bluuebunny/crossref_metadata_2025 huggingface dataset:
The inductance of the discharge in a spark gap
Authors: Persephonis P. , Vlachos K. , Georgiades C. , Parthenios J.
Published: July 2002

The inductance of an arc discharge has been modeled and a mathematical formula given for the arc inductance in terms of its geometrical dimensions. This formula agrees with the experimental results and is used to determine the arc-channel radius. Inductance measurements have been taken for various values of pressure, interelectrode distance and applied voltage in a triggered spark gap. The dependence of inductance upon these parameters has been explained through the existing variations of the arc-channel cross section. Two mechanisms are responsible for the variations of inductance. The first is diffusion and the second is the tendency of the current channel to vary diameter with pressure, interelectrode distance, and applied voltage. The time history of the arc-channel inductance has been investigated. Finally, the ambipolar diffusion coefficient and the total charge number of the arc channel has been obtained.
Liquid silicide formation on the Si wafer free surface during Ni diffusion at 1200 °C
Authors: Li Jianxing , Yang Woun-Suck , Tan Teh Y. , Chevacharoenkul Sopa , Chapman Richard
Published: July 2002

An investigation of nickel diffusion in silicon wafers at 1200 °C, which is above a few Ni-Si eutectic and peritectic temperatures, has been performed. Czochralski Si wafers with nickel film on the back surfaces and mechanical damages on the front surfaces were annealed in different ambients for various periods of time. The wafers annealed in vacuum showed that a liquid phase rich in nickel has been formed at their front (initially free) surfaces at the annealing temperature. The amount of the liquid-phase material increases with an increase of the annealing time. After cooling down, the solidified materials showed various morphologies: spheres, polygons, dendrites, and terraces. The thermodynamic driving force leading to this phenomenon is attributed to the Si (free) surface energy.
Thin-film junctions of cadmium telluride by metalorganic chemical vapor deposition
Authors: Chu T. L. , Chu Shirley S. , Ferekides C. , Britt J. , Wu C. Q.
Published: July 2002

Polycrystalline films of cadmium telluride (CdTe) deposited by the metalorganic chemical vapor deposition (MOCVD) technique using the reaction of dimethylcadmium (DMCd) and di-isopropyltellurium (DIPTe) can be p type or n type, depending on the DMCd/DIPTe molar ratio in the reaction mixture. Extrinsic CdTe films have been deposited by using group III and group V compounds as dopants during the MOCVD process. Gallium can be incorporated into CdTe films to yield a dark resistivity of about 1000 Ω cm and a carrier concentration of about 2×1017 cm−3; however, the incorporation of arsenic or antimony is considerably more difficult, and low resistivity p-CdTe films cannot be obtained. Extrinsically doped CdTe films show significantly different photoluminescence spectra from intrinsic films of the same conducting type. Heterojunctions have been prepared by depositing p-CdTe films on CdS-coated SnO2:F/glass substrates. The junction properties and the post-deposition treatments have been investigated. Large-area (≳1 cm2) MOCVD CdTe thin-film solar cells of near 10% efficiency have been reported for the first time. Homojunctions have also been prepared by the successive in situ deposition of n- and p-CdTe films on SnO2:F/glass substrates, and their photovoltaic characteristics evaluated.
As-grown preparation of high-critical-temperature superconducting Y1Ba2Cu3O7−x films by radio-frequency plasma flash evaporation
Authors: Terashima Kazuo , Akagi Tatsuya , Komaki Hisashi , Yoshida Toyonobu
Published: July 2002

As-grown superconducting Y1Ba2Cu3O7−x films were successfully synthesized on (100) MgO and (100) SrTiO3 substrates by radio-frequency (rf) plasma flash evaporation, in which fine powder mixtures of constituents are continuously injected into an rf plasma, co-evaporated, and co-deposited onto a substrate. In particular, films deposited on a (100) SrTiO3 substrate at 670 °C exhibited a superconducting transition temperature Tc of 92 K and critical current density Jc of 3×105 A/cm2 under zero magnetic field at 77 K. The excitation temperatures of several species during the depositions were estimated from optical emission spectroscopy. The temperature of Y+ was a few thousand degrees lower than that of Ba+ and Cu. The difference in these excitation temperatures revealed that this plasma process was a nonequilibrium deposition.
A study of the coupling efficiency versus grating periodicity in a normal incident GaAs/AlGaAs multiquantum well infrared detector
Authors: Yu Larry S. , Li Sheng S. , Wang Y. H. , Kao Y. C.
Published: July 2002

A detailed study of the dependence of coupling quantum efficiency on the grating periodicity of planar metal grating coupled GaAs/AlGaAs quantum well infrared photodetectors has been made in this work. Five different detector samples with grating periodicities of Λ=1.1, 3.2, 5, 7, and 10 μm have been fabricated for the present study. The results showed that the device with a 5 μm grating periodicity gave the best front-side coupling efficiency, which was in good agreement with our theoretical prediction. A single pass quantum efficiency (η) of 11% was achieved for the front-side illumination at λp=9.8 μm and T=77 K.
MeV B compensation implants into n-type GaAs and InP
Authors: Nadella Ravi K. , Vellanki Jayadev , Rao Mulpuri V. , Holland O. W.
Published: July 2002

High energy B implantations were performed into n-type GaAs and InP at room temperature in the range of energies from 1 to 5 MeV and fluences from 1011 to 1016 cm−2. The material did not become amorphous for any of the fluences used. Buried layers with resistivities as high as 108 Ω cm and 106 Ω cm were obtained in GaAs and InP, respectively, after heat treatments. The breakdown voltages corresponding to the highest resistivities are 80 and 35 V, respectively, in GaAs and InP. In GaAs, the Rutherford backscattering analysis on the annealed samples showed an aligned yield close to that of a virgin sample, whereas, the yield in InP is more than that of the as-implanted sample.
Investigation of the kinetic mechanism of the Ar+-ion-enhanced Cl2/GaAs{110} etch rate phenomenon: Dependence on the reactant flux ratio
Authors: DeLouise L. A.
Published: July 2002

The effect of 1 keV Ar+-ion bombardment on the reaction of a high-intensity pulsed Cl2 supersonic beam with GaAs{110} at room temperature has been investigated by measuring time-of-flight wave forms of the unreacted Cl2 scattered from the surface as a function of the incident Ar+/Cl2 flux ratio and the kinetic energy of the Cl2 beam. Measurements made with a high-kinetic-energy Cl2 beam (Ei = 0.48 eV) show that ion bombardment enhances the dissociative chemisorption of Cl2 species trapped at the surface in a weakly adsorbed precursor state and has a negligible affect on Cl2 species that enter the direct-inelastic scattering channel. Quantitative analysis of the room-temperature etch rate, determined using a low-energy Cl2 beam (Ei = 0.14 eV), indicates a strong dependence on the incident Ar+/Cl2 flux ratio. Typical values range between 0.02 and 0.10 μm/min. These etch rates are ∼100 times larger than the Ar+-ion sputter desorption rate expected for the ion flux used in these measurements. Under high Cl2 flux conditions the kinetics of the ion-assisted etch reaction exhibits a first-order dependence on Ar+ flux. Postetch surface stoichiometric measurements indicate a correlation of an increasingly As-rich surface layer with an increasing etch rate. An As-rich surface layer indicates that the relative Ga to As product desorption rate is larger under the condition of stoichiometric ion-assisted etching. It is proposed that the ion-enhanced etch rate phenomenon results from the fact that ion bombardment opens a momentum desorption channel facilitating the nonthermal preferential desorption of a thermal rate-limiting Ga-intermediate species. In addition, ion bombardment creates Cl2 dissociation sites by both a physical and a chemical mechanism which leads to an increase in the concentration of reactive surface intermediate species.
Some band structure related optical and photoelectrical properties of Pb1−xEuxSe (0≤x≤0.2)
Authors: Herrmann K. H. , Möllmann K.-P. , Tomm J. W. , Böttner H. , Lambrecht A. , Tacke M.
Published: July 2002

The results of an experimental study of photoelectrical and optical properties of molecular beam epitaxy grown Pb1−xEuxSe are presented. The data obtained are discussed with respect to band structure of the mixed crystal obtained by a linear interpolation between the relevant energy states in the binaries PbSe and EuSe.
Generation of divacancies in silicon by MeV electrons: Dose rate dependence and influence of Sn and P
Authors: Svensson Bengt G. , Lindström J. Lennart
Published: July 2002

Silicon samples doped with tin and/or phosphorus have been irradiated at room temperature by 2.0 MeV electrons in order to study the generation of divacancy centers (V2). The samples were analyzed using low-temperature infrared absorption measurements and deep level transient spectroscopy. The production of V2 is found to be very small in the tin-doped samples, which suggests that the generation process hinges strongly on the existence of mobile monovacancies. Substitutional Sn atoms act as efficient traps for migrating vacancies and form stable vacancy-tin centers with a corresponding reduction in the formation of ‘‘ordinary’’ vacancy-related defects. Indeed, at temperatures above ∼150 °C where the vacancy-tin centers dissociate, a pronounced increase in the V2 concentration occurs. On the other hand, measurements of the V2 concentration in the non tin-doped samples as a function of electron dose using different dose rates reveal that the steady-state concentration of monovacancies during irradiation has no influence on the generation of V2. Thus, pairing of two single vacancies created by different bombarding electrons can be excluded as a major formation mechanism for V2. Evidence is presented for a new model where the formation of V2 is predominantly due to migration and subsequent agglomeration of two single vacancies created directly (electron-atom collision) and indirectly (recoiling atom-atom collision), respectively, by the same impinging electron. Finally, the presence of substitutional phosphorus atoms in high-purity float zone Si samples is shown to enhance the production of V2. The experimental data exhibit excellent relative agreement with computer simulations assuming a defect reaction model where annihilation of vacancy-phosphorus centers by Si self-interstitials is taken into account, i.e., the role of phosphorus is to provide an alternative recombination center for the Si self-interstitials with a corresponding growth of surviving V2 centers.
Crystallographic and magnetic properties of the spinel phase for NixFe1−xCr2S4
Authors: Kim Chul Sung , Kim Dae Yong , Ko Heung Moon , Kim Jwa Kyum , Park Jae Yun
Published: July 2002

Ferrimagnetic NixFe1−xCr2S4 is found to crystallize with a pure spinel structure in the composition range 0≤x≤0.4. 57Fe Mössbauer spectra of NixFe1−xCr2S4 have been taken at various temperatures ranging from 13 K to room temperature. Analysis of Mössbauer spectra and x-ray crystallographic data indicate that Ni ions occupy the tetrahedral site, and the lattice constant is in good agreement with Vegard’s law. The isomer shifts indicate that the valence states of the Fe ions have a ferrous character. The Néel temperature increases linearly with Ni concentration, suggesting that the superexchange interaction for Ni-S-Cr link is stronger than that for Fe-S-Cr link. The anomalous reduction of the magnetic hyperfine field below 70 K could be explained in terms of the cancellation effect between mutually opposite orbital current field HL and the Fermi contact field HD. It is notable that, as the temperature decreases below the Néel temperature, both quadrupole shift and asymmetrical line broadening appear and increase with decreasing temperature, suggesting the presence of an electric-field gradient and accompanying relaxation effects.
Strength of titanium diboride under shock wave loading
Authors: Dandekar Dattatraya P. , Benfanti Daniel C.
Published: July 2002

Strength of TiB2 under plane shock wave loading is assessed in terms of its spall threshold and the shear stress sustained by it under shock compression to 60 GPa. The results of experiments support the thesis that the observed cusp in TiB2 at 4.5–7.0 GPa is of mechanical nature and its effect is to decrease the spall threshold values at stresses above the cusp but below the accepted Hugoniot elastic limit (HEL) value of 13–17 GPa. A comparison of shock Hugoniot data on two different TiB2 obtained from Grady [Dynamic Material Properties of Armor Ceramics, Sandia Laboratories, SAND 91-0147. UC-704 (1991)] and the adiabats constructed from high pressure ultrasonic wave velocity measurements show that these materials sustain increasing shear stresses with increasing values of shock stress. For example, results of data analysis on the denser TiB2 show that it sustains around three times the shear stress at 60 GPa than at its HEL, i.e., 17 GPa.
Magnetic properties of ultrathin Co/Cu(001) films during growth
Authors: Schumann F. O. , Bland J. A. C.
Published: July 2002

We have investigated the evolution of the in-plane magnetic hysteresis behavior of Co/Cu(001) films (in the monolayer range) by means of the transverse magneto-optical Kerr effect. Shortly after the onset of long-range order, the deposition was stopped and the almost square hysteresis loop which resulted revealed a small coercive field (Hc=0.9 G) indicating a Curie temperature Tc close to room temperature. Further Co deposition in small increments showed a dramatic increase of Hc. An increase of 9% in the thickness resulted in a variation of Hc in the range 0.9 G<Hc<44.6 G to which a power law of the form (d/dc−1)α with α=0.58±0.07 could be attributed. This empirical fit suggests that the dramatic increase of the coercivity is thermodynamic in origin and related to the thickness dependence of the magnetization M.
Nonlinear optical properties of CdSe quantum dots
Authors: Park S. H. , Casey Michael P. , Falk Joel
Published: July 2002

The nonlinear optical properties of CdSe quantum dots are investigated using self-saturation and degenerate four-wave mixing techniques. The saturation of the room temperature absorption coefficient, for a wavelength in the vicinity of the first exciton peak, is measured. The measured data can be predicted from a description of the quantum dot as a two level, homogeneously broadened, saturating system. The variation of the four-wave mixing reflectivity with intensity and with wavelength is reported. All four-wave mixing data are also consistent with the homogeneously-broadened two-level model.
Carbon incorporation during growth of GaAs by TEGa-AsH3 base low-pressure metalorganic chemical vapor deposition
Authors: Chen H. D. , Chang C. Y. , Lin K. C. , Chan S. H. , Feng M. S. , Chen P. A. , Wu C. C. , Juang F. Y.
Published: July 2002

Heavily carbon-doped GaAs (1×1018∼1×1020 cm−3) grown by low-pressure metalorganic chemical vapor deposition using triethylgallium and arsine as sources and liquid carbon-tetrachloride (CCl4) as dopant has been investigated. The carrier concentration was verified at various growth temperatures, V/III ratios, and CCl4 flow rates. Dopant concentration first increased from 550 °C and reached a maximum at 570 °C growth temperature (Tg) and then decreased monotonously. Carbon incorporation was strongly enhanced when the V/III ratio was less than 30 at Tg=590 °C or less than 40 at Tg=630 °C. Hole concentration increased and then decreased as CCl4 flow rate increased. Growth rate of layers decreased as growth temperature and flow rate of CCl4 increased. The doping efficiency of epitaxial layers grown on the (100) substrate was higher than that on the 2° off toward <110≳ misoriented substrate. Carbon-doped GaAs films had higher Hall mobility than zinc-doped GaAs films at high doping levels due to less self-compensation. The highest dopant concentration in this system was 2.3×1020 cm−3 at Tg=580 °C and V/III=10.
Relationship between structural phase transitions and elastic anomalies in metallic superlattices
Authors: Fullerton Eric E. , Schuller Ivan K. , Parker F. T. , Svinarich Kathryn A. , Eesley Gary L. , Bhadra R. , Grimsditch M.
Published: July 2002

A detailed study of the structural and elastic properties of Fe/Cu superlattices has been performed. These superlattices exhibit a structural phase transition as a function of layer thickness in which bcc α-Fe transforms into fcc γ-Fe. This structural phase transition in which the Fe layers become coherent with the Cu layers is signaled by clear-cut changes in elastic and magnetic properties. The elastic properties studied through this transition indicate that in-plane coherency plays an important role in the elastic behavior of metallic superlattices.
Formation of void/Ga-precipitate pairs during Zn diffusion into GaAs: The competition of two thermodynamic driving forces
Authors: Jäger W. , Rucki A. , Urban K. , Hettwer H.-G. , Stolwijk N. A. , Mehrer H. , Tan T. Y.
Published: July 2002

An experiment of diffusing Zn into GaAs has been conducted at 900 °C using Zn metal as the source material in a quartz ampoule, with or without As being included. For cases without further including As in the ampoule, the Zn profile is box shaped and the Zn-diffused region contains dislocations and void/Ga-precipitate pairs, with the void to precipitate volume ratio being essentially constant throughout the Zn-diffused region. For cases including As in the ampoule, the Zn profile is of the kink-and-tail type with the Zn-diffused region containing dislocations and void/Ga-precipitate pairs. In the profile tail region, the Ga-precipitate to void volume ratio is substantial, while in the profile kink region of high Zn concentration near the surface only voids are left. The results are interpreted in accordance with Zn and diffusion-ambient-induced Ga-As-Zn ternary alloy system thermal equilibrium requirements in general, and the consequential solid phase composition variations in particular. Without As inclusion in the ampoule, the overall Ga-As material Ga concentration in the Zn-diffused region has exceeded the allowed limit of the GaAs crystal, with the crystal composition at a limit. With the inclusion of As in the ampoule, the GaAs crystal composition in the high Zn concentration region near the surface has reached an allowed As concentration limit, but in the profile tail region the overall Ga-As material Ga concentration has exceeded an allowed limit of the GaAs crystal. This is because the vapor phase Zn and As species constitute two conflicting thermodynamic driving forces for producing GaAs crystals with two opposite and extreme compositions: As for producing As-rich crystals, and Zn for producing Ga-rich materials. The common assumption that the inclusion of As in the diffusion ampoule will ensure the whole GaAs crystal essentially at an As-rich composition does not hold during high-concentration Zn indiffusion.
Measurement of laser ablation threshold on doped BiSrCaCuO high-temperature superconductors by the pulsed photothermal deflection technique
Authors: Rajasree K. , Vidyalal V. , Radhakrishnan P. , Nampoori V. P. N. , Vallabhan C. P. G.
Published: July 2002

Laser-induced damage and ablation thresholds of bulk superconducting samples of Bi2(SrCa)xCu3Oy(x=2, 2.2, 2.6, 2.8, 3) and Bi1.6 (Pb)xSr2Ca2Cu3 Oy (x=0, 0.1, 0.2, 0.3, 0.4) for irradiation with a 1.06 μm beam from a Nd-YAG laser have been determined as a function of x by the pulsed photothermal deflection technique. The threshold values of power density for ablation as well as damage are found to increase with increasing values of x in both systems while in the Pb-doped system the threshold values decrease above a specific value of x, coinciding with the point at which the Tc also begins to fall.
Experimental study and computer simulation of collimated sputtering of titanium thin films over topographical features
Authors: Liu D. , Dew S. K. , Brett M. J. , Janacek T. , Smy T. , Tsai W.
Published: July 2002

An experimental study and computer simulation of collimated sputtering of titanium (Ti) thin films onto submicrometer trenches is presented. The effect of square grid collimators with aspect ratios varying from 0.5 to 2 has been studied. Simulation of collimated sputtering involves the combination of the simulation of sputter distributions vapor transport model and the simulation by ballistic deposition film growth model. This combination is able to simulate the effect of collimation on the spatial and angular distributions of deposited atoms, and is able to predict the film coverage, deposition rate change, and microstructure of the deposited films. Both experimental and simulation results show that bottom coverage in trenches of aspect ratio 1.2 can be significantly improved from 50% to more than 80% using collimation. However, a penalty is paid by a corresponding decrease in deposition rate down to 15% of the uncollimated value. Additionally, the microstructure (grain size and orientation) is altered by collimation. The good agreement between the simulation and experimental results indicates that the model will be very useful for predicting and optimizing the properties of films deposited by collimated sputtering over topographical features.
High-coercivity Sm–Fe–Ga–C compounds with Th2Zn17 structure by melt spinning
Authors: Kong Lin-Shu , Shen Bao-gen , Wang Fang-wei , Cao Lei , Guo Hui-qun , Ning Tai-shan
Published: July 2002

The magnetic hardening of the Sm2Fe14Ga3Cx was investigated by melt spinning. It was found that high coercivities can be achieved by direct quenching at the optimum substrate velocity. The coercivities of 12.6–15.0 kOe were obtained in Sm2Fe14Ga3Cx ribbons with carbon contents x from 1.0 to 2.5. The as-quenched Sm2Fe14Ga3C1.5 ribbons had a coercivity exceeding 13 kOe within a large range of quench rates between 18 and 30 m/s. X-ray diffraction experiments indicate that the ribbons are almost entirely comprised of the Th2Zn17 phase. It is concluded that Ga not only stabilizes the hard magnetic phase but also is very effective in raising the coercivity in the Sm2Fe14Ga3Cx melt-spun ribbons.
Influence of rf magnetron sputtering conditions on the magnetic, crystalline, and electrical properties of thin nickel films
Authors: Miller M. S. , Stageberg F. E. , Chow Y. M. , Rook K. , Heuer L. A.
Published: July 2002

Thin nickel films with thicknesses ranging from 30 to 150 nm were deposited via radio frequency (rf) magnetron sputtering. The influence of argon pressure, film thickness, rf input power, and deposition rate on the magnetic, crystalline, and electrical properties of the films was evaluated. Depending on deposition conditions, film coercivity could be varied from 2 to 290 Oe while saturation magnetization could be varied from 280 to 500 emu/cm3. Higher argon pressures produced lower coercivity films. The films exhibited a dominant fcc(111) orientation. Lower argon pressures and higher rf input powers increased nickel grain sizes. Classical models based on domain wall energy considerations and film microstructure are used to intrepret the observed experimental results.
Nonuniform switching of single domain particles at finite temperatures
Authors: Braun Hans-Benjamin , Bertram H. Neal
Published: July 2002

It has recently been shown that nonuniform thermal fluctuations are able to significantly reduce the coercivity of elongated ferromagnetic particles compared to the uniform rotation model of Néel and Brown. In particular, this theory revealed that even for particles that are single domain in the remanent state, a nonuniform energy barrier exists which is proportional to the product of the particle cross-sectional area and the domain wall energy. For sufficiently long particles, this energy barrier is therefore always lower than that of uniform reversal. Here, several implications of this theory are discussed. It is shown that the coercivity of a particle with fixed volume decreases with increasing aspect ratio. For a fixed particle shape, the coercivity is lower with decreasing exchange constant. For small particle aspect ratios the theory is shown to merge continuously into the Néel–Brown theory. The angular dependence of the coercivity is evaluated explicitly. The reduction from the Stoner–Wohlfarth value is maximal for external fields parallel and perpendicular to the particle axis and the coercivity shows an asymmetric angular dependence. Both features are in accordance with recent experiments on an elongated single domain particle.
Hole defects in molecular beam epitaxially grown p-GaAs introduced by alpha irradiation
Authors: Goodman S. A. , Auret F. D. , Meyer W. E.
Published: July 2002

Epitaxial aluminum Schottky barrier diodes on molecular beam epitaxially grown p-GaAs with a free carrier density of 2×1016 cm−3 were irradiated with alpha particles at room temperature using an americium-241 (Am-241) radio nuclide. For the first time, the radiation induced hole defects are characterized using conventional deep level transient spectroscopy (DLTS). The introduction rates and DLTS ‘‘signatures’’ of three prominent radiation induced defects Hα1, Hα4, and Hα5, situated 0.08, 0.20, and 0.30 eV above the valence band, respectively, are calculated and compared to those of similar defects introduced during electron irradiation.
Giant magnetoresistance and microstructural characteristics of epitaxial Fe-Ag and Co-Ag granular thin films
Authors: Thangaraj N. , Echer C. , Krishnan Kannan M. , Farrow R. F. C. , Marks R. F. , Parkin S. S. P.
Published: July 2002

The microstructure and magnetic properties of phase-separated Fe-Ag and Co-Ag granular alloy films, grown epitaxially on (001) NaCl substrates and exhibiting giant magnetoresistance (GMR), have been investigated. Surprisingly, two Fe-Ag films of similar composition grown under identical conditions are found to have substantially different microstructures yet display similar GMR. The microstructure of the films is characterized by Fe-rich or Co-rich regions, respectively, 350–700 nm in extent, surrounded by a Ag-rich matrix. Within the Ag-rich regions, the Fe concentration varies from 20 to 25 at % and the Co concentration is ≂16 at %. Within these regions essentially pure fcc Co particles and bcc Fe particles are in parallel and rotated epitaxial alignment respectively with the fcc silver matrix. The Co and Fe particles are ≂15–25 Å in diameter. It is these small particles which most likely account for the giant magnetoresistance exhibited by these alloys. This suggests that a size distribution of magnetic particles, sharply peaked at the optimum size with limited bulk segregation, might give rise to larger GMR values.
Short-range order, microstructure and their correlation with light-induced degradation in hydrogenated amorphous silicon deposited at high growth rates by cathode heating technique
Authors: Chattopadhyay S. , Sharma S. N. , Banerjee Ratnabali , Bhusari D. M. , Kshirsagar S. T. , Chen Yan , Williamson D. L.
Published: July 2002

Hydrogenated amorphous silicon (a-Si:H) films were deposited at high growth rates by increasing the rf power density in a (SiH4+H2) discharge, while powder formation due to gas phase polymerization was controlled by heating the cathode together with the anode. A combination of Raman scattering, infrared absorption, and small angle x-ray scattering experiments was used to study the short-range order and microstructure of films deposited in different (dusty or otherwise) plasma conditions. The results were correlated with initial and light-soaked photoresponse to demonstrate that films with more microstructure and less short-range order were generally poorer.
Characterization of buried pseudomorphic InGaAs layers using high-resolution x-ray diffraction
Authors: Meshkinpour M. , Goorsky M. S. , Matney K. M. , Streit D. C. , Block T. R.
Published: July 2002

The thickness and composition of the InGaAs layer in GaAs/AlGaAs/InGaAs/AlGaAs/GaAs high-electron-mobility transistor devices were determined to within ±5 Å and ±0.003, respectively, using high-resolution x-ray diffraction. The combined thickness of the capping AlGaAs and GaAs layers were also determined to within ±5 Å. Although the interference effects near the substrate peak in the diffraction pattern may be identical for structures with different InGaAs thicknesses, the peak from the buried InGaAs layer will be different. In other words, if the diffraction from the buried layer is measured, one can readily distinguish between structures whose interference peaks are otherwise the same. It is also shown that the use of different reflections removes the ambiguity associated with interference peaks.
Molecular-beam-epitaxial growth and characterization of high-quality alloys and multiple quantum wells on InP substrates using a post-evaporation-heated arsenic source
Authors: Leavitt R. P. , Bradshaw J. L.
Published: July 2002

We report the growth of high-quality As-based ternary and quaternary alloys lattice matched to InP using a valved arsenic source that can post-heat the As beam after evaporation. We find that the optimum group-V-to-group-III beam-equivalent pressure ratio for growth of (In,Ga)As alloys using this source is considerably lower than values reported previously for growth using conventional As4 sources. Consequently, high-quality (In,Ga)As, (In,Al)As, and (In,Al,Ga)As alloys (and quantum wells made from these alloys) can be grown under the same growth conditions, i.e., substrate temperatures between about 525 °C and 540 °C and V/III pressure ratios between 10:1 and 15:1. Thick-film alloys and multiple-quantum-well structures grown under these conditions show superior structural and optical quality. Strong excitonic features are observed in the room-temperature absorption spectra of a number of multiple-quantum-well structures with well widths ranging from 30 Å to 170 Å . Calculations of the exciton transition energies using a simple empirical two-band model are in excellent agreement with experiment, even for a structure containing quantum wells in tensile strain in which the ordering of ground-state light- and heavy-hole excitons is reversed. The optical absorption spectrum of a 50-Å -period (In,Ga)As/(In,Al)As superlattice shows room-temperature excitons involving electronic states at both the bottom and top of the minibands. Exciton line widths for these quantum-well structures, measured using low-temperature photoluminescence, are consistent with the limits imposed by random alloy fluctuations. We tentatively explain the lower optimum V/III pressure ratio for growth of (In,Ga)As in terms of the increase in kinetic energy of As4 molecules (compared with the kinetic energy of molecules from a conventional As4 source) and the consequent enhancement in the efficiency of dissociation of As4 molecules into As2 molecules at the growing surface.
A study of epitaxially stabilized FeSi2 by surface enhanced Raman scattering
Authors: Mo Yujun , von Känel H. , Onda N. , Wachter P. , Mattei G.
Published: July 2002

Epitaxially stabilized films with the defect CsCl structure, that were grown by molecular beam epitaxy, have been studied by surface enhanced Raman scattering using a silver overlayer. We have observed that the defect-induced phonon density of states features in the Raman signal shift from 256 cm−1 for a coherently strained film to 263 cm−1 for a relaxed one. The lower energy observed for the former can qualitatively be explained by the expansive trigonal distortion arising from the misfit of −0.5%.
Some properties of semiconducting IrSb3
Authors: Slack Glen A. , Tsoukala Veneta G.
Published: July 2002

Polycrystalline p-type samples of IrSb3 and Ir0.5Rh0.5Sb3 have been made by hot isostatic pressing of powders. A number of properties such as thermal expansion coefficient, sound velocity, thermal conductivity, electrical conductivity, Seebeck coefficient, and carrier concentration have been measured. These compounds show promise as thermoelectric materials.
GaAs/AlGaAs multiquantum well vertical cavity tunable photodetector
Authors: Lai K. , Hansing C. , Streetman B. G. , Campbell J. C. , Leavitt R. , Simonis G.
Published: July 2002

A new electrically tunable photodetector is described. Wavelength tunability has been achieved by utilizing the quantum confined Stark effect in a low-Q vertical cavity to shape the exciton absorption characteristic. The absorption peak has been tuned approximately 17 nm with a bias of 8 V. Results are compared with theoretical calculations.
Electrostatic forces between metallic tip and semiconductor surfaces
Authors: Hudlet S. , Saint Jean M. , Roulet B. , Berger J. , Guthmann C.
Published: July 2002

Atomic force microscopy used in the resonant mode is a powerful tool for measuring local surface properties: for example, the quantitative analysis of the electrical forces induced by the application of an electric field between a conductive microscope tip and a surface allows the determination of the tip/surface capacitance and the local surface work function. However, these quantitative analyses require knowledge of tip geometry. In this article, we show that the simple procedure of evaluating the tip curvature radius by fitting the variations of the electrostatic force with the tip-surface distance is not always adapted to the case where one of the tip-surface system elements is a semiconductor. However, particular experimental conditions are determined to overcome these difficulties.
The effect of beamwidth on the analysis of electron-beam-induced current line scans
Authors: Luke Keung L.
Published: July 2002

A real electron beam has finite width, which has been almost universally ignored in electron-beam-induced current (EBIC) theories. Obvious examples are point-source-based EBIC analyses, which neglect both the finite volume of electron–hole carriers generated by an energetic electron beam of negligible width and the beamwidth when it is no longer negligible. Gaussian source-based analyses are more realistic but the beamwidth has not been included, partly because the generation volume is much larger than the beamwidth, but this is not always the case. In this article Donolato’s Gaussian source-based EBIC equation is generalized to include the beamwidth of a Gaussian beam. This generalized equation is then used to study three problems: (1) the effect of beamwidth on EBIC line scans and on effective diffusion lengths and the results are applied to the analysis of the EBIC data of Dixon, Williams, Das, and Webb; (2) unresolved questions raised by others concerning the applicability of the Watanabe–Actor–Gatos method to real EBIC data to evaluate surface recombination velocity; (3) the effect of beamwidth on the methods proposed recently by the author to determine the surface recombination velocity and to discriminate between the Everhart–Hoff and Kanaya–Okayama ranges which is the correct one to use for analyzing EBIC line scans.
Elastic properties of GaAs during amorphization by ion implantation
Authors: Mutti P. , Sklar Z. , Briggs G. A. D. , Jeynes C.
Published: July 2002

Line-focus beam acoustic microscopy has been used to study changes in the elastic constants of GaAs during amorphization produced by implantation with Si+ ions at liquid-nitrogen temperature. The distribution of amorphous material was determined by Rutherford backscattering and channeling. Values of c11 and c44 were estimated by fitting theoretical curves to the measured angular dispersion of surface acoustic waves in the (001) plane. The implanted material was modeled as a statically stressed anisotropic layer on an unmodified GaAs substrate. The values of c11 and c44 were found to decrease with increasing ion fluence. At the highest fluence the implanted region was completely amorphous, and it was observed that the softening of c44(41%) was significantly greater than that of c11(17%).
Novel oscillating cup viscometer–application to molten HgTe and Hg0.8Cd0.2Te
Authors: Mazuruk Konstantin , Su Ching-Hua , Lehoczky Sandor L. , Rosenberger Franz
Published: July 2002

A novel oscillating-cup viscometer, employing strain gauges for electronic signal acquisition, was developed, tested with gallium and applied to molten II-VI compounds. Kinematic viscosity data were obtained for Ga from 60 to 815 °C, for HgTe from 700 to 790 °C, and for Hg0.8Cd0.2Te from 790 to 850 °C.
Growth of abrupt InGaAs(P)/In(GaAs)P heterointerfaces by gas source molecular beam epitaxy
Authors: Shiau Guang-Jye , Chao Chih-Ping , Burrows Paul E. , Forrest Stephen R.
Published: July 2002

We demonstrate a novel method for the growth of abrupt InGaAs(P)/In(GaAs)P heterojunctions by gas source molecular beam epitaxy. We find that exposure of freshly grown InP to an As flux during growth interruption between layers of different compositions results in the substitution of surface P atoms and As atoms, thereby generating a strained transition layer at each interface. By assuring a group-III stabilized surface during interruption required to grow InGaAs(P)/In(GaAs)P heterointerfaces, As/P substitution can be avoided, thereby resulting in improved interface quality. Heterointerface abruptness was examined by double-crystal x-ray diffraction and low temperature photoluminescence. The results show that the interfaces grown with the modified switching sequence are considerably more abrupt than those obtained using conventional sequences where As/P interdiffusion extends over several monolayers.
Lattice accommodation in heteroepitaxial semiconductor layers grown beyond critical thickness
Authors: Tabuchi M. , Kumamoto T. , Takeda Y.
Published: July 2002

The accommodation of lattice mismatching in InPAs layers with thickness from one monolayer to 200 Å grown on InP substrates is measured by extended x-ray-absorption fine structure. Measured critical thicknesses agree with model calculations. In the layers beyond the critical thickness, gradual lattice accommodation with increase of the layer thickness is observed. The lattice accommodation mechanism beyond the critical thickness is discussed by comparison with the calculations.
The roles played by Ag and Al dopants in controlling the electrical properties of ZnO varistors
Authors: Fan Jiwei , Freer Robert
Published: July 2002

Four sets of ZnO-based ceramic varistors (reference samples-ZNR; doped with aluminium-AL; doped with silver-AG; doped with aluminium and silver-AA) have been prepared by the conventional mixed oxide route. The current-voltage (I-V) characteristics, determined at current densities up to 1 mA/cm2, yielded nonlinear coefficients (α) of 38, 60, 22 and 56, respectively. Dc and ac degradation tests were performed at 115 °C. Ac impedance analysis was used to determine grain (rg) and grain boundary (Rb) resistances; capacitance-voltage analysis enabled the donor density (Nd) and the barrier height (φ) to be determined. Doping the varistors with Al increased Nd, reduced rg, and improved the I-V characteristics, but caused an increase in Zn interstitials which degraded the stability. In contrast, Ag acts as an amphoteric dopant in ZnO causing a decrease in Nd, an increase in rg and Rb, and a reduction in α ;  Ag located in the interstitial sites is able to block the formation and migration of new Zn interstitials, thereby improving the stability. Doping with both Al and Ag at optimum levels gives benefits from both additives, i.e., Al doping (increases Nd and decreases rg) improves nonlinearity, while Ag doping (blocks the formation and migration of Zn interstitials) improves the stability of ZnO varistors.
Thomas–Fermi approach to resonant tunneling in δ-doped diodes
Authors: Diez Enrique , Domínguez-Adame Francisco , Sánchez Angel
Published: July 2002

We study resonant tunneling in B-δ-doped diodes grown by Si-molecular beam epitaxy. A Thomas–Fermi approach is used to obtain the conduction-band modulation. Using a scalar Hamiltonian within the effective-mass approximation we demonstrate that the occurrence of negative differential resistance (NDR) only involves conduction-band states, whereas interband tunneling effects seem to be negligible. Our theoretical results are in very good agreement with recent experimental observations of NDR in this type of diode.
Interactions between hydrogen and group VI donors in GaAs and GaAlAs
Authors: Theys B. , Machayekhi B. , Chevallier J. , Somogyi K. , Zahraman K. , Gibart P. , Miloche M.
Published: July 2002

GaAs and GaAlAs layers doped with different group VI donors (S, Se, Te) have been exposed to hydrogen plasma. By secondary ion mass spectroscopy, it is shown that, as in Si-doped materials, the hydrogen diffusion strongly depends on the AlAs content. Electronic transport measurements indicate that after hydrogen diffusion the electron concentration systematically decreases and their mobility increases, demonstrating clearly the passivation of the group VI donors by hydrogen.
Low temperature photoluminescence study in AlxGa1−xAs alloys in the indirect band gap region (x≳0.4)
Authors: Torres-Delgado G. , Castanedo-Perez R. , Diaz-Arencibia P. , Mendoza-Alvarez J. G. , Orozco-Vilchis J. L. , Murillo-Lara M. , Serra-Jones A.
Published: July 2002

Low-temperature photoluminescence spectroscopy was used to study AlxGa1−xAs intrinsic epitaxial layers in the range of aluminum content between 0.45<x<0.9 in the region where the band gap minimum is indirect. Nonintentionally doped samples were grown by liquid phase epitaxy very near the saturation temperature (∼800 °C). The heterostructures consisted of an AlxGa1−xAs layer grown on top of a GaAs Si-doped (n∼2×1018 cm−3) substrate with orientation (100) 2° toward the (110). There is little information on the zero phonon excitonic lines for Al compositions x≳0.6, because the appearance of these lines depends on several factors such as internal strength, crystalline defects, and impurities. In our samples the excitonic lines were observable up to x=0.9. Transitions related to bound-to-free carrier as well as their phonon replicas were observable too. The data obtained from the spectra fitted with multigaussian lines allowed us to estimate the dependencies on the Al composition, x, of the bound exciton peak, the bound exciton binding energy, and the exciton localization energy. The acceptor ionization energy attributed to carbon residual impurities in As sites and its dependence on x was also obtained. Fitting by Gaussian lines the phonon region, the PL spectra fitted better with the TO(X) AlAs-type branch rather than the LO(X) AlAs-type, with a linear dependence with x in the studied range. The phonon energies of the GaAs type seem to behave almost constant and it was difficult to assign them to the LO(X) GaAs type or to TO(X) GaAs type due to the scatter of the data for high values of x. Other phonon replicas from the bound exciton and the electron-to-acceptor carbon transition (e-A°) coincide with the LO(X) AlAs type and TA(X), TA(X)+LA(X) and two LA(X) phonon energies. All the dependencies we present here are valid for aluminum compositions in the range 0.48<x<0.90.
Fabrication of self-assembled multilayer heterostructure of poly(p-phenylene vinylene) and its use for an electroluminescent diode
Authors: Onoda Mitsuyoshi , Yoshino Katsumi
Published: July 2002

Multilayer heterostructure thin films of electroactive polymers such as poly(p-phenylene vinylene) (PPV)-precursor and sulfonated polyaniline (SPAn) have been successfully fabricated via a process based on the spontaneous self-assembly of conjugated polyion on a substrate. We have prepared SPAn‖PPV multilayer heterostructure light-emitting diodes (LEDs) using this electrostatic self-assembly technique and discussed the electroluminescent (EL) properties. The SPAn‖PPV monolayer LED emits greenish-yellow light, but the SPAn‖PPV multilayer emits bluish-green light and there is a slight shift between the two spectra of these devices. These results can be interpreted in terms of the confinement effect of carriers in the superlattice structure consisting of a SPAn‖PPV multilayer system.
Chemical oxidation of hydrogen passivated Si(111) surfaces in H2O2
Authors: Neuwald U. , Feltz A. , Memmert U. , Behm R. J.
Published: July 2002

The initial stages of oxide formation on atomically flat, monohydride terminated Si(111) surfaces by oxidation in 30% hydrogen peroxide solution (H2O2) were investigated by scanning tunneling microscopy and x-ray photoelectron spectroscopy. The reaction proceeds via homogeneous nucleation of small oxide clusters on the surface and subsequent lateral growth of these clusters within the surface bilayer. The oxidation of the topmost Si(111) bilayer in H2O2 solution is completed after 30 min, leading to a SiO1.2 average layer composition. For the next bilayer the oxidation rate decreases drastically—after 2 months only 60% of the second bilayer are oxidized. An inverse logarithmic rate of the second bilayer oxidation is consistent with a field assisted growth mechanism. The significant differences in the oxidation rates between the first and the second bilayer allow to produce well defined oxide layers of about 5 Å thickness.
Voltage dependence of microsecond switching in a nematic optical phase modulator
Authors: Baier-Saip J. A. , Bostanjoglo O. , Eichler H. J. , Macdonald R.
Published: July 2002

A study of the temporal response of a biased planar nematic liquid crystal to short (∼10 μs) voltage pulses is presented. The resulting optical phase shift varies quadratically with time during the first 2 μs after switching the voltage on and then linearly. A theoretical model is developped starting from the Leslie-Ericksen theory which describes the investigated phase modulation in the microsecond time scale.
Structure and magnetic properties of CeFe13−xSix compounds
Authors: Zhao Yanming , Liang Jingkui , Tang Weihua , Guo Yongquan , Rao Guanghui
Published: July 2002

New iron-based rare-earth CeFe13−xSix (2.4≤x≤2.6) intermetallic compounds which crystallized in the NaZn13-type structure with Fm3c(O6h) space-group symmetry have been found. The ac susceptibility and the magnetization curve measurements are reported. The substitution of silicon for iron leads to an increase of the Curie temperature and a reduction of the unit-cell volume. The saturation magnetic moment decreases from 15.7μB/f.u. to 13.4μB/f.u. with x=2.4–2.6.
Temperature dependence of dislocation efficiency as sinks for self-interstitials in silicon as measured by gold diffusion
Authors: Yakimov E. , Mariani G. , Pichaud B.
Published: July 2002

The substitutional gold concentration introduced in silicon by a diffusion step between 850 and 1000 °C was measured by deep level transient spectroscopy both in floating zone (FZ) and Czochralski (Cz) silicon containing different dislocation densities. The comparison, in the same sample, of dislocated and undislocated regions allows the efficiency of dislocations as sinks for self-interstitials γ to be measured as a function of diffusion temperature. In FZ silicon γ was found independent of temperature whereas in Cz silicon a remarkable temperature dependence for γ was observed which can be attributed to the release of dislocations by a thermally stimulated climbing mechanism from obstacles (i.e., oxygen segregation or precipitation).