bluuebunny/crossref_metadata_2025 huggingface dataset:
Grain size dependence of the critical current density in YBa2Cu3Ox superconductors
Authors: Kuwabara M. , Shimooka H.
Published: July 2002

The grain size dependence of the critical current density in bulk single-phase YBa2Cu3Ox ceramics was investigated. The grain size of the materials was changed to range approximately from 1.0 to 25 μm by changing the conditions of power processing and sintering, associated with an increase in the sintered density of the materials with increasing grain size. The critical current density has been found to exhibit a significant grain size dependence, changing from 880 A/cm2 to a value of 100 A/cm2 with a small increase in the average grain size from 1.2 to 2.0 μm. This seems to provide information about the nature of the weak link between superconducting grains which might govern the critical current density of the materials.
Time-resolved spectroscopic study of the KrF laser-induced plasma plume created above an YBaCuO superconducting target
Authors: Girault C. , Damiani D. , Aubreton J. , Catherinot A.
Published: July 2002

The laser-induced plasma plume created above an YBaCuO superconducting target by a KrF laser beam (248 nm) is investigated by time-resolved spectroscopy. High-resolution spectra are obtained and ejection velocities of ablated species are deduced from temporal evolution of spatially resolved spectroscopic measurements.
Characterization of GaAs/Ga1−xAlxAs heterojunction bipolar transistor structures using photoreflectance
Authors: Yin X. , Pollak Fred H. , Pawlowicz L. , O’Neill T. , Hafizi M.
Published: July 2002

We have studied the photoreflectance spectra at 300 K from a number of GaAs/Ga1−x AlxAs heterojunction bipolar transistor (HBT) structures grown by molecular beam epitaxy and metalorganic chemical vapor deposition. From the observed Franz–Keldysh oscillations we have been able to evaluate the built-in dc electric fields Fdc in the Ga1−x Alx As emitter as well as the n−-GaAs collector region. In addition, the Ga1−x Alx As band gap (and hence Al composition) has been determined. The obtained values of Fdc are in good agreement with numerically computed values for the analyzed HBT structures, thus making it possible to deduce doping levels in these sections.
The 193 nm fragmentation and ionization of trimethylaluminum: Evidence for photoinduced α-hydrogen elimination
Authors: Brum Jeffrey L. , Tong Ping , Koplitz Brent
Published: July 2002

Results are reported on the 193 nm excitation of trimethylaluminum under collisionless conditions. Time-of-flight mass spectra are monitored at several different excimer laser powers. At relatively low powers, the mass spectra consist solely of masses 27 and 57, presumably the Al+ and Al(CH3)+2 ions. At higher laser powers, however, mass 58 is readily observed, suggesting the presence of a reaction channel involving α-hydrogen elimination to form the AlH(CH3)2 photofragment. This observation is interpreted in the context of recent theoretical calculations by Higashi and Steigerwald [Appl. Phys. Lett. 54, 81 (1989)].
Generation of highly vibrationally excited H2 and detection by 2+1 resonantly enhanced multiphoton ionization
Authors: Robie Daniel C. , Jusinski Leonard E. , Bischel William K.
Published: July 2002

We report the first detection by optical means of highly vibrationally excited H2 X1Σ+g(vx=6–11). Vibrationally excited H2 was generated using a recently discovered hot-wire effect in H2 gas, and was detected in 40 bands with 2+1 resonantly enhanced multiphoton ionization via the EF state (vEF=0–14). Rotational temperatures are in the range 200–650 K, well below that required for thermal excitation of the observed vibrational levels.
Use of trilevel resists for high-resolution soft-x-ray projection lithography
Authors: Berreman D. W. , Bjorkholm J. E. , Becker M. , Eichner L. , Freeman R. R. , Jewell T. E. , Mansfield W. M. , MacDowell A. A. , O’Malley M. L. , Raab E. L. , Silfvast W. T. , Szeto L. H. , Tennant D. M. , Waskiewicz W. K. , White D. L. , Windt D. L. , Wood O. R.
Published: July 2002

A projection optical system with 20:1 reduction has been used with radiation at ∼36 nm to evaluate resists for use in soft-x-ray projection lithography. The high absorption of soft x rays by carbon-based polymers requires that an imaging resist layer be very thin. The sensitivities and contrasts of several such resists are reported. By incorporating a thin imaging layer into a trilayer resist scheme, we have exposed, developed, and transferred features as small as 0.2 μm into silicon.
Alternative approach to nanocomposite synthesis by sputtering
Authors: Chow G. M. , Holtz R. L. , Pattnaik A. , Edelstein A. S. , Schlesinger T. E. , Cammarata R. C.
Published: July 2002

A new method is presented which circumvents the usual thermodynamic limitations (alloying and compound formation) in fabricating phase-separated materials. This opens whole new classes of materials that can be prepared as particulate composites.This method utilizes sputtering at high pressures (0.2–0.6 Torr) in a thermal gradient to produce nanoscale (<15 nm diameter) particles, which are then embedded in a matrix produced by normal sputtering. The microstructure and microhardness of 0.5-μm-thick composites of molybdenum particles (3–12 nm average particle size) in aluminum are presented as examples. This system cannot be prepared by the conventional phase separation technique of cosputtering.
Microwave properties of highly oriented YBa2Cu3O7−x thin films
Authors: Inam A. , Wu X. D. , Nazar L. , Hegde M. S. , Rogers C. T. , Venkatesan T. , Simon R. W. , Daly K. , Padamsee H. , Kirchgessner J. , Moffat D. , Rubin D. , Shu Q. S. , Kalokitis D. , Fathy A. , Pendrick V. , Brown R. , Brycki B. , Belohoubek E. , Drabeck L. , Gruner G. , Hammond R. , Gamble F. , Lairson B. M. , Bravman J. C.
Published: July 2002

We have performed intra- and extra-cavity microwave frequency (1–100 GHz) measurements on high quality Y1Ba2Cu3O7−x superconducting thin films on (100) LaAlO3 substrates. The ∼0.3 μm thin films fabricated by the pulsed laser deposition technique exhibit superconducting transition temperatures >90 K, as determined by resistivity and ac susceptibility measurements, and critical current densities of 5×106 A/cm2 at 77 K. Moreover, ion beam channeling minimum yields of ∼3% were measured, indicating the extremely high crystalline quality of films grown on the LaAlO3 substrate. Microwave surface resistance values at 77 K for these films are found to be more than one to two orders of magnitude lower than for copper at 77 K for almost the entire frequency range explored. We postulate that the reason we observe such low surface resistances in these films is the virtual absence of grain and phase boundaries coupled with the high degree of crystallinity. Furthermore, we believe that the residual resistance measured below Tc is at present dominated by losses occurring in the substrate and the cavities rather than by losses intrinsic to the Y-Ba-Cu oxide superconductor.
Phonons in epitaxially grown α-Sn1−xGex alloys
Authors: Menéndez J. , Sinha K. , Höchst H. , Engelhardt M. A.
Published: July 2002

We present a Raman scattering study of optical phonons in α-Sn1−xGex alloys (x≤0.08) grown by molecular beam epitaxy on CdTe (100) substrates. The Raman spectra provide strong evidence for the growth of high quality films in the diamond-structure phase. The composition dependence of the Raman modes shows some qualitative differences with results from the isomorphic Ge1−xSix system. We show that these differences can be understood in terms of a simple model that considers confinement and strain effects.
Metal/semiconductive polymer Schottky device
Authors: Gupta Rajiv , Misra S. C. K. , Malhotra B. D. , Beladakere N. N. , Chandra Subhas
Published: July 2002

Development of a metal/organic-semiconductor Schottky junction as an alternative to the metal/inorganic-semiconductor junction is reported. Metal/polypyrrole (PP) junctions have been prepared with electrochemically deposited doped PP films of different thickness and various metals (In, Sn, Ti, and Al) as electrodes. The electrical characteristics of the junction depend upon the work functions of PP and the metal. It has been possible to prepare Schottky barriers on the PP films with a metal electrode having a work function lower than that of the polymer. Various physical characteristics of the polymer, work function, Fermi level, and carrier concentration have been estimated.
Effects of strained-layer structures on the threshold current density of AlGaInP/GaInP visible lasers
Authors: Hashimoto Jun-ichi , Katsuyama Tsukuru , Shinkai Jiro , Yoshida Ichiro , Hayashi Hideki
Published: July 2002

We have investigated the effects of strained-layer structures on the reduction of the threshold current density of AlGaInP/GaInP visible laser diodes. It was found that a remarkable reduction of the threshold current density could be realized by incorporating strained single quantum well structures in their active regions. The minimum threshold current density at room temperature under pulsed conditions is 215 A/cm2, which is by far the lowest value ever reported for AlGaInP/GaInP visible lasers and comparable to those of AlGaAs/GaAs lasers.
Evidence for ledge growth and lateral epitaxy of diamond single crystals synthesized by the combustion flame technique
Authors: Ravi K. V. , Joshi A.
Published: July 2002

The synthesis of diamond at high temperatures (≳1000 °C) utilizing the combustion flame technique is shown to result in the growth of diamond single crystals with a {100} habit. Evidence for ledge growth and lateral epitaxy of diamond on nondiamond substrates is presented and it is postulated that the propagation of {100} faces is enhanced by the presence of oxidizing specie in the flame ambient that preferentially remove nondiamond bonded species from the deposits as well as etch the diamond surface to create growth ledges. The ledges are most readily formed on {100} surfaces and single-crystal growth occurs by the lateral propagation of the ledges created by oxidation.
X-ray radiation damage and a reliability study on bipolar devices
Authors: Boyne D. , Hsia L. , Wachnik R. , Decker R. , Wasik C.
Published: July 2002

The radiation damage and reliability of bipolar transistors have been studied using the vacuum ultraviolet storage ring of the National Synchrotron Light Source at the Brookhaven National Laboratory. The bipolar devices under study were exposed to x-ray radiation in the range of 1–2 keV. The device parameters were measured before and after irradiation, and were measured again after the devices were annealed. Finally, the devices underwent a reliability test. The study shows that x-ray radiation degrades the common emitter current gain. Upon annealing, the current gain recovers its initial value. Furthermore, the current gain exhibits the same degree of reliability as observed on nonirradiated devices.
Crystal forms by solid-state recrystallization of amorphous Si films on SiO2
Authors: Noma T. , Yonehara T. , Kumomi H.
Published: July 2002

The recrystallization behavior of amorphous silicon films formed by Si+ ion bombardment onto polycrystalline silicon films has been studied. Two crystal forms have been identified by transmission electron microscopy and electron diffraction for the first time. Disk-shaped crystals are formed as a result of the presence of {111} twin planes parallel to the film surface. Threefold symmetric crystals are formed by the presence of the three {111} twin planes that are not parallel to the film surface. Their feature looks less dendritic due to restricted space of film thickness in which crystal branches may grow. These two crystal forms have 〈111〉 direction normal to the film surface. A model for the formation of these crystals is proprosed in the process of solid-state recrystallization.
Laser-compatible InGaAs/GaAs strained layer waveguide electroabsorption modulators
Authors: Moss D. , Williams R. L. , Dion M. , Landheer D.
Published: July 2002

We report single and double quantum well electroabsorption waveguide modulators that achieve exceptional performance as both lasers and modulators. Contrast ratios of 19 and 26 dB for a 400 μm long device have been achieved for the single and double well samples, respectively. When operated as lasers, the devices exhibit broad area threshold current densities of 56 A/cm2 and 87 A/cm2 for the single and double well samples, for cavity lengths longer than 3 mm. In addition, measurements of lasing wavelength versus cavity length show that both devices can operate as lasers at wavelengths compatible with their respective modulator performance.
Long-wavelength (1.3 μm) luminescence in InGaAs strained quantum-well structures grown on GaAs
Authors: Roan E. J. , Cheng K. Y.
Published: July 2002

Long-wavelength (≳1.1 μm) optical emission has been achieved in pseudomorphic InGaAs-on-GaAs quantum-well structure by replacing InGaAs random alloy quantum well with (InAs)n/(GaAs)n short period superlattice (SPS). With the same quantum-well width, the photoluminescence peak energy of the SPS structure is always smaller than that of the In0.5Ga0.5As random-alloy structure. Strong photoluminescence was observed in (InAs)1/(GaAs)1 SPS quantum wells with thickness up to 84 Å. The longest optical-emission wavelength observed in (InAs)1/(GaAs)1 SPS quantum-well structures at room temperature was 1.34 μm.
Application of oxidation to the structural characterization of SiC epitaxial films
Authors: Powell J. A. , Petit J. B. , Edgar J. H. , Jenkins I. G. , Matus L. G. , Choyke W. J. , Clemen L. , Yoganathan M. , Yang J. W. , Pirouz P.
Published: July 2002

Both 3C-SiC and 6H-SiC single-crystal films can be grown on vicinal (0001) 6H-SiC wafers. We have found that oxidation can be a powerful diagnostic process for (1) ‘‘color mapping’’ the 3C and 6H regions of these films, (2) decorating stacking faults in the films, (3) enhancing the decoration of double positioning boundaries, and (4) decorating polishing damage. Contrary to previously published oxidation results, proper oxidation conditions can yield interference colors that provide a definitive map of the polytype distribution for both the Si face and C face of SiC films. Defects were more effectively decorated by oxidation on the Si face than on the C face.
Gallium arsenide surface reconstructions during organometallic vapor-phase epitaxy
Authors: Lamelas F. J. , Fuoss P. H. , Imperatori P. , Kisker D. W. , Stephenson G. B. , Brennan S.
Published: July 2002

In situ surface x-ray scattering studies of the GaAs(001) surface were used to determine whether specific surface reconstructions occur during organometallic vapor-phase epitaxy. Prior to growth, we find that surfaces heated in the presence of As form a c(4×4) structure, while those heated in the absence of organometallics or in Ga form two similar fourfold reconstructions. We find no evidence for the presence of any surface reconstruction during the actual layer-by-layer growth process.
Reduction of transient boron diffusion in preamorphized Si by carbon implantation
Authors: Nishikawa Satoshi , Tanaka Akira , Yamaji Tetsuo
Published: July 2002

Boron diffusion in preamorphized Si is studied as a function of dose of carbon ion implantation. The boron was implanted at 20 keV with a dose of 1×1015 cm−2, and carbon was implanted at 60–90 keV. The preamorphized depth was 230 nm. It is shown that transient enhanced diffusion occurs even in the preamorphized region without C+ implantation. The diffusion constant is larger than the standard one by about one order of magnitude in the case of 1000 °C annealing for 15 s. This enhancement is eliminated by C+ implantation at a dose of about 1015 cm−2. This implantation also reduces the defects at the amorphous/crystal interface. These findings indicate that the implanted carbon acts as a sink of excess interstitials.
Distribution of trap levels in field stimulated exoelectron emission
Authors: Tagawa Masahito , Tsuzuki Koji , Mori Mikio , Ohmae Nobuo , Umeno Masataka
Published: July 2002

Trap levels of exoelectrons in high purity aluminum were estimated from the emission properties of field stimulated exoelectrons. The trap level of exoelectron in 99.9999% pure aluminum was found at about 0.2 eV higher than the Fermi level of aluminum and those in less pure aluminum were distributed in a higher energy region. The observed distribution of exoelectron trap levels agrees with the trap level determined by photostimulated exoelectron emission and also explains the glow curves of field stimulated exoelectron emissions from aluminum specimens of different purities.
Observation of interstitial carbon in heavily carbon-doped GaAs
Authors: Höfler G. E. , Hsieh K. C.
Published: July 2002

Nuclear reaction analysis, utilizing the 12C(d, p)13C reaction, in conjunction with Rutherford backscattering spectrometry in the channeling geometry were used to determine the lattice locations of carbon in heavily carbon-doped GaAs samples with carbon concentrations greater than 5×1019 cm−3. The data unambiguously show that at least 25% of the carbon atoms occupied interstitial sites within the channels of the GaAs crystal. The existence of interstitial carbon is important for practical applications since the atomic diffusivity of interstitial carbon may be different from that observed in GaAs when carbon is only incorporated substitutionally.
Degradation and breakdown of silicon dioxide films on silicon
Authors: DiMaria D. J. , Arnold D. , Cartier E.
Published: July 2002

Destructive breakdown in silicon dioxide is shown to be strongly correlated to the oxide degradation caused by hot-electron-induced defect production and charge trapping near the interfaces of the films. Two well-defined transitions in the charge-to-breakdown data as a function of field and oxide thickness are shown to coincide with the onset of trap creation and impact ionization by electrons with energies exceeding 2 and 9 eV, respectively.
Theory of giant magnetoresistance in magnetic granular films
Authors: Zhang Shufeng
Published: July 2002

It has been recently found that giant magnetoresistance can also be obtained in metallic films containing magnetic particles. Based on spin-dependent scattering, we propose a model that allows us to explain various features of the magnetoresistance and to predict new features. The model is further extended to include the temperature dependence of the magnetoresistance.
All-optical, high contrast GaAlInAs multiple quantum well asymmetric reflection modulator at 1.3 μm
Authors: Krol M. F. , Ohtsuki T. , Khitrova G. , Boncek R. K. , McGinnis B. P. , Gibbs H. M. , Peyghambarian N.
Published: July 2002

A high contrast, low intensity GaAlInAs/AlInAs multiple quantum well asymmetric Fabry–Perot reflection modulator for operation at 1.3 μm has been demonstrated. The reflection modulator takes advantage of the large absorptive and refractive nonlinearities associated with saturating the heavy-hole exciton resonance. We achieve an on/off contrast ratio in excess of 1000:1 (30 dB) and an insertion loss of 2.2 dB at a pump intensity of 30 kW/cm2, corresponding to a carrier density of 4.5×1017 cm−3. The modulator was demonstrated to have a large operating bandwidth, achieving an on/off contrast ratio of greater than 100:1 over a 5 nm optical band. The operating speed of the modulator was measured and found to approach 1 GHz.
Carrier pinning by mode fluctuations in laser diodes
Authors: O’Gorman J. , Chuang S. L. , Levi A. F. J.
Published: July 2002

We show that fluctuations into cavity modes give rise to substantial subthreshold carrier pinning in laser diodes. These fluctuations, which extract a current Ifl from the device, play an increasingly important role with increasing temperature.
Photoluminescence excitation spectroscopy on intermixed GaAs/AlGaAs quantum wires
Authors: Prins F. E. , Lehr G. , Burkard M. , Schweizer H. , Pilkuhn M. H. , Smith G. W.
Published: July 2002

Quantum wires with widths down to 45 nm have been realized by implantation induced intermixing of a surface near GaAs/AlGaAs quantum well. A very steep lateral potential has been achieved together with extreme low damage in the wire regions. As a result the optical transitions in photoluminescence excitation spectroscopy could be observed for all wire widths. With decreasing wire width an increasing Stokes shift has been determined due to the increasing importance of fluctuations in wire dimensions. A weak wire width dependence of transitions near the former two-dimensional light-hole level was observed, which is attributed to the predicted reduced energy shift of states near this level.
Simultaneous scanning tunneling and optical near-field imaging with a micropipette
Authors: Lieberman Klony , Lewis Aaron
Published: July 2002

Cross correlated near-field scanning optical microscopy and tunneling images are presented. These images were obtained simultaneously using a metal coated micropipette as both a tunneling and near-field optical probe. The optical and tunneling images exhibit a strong correlation but also contain differences representing real surface features which either technique alone would not indicate. The effect of crosstalk between the tunneling and optical images and possible artifacts introduced to the optical image by the feedback are discussed.
Effect of powder synthesis procedure on calcium site occupancies in barium calcium titanate: A Rietveld analysis
Authors: Krishna P. S. R. , Pandey Dhananjai , Tiwari V. S. , Chakravarthy R. , Dasannacharya B. A.
Published: July 2002

(Ba,Ca)TiO3 samples with the ratio of (Ba+Ca) to Ti as unity but prepared by (i) a semiwet route involving (Ba,Ca)CO3 solid solution precursors and TiO2 and (ii) the conventional dry route involving BaCO3, CaCO3, and TiO2 powders, exhibit diffuse and BaTiO3-type sharp ferroelectric phase transitions, respectively. Rietveld analysis of neutron powder diffraction data from the two types of samples shows that, whereas in the samples prepared by the semiwet route, Ca2+ exclusively occupies the Ba2+ site, nearly half of the substituted Ca2+ occupies the Ti4+ site in the case of samples prepared by the dry route. We show that Ca replacement at the Ba site leads to diffuseness whereas Ca introduction at the Ti site leads to sharpening of the transition. To our knowledge, this is the first clear demonstration of the powder synthesis procedure affecting the substitutional sites in multicomponent ceramics and this, in turn, influencing their physical behavior drastically.
High temperature (77 °C) operation of 634 nm InGaAlP multiquantum-well laser diodes with tensile-strained quantum wells
Authors: Watanabe Minoru , Rennie John , Okajima Masaki , Hatakoshi Gen-ichi
Published: July 2002

634 nm InGaAlP multiquantum-well (MQW) laser diodes, with tensile-strained InGaP well layers, have been investigated. A threshold current as low as 59 mA was achieved, at 20 °C, under continuous wave (cw) operation. These MQW laser diodes were able to operate up to temperatures as high as 77 °C under cw conditions. This maximum cw operation temperature is the highest ever reported for 634 nm laser diodes. These MQW laser diodes have shown stable cw operation over 2000 h at an output power of 3 mW, at an ambient temperature of 50 °C.
Multifunctional probe microscope for facile operation in ultrahigh vacuum
Authors: Howald L. , Meyer E. , Lüthi R. , Haefke H. , Overney R. , Rudin H. , Güntherodt H.-J.
Published: July 2002

A scanning force/tunneling microscope (SFM/STM) for remote controlled operation in ultrahigh vacuum (UHV) is described. The lateral forces, normal forces, and tunneling currents between probe tip and sample can all be measured simultaneously. The optical beam deflection detector and the sample position can be adjusted by means of three compact inertial stepping motors. An UHV-compatible light emitting diode is introduced as a general alternative to the widely used laser diode in the detector. Images, taken at 5×10−11 mbar on Si(111) with STM and noncontact SFM, and on NaF(001) with contact SFM, are presented.
High current density carbon-doped strained-layer GaAs (p+)-InGaAs(n+)-GaAs(n+) p-n tunnel diodes
Authors: Richard T. A. , Chen E. I. , Sugg A. R. , Höfler G. E. , Holonyak N.
Published: July 2002

Data are presented showing that a GaAs p-n tunnel diode can be modified, and improved, with the introduction of an InxGa1−xAs layer (Lz∼100 Å) in the barrier region to reduce the energy gap (and carrier mass) and increase the tunneling probability without sacrificing the high injection barrier and voltage of GaAs. Peak tunnel current densities in the range (1–1.5)×103 A/cm2 are obtained, with peak-to-valley current ratios of ∼20:1 and voltage ‘‘swings’’ from peak tunnel current to equal injection current of ≳1 V (≤1 V for GaAs). The C-doped GaAs(p+)-InGaAs(n+)-GaAs(n+) diodes are grown by metalorganic chemical vapor deposition and are compared to GaAs tunnel diodes fabricated by the usual alloy process (i.e., local liquid phase epitaxy).
Identity of the light-emitting states in porous silicon wires
Authors: Yeh Chin-Yu , Zhang S. B. , Zunger Alex
Published: July 2002

We present empirical pseudopotential calculations of the electronic structure of [001] silicon quantum wires, aiming at identification of the states that couple radiatively to the valence-band maximum. We find that the near-gap wave functions differ qualitatively from effective-mass depictions. Instead, they can be described as off-Γ bulk states. The effects of H chemisorption on the wire energies and wave functions are studied. We find that the Si skeleton dominates the emission. The resulting lifetimes versus energy relation agrees with the experimental measurements only if one assumes coexistence of quantum wires with quantum dots.
Oscillations in the magnetoresistance in electrostatically controlled double-quantum-wire electron interferometers
Authors: Okuda Masahiro
Published: July 2002

Oscillations in the magnetoresistance were observed in electrostatically controlled double-quantum-wire electron interferometers. The interferometers are constituted by closely located two quantum wires in which the electron wave function is electrostatically controlled by triple Schottky gates, so that the Aharonov–Bohm (AB) type interferometers are effectively formed. The AB oscillations and doubled frequency oscillations are observed in measured samples when the subgates (two side gates) are reverse biased so that the second subband level is depleted in both subgate regions.
Efficient keV x-ray emission from plasmas generated with 4 ps KrF excimer laser pulses
Authors: Turcu I. C. E. , Ross I. N. , Tallents G. J.
Published: July 2002

A high x-ray conversion efficiency is observed from plasmas produced with 4 ps pulses at 249 nm using commercial KrF lasers. Copper plasmas convert 11% of laser energy into hν∼1.2 keV photons, carbon plasmas 6.8% into ‘‘water window’’ photons (0.28 keV<hν<0.53 keV) and aluminum plasmas 0.4% into hν∼1.6 keV photons.
Study of current-voltage characteristic in a ZnSe-based II-VI laser diode
Authors: Suemune Ikuo
Published: July 2002

High operating voltage in a blue-green laser diode was explained quantitatively by the tunneling process through the barrier at a metal/p-ZnSe interface. The barrier height at the metal/p-ZnSe interface was evaluated considering the tunneling and thermionic emission processes in the realistic diode structure.
Refractive indices of ZnMgSSe alloys lattice matched to GaAs
Authors: Ukita M. , Okuyama H. , Ozawa M. , Ishibashi A. , Akimoto K. , Mori Y.
Published: July 2002

We have experimentally investigated refractive indices n of ZnxMg1−xSySe1−y, using the ellipsometry method and reflection-spectrum measurement. The samples are epitaxial films of undoped ZnxMg1−xSySe1−y grown by molecular beam epitaxy on semi-insulating GaAs substrates. The obtained dispersion relations of n in the transparent region are classified by the band-gap energy Eg. We have found that the refractive index n of ZnxMg1−xSySe1−y decreases as Eg increases. These results will be available for the design of blue laser diodes containing ZnMgSSe.
Photoluminescence of Sm doped porous silicon—evidence for light emission through luminescence centers in SiO2 layers
Authors: Lin J. , Zhang L. Z. , Huang Y. M. , Zhang B. R. , Qin G. G.
Published: July 2002

After oxidation promoted by gamma-ray irradiation, in the photoluminescence (PL) spectra of Sm doped porous silicon (PS), there are three sharp peaks, superimposed on a broad band, with wavelengths near to those of the Sm doped SiO2 [R. Morimo, T. Mizushima, and H. Okumura, J. Electrochem. Soc. 137, 2340 (1990)]. The experimental results indicate that Sm-related luminescence centers can be created within the oxide of porous silicon, and only in porous silicon with high porosity can the Sm-related luminescence be found in the SiO2 layer. This experimental result can be explained by the fact that the excitation of electron-hole pairs occurs in nanoscale silicon, and the recombination occurs at the Sm-related luminescence centers in SiO2 layers covering nanoscale silicon.
High-frequency harmonics generation by Bi2Sr2Ca2Cu3Oy ceramic carrying alternating transport current
Authors: Grishin A. M. , Korenivski V. N. , Rao K. V. , Ulyanov A. N.
Published: July 2002

We have detected the nonlinear magnetic flux generated by a superconducting slab in the presence of an alternating transport current alone, and with no external fields around during the detection. The temperature dependence of induced flux is found to be analogous in its main features to the nonlinear susceptibility: (1) only odd harmonics exist, (2) the third-harmonic signal is bell shaped with temperature, and (3) it is quite similar to the temperature dependence of the lossy component of the susceptibility at the fundamental frequency. Furthermore, two regimes, intra- and intergranular supercurrent contributions, can also be distinguished. These observations suggest a new method for the determination of critical current and its temperature dependence. The observed effective higher harmonics generation may be used in developing novel nonlinear superconducting components such as new types of frequency multipliers and multifrequency generators.
Enhancement of high-temperature photoluminescence in strained Si1−xGex/Si heterostructures by surface passivation
Authors: St. Amour A. , Sturm J. C. , Lacroix Y. , Thewalt M. L. W.
Published: July 2002

The photoluminescence from strained Si1−xGex alloy quantum wells on Si(100) has been measured from 6 to 300 K. It is shown that the high-temperature photoluminescence of Si1−xGex quantum wells can be increased by over an order of magnitude by passivation of the top silicon surface. Through experiments and a model, it is clearly demonstrated that the decay of the Si1−xGex photoluminescence at high temperature is controlled by surface recombination, not by an intrinsic property of Si1−xGex. By applying proper conditions, nearly constant Si1−xGex photoluminescence can be achieved from 77 to 250 K.
Transient component of leakage current in silicon pn junctions
Authors: Murakami Yoshio , Shingyouji Takayuki
Published: July 2002

We found that a transient component exists in the pn junction leakage current formed in Czochralski (CZ) silicon wafers. This transient component is found to be an area effect by measuring the area dependence of the leakage current; this can only be observed after a hole injection occurs by applying the appropriate bias to the junction. Furthermore, this component is not observed in epitaxial, float-zone, or magnetic-field-applied Czochralski wafers that have low oxygen concentrations. From the analysis of the time dependence of the current, we concluded that the transient component observed in CZ silicon is hole emission current from oxygen-related hole traps that have an exponential trap distribution.
Low-energy-electron diffraction by nano-objects in projection microscopy without magnetic shielding
Authors: Thien Binh Vu , Semet V. , Garcia N.
Published: July 2002

This paper shows that by using nanotips as nanosources of electrons, Fresnel diffraction by nano-objects is observed in a low-energy-electron projection microscope without any magnetic field shielding. The results shown are consistent with the Aharonov-Bohm effect and Lorentz force deviation due to the magnetic stray fields.
Velocity selection of fast laser ablated aluminum atoms by temporally and spatially specific photoionization
Authors: Macler Michel , Fajardo Mario E.
Published: July 2002

The successful demonstration of velocity selection of fast aluminum atoms by a novel, nonmechanical technique is reported. Pulses of atoms with broad velocity distributions are produced by laser ablation of aluminum metal. A second pulsed laser, delayed by ∼1 μs and crossed at a right angle to the atomic beam, is used to photoionize only those atoms with unwanted velocities, i.e., atoms moving too fast or too slow to be hidden behind an opaque mask placed ∼1 cm from the ablated surface. The photoions are subsequently deflected from the beam by a static magnetic field. Velocity selected Al atom fluxes equivalent to Φ∼1011 atoms/(cm2 eV pulse) at a working distance of 10 cm are demonstrated.
Activation energy for C94 and C54 TiSi2 formation measured during rapid thermal annealing
Authors: Colgan E. G. , Clevenger L. A. , Cabral C.
Published: July 2002

The activation energies Ea for C49 and C54 TiSi2 formation were determined using in situ resistance measurements during rapid thermal annealing. Ti films were evaporated on undoped polycrystalline Si (poly-Si) and single-crystal Si on sapphire (SOS) substrates. The resistance was monitored for heating rates from 1 to 95 °C/s up to 1000 °C. The Ea’s determined from Kissinger plots were 1.86±0.23 and 1.65±0.31 eV for C49 TiSi2 formation and 3.30±0.16 and 3.67±0.13 eV for C54 TiSi2 formation for Ti/poly-Si and Ti/SOS samples, respectively. These are the first reported measurements of Ea’s for C49 and C54 TiSi2 formation at such high heating rates. The formation sequence remained the same for the range of heating rates examined.
Evidence that blue luminescence of oxidized porous silicon originates from SiO2
Authors: Kontkiewicz A. J. , Kontkiewicz A. M. , Siejka J. , Sen S. , Nowak G. , Hoff A. M. , Sakthivel P. , Ahmed K. , Mukherjee P. , Witanachchi S. , Lagowski J.
Published: July 2002

We have analyzed red and blue luminescence from porous silicon as a function of oxidation parameters and feature dimension determined with an atomic force microscope. We have found correlation between blue luminescence intensity and the increase in feature size caused by oxidation. We have further shown that blue luminescence, is identical, with respect to spectrum and fast decay, to that of high microelectronic quality SiO2 grown on crystalline silicon using dry oxygen plus an organic chlorine compound. Thus, we conclude that blue luminescence originates from SiO2 film rather than from the silicon nanocrystals in the porous material. Intensity enhancement, as compared to SiO2 on crystalline wafers, comes from the gigantic surface area of porous silicon.
Donor-related deep level in bulk GaSb
Authors: Dutta P. S. , Koteswara Rao K. S. R. , Sangunni K. S. , Bhat H. L. , Kumar Vikram
Published: July 2002

Deep level transient spectroscopy (DLTS) and thermally stimulated capacitance (TSCAP) studies on bulk grown tellurium- selenium-, and sulphur-doped gallium antimonide reveal the presence of deep level intrinsic of the dopant species. The trap densities in Te- and Se-doped samples were found to be at least two orders of magnitude lower than the shallow donor concentration. Interestingly, the DLTS spectrum of S:GaSb exhibits DX-like nature with the trap concentration comparable to that of shallow donor concentration. However, the Te and Se related levels do not exhibit DX-like nature. The DLTS and TSCAP results are in good agreement with each other.
Enhanced J
c’s of YBa2Cu3O7−x–Ag ex situ annealed coevaporated films on LaAlO3 (100) substrates
Authors: Clausen T. , Ejrnæs M. , Olesen M. , Hilger K. , Skov J. L. , Bodin P. , Kühle A. , Chorkendorff I.
Published: July 2002

A 5× increase of the critical current density (Jc) at 77 K was obtained by coating a coevaporated 500 nm thick Y, BaF2, Cu film with 50 nm Ag prior to the ex situ annealing. Jc increased from 0.2 for uncoated samples to 1 MA/cm2 for the Ag-coated sample without severely affecting the zero resistance transition temperature (Tc0). Scanning electron microscopy showed that the surface morphology was improved and that the normally observed trellislike structure was greatly reduced. By combining electron microscopy and sputter assisted Auger analysis it was found that the Ag nucleated in droplets on the surface of the superconductor with only small amounts of Ag in the superconductor matrix. X-ray diffraction confirmed that the Ag-coated film was highly c-axis oriented. The increase in Jc is believed to be due to the improved surface properties of the superconductor, indicating that a larger amount of the film is c-axis oriented or that the single-crystalline grains are larger.
Observation of carrier concentration saturation effect in n-type AlxGa1−xAs
Authors: Du A. Y. , Li M. F. , Chong T. C. , Chua S. J.
Published: July 2002

In a series of Al0.3Ga0.7As epitaxial layers with Si doping concentrations varied from 1×1017 to 1.5×1018 cm−3, carrier concentration saturation effect was observed by Hall measurements. When Si doping concentration was increased, the carrier concentration tended to saturate. This is due to the negative U property of the donor DX center. The Fermi energy tends to be pinned at the free energy level of the DX center. This carrier concentration saturation effect should not be limit to only n-AlxGa1−xAs semiconductors. It is a general effect in n-type compound semiconductors when donor impurities induce negative U DX levels, and will have a great influence in designing optoelectronic and fast speed microelectronic devices.
Photoemission capacitance transient spectroscopy of n-type GaN
Authors: Götz W. , Johnson N. M. , Street R. A. , Amano H. , Akasaki I.
Published: July 2002

Electronic defects in n-type GaN were characterized by photoemission capacitance transient spectroscopy. Conventional deep level transient spectroscopy is of limited use in semiconductors with wide band gaps (e.g., 3.4 eV for GaN at 300 K) because it utilizes thermal energy for charge emission which restricts the accessible range of bandgap energies to within ∼0.9 eV of either band edge, for practical measurement conditions. For electron photoemission to the conduction band, four deep levels were detected at optical threshold energies of approximately 0.87, 0.97, 1.25, and 1.45 eV. It is suggested that the above photodetected deep levels may participate in the 2.2 eV defect luminescence transitions, which are also demonstrated for our material.
Thermal stability of AlInAs/GaInAs/InP heterostructures
Authors: Hayafuji N. , Yamamoto Y. , Yoshida N. , Sonoda T. , Takamiya S. , Mitsui S.
Published: July 2002

The origin of the thermal instability of the AlInAs/GaInAs system is identified and a novel method to recover the thermal degradation is also demonstrated. The thermal diffusion of fluorine into the Si-doped AlInAs layer is found to be the main cause of the electrical deterioration of this system. This finding has led to a method to recover the thermal degradation by purging the fluorine off using the reannealing in the ultrahigh-vacuum condition. This method is now potentially becoming a good candidate as a tip for the AlInAs/GaInAs devices fabrication including laser diode and high electron mobility transistor.
Intrinsic multiple quantum well spatial light modulators
Authors: Rabinovich W. S. , Bowman S. R. , Katzer D. S. , Kyono C. S.
Published: July 2002

Large improvements are reported in the sensitivity of optically addressed multiple quantum well spatial light modulators. In prior work with these materials the quantum well region has been made semi-insulating. It is shown that this is unnecessary and in fact detrimental to performance. By placing layers containing high trap concentrations at the ends of the structure and leaving the active quantum well layers intrinsic the speed of the device at a given illumination is improved by more than four times, diffraction efficiency is enhanced and spatial resolution is almost the same.
Ultrahigh-vacuum evaporation system with low-temperature measurement capability
Authors: Orr B. G. , Goldman A. M.
Published: July 2002

An ultrahigh-vacuum system has been constructed and outfitted with molecular beam sources and surface analysis equipment [Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED)]. The sample insertion system is a low-temperature apparatus. With this equipment, films can be grown in the 10−11–10−9-Torr range with substrate temperatures at 4.2–20 K. A modification of the system will permit growth at elevated temperatures followed by in situ low-temperature measurements. The design of the low-temperature apparatus permits extensive measurements to be taken in an environment in which the temperature can be controlled over a range from 0.3 to 300 K and the magnetic field from 10−7 to 6 T.
Ultrahigh-vacuum cryostat and sample manipulator for operation between 5 and 800 K
Authors: Shayegan M. , Cavallo J. M. , Chottiner G. , Glover R. E.
Published: July 2002

Design and performance of a cryostat with a rotatable sample stage for operation in ultrahigh vacuum is reported. The cryostat, which is designed for adsorption studies on single-crystal metals at low temperatures, has a temperature range of 5.5≤T≤800 K. Sample cooling from 800 to 5.5 K is achieved in about 90 min.
Optimum angle of deflection electrodes of a cylindrical electrostatic analyzer
Authors: Oshima C. , Souda R. , Aono M. , Ishizawa Y.
Published: July 2002

The optimum deflection angle of the electrodes in a cylindrical electrostatic analyzer with real slits has been discussed on the basis of numerically calculated electron trajectories. The effect of fringing fields has been included exactly in this calculation. The optimum angle is 126.5°, being independent of the gap between the inner and outer electrodes. By this optimization, the first-order angle aberration can be eliminated completely.
Automatic topographic examination of semiconductors using modulated reflectance
Authors: Aubel J. L. , Pidick G. , Reddy U. K. , Sundaram S.
Published: July 2002

A modified cell design for studies of electrolyte electroreflectance of solids is reported. A new type of electrolyte cell design for ensuring easy and proper contacts with the surface of the sample wafer is described. The modified system is shown to be suitable for fast and convenient acquisition and display of spectra, or the variation of composition or carrier concentration as a function of position on the sample. Typical results for GaAs:Si and MBE-grown Ga1−xAlxAs/GaAs samples are shown.
Multichannel far-infrared phase imaging for fusion plasmas
Authors: Young P. E. , Neikirk D. P. , Tong P. P. , Rutledge D. B. , Luhmann N. C.
Published: July 2002

A 20-channel far-infrared imaging interferometer system has been used to obtain single-shot density profiles in the UCLA Microtor tokamak. This system differs from conventional multichannel interferometers in that the phase distribution produced by the plasma is imaged onto a single, monolithic, integrated microbolometer linear detector array and provides significantly more channels than previous far-infrared interferometers. The system has been demonstrated to provide diffraction-limited phase images of dielectric targets.
New flow tube design for oxidant+metal vapor experiments
Authors: Bachar J. , Rosenwaks S.
Published: July 2002

A new flow tube which was designed for O2(1Δ)+metal vapor experiments is described. The combination of the tube with a metal vapor furnace solved two major problems in the experimental system: (a) it prolonged the time available for data collection from 1 to 60 h and (b) furnace outlet clogging has been eliminated.
Janus, a bidirectional, multifunctional plasma diagnostic
Authors: Wan A. S. , Yang T. F. , Lipschultz B. , LaBombard B.
Published: July 2002

Janus is a multiple-function edge probe used to diagnose the Alcator C limiter shadow plasma whose components include two sets of identical diagnostics, each set facing a different direction. Included within each set of diagnostics are a retarding-field energy analyzer (RFEA), Langmuir probe, and calorimeter. Janus is constructed to make measurements both parallel and antiparallel to a magnetic field. It can withstand high heat fluxes and can be scanned perpendicular to the magnetic field in the limiter shadow. The RFEA can alternatively sample both the ion and electron parallel energy distribution functions during a tokamak discharge. From the Langmuir probe, one can infer electron temperature, density, and the plasma floating potential. The calorimeter independently detects the total parallel heat flux incident on an electrically floating plate. Together these three diagnostics enable detailed, localized edge plasma studies on Alcator C. This paper presents the design considerations for each of the diagnostics along with a brief summary of the analysis techniques. Some experimental results obtained using Janus will also be presented.
SPRED spectrograph upgrade: High-resolution grating and improved absolute calibrations
Authors: Stratton B. C. , Fonck R. J. , Ida K. , Jaehnig K. P. , Ramsey A. T.
Published: July 2002

Two improvements to the SPRED multichannel VUV spectrographs used on the TFTR and PBX tokamaks have been made: (1) a new 2100-g/mm grating covering the 100–320-Å region with 0.4-Å resolution (FWHM) has been added to the existing 450-g/mm grating (100–1100 Å with 2-Å resolution), and (2) the TFTR SPRED has been absolutely calibrated using synchrotron radiation from the NBS SURF II facility, while the PBX system has been calibrated using conventional branching ratios along with line ratios from charge-exchange-recombination excited lines. The availability of high-resolution spectra in the 100–320-Å range provides improved measurements of metallic ion emissions and, when the instrument views across a neutral beam as in PBX, allows carbon and oxygen densities to be measured via charge-exchange-recombination spectroscopy.
General distributed control system for fusion experiments
Authors: Klingner P. L. , Levings S. J. , Wilkins R. W.
Published: July 2002

A general control system using distributed LSI-11 microprocessors is being developed. Common software resides in each LSI-11 and is tailored to an application by control specifications downloaded from a host computer. The microprocessors, their control interfaces, and the micro-to-host communications are CAMAC based. The host computer also supports an operator interface, coordination of multiple microprocessors, and utilities to create and maintain the control specifications. Typical applications include monitoring safety interlocks as well as controlling vacuum systems, high-voltage charging systems, and diagnostics.