bluuebunny/crossref_metadata_2025 huggingface dataset:
A simple model to determine multiplication and noise in avalanche photodiodes
Authors: Ong D. S. , Li K. F. , Rees G. J. , David J. P. R. , Robson P. N.
Published: July 2002

Avalanche multiplication and noise in 1.0, 0.5, 0.1, and 0.05 μm GaAs p+-i-n+ diodes have been calculated for both electron and hole initiated multiplication using a simple model which incorporates a randomly-generated ionization path length (RPL) and a hard-threshold dead space. We find that the mean multiplication obtained using this RPL model is in excellent agreement, even for the shortest structure, with that obtained from an analytical-band structure Monte Carlo (MC) model, which incorporates soft-threshold effects. However, it predicts slightly lower avalanche noise in the shorter devices. This difference results from the narrower ionization path length probability distribution and larger dead space of the hard-threshold RPL model at high electric fields as compared to the more realistic distribution function associated with the relatively sophisticated MC model.
A biaxial film-compensated thin homogeneous cell for reflective liquid crystal display
Authors: Wu Shin-Tson , Wu Chiung-Sheng
Published: July 2002

Computer simulations on the display characteristics of a biaxial film compensated thin homogeneous liquid crystal cell are performed. Results indicate that the biaxial film-compensated reflective displays exhibit a relatively weak wavelength dispersion, wide viewing angle, high contrast ratio, low operation voltage, abundant gray scale, and fast response time. Their potential applications for reflective direct-view and color-sequential projection displays are emphasized.
Oriented Sm2(Co, Cu, Fe, Zr)17 permanent magnets produced by solidification in a magnetic field
Authors: Legrand B. A. , de la Bathie R. Perrier , Tournier R.
Published: July 2002

Oriented samples of Sm2(Co, Cu, Fe, Zr)17 have been produced by solidification from the liquid state in the presence of a magnetic field of several Tesla, their easy magnetization axis being in the direction of the applied magnetic field. This orientation allowed the development of a new process to produce anisotropic permanent magnets without using the metallurgical powder technology. This is the case for the samarium–cobalt magnets presented in this paper. In particular, it is shown that the chemical composition used to produce them might be adapted from those used for sintered Sm2(Co, Cu, Fe, Zr)17 magnets, in order to optimize their magnetic properties.
Microstructure and microwave dielectric properties of epitaxial SrTiO3 films on LaAlO3 substrates
Authors: Ryen L. , Olsson E. , Madsen L. D. , Wang X. , Edvardsson C. N. L. , Jacobsen S. N. , Helmersson U. , Rudner S. , Wernlund L.-D.
Published: July 2002

Sr-deficient and stoichiometric epitaxial (001) SrTiO3 films, deposited on (110)rhombohedral LaAlO3 substrates by rf magnetron sputtering, have been characterized using high-resolution transmission electron microscopy. A subsequent heat treatment in oxygen had a positive influence on the dielectric properties. Sr-deficiency had a large negative impact on the microwave dielectric constant of the films. These changes were correlated to changes in lattice parameters. In all samples, at the film/substrate interface, were misfit dislocations present. The residual elastic strain compressed the SrTiO3 unit cell in the substrate surface plane and expanded it an equal amount in the [001] direction. X-ray diffraction revealed that the tetragonal distortion, due to the mismatch strain, was concentrated to a narrow region closest to the film/substrate interface.
Particle-size effect on the ferroelectric phase transition in PbSc1/2Ta1/2O3 ceramics
Authors: Park Yung , Knowles Kevin M. , Cho Kurn
Published: July 2002

The particle-size dependence on the ferroelectric phase transition in nanocrystalline PbSc1/2Ta1/2O3 ceramics has been investigated for particles in the 0.01–0.16 μm size range. The phase transition was monitored through measurement of the dielectric constant as a function of temperature, variable temperature x-ray diffraction, and differential scanning calorimetry. The c/a tetragonality ratio decreased monotonically with particle size, reducing to unity at 0 °C for particles ⩽0.053 μm in size. The ferroelectric transition temperature Tc decreased gradually and the phase transition became more diffuse in nature as the particle size decreased. Below a mean particle size of 0.02 μm there was no peak in either the dielectric constant or the differential scanning calorimetry curve as a function of temperature, even though variable temperature x-ray diffraction could detect tetragonality indicative of ferroelectric distortions at sufficiently low temperatures down to the smallest crystallite size of 0.01 μm.
Experimental characterization of quasi static and shock wave behavior of porous aluminum
Authors: Bonnan Stephane , Hereil Pierre-Louis , Collombet Francis
Published: July 2002

Experiments of quasi static hydrostatic and uniaxial strain compression, and of shock wave propagation performed on 9% and 17% porous aluminum are presented, analyzed, and compared. Quasi static experiments show the influence of coupling between void collapse and plasticity induced in the matrix on the material macroscopic behavior. The amount of pore compaction appears to be enhanced by the deviatoric stress component present in the uniaxial strain tests and not in the hydrostatic ones. The originality of the plate impact setup and its associated metrology [velocity interferometer system for any reflector (VISAR) interferometry and polyvinylidene fluoride (PVDF) piezoelectric gages] exhibits also the influence of these local physical mechanisms on shock wave propagation in porous aluminum. More, the variations observed between the rise times of shocks seem to point out a preponderance of the dynamic effects (inertia or strain rate) over the material behavior. We observe indeed that the higher the stress in the material, the shorter the shock rise time. This point is confirmed by comparing quasi static and dynamic responses of porous aluminum. Comparison of these experimental results to numerical simulations should be interesting to prove or not this hypothesis.
Surface roughness in InGaAs channels of high electron mobility transistors depending on the growth temperature: Strain induced or due to alloy decomposition
Authors: Peiró F. , Cornet A. , Morante J. R. , Beck M. , Py M. A.
Published: July 2002

InAlAs/InGaAs/InP based high electron mobility transistor devices have been structurally and electrically characterized, using transmission electron microscopy and Raman spectroscopy and measuring Hall mobilities. The InGaAs lattice matched channels, with an In molar fraction of 53%, grown at temperatures lower than 530 °C exhibit alloy decomposition driving an anisotropic InGaAs surface roughness oriented along [11̄0]. Conversely, lattice mismatched channels with an In molar fraction of 75% do not present this lateral decomposition but a strain induced roughness, with higher strength as the channel growth temperature increases beyond 490 °C. In both cases the presence of the roughness implies low and anisotropic Hall mobilities of the two dimensional electron gas.
Characterization of phosphorus doped n-type 6H-silicon carbide epitaxial layers produced by nuclear transmutation doping
Authors: Heissenstein Hans , Peppermueller Christian , Helbig Reinhard
Published: July 2002

Aluminum doped 6H-SiC epitaxial layers (p-type net doping: NA−ND=5.2×1015 cm−3, thickness 3 μm) on a p+ doped 6H-SiC substrate (NA−ND=1018 cm−3) were irradiated with neutrons in a nuclear reactor. The neutron fluences applied were 9.4×1019, 3.5×1020, and 6.4×1020 cm−2. The phosphorus impurity in SiC is produced by a nuclear (n,γ) reaction with the decay of Si31 to P31. The irradiated samples were subsequently annealed at temperatures between 800 and 1850 °C. The annealing behavior was studied by low temperature photoluminescence, Fourier transform infrared spectroscopy, and Hall effect and I–V and C–V measurements. After the annealing process the 6H-SiC p-type epilayer changed to n type resulting in a pn junction within the material. The properties of the pn junction were characterized.
Evolution of strain-dependent transport properties in ultrathin La0.67Sr0.33MnO3 films
Authors: Ju H. L. , Krishnan Kannan M. , Lederman D.
Published: July 2002

We report on a systematic investigation of strain-induced lattice distortion effects on the crystal structure and transport properties of as-grown and postannealed ultrathin La0.67Sr0.33MnO3 epitaxial films grown on LaAlO3 (001) substrates by pulsed laser deposition. The resistivity of the as-grown films is critically dependent on the thickness, i.e., 100 Å thick films show insulating behavior, 300 Å thick films show metal-insulator transitions, and 500 Å thick films show metallic behavior. However, all the annealed films show identical metallic behavior. Conventional θ–2θ x-ray scans with momentum transfer (q) along the [001] direction, and θ–2θ scans of the (103) and (113) peaks, with q having a component perpendicular to the growth direction, were used to measure the out-of-plane and in-plane lattice parameters of the sample. Φ scans of the (103) and (113) peaks revealed a fourfold symmetry which is consistent with a tetragonal unit cell. These data conclusively show that significantly elongated tetragonal structures (c/a=1.02–1.04) are insulating whilst films with cubic unit cells, relaxed either due to annealing or as a function of thickness, have metallic characteristics.
Correlation between rub-induced grooves in a polyimide-treated substrate and microstructure of rubbing fiber: An atomic force microscopy study
Authors: Mahajan Milind P. , Rosenblatt Charles
Published: July 2002

Rubbed polyimide surfaces, which are used for liquid crystal alignment, generally exhibit microscopic grooves which lie parallel to the rubbing direction. Using atomic force microscopy we examined both the grooves and the fibers that create the grooves. We find that for a wide range of rubbing strengths, the microstructure of the grooves, as determined by their radii of curvature, correlates well with the microscopic topography of the fibers. This result indicates that the rubbing-induced topography depends on not only the characteristic rubbing strength, but on the structure of the rubbing fiber as well.
The study of deep levels in InSb and HgCdTe by transient gratings at two photon excitation
Authors: Deb Paramita
Published: July 2002

Transient grating measurements on n-InSb and n-HgCdTe are described. A simple yet sensitive technique has been devised, that locates the deep levels within the forbidden gap with an accuracy of about 0.2 meV. The results are in agreement with the results of other techniques and provides a direct method of detecting the presence of impurities or defects.
Out-diffusion of Zn from Si: A method to study vacancy properties in Si
Authors: Giese A. , Bracht H. , Stolwijk N. A. , Walton J. T.
Published: July 2002

We report out-diffusion experiments of Zn from homogeneously Zn-doped Si samples which were performed at 1107 °C. Depth distributions of Zn recorded by spreading-resistance profiling are accurately described on the basis of simultaneous contributions of the kick-out and dissociative diffusion mechanism. Analysis of the profiles reveals that Zn out-diffusion is mainly mediated by the dissociative mechanism. Fitting of Zn profiles yields data for the transport capacity of vacancies CVeqDV in Si and their thermal equilibrium concentration CVeq.
Ohmic contact formation mechanism of the PdGeAu system on n-type GaSb grown by molecular beam epitaxy
Authors: Vogt A. , Simon A. , Hartnagel H. L. , Schikora J. , Buschmann V. , Rodewald M. , Fuess H. , Fascko S. , Koerdt C. , Kurz H.
Published: July 2002

We have fabricated Pd/Ge/Au/Pt/Au ohmic contacts on n-type GaSb grown by molecular beam epitaxy. The annealed contacts show specific contact resistivities which are as low as 4.9×10−6 Ω−cm2 at an annealing temperature of 300 °C for 45 s. The resistivity is very sensitive to the annealing conditions and the thickness of the constituents. The surface remains smooth after annealing. Microstructure analysis with cross-sectional transmission electron microscopy and sputtered neutral mass spectrometry elucidates the solid phase reactions and interdiffusion of the constituents. The formation of a PdGe phase and the diffusion of Ge into GaSb seem to be the decisive and annealing-dependent processes leading to the ohmic behavior.
Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapor deposition
Authors: Costa J. , Roura P. , Morante J. R. , Bertran E.
Published: July 2002

Previous results concerning radiative emission under laser irradiation of silicon nanopowder are reinterpreted in terms of thermal emission. A model is developed that considers the particles in the powder as independent, so under vacuum the only dissipation mechanism is thermal radiation. The supralinear dependence observed between the intensity of the emitted radiation and laser power is predicted by the model, as is the exponential quenching when the gas pressure around the sample increases. The analysis allows us to determine the sample temperature. The local heating of the sample has been assessed independently by the position of the transverse optical Raman mode. Finally, it is suggested that the photoluminescence observed in porous silicon and similar materials could, in some cases, be blackbody radiation.
Photoluminescence investigation of GaInP/GaAs multiple quantum wells grown on (001) and (311) B GaAs surfaces by gas source molecular beam epitaxy
Authors: Li Xiaobing , Sun Dianzhao , Dong Jianrong , Li Jianping , Kong Meiying , Yoon S. F.
Published: July 2002

Photoluminescence (PL) investigation was carried out on GaInP/GaAs multiple quantum wells structures grown on (001) and (311) B surfaces of GaAs by gas source molecular beam epitaxy. Superlattice structures of GaAs/GaInP grown on (001) GaAs substrate were also studied in comparison. Deep-level luminescence was seen to dominate the PL spectra from the quantum wells and superlattice structures that were grown on (001) GaAs substrate. In contrast, superior optical properties were exhibited in the same structures grown on (311) B GaAs surfaces. The results suggested that GaAs/GaInP quantum well structures on (311) B oriented substrates could efficiently suppress the deep-level emissions, result in narrower PL peaks indicating smooth interfaces.
On the influence of the gas velocity on dissociation degree and gas temperature in a flowing microwave hydrogen discharge
Authors: Chabert P. , Rousseau A. , Gousset G. , Leprince P.
Published: July 2002

We report on the influence of the gas velocity on the discharge equilibrium in a low-pressure (1 Torr) microwave driven hydrogen plasma. We show that the gas velocity has a great influence on the dissociation degree, the key parameter being the residence time of the molecules in the plasma with respect to their effective dissociation time. The power balance (plasma length, electric field…) is substantially modified by the variations of the dissociation degree induced by the change in gas velocity. On the contrary, the gas temperature, determined by Doppler broadening of H atom Balmer lines, is not directly dependent on the gas velocity. Simple calculations confirm that the gas temperature is determined by local parameters and that heat is not axially transported. Surprisingly, dissociation of the H2 molecules does not seem to play any significant role for the heating of the gas.
Low temperature laser cooling with a rare-earth doped glass
Authors: Lamouche G. , Lavallard P. , Suris R. , Grousson R.
Published: July 2002

A theoretical study of laser cooling at low temperature by anti-Stokes luminescence in a rare-earth doped glass is performed. A model is developed to evaluate the absorption and emission spectra of rare-earth ions in a glass matrix. This model allows the evaluation of the inhomogeneously broadened spectra at any temperature. It takes into account the saturation effects that occur at high excitation. The model is used to evaluate the cooling capability at low temperature of a ytterbium-doped fluorozirconate glass, the latter having been proposed in the literature as a good candidate for the cooling element of a cryocooler. Results are compared with previous estimations, confirming that one could expect a useful cooling efficiency from this material, but with smaller performances than previously estimated. Limitations to the cooling process are discussed. The reabsorption of luminescence is identified as one of the main limitations to the performance of a potential cryocooler.
New bistable twisted nematic liquid crystal displays
Authors: Xie Z. L. , Kwok H. S.
Published: July 2002

We have developed new bistable twisted nematic (BTN) displays that operate between the −π/2 and 3π/2 twist states and between the π/2 and 5π/2 twist states. Together with the (0,2π) BTN, this forms a set of all possible transmittive BTNs. Experimentally, it was confirmed that several switching wave forms could be used to switch the BTN from one state to another. Voltages below 10 V are sufficient to reset and switch these BTNs. The effect of the d/P0 ratio on the bistable switching characteristics is also investigated.
Different paths to tunability in III–V quantum dots
Authors: Leon R. , Lobo C. , Clark A. , Bozek R. , Wysmolek A. , Kurpiewski A. , Kaminska M.
Published: July 2002

Tunability in the concentration and average dimensions of self-forming semiconductor quantum dots (QDs) has been attained. Three of the approaches examined here are: variations with temperature, group V partial pressure and with substrate miscut angle. Thermally activated group III adatom mobilities result in larger diameters and lower concentrations with increasing deposition temperatures. These variations are presented for InGaAs/GaAs and AlInAs/AlGaAs, where striking differences were seen. Tunability in the InGaAs/GaAs QD concentration was also obtained in metalorganic chemical vapor deposition by varying the arsine flow. The latter gave widely varying concentrations and similar sizes. Substrate orientation was found to also be a key factor in island nucleation: Changes in vicinal orientation near (100) can be used to exploit the preferential step edge nucleation at mono and multi-atomic steps, so varying miscut angle (θm) can be used to change island densities and sizes. Anisotropies in island nucleation producing n-dot strings aligned with multiatomic step edges are observed for θm⩾0.75° and up to 2°. Quantum mechanical coupling from such island strings result in non-Gaussian shapes in the inhomogeneously broadened photoluminescence peaks. The effects of some of the other morphological differences presented here on the luminescence emission from QD ground states is discussed for InGaAs/GaAs QDs.
Monitoring of the ion energy and current density at the surface of films grown by excimer laser ablation
Authors: Franghiadakis Y. , Fotakis C. , Tzanetakis P.
Published: July 2002

A simple and easy to implement, ion time-of-flight (TOF), detection system has been developed and used to monitor the ions ejected during pulsed excimer laser ablation of solid and molten Si and Ge targets. The setup employs a Faraday cup (FC) detector with a high gain-bandwidth preamplifier and an adjustable voltage electrostatic barrier. The FC is capable of very long time, undisturbed, operation even with significant deposition of material on it. The analysis of the TOF ion signal and its modification by the barrier potential yields valuable quantitative information about the ion flux and kinetic energy at the surface of the growing film. The technique is capable of resolving atomic/cluster components of different charge to mass ratios. High ion fractions and ion energies are observed, in all cases studied, when the laser fluence is clearly above the ablation threshold.
Elemental characterization of electroluminescent SrS:Ce thin films
Authors: Li Wei-Min , Ritala Mikko , Leskelä Markku , Lappalainen Reijo , Jokinen Janne , Soininen Erkki , Hüttl Bernd , Nykänen Erja , Niinistö Lauri
Published: July 2002

Impurities in SrS:Ce and SrS:Ce,Na thin films prepared by atomic layer epitaxy (ALE) and SrS:Ce,Mn,Cl prepared by reactive evaporation have been studied by combining various ion beam techniques, viz. Rutherford backscattering spectroscopy, nuclear resonance broadening, proton induced x-ray emission, particle induced gamma-ray emission, deuteron induced reactions and the two variants of elastic recoil detection analysis (ERDA), i.e., ERDA with an absorber and time-of-flight (TOF)-ERDA. In both ALE and reactively evaporated films the SrS or (Sr+Mn)/S ratios were close to unity and Ce concentrations varied from 0.1–0.4 at. %. The major impurities in the SrS bulk were found to be H, C, and O. The concentrations of these impurities in the state-of-the-art ALE SrS:Ce films were ∼0.3 at. % for H, <0.2 at. % for C and ∼0.1 at. % for O, whereas 0.6–1.8 at. % H, 0.2–0.7 at. % C and 0.5–1.8 at. % O were found from the films prepared by the reactive evaporation. In the ALE SrS:Ce samples high electroluminescence (EL) brightness was found to correlate with low overall impurity content. However, though reactively evaporated SrS:Ce,Mn,Cl films contain higher level of impurities, they usually have better EL brightness than the ALE made SrS:Ce films. In many ALE SrS:Ce,Na samples Na was found to concentrate at the interface between upper insulator and the SrS:Ce,Na film. The EL performances of the corresponding thin-film electroluminescent stacks were poor.
A luminescence versus temperature study of fresh and oxidized porous silicon layers under different atmospheres
Authors: Létant S. , Vial J. C.
Published: July 2002

Continuous and time resolved photoluminescence measurements of respectively oxidized and fresh porous silicon layers versus the temperature have been performed. Oxidized samples show an increase of both photoluminescence intensities and lifetimes when the temperature decreases, usually attributed to the reduction of the recombination rate on nonradiative centers at low temperatures. A drastically opposite behavior is observed on fresh samples which show correlated decreases of the photoluminescence intensities and of the lifetimes. Between the ambient and the liquid nitrogen temperatures, the luminescence signal is divided by 70. Moreover, the quenching is observed under nitrogen, helium and vacuum, but is totally inefficient when a film of pentane recovers the surface of the layer. We explain those experimental results by the nonradiative energy transfer from confined photocreated carriers in the silicon crystallites to Si-H surface vibrations which cannot happen when the oscillators are caged by pentane or isolated by the oxide layer.
Strain effect and photoluminescence of ZnS epilayers grown on GaP(100) substrates by hot-wall epitaxy
Authors: Nam Sungun , O Byungsung , Lee Ki-Seon , Choi Yong Dae , Kim Chang-Soo
Published: July 2002

ZnS epilayers were grown on GaP (100) substrates by hot-wall epitaxy and their properties were investigated. The films grown under the optimum conditions show high quality. The full width at half-maximum (FWHM) of the double-crystal rocking curves and the lattice parameter show that the epilayers grow coherently up to 0.035 μm. And it is also found that in ZnS epilayers thicker than 1.5 μm the lattice mismatch induced strain is almost relaxed. The strong near-band excitonic peaks and the hardly observable deep level emission in photoluminescence spectra indicate the high quality of films. The room temperature free exciton line was observed at 3.6682 eV with a FWHM of 40 meV.
Cathodoluminescence study of disordering of GaAs/AlGaAs quantum wells using an AlAs native oxide and thermal annealing technique
Authors: Zhang X. , Rich D. H. , Lin C.-K. , Dapkus P. D.
Published: July 2002

GaAs/AlGaAs quantum wells (QWs), selectively disordered using an AlAs native oxide and thermal annealing technique, were studied using spectrally, spatially, and temporally resolved cathodoluminescence (CL). The spectral shift of the QW luminescence was determined as a function of annealing temperature in the oxide and nonoxide regions. Time-resolved CL was used to assess the impact of defects and the built-in field near the oxide/nonoxide transition region on the carrier lifetime. Spatially resolved CL spectroscopy was used to examine changes in the QW luminescence intensity near the transition region. The carrier lifetime was found to increase in the transition region, owing to the enhanced spatial separation of electrons and holes in this region. From CL images and line scans of three samples annealed at different temperatures, a partial dead region is found between oxide and nonoxide regions. Details of the native oxide formation are discussed.
Modeling of nucleation and growth of voids in silicon
Authors: Plekhanov P. S. , Gösele U. M. , Tan T. Y.
Published: July 2002

During Si crystal growth, nucleation and growth of voids and vacancy-type dislocation loops under Si vacancy supersaturation conditions have been modeled. From nucleation barrier calculations, it is shown that voids can be nucleated, but not dislocation loops. The homogeneous nucleation rate of voids has been calculated for different temperatures by assuming different enthalpy of Si vacancy formation. The void growth process has been calculated using a moving boundary formulation. Matching the results of void nucleation and growth simulations and taking into account the competition between the two processes, limited time available, and the crystal cooling rate, it is shown that the experimentally observed void density and size data can be explained if the Si vacancy formation enthalpy is in the range of 2.8–3.4 eV and the void nucleation temperature is in the range of 970–1060 °C.
Observations of the Hugoniot curves for glasses as measured by embedded stress gauges
Authors: Millett Jeremy , Bourne Neil , Rosenberg Zvi
Published: July 2002

The shock Hugoniots of soda-lime glass and the lead-filled glass, DEDF have been measured by embedding manganin stress gauges between tiles and impacting them with copper flyer plates. We have observed, however, that when the impact conditions are reversed (that is, glass flyer plates on copper targets), above the soda-lime Hugoniot elastic limit (HEL) of ∼6 GPa, and the DEDF HEL of ∼4.5 GPa, the measured stress is significantly higher than would be expected. We have repeated these experiments in mild steel, both above and below the α–ε phase transition, and no differences have been noted. We have also used this technique to measure the Hugoniot of copper, and compared the results to accepted data. The agreement is very good, and thus validates our results for glass and steel. Our observations should be of interest to theoreticians in the design of constitutive models for glasses.
Alternating current conductivity and dielectric dispersion in copper-silica nanocomposites synthesized by electrodeposition
Authors: Banerjee S. , Chakravorty D.
Published: July 2002

Composites of nanometer-sized copper metal with diameters varying from 3.2 to 11.4 nm dispersed in a silica gel medium were synthesized by an electrodeposition method. The ac conductivity and dielectric dispersion of these nanocomposites were measured over the frequency range 0.2 kHz–1.5 MHz at temperatures varying from 150 to 300 K. The ac conductivity showed a frequency dependence of ∝ωn where ω is the angular frequency and n∼0.62 the latter being temperature independent. The quantum mechanical tunneling model was used to explain this result. The dielectric modulus data were analyzed on the basis of a stretched exponential relaxation function. The values of the exponent β as extracted from such analysis were found to be in the range 0.31–0.42 and were temperature independent for different gel compositions. The activation energies were estimated from the temperature variation of frequency at which the imaginary part of the dielectric modulus was maximum. The activation energy value ∼0.24 eV could be explained satisfactorily on the basis of an electron tunneling mechanism.
Ohmic Pd/Zn/Au/LaB6/Au contacts on p-type In0.53Ga0.47As: Electrical and metallurgical properties
Authors: Ressel P. , Leech P. W. , Veit P. , Nebauer E. , Klein A. , Kuphal E. , Reeves G. K. , Hartnagel H. L.
Published: July 2002

The development of a shallow and low-resistive contact on moderately doped (p≈5×1018 cm−3) In0.53Ga0.47As is demonstrated. By reducing the layer thicknesses of a conventional Pd/Zn/Pd/Au scheme to a minimum and coupling this system to an outer Au layer via an amorphous LaB6 diffusion barrier, contact resistivities ⩽1×10−6 Ω cm2 were achieved only slightly exceeding that of the conventional scheme (2–4×10−7 Ω cm2). The contact reaction depth, however, could be reduced from several hundred to well below 100 nm, since the LaB6 barrier effectively prevents the outer Au layer from reaction with the semiconductor during contact formation. The influence of Zn content on electrical and metallurgical properties has been studied by varying it over orders of magnitude using both implantation and evaporation as a means of introducing Zn into the metallization. Implanted contacts with low Zn content annealed at 375 °C exhibit a reaction depth as low as 55 nm with the Zn diffusion depth practically coinciding with the reaction depth. If Zn is evaporated, the reaction depth is enhanced and the Zn diffusion depth exceeds the reaction depth noticeably. Secondary ion mass spectrometry demonstrated the onset of ohmic behavior to be correlated with the buildup of high interfacial Zn concentrations suggesting that these contacts conform to the standard model of interfacial doping. Simultaneously a solution phase emerges, reported here first, namely (Pd,Au)12(Ga,In)5As2 based on hexagonal Pd12Ga5As2 as revealed by cross-sectional transmission electron microscopy and x-ray diffraction analysis. Contacts with evaporated Zn exhibit stable resistivity during thermal stressing at 400 °C for 24 h. The stability loss for longer times is correlated with the appearance of (Au,Pd)9In4, a solution phase based on cubic Au9In4, evolving from indiffused Au and replacing increasingly the low-resistive (Pd,Au)12(Ga,In)5As2/p+-In0.53Ga0.47As junction areas.
Electronic structure of layered silver carboxylates
Authors: Chadha Monica , Dunnigan Michael E. , Sahyun M. R. V. , Ishida Takuzo
Published: July 2002

A series of silver salts of straight-chain organic monoacids, [CH3(CH2)nCO2Ag]2, and diacids, Ag[O2C(CH2)nCO2Ag]m, have been prepared. Powder deposits of these layered organic dielectric sandwich-type solids have been characterized by reflection spectroscopy. From analysis of the absorption edges we infer in all cases an indirect optical band gap. For interlayer spacings, d⩽30 Å (n⩽10), superlattice formation occurs with concomitant redshift in the absorption edge, by 0.135 eV (∼10 nm) for the limiting monoacid case, n=0. These results have been rationalized by tight-binding band-structure calculations from the literature and by a fragment orbital in the solid calculation at the extended Hückel level of approximation.
Magnetic properties of the normal state of YBa2Cu3−xLixOz
Authors: Nicolas-Francillon M. , Maury F. , Ollitrault-Fichet R. , Nanot M. , Legeay P.
Published: July 2002

Resistivity and magnetization measurements were carried out on two series of YBa2Cu3−xLixOy compounds annealed under either air or oxygen atmosphere, with 0<xLi<0.6. A decrease of 1.14 oxygen. Li mol−1 is observed. All the samples are orthorhombic and remain in the metallic range. The materials obey a Curie or Curie–Weiss law. As long as xLi<0.30, lithium is assumed to substitute for copper of the chains; the dρ/dT slope is positive and Tc decreases very slowly in a quasiplateau. If the observed effective magnetic moment is assumed to be carried by lithium, this moment is constant and equal to 1.1 μB/Li+; if it is assumed to be carried by all the copper ions, it increases at a rate of ≅1.2 μB/Li [i.e., 1 μB/lost O−−]. When xLi>0.30, lithium probably occupies a second site; the resistivity is characterized by a metal-like behavior above 100 K and by a logarithmic increase below 100 K; the effective moment increases up to peff/Li=1.6 μB or peff/Cu=0.8 μB. The spin susceptibility decreases as a function of the doping level showing the gradual transition from a YBCO7-type behavior towards a still metallic but rather YBCO6.5-like material. The Cu(1) substitution by Li leads to a gradual decoupling of the planes and to a weak localization of the charge carriers, in that case, the magnetic moments induced by the doping do not seem to have any influence on the Tc behavior at least when xLi<0.30.
Magnetic anisotropy and microstructure in molecular beam epitaxial FePt (110)/MgO (110)
Authors: Farrow R. F. C. , Weller D. , Marks R. F. , Toney M. F. , Smith David J. , McCartney M. R.
Published: July 2002

We compare the growth, structural, and magnetic properties of FexPt1−x(x≃0.6) films grown by molecular beam epitaxy on MgO (110) substrates held at 300 and 500 °C. Growth at both temperatures yields nearly single orientation epitaxial films with FePt, Pt [001]//MgO[001] and FePt, Pt (110)//MgO (110). For growth at 300 °C, chemical ordering to the L10 (CuAu I) tetragonal phase is incomplete (long-range order parameter, S=0.38) in contrast with growth at 500 °C for which ordering is nearly complete (S=0.81). Despite incomplete chemical ordering, a large two-fold, in-plane magnetic anisotropy (Ku=1.5×107 erg/cm3) is present for the film grown at 300 °C. Transmission electron microscopy for the 300 °C film confirms epitaxial growth with few extended defects but also reveals that chemical ordering is distributed inhomogeneously within the film. The direction of ordering is generally uniform (c axis along MgO [001]) but variations in the degree of ordering are evident on a length scale of 1–10 nm.
Resonant tunneling spin valve: A novel magnetoelectronics device
Authors: Bruno P. , Wunderlich J.
Published: July 2002

The concept of a novel magnetoelectronics device, the resonant tunneling spin valve, which combines the spin-valve tunneling junction and the double barrier resonant tunneling diode, is proposed. Model calculations are performed in order to investigate the spin-valve effect in this structure.
Phonon assisted intersubband transitions in step quantum well structures
Authors: Teng H. B. , Sun J. P. , Haddad G. I. , Stroscio Michael A. , Yu SeGi , Kim K. W.
Published: July 2002

We evaluate effects of heterointerfaces on optical phonon modes and phonon assisted electron intersubband transition rates in step quantum well structures for intersubband lasers. Various phonon modes and electron–phonon interaction Hamiltonians, including the interface modes, confined longitudinal-optical modes, and half space modes in the quantum well structures are calculated based on the macroscopic dielectric continuum model and microscopic analysis. The transfer matrix method is used to calculate the interface modes. The intersubband transition rates due to electron–phonon scattering by these phonon modes are evaluated using Fermi’s golden rule, with the electron wave functions obtained by solving the Schrödinger equation for the heterostructures under investigation. Our results show that, compared with the transition rates in the same structures calculated using the bulk phonon modes and the bulk Fröhlich interaction Hamiltonian, the electron interface–phonon interactions give significantly larger transition rates up to an order of magnitude. Therefore, the effects of localized phonon modes, especially the interface modes, must be taken into consideration for optimal device design.
Dependence of (0001) GaN/AlN valence band discontinuity on growth temperature and surface reconstruction
Authors: King S. W. , Ronning C. , Davis R. F. , Benjamin M. C. , Nemanich R. J.
Published: July 2002

X ray and ultraviolet photoelectron spectroscopies have been used to determine the heterojunction valence band discontinuity at the (0001) GaN/AlN interface. Type I discontinuity values of 0.5±0.2 eV were determined for GaN grown on AlN at 650 °C and 0.8±0.2 eV for GaN grown on AlN at 800 °C. These values are critically evaluated with respect to film quality, the results of other experimental studies, and theory.
A method to interpret micro-Raman experiments made to measure nonuniform stresses: Application to local oxidation of silicon structures
Authors: Pinardi K. , Jain S. C. , Willander M. , Atkinson A. , Maes H. E. , Van Overstraeten R.
Published: July 2002

A method is described to calculate the Raman spectrum from a nonuniformly strained sample taking into account the effects that arise due to finite depth of penetration and diameter of the laser beam. Both the parallel and the focused beams are considered. The case of stress in a Si substrate decaying monotonically with depth z (rapidly near the interface and slowly at larger depths) is considered in detail. The predicted Raman shifts are found to be sensitive to both the distribution of stress and to the absorption coefficient α for the laser light wavelength used. It is found that light scattered from distances much larger than 1/α still contribute significantly to the observed Raman spectrum. The observed shift in the peak of the spectrum does not correspond to the stress close to the interface. If the stress decays more rapidly than the light intensity, the Raman line that originates from the unstrained lower part of the substrate dominates. For transparent material (α=0) and unfocused beam the Raman spectrum consists of only the unstrained Si line; the contribution to Raman line from the strained interface region is completely masked. For measurements of stresses near the interface short wavelength light with an absorption depth of 5–10 nm is recommended. The calculated and observed Raman shifts in a local oxidation of silicon (a processing technique for isolation) with polysilicon buffer between the nitride stripe and the Si substrate are compared. The agreement between the calculated and the observed Raman shifts is very good. The salient points of our approach which enabled us to obtain this agreement are: We took into account the effects of laser beam width, penetration depth, and focusing; we included the stresses in the polysilicon layer and near the polysilicon/silicon interface, and we included contributions from large depths.
Nitrogen engineering of ultrathin oxynitrides by a thermal NO/O2/NO process
Authors: Gusev E. P. , Lu H. C. , Garfunkel E. , Gustafsson T. , Green M. L. , Brasen D. , Lennard W. N.
Published: July 2002

The paper discusses nitrogen engineering of ultrathin (<5 nm) oxynitride gate dielectrics. The dielectric film that we have aimed for has two nitrogen enhanced layers: one at the SiO2/polysilicon interface to retard boron diffusion from the gate, and a smaller one peaked at the Si/SiO2 interface to increase the hot electron degradation resistance. We were able to produce this dielectric by a thermal (NO/O2/NO) process, aided by an understanding of the kinetics and thermodynamics of nitrogen incorporation in SiO2.
Stress and thermomechanical properties of amorphous hydrogenated germanium thin films deposited by glow discharge
Authors: Marques F. C. , Wickboldt P. , Pang D. , Chen J. H. , Paul W.
Published: July 2002

Stress measurements of a-Ge:H thin films deposited by rf glow discharge using a large variety of deposition conditions are reported. It was observed that the stress of the films is strongly related to their structure. Tensile films are usually porous or have many defects, while compressive films are usually homogeneous. High quality films are always compressive. A strong correlation of the stress in the films with the unbonded hydrogen concentration was observed, which may explain the origin of the compressive stress. There was no systematic or consistent link between the stress and the bonded hydrogen content or the deposition rate. The thermal expansion coefficient and the elastic constant were determined for high quality films.
Ballistic electron emission microscopy study of barrier height inhomogeneities introduced in Au/n-Si Schottky contacts by a HF pretreatment
Authors: Detavernier C. , Van Meirhaeghe R. L. , Donaton R. , Maex K. , Cardon F.
Published: July 2002

The distribution of Schottky barrier heights over the contact area in Au/n-Si diodes was determined by ballistic electron emission microscopy. For samples on which an aqueous HF pretreatment of the Si substrate was applied, the histogram contains several high barrier Gaussian distribution components. After a short rinse, in de-ionized water or methanol, it was mainly the most important lower Gaussian component which was left. Using additional x-ray photoemission spectroscopy and atomic force microscopy measurements allowed us to propose a model, wherein negatively charged species containing F at the interface, are thought to be responsible for the high barrier Gaussian components.
Magnetic domains and magnetization reversal in epitaxial Fe layers patterned by the atomic saw method
Authors: Jaffrès H. , Ressier L. , Peyrade J. P. , Fert A. R. , Gogol P. , Thiaville A. , Schuhl A. , Nguyen Van Dau F.
Published: July 2002

We present a study of the magnetic domains structure and the magnetization reversal in systems of stripes and dots patterned on Fe films by the atomic saw method. Continuous epitaxial 50 and 20 Å Fe films have been grown by molecular beam epitaxy deposition method, then by applying strain on MgO monocrystalline substrate and controlling the dislocation slipping, micronic iron stripes and dots have been obtained. For the system of 1.4 μm wide stripes characterized by a strong uniaxial magnetic anisotropy (Ha=1500 Oe) resulting from a uniaxial relaxation of Fe lattice parameter, large magnetic domains have been observed by Kerr microscopy imaging. This structure is not directly correlated to the geometry of ribbons. It reveals the strong influence of the mutual dipolar coupling leading to a quasi-collective magnetization reversal. In contrast, Kerr microscopy observations done on samples structured into 1–3 μm dots show that the domain wall propagation is hindered by the net of orthogonal steps generated by the process. The domain architecture is then directly twinned to the geometry of dots. Both dot mean size and effective size dispersion which is imposed by the dipolar field seem to be the two relevant parameters that govern the two steps magnetization reversal. Calculations of coupling dipolar field are done showing its strong influence upon the magnetization reversal in the stripes case.
Single electron tunneling and suppression of short-channel effects in submicron silicon transistors
Authors: Peters M. G. , den Hartog S. G. , Dijkhuis J. I. , Buyk O. J. A. , Molenkamp L. W.
Published: July 2002

We report measurements on submicron metal–oxide–semiconductor field effect transistors equipped with a gate on three sides of the channel. At room temperature, a strong suppression of short-channel effects has been achieved for the narrowest channels. At liquid helium temperatures, the same devices exhibit clear conductance oscillations in the subthreshold regime, indicating that a quantum dot has formed in the disordered channel.
Minority carrier capture at DX centers in AlGaSb Schottky diodes
Authors: Gombia E. , Mosca R. , Franchi S. , Ghezzi C. , Magnanini R.
Published: July 2002

Hole capture at DX centers is evidenced in Schottky diodes made on Te-doped AlGaSb under forward bias conditions. Capacitance versus voltage measurements performed at low temperatures show that the occupancy of the DX centers is affected by sufficiently large forward biases. The current densities required to achieve such modifications are at least one order of magnitude smaller than those needed in Si-doped AlGaAs. Positive deep level transient spectroscopy signals are systematically detected in all the samples studied when forward-bias filling pulses are used. These observations are discussed in terms of minority carrier (hole) injection and subsequent capture by the DX centers.
Effects of introduction of sodium and water on second-order nonlinearity in poled synthetic silica glass
Authors: Imai Hiroaki , Horinouchi Suguru , Asakuma Naoko , Fukao Kazuhiro , Matsuki Daizaburo , Hirashima Hiroshi , Sasaki Keisuke
Published: July 2002

Second harmonic generation (SHG) from silica glasses doped with sodium or water was investigated to clarify the effects of the impurities on the second-order nonlinearity. Sodium ions were introduced into pure silica glasses from a polymer coating containing a trace amount of sodium with electrical poling. The doping of sodium drastically increased the SHG intensity from the poled silica glasses. The nonlinearity localized in the anode surface region is suggested to be ascribed to a frozen electric field formed by the drift of sodium ions. Introduction of water molecules into silica glasses was performed by hydrothermal treatment in purified water using an autoclave. The SHG intensity from the electrically poled glasses also increased with the increasing content of water molecules. In this case, the SHG was emitted from the entire region of the sample plate. The origin of the nonlinearity is tentatively assumed to be due to a frozen electric field created by charge separation with protonic conduction.
Characteristics of populations and gains in neon-like argon (Ar IX)
Authors: Kim Dong-Eon , Kim Dae-Soung , Osterheld Albert L.
Published: July 2002

The characteristics of the populations and gain coefficients in Ne-like Ar have been investigated considering the 37 levels of the 2p6, 2p53s, 2p53p, 2p53d, 2s2p63s, 2s2p63p, and 2s2p63d configurations. It was found that large gains on the 3p 1S0−3s 1P1, 3p 3P2−3s 1P1, and 3p 3D2−3s 3P1 transitions are formed for the density between 1018 and 1019 cm−3. The effect of the opacities of the 2p 1S0−3s 3P1, 2p 1S0−3s 1P1, 2p 1S0−3d 3D1, 2p 1S0−3d 1P1, and 2p 1S0−2s3p 1P1 transitions becomes important for a density higher than 1018 cm−3 and even increases the gain on the 3p 1S0−3s 1P1 transition for the opacity of the 2p 1S0−3s 1P1 transition of up to 2.
Plasma-sheath resonances and energy absorption phenomena in capacitively coupled radio frequency plasmas. Part II. The Herlofson paradox
Authors: Ku Victor P. T. , Annaratone Beatrice M. , Allen John E.
Published: July 2002

The existence of plasma-sheath resonances in a capacitively coupled plasma reactor, which contains a fairly low pressure discharge, has been demonstrated. Under this kind of pressure the plasma is approximately collisionless and inhomogeneous. Two different models have been used to resolve the resonances successfully, a uniform and a nonuniform plasma model. It is shown that the uniform plasma model can explain well the experimental observations; the transmission of rf current circulating in the plasma system is maximum at the plasma-sheath resonance at a frequency below the electron plasma frequency. It was found during further analysis using the nonuniform plasma model, with experimental values for the plasma width and sheath thickness, that the plasma-sheath resonance coincides with the local plasma frequency at two planes within the discharge. At these two resonance planes, the plasma impedance function has a pole. The model predicts a resonant energy absorption in this region and, furthermore, the amount of absorbed energy is essentially determined by the steepness of the electron density profile within the resonance plane. This result is in good agreement with Crawford and Harker [J. Plasma Phys. 8, 261 (1972)].