axis oriented films. These findings may have important bearing on fabrication of high Tc proximity coupled superconductor/normal metal/superconductor Josephson devices.
Faraday rotation from Zn1−xFexSe epilayers
Authors: Fu L. P. , Schmiedel T. , Petrou A. , Warnock J. , Jonker B. T.
Published: July 2002

We present Faraday rotation measurements on Zn1−xFexSe films grown by molecular beam epitaxy. The samples exhibit giant Faraday rotation characteristic of diluted magnetic semiconductors. Measurements were carried out at T=4.2 K and in magnetic fields up to 8 T. The Faraday rotation angle shows a strong resonance for photon energies close to the band gap. The resonance curves were fitted by a simple dielectric model using the exciton oscillator strength as a fitting parameter.
Interface traps and interface recombination in AlGaAs/GaAs quantum well laser diodes
Authors: Xie K. , Wie C. R. , Varriano J. A. , Wicks G. W.
Published: July 2002

The current-voltage characteristics and deep traps of various GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures are studied as a function of the growth temperature and threshold current. It is shown that the interface nonradiative recombination processes cause a high threshold current. An impurity-related deep level with activation energy Ea=0.48 eV at the interface region and an interface state with wide energy distribution were found in the high threshold current diodes. The current-voltage characteristics show that interface recombination, rather than bulk recombination, is the dominant carrier transport process in the diodes and is responsible for the high threshold current.
Improved crystalline quality of Si1−xGex formed by low-temperature germanium ion implantation
Authors: Shoji Ken-ichi , Fukami Akira , Nagano Takahiro , Tokuyama Takashi , Yang Cary Y.
Published: July 2002

Improvement of crystalline quality in Si1−xGex formed by germanium ion implantation has been found. End-of-range defects were drastically reduced in number by lowering the substrate temperature during implantation with doses on the order of 1016 cm−2. This improvement was confirmed by electrical characterization of p-n junctions formed in the SiGe layer as well as by transmission electron microscopy.
Effects of compressive and tensile uniaxial stress on the operation of AlGaAs/GaAs quantum-well lasers
Authors: Tiwari Sandip , Bates Richard S. , Harder Christoph S. , Behfar-Rad A.
Published: July 2002

The consequences of strain on the threshold current, relaxation oscillation frequency, and wavelength of operation of lasers have been investigated using an external uniaxial stress impressed on Ga1−xAlxAs/GaAs ridge-guide quantum-well lasers employing graded index and separate confinement. Under the conditions of small strain (≤0.1%), the threshold current increases monotonically from compressive to tensile conditions by as much as 30%, the relaxation oscillation frequency decreases monotonically from compressive to tensile conditions by as much as 100% when operating close to threshold conditions, and the mode spectrum shifts in accord with the shifts expected in the subband energy levels. These observations indicate that changes in effective mass, and hence in transparency and differential gain, are the dominant causes of this variation. For smaller band-gap structures, such as Ga1−xInxAs1−yPy lasers, where carrier lifetime effects should be expected to dominate, we speculate that the threshold current density may show a decrease for both compressive and tensile strain.
New AlGaAs/GaAs double heterostructure-emitter bipolar transistor prepared by molecular beam epitaxy
Authors: Liu Wen-Chau , Lour Wen-Shiung , Guo Der-Feng
Published: July 2002

A new AlGaAs/GaAs double heterostructure-emitter bipolar transistor (DHEBT) prepared by molecular beam epitaxy (MBE) has been demonstrated. Due to the symmetric structure with respect to the base layer, the device would operate as a bi-directional transistor and switch. The common-emitter current gain for up to 18, with an offset voltage smaller than 0.1 V, is obtained. Furthermore, the undesired knee-shaped characteristics and the reachthrough effect observed in the conventional double heterojunction bipolar transistor (DHBT) are avoided. The bi-directional operations of this structure give a significant flexibility in the circuit applications. It is believed that with adequate adjustments on structural parameters, especially in the minimization of the base-collector junction area, a more symmetric and an improved electrical performance may be achieved.
Atomic layer epitaxy of AlAs using thermally decomposed gas molecules
Authors: Yokoyama Haruki , Shinohara Masanori , Inoue Naohisa
Published: July 2002

A new method of atomic layer epitaxy (ALE) using a thermal-cracking tube is investigated and applied to AlAs growth. ALE growth of AlAs is accomplished in a substrate temperature range from 460 to 480 °C using an alternate gas supply of AsH3 and thermally decomposed alkylaluminum generated from trimethylaluminum (TMA). An analysis of the cracking kinetics using a quadrupole mass spectrometer reveals that ALE results from the chemisorption enhancement of column III materials related to decomposed alkylaluminum.
Uniform, high-gain AlGaAs/InGaAs/GaAs heteroemitter bipolar transistors using chemical passivation and nonalloyed ohmic contacts
Authors: Tseng H. C. , Li S. S. , Lin Y. W. , Park R. M.
Published: July 2002

An Al0.25Ga0.75As/In0.08Ga0.92As/GaAs heteroemitter bipolar transistor (HEBIT) with a uniformly high common-emitter current gain (β) of 500 was demonstrated for the first time. Using a simple chemical treatment [P2S5/(NH4)2S], we have greatly reduced the surface recombination in the emitter-base junction perimeters, resulting in a tenfold increase in the maximum current gain of the device at low collector currents. InGaAs nonalloyed ohmic contacts were also employed to form both emitter and base electrodes in a one-step process. To evaluate the performance of the HEBIT, we conducted current-voltage measurements and compared the Gummel plots at 300 and 77 K.
Critical current behavior in the Sb2O3-doped YBa2Cu3O7−δ superconductor
Authors: Jin S. , Tiefel T. H. , Fastnacht R. A. , Kammlott G. W.
Published: July 2002

The effect of Sb2O3 doping on Tc and Jc behavior of the polycrystalline YBa2Cu3O7−δ superconductor has been studied. While the overall Jc remained relatively low, the addition of a small amount of Sb2O3 (e.g., 0.5 wt %) noticeably improved transport Jc at 77 K by a factor of ∼6 at H=0 and of ∼80 at H=200 Oe. The magnetization hysteresis (ΔM) at H=0.9 T also increased by a factor of ∼3. The Jc improvement, which occurs in spite of the increased porosity, reduced grain size, and slightly suppressed Tc in the Sb2O3-doped samples, is attributed to the stronger coupling at grain boundaries in view of the significantly reduced field dependence of transport Jc. Another beneficial effect of the Sb2O3 doping, i.e., accelerated kinetics of oxygen absorption during the tetragonal-orthorhombic transformation, is also discussed. The Sb2O3 doping, if properly optimized, may be useful for alleviating both the grain boundary weak-link behavior and the oxygen-supply problem typically present in thick, dense, or metal-clad YBa2Cu3O7−δ superconductors.
Proton exchanged imbalanced Ti:LiNbO3 Mach-Zehnder modulator
Authors: Mollier P. , Porte H. , Goedgebuer J. P.
Published: July 2002

We describe the fabrication process of a LiNbO3 integrated Mach-Zehnder interferometer featuring a large path-imbalance between the arms. This path-imbalance is created by the proton exchange located on one arm. The proton exchange increases the extraordinary refractive index ne of lithium niobate and modifies the effective index of the guided mode. Path-differences of 80 μm per mm of proton exchange length are obtained with the fabrication process reported here.
Polarized excitation luminescence of semiconductor quantum wells
Authors: Baliga Arvind , Anderson Neal G.
Published: July 2002

A new photoluminescence excitation (PLE) spectroscopy technique for the characterization of quantum-well structures is described. The technique, which requires a standard PLE setup modified to allow for variable control of the excitation beam polarization, provides an unusually simple and unambiguous means for distinguishing between electron-to-light-hole (e-lh) and electron-to-heavy-hole (e-hh) near-gap excitonic optical transitions. Transition types are identified by inspection of the quotient of two PLE spectra obtained using orthogonal incident beam polarizations. In this letter we describe the experimental setup and procedure for this technique, discuss the theoretical foundation for our interpretation scheme, and present experimental results for tensile-strained GaAsyP1−y-AlxGa1−xAs and lattice-matched GaAs-AlxGa1−xAs quantum-well structures.
Effects of superradiant modes on the performance of free-electron laser amplifiers
Authors: Lin A. T. , Lin C. C. , Chu K. R.
Published: July 2002

Computer simulation results reveal that the superradiant modes in free-electron laser amplifiers with high gain and long interaction length tend to degrade the amplifier performance. If these modes are kept at low levels, the amplifier efficiency can be significantly improved. This observation can be employed to interpret the recent MIT reversed guide field free-electron laser experiment.
Preparation of superconducting Ba2YCu3O7−y-Ag composite films on sapphire by the dipping pyrolysis process
Authors: Manabe T. , Kondo W. , Mizuta S. , Kumagai T.
Published: July 2002

Superconducting Ba2YCu3O7−y(BYCO)-Ag composite films were prepared on sapphire substrates without a buffer layer by the dipping pyrolysis process. Prefired films of a BaCO3-Y2O3-CuO-Ag mixture (Ba:Y:Cu:Ag=2:1:3:0.2 in molar ratio) were heat treated at 740 °C under a low oxygen partial pressure [p(O2)=2×10−4 atm] followed by O2 annealing at 450 °C. The resulting 0.8–1.8 μm thick films showed high superconducting transition temperatures of Tc, zero=88–89 K.
Growth and characterization of ZnSe crystals grown by the solution growth method
Authors: Okuno Yasuo , Sano Michihiro , Kato Hiroyuki , Maruyama Tsuyosi
Published: July 2002

Bulklike ZnSe crystal with crystallographic quality the same as commercial GaAs can be grown at a growth temperature of 950 °C by the solution growth method. The growth rate of the crystal depends on the temperature gradient in the solvent, the length of the heat sink and the mol % of Se in the solvent. The full width at half maximum (FWHM) of x-ray rocking curve of the ZnSe crystal grown by this method is nearly the same as that of GaAs. The crystal grown from a Te/Se solvent of Te≤70 mol % can be regarded as essentially ZnSe from the cathodoluminescence measurement.
Preparation of YBa2Cu3O7−δ thin films with thickness gradients and investigations of growth stages by scanning tunneling microscopy
Authors: Haefke H. , Lang H. P. , Leemann G. , Güntherodt H.-J.
Published: July 2002

The growth of (001) oriented YBa2Cu3O7−δ films on LaAlO3(100) has been studied by scanning tunneling microscopy. Using a special half-shadow technique different growth stages have been obtained simultaneously on a single substrate crystal: nucleation and spreading of two-dimensional islands, their coalescence and growth, formation of well-developed growth hills and finally of a continuous film. The growth steps were determined to be one unit cell spacing in height regarding the crystallographic c-axis.
Improvement of flux pinning in the Ag-clad Bi-Pb-Sr-Ca-Cu-O wires through the use of a short period melt processing
Authors: Dou S. X. , Liu H. K. , Guo Y. C.
Published: July 2002

A significant enhancement of the Jc in magnetic field in the Ag-clad Bi-Pb-Sr-Ca-Cu-O (BPSCCO) wires has been achieved by using a short period melt processing. The weak links have been largely eliminated in the melt-processed tapes which exhibits a 3–10-fold increase in the Jc at 77 K and 1 T over the normally processed tapes. For tapes with a thickness 80 μm 60% of melt-processed samples have the Jc values between 10 000 and 27 000 A/cm2 at 77 K in zero field. The irreversibility lines determined using ac susceptibility measurements under dc fields confirmed the improvement of flux pinning in the melt-processed tapes compared with the normally processed tapes. The improved Jc-H characteristics may be attributed to the desirable microstructures produced from the high Tc phase formation-decomposition-recovery process through the use of a short period melting.
Electron beam pumping of the 546.1 nm mercury laser
Authors: Rhoades R. L. , Verdeyen J. T.
Published: July 2002

The first known instance of electron beam pumped lasing on the 546.1 nm transition in mercury (Hg) is reported. This has been achieved using high-energy electrons to create excitation and ionization in a coaxial diode chamber containing a mixture of noble gases with a small amount of mercury vapor. The kinetic processes within the resulting partially ionized plasma favor the production of mercury ions and ion complexes containing mercury with subsequent recombination with electrons favoring the production of the 73S state of Hg, the upper laser level.
Annealing effect on the shape of CdTe/ZnTe quantum wells
Authors: Hamoudi A. , Ligeon E. , Saminadayar K. , Cibert J. , Dang Le Si , Tatarenko S.
Published: July 2002

We report on the effect of thermal annealing on thin (001) CdTe/ZnTe single quantum wells. We found that the interdiffusion strongly modifies the optical spectra: (i) besides the expected blue shift, a strong reduction of the linewidth is observed in a first stage of annealing; we attribute this effect to a complete elimination of the larger islands present at the interface, (ii) in a second stage, ‘‘usual’’ interdiffusion occurs, with a slow rate in agreement with the diffusion coefficient extrapolated from reported self-diffusion data at higher temperatures.
High-temperature x-ray diffraction analysis for Bi2Sr2CaCu2Oy
Authors: Hasegawa T. , Kitamura T. , Kobayashi H. , Kumakura H. , Kitaguchi H. , Togano K.
Published: July 2002

In the case of Bi-2212/Ag composite tapes, the partial-melting technique is useful to obtain a highly textured microstructure. The liquid phase plays an important role during this process. In this letter, we studied the phase transition of oxide phases in the Bi-2212/Ag composite tapes during partial melting and solidification by using the high-temperature x-ray diffraction technique. In the partially molten state, the liquid phase coexists with various kinds of (Ca,Sr)-Cu oxides whose Cu content decreases with increasing temperature. The Bi-2212 phase forms directly from the coexistent state of the liquid phase and the (Ca,Sr)2CuO3 compound. c-axis alignment of Bi-2212 phase starts subsequently to the beginning of precipitation. Bi-2212 phase decomposes and the Bi-2201 phase appears below 800 °C. At lower temperature, the Bi-2201 phase is more stable than Bi-2212 phase and can easily form in the super-cooled liquid phase.
Diamond growth in a novel low pressure flame
Authors: Glumac N. G. , Goodwin D. G.
Published: July 2002

Diamond growth using a new low-pressure combustion technique is reported. A large-area hydrogen/oxygen flame is used as the source of atomic hydrogen. Methane diluted in hydrogen is injected into the flame near a heated silicon substrate, on which diamond crystallites nucleate and grow. This technique is potentially capable of large-area film growth, since atomic hydrogen can be generated uniformly over arbitrarily large areas.
Resonant multiphoton electron emission from a quantum well
Authors: Sirtori Carlo , Capasso Federico , Sivco Deborah L. , Cho Alfred Y.
Published: July 2002

Direct observation of resonant multiphoton electron emission from a quantum well is reported. Photocurrent measurements as a function of photon energy and polarization in an AlInAs/GaInAs quantum well heterostructure demonstrate that electrons are photoexcited out of the wells via a three-photon transition into a resonant state mainly localized in the barriers. This effect is strongly enhanced by the presence of intermediate energy levels.
Variation of microtwin content with thickness in cadmium telluride films grown on sapphire by metalorganic chemical vapor deposition
Authors: Glass H. L. , Appleby Woods M. R.
Published: July 2002

X-ray diffraction was used to measure microtwinning in (111) CdTe epitaxial films grown on (00⋅1) sapphire substrates by metalorganic chemical vapor deposition. The concentration of microtwins is high near the interface but drops off rapidly with distance. For films grown to thicknesses up to ≊10 μm, the microtwin content is constant and relatively low (much less than 1 vol %) away from the interface. For films grown more than ≊10 μm thick there is a substantial increase in the microtwin content. These microtwins are concentrated near the film surface.