bluuebunny/crossref_metadata_2025 huggingface dataset:
Microstructure and properties of YBa2Cu3O7−δ thin films with BaO precipitates
Authors: Tian Y. J. , Guo L. P. , Li L. , Zhou Y. Q. , Yang Y. , Zhao Z. X. , Xu S. F. , Lu H. B. , Zhou Y. L. , Chen Z. H. , Cui D. F. , Yang G. Z.
Published: July 2002

BaO precipitates with sizes between 10–100 nm in laser ablated YBa2Cu3O7−δ (YBCO) thin films on Y-stabilized Zirconia substrates have been identified by x-ray diffraction and transmission electron microscopy. The precipitates exhibit equiaxed shapes and grow epitaxially inside and on the surface of the YBCO films, with (001)BaO plane parallel to the a,b plane of YBCO. Some of these smaller precipitates and BaO/YBCO boundaries probably provide potential pinning sites for magnetic flux lines, which might contribute to the observed increase of critical current density with magnetic field B under B≤500 G in the case of B perpendicular to the c axis of the film.
Determination of heterojunction band discontinuities in strained AlxIn1−xP/lnP systems
Authors: Lammasniemi J. , Tappura K. , Smekalin K.
Published: July 2002

Conduction and valence band discontinuities were determined in the strained AlxIn1−xP/InP heterojunction system by absorption measurements from multiquantum well (MQW) and superlattice (SL) structures. It was found that the conduction band offsets were 0.337 and 0.260 eV for systems having Al concentrations of x=0.20 (MQW) and x=0.15 (SL), respectively. The valence band discontinuities were 85 and 49 meV for the heavy-hole valence band and −29 and −35 meV for the light-hole valence band, respectively, i.e., the light-hole valence band of AlxIn1−xP forms a staggered type II heterojunction with InP.
Large photorefractive effect in a thermally decomposed polymer compared with that in molecularly doped systems
Authors: Yokoyama Kenji , Arishima Koichi , Sukegawa Ken
Published: July 2002

Photorefractive polymers with the same electro-optic effect were fabricated to investigate the photorefractive effects in different photoconductive systems. The photoconduction in the polymers was varied by the addition of squarylium dye to diethylaminobenzaldehyde-diphenylhydrazone (DEH), by the formation of a charge-transfer complex between tetracyanoquinodimethane and DEH, and by the thermal decomposition of DEH. The largest photorefractive effect was observed in the thermally decomposed polymer among these polymers. A diffraction efficiency of 1.1% and a beam-coupling gain coefficient of 10 cm−1 were achieved in a 34.9 V/μm dc electric field.
Dielectric response of As-stabilized GaAs surfaces observed by surface photo-absorption
Authors: Uwai Kunihiko , Kobayashi Naoki
Published: July 2002

This letter discusses changes in the isotropic and anisotropic surface dielectric response caused by surface conversion from c(4×4) to (2×4) of a GaAs(001) As-stabilized surface, observed using surface photo-absorption. The anisotropic part quantitatively reproduces the change in the surface dielectric anisotropy spectra observed in reflectance difference measurements during this surface conversion. The isotropic part can be measured only by surface photo-absorption and comparison is made with a calculation using a three-phase model consisting of vacuum/As/GaAs.
Collisionless electron heating by radio-frequency plasma sheaths
Authors: Surendra M. , Vender D.
Published: July 2002

Collisionless rf sheaths have been investigated in a self-consistent particle simulation. The one-dimensional simulation is bounded by an rf driven electrode on one side and quasineutral plasma on the other so that the sheath can be examined in isolation from other discharge processes. The simulations indicate that electron velocity distributions arriving at the electrode are distorted when the ratio of the maximum velocity of the moving electron front in the rf sheath to the thermal velocity is greater than ∼0.5. Both electron heating and power loss to the electrode increases as this ratio is increased. At ratios greater than ∼0.8–1 a positive net energy flux returning from the rf sheath is observed. Electron inertia is not negligible in this regime.
Epitaxial growth of Fe-doped sapphire thin films from amorphous Al oxide layers deposited on sapphire substrates
Authors: Yu Ning , Nastasi Michael
Published: July 2002

Thin films of amorphous Al2O3, about 280 nm thick, with Fe-cation concentrations of 0–4 at. % were deposited onto alpha-alumina [0001] substrates. Epitaxial regrowth of the thin films was found to occur during a postannealing process at temperatures of 950 and 1400 °C. The regrowth quality was determined by Rutherford backscattering spectrometry and ion channeling measurements. Perfect regrowth was found in the undoped samples after annealing at 1400 °C with a minimum backscattering yield of 2% in the Al sublattice. Furthermore, ion channeling angular scans revealed that Fe dopants occupied the substitutional sites of Al sublattice upon thermal anneal. This simple method of incorporating dopants into single-crystal alumina has potential in the fabrications of thin-film planar optical waveguides.
Strong field, in-plane poling for nonlinear optical devices in highly nonlinear side chain polymers
Authors: Otomo Akira , Stegeman George I. , Horsthuis Winfried H. G. , Möhlmann Gustaaf R.
Published: July 2002

The conditions essential for high field, in-plane poling were investigated for nonlinear optical (NLO), planar devices with 4-dimethylamino-4′-nitrostilbene (DANS) side-chain polymers. Applied poling field as large as 370 V/μm were achieved before breakdown occurred and a nonlinearity of d22=153 pm/V at 1064 nm (near resonant) was obtained with stable poling at 300 V/μm.
Polarization reversal in LiNbO3 crystals under asymmetric diffusion conditions
Authors: Kugel V. D. , Rosenman G.
Published: July 2002

It is shown that domain inversion in LiNbO3 crystals stimulated by heat treatment depends on boundary conditions. Limitation of outdiffusion from −C surface leads to the appearance of an inverted layer with the thickness more than one-half of a crystal thickness. Consequent indiffusion preserves the bidomain structure but reduces the inverted layer depth. The phenomenon is explained in terms of the built-in electric field caused by diffusion process.
High-resolution spectroscopy using high-order derivative techniques
Authors: Dharamsi A. N. , Lu Y.
Published: July 2002

A novel technique, employing high-order derivative absorption spectroscopy, is used to demonstrate that the resolution of nearly overlapping lines, with disparate oscillator strengths, can be obtained. The method employs tunable diode lasers and does not require expensive wavelength-resolution apparatus. An experimental demonstration involving nearly overlapping lines of the weak (doubly forbidden by the spin selection rule and the lack of an electric dipole oscillator strength) A band of molecular oxygen is provided. The advantages of the method in being able to inexpensively monitor the densities (and other properties) of more than one specie simultaneously, and in the investigation of line-shape profiles, are briefly discussed also.
Vertical cavity amplifying photonic switch
Authors: Raj R. , Oudar J. L. , Bensoussan M.
Published: July 2002

A 30 dB gain with multi-GHz modulation bandwidth is obtained in an optically pumped vertical cavity Fabry–Pérot structure. This device may be called a vertical cavity amplifying photonic switch (VCAPS). The amplification configuration has been used for a direct measurement of the carrier induced refractive index change over the absorption as well as the gain regime.
Fabrication of a periodically domain-inverted structure in LiNbO3 thin film by a domain-transfer technique
Authors: Momiji Hiroshi , Kawamoto Kazumi , Ito Kenchi
Published: July 2002

A new technique to fabricate a periodically domain-inverted structure in LiNbO3 is reported. This structure, fabricated by Ti indiffusion on a LiNbO3 substrate, is transferred in a LiNbO3 thin film grown by liquid phase epitaxy. As the growth temperature decreases below 690 °C, the cross section of the inverted domain changes from a reversed trapezoidal to a rectangular shape.
Effect of the boron doping concentration and Fermi level on the generation of crystal originated particles in p-type Czochralski silicon wafers
Authors: Wijaranakula W. , Archer S.
Published: July 2002

The effect of the boron doping concentration on the generation of crystal originated particles in p-type 200-mm-diam Czochralski silicon wafers was examined. The results indicate that the generation of crystal originated particles is suppressed by heavily doping with boron. Based upon the band-gap theory and from the argument that the defects are vacancy type, the generation of crystal originated particles could occur via vacancy aggregation at temperatures below 1000 °C. In the present model, negatively charged vacancies which are dominant at high temperatures are hypothesized to be the point defect species contributing to the generation of crystal originated particles.
Flux pinning and weak links in the behavior of the critical current of a-axis and c-axis EuBa2Cu3O7 superconducting thin films
Authors: Velez M. , Martín J. I. , Vicent J. L.
Published: July 2002

Pure a-axis and c-axis oriented EuBa2Cu3O7 films have been grown by a dc sputtering technique. The a-axis films show critical current values typically one order of magnitude lower than c-axis films. The Jc(T) behavior could be governed, in a-axis films, by the microstructure of these films with small domains and 90° boundaries, but Jc(B) does not seem to be limited by weak links. The magnetic-field dependence of Jc shows a similar behavior in a-axis and c-axis films with a leading role of pinning in controlling the critical current in the high-field range. Jc(B) is limited by flux creep with a similar distribution of pinning energies in both types of films, in spite of the very different microstructure.
Novel tunable semiconductor lasers using continuously chirped distributed feedback gratings with ultrahigh spatial precision
Authors: Hillmer H. , Zhu H.-L. , Grabmaier A. , Hansmann S. , Burkhard H. , Magari K.
Published: July 2002

We present experimental and theoretical results obtained from a new type of tunable two- and three-section semiconductor distributed feedback (DFB) laser using tailored chirped DFB gratings. The chirped gratings are defined by bent waveguides on homogeneous grating fields with ultrahigh spatial precision using bent waveguides. Arbitrary and continuous chirping functions can be obtained by appropriate mathematical bending functions. Our method is applied for tunable lasers with optimized bending shapes revealing a considerably enlarged wavelength tuning range of 5.5 nm without wavelength gaps.
Polymerization to graphite-like nitride clusters from tetracyanoethylene vapor
Authors: Yanagi H. , Itoh H. , Ueda Y. , Ashida M.
Published: July 2002

Various types of graphite-like nitride clusters were synthesized by chemical vapor deposition of tetracyanoethylene (TCE) onto a substrate of cleaved KCl crystals at 230–300 °C. At a low reaction temperature of 230 °C, TCE was polymerized to ribbon-like clusters, which resembled curved graphitic sheets. Upon increasing the reaction temperature to 250–300 °C, polymerized TCE condensed on KCl to form spheric clusters. The most regular structure of the clusters was obtained at 300 °C in a nanoscale needle-like morphology. High-resolution electron microscopy suggested that these clusters were composed of polymerized TCE layers with a interlayer distance of 0.34–0.35 nm.
Inductance measurements in multilevel high T
c step-edge grain boundary SQUIDS
Authors: Forrester M. G. , Davidson A. , Talvacchio J. , Gavaler J. R. , Przybysz J. X.
Published: July 2002

Multilevel high Tc SQUIDs, suitable for digital circuit applications, have been fabricated and tested. The devices employ a YBa2Cu3O7−δ (YBCO) ground plane, an epitaxial SrTiO3 insulator, and a YBCO microstrip layer. Junctions are formed by the step-edge grain boundary process, with a ground plane contact for the ‘‘low’’ side of each junction, using only isotropic sputtering and milling techniques. Control current is directly injected in a microstrip segment of the SQUID loop, allowing us to measure the microstrip inductance, and thus to infer the magnetic penetration depth of the YBCO. The SQUIDs are operational above 77 K, at which temperature we infer a penetration depth of 350 nm. The temperature dependence of the penetration depth is found to be in reasonable agreement with the Gorter–Casimir form close to Tc.
Performance assessment of quantum well infrared photodetectors
Authors: Choi K. K. , Lee C. Y. , Tidrow M. Z. , Chang W. H. , Gunapala S. D.
Published: July 2002

The performance of GaAs/AlGaAs quantum well infrared photodetectors specified in terms of background limited temperature Tb and specific detectivity D* has been calculated based on realistic detector parameters. It is found that for a detector with an external quantum efficiency η of 6.9%, Tb is 76 K for a cutoff wavelength of 10 μm. This value of Tb agrees with the recent experimental result and is significantly higher than the previous estimation given by M. A. Kinch and A. Yariv [Appl. Phys. Lett. 55, 2093 (1989)]. If η is unity, the projected Tb can be as high as 88 K with a D* of 2.2×1011 cm√Hz/W. For a lower temperature operation, D* increases to 7.5×1011 cm√Hz/W at 77 K, comparable to that of a HgCdTe detector.
The electrostatic polarization of ferroelectric liquid crystals: A new interpretation of the triangle-wave technique
Authors: Ruth J. , Selinger J. V. , Shashidhar R.
Published: July 2002

We show that the electrostatic polarization measured in ferroelectric liquid crystals using the standard triangle-wave technique is not the total polarization of the sample, but rather a specific nonlinear component of the total polarization. We present a Landau theory to calculate this nonlinear component explicitly. In the high-temperature limit, above the smectic-A–smectic-C* phase transition, the theory predicts that the observed polarization scales as E3/(T−TAC)4, where T is the temperature and E the applied electric field. In the low-temperature limit, the observed polarization approaches the total polarization. These theoretical predictions are consistent with experimental measurements.
20-fold pulse compression in a synchronously pumped optical parametric oscillator
Authors: Khaydarov John D. V. , Andrews James H. , Singer Kenneth D.
Published: July 2002

We have developed a model of pulses interacting in a synchronously pumped optical parametric oscillator to predict the pump intensity dependence of generated pulse compression due to pronounced group velocity walk off between the pump, signal, and idler pulses in the nonlinear crystal. We have experimentally demonstrated up to 20-fold compression of the generated pulse relative to an 11 ps pump, and measured transform-limited spectral broadening of the compressed pulse.
High-power, single-frequency diode-pumped Nd:YAG microcavity lasers at 1.3 μm
Authors: Gavrilovic P. , O’Neill M. S. , Zarrabi J. H. , Singh S. , Williams J. E. , Grodkiewicz W. H. , Bruce A.
Published: July 2002

A single-frequency output power of 210 mW on the 1.3 μm transition was obtained from a monolithic YAG laser doped with 4.2% Nd. Lasers fabricated from high Nd-concentration YAG crystals oscillated simultaneously on the 1.319 and 1.338 μm lines at high pump power, but in a single axial mode at both wavelengths. The oscillation frequency was continuously temperature-tunable at a rate of 3.6 GHz/ °C over a range of 170 GHz (10 Å). The emission was linearly polarized with an extinction ratio ≳1000:1. In two-wavelength operation, the 1.319 and 1.338 μm transitions were orthogonally polarized.
Crystallization at initial stage of low-temperature polycrystalline silicon growth using ZnS buffer layer with 〈111〉 preferred orientation
Authors: Matsumoto T. , Nagahiro Y. , Nasu Y. , Oki K. , Okabe M.
Published: July 2002

We developed a low-temperature growth technique for polycrystalline silicon (poly-Si). When Si is deposited on glass substrates at 450 °C, it crystallizes as thickness increases, but 10-nm-thick layers of Si are mainly amorphous. Use of a ZnS buffer layer with 〈111〉 preferred orientation facilities crystallization of Si during the initial growth stages. The preferred orientation of poly-Si on glass substrates is 〈110〉, while that of poly-Si on the ZnS buffer layer is 〈111〉. This is probably due to local epitaxial growth on polycrystalline ZnS grains with 〈111〉 preferred orientation. Raman spectroscopy showed that the ZnS buffer layer significantly improved the crystallinity of 25-nm-thick Si layers.
Novel ‘‘serially grafted’’ connection between functional and passive polymer waveguides
Authors: Watanabe Toshio , Amano Michiyuki , Hikita Makoto , Shuto Yoshito , Tomaru Satoru
Published: July 2002

A serially grafted connection between polymer waveguides is realized. This technique allows a passive polymer waveguide to be connected to a functional polymer waveguide with a connection loss of 0.005 dB. It is also applied to a Mach–Zehnder modulator using an electro-optic polymer, and optical intensity modulation in the serially grafted modulator is successfully demonstrated.
Novel absorption line shape induced by Tamm states in a finite-length GaAs/AlGaAs superlattice
Authors: Yu Roger H.
Published: July 2002

The absorption coefficient of a finite-length GaAs/AlGaAs superlattice is calculated using the density-density response function χ(q∥,ω;z,z′). The surface localized Tamm states yield an extra peak in the spectrum. The Coulomb interaction among the electrons shifts the resonant energy upward, changes the line shape, and leaves a weak but well-defined structure behind.
Nucleation of Co silicide on H passivated Si(111)
Authors: Copel M. , Tromp R. M.
Published: July 2002

We have investigated the effect of H passivation on the deposition of Co on Si(111). The H terminated surface has fewer nucleation sites for silicide formation than either the bare (7×7) surface or the boron (√3×√3)R30°. This leads to a growth mode dominated by the formation of sparse nonepitaxial islands, which grow laterally to merge. The H passivated (1×1) surface does not contain Si adatoms, unlike the (7×7) and boron (√3×√3)R30° surfaces, which must be the nucleation site for cobalt silicide formation on Si(111).
Ultrafast band-edge carrier dynamics in In0.65Ga0.35As
Authors: Pelouch Wayne S. , Schlie L. A.
Published: July 2002

The ultrafast band-edge carrier dynamics in In0.65Ga0.35As/GaAs are studied using a transmission correlation technique in the 77–300 K temperature range. The experiment is performed using a femtosecond optical parametric oscillator tunable in the 1.5–1.8 μm spectral range with sub-100-fs pulses. Rapid recovery (≤10 ps) of the nonlinear absorption is observed, suggesting an exceptionally high rate of recombination predominantly due to dislocations at the InGaAs/GaAs heterojunction. The phonon-scattering time is experimentally identified and measured to be 200–400 fs.
Transport through 90° [010] basal-plane-faced tilt and twist grain boundaries in YBa2Cu3O7−δ thin films
Authors: Lew D. J. , Suzuki Y. , Marshall A. F. , Geballe T. H. , Beasley M. R.
Published: July 2002

We have fabricated controlled grain boundaries between a- and c-axis films of YBa2Cu3O7−δ (YBCO). By selectively depositing c-axis and in-plane aligned a-axis YBCO on the same substrate, we have grown nominally 90° [010] basal-plane-faced tilt and twist grain boundaries (BFGB’s and TGB’s) on LaSrGaO4 (LSGO) substrates and have studied their transport. We find that the TGB is very robust, exhibiting high critical current density Jc and weak field dependence, and is as well coupled as our [010] antiphase boundaries. The BFGB is not as strong, but couples better than high-angle [001] tilt grain boundaries in c-axis films. Although the BFGB’s do limit Jc, they do not reduce Jc by more than an order of magnitude and the field dependence is very weak. We discuss these grain boundaries in terms of weak links.
Optical waveguides for evanescent field sensing
Authors: Rehouma F. , Persegol D. , Kevorkian A.
Published: July 2002

A new fabrication process of ion-exchange waveguides well suited for integrated-sensor applications is presented. The process consists in burying each extremity of the waveguide few microns below the surface while its central region is located at the surface. For a single-mode waveguide at 785 nm, we measured propagation losses of 0.13 dB/cm for the buried portion, 2.80 dB/cm for the surface portion, and losses of 0.40 dB for the transition region. For this structure, we also observed a deformation of the fundamental mode. The proposed method is easy to implement and permits one to control precisely the topography of the elaborated waveguides.
Interfacial microstructures of ultrathin Ge layers on Si probed by x-ray scattering and fluorescence yield
Authors: Ming Z. H. , Soo Y. L. , Huang S. , Kao Y. H. , Tsang J. C. , Iyer S. S.
Published: July 2002

Angular dependence of grazing-incidence x-ray scattering and Ge Kα fluorescence yield were measured for buried ultrathin Ge layers grown on bulk Si by molecular beam epitaxy. Results obtained for samples with different Ge layer thickness are compared. The data reveal information on microstructures in these layered materials in terms of the average interfacial roughness, correlation lengths of height fluctuations, and Ge density profile. Structural parameters are obtained by comparison of experimental data with theoretical models.The results also indicate that the interfacial roughness at neighboring interfaces is highly correlated. Significant changes of microstructures in the Ge epilayer are found as the layer thickness approaches the critical thickness. The x-ray scattering techniques are demonstrated to be capable of detecting a precursor of lattice relaxation in multilayers of lattice-mismatched compound semiconductors.
Nanocolumns composed of GaAs-InAs jointed whiskers and SiO2 covers
Authors: Yazawa M. , Haraguchi K. , Koguchi M. , Katsuyama T. , Hiruma K. , Ohta H.
Published: July 2002

A dense array of nanocolumns composed of GaAs-InAs jointed whiskers and SiO2 covers has been fabricated on InAs substrates. The cylindrical GaAs whisker with a 20 nm diameter and 1.5 μm long is jointed on top of 0.3-μm-long InAs whisker by vapor-liquid-solid epitaxy. The nanocolumns array exhibited photoluminescence at 14 K.
Band discontinuities in the (Pb,Eu)Se system determined by frequency-dependent admittance analysis
Authors: Xu J. , Steiner K. , Tacke M.
Published: July 2002

Band discontinuities in anisotype Pb1−xEuxSe/PbSe single heterojunctions are determined using frequency-dependent admittance analysis. A four component small signal equivalent circuit model is proposed to describe the diode admittance. Good agreement between the simulation results and experimental data have been obtained. The band discontinuities are evaluated at 130 K for x<0.05.
Gap voltage of Nb-Al/AlOx-Nb tunnel junctions
Authors: Lehnert T. , Schuster K. , Gundlach K. H.
Published: July 2002

The gap voltage of Nb-Al/AlOx-Nb junctions was studied as a function of the Nb electrode thickness. The gap parameter increased with the electrode thickness. The I(V) curves of these junctions, employed as quasiparticle frequency mixers, are discussed. For 120-nm-thick Nb electrodes, a gap voltage 2Δ/e of 2.95 mV at 3 K is found in junctions with a critical current density of 2.5 kA/cm2. This is close to the ideal bulk value of Nb. The corresponding gap frequency of 714 GHz is clearly above 691 GHz, an important radioastronomical frequency. Complementary Tc measurements of Nb films have been carried out. The results agree with a model based on the proximity effect between the ideal metallic layer and a nonideal surface layer.
Optimization of diamond growth with statistical experiment design in an electron cyclotron resonance plasma
Authors: Ring Z. , Mantei T. D. , Choo A. G. , Jackson H. E.
Published: July 2002

Multivariable statistically designed experiments have been used to optimize diamond growth in a H2-CH4-O2 magnetoactive plasma at 1–3 Torr total pressure. The input process parameters included total pressure, microwave power, substrate temperature, methane and oxygen fractions, and applied substrate bias, and the experimental response outputs were diamond film growth rate and crystallinity. A clearly defined characteristic 1333 cm−1 Raman peak with a 5 cm−1 full width at half-maximum identified the optimum diamond crystal growth conditions of H2 (75%)-CH4 (13%)-O2 (12%), 3 Torr total pressure, and 650 °C substrate temperature.
Boron and aluminum implantation in α-SiC
Authors: Ahmed S. , Barbero C. J. , Sigmon T. W. , Erickson J. W.
Published: July 2002

In this letter, we report the comparison of experimental implantation profiles in 6Hα-silicon carbide (SiC) with the theoretical results obtained using the widely used trim profile simulator. Our results for the projected range of Al and B implanted into this material agree well with those predicted by trim, in contrast to previously published results. Profiles for 40–250 keV Al and B implantation, off-crystal axis to minimize channeling in SiC, are obtained using secondary ion mass spectroscopy. The Pearson I and VI theoretical profiles, constructed using the first four moments of the distribution generated by the trim simulator, are then compared to the experimental ones.
Background limited infrared performance of n-type Si-SiGe(111) quantum well infrared photodetector
Authors: Shadrin V. D.
Published: July 2002

The quantum well infrared photodetector (QWIP), based on n-type Si-SiGe structures grown in [111] crystallographic direction, possesses low dark current performance in a wide range of impurity doping concentrations. In addition, when compared to GaAs-AlGaAs QWIP, it has an enhanced photoabsorption efficiency. The background limited infrared performance (BLIP) is analyzed theoretically for both n-Si-SiGe(111) and GaAs-AlGaAs QWIPs. The BLIP temperature drop which is due to the Fermi level rise associated with an increase in the doping impurity concentration, is found to begin at higher concentration in n-Si-SiGe than in GaAs-AlGaAs QWIPs. The concentration dependency of the QWIP performance characteristics in BLIP is compared for n-Si-SiGe and GaAs-AlGaAs.
Analysis of T0 in 1.3 μm multi-quantum-well and bulk active lasers
Authors: Ackerman D. A. , Morton P. A. , Shtengel G. E. , Hybertsen M. S. , Kazarinov R. F. , Tanbun-Ek T. , Logan R. A.
Published: July 2002

Temperature dependence of threshold in 1.3 μm semiconductor lasers is analyzed in terms of contributions due to gain, internal efficiency, internal loss, and nonradiative recombination. Rapid decrease of differential gain and roughly proportional increase in transparency carrier density are determined to dominate temperature dependence of threshold current. Auger recombination is found to play a secondary role in reducing T0 by compounding the effects of rapidly increasing threshold carrier density.
Femtosecond hole thermalization in bulk GaAs
Authors: Tommasi R. , Langot P. , Vallée F.
Published: July 2002

Ultrafast hole relaxation dynamics is selectively investigated in intrinsic bulk GaAs using a high sensitivity two-wavelength pump-probe technique. Nonequilibrium carriers are photoexcited close to the bottom of their respective bands and hole heating is followed by monitoring the transient bleaching of optical transitions involving higher momentum states. Hole heating is found to be dominated by hole-phonon interactions with a thermalization time of the order of 150 fs for carriers densities in the 1017 cm−3 range.
Structural changes induced by hydrogen absorption in palladium and palladium–ruthenium alloys
Authors: Cabrera A. L. , Morales L. Erie , Hasen J. , Schuller Ivan K.
Published: July 2002

The structural changes in Pd and Pd–Ru alloys induced by the repeated absorption–desorption of hydrogen have been studied. It is found that absorption–desorption cycles produce structural changes in Pd whereas the addition of small amounts of Ru inhibits these hydrogen-induced changes. The experimental results show that bulk hydrogen absorption occurs in Pd, while hydrogen surface adsorption becomes dominant over bulk absorption, in the Pd–Ru alloy.
Representative reflectance anisotropy spectra from Al(x)Ga(1−x)As layers (x=0 to 1.0) grown on GaAs(001) by molecular beam epitaxy
Authors: Rose K. C. , Morris S. J. , Westwood D. I. , Woolf D. A. , Williams R. H. , Richter W.
Published: July 2002

We present in this study reflectance anisotropy spectra also termed reflectance difference spectra for the surfaces of thick Al(x)Ga(1−x)As (x=0.0, 0.25, 0.50, 0.75, 1.00) layers grown on GaAs(001) by molecular beam epitaxy (MBE). Layers ≥2 μm thick were grown in order to minimize the interference effects from the buried interface and to obtain spectra representative of bulk Al(x)Ga(1−x)As surfaces. All surfaces were also independently characterized using reflection high energy electron diffraction (RHEED), and found to exhibit a c(4×4) reconstruction. The reflectance anisotropy spectra were qualitatively similar to one another, but showed clear dependence of energy and magnitude upon the mole fraction x. These spectra can serve as reference for determining stochiometries in MBE growth of Al(x)Ga(1−x)As and probably should be useful for future comparison to metalorganic vapor phase epitaxy (MOVPE) Al(x)Ga(1−x)As growth.
Wide band gap MgZnSSe grown on (001) GaAs by molecular beam epitaxy
Authors: Wu B. J. , DePuydt J. M. , Haugen G. M. , Höfler G. E. , Haase M. A. , Cheng H. , Guha S. , Qiu J. , Kuo L. H. , Salamanca-Riba L.
Published: July 2002

We report molecular beam epitaxial study of wide band gap (≳2.9 eV at room temperature) MgZnSSe on (001) oriented GaAs using ZnS, Mg, Zn, and Se sources. Although the growth is under group II rich condition, the compositions of S and Mg in the MgyZn1−ySxSe1−x are linear functions of flux ratios, PZnS/PSe and PMg/PZnS, up to 35%, respectively. Mirrorlike surface and low defect density (5×104 cm−2) MgZnSSe with band gap close to 3.1 eV can be achieved. Composition modulation, tweedlike contrasts and strain contrasts in the MgZnSSe are observed from transmission electron microscope analysis. For the first time, a miscibility gap at high S and Mg compositions is reported. Nitrogen-free radicals are used as the p-type dopant for the doping study. For the MgZnSSe with room-temperature band gap energy higher than 2.9 eV, the net acceptor concentration decreases as the band gap energy increases.
Gettering of iron impurities in p/p+ epitaxial silicon wafers with heavily boron-doped substrates
Authors: Aoki Masaki , Itakura Toru , Sasaki Nobuo
Published: July 2002

We studied gettering effects in p/p+ silicon epitaxial wafers that have no dislocations near the epi/sub interface and oxygen precipitates inside the p+ substrate. The wafers were contaminated with high- and low-level Fe surface concentrations to clarify the relationship between gettering effects and Fe concentration. After annealing at 1000 °C for 60 min followed by quenching, the p/p+ epitaxial wafers showed the same gettering effect independent of the Fe contamination level. The intrinsic gettered reference wafers, however, did not show the gettering effect because Fe impurities do not supersaturate at 1000 °C. We concluded that the gettering effect of Fe in the p/p+ epitaxial wafers is due to the difference in solubility between the p epitaxial layer and the p+ substrate.
Thickness-field excited thickness-shear resonators in (110) GaAs
Authors: Hjort Klas , Schweeger Giorgio , Dehé Alfons , Fricke Klaus , Hartnagel Hans L.
Published: July 2002

The authors present experimental evidence for thickness-field excitation of thickness-shear vibrations in GaAs as an example material for zincblende-crystal semiconductors. The resonance frequency is shown to agree perfectly with theory. This gives an alternative to the composite resonators in miniaturized local oscillators for, e.g., monolithic microwave integrated circuit applications.
Spectral and spatial resolution of photopumped II–VI single quantum well lasers
Authors: Bagnall D. M. , Ralston A. , O’Donnell K. P. , Smith P. C. , Wright P. , Cockayne B.
Published: July 2002

Simple II–VI laser cavities were formed by cleaving epitaxial ZnSe samples containing a single CdSe quantum well of mean thickness about 1 monolayer. Time-integrated stimulated emission spectra of these lasers show little evidence of mode structure. A video camera enables temporal resolution of the stimulated emission spectra. Mode structure appears on the time-resolved spectra. In addition, however, video spectroscopy reveals spatial variations of wavelength and intensity in the laser output which are not well understood.
Analytical solution of wave mixing between short optical pulses in a semiconductor optical amplifier
Authors: Shtaif M. , Eisenstein G.
Published: July 2002

An analytical solution to the problem of nondegenerate four wave mixing in a semiconductor optical amplifier emphasizing operation with short optical pulses is described. Calculated conversion efficiencies for pulses are significantly larger than for cw fields and are pulse width dependent. Conditions for optimum conversion efficiencies in terms of the pump input energy and the relative temporal alignment between the pulses are presented.
Homoepitaxial diamond film deposition on a brilliant cut diamond anvil
Authors: McCauley Thomas S. , Vohra Yogesh K.
Published: July 2002

A thick, homoepitaxial diamond film was deposited on the microcracked (100) tip of a brilliant cut type Ia natural diamond anvil by microwave plasma-assisted chemical vapor deposition (MPCVD). The rapid (up to 20 μm/h), highly anisotropic growth resulted in a modification of the original geometry of the central flat and culet regions. The size of the diamond culet grew from 350 to ∼800 μm in diameter in about 8 h. The deposited film was characterized by optical microscopy, micro-Raman and photoluminescence (PL) spectroscopy. Raman analysis of the deposit confirms that its crystalline quality is comparable to that of the substrate, and that it contains no amorphous or graphitic phases of carbon. The epitaxial overlayer exhibits two prominent PL bands not observed from the substrate with zero-phonon (ZPL) lines at 1.945 and 2.156 eV, which are attributed to nitrogen-related defect complexes. The present work shows the potential viability of high growth rate homoepitaxy by MPCVD for numerous industrial and scientific applications, such as high pressure research.
Free-carrier induced saturation of the Faraday rotation in semimagnetic semiconductors
Authors: Hugonnard-Bruyère S. , Buss C. , Frey R. , Flytzanis C.
Published: July 2002

Large photoinduced Faraday rotations of the polarization direction of a probe beam traveling through a semimagnetic semiconductor are obtained at temperatures higher than that of liquid nitrogen when a pump pulse is sent through the material. These rotations are quasiproportional to the photoinduced free-carrier density and the applied magnetic field and are strongly resonant with the band-gap-probe frequency detuning. A model based on the free-carrier induced saturation of the refractive indices explains well the experimental observations. The high values obtained for the photoinduced polarization rotations (more than 1° pJ−1 μm2) make such a process attractive for the design of optical signal processing devices.
Optical gain in ordered GaInP/AlGaInP quantum wells
Authors: Moritz A. , Hangleiter A.
Published: July 2002

In this letter we present optical gain calculations for strained and ordered GaInP/AlGaInP quantum-wells which are based on a 6-band kp theory. The calculations were performed with a set of parameters that show an excellent agreement with gain measurements on unstrained and disordered samples. We show that ordering and strain together have a great influence on the polarization of the gain and the gain versus carrier density characteristics, and may be used to further improve properties of lasers in this material system.
Direct measurements of magnetostrictive process in amorphous wires using scanning tunneling microscopy
Authors: Costa J. L. , Nogués J. , Rao K. V.
Published: July 2002

We demonstrate a versatile capability to measure directly the magnetostrictive properties through the magnetization process on a nanometric scale using a modified scanning tunneling microscope. Single 10 mm long, 125 μm diam amorphous wires of both positive and negative magnetostriction have been studied and the data are compared with the hysteretic loops determined by both ac and SQUID magnetic measurements. This improved technique promises interesting possibilities, from both fundamental and applications points of view, in a number of scientific disciplines especially of interest in life and environmental sciences.
Simultaneous and direct measurement of gas–liquid density at high temperature and high pressure using the nuclear-spin projection method
Authors: Shimokawa Shigezo , Yamada Eiji
Published: July 2002

A new technique to measure gas–liquid densities using the one-dimensional projection method is presented. The measurements were made in an NMR autoclave under high-temperature and high-pressure conditions. Some of the results of densities of cyclopentane determined from the room temperature to the supercritical regions are reported.
Constant-volume valve suitable for use in gas thermometry
Authors: Kemp R. C.
Published: July 2002

An improved design for a simple constant-volume valve is described. Experience with the valve over several years of use confirms that it is suitable for use in gas thermometry or similar applications.
Magnetic alignment of the Tara tandem mirror
Authors: Post R. S. , Coleman J. W. , Irby J. H. , Olmstead M. M. , Torti R. P.
Published: July 2002

Techniques developed for the alignment of high-energy accelerators have been applied to the alignment of the Tara tandem mirror magnetic confinement device. Tools used were: a transit/laser surveyor’s system for establishing an invariant reference; optical scattering from ferromagnetic crystallites for establishing magnetic centers in the quadrupole anchor/transition modules; an electron-optical circle-generating wand for alignment of the solenoidal plug and central cell modules; and four differently configured electron emissive probes, including a 40-beam flux mapping e gun, for testing the alignment of the coils under vacuum. Procedures are outlined, and results are given which show that the magnetic axes of the individual coils in the Tara set have been made colinear with each other and with the reference to within ±1.0 mm over the length of the machine between the anchor midplanes.
Measurement of proton autoneutralization potential
Authors: Garcia Manuel
Published: July 2002

A proton space charge having multi-MeV kinetic energy was injected through a thin ground plane to extract electrons and produce a time-dependent autoneutralization space potential. An electron-emitting floating-potential resistive divider was used to measure the space potential during 20 ns of the proton current pulse. During this time, proton kinetic energy fell from 10.6 to 8.5 MeV and thus the space potential (taken as 1.09× the floating potential) fell from 5.8 to 4.6 kV.
Precipitation in heavily arsenic-implanted GexSi1−x alloys
Authors: Fan T. W. , Nejim A. , Zhang J. P. , Wang Z. G. , Hemment P. L. F. , Chescoe D.
Published: July 2002

We report a study of precipitation and diffusion in Ge0.5Si0.5 alloy implanted with As+ ions at an energy of 100 keV with a dose of 6×1016 cm−2 and subsequently annealed at 800 and 1000 °C for 1 h. The samples were analyzed by transmission electron microscopy and x-ray energy dispersive spectrometry. A high density of precipitates was observed near the surface of the samples after annealing both at 800 and 1000 °C. The behavior of the precipitation is strongly dependent on the anneal temperature. When the anneal temperature increases, the average size of precipitates increases and the distribution of the precipitates is localized in the vicinity of the surface. X-ray spectra show that most of the largest precipitates formed during the annealing are arsenides. Rodlike precipitates formed during the thermal annealing at 800 °C are tentatively identified as monoclinic GeAs by selected electron diffraction patterns.
Computer program for predicting tandem parameters
Authors: Weitkamp W. G.
Published: July 2002

A computer program has been written to predict all beam-optics parameters as well as to estimate beam currents and radiation levels for a tandem accelerator. The program is useful for testing the feasibility of unusual beams and for setting up the accelerator.
350-GHz phase/frequency locked loop for use with a carcinotron backward wave oscillator
Authors: van Ardenne A. , Woestenburg E. E. M. , van der Ree L. J.
Published: July 2002

A phase/frequency locked loop for use in a millimeter-wave radio-astronomy receiver is described for locking a carcinotron backward wave oscillator (BWO) operating in the 330–370-GHz range to a fixed 150-MHz reference source. The appropriate 29th–31st harmonic of a stable and tunable frequency source around 11.75 GHz acts as the local oscillator to down convert the high BWO frequency to 150 MHz prior to phase/frequency comparison. A double loop is required, whereby the low-frequency part of the spectrum of the loop error voltage is applied to the line voltage circuit of the carcinotron power supply over more than 30 m distance. The high end of the spectrum is coupled to this voltage via a voltage transformer located next to the carcinotron. Under these conditions a 135-kHz loop bandwidth is easily obtained. The locked signal shows excellent stability, and the measured performance level in terms of the Allan variance to characterize fractional frequency stability is about 10−16 for integration times in the range 100–1000 s. The performance in general satisfies all applications where a high level of phase/frequency stability is required, such as in plasma physics and in presently planned and future uses in radio astronomy.
Dynamic balancing of Wheatstone bridge with two thin-film arms: Theory and its experimental verification
Authors: Chaturvedi U. K. , Shrinet V. , Nigam A. K.
Published: July 2002

A theory for the dynamic balancing of the double thin-film arms Wheatstone bridge, when both of its sensor arms are growing in thickness in vacuum, has been developed. Rate of growth of the imbalancing signal, bridge sensitivity at the null balance, and its decay with the growth of the sensor arm thickness are derived and discussed. It is shown that as long as the bridge remains dynamically balanced, the ratio of the deposition rates of the films on the sensor arms R and S is proportional to the external resistance ratio P/Q. Hence, if the bridge is kept balanced by the feedback system, the composition of the film codeposited on a common substrate will remain constant which can, however, be varied by changing the ratio P/Q. An experiment was performed to test the validity of this theory by evaporating Cu and Cr separately on the two sensor arms of the bridge in the same vacuum chamber. The vapor pressures of both the metals were recorded simultaneously by a RGA scanning 50–70-amu range. It was found that as long as the bridge remains balanced, the ratio pCu /pCr remains constant. In contrast, when the bridge is imbalanced with either polarity, pCu /pCr varied in either direction from this constant value. Further, for a ±10% deviation in the pCu /pCr from the constant ratio, the signal grows at a rate of ∼10 μV s−1. This signal is found to be capable of electronic feedback control of the evaporation system.
Using a calorimeter and spectrometer to measure radiant efficiencies of infrared sources
Authors: Pargellis A. N.
Published: July 2002

The radiant efficiency of a heater is observed to decrease when an intervening window is used to protect the infrared source. A power balance equation is used to calculate the theoretical radiant efficiency, defined as the fraction of electrical power into the heater emitted as infrared radiation. The calculations are based on measured temperatures of the different heater faces and indicate the losses are mostly due to convective losses from the heater face and top panel. A spectrometer measures the spectral radiance emitted from the heater. The area under the spectral radiance curve gives a second estimate for the radiant efficiency. A large calorimeter is described which directly measures the emitted infrared radiation from the heater’s face. Finally, the surface temperatures of the different window materials are measured and compared with calculated values.
Production of vibrationally cold ions using a radio-frequency storage ion source
Authors: Sen Amarjit , Mitchell J. B. A.
Published: July 2002

A storage ion source has been built for the production of vibrationally cold H+2 and H+3 ions. Using a gas mixture of H2 and rare gases (Ne and He) in the source at high pressure, higher vibrational states of these ions have been effectively depleted by ion–molecule reactions and collisional deactivation. The internal energy of the cold ions was determined by the threshold measurement of collision-induced dissociation. With a mixture of H2 and Ne (ratio 1 : 5) in the source operated at 80 mTorr, H+2 ions were found to be in v=0 and 1 states only and the H+3 ions were in the ground vibrational state.
Combined mu-metal and niobium superconductor shielding for dc SQUID operation
Authors: Xu Bu-xin , Hamilton W. O.
Published: July 2002

In this note, we report the use of a combined mu-metal and niobium superconductor shielding which results in much improved dc SQUID operation. The configuration is suitable for large experiments and is much cheaper than the mu-metal shielding conventionally used.
Optical properties of the retroreflective array in spectroscopic instrumentation
Authors: Peck Konan , Morris Michael D.
Published: July 2002

The distribution of retroreflected light from an array of corner cube prisms is measured as a function of angle of incidence and beam collimation. The imaging properties of the array are compared to those of a plane mirror in the presence and absence of phase perturbations. The implications for the use of a retroreflective array in absorbance measuring instrumentation are discussed.
High gain, multiatmosphere CO2 laser amplifier
Authors: Stuart G. C. , Houtman H. , Meyer J.
Published: July 2002

A novel TE discharge, 15-mm aperture, multiatmosphere, CO2 laser amplifier is described, with measured electrical characteristics and gain measurements on the 9.294-μm, 9R(16) line. The electrical circuit used in this amplifier is a realistic alternative to the Marx bank or conventional LC inversion circuit and, similarly, it would be useful for excitation of other gas lasers as well. This automatically preionized, double-sided, fourfold LC inversion circuit uses only one spark gap, and it is shown to provide small-signal gains of 5.7% cm−1, at 120 J l−1 atm−1 and 10 atm. The generalization to an n–stage device, which would be suitable for higher pressures, and larger apertures, is discussed.