bluuebunny/crossref_metadata_2025 huggingface dataset:
Magnetic transitions and electrical transport in lanthanum strontium manganite: Effects of substitutions and high pressure
Authors: Srinivasan G. , Hanna D.
Published: July 2002

The observation of a substitution-induced ferromagnetic order (Tc=200 K) is reported in antiferromagnetic La0.9Sr0.1MnO3 (TN=130 K) when 0.05 of La is replaced by yttrium. Important results of ferromagnetic resonance (FMR) and high-pressure transport studies are as follows. (i) Data on FMR linewidth and resistivity provide evidence for the presence of microscopic magnetic inhomogeneities, possibly La0.9Sr0.1MnO3. (ii) For zero external pressure, a resistivity peak at 130 K due to intergrain tunneling, a secondary peak close to Tc, and magnetoresistance (MR) on the order of 30% are observed. (iii) At high pressures, the resistivity peaks shift to higher temperatures at the rate of 24–26 K/GPa and MR values decrease. (iv) An internal pressure of 3 GPa due to Y substitution is inferred from high-pressure transport studies.
Optical measurement of large transient mechanical displacements
Authors: Barrière Christophe , Royer Daniel
Published: July 2002

A simple method suitable for extracting large mechanical displacements from the phase modulation of an optical beam reflected from the moving surface is presented. In the MHz range, transient displacements larger than 1 μm have been measured with a standard heterodyne interferometer.
Self-limiting atomic-layer deposition of Si on SiO2 by alternate supply of Si2H6 and SiCl4
Authors: Yokoyama Shin , Ohba Kenji , Nakajima Anri
Published: July 2002

Atomic-layer deposition of Si on SiO2 with a self-limiting growth mode was achieved at substrate temperatures between 355 and 385 °C by means of alternate supply of Si2H6 and SiCl4 gas sources. The growth rate was saturated at 2 ML per cycle at these temperatures and for Si2H6 exposure time over 120 s. The smooth surface (∼0.26 nm in arithmetic average roughness) was obtained under the self-limiting condition irrespective of a film thickness up to 6.5 nm.
Optimum Er concentration for in situ doped GaN visible and infrared luminescence
Authors: Lee D. S. , Heikenfeld J. , Steckl A. J. , Hommerich U. , Seo J. T. , Braud A. , Zavada J.
Published: July 2002

GaN thin films have been doped with varying Er concentrations (0.01–10 at. %) during molecular-beam-epitaxy growth. As expected, the visible and infrared (IR) emissions, from photoluminescence (PL) and electroluminescence (EL), are a strong function of Er concentration. We report on the determination of an optimum Er doping level for PL and EL intensity. Secondary ion mass spectroscopy and Rutherford backscattering measurements showed that the Er concentration in GaN increased exponentially with Er cell temperature. PL and EL intensity of green emission at 537 and 558 nm, due to Er 4f–4f inner shell transitions, exhibited a maximum at ∼1 at. % Er. IR PL intensity at 1.54 μm, due to another Er transition, revealed the same maximum for ∼1 at. % Er concentration. PL lifetime measurements at 537 nm showed that samples with Er concentration <1 at. % had a lifetime of ∼5 μs. For Er concentration ⩾1 at. %, the lifetime decreased rapidly to values below 1 μs. This concentration quenching is believed to be due to a combination of Er cross relaxation and energy transfer to GaN defects, eventually followed by precipitation. This conclusion is supported by x-ray diffraction measurements. As a result, we have determined that the optimum Er doping concentration into GaN is ∼1 at. %.
Coplanar strips for Josephson voltage standard circuits
Authors: Schubert M. , May T. , Wende G. , Fritzsch L. , Meyer H.-G.
Published: July 2002

We present a microwave circuit for Josephson voltage standards. Here, the Josephson junctions are integrated in a microwave transmission line designed as coplanar strips (CPS). The new layout offers the possibility of achieving a higher scale of integration and to considerably simplify the fabrication technology. The characteristic impedance of the CPS is about 50 Ω, and this should be of interest for programmable Josephson voltage standard circuits with SNS or SINIS junctions. To demonstrate the function of the microwave circuit design, conventional 10 V Josephson voltage standard circuits with 17 000 Nb/AlOx/Nb junctions were prepared and tested. Stable Shapiro steps at the 10 V level were generated. Furthermore, arrays of 1400 SINIS junctions in this microwave layout exhibited first-order Shapiro steps.
Twin crystallization in La2/3Ba1/3MnO3+δ oxide and its effect on the saturation magnetization
Authors: Li S. , Zhang Y. B. , Gao W. , Sun C. Q. , Widjaja S. , Hing P.
Published: July 2002

Crystallization of La2/3Ba1/3MnO3+δ oxide, which resulted in a twin-type lamella microstructure, was studied in the present work. Thermomechanical and high-temperature x-ray diffraction analyses revealed that the twin crystallization occurred at 548 K. The saturation magnetization of the twin-grained La2/3Ba1/3MnO3+δ oxide was shown much higher than the nontwin oxides. This is probably due to the easy alignment of the domains by the rearrangement of the dislocations in the twin-grain boundaries.
Two-dimensional assembly of gold nanoparticles with a DNA network template
Authors: Maeda Y. , Tabata H. , Kawai T.
Published: July 2002

Gold nanoparticles have been assembled into two-dimensional complexes using a DNA network template. Atomic force microscope images indicate that gold nanoparticles can be artificially arranged using a DNA molecular template with an average separation of 260 nm. Furthermore, the pattern of the complex can be controlled by changing the concentration of the DNA solution. The results suggest that this method is effective in achieving positional control of nanoscale arrangements for a wide range of applications.
Measurements of geometric enhancement factors for silicon nanopillar cathodes using a scanning tunneling microscope
Authors: Lewis P. A. , Alphenaar B. W. , Ahmed H.
Published: July 2002

High-density silicon nanopillar cathodes were fabricated using a self-assembling colloidal gold etch mask. Scanning tunneling microscopy experiments were performed to locate individual nanopillars and to investigate their field emission properties. Emission characteristics were obtained over a range of fixed separations from the nanopillar apex, allowing the empirical determination of the geometric field enhancement factors from the resulting Fowler–Nordheim plots. The geometric enhancement factors were found to increase dramatically for decreasing anode–cathode separation and the rate of increase is dependent on the nanopillar geometry.
Incorporation kinetics of indium in indium gallium nitride at low temperature
Authors: Storm D. F. , Adelmann C. , Daudin B.
Published: July 2002

Recently, a phenomenological model of the incorporation kinetics of indium in indium gallium nitride was proposed based on published data from InGaN growth at relatively high fluxes and high substrate temperature, TS=650 °C. We present data from growths at much lower fluxes and at a significantly lower substrate temperature, which are also very well described by this model, as well as data for which the model appears less satisfactory. The nature of the indium self-blocking mechanism and the dependence of the model’s fitting parameters on the substrate temperature are discussed.
Trends in bonding configuration at SiC/III–V semiconductor interfaces
Authors: Zheng Jin-Cheng , Wang Hui-Qiong , Wee A. T. S. , Huan C. H. A.
Published: July 2002

The structural and electronic properties of interfaces between β-SiC and III–V semiconductors are studied by first-principles calculations. Favorable bonding configurations are found to form between Si–V and C–III (model A) for BN, AlN, AlP, AlAs, GaN, GaP, GaAs, InN, InP, InAs, and InSb, and Si–III and C–V (model B) for BP, BAs, BSb, AlSb, and GaSb. The relationship between the formation energy difference and lattice constant difference, as well as the charge distribution, for these two models is found. The origin of bonding configurations can be explained in terms of the ionicity of III–V semiconductors, electrostatic effect, charge distribution, and band-structure component.
Antiphase-boundary extension in single-variant CuPt–B ordered Ga0.47In0.53As on InP
Authors: Ahrenkiel S. P. , Hanna M. C.
Published: July 2002

We describe the extension (tilt) of antiphase boundaries (APBs) in single-variant, CuPt–B ordered, epitaxial Ga0.47In0.53As films grown by low-pressure metalorganic chemical vapor deposition on (001) InP miscut 6° toward (111)B. The mean extension angles [measured from (001) toward the (111)B ordering plane] range from approximately 90° to 123° in these samples. We suggest that APBs propagate in step-flow growth mode as advancing step edges traverse the intersections of extant APBs and the vicinal (001) surface.
Transport behavior and critical current densities in MgB2 wires
Authors: Pradhan A. K. , Feng Y. , Zhao Y. , Koshizuka N. , Zhou L. , Zhang P. X. , Liu X. H. , Ji P. , Du S. J. , Liu C. F.
Published: July 2002

We report on the transport and magnetization properties of MgB2 wires fabricated by a powder-in-tube (PIT) technique. Temperature and magnetic-field-dependent resistivity displays a high conductivity and upper critical field Hc2 generally observed in dense samples. The electronic mass anisotropy γ≈1.3±0.15 predicts some texturing in the wire. Our data on transition temperature TC, Hc2, and both magnetic and transport critical current density Jc indicate that MgB2 can be manufactured in a wire form using a PIT technique and required engineering Jc can be achieved on further optimization.
Ultraviolet and visible resonance-enhanced Raman scattering in epitaxial Al1−xInxN thin films
Authors: Naik V. M. , Weber W. H. , Uy D. , Haddad D. , Naik R. , Danylyuk Y. V. , Lukitsch M. J. , Auner G. W. , Rimai L.
Published: July 2002

We report the observation of ultraviolet and visible near-resonance enhanced Raman scattering in epitaxial wurtzite Al1−xInxN (0001) (0⩽x<0.7) thin films. The films (thickness∼150 nm) were grown by plasma source molecular beam epitaxy on sapphire (0001) substrates. A substantial spectral enhancement is seen for Al-rich samples using 244 nm (5.01 eV) radiation due to the closeness of their band gap energy to the excitation energy. On the other hand, samples with x∼0.6 (energy band gap ∼2.5 eV) show significant enhancement with 514.5 nm (2.41 eV) excitation. The A1(LO) and E2 zone center phonons have been observed for all the samples. The A1(LO) phonon frequency shows the expected decrease with increasing x. The E2 mode shows a two-mode behavior supporting the recent theoretical predictions. Due to increased resonance enhancement, strong second- and third-order spectra are seen in some films.
Room-temperature-grown rare-earth-doped GaN luminescent thin films
Authors: Lee D. S. , Steckl A. J.
Published: July 2002

Visible emission has been observed from rare-earth (RE)-doped GaN electroluminescent devices (ELDs) as-grown near room temperature on Si (50–100 °C): red from GaN:Eu, green from GaN:Er, and blue from GaN:Tm. Green emission at 537/558 nm from GaN:Er ELD had a measured brightness of ∼230 cd/m2 at 46 V bias. X-ray diffraction indicates that the low-temperature-grown GaN:Er structure was oriented with the c axis perpendicular to the substrate. Scanning electron and atomic force microscopy indicate that the films had a rough surface and a compact structure consisting of small grains. Electroluminescence intensity of GaN:RE was significantly improved with postgrowth annealing. For GaN:Er films, after 800 °C annealing, the green emission brightness efficiency increased by ∼10×.
Deep levels of tantalum in silicon carbide and incorporation during crystal growth
Authors: Grillenberger J. , Pasold G. , Witthuhn W. , Achtziger N.
Published: July 2002

Band-gap states of tantalum in n-type 6H– and 15R–silicon carbide (SiC) were investigated by deep-level transient spectroscopy (DLTS). The samples were doped with Ta by ion implantation followed by an annealing procedure. DLTS measurements reveal two implantation-induced band-gap states in each polytype. These donor-like levels (located at EC−0.46 eV and EC−0.49 eV in 6H– and EC−0.43 eV and EC−0.46 eV in 15R–SiC) are assigned to Ta occupying inequivalent lattice sites in the SiC crystals. The investigation of 6H–SiC bulk material grown in an atmosphere containing Ta indicates the incorporation of Ta in these crystals during growth.
Near-field spectroscopy with white-light illumination
Authors: Seidel J. , Grafström S. , Loppacher Ch. , Trogisch S. , Schlaphof F. , Eng L. M.
Published: July 2002

We report on near-field optical spectroscopy based on the illumination of the sample with white light from a Xe arc lamp through a tapered optical fiber. The light transmitted through the sample is analyzed with a grating spectrometer in the spectral range between 400 and 750 nm. Our setup provides a unique possibility for recording detailed spectroscopic information within a short acquisition time. Near-field spectra acquired on gold clusters measuring 100 nm in diameter and 20 nm in height reveal a wavelength-dependent transmittivity with both reduced and enhanced light intensities probably stemming from surface plasmon excitation.
Scanning x-ray microtopographs of misfit dislocations at SiGe/Si interfaces
Authors: Mooney P. M. , Jordan-Sweet J. L. , Christiansen S. H.
Published: July 2002

Misfit dislocations at Si1−xGex/Si interfaces have been imaged by x-ray microdiffraction using the 004 diffraction peak of both the Si1−xGex layer and the Si(001) substrate. At the Si1−xGex layer peak, a decrease in the diffracted intensity is found at dislocations, with features as narrow as 4 μm. Similar features are seen using the Si peak; however, the diffracted intensity increases at the dislocations. We discuss the intensity contrast mechanisms and demonstrate that the distortion of the crystal lattice from the dislocations extends throughout the entire epitaxial layer structure.
On the dynamics of periodically-poled lithium niobate formation by off-center Czochralski technique
Authors: Sada C. , Argiolas N. , Bazzan M.
Published: July 2002

The formation mechanisms which might be responsible for the domain structures observed in periodic-poled lithium niobate crystals grown by the off-center Czochralski technique were investigated varying the process parameters. We show that the periodic inverted domains structure does not maintain the polarization over the whole crystal, as commonly accepted. In particular, a “separation plane” parallel to the growth axis exists and divides the crystal in two halves, each with periodic structure but with opposite domain sequences. A possible explanation of the phenomenon is presented invoking both the thermoelectric effect and its correlation to the dynamic temperature fluctuation experienced by the growing crystal.
Magnetostriction and stress of Ta/(Cu/Co90Fe10)×N/Ta multilayer films
Authors: Jo S. , Seigler M.
Published: July 2002

The magnetostriction and stress of multilayer Ta 50 Å/(Cu/Co90Fe10)×N/Ta 50 Å films were investigated. As the number of Cu 15 Å/CoFe 15 Å bilayers was increased, the saturation magnetostriction decreased from −2×10−6 at 2 bilayers to −7×10−6 at 20 bilayers. A change of CoFe thickness from 5 to 25 Å caused a decrease in the magnitude of the magnetostriction of about 1×10−6. Stress measurements showed a decrease in the magnitude of compressive stress from 1 100 MPa to nearly zero as the number of Cu 15 Å/CoFe 15 Å bilayers increased from 2 to 20. The maximum magnetostrictive anisotropy that could be developed due to nonzero magnetostriction and stress is calculated to be 45.0 Oe when the number of Cu 15 Å/CoFe 15 Å bilayers is 10.
Observation of quantum size and alloying effects of single InGaAs quantum dots on GaAs(001) by scanning tunneling spectroscopy
Authors: Yamauchi T. , Ohyama Y. , Matsuba Y. , Tabuchi M. , Nakamura A.
Published: July 2002

We have investigated the morphologies and gap energies of In0.46Ga0.54As quantum dots (QDs) by using scanning tunneling microscopy/spectroscopy. The measured gap energy increases with decreasing dot height in the range of 1.7–6.6 nm. The gap energy has been calculated using a one-dimensional quantum well model taking into account a variation of In composition. Comparison of the observed height dependence and the calculation indicates the one-dimensional quantum confinement of carriers and the In enrichment in a QD.
Real-time measurement of pressure inside field-emission displays
Authors: Chalamala Babu R. , Reuss Robert H. , Dean Kenneth A.
Published: July 2002

We present a method for nondestructive, real-time measurement of pressure inside sealed flat-panel field-emission displays. Analogous to an ion gauge, field-emitted electrons were used to ionize residual gases within the interelectrode region and the resulting ion current was measured. The procedure was calibrated in a vacuum chamber with known gas pressure. Next, we demonstrated the pressure measurement method inside sealed field-emission displays outfitted with spinning rotor gauge attachments for comparison. We found good correlation between the spinning rotor gauge measurement and the pressure measured by our method. In addition, we observed that the operation of the field-emission display resulted in a rapid drop in pressure similar to ion pumping effects observed in other vacuum electron tubes.
Ion-beam-assisted growth of CoPt3 films
Authors: Vasumathi D. , Maranville B. B. , Hellman F.
Published: July 2002

CoPt 3 films are known to exhibit room-temperature perpendicular magnetic anisotropy when grown between 200 °C–400 °C. We have used Ar ion-beam-assisted growth of CoPt3 films in an effort to enhance the anisotropy and also lower the temperature range where it occurs. We present a systematic study of the dependence of magnetic properties of the films on substrate temperature during growth, ion beam energy, and ion to atom arrival ratio, R, at the substrate. We find a significant increase of the anisotropy in films grown at 100 °C compared to the R=0 value resulting in perpendicular anisotropy at moderate growth temperatures. At 250 °C and 400 °C, however, there is a monotonic decrease in anisotropy with R for all energies.
Hillock formation on copper at room temperature by cleaning in ammonia vapor
Authors: Herley P. J. , Greer A. L. , Jones W.
Published: July 2002

Rapidly solidified copper particles are formed by electron-beam decomposition of copper hydride. When exposed to aqueous ammonia vapor at room temperature, the surface of the particles is cleaned and etched, and it develops hillocks and incipient whiskers. Damage of this kind is associated with compressive stress in integrated-circuit metallization. The development of such damage without any elevated temperature appears facilitated by the surface cleaning, and may have implications for device processing. Some links with surface diffusivity and its proposed role in device reliability are also explored.
Nanolithography of silicon: An approach for investigating tip-surface interactions during writing
Authors: Ramsier R. D. , Ralich R. M. , Lyuksyutov S. F.
Published: July 2002

We report an approach for monitoring tip-surface interactions. The approach is based on power spectrum analysis of atomic force microscope (AFM) tip oscillations during scanning probe nanolithography on Si surfaces. A single mode harmonic oscillator model allows us to determine the main characteristics of the AFM tip-surface interaction: the amplitude of oscillation, the resonant frequency, and the damping factor, during the writing process. We measure these quantities in scanning probe anodization versus the bias voltage. By fixing the length of lithographically patterned lines, and the energy deposited into each line, we search for trends which may reveal the major factors controlling the quality of AFM-written nanostructures. Our data are consistent with the concepts that a water meniscus and electrostatic tip-surface interactions dominate contact AFM lithography.
Tunnel-regenerated multiple-active-region light-emitting diodes with high efficiency
Authors: Guo Xia , Shen Guang-Di , Wang Guo-Hong , Zhu Wen-Jun , Du Jin-Yu , Gao Guo , Zou De-Shu , Chen Yong-Hai , Ma Xiao-Yu , Chen Liang-Hui
Published: July 2002

Tunnel-regenerated multiple-active-region (TRMAR) light-emitting diodes (LEDs) with high quantum efficiency and high brightness have been proposed and fabricated. We have proved experimentally that the efficiency of the electrical luminescence and the on-axis luminous intensity of such TRMAR LEDs scaled linearly approximately with the number of the active regions. The on-axis luminous intensity of such TRMAR LEDs with only 3 μm GaP current spreading layer have exceeded 5 cd at 20 mA dc operation under 15° package. The high-quantum-efficiency and high-brightness LEDs under the low injection level were realized.
Fragmentation of cobalt layers in Co/Cu multilayers monitored by magnetic and magnetoresistive measurements
Authors: Spizzo F. , Angeli E. , Bisero D. , Vavassori P. , Ronconi F.
Published: July 2002

We have monitored the structural evolution of Co(tCo)/Cu(4×tCo) multilayers when tCo ranges from 12 to 2 Å. The investigation has been performed by studying their magnetization and giant magnetoresistance, since these properties are complementary in providing information about the structure of the magnetic species into the samples. In particular, in the intermediate range of thickness, we observed no correspondence between magnetic and magnetoresistive behavior. Finally, at sufficiently low thickness, the samples exhibit noninteracting superparamagnetic features. This kind of evolution has been ascribed to the progressive fragmentation of Co layers.
Organic thin-film transistors based on bis(1,2,5-thiadiazolo)-p-quinobis (1,3-dithiole)
Authors: Xue Jiangeng , Forrest Stephen R.
Published: July 2002

Organic thin-film transistors (OTFTs) based on the planar stacking sulfur-containing organic compound, bis(1,2,5-thiadiazolo)-p-quinobis(1,3-dithiole) (BTQBT), and grown using the ultrahigh vacuum process of organic molecular-beam deposition have been demonstrated. Effective room-temperature field-effect mobilities as high as (0.044±0.006) cm2/V s and on/off ratios of 103 are achieved. The mobilities depend on the BTQBT growth rate, the gate voltage, and the operating temperature. Coating the gate dielectric with a self-assembled monolayer of octadecyltrichlorosilane leads to a three-fold increase in mobility compared with that of an uncoated dielectric. The planar BTQBT molecules form stacks whose axes are normal to the substrate surface. As a result of its unusual crystal structure, the field-effect mobility of BTQBT OTFTs is remarkably high compared with other planar stacking molecules.
Improvement of CoSi2 thermal stability by cavity formation
Authors: Alberti A. , La Via F. , Ravesi S. , Rimini E.
Published: July 2002

We propose a method to improve the thermal stability of thin CoSi2 layers on polycrystalline silicon substrates. Nitrogen atoms have been implanted at 55 keV to a dose of 5×1015/cm2 through a 70 nm silicide layer in order to locate the implanted peak near the silicide/silicon interface. The large band of cavities created at the interface has extended the thermal stability window by 125 °C with respect to the standard process. The improvement has been related to the silicide grain-boundary pinning due to the increase of the interface free energy contribution.
Ferroelectric SrBi4Ti4O15 thin films with high polarization grown on an IrO2 layer
Authors: Sohn D. S. , Xianyu W. X. , Lee W. I. , Lee I. , Chung I.
Published: July 2002

Ferroelectric strontium bismuth titanate (SrBi4Ti4O15) thin films with a high remanent polarization were produced by a chemical solution deposition method. Pt and IrO2 layers were used as substrates. It was found that ferroelectric SrBi4Ti4O15 films can be successfully fabricated on IrO2: They demonstrate a saturated hysteresis loop at 5 V with remanent polarization (Pr) of 19 μC/cm2 and coercive field (Ps) of 116 kV/cm. SrBi4Ti4O15 films grown on IrO2 show larger and denser grains and controlled surface morphology. The grains are random oriented, while those of films on Pt are mainly c-axis oriented. It is concluded that the high remanent polarization of the films grown on IrO2 originates from the relatively high concentration of a- and b-axis orientations.
Controlled striped phase formation on ultraflat Si(001) surfaces during diborane exposure
Authors: Nielsen J.-F. , Pelz J. P. , Hibino H. , Hu C.-W. , Tsong I. S. T. , Kouvetakis J.
Published: July 2002

We have used low-energy electron microscopy to study spontaneous step formation in “striped” domains on ultraflat Si(001)-(2×1) surfaces during B2H6 exposure at elevated temperatures. We show that the size and arrangement of striped domains are kinetically limited, and propose that the limiting factor is the supply of diffusing Si surface adatoms. By adding controlled amounts of extra Si to ultraflat terraces, it is possible to foster the formation of very large (>5 μm) single-domain striped regions with adjustable stripe widths.
On the stability of organic field-effect transistor materials
Authors: Schön Jan Hendrik
Published: July 2002

Stability and degradation of transistor performance of devices based on pentacene and α-sexithiophene are investigated. In order to distinguish between effects at grain boundaries and material issues, macroscopic bicrystals were used, where transistors were prepared on a single grain as well as across a single grain boundary. The main reason for performance instabilities is the formation of oxygen-related trapping states at the grain boundary upon exposure to air. However, especially in the case of pentacene, stable hole transport properties are observed.
A self-passivated Cu(Mg) gate electrode for an amorphous silicon thin-film transistor
Authors: Lee W. H. , Cho B. S. , Kang B. J. , Yang H. J. , Lee J. G. , Woo I. K. , Lee S. W. , Jang J. , Chae G. S. , Soh H. S.
Published: July 2002

The feasibility of using Cu(Mg) alloy film as a gate electrode for thin-film transistor (TFT) liquid crystal displays has been investigated. When pure Cu was used as a gate electrode, severe interdiffusion occurred between Cu and the gases SiH4, NH3, and CF4 during plasma-enhanced chemical vapor deposition of a gate dielectric, SiNx, and dry etching of the SiNx. On the other hand, the deposition of a Cu(Mg) alloy film gives rise to the formation of a MgO/Cu bilayer structure with low Cu resistivity, good adhesion to SiO2, higher leakage current density, and excellent passivation capability. A hydrogenated amorphous silicon TFT with a MgO encapsulated Cu gate exhibited a gate voltage swing of 0.91 V/dec. and a threshold voltage of 6.8 V, resulting in a reduction of process steps and better performance.
Thermal stability study of Ni/Ta n-GaN Schottky contacts
Authors: Chen G. L. , Chang F. C. , Shen K. C. , Ou J. , Chen W. H. , Lee M. C. , Chen W. K. , Jou M. J. , Huang C. N.
Published: July 2002

The Schottky behavior of Ni/Ta and Ni contacts on n-GaN was investigated under various annealing conditions by current–voltage measurements. It is found that with the addition of Ta between the Ni layer and the GaN substrate the thermal stability properties of devices can be significantly improved. Experimental results indicate that a high quality Ni/Ta n-GaN Schottky diode with an ideality factor and barrier height of 1.16 and 1.24 eV, respectively, can be obtained under 1 h annealing, at 700 °C.
Self-assembly of epitaxially grown Ge/Si quantum dots enhanced by As ion implantation
Authors: Gaiduk P. I. , Larsen A. Nylandsted , Hansen J. Lundsgaard , Mudryj A. V. , Samtsov M. P. , Demenschenok A. N.
Published: July 2002

Ge/Si islands, grown using molecular-beam epitaxy on a Si/Si0.5Ge0.5 buffer structure modified with in situ ion implantation of 1 keV As+, are investigated by transmission electron microscopy (TEM), photoluminiscence (PL), and Raman spectroscopy. Vertically correlated Ge islands are observed by TEM as a result of the implantation. A 0.8 μm PL peak is detected from the layers of self-assembled Ge quantum dots. A nonhomogeneous distribution of strain around the Ge/Si islands is deduced from the Raman scattering data. This strain is assumed to be responsible for the PL emission.
Imaging the interlayer interactions of multiwall carbon nanotubes using scanning tunneling microscopy and spectroscopy
Authors: Hassanien A. , Mrzel A. , Tokumoto M. , Tománek D.
Published: July 2002

Using atomically-resolved scanning tunneling microscopy and spectroscopy, we probe the nature of interwall interactions within multiwall carbon nanotubes at room temperature. We find that, at low bias voltages, the tunnel current depends strongly on the atomic position, introducing visibility differences between adjacent lattice sites. Since all atoms are equally visible in analogous measurements on single-wall nanotubes, we conclude that these modulations are introduced by the interwall interactions and provide unique information about the stacking nature.
Grazing-angle intersubband absorption and photocurrent in thick n-doped GaAs multiple-quantum-well structures
Authors: Dupont E. , Liu H. C. , Schmidt S. R. , Seilmeier A.
Published: July 2002

We present an experimental study of the absorption and photocurrent in a thick n-doped GaAs/AlGaAs quantum-well infrared photodetector (QWIP). At grazing-angle incidence, we observed peaks that are far detuned from the intersubband resonance. These phenomena can be well explained by the resonant coupling of the light to leaky waveguide modes in the multiple-quantum-well stack. We propose taking advantage of these waveguiding effects for the design of QWIP structures operated at a high temperature.
Responsivities of n-type GaAs/InGaAs/AlGaAs step multiple-quantum-well infrared detectors
Authors: Cheah C. W. , Karunasiri G. , Tan L. S. , Zhou L. F.
Published: July 2002

The responsivities of bound-to-bound transitions in an n-type Al0.15Ga0.85As/GaAs/In0.15Ga0.85As multiple-quantum-well infrared photodetector had been measured, using both the 45° facet edge coupling scheme as well as direct back side illumination. It was found that the transverse electric (TE) mode responsivity was slightly redshifted in the 9 μm spectral region, and its magnitude was about 1%, with respect to the mixed TE and transverse magnetic (TM) mode infrared radiation, when direct back side illumination was employed. These observations were in good agreement with theoretical calculations using a 14-band k.p model. The much larger TE response observed when the 45° facet edge coupling scheme was employed was probably due to mesa edge scattering.
Boron carbonitride nanojunctions
Authors: Guo J. D. , Zhi C. Y. , Bai X. D. , Wang E. G.
Published: July 2002

Boron carbonitride (BCN) nanometric heterojunctions are controllably fabricated by bias-assisted hot-filament chemical vapor deposition with a pause-reactivation two-stage (PRTS) process. Tailored composition revulsion across the nanotube junction is obtained by simply varying the concentration of the gaseous precursor between the two stages of the PRTS process. The critical effect of the plasma power density in the reactivation process on continuous growth of the nanotubes is realized and controlled, leading to successful synthesis of the Y-shaped BCN nanojunctions.
Defect reduction with quantum dots in GaN grown on sapphire substrates by molecular beam epitaxy
Authors: Huang D. , Reshchikov M. A. , Yun F. , King T. , Baski A. A. , Morkoç H.
Published: July 2002

The GaN films grown on buffer layers containing quantum dots by molecular beam epitaxy on sapphire substrates were investigated. The density of the dislocations in the films was determined by wet chemical etching and atomic force microscopy. It was found that the insertion of a set of multiple GaN quantum-dot layers in the buffer layer effectively reduces the density of the dislocations in the epitaxial layers. As compared to the dislocation density of ∼1010 cm−2 in the typical GaN films grown on AlN buffer layer, a density of ∼3×107 cm−2 was demonstrated in the GaN films grown with quantum dot layers.
Elastic properties of hexagonal columnar mesophase self-organized from amphiphilic supramolecular columns
Authors: Jung Hee-Tae , Kim Sang Ouk , Hudson S. D. , Percec V.
Published: July 2002

By direct imaging, we evaluate the relative stiffness of a hexagonal columnar mesophase induced by the self-organization of amphiphilic columnar supramolecules, both quantitatively and qualitatively, based on examination of the planar texture, and of defects contained therein. Quantitative measurement was performed by an analysis of distortions around edge dislocations. The elastic anisotropy varies significantly with the degree of molecular association. The columnar supramolecule containing rigid tapered molecules is stiffer than weakly associated self-assembled macromolecules by an order, resulting in different stability of the mesophase. In addition, defect characteristics are examined to further understand these supramolecular assemblies and their elastic properties. Highly resolved +1/2 disclinations have been observed in the flexible columnar supramolecules.
High carrier injection optical switch based on two-mode interference in SiGe alloy
Authors: Li Baojun , Chua Soo-Jin
Published: July 2002

Based on the two-mode interference principle, the high carrier injection effect, and the free-carrier plasma dispersion effect, a directional coupling active optical switch in SiGe/Si alloy has been fabricated. Its insertion loss and crosstalk were measured to be 2.74 and −15.5 dB, respectively, at the wavelength of 1.3 μm and the total switching current of 110 mA. The fastest response time of the switch is up to 30 ns.
Light coupling mechanism of quantum grid infrared photodetectors
Authors: Mao J. , Majumdar Amlan , Choi K. K. , Tsui D. C. , Leung K. M. , Lin C. H. , Tamir T. , Vawter G. A.
Published: July 2002

Rigorous electromagnetic modeling based on a modal solution of the pertinent boundary-value problem was performed to study the light coupling mechanism in quantum grid infrared photodetectors with lamellar patterns. Our theory shows that vertical field component and absorption quantum efficiency η can be strongly enhanced by judiciously adjusting the width w of individual grid lines. This enhancement is further increased if the top of the grid lines is covered by metal, which behave as a collection of dipole scatterers. We have experimentally verified the dipole scattering characteristics with different w, and found that the variation of η agrees very well with the theory. We also found that, as expected, the conductivity of the metal strips affects η significantly due to internal dissipation.
Control of spectral distribution of Raman sidebands in impulsively stimulated rotational Raman scattering
Authors: Kawano Hiroyuki , Suda Akira , Midorikawa Katsumi
Published: July 2002

We investigate the dependence of the intensity distribution of the impulsively stimulated rotational Raman sidebands from hydrogen gas on the polarization of the prepump and pump pulses. The results show that the energy flow direction of the pump pulse can be confined to either the Stokes side or the anti-Stokes side by changing the polarization of the 60 fs pump pulse. It is found that the use of a linearly polarized pump pulse is the most effective way to generate the largest number of Raman components as possible.
Field-emission properties of multihead silicon cone arrays coated with cesium
Authors: Wong W. K. , Meng F. Y. , Li Q. , Au F. C. K. , Bello I. , Lee S. T.
Published: July 2002

Field emission from multihead silicon (Si) cones was substantially improved by cesium (Cs) coating. Increasing the Cs coating lowered the emission turn-on field (for 10 μA/cm2) from 25 V/μm to a saturated value of 13 V/μm, while the threshold field (for 10 mA/cm2) decreased by 30%, dropping from 27 V/μm for Si cones coated with 1.8 monolayers (ML) of Cs to a saturated value of 19 V/μm with 4.1 ML of Cs. The Cs-treated Si cones could give an emission current density that was three to ten times that delivered by bare Si cones. The work function reduced by a factor of 1.43 for Si cones coated with 4.9 ML of Cs with reference to the untreated Si cones. From the slope of Fowler–Nordheim plot, the field enhancement factor β was found to increase by a factor of 2.02 for Si cones coated with 2.5 ML of Cs and then reduce to 1.57 after the 4.9 ML of Cs deposition. Reduction of the factor β might occur because of a thick Cs layer, which could flatten the sharp cone features. Stability test showed that no current decay was observed at a current density of 0.8 mA/cm2 under a constant applied field of 16 V/μm during the 10 h investigation.
Direct measurement of electron transport in GaN/sapphire interface layer grown by metalorganic chemical vapor deposition
Authors: Kim K. S. , Cheong M. G. , Cho H. K. , Suh E. K. , Lee H. J.
Published: July 2002

Hall effect and capacitance–voltage measurements confirm a conductive thin layer near the GaN/sapphire interface. The temperature-dependent Hall effect of the interface layer was directly measured at temperatures above 100 K, and the results were satisfactorily described by solving the Boltzmann transport equation with various scattering mechanisms. Transport occurs in the conduction band of the layer and is characterized by two dominant scattering mechanisms due to space charge and ionized impurity. The high acceptor density and large space charge effect are related with the dislocations in the interface layer.
Thermochromic effect in YH3−δ and Mg0.1Y0.9H2.9−δ
Authors: Giebels I. A. M. E. , van der Molen S. J. , Griessen R. , Di Vece M.
Published: July 2002

Pure YH3−δ and Mg0.1Y0.9H2.9−δ exhibit large differences in their thermochromic properties. Whereas the transmittance of YH3−δ decreases by 9% to 36%, in Mg0.1Y0.9H2.9−δ it decreases by 64% to 83% between 30 °C and 160 °C. This thermochromic effect is caused by hydrogen desorption upon temperature increase. The main difference between the two systems is that the face-centered-cubic to hexagonal-close-packed phase transition between YH2+δ and YH3−δ is absent in Mg0.1Y0.9Hx, i.e., the YHx stays cubic for all 2⩽x⩽3. Because of this, there are no plateaus in the pressure–composition isotherms of Mg0.1Y0.9Hx and the steepness of the optical transition upon hydrogen desorption increases drastically compared to pure YHx.
Investigation of electron and ion emission from the metal point contact on ferroelectrics
Authors: Sroubek Zdenek
Published: July 2002

Negative ion and electron emission from the point contact, formed by a tungsten tip on lead–zirconium–titanate, has been produced by the application of two successive negative and positive voltage pulses and analyzed by a time-of-flight (TOF) mass spectrometer. No positive ions were detected with the used small pulse amplitudes. The electron emission is strong and in a form of very short (<3.5 ns) pulses. The TOF analysis indicates that both ions and electrons are emitted by Coulomb repulsion of uncompensated surface charges around the W tip.
Increasing the spin-lattice relaxation time of hyperpolarized xenon ice at 4.2 K
Authors: Lang Stephen , Moudrakovski Igor L. , Ratcliffe Christopher I. , Ripmeester John A. , Santyr Giles
Published: July 2002

After cryogenic trapping of hyperpolarized xenon produced by optical pumping, significant increases in the spin-lattice relaxation time (T1) of Xe129 in solid xenon at 4.2 K can be achieved by annealing the solid at an appropriate temperature. Thus, T1 at 4.2 K in a field of 180 G increased from 20.4 to 35 h on warming a sealed sample initially condensed at 77 K in an isopentane bath at 113 K for 10 min. This provides further confirmation that the primary relaxation mechanism for Xe129 at low temperature is cross relaxation to Xe131 and demonstrates that long term storage of hyperpolarized xenon produced using flow polarizers is feasible.
Enhanced magnetization of nanostructured granular Ni/[Cu(II)–C–O] films
Authors: Chow G. M. , Ding J. , Zhang J.
Published: July 2002

Enhanced saturation magnetization was observed in granular nanostructured Ni/[Cu(II)–O–C] films deposited by the nonaqueous, electroless polyol process. The saturation magnetization of Ni–Cu complex films at room temperature was 112 emu/g, which was about two times that of bulk Ni (54.4 emu/g). It is suggested that the enhancement was caused by the magnetic interaction between the Ni particles and the amorphous Cu(II) complex. The observed shifted Curie temperature, large anisotropy field, and strong temperature dependence of saturation magnetization support this argument.
Logarithmically perturbed two dimensional oscillator model of a quantum-dot nanostructure
Authors: Coffey Mark W.
Published: July 2002

We present a logarithmically perturbed two-dimensional isotropic oscillator model of a quantum-dot nanostructure. The logarithmic potential has an ionic line charge physical origin. We estimate the magnitude of this line charge and we obtain the matrix elements required by perturbation theory. The measured ground-state energy depression for a GaAs/AlGaAs/InGaAs heterostructure can be quantitatively accounted for by the model (although not uniquely). We include the effects of nonzero angular momentum states, which play a significant role in determining the dot current–voltage curves. We mention the immediate selection rule imposed by a perturbing potential of rotational symmetry, together with further analytic results.
Pinning effect of a LaFeO3 buffer layer on the magnetization of a La1−xPbxMnO3 layer
Authors: Yamada Y. , Kusumori T. , Muto H.
Published: July 2002

Epitaxial bilayered films of La0.55Pb0.45MnO3/LaFeO3 were fabricated on LaAlO3–Sr2AlTaO6 (111) and (001) single-crystal substrates by a pulsed-laser ablation-deposition method to examine the influence of spin orientation in the antiferromagnetic LaFeO3 buffer layer on the magnetization of manganite. The metal–insulator transition temperature (Tρmax) of the bilayered films with (111) orientation increased remarkably compared with that of the monolayered manganese-oxide films, while the Tρmax of the (001) oriented films increased only slightly. Since the spins on the (111) and (001) planes of LaFeO3 are aligned parallel (uncompensated) and antiparallel, respectively, the remarkable increase of Tρmax observed for the former is mainly ascribed to strong pinning of the manganite layer’s magnetization by the uncompensated spins on the (111) plane. This strong pinning was confirmed by ferromagnetic resonance measurements of the bilayered and monolayered films.
Electric field-assisted dissociation of singlet excitons in tris-(8-hydroxyquinolinato) aluminum (III)
Authors: Szmytkowski J. , Stampor W. , Kalinowski J. , Kafafi Z. H.
Published: July 2002

Results based on the electric-field dependence of electro-modulated fluorescence and steady-state photoconduction in films of tris-(8-hydroxyquinolinato) aluminum (III) (Alq3) are consistent with a model where singlet excitons dissociate into electron–hole pairs through charge transfer state. Dissociation into free carriers or predominantly geminate recombination results from this intermediate state. The quantum efficiency of charge separation is in agreement with predictions based on a model using the three-dimensional Onsager theory of geminate recombination combined with volume (bimolecular) recombination of the photogenerated space charge. This model is used to account for the decrease in the electroluminescence quantum efficiency of Alq3-based organic light-emitting diodes at high electric fields.
Morphology evolution of ZnO(000 1̄) surface during plasma-assisted molecular-beam epitaxy
Authors: Chen Yefan , Ko Hang-Ju , Hong Soon-Ku , Yao Takafumi , Segawa Yusaburo
Published: July 2002

Morphology evolution of ZnO(000 1̄) surface during plasma-assisted molecular-beam epitaxy was studied. In a step-flow growth mode, terraces bounded by anisotropic steps along 〈11 2̄0〉 characterize the surface of ZnO epilayers. Adatoms favorably incorporate at upsteps, which translates hexagonal islands to regular terraces with equalized width. Surface morphology is sensitive to the Zn/O ratio. Oxygen stabilizes mobile Zn adatoms hence oxygen rich conditions result in rough step edges and irregular hexagonal terraces. Steps become smoother with increasing Zn/O ratio until the stoichiometric condition is achieved. As the Zn flux exceeds the stoichiometry, faceted hexagonal pits form on the ZnO(000 1̄) surface. Rather than the lack of surface mobility, the lack of stabilization of adatoms is responsible to the formation of pit.
Phonon excitations and thermodynamic properties of cubic III nitrides
Authors: Talwar D. N.
Published: July 2002

A comprehensive study of phonon excitation and thermodynamic properties of cubic GaN and AlN is reported using a rigid-ion model (RIM) in the quasiharmonic approximation. The data on elastic constants and phonon modes at critical points at ambient and high pressures have allowed us to optimize RIM parameters to obtain accurate values of phonon dispersions, one-phonon density of states, mode Grüneisen parameters γ(q), specific heat Cv(T), and thermal expansion α(T) coefficients. Despite the small softening of TA modes in GaN and AlN, the variations of thermal expansion coefficients are seen much like that of Cv(T), and unlike other III–V compounds it exhibits no negative values at lower temperatures.
Tuning polar anchoring energy through chemical modification of photodimerized surfaces
Authors: Shenoy Devanand , Beresnev Leonid , Holt David , Shashidhar Ranganathan
Published: July 2002

A detailed study of the dependence of the polar anchoring energy of liquid crystals on the chemical structure of photoaligning layers is presented. The monolayer alignment layers consist of different chemical groups at the terminus so that the interaction of the alignment layer with the liquid-crystal molecules is systematically varied. The results demonstrate the ability to tune the polar anchoring energy by chemical modification of the alignment layers.
Metalorganic vapor phase epitaxy-grown GaP/GaAs/GaP and GaAsP/GaAs/GaAsP n-type resonant tunnelling diodes
Authors: Wernersson L.-E. , Borgström M. , Gustafson B. , Gustafsson A. , Pietzonka I. , Pistol M.-E. , Sass T. , Seifert W. , Samuelson L.
Published: July 2002

We have studied GaP/GaAs/GaP and GaAsxP1−x/GaAs/GaAsxP1−x double-barrier resonant tunnelling diodes grown by metalorganic vapor phase epitaxy. We find that GaP tensile strained barriers in GaP/GaAs/GaP diodes may be grown with a barrier thickness below the critical thickness of about 12 monolayers. However, a corrugation of the strained barrier is observed by transmission electron microscopy. This variation may explain the low peak-to-valley ratio of the diodes (about 2). In contrast, GaAsxP1−x/GaAs/GaAsxP1−x resonant tunnelling diodes have been grown with a homogeneous thickness of the barriers, consequently showing a substantially improved electrical performance compared to the GaP diodes with peak-to-valley ratios >5.
Spontaneous vacancy array formation on FeSi2 and CoSi2 formed on Si(100) 2×n surface
Authors: Wang Jun-Zhong , Jia Jin-Feng , Liu Hong , Li Jian-Long , Liu Xi , Xue Qi-Kun
Published: July 2002

Atomic structure of FeSi2 or CoSi2 grown on the Si(100) 2×n surface has been investigated by scanning tunneling microscopy. After annealing the Fe or Co covered Si(100) 2×n substrate at ∼800 °C, an ordered adatom vacancy array appears on the nominal 1×1 surface of the formed FeSi2 or CoSi2 islands, which has not been observed for silicide on the Si(100)–2×1. Upon further annealing to ∼1100 °C, the vacancies coalesce into striped domains along one of the 〈011〉 directions. These nanostructured features are a result of the Ni impurities, and can be a promising template for fabricating nanodot arrays.