bluuebunny/crossref_metadata_2025 huggingface dataset:
Leaves of Lianas and Self‐Supporting Plants Differ in Mass per Unit Area and in Nitrogen Content
Authors: Kazda M. , Salzer J.
Published: July 2002

Abstract: The aim of this study was to test the hypothesis that the reduction in supporting tissues in climbers compared to self‐supporting plants is also true for their leaves, and that climbers generally require higher leaf nitrogen than self‐supporting plants to accomplish fast growth. This hypothesis was tested using paired samples of both growth forms with assessment of leaf area index above the sampled plants (LAIa) in a tropical rain forest in Gabon. The sampling protocol ensured that within a highly fluctuating low canopy environment, the growth conditions were identical for each pair sampled. The results confirmed the hypothesis. Lianas had significantly lower leaf mass per unit leaf area (LMA) than their supporters. Liana leaves also contained significantly more nitrogen than host tree leaves. The differences in nitrogen concentration between liana and tree leaves reversed for the most shaded sites, when nitrogen was expressed on a leaf area base (Narea). Significant regression between leaf nitrogen and LAIa was found for the climbers on the shaded sites but not for their supporters. This indicated better acclimation of climbers to prevailing light conditions. Better nitrogen allocation at low LMA, together with lower carbon costs for building supporting tissues, makes lianas highly competitive, especially where high nitrogen availability is assured.
Influence of Chilling Stress on the Intercellular Distribution of Assimilatory Sulfate Reduction and Thiols in Zea mays
Authors: Kopriva S. , Jones S. , Koprivova A. , Suter M. , von Ballmoos P. , Brander K. , Flückiger J. , Brunold C.
Published: July 2002

Abstract: The effect of chilling on the intercellular distribution of mRNAs for enzymes of assimilatory sulfate reduction, the activity of adenosine 5′‐phosphosulfate reductase (APR), and the level of glutathione was analysed in leaves and roots of maize (Zea mays L). At 25 °C the mRNAs for APR, ATP sulfurylase, and sulfite reductase accumulated in bundle‐sheath only, whereas the mRNA for O‐acetylserine sulfhydrylase was also detected in mesophyll cells. Glutathione was predominantly detected in mesophyll cells; however, oxidized glutathione was equally distributed between the two cell types. Chilling at 12 °C induced oxidative stress which resulted in increased concentrations of oxidized glutathione in both cell types and a prominent increase of APR mRNA and activity in bundle‐sheath cells. After chilling, mRNAs for APR and sulfite reductase, as well as low APR activity, were detected in mesophyll cells. In roots, APR mRNA and activity were at higher levels in root tips than in the mature root and were greatly increased after chilling. These results demonstrate that chilling stress affected the levels and the intercellular distribution of mRNAs for enzymes of sulfate assimilation.
Compensation mechanism in semi-insulating GaAs: The role of intrinsic acceptor defects
Authors: von Bardeleben H. J. , Bourgoin J. C. , Stievenard D.
Published: July 2002

We present experimental evidence for the presence in semi-insulating undoped GaAs of the double-acceptor defect with ionization energies of 78 and 203 meV, which is currently attributed to the GaAs antisite in concentrations of at least 2×1015 cm−3. We then discuss the implications of this result for the compensation mechanism in these materials.
Identification of residual donors in high-purity undoped p-type epitaxial GaAs by magnetophotoluminescence
Authors: Bose S. S. , Kim M. H. , Stillman G. E.
Published: July 2002

The residual donor species, Si, S, and Ge, have been identified in high-purity undoped p-type epitaxial GaAs grown by metalorganic chemical vapor deposition and arsenic trichloride vapor phase techniques using the magnetic splittings of ‘‘two-electron’’ replicas of donor bound exciton transitions at low temperature (∼1.8 K) and at a high magnetic field (9.0 T). This technique permits identification of donors in certain high-purity p-type GaAs samples in which donor species cannot be identified by photothermal ionization spectroscopy or any other technique.
Highly oriented superconducting Tl-Ca-Ba-Cu oxide thin films with 2-1-2-2 phase
Authors: Ichikawa Yo , Adachi Hideaki , Setsune Kentaro , Hatta Shin-ichiro , Hirochi Kumiko , Wasa Kiyotaka
Published: July 2002

Thin films of the Tl-Ca-Ba-Cu-O system with 2-1-2-2 phase have been prepared stably on MgO substrates by rf magnetron sputtering and post-annealing. The post-annealing of the films was carefully carried out in order to reduce the effect of evaporation of Tl. The annealed films showed a c-axis-oriented structure perpendicular to the substrates. The highest zero-resistance temperature of the films was 102 K. The highest critical current density obtained so far was 1.2×105 A/cm2 at 77 K.
Unified planar process for fabricating heterojunction bipolar transistors and buried-heterostructure lasers utilizing impurity-induced disordering
Authors: Thornton R. L. , Mosby W. J. , Chung H. F.
Published: July 2002

We describe results on a novel geometry of heterojunction bipolar transistor that has been realized by impurity-induced disordering. This structure is fabricated by a method that is compatible with techniques for the fabrication of low threshold current buried-heterostructure lasers. We have demonstrated this compatibility by fabricating a hybrid laser/transistor structure that operates as a laser with a threshold current of 6 mA at room temperature, and as a transistor with a current gain of 5.
Simulation of saturation and relaxation of intersubband absorption in doped quantum wells
Authors: Newson D. J. , Kurobe A.
Published: July 2002

We have simulated the saturation and relaxation of intersubband absorption in doped GaAs quantum wells, in which radiation excites electrons between the ground and higher subbands. Screened longitudinal optic (LO) phonon, hot LO phonon, acoustic phonon, intrasubband scattering, conduction-band nonparabolicity, and intervalley scattering were included in our simulation. We find that the change in absorption relaxes with a time constant of about 5 ps after pulsed excitation. Under continuous excitation the absorption saturates as a nearly homogeneously broadened system.
Formation of epitaxial Si1−xGex films produced by wet oxidation of amorphous SiGe layers deposited on Si(100)
Authors: Prokes S. M. , Tseng W. F. , Christou A.
Published: July 2002

Epitaxial SiGe/Si structures have been formed by wet oxidation of amorphous SiGe films. Amorphous, 1000-Å-thick Si0.86Ge0.14 films were electron beam evaporated onto RCA cleaned Si(100) substrates at a background pressure of 1×10−7 Torr. They were then wet oxidized in an open tube furnace, at 900 °C for various times. They have been examined by reflective high-energy electron diffraction and Rutherford backscattering. Results indicate the formation of an epitaxial SiGe layer following the oxidation, whereas a polycrystalline layer forms following a vacuum or nitrogen ambient anneal. It is suggested that the oxide contamination at the amorphous SiGe/Si interface is too high to allow solid phase epitaxial growth to occur in an oxygen-free ambient, but during the oxidation process, some native oxide is dissolved due to a gradient of silicon from the substrate to the growing SiO2 on the surface. This allows grains of the SiGe alloy to orient with respect to the substrate, and secondary grain growth occurs during the oxidation process.
Non-Newtonian strain relaxation in highly strained SiGe heterostructures
Authors: Dodson Brian W. , Tsao Jeffrey Y.
Published: July 2002

Semiconductor strained-layer structures routinely incorporate layer stresses in the GPa regime. At these stresses, models for strain relaxation cannot be based on material parameters determined at small stresses. A new analysis of experimental data on strain relaxation in metastable SiGe/Ge(001) strained-layer structures allows the effects of high stresses on relaxation kinetics to be determined. The activation energy for plastic flow is found to decrease roughly linearly with increasing stress, in a manner analogous to non-Newtonian viscous flow in fluids. As a result, macroscopic relaxation occurs more easily and becomes dramatically more abrupt as the growth temperature is reduced than would be extrapolated from the small-stress behavior. Implications for design and fabrication of metastable strained-layer devices are discussed.
Infrared absorption of electron irradiation induced deep defects in semi-insulating GaAs
Authors: Manasreh M. O. , Fischer D. W.
Published: July 2002

Electron irradiation induced defects in semi-insulating GaAs grown by the liquid-encapsulated Czochralski technique were studied using infrared absorption spectroscopy. A broad peak (P1) and a shoulder (P2) were observed in the infrared absorption spectra of the irradiated materials at 0.98 and 0.78 eV, respectively. The electron-phonon coupling strength as well as the Franck–Condon shift of P1 was estimated from the temperature dependence of the linewidth. The annealing kinetics between 375 and 450 °C show that the P1 defect vanishes by a long-range migration process with an enthalpy of 0.78±0.02 eV. A speculation for the atomic structure of P1 is presented.
Li-doped ZnSe epitaxial layers by ion implantation
Authors: Yodo Tokuo , Yamashita Ken
Published: July 2002

Li+ ions were implanted into undoped high crystalline quality ZnSe epilayers grown by metalorganic vapor phase epitaxy. Thermal annealing after ion implantation was performed between 451 and 757 °C. The total photoluminescence intensity (measured at 77 K) of implanted layers before annealing was two orders of magnitude less than that of an as-grown one because of the Li+ ion damage. As the annealing temperature was increased, however, the damage was reduced and the photoluminescence intensities of two main peaks (band-edge emission and strong free to acceptor transition emission) became intense without generating deep centers. At 4.2 K, the only emission observed was associated with acceptor bound exciton and donor acceptor pair recombination in the near-band-edge region.
New low-temperature process for growth of GaAs on Si with metalorganic molecular beam epitaxy assisted by a hydrogen plasma
Authors: Suemune I. , Kunitsugu Y. , Tanaka Y. , Kan Y. , Yamanishi M.
Published: July 2002

A new low-temperature cleaning as well as a growth method using a hydrogen (H) plasma was successfully applied to the growth of GaAs on Si. The H plasma is expected to play several roles, such as cleaning the Si surface just prior to growth, decomposition of the metalorganic sources introduced without thermal cracking for the GaAs growth, supplying H radicals to terminate the organic-alkyl radicals, and enhancement of the mobilities for the surface migration. The almost atomically flat Si clean surface was prepared at a temperature as low as 300 °C using the H plasma. A single-domain GaAs epilayer was successfully realized at 400 °C after growth of 100 nm on a two-domain (001) Si surface using triethylgallium and triethylarsenic introduced without thermal cracking. The films were grown just after surface cleaning with the H plasma in the same chamber. Therefore, the reduction of the temperature for the whole growth process is possible with this new method.
Thermal stability of sawtooth-shaped HgCdTe superlattices
Authors: Cheung Jeffrey T. , Chen John-Sea
Published: July 2002

Steps in optical absorption spectra due to transitions to different subbands have been observed in HgCdTe-based nonrectangular superlattices. The sawtooth-shaped superlattice band gap shows much less thermal degradation than the conventional rectangular superlattice. The improved thermal stability can be attributed to the enhanced quantization effect in which the dependence of superlattice band gap on quantum well size is less sensitive for a sawtooth quantum well than for a rectangular quantum well.
Substrate misorientation effects on the structure and electronic properties of GaInAs-AlInAs interfaces
Authors: Hu Yu-Peng , Petroff P. M. , Qian Xueyu , Brown A. S.
Published: July 2002

The effects of substrate misorientation on the interface quality of Ga0.47In0.53As/Al0.48In0.52As quantum well structures grown by molecular beam epitaxy lattice matched on InP substrates have been investigated. Transmission electron microscopy and low-temperature (15 K) cathodoluminescence spectra have been used to characterize the GaInAs/AlInAs layers and analyze the effects of nucleation and growth kinetics on the heterojunction interface quality. The quantum well luminescence line shape correlates with the presence of compositional fluctuations in the AlInAs layers and thickness variations of the quantum wells for structures deposited on misoriented substrates. Rapid thermal annealing of these samples improves the quantum well luminescence characteristics.
Contact resistance to undoped and phosphorus-doped hydrogenated amorphous silicon films
Authors: Kanicki Jerzy
Published: July 2002

An extensive systematic study of contact properties to as-deposited undoped and phosphorus-doped hydrogenated amorphous silicon (a-Si:H) in metal/a-Si:H diode configuration has shown that the magnitude of the contact resistance can be adjusted to some degree by the proper choice of metal work function. It is also obvious from our experimental data that the film doping (or bulk resistivity) is the most important factor in controlling the value of contact resistance for a given metallization. The lowest contact resistance values for both undoped and doped films have been achieved for Eu, Y, Sc, and Mg. Reasonable values for heavily doped films have also been obtained for Ti, Ta, Mo, and Al contacts. We have also shown, for the first time, that a further decrease of contact resistance can be achieved by increasing the doping efficiency of the phosphorus-doped layer.
Trends of deep level electron traps in AlxGa1−xAs grown by molecular beam epitaxy
Authors: Puechner R. A. , Johnson D. A. , Maracas G. N.
Published: July 2002

Deep level transient spectroscopy is used to study one of the deep electron traps commonly seen in AlxGa1−xAs grown by molecular beam epitaxy (MBE). This trap, having the largest capture cross section of the commonly observed levels in MBE material, is measured in samples with Al composition varying from 8 to 40%. It is shown that the activation energy remains fixed with respect to the valence band, over the composition range studied. The trap is tentatively identified as a group III related vacancy. The effects of varying MBE growth parameters are discussed.
Evidence for large-area superemission into a high-current glow discharge
Authors: Hartmann W. , Dominic V. , Kirkman G. F. , Gundersen M. A.
Published: July 2002

This letter presents evidence for large-area (≊1 cm2) cathode superemission (∼10 000 A/cm2) into a high-current glow discharge in a pseudospark or back lighted thyratron switch. Cathodes studied with a scannning electron microscope following operation at 6–8 kA, ≊1 μs pulse length, and 105 pulses in a low-pressure H2 discharge show evidence of melting of a thin surface layer within a radius of ∼4 mm, indicating that the discharge is a superdense glow with a cross-sectional area of the order of 1 cm2, rather than an arc. Further supporting evidence is provided by streak camera data. An ion beam present during the avalanche phase of the discharge is responsible for heating the cathode surface resulting in a significant field-enhanced thermionic emission.
Growth and diffusion of abrupt beryllium-doped profiles in gallium arsenide by organometallic vapor phase epitaxy
Authors: Tejwani M. J. , Kanber H. , Paine B. M. , Whelan J. M.
Published: July 2002

We have grown abrupt beryllium-doped profiles in gallium arsenide by organometallic vapor phase epitaxy. Secondary-ion mass spectroscopy is used to study the annealing behavior of profiles with beryllium concentrations of 2×1018 cm−3. The diffusion fronts are non-Gaussian and abrupt. Estimates of the diffusion coefficient of beryllium in gallium arsenide are obtained assuming a quadratic dependence on concentration. The beryllium diffusion coefficient is approximately 10−15 cm2/s at 825 °C and is at least an order of magnitude less than that reported for zinc profiles grown by organometallic vapor phase epitaxy. In addition, we have also observed anomalous surface tailing during growth which is very similar to that reported during beryllium doping by molecular beam epitaxy.
(111) oriented (GaAs)n(AlAs)n superlattices are direct band-gap materials for all n’s
Authors: Wei Su-Huai , Zunger Alex
Published: July 2002

Total energy calculations show that the (111) (AlAs)n(GaAs)n superlattice has a lower formation enthalpy (i.e., is stabler) than either the (001) or (110) superlattices. Self-consistent band structure calculations further show that while the (001) superlattice is direct only for n>7, the (111) superlattice has (i) a smaller and (ii) a direct (not pseudodirect) gap for all n’s. Contrary to the expectations based on particle in a box models, the confined states at the zone center are strongly localized even for the monolayer superlattice.
Linear and nonlinear optical properties of 2-(N-prolinol)-5-nitropyridine
Authors: Sutter K. , Bosshard Ch. , Wang W. S. , Surmely G. , Günter P.
Published: July 2002

The optical and nonlinear optical properties of 2-(N-prolinol)-5-nitropyridine (PNP) have been investigated. The refractive indices show strong dispersion and high birefringence in the spectral range from 488 to 1064 nm. At 632 nm the main refractive indices are nx=1.990, ny=1.788, and nz=1.467. The two major nonlinear optical susceptibilities were determined (dyxx=48 pm/V, dyyy=17 pm/V). Angle-tuned phase-matched second-harmonic generation was demonstrated at 1064 nm. Type I noncritical phase matching using the nonlinear optical susceptibility dyxx is predicted for a fundamental wavelength λ=1020 nm.
Reaction of Ba2YCu3O6.9 films with yttria-stabilized zirconia substrates
Authors: Cima M. J. , Schneider J. S. , Peterson S. C. , Coblenz W.
Published: July 2002

Degradation of Ba2YCu3O6.9 films on yttria-stabilized zirconia substrates is shown to proceed by barium depletion from the film and barium zirconate formation. This process is accompanied by a decrease in the temperature of zero resistivity and ultimately produces Y2Cu2O5 and CuO. BaZrO3 is, however, found to be inert to Ba2YCu3O6.9 at 950 °C. Use of a truly chemically inert substrate material may permit extended heat treatment of films without degradation due to substrate interactions.
Imaging of layered semiconductor clusters by scanning tunneling microscopy: Bi2S3 on graphite and gold substrates
Authors: Jing T. W. , Ong N. P. , Sandroff C. J.
Published: July 2002

We present scanning tunneling microscope images of Bi2S3 clusters deposited on the surfaces of graphite and gold. We observed similar images on both substrates, discerning the disk-like structures expected for clusters having layered symmetry. The images of highest quality were obtained for 600-atom clusters roughly 100 Å in lateral extent and 25 Å thick. The difficulty in characterizing clusters with smaller lateral dimension suggests that clusters must be bound to the surface with a critical adsorption energy in order to obtain stable images.
Formation of Tl-Ca-Ba-Cu-O films by diffusion of Tl into rf-sputtered Ca-Ba-Cu-O
Authors: Qiu C. X. , Shih I.
Published: July 2002

Using a rf sputtering method with a single Ca-Ba-Cu-O target (Ca:Ba:Cu=2:2:3), thin films with a thickness of about 1 μm have been deposited on ZrO2 and Si substrates. Diffusion experiments showed that the resistivity can be largely reduced by treating the films in an environment containing O2 and Tl. Treatment experiments at different temperatures further showed that the resistivity was affected by the temperature. By treating at 730 °C for about 1 h, a film on ZrO2 with a transition temperature of about 110 K has been obtained. Zero resistance was achieved at 75 K.
Role of higher silanes in the plasma-induced deposition of amorphous silicon from silane
Authors: Heintze M. , Vepřek S.
Published: July 2002

Time-resolved mass spectrometric data of the decay of monosilane concentration and of the formation and decay of disilane and trisilane are presented together with the silicon deposition rates measured under the same conditions. The data can be quantitatively explained by the mechanism involving, as a first step, the fragmentation of silane into SiH2 and H2, and deposition of a-Si via higher silanes which are formed by a fast insertion reaction of silene into gaseous silanes. Deposition via a SiH3 radical, which has been suggested by several groups, cannot explain the experimental data.
Layer intermixing in 1 MeV implanted GaAs/AlGaAs superlattices
Authors: Lee S.-Tong , Chen Samuel , Rajeswaran G. , Braunstein G. , Fellinger P. , Madathil J.
Published: July 2002

Layer intermixing in MeV Si-implanted GaAs/AlGaAs superlattices (SLs) with doses between 3×1015 and 1×1016 /cm2 has been examined by transmission electron microscopy and secondary-ion mass spectrometry. After either rapid thermal annealing at 1050 °C for 10 s or furnace annealing at 850 °C for 3 h, all the SLs showed a highly crystalline, defect-free zone in the near-surface region followed by a band of secondary defects, with the maximum density located about 1 μm below the surface. A totally mixed region, within the secondary defect band, occurred only in the SL implanted to 1×1016 Si/cm2 and annealed at 850 °C for 3 h. At lower doses or under rapid thermal annealing, only slight Al/Ga interdiffusion was observed, primarily in the layers that contained the high density of dislocation defects. For either annealing condition, the Si concentration profiles showed only slight broadening and they correlated with the distribution of secondary defects as well as with the depth of the intermixed layer. The effects of dynamic annealing and surface on the implantation energy dependence, i.e., MeV vs keV, of layer intermixing are discussed.
Selective epitaxy in the conventional metalorganic vapor phase epitaxy of GaAs
Authors: Kuech T. F. , Tischler M. A. , Potemski R.
Published: July 2002

The selective epitaxy of GaAs was demonstrated in the metalorganic vapor phase epitaxy of GaAs utilizing diethylgallium chloride [Ga(C2H5)2Cl] and AsH3. No GaAs will deposit on SiO2, Si3N4, or SiONx under normal growth conditions, i.e., 600–800 °C at 0.1 atm reactor pressure. Unlike other forms of selective epitaxy, there is no enhanced growth rate at the edge of the selectively grown regions. The selectivity is a result of the reduced adsorption of the growth precursor, probably GaCl, on the masking material relative to the exposed GaAs areas. Similar selectivity should be possible for Al and In containing semiconductors using an analogous growth chemistry.
GaSb/GaInSb quantum wells grown by metalorganic vapor phase epitaxy
Authors: Haywood S. K. , Chidley E. T. R. , Mallard R. E. , Mason N. J. , Nicholas R. J. , Walker P. J. , Warburton R. J.
Published: July 2002

Single and multiple quantum wells of GaSb/GaInSb were grown by metalorganic vapor phase epitaxy. X-ray diffraction on an 80 Å single well confirmed the Ga1−xInxSb composition to be x=0.15, for which the lattice mismatch is ≊1.0%. Photoluminescence and photoconductivity from this sample both showed a signal due to carriers in the well, the position of which was in good agreement with the calculated band diagram. Shubnikov–de Haas oscillations in the transverse magnetoresistance (ρxx) of a four-period multiquantum well, and the associated quantum Hall effect, indicated that a two-dimensional hole gas was present in one of the wells. Unusually, the strongest oscillations were seen for occupancy of an odd number of (spin split) Landau levels (ν=1,3,5,...,etc.) This sample also showed luminescence peaks at 738 and 755 meV which were attributed to recombination in the wells.
Low-noise thin-film TlBaCaCuO dc SQUIDs operated at 77 K
Authors: Koch R. H. , Gallagher W. J. , Bumble B. , Lee W. Y.
Published: July 2002

We have made a series of single-level dc superconducting quantum interference devices (SQUIDs) from 4-μm-thick TlBaCaCuO films with large grain sizes and operated them in liquid nitrogen. Although device characteristics could not be precisely controlled, some devices had white-noise levels that approached thermally limited noise above ∼1000 Hz. In addition, devices with 5 and 80 pH loop inductances had 1/ f noise levels at 10 Hz of 2×10−29 and 5×10−29 J/Hz, respectively. The noise levels at these frequencies are comparable to commercial rf SQUIDs operating in liquid helium, but the hysteresis of the voltage-flux characteristic of the high Tc SQUIDs remains large.
Optogalvanic signals from argon metastables in a rf glow discharge
Authors: Murnick D. E. , Robinson R. B. , Stoneback D. , Colgan M. J. , Moscatelli F. A.
Published: July 2002

Laser optogalvanic (LOG) signals at 667.7, 751.5, and 696.5 nm from the 3 P1 and 3 P2 levels of Ar were studied at a pressure of 250 mTorr in a rf glow discharge. Signals with unexpected signs and time dependences were found. The results are interpreted as being due to radiative trapping effects and collisional mixing between resonance and metastable levels. An average electron energy of 2.1 eV is derived from modeling the data.
Grain boundary amorphization reaction in thin films of elemental Cu and Y
Authors: Johnson R. W. , Ahn C. C. , Ratner E. R.
Published: July 2002

Compositionally modulated thin films of Cu and Y were prepared in an ultrahigh vacuum dc ion beam deposition chamber. Room-temperature growth of an amorphous Cu-Y phase was observed with interdiffusion of the elemental Cu and Y. Transmission electron microscopy of as-prepared samples revealed a novel growth morphology; amorphous phase formation was observed both at the original Cu/Y interface and between the grains of the elemental Y. Estimates for the thermodynamic and kinetic factors underlying the grain boundary amorphization reaction are presented.
Properties of Si layers grown by molecular beam epitaxy at very low temperatures
Authors: Jorke H. , Kibbel H. , Schäffler F. , Casel A. , Herzog H.-J. , Kasper E.
Published: July 2002

(100) silicon molecular beam epitaxy films with etch pit densities below 103 cm−2 and χmin values of 3.3–3.9% were grown at very low temperatures (Ts =250–350 °C). Although dopant activation is significantly below unity at n=1018 Sb atoms/cm3 Hall mobilities of homogeneously Sb-doped samples (Ts =250 °C, 300 °C) are found to match reasonably bulk values. δ doping at a monolayer Sb deposition shows a dopant activation of 0.45–0.81 with no detectable broadening at Ts =200 °C.
Room-temperature confinement and photoluminescence near 3 μm from HgCdTe multiple quantum wells
Authors: Cesar C. L. , Islam M. N. , Feldman R. D. , Spitzer R. , Austin R. F. , DiGiovanni A. E. , Shah J. , Orenstein J.
Published: July 2002

We report room-temperature confinement and photoluminescence near 3 μm in a mercury cadmium telluride multiple quantum well. The absorption spectra show transitions from the first heavy and light hole confined levels to the first conduction electron confined level. Photoluminescence is present even at room temperature. The transition energies and temperature dependence of our data can be described by a square well model provided that a temperature-independent value of approximately 400 meV is used for the HgTe-CdTe valence-band offset.
Dependence of apparent barrier height on barrier thickness for perpendicular transport in AlAs/GaAs single-barrier structures grown by molecular beam epitaxy
Authors: Kyono C. S. , Kesan V. P. , Neikirk D. P. , Maziar C. M. , Streetman B. G.
Published: July 2002

Current-voltage characteristics of single-barrier AlAs/GaAs heterostructures measured over a wide temperature range are used to elucidate the mechanisms governing electron transport through these barriers. Five barriers, ranging in thickness from 14.2 to 150 Å, are examined. The results clearly illustrate Γ-band, elastic-tunneling-dominated transport for the thinnest barrier (14.2 Å) devices and thermionic emission characteristics for the thickest barrier (150 Å) devices. However, devices with an intermediate barrier thickness exhibited tunneling-like currents larger than calculated low-temperature elastic tunneling currents. This effect is apparently due to inelastic tunneling from the GaAs Γ band through the AlAs X-point barrier.
Transport properties of two-dimensional electron gas systems in delta-doped Si:In0.53Ga0.47As grown by organometallic chemical vapor deposition
Authors: Hong W-P. , DeRosa F. , Bhat R. , Allen S. J. , Hayes J. R.
Published: July 2002

We have investigated the transport properties of a two-dimensional electron gas formed in delta-doped In0.53 Ga0.47 As grown by metalorganic chemical vapor deposition technique. Very high free-electron concentrations of 1.4×1013 and 9.6×1012 cm−2 have been obtained at 300 and 77 K, respectively. Hall mobilities of 9300 and 14 600 cm2 /V s were measured with carrier concentrations of 3.7×1012 and 3.0×1012 cm−2 at 300 and 77 K, respectively. This is a factor of 3 higher than is expected for homogeneously doped materials having a similar doping. Schubnikov–de Haas oscillations confirmed the two-dimensional nature of the electronic structure in these delta-doped materials, and electron effective masses were determined from cyclotron resonance measurements.
Hydrogenation and subsequent hydrogen annealing of GaAs on Si
Authors: Hsieh K. C. , Feng M. S. , Stillman G. E. , Holonyak N. , Ito C. R. , Feng M.
Published: July 2002

The effects of hydrogenation and subsequent annealing on unintentionally doped GaAs layers grown directly on Si substrates by metalorganic chemical vapor deposition have been characterized by capacitance-voltage measurements, Hall effect measurements, transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). Significant reduction of the carrier concentration in the GaAs layers after hydrogen plasma exposure is obtained. TEM shows that the hydrogen plasma slightly etches the surface of the GaAs layers, and EDS demonstrates that the etched area becomes arsenic deficient and contains minute Ga particles. In addition, atomic hydrogen diffuses deeply along threading dislocations and microtwin interfaces into the GaAs layers and reacts with GaAs locally around the defects.
Thermal recovery process of the midgap-state profile of light-soaked undoped hydrogenated amorphous silicon
Authors: Matsuura Hideharu
Published: July 2002

Changes of midgap-state profiles of light-soaked undoped hydrogenated amorphous silicon are measured in the process of a 150 °C annealing by transient heterojunction-monitored capacitance measurements. Monomolecular annealing kinetics are found to be suitable for explaining the results, and the thermal activation energy for annealing is determined at each energy position of midgap states. This activation energy decreases with an increase in the energy position measured from the conduction-band edge.
Wideband scanning of a guided-light beam and spectrum analysis using magnetostatic waves in an yttrium iron garnet-gadolinium gallium garnet waveguide
Authors: Tsai C. S. , Young D.
Published: July 2002

Wideband scanning of a light beam through the use of magnetostatic waves has been accomplished for the first time. Specifically, a guided-light beam at a wavelength of 1.317 μm in an yttrium iron garnet-gadolinium gallium garnet waveguide was Bragg diffracted by magnetostatic forward volume waves (MSFVWs) operating at a frequency range of 2–7 GHz. Wideband scanning of the light beam was accomplished simply by changing the frequency of the MSFVW continuously from center frequencies of 2.5 and 6.0 GHz while keeping the dc magnetic field fixed or by continuously tuning the dc magnetic field while keeping the frequency of the MSFVW fixed at the center frequencies. A large number of resolvable scan spot positions were obtained in both cases. The resulting wideband magneto-optic Bragg cell or light beam scanner was also used to perform spectral analysis of wideband rf signals at a center frequency of 3.2 GHz. Potential advantages of the magneto-optic Bragg cell and scanner over the existing acousto-optic counterparts also identified.
Degradation kinetics of GaAs quantum well lasers
Authors: Madhava-Menon Eswar C. , Petroff P. M. , Waters R. G.
Published: July 2002

Degradation-induced dark line defect growth velocities have been found to be inversely proportional to the operating temperature in quantum well lasers under current injection. A model has been developed, which considers the differing band structure and role of deep levels in quantum well structures as opposed to conventional double-heterostructure lasers. Our findings have significant implications for device yield since they suggest nontraditional screening strategies that recognize temperature as a decelerant for the sudden failure mode.
Generation of strong composition-modulated structures and absence of ordered structures in InGaP crystals grown on (110) GaAs substrates by metalorganic chemical vapor deposition
Authors: Ueda O. , Takechi M. , Komeno J.
Published: July 2002

InGaP crystals grown on (110) GaAs substrates by chemical vapor deposition have been structurally investigated by transmission electron microscopy. No superstructure spots are present in electron diffraction patterns from (110) plan view and (11̄0) and (001) cross sections. However, composition-modulated structures with strong amplitude are found not only in the 〈001〉 but in the 〈11̄0〉 directions. They are found to be stable in the form of columnar domains during crystal growth. These results strongly suggest that atomic ordering and spinodal decomposition of the crystal are competing on the growth surface via surface diffusion of deposited atoms.
Amorphous silicon thin-film transistors with two-layer gate insulator
Authors: Kim Nam-Deog , Kim Choong-Ki , Jang Jin , Lee Choochon
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) thin-film transistors (TFTs) which utilize two layers of amorphous silicon nitride with different composition as a gate insulator have been studied. The field-effect mobility, subthreshold slope, and stability of an a-Si:H TFT are enhanced by inserting a thin silicon-rich nitride layer between the a-Si:H and the gate insulator. The improvement of these characteristics appears to be due to both the decrease of the interface state density between the a-Si:H and the top silicon-rich nitride layer, and the good dielectric quality of the bottom nitride layer.
High-power vertical-cavity surface-emitting AlGaAs/GaAs diode lasers
Authors: Zinkiewicz L. M. , Roth T. J. , Mawst L. J. , Tran D. , Botez D.
Published: July 2002

Vertical-cavity surface emitters with rear mirrors made of conductive semiconductor stack reflectors (Rr =98%) were developed. Current confinement is obtained via an etch and regrowth technique with no need for dielectrics. Peak powers of 120 mW were achieved at room temperature. The external differential quantum efficiency is 15%.
Atomic layer epitaxy of GaAs using triethylgallium and arsine
Authors: Ohno H. , Ohtsuka S. , Ishii H. , Matsubara Y. , Hasegawa H.
Published: July 2002

Atomic layer epitaxy (ALE) of GaAs is realized using triethylgallium (TEG) and arsine in a conventional atmospheric pressure metalorganic vapor phase epitaxy reactor. It is shown that the use of TEG and arsine resulted in ALE growth of GaAs in rather limited ranges of substrate temperature and the TEG supply rate. The mechanism of ALE is also discussed with the aid of x-ray photoelectron spectroscopy on the surfaces of clean GaAs before and after exposure to TEG.
Measurements of the nonlinear damping factor in 1.5 μm distributed feedback lasers
Authors: Tsuji S. , Vodhanel R. S. , Choy M. M.
Published: July 2002

We demonstrate a new method to measure the nonlinear damping factor in 1.5 μm distributed feedback lasers from the phase difference between FM and AM signals due to carrier density modulation. The damping factor is found to have the same linear relation to the square of the resonance frequency for devices having different wavelength detuning.
Room-temperature two-dimension exciton exchange and blue shift of absorption edge in GaAs/AlGaAs superlattices under an electric field
Authors: Yan R. H. , Simes R. J. , Ribot H. , Coldren L. A. , Gossard A. C.
Published: July 2002

The first room-temperature two-dimensional heavy hole exciton red shift, greater than 25 meV due to the exchange of Van Hove-type M1 and quantum well excitons, was observed by photocurrent measurements in molecular beam epitaxy grown superlattices with periods of 30 Å GaAs/25 Å Ga0.5Al0.5As. At a photon energy of 1.66 eV, a 3000 cm−1 absorption change due to the blue shift of the superlattice absorption edge was also observed at room temperature in superlattices with periods of 20 Å GaAs/20 Å Ga0.5Al0.5As.
Novel characterization of implant damage in SiO2 by nuclear-deposited energy
Authors: Hiraiwa A. , Usui H. , Yagi K.
Published: July 2002

Etching enhancement in through-implanted SiO2 has been characterized by nuclear-deposited energy independently of implant conditions. An empirical expression has been proposed to describe the etching rate for any implantation. The enhanced etching has been related to the Si-O vibrational frequency shift. Etching enhancement has been found to reflect the structural change in SiO2, and to be a good measure of degradation. The structural change of SiO2 stops and the etching rate reaches a maximum for an ion dose corresponding to nuclear-deposited energy larger than 3.4×1023 eV/cm3. This energy is equal to the total SiO bonding energy (3.8 eV) in a unit volume of SiO2.
Preparation of superconducting Bi-Sr-Ca-Cu-O thin films by sequential electron beam evaporation and oxygen annealing
Authors: Steinbeck J. , Tsaur B-Y. , Anderson A. C. , Strauss A. J.
Published: July 2002

Superconducting films with nominal composition Bi2Sr2Ca1Cu2Ox have been prepared on 〈100〉 MgO substrates by sequential electron beam evaporation of Cu, Bi, and Sr2/3 Ca1/3 F2, followed by annealing in flowing wet, then dry, O2. X-ray diffraction data show that the films contain the two Bi-Sr-Ca-Cu-O phases that have been identified in the literature as a superconducting phase with c∼31 Å and a semiconducting phase with c∼24 Å. Both phases are strongly textured with the c axis perpendicular to the substrate. For the best film, which was annealed at 870 °C for 30 min, zero resistance was observed at 90 K, and the critical current density increased from 0.8×105 A/cm2 at 77 K to 2.3×105 A/cm2 at 4.2 K.
Reduced defect density in silicon-on-insulator structures formed by oxygen implantation in two steps
Authors: Margail J. , Stoemenos J. , Jaussaud C. , Bruel M.
Published: July 2002

Up to now, the best quality silicon-on-insulator material formed by high-dose oxygen ion implantation has been produced by conventional one step implantation and subsequent high-temperature annealing. Nevertheless, it still contains structural defects: threading dislocations in the top silicon film and silicon islands inside the buried SiO2 layer. This letter presents a new method for producing silicon-on-insulator structures using sequences of oxygen ion implantation and high-temperature annealing steps. The structures obtained using this method are nearly free from defects observable by electron microscopy. In particular, no threading dislocations could be observed and the density of silicon islands is reduced by several orders of magnitude. A mechanism of Si interstitial atoms migration is proposed to explain these observations in accordance with other studies.
Elucidation of x-ray diffraction data on the nature of the ordering of GaInP2 and how the ordering relates to changes in the optical properties
Authors: Kurtz Sarah R. , Olson J. M. , Kibbler A.
Published: July 2002

The relationship between anomalous changes in the band gap and ordering of the group III sublattice of GaInP2 is discussed. X-ray diffraction data are reported for the first time which show significant long-range order of the CuPt type, i.e., alternating {111} Ga and In planes, in agreement with published electron diffraction data. However, the x-ray and electron diffraction data differ as to which of the four possible orientations are observed and as to the directions of the streaks. No correlation was observed between the long-range order parameter and the band-gap anomalies.
Highly oriented Tl2Ba2Ca2Cu3O10 thin films by pulsed laser evaporation
Authors: Liou S. H. , Aylesworth K. D. , Ianno N. J. , Johs B. , Thompson D. , Meyer D. , Woollam John A. , Barry Colleen
Published: July 2002

We have fabricated superconducting thin films on MgO(100) substrates with nearly pure Tl2Ba2Ca2Cu3O10 (2:2:2:3) phase using pulsed laser evaporation and post-annealing. The films had c axes perpendicular to the substrates. Superconducting films with onset temperatures of 125 K and zero resistance at 110 K were obtained. X-ray microprobe fluorescence measurements indicate that a typical composition of films is Tl0.66Ba1.77Ca1.46Cu3Ox, which is low in Tl compared to that expected for the 2:2:2:3 phase. A typical grain size is greater than 10 μm as revealed by scanning electron microscopy.
Preparation of high T
c Tl-Ba-Ca-Cu-O thin films by pulsed laser evaporation and Tl2O3 vapor processing
Authors: Johs B. , Thompson D. , Ianno N. J. , Woollam John A. , Liou S. H. , Hermann A. M. , Sheng Z. Z. , Kiehl W. , Shams Q. , Fei X. , Sheng L. , Liu Y. H.
Published: July 2002

Tl-Ba-Ca-Cu-O superconducting thin films with zero-resistance temperatures up to 115 K have been prepared using a Tl2 O3 vapor process on Ba-Ca-Cu-O precursor thin films. The Ba-Ca-Cu-O thin films were made by laser deposition on Y-stabilized ZrO2 substrates. This technique minimizes problems caused by the toxicity of Tl2 O3, and its subsequent decomposition to the volatile and toxic Tl2 O upon heating. Therefore, it may have practical application in the fabrication of high Tc Tl-Ba-Ca-Cu-O superconducting thin-film devices.
Growth of textured films of Bi2Sr2CaCu2O8+x from KCl solution
Authors: Balestrino G. , Paoletti A. , Paroli P. , Romano P.
Published: July 2002

Films of Bi2Sr2CaCu2O8+x have been grown from KCl solution. Gadolinium gallium garnet substrates (1 1 1) oriented have been used. The films resulted highly textured with the c axis perpendicular to the substrate surface. The onset of the resistive transition was 85 K, while zero resistance was obtained at 50 K. This technique is particularly promising in view of the possibility to grow large epitaxial films of both the Bi2Sr2CaCu2O8+x and the Bi2Sr2Ca2Cu3O10+x phases.
Resistive evaporation of superconducting Y-Ba-Cu-O thin films from a single source
Authors: Azoulay Jacob , Goldschmidt Dan
Published: July 2002

A new evaporation method of high-temperature superconducting films, the resistive evaporation from a single source, is reported here for the first time. The source material, inserted into a tungsten boat in a conventional vacuum system, consisted of a pulverized mixture of Cu, YF3, and BaF2. The handling of the source material required only grinding and mixing of the raw materials. Its deposition onto SrTiO3 substrates yielding superconducting films with properties very similar to those obtained in a layer-by-layer resistive evaporation of these materials. In particular, a resistive transition onset at 75 K and zero resistance at ∼40 K, and critical currents of 2000 A/cm2 at ≊10 K have been measured. The broad transition may be attributed to a copper concentration gradient, as measured by Auger depth profiling, or to a residual fluorine-rich phase.
Spectral characteristics of plasma emission during ArF excimer laser ablation of YBa2Cu3O7
Authors: Geyer T. J. , Weimer W. A.
Published: July 2002

In order to characterize the laser ablation process as it relates to the deposition of high Tc superconducting thin films, optical emission spectra produced during ArF excimer laser ablation of the high Tc superconductor YBa2Cu3O7 were studied. The population of excited states within the gas phase blow-off material depends strongly on excimer laser fluence at 193 nm. Ablation using relatively low excimer laser fluences produces a significant fraction of YO in the gas phase. The plasmas derived from ablation of targets made from the high Tc material are similar to plasmas observed from ablation of the individual components BaCO3, CuO, and Y2O3.
Reactivity of Bi-Sr-Ca-Cu-O superconductor surface with indium studied by photoelectron spectroscopy
Authors: Kulkarni Pramada , Mahamuni Shailaja , Chandrachood Madhavi , Mulla I. S. , Nigavekar Arun S. , Sinha A. P. B. , Kulkarni S. K.
Published: July 2002

Interfacial interaction between Bi2Sr2CaCu2O8+δ high Tc superconductor and indium metal has been investigated using photoemission measurements. The results indicate that a sharp interface is possible and the interfacial interaction is restricted to a region of ∼5 Å thickness. In this region, there is a substantial CuO and SrO bond breaking.
Chemical instability of YBa2Cu3O7−δ at high mechanical pressures
Authors: Hendrix B. C. , Abe T. , Borofka J. C. , Tien J. K. , Caulfield T.
Published: July 2002

The high-temperature superconducting phase YBa2Cu3O7−δ has been found to be unstable with respect to and decompose into other metal oxides (Y2BaCuO5, CuO, Cu2O, BaCuO2, and others) at stresses greater than 100 MPa at temperatures from 700 to 950 °C. Hot isostatic pressing and hot uniaxial pressing were used to apply pressure to and to densify porous YBa2Cu3O7−δ. X-ray diffraction and scanning electron microscopy/x-ray spectroscopy were used to determine phase purity before and after pressing. The instability of the superconducting phase should be considered during the development of hot mechanical densification and shape-forming processes, since a limit exists for the time at temperature and pressure that the YBa2Cu3O7−δ phase can withstand before decomposing.
As-grown superconducting Bi-Sr-Ca-Cu-O thin films by coevaporation
Authors: Satoh T. , Yoshitake T. , Miura S. , Fujita J. , Kubo Y. , Igarashi H.
Published: July 2002

Superconducting Bi-Sr-Ca-Cu-O thin films have been prepared on (100) MgO substrates at about 600 °C by coevaporation. The c-axis lattice constant of this system was controlled to the values of 24–43 Å by changing film composition. Superconducting transition temperatures of these films were affected by substrate temperature and by a post-deposition annealing at a low temperature. The highest zero resistance temperature (Tc, zero) of the as-grown Bi2(Sr,Ca)3Cu2Ox film was 79 K. The best Bi2(Sr, Ca)4Cu3Ox film showed an onset temperature of 105 K and Tc, zero zero of 78 K after annealing at 400 °C for 1 h.
Secondary-ion mass spectrometry study of the migration of Si in planar-doped GaAs and Al0.25Ga0.75As
Authors: Lanzillotto A-M. , Santos M. , Shayegan M.
Published: July 2002

We have investigated the effect of substrate temperature Ts during growth by molecular beam epitaxy on the migration of Si atoms in δ-(or planar) doped GaAs and Al0.25Ga0.75As using secondary-ion mass spectrometry (SIMS). Our results for δ-doped GaAs illustrate a measurable spread of Si that increases by ∼80 Å as Ts is varied from 580 to 640 °C. For comparable Ts, the Si spreads further (by ∼350 Å) in δ-doped Al0.25Ga0.75As. For Ts<580 °C, the width of the Si profiles is determined by the resolution of our SIMS measuring technique. Magnetotransport measurements were also performed on these structures in order to determine the spreading of the dopants. The Si migration measured by SIMS is in qualitative agreement with the transport results; however, the SIMS data indicate larger Si areal densities. Two mechanisms, autocompensation and the electron localization by a DX center, are believed to be responsible for the latter observation.