bluuebunny/crossref_metadata_2025 huggingface dataset:
Rapid thermal low-pressure chemical vapor deposition of SiOx films onto InP
Authors: Katz A. , Feingold A. , Pearton S. J. , Chakrabarti U. K.
Published: July 2002

High quality SiOx films have been deposited onto InP substrates by means of a rapid thermal low-pressure chemical vapor deposition technique, using oxygen (O2) and 2% diluted silane (SiH4) in argon (Ar) in the gas sources. Rapid deposition rates in the range of 15–40 nm s−1 with apparent activation energies of 0.12–0.15 eV were obtained at temperatures in the range of 350–550 °C, pressures between 5 and 15 Torr, and O2:SiH4 ratios in the range of 5:1–20:1. The SiOx films had refractive indexes between 1.44 and 1.50, densities of 2.25–2.37 g cm−3, internal compressive stresses of −0.5×109 to −3×109 dyn cm−2, and exhibited wet etch rates of 0.2–0.8 nm s−1 through the standard p-etch process. The influence of the various process parameters on all the SiOx film parameters such as morphology and microstructure was examined.
Optimum implantation conditions for ion beam synthesis of buried cobalt silicide layers in Si(100)
Authors: Dekempeneer E. H. A. , Ottenheim J. J. M. , Vandenhoudt D. W. E. , Bulle-Lieuwma C. W. T. , Lathouwers E. G. C.
Published: July 2002

Ion beam synthesis of buried CoSi2 layers in Si(100) (Co+ energy=170 keV, dose=1.7×1017 ions cm−2) is studied as a function of implantation temperature (250→500 °C) and beam current density (1.6→3 μA cm−2). Conventional cross-section transmission electron microscopy and Rutherford backscattering spectrometry are used to correlate the experimental conditions with the amount of pinholes in the silicide layer and the flatness of the CoSi2/Si interfaces after annealing. Optimum implantation conditions yielding a pinhole-free buried silicide layer with flat interfaces are obtained.
Large nonresonant third-order hyperpolarizabilities of organic charge-transfer complexes
Authors: Gong Qihuang , Xia Zongju , Zou Y. H. , Meng Xiansheng , Wei Lin , Li Fu-mian
Published: July 2002

Large nonresonant third-order hyperpolarizabilities, γ1111, for solutions of three organic charge-transfer complexes in tetrahydrofuran have been measured. The values obtained for nonresonant γ1111 are 5.8×10−32 esu for Perylene/tetracyanoethylene, 1.2×10−32 esu for Pyrene/tetracyanoethylene, and 7.2×10−33 esu for Naphthanthracene/tetracyanoethylene. The corresponding values of the third-order susceptibility χ(3)1111 for the solid state are estimated to be 1.0×10−9 esu, 2.3×10−10 esu, and 1.3×10−10 esu, respectively.
Reactive ion etching of SiGe alloys using HBr
Authors: Bestwick Tim D. , Oehrlein Gottlieb S. , Zhang Ying , Kroesen Gerrit M. W. , de Frésart Edouard
Published: July 2002

We have studied reactive ion etching of Si1−xGex alloys with x≤0.15 and Ge in HBr plasmas. The etch rate of SiGe increases monotonically with the Ge content of the alloy and for a Si85Ge15 alloy was ≂50% greater than for Si. Etch profiles are identical to those formed in singe-crystal Si. X-ray photoelectron spectroscopy studies shown that the surface of the etched SiGe alloys are depleted in Ge and consist of about one monolayer of brominated Si over the alloy.
Endotaxial growth of CoSi2 within (111) oriented Si in a molecular beam epitaxy system
Authors: George T. , Fathauer R. W.
Published: July 2002

A new mode of growth is reported in which buried metallic layers can be fabricated within a single-crystal semiconductor through preferential subsurface growth on previously-grown ‘‘seed’’ regions. The deposition of Co at 800 °C at a rate of 0.01 nm/s on (111) Si substrates containing buried CoSi2 columns 40–100 nm below the Si surface results in the growth and coalescence of these subsurface columns. The formation of a CoSi2 layer on the Si surface is suppressed by this growth mode. It is proposed that the high diffusion rate of Co at 800 °C, coupled with the high growth rate of CoSi2 at the subsurface columns, is responsible for this preferred ‘‘endotaxial’’ growth mode. This growth technique was used to produce a continuous buried single-crystal layer of CoSi2 under a single-crystal Si capping layer.
Nd3+ incorporation in CaF2 layers grown by molecular beam epitaxy
Authors: Bausá L. E. , Legros R. , Muñoz-Yagüe A.
Published: July 2002

Molecular beam epitaxy of Nd3+-doped CaF2 monocrystalline layers on CaF2 substrates is demonstrated. Nd concentration is controlled by the temperature of an evaporation cell containing NdF3. Photoluminescence spectra of the samples show emissions from Nd3+ centers in tetragonal symmetry sites as a consequence of the charge compensation mechanism provided by interstitial F− ions.
Preparation of PbTiO3 thin films at low temperature by an excimer laser ablation technique
Authors: Tabata Hitoshi , Kawai Tomoji , Kawai Shichio , Murata Osamu , Fujioka Junzo , Minakata Shun-ichi
Published: July 2002

We have formed PbTiO3 thin films on (100) SrTiO3 substrates at a temperature of 350 °C using an ArF excimer laser ablation technique. Until now, PbTiO3 films have not been formed at temperatures lower than 500 °C using other thin-film techniques. The important points in the present study are the laser excitation of the substrate during the film growth and the lattice matching between the film and the substrate. The film deposited on the SrTiO3 substrate shows preferential orientation of the c-axis perpendicular to the substrate surface.
Carbon acceptor incorporation in GaAs grown by metalorganic chemical vapor deposition: Arsine versus tertiarybutylarsine
Authors: Watkins S. P. , Haacke G.
Published: July 2002

Undoped p-type GaAs epilayers were grown by low-pressure metalorganic chemical vapor deposition (MOCVD) at 650 °C and 76 Torr using either arsine or tertiarybutylarsine (TBA), and trimethylgallium (TMG). Extremely high-purity precursors were used in order to eliminate extrinsic doping effects. Carbon acceptors from the TMG were the dominant residual electrical impurities under all growth conditions. Temperature-dependent Hall measurements were used to make a quantitative comparison of the carbon acceptor concentrations for arsine- and TBA-grown epilayers over a range of As partial pressures. For a given group V partial pressure, we report a significant reduction in carbon acceptor incorporation using TBA compared with arsine under identical growth conditions.
Antireflection coatings for advanced semiconductor device metallization using laser reflow and chemical mechanical planarization
Authors: Yu C. , Sandhu G. S. , Doan T. T.
Published: July 2002

We have investigated titanium tungsten (TiW), titanium nitride (TiN), and chemical vapor deposited (CVD) tungsten (W) films as antireflection coatings (ARC) on aluminum (Al) alloy films to widen the laser planarization process window for contact via filling. ARCs lowered the minimum laser fluence required to accomplish complete contact filling and the maximum laser fluence before the onset of optical ablation, resulting in a significant increase in the process window. This increase closely correlated with the optical and thermal properties of the ARCs. The observed increase in resistivity of laser processed Al films due to intermixing with the ARCs does not preclude its use as via stud metal. A newly developed Al plug technology utilizing chemical mechanical polishing to remove the laser processed film from the surface is presented.
Epitaxial growth of Bi12GeO20 thin-film optical waveguides using excimer laser ablation
Authors: Youden K. E. , Eason R. W. , Gower M. C. , Vainos N. A.
Published: July 2002

Thin-film optical waveguides of the photorefractive optical material bismuth germanium oxide (Bi12GeO20) have been epitaxially grown onto heated zirconia substrates by excimer laser ablative sputtering. The epitaxial nature and stoichiometry of the films were verified using x-ray diffraction analysis. Waveguide modes were observed for effective refractive indices in close agreement with theoretical predictions.
Disk-shaped vacuum ultraviolet light source driven by microwave discharge for photoexcited processes
Authors: Yoshizawa K. , Taki M. , Tachibana K. , Moriyama S.
Published: July 2002

An efficient vacuum ultraviolet light source of large diameter has been developed for use in photoexcited processes. It has a novel structure in which microwaves propagating through a sapphire window break down Xe gas enclosed in a disk-shaped volume and thereby produce intense emission from Xe at a wavelength of 147 nm. By optimizing the pulse width and the repetition rate, we obtained an illuminance of more than 3 mW/cm2 at the window surface. When this lamp was used as a source for photoinduced chemical vapor deposition of hydrogenated amorphous silicon films, a deposition rate of 5 nm/min was attained.
Submicron periodic structures produced on polymer surfaces with polarized excimer laser ultraviolet radiation
Authors: Bolle Matthias , Lazare Sylvain , Le Blanc Monique , Wilmes Aloyse
Published: July 2002

For the first time, pure laser induced periodic structures (without any ablation or any larger structure) of submicron size (spacing and amplitude of 0.2 μm) are developed on polymer surfaces [poly(ethylene terephthalate), poly(butylene terephthalate) and polystyrene] by irradiation with one thousand pulses of the polarized beam of the excimer laser (193 and 248 nm). Fluence is below the ablation threshold and must be chosen in a narrow window which depends on the polymer and the wavelength. The obtained relief is observed by SEM, TEM and characterized by ellipsometry. Structures are obtained also by irradiation of thin films (2000 Å) coated on silicon wafers.
Chemical vapor deposition of diamond films from water vapor rf-plasma discharges
Authors: Rudder R. A. , Hudson G. C. , Posthill J. B. , Thomas R. E. , Hendry R. C. , Malta D. P. , Markunas R. J. , Humphreys T. P. , Nemanich R. J.
Published: July 2002

Polycrystalline diamond films have been deposited from water vapor rf-plasma discharges at 1.0 Torr containing various alcohol vapors. No other gases such as H2, F2, or Cl2 were admitted to the growth chamber. Scanning electron microscopy and Raman spectroscopy have been used to characterize the diamond films. In addition, a water-ethanol mixture has been used for homoepitaxial deposition with a full-width-half-maximum narrower than the bulk substrate (2.60 and 2.75 cm−1, respectively). This technique represents a remarkable new approach to the growth of diamond which does not depend on delivery of hydrogen, fluorine, hydrocarbon, or halocarbon gases that have been typically used by other workers. The nucleation density and topography of the polycrystalline diamond films deposited from the water alcohol mixtures are quite sensitive to the choice of alcohol. Water vapor discharges, by producing H atoms and OH radicals, become the functional equivalent to molecular H2 discharges producing H atoms characteristic of many other diamond chemical vapor deposition techniques.
Epitaxial regrowth of Sb implanted Si1−xGex alloy layers
Authors: Atzmon Z. , Eizenberg M. , Revesz P. , Mayer J. W. , Hong S. Q. , Schäffler F.
Published: July 2002

Solid phase epitaxial regrowth of Sb implanted strained Si1−xGex alloy layers is reported. A set of Si0.92Ge0.08 alloys, MBE grown on (100)Si substrates, were implanted at room temperature with Sb+ ions at an energy of 200 keV and a dose of 1015 ions/cm2. These alloys were annealed in a RTA system at temperatures of 525, 550, and 575 °C for durations between 30 s and 10 min. Backscattering (channeling) measurements show an increase in the regrowth rate compared to furnace annealed Sb implanted (100)Si. Hall effect measurements show an increase in the carrier concentration as the amorphous/crystalline interface propagates to the surface. The concentration reaches a maximal value which exceeds the maximum solid solubility of Sb in Si at the annealing temperature by a factor of six. There is a reverse annealing tendency for longer annealing times.
All-optical switching devices based on large nonlinear phase shifts from second harmonic generation
Authors: Assanto Gaetano , Stegeman George , Sheik-Bahae Mansoor , Van Stryland Eric
Published: July 2002

We show that the large nonlinear phase shifts obtained from phase-mismatched second harmonic generation can be used to implement all-optical switching devices such as a nonlinear Mach–Zehnder interferometer and a nonlinear directional coupler.
New ultraviolet-transport electroconductive oxide, ZnGa2O4 spinel
Authors: Omata Takahisa , Ueda Naoyuki , Ueda Kazushige , Kawazoe Hiroshi
Published: July 2002

ZnGa2O4 having the spinel structure was prepared, and optical and electrical properties of the material were measured. By the measurements of diffuse reflectance spectra, ZnGa2O4 was found to have a wider band gap (∼5 eV) than ITO (indium tin oxide). Electrical conductivity of the H2-annealed ceramic of ZnGa2O4 was 3×101 S cm−1. Thus the ZnGa2O4 spinel was found to be a new UV-transparent electronic conductor.
Double rotor for solid-state NMR
Authors: Samoson A. , Pines A.
Published: July 2002

An NMR double-rotation probe providing for reorientation of a sample around two axes is described, and the key design features for low-friction motion are discussed. By means of double rotation, second-order broadening is averaged away, thus enhancing the resolution of spectra for quadrupole nuclei in solid-state NMR.
Infrared synchrotron radiation measurements at Brookhaven
Authors: Williams G. P. , Hirschmugl C. J. , Kneedler E. M. , Sullivan E. A. , Siddons D. P. , Chabal Y. J. , Hoffman F. , Moeller K. D.
Published: July 2002

We describe the spectrometer and experimental stations on the infrared beamline (IR4) at the National Synchrotron Light Source, Brookhaven. We also report measurements of the source characteristics in the range 30–400 μm (25–350 cm−1), including both flux and brightness and compare them with corresponding measurements from a 1000-K blackbody source.
Low voltage drop plasma switch for inverter and modulator applications
Authors: Goebel D. M. , Poeschel R. L. , Schumacher R. W.
Published: July 2002

A low forward voltage drop plasma switch has been developed for high-efficiency inverter and modulator applications. The switch, called the HOLLOTRON, is based on a grid-controlled, thermionic hollow-cathode discharge. A low forward voltage drop (10–20 V) is achieved by operating the hollow-cathode discharge in a static gas pressure of xenon. The dense plasma generated in the Ba-oxide dispenser hollow cathode is spread over a relatively large control grid area by a diverging magnetic field superimposed on the discharge. Interruption of the discharge current at high current densities (≳4 A/cm2) over the grid area is achieved by biasing the control grid sufficiently negative with respect to the plasma. The HOLLOTRON switch has demonstrated voltage stand-off of up to 20 kV, switching times of ≤0.3 μs, and pulse repetition frequencies of 20 kHz at 50% duty.
NSLS transvenous coronary angiography beamline upgrade and advanced technology initiatives
Authors: Gmür N. F. , Chapman D. , Thomlinson W. , Thompson A. C. , Lavender W. M. , Scalia K. , Malloy N. , Mangano J. , Jacob J.
Published: July 2002

Since October 1990, the coronary anatomies of a total of 16 patients (male and female) have been imaged at the National Synchrotron Light Source (NSLS) as part of the Dual Energy Digital Subtraction Transvenous Coronary Angiography research program. This program takes place in the Synchrotron Medical Research Facility (SMERF) on the X17B2 wiggler beamline. Encouraged by the success of the initial patient images, the NSLS has recently embarked on an ambitious upgrade effort. This effort covers all aspects of the X17B2 beamline and includes improved radiation shielding, a Laue monochromator assembly, a computer-controlled five motion patient scanning chair assembly, a fast low-noise image acquisition system, and a modularized patient safety system. These improvements will allow major advances in imaging patients based on ECG signal gating and multiple view imaging. Two advanced technology initiatives are underway with industrial collaborators. One will develop real-time image acquisition and display of the subtracted digital images. The second will develop a compact x-ray source of medical imaging. The source will be a linear electron accelerator creating characteristic radiation line emissions.
Actively quenched single-photon avalanche diode for high repetition rate time-gated photon counting
Authors: Spinelli A. , Davis L. M. , Dautet H.
Published: July 2002

This paper reports an experimental characterization of the EG&G SPCM-AQ single-photon avalanche diode module with an active quenching bias circuit that gives a dead time of ∼35 ns for use in high count rate, fast timing applications. A quantum efficiency of ≳70% and an optimal timing jitter with a full width at one-thousandth maximum of 1.5 ns is obtained if the light is tightly focused to a spot of <25 μm in the center of the active region, if the signal from the diode before the active quenching circuitry is used as input to the timing electronics, and if an external dead time of ∼55 ns is imposed. Under these conditions, the total probability of obtaining an afterpulse is found to be ∼2×10−3. Limitations of existing time-correlated single-photon counting instrumentation for count rates exceeding 106 s−1 are discussed.
High repetition rate probing laser for Thomson scattering diagnostics
Authors: Kantor M. Yu. , Kouprienko D. V.
Published: July 2002

A new laser based on a multipass intracavity probing system for Thomson scattering diagnostics is presented in the article. The laser system provides a considerable gain in both repetition rate of laser pulses and the diagnostics sensitivity. These capabilities have their origin in a highly effective pumping-to-probing energy conversion which is intrinsic to the system suggested. Main factors affecting the performance of the system are considered. The application of the system in high temperature plasma diagnostics is tested experimentally, and the results are discussed in this article.
Deep level transient spectroscopic and Hall effect investigation of the position of the vanadium acceptor level in 4H and 6H SiC
Authors: Jenny J. R. , Skowronski J. , Mitchel W. C. , Hobgood H. M. , Glass R. C. , Augustine G. , Hopkins R. H.
Published: July 2002

Hall effect, deep level transient spectroscopy (DLTS) and optical absorption measurements were employed in concert to determine the position of the vanadium acceptor level in vanadium and nitrogen doped 6H and 4H SiC. Hall effect results indicate that the acceptor position in 4H SiC is at 0.80 eV beneath the conduction band edge, and 0.66 eV for the 6H polytype. The DLTS signature of the defect in the 4H polytype showed an ionization energy of 0.80 eV and a capture cross section of 1.8×10−16 cm−2. The optical absorption measurements proved that the levels investigated are related to isolated vanadium, and therefore the vanadium acceptor level. Based on the DLTS measurements and secondary ion mass spectroscopy data, the maximum solubility of vanadium in SiC was determined to be 3.0×1017 cm−3. At these incorporation limits and with the depth of the level, the vanadium acceptor level could be used in the creation of semi-insulating silicon carbide.
Silicon heterointerface photodetector
Authors: Hawkins Aaron R. , Reynolds Thomas E. , England Derek R. , Babic Dubravko I. , Mondry Mark J. , Streubel Klaus , Bowers John E.
Published: July 2002

We report the demonstration of an infrared avalanche photodetector that uses an InGaAs absorption layer and a Si avalanche multiplication layer bonded by wafer fusion. Photocurrent measurements of the silicon heterointerface photodetector showed high response to 1.3 μm light and gains of up to 130. Frequency response measurements for the detectors yielded 3 dB bandwidth products of up to 81 GHz.
20 GHz high performance planar Si/InGaAs p-i-n photodetector
Authors: Levine B. F. , Hawkins A. R. , Hiu S. , Tseng B. J. , King C. A. , Gruezke L. A. , Johnson R. W. , Zolnowski D. R. , Bowers J. E.
Published: July 2002

Planar Si/InGaAs wafer fused p-i-n photodetectors were fabricated and measured. They show high internal quantum efficiency, high speed, record low dark current, and no evidence of charge trapping, recombination centers, or a bandgap discontinuity at the heterointerface.
Reduction in defect density by annealing in hydrogenated tetrahedral amorphous carbon
Authors: Conway N. M. J. , Ilie A. , Robertson J. , Milne W. I. , Tagliaferro A.
Published: July 2002

Electronic applications of diamond-like carbon have been limited by its relatively high disorder and defect density. We find that the density of paramagnetic defects in hydrogenated tetrahedral amorphous carbon and the Urbach slope of the optical absorption edge can be reduced by annealing at 300 °C, with little effect on the optical gap. This leads to a reduction in the dark conductivity and an increase in the photosensitivity. The effect is attributed to the migration of hydrogen through the C–C network, to allow better passivation of dangling bonds and a modification of the more weakly bonded sp2 clusters with narrower local band gaps.
Frequency tuning of optical parametric generator in periodically poled optical superlattice LiNbO3 by electro-optic effect
Authors: Lu Yan-qing , Zheng Jian-jun , Lu Ya-lin , Ming Nai-ben , Xu Zu-yan
Published: July 2002

Frequency tuning of optical parametric generators in periodically poled LiNbO3 by applying a periodic electric field was demonstrated. Remarkable wavelength change was achieved. The dependence of the wavelength shift on the applied field shows a linear relationship. The tuning rates exceeding 3 nm/(kV/mm) were obtained. The phenomenon of dispersion in electro-optic tuning was predicted. Possible applications were discussed.
Phase stability for the in situ growth of Nd1+xBa2−xCu3Oy films using pulsed-laser deposition
Authors: Cantoni C. , Norton D. P. , Kroeger D. M. , Paranthaman M. , Christen D. K. , Verebelyi D. , Feenstra R. , Lee D. F. , Specht E. D. , Boffa V. , Pace S.
Published: July 2002

We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth.
AlGaN metal–semiconductor–metal photodiodes
Authors: Monroy E. , Calle F. , Muñoz E. , Omnès F.
Published: July 2002

We report on the fabrication and characterization of AlGaN metal–semiconductor–metal photodiodes with sharp cutoff wavelengths from 365 to 310 nm. The detectors are visible blind, with an ultraviolet/visible contrast of about 4 orders of magnitude. The photocurrent scales linearly with optical power for photon energies both over and below the band gap, supporting the absence of photoconductive gain related to space-charge regions. No persistent photoconductivity effects have been detected. Time response is limited by the RC product of the measurement system, the transit time of the device being far below 10 ns. The normalized noise equivalent power at 28 V bias is lower than 17 pW/Hz1/2 in GaN detectors, and about 24 pW/Hz1/2 in Al0.25Ga0.75N photodiodes.
A monohydride high-index silicon surface: Si(114):H-(2×1)
Authors: Laracuente A. , Erwin S. C. , Whitman L. J.
Published: July 2002

We describe the adsorption of H on Si(114)-(2×1) as characterized by scanning tunneling microscopy and first-principles calculations. Like Si(001)—and despite the relative complexity of the (114) structure—a well-ordered, low-defect-density monohydride surface forms at ∼400 °C. Surprisingly, the clean surface reconstruction is essentially maintained on the (2×1) monohydride surface, composed of dimers, rebonded double-layer steps, and nonrebonded double-layer steps, with each surface atom terminated by a single H. This H-passivated surface can also be easily and uniformly patterned by selectively desorbing the H with low-voltage electrons.
Hydrogenation of ZnS passivation on narrow-band gap HgCdTe
Authors: White J. K. , Musca C. A. , Lee H. C. , Faraone L.
Published: July 2002

Due to the narrow band gap of HgCdTe required for infrared photodetectors, the device performance is readily influenced by surface effects. This letter examines the effect that hydrogenation has on the quality of industry-standard ZnS passivating films. The hydrogenation is achieved by exposing the samples to a H2/CH4 plasma that is present during a reactive ion etching process. The results show a marked improvement of the passivant/substrate interface for hydrogenated devices with a reduction of the average fixed interface charge density to 3.5×1010 cm−2, accompanied by a sixfold decrease in the standard deviation. The advantage of this method of hydrogenation is that it is integrated into the reactive ion etch processing for mesa formation or p-type to n-type conversion in photoconductive or photovoltaic device fabrication, respectively. With the improvement of the ZnS passivation with hydrogenation, this method may alleviate the need for complex epitaxial passivation processing.
Investigation of the formation of the 2.8 eV luminescence band in p-type GaN:Mg
Authors: Shahedipour F. , Wessels B. W.
Published: July 2002

The stability of defects present in GaN:Mg has been investigated using photoluminescence (PL) spectroscopy. The two dominant defect-related PL emission bands in p-type GaN were investigated, the blue band at 2.8 eV and the ultraviolet (UV) emission band at 3.27 eV. The intensity of the 3.27 eV PL band increases with increasing resistivity, whereas the 2.8 eV PL band intensity increases with a decrease in resistivity. The luminescence data is explained by a model whereby the concentration of luminescent centers depends on the Fermi level position. The shallow donor responsible for the UV band is attributed to hydrogen, whereas the deep donor defect responsible for the 2.8 eV band is attributed to a nitrogen vacancy complex.
Optical characterization of high-purity cubic GaN grown on GaAs (001) substrate by metalorganic chemical vapor deposition
Authors: Xu Dapeng , Yang Hui , Li J. B. , Zhao D. G. , Li S. F. , Zhuang S. M. , Wu R. H. , Chen Y. , Li G. H.
Published: July 2002

The optical properties of cubic GaN films have been investigated in the temperature range of 10–300 K. Five peaks were observed at 10 K. From the dependence of photoluminescence emissions on the temperature and excitation intensity, we have assigned two of the five peaks (2.926 and 2.821 eV) to donor–acceptor pair (DAP) transitions. Furthermore, these two peaks were found to be related to a common shallow donor involved in the peak position previously reported at 3.150 eV. The intensities of DAP transitions were much weaker than that of excitonic emission even at low temperature, indicating a relatively high purity of our samples.
On the loss of smoothness of the solutions of the Euler equations and the inherent instability of flows of an ideal fluid
Authors: Yudovich V. I.
Published: July 2002

Certain classes of flows of an ideal incompressible liquid which with time gradually lose their smoothness are studied. The loss of smoothness is expressed as infinite growth of the vorticity as t→∞ for three-dimensional flows and an increase of the gradient of the vorticity for planar and axisymmetric flows. Examples of such flows in the planar and axisymmetric cases are flows with a rectilinear streamline; this can be established using a special local Lyapunov function. Incompressible flows of a dusty medium are another example (it turns out that collapse is impossible for such flows, but the vorticity and the rate of deformation, as a rule, grow with no limit). Other examples can be constructed by composition of shear flows. It is shown that in the vorticity metric almost all stationary planar flows are unstable with respect to three-dimensional disturbances and in the vorticity gradient metric planar and axisymmetric flows with a rectilinear streamline are unstable.
Rate equations for collision-induced desorption and abstraction in the reaction system H(g)+D/Si(100)→D2,HD at 573 K
Authors: Khanom F. , Shimokawa S. , Inanaga S. , Namiki A. , N.-Gamo M. , Ando T.
Published: July 2002

Collision-induced desorption (CID) and Eley–Rideal (ER) type abstraction have been investigated in the reaction system, H(g)+Dad/Si(100)→D2,HD, at surface temperature of 573 K where dihydride phase is unstable. Rate equations for CID were obtained based on a second-order kinetics with respect to doubly occupied Si dimers DSi–SiD, DSi–SiH, and HSi–SiH, which are considered as parent species of transiently formed dihydrides, DSiD, DSiH, and HSiH, respectively. The coverages of the doubly occupied dimers were determined as a function of H exposure time by means of temperature-programmed-desorption (TPD), and then the rate equations were calculated. As a result, the experimental D2 and HD rate curves were reasonably fit with the rate equations formulated. This fact indicates that CID is induced when two dihydride species encounter during their propagation via dihydride–monohydride isomerization reactions. It was found that HSi–SiH does not play a role in CID of both D2 and HD. Possible origins of this isotope effect were discussed in terms of quantum effects on associative desorption and diffusion processes. The HD rate curve due to direct ER abstraction channel was fit with a first-order kinetics in Dad coverage for a low coverage regime where dangling bonds exist. However, for a high coverage regime where the surface is saturated, the HD rate curve was fit with DSi–SiD coverages, suggesting that Dad that are paired up with Had in the same Si dimers are not abstracted.
On the initial steps in the decomposition of energetic materials from excited electronic states
Authors: Im H.-S. , Bernstein E. R.
Published: July 2002

Decomposition studies of hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX-C3H6N6O6, see Fig. 1) isolated in the gas phase and cooled in a supersonic expansion are reported for the excited electronic state near 225 nm. The RDX is handled safely and effectively through matrix-assisted laser desorption (MALD) of a thin film of RDX/R6G laser dye (1:1) adsorbed on an aluminum oxide coating on an aluminum drum. The aluminum oxide coating is generated by plasma electrolytic oxidation of aluminum. The combination of MALD and supersonic molecular beam techniques generates intact and cold RDX molecules isolated in the gas phase. Two basic conclusions are reached in this study. (1) Photodissociation of RDX at ≈225 nm generates NO as an initial product. (2) Nascent NO thus generated is vibrationally hot (Tvib∼1800 K) and rotationally cold (Trot∼20 K).
Review of physics and applications of relativistic plasmas driven by ultra-intense lasers
Authors: Umstadter Donald
Published: July 2002

As tabletop lasers continue to reach record levels of peak power, the interaction of light with matter has crossed a new threshold, in which plasma electrons at the laser focus oscillate at relativistic velocities. The highest forces ever exerted by light have been used to accelerate beams of electrons and protons to energies of a million volts in distances of only microns. Not only is this acceleration gradient up to a thousand times greater than in radio-frequency-based sources, but the transverse emittance of the particle beams is comparable or lower. Additionally, laser-based accelerators have been demonstrated to work at a repetition rate of 10 Hz, an improvement of a factor of 1000 over their best performance of just a couple of years ago. Anticipated improvements in energy spread may allow these novel compact laser-based radiation sources to be useful someday for cancer radiotherapy and as injectors into conventional accelerators, which are critical tools for x-ray and nuclear physics research. They might also be used as a spark to ignite controlled thermonuclear fusion. The ultrashort pulse duration of these particle bursts and the x rays they can produce, hold great promise as well to resolve chemical, biological or physical reactions on ultrafast (femtosecond) time scales and on the spatial scale of atoms. Even laser-accelerated protons are soon expected to become relativistic. The dense electron–positron plasmas and vast array of nuclear reactions predicted to occur in this case might even help bring astrophysical phenomena down to Earth, into university laboratories. This paper reviews the many recent advances in this emerging discipline, called high-field science.
Vibrational and rotational energy transfer in collisions of vibrationally excited HF molecules with Ar atoms
Authors: Krems Roman V. , Nordholm Sture
Published: July 2002

This work presents converged vibrational close coupling-rotational coupled states calculations of cross sections and rate constants for rotational and vibrational transitions in collisions of vibrationally excited HF molecules with Ar atoms. Reduced channel basis sets assuming both a lower and an upper cutoff in vibrational quantum number are used for calculations at high internal energies of the diatomic molecule. The most recent potential energy surface is employed for the calculations and the correspondence of the results to the previous investigation of rovibrational dynamics in collisions of HF(v=1) with Ar is examined. It is shown that initial vibrational excitation stimulates vibrationally inelastic transitions to a great extent while the rotational energy transfer is essentially unaffected by initial v-number. The cross sections for vibrational relaxation of different vibrational levels of HF are shown to exhibit a strong dependence on initial rotational energy which is, however, of different magnitude for different vibrational states. The dependence of the vibrational relaxation of HF(v=1,3,6) on the translational energy of the atomic collision partner is different by an energy independent factor for different vibrational levels in the high energy limit and shows minima at low collision energies.
Computational band-structure engineering of III–V semiconductor alloys
Authors: Geller Clint B. , Wolf Walter , Picozzi Silvia , Continenza Alessandra , Asahi Ryoji , Mannstadt Wolfgang , Freeman Arthur J. , Wimmer Erich
Published: July 2002

Accurate band structures of binary semiconductors AB (A=Al, Ga, In and B=P, As, Sb) and selected ternary III–V semiconductors were calculated using an all-electron screened exchange approach within the full potential linearized augmented plane-wave method. Fundamental band gaps and Γ–L and Γ–X separations in higher-lying conduction bands are predicted with an accuracy of a few tenths of 1 eV. Screened exchange also performs better than the local density approximation for calculating conduction-band effective masses. Highly n-doped InPAs materials with compositions near InP0.2As0.8 offer lower effective masses, greater optical band-gap shifts, and potentially higher electron mobility than n-doped InGaAs materials with comparable band gaps.
Degradation kinetics of thermal oxides
Authors: Irrera Fernanda
Published: July 2002

In this letter I propose a kinetics model of trap creation in thermal oxides under electrical stress. The model is based on the idea that electrons drifted in the oxide conduction band by the stress field undergo scattering by existing defects. The process controls the creation of additional traps. The kinetics model predicts a square root dependence on the stress time. Model results are validated by experimental data of stress induced leakage current. Defect concentrations calculated with my kinetics model were used in a trap-assisted model and yielded excellent fits of the low-field conductivity
Evaporation of carbon nanotubes during electron field emission
Authors: Dean Kenneth A. , Burgin Timothy P. , Chalamala Babu R.
Published: July 2002

Extracting large field-emission currents from individual single-walled carbon nanotubes degrades their current–voltage behavior and changes the nanotube structure. Field-emission microscopy observations and field-emission current–voltage measurements revealed behavior characteristic of local heating, followed by thermally assisted field evaporation of the atoms on the end of the nanotube, and subsequent restructuring of the cap. Field evaporation of carbon was found to result in reduction of the length of the tube without catastrophic arcing events. This process defines the upper limit for the field-emission current that can be obtained from an individual single-walled nanotube.
Large magnetoresistance in Fe/MgO/FeCo(001) epitaxial tunnel junctions on GaAs(001)
Authors: Bowen M. , Cros V. , Petroff F. , Fert A. , Martı́nez Boubeta C. , Costa-Krämer J. L. , Anguita J. V. , Cebollada A. , Briones F. , de Teresa J. M. , Morellón L. , Ibarra M. R. , Güell F. , Peiró F. , Cornet A.
Published: July 2002

We present tunneling experiments on Fe(001)/MgO(20 Å)/FeCo(001) single-crystal epitaxial junctions of high quality grown by sputtering and laser ablation. Tunnel magnetoresistance measurements give 60% at 30 K, to be compared with 13% obtained recently on (001)-oriented Fe/amorphous-Al2O3/FeCo tunnel junctions. This difference demonstrates that the spin polarization of tunneling electrons is not directly related to the density of states of the free metal surface—Fe(001) in this case—but depends on the actual electronic structure of the entire electrode/barrier system.
Photodissociation study of CS2 at 193 nm using slice imaging
Authors: Kitsopoulos Theofanis N. , Gebhardt Christoph R. , Rakitzis T. Peter
Published: July 2002

The photodissociation of CS2 at 193 nm (51 700) cm−1) is studied using the slice imaging technique, and velocity distributions for state selected S(3P2,1,0) and S(1D2) photofragments are measured. From the analysis of the speed distributions we determine the S(3P2,1,0)/S(1D2) branching ratio to be 1.5±0.4. The partially resolved S-atom speed distributions peak at intermediate speeds, indicating substantial rovibrational excitation of the CS sibling fragment. By modeling this CS propensity using a Franck–Condon analysis between the X(1Σg+) and the excited B21 state, we estimate the origin of the excited state to be located at ∼45 600 cm−1. From the S-atom angular distributions, we determine the spatial anisotropy parameter β, and find that the value for S(3P2,1,0) is slightly speed dependent with an average value of ∼0.2. The β values for S(1D2) are strongly speed dependent, reaching the value of 0.6 at intermediate speeds and an average value of 0.2 at low and high speeds. From the observed β values we conclude that the CS2 molecule is highly bent (∼115°) prior to dissociation, while the dissociation lifetime is on the order of a rotational period.
Variable temperature study of the passivation of dangling bonds at Si(100)-2×1 reconstructed surfaces with H and D
Authors: Hersam M. C. , Guisinger N. P. , Lee J. , Cheng K. , Lyding J. W.
Published: July 2002

The passivation of the Si(100) surface with H and D is studied with scanning tunneling microscopy (STM). During the passivation process, the clean Si(100) surface is exposed to a gas phase mixture of atomic H and D. By directly observing the dramatic isotopic difference in STM-induced electron stimulated desorption rates, the relative surface concentrations of H and D is discerned with atomic resolution. The ratio of D to H on the Si(100) surface is found to vary by more than an order of magnitude following monolayer passivation at temperatures between 300 and 700 K. A statistical thermodynamics model attributes this behavior to the difference in the vibrational frequencies of H and D on silicon surfaces.
The level position of a deep intrinsic defect in 4H-SiC studied by photoinduced electron paramagnetic resonance
Authors: Zvanut M. E. , Konovalov V. V.
Published: July 2002

Photoinduced electron paramagnetic resonance studies performed on nominally semi-insulating, high purity 4H-SiC have revealed charge transfer from an intrinsic defect (ID) to both the shallow boron acceptor and nitrogen donor. At 4 K, incident photon energy between 1.0 and 1.7 eV produces an increase in paramagnetic boron of approximately the same magnitude as the decrease in the paramagnetic defect concentration. For T<80 K, both spectra remain unchanged after removing the light. Illumination with energy greater than 1.8 eV at 4 K increases the nitrogen, boron, and ID spectra simultaneously, but after blocking the light all three signals return to the pre-illumination level. A model based on excitation to and from the bandedges places the defect level 1.1±0.2 eV above the valence bandedge.
Site-occupying behavior of boron in compensated p-type 4H–SiC grown by sublimation epitaxy
Authors: Kakanakova-Georgieva A. , Yakimova R. , Linnarsson M. K. , Janzén E.
Published: July 2002

Results from electrical and optical measurements of boron in compensated p-type 4H–SiC layers doped with Al, N, and B are reported. The layers were produced by sublimation epitaxy and characterized by secondary ion mass spectrometry, capacitance–voltage, and cathodoluminescence techniques. The boron-related contribution to the net acceptor concentration in the layers as well as the boron-related emission at ∼505 nm are detected for various growth conditions. The effect of the concentrations of the attendant impurities Al and N, concentration ratio of Al to N atoms, and growth rate on the site-occupying behavior of boron in the layers is discussed.
Plasma molding over surface topography: Energy and angular distribution of ions extracted out of large holes
Authors: Kim Chang-Koo , Economou Demetre J.
Published: July 2002

Plasma molding over surface topography was investigated by measuring the energy and angular distribution of ions extracted from a hole in contact with a high density plasma. Holes with diameter larger than as well as smaller than the local sheath thickness were studied in argon or deuterium gas. When the hole diameter (10 μm) was much less than the sheath thickness, the plasma was not perturbed by the presence of the hole. The ion energy distribution (IED) had multiple peaks due to ions sampling the time-varying potential while crossing the sheath. The ion angular distribution (IAD) was Gaussian, peaking at zero angle with respect to the surface normal. These results agree with reported studies. At the other extreme, when the hole diameter (1270 μm) was larger than the sheath thickness, plasma “leaked” into the hole. The IED had a single peak since ions now experience an average sheath potential. The IAD was quite broad extending beyond 30° off normal. When the hole diameter (508 μm) was comparable to the sheath thickness, the shape of the IED and IAD was in-between the two extremes mentioned above. The IAD became more isotropic with increasing power, suggesting that the plasma leaked only partly through the hole (the plasma–sheath meniscus was located inside the hole). For all cases, increasing pressure resulted in lower ion energy in argon plasmas due to ion–neutral collisions. Increasing pressure had little effect on the ion energy for deuterium plasmas, for hole diameter less than 508 μm. This is due to the smaller ion–neutral collision cross section for deuterium.
Study of charge distribution across interfaces in GaN/InGaN/GaN single quantum wells using electron holography
Authors: Cai Juan , Ponce F. A.
Published: July 2002

Strong internal electric fields (of the order of MV/cm) influence the electronic properties and light emission process of InGaN quantum wells. We have used electron holography to directly profile the potential and charge distribution across a GaN/In0.18Ga0.82N/GaN quantum well structure, and present here an analysis of the fine-scale potential variations at the quantum well. The potential profiles show a drop of 0.6±0.2 V across, and an internal electric field of −2.2±0.6 MV/cm in the quantum well. An analysis based on Poisson’s equation suggests that the field is caused by electronic charges with a peak density of 8×1020 cm−3, corresponding to a sheet charge density of 0.027 C/m2. Free electron and hole densities of the order of 1020 cm−3 are confined separately in the quantum well. These free carriers screen only part of the electric field due to the polarization effect.
What is the role of chaotic scattering in irreversible processes?
Authors: Gaspard Pierre
Published: July 2002

We study kinetic properties of simple mechanical models of deterministic diffusion like the scattering of a point particle in a billiard of Lorentz type and the multibaker area-preserving map. We show how dynamical chaos and, in particular, chaotic scattering are related to the transport property of diffusion. Moreover, we show that the Liouvillian dynamics of the multibaker map can be decomposed into the eigenmodes of diffusive relaxation associated with the Ruelle resonances of the Perron–Frobenius operator.
Thermally annealed silicon nitride films: Electrical characteristics and radiation effects
Authors: Stein Herman J.
Published: July 2002

Electrical characteristics, including retention under 60Co γ-ray irradiation of MNOS (metal-nitride-oxide-semiconductor) structures with LPCVD (low-pressure chemical-vapor-deposited) silicon nitride have been investigated. Capacitance-voltage techniques were used to measure injected, retained, and equilibrium charge. Current-voltage techniques were used to measure voltage and temperature dependence of charge transport. Measurements were made on MNOS with the following nitride annealing histories: (1) as-deposited at 750 °C, (2) 950 °C in N2, and (3) 950 °C in N2 followed by 900 °C in H2. Internal IR reflection techniques were used to measure chemically bonded hydrogen in the nitride films. Annealing at 950 °C in N2 (1) decreased the concentration of hydrogen, (2), decreased equilibrium positive charge, and (3) increased low- and high-field transport. Partial restoration of the as-deposited characteristics was achieved by subsequent annealing in H2 at 900 °C. Charge loss (retention) under 60Co γ-ray irradiation is essentially independent of high-temperature annealing whereas degradation is observed for net negative charge retention measured in the absence of ionizing radiation. Effects of annealing on MNOS structures and models to explain the results are discussed.
Effect of melt temperature on the dc conductivity of GeO2 glasses
Authors: Magruder R. H. , Kinser D. L. , Weeks R. A. , Jackson J. M.
Published: July 2002

The dc electrical properties of a series of high purity GeO2 glasses fused and equilibrated at various temperatures (Tφ) in air were measured. Tφ ranged from 1200 to 1690 °C. The observed changes are not correlated changes in concentration of any of the impurities as determined by neutron activation analyses or IR measurements of OH concentrations. The resistivity was found to obey an Arrhenius function with enthalpy of activation of approximately 1.0 eV for all Tφ’s except for Tφ =1200 °C. The charge carrier was concluded to be the Na+ ion. The mobility of the Na+ ion was calculated and was found to be a function of Tφ. We suggest that the change in Tφ results in a change in the configuration coordinates of the average interstitial sites through which the Na+ ion moves. This change results in differences in the vibrational energy structure of the interstitial sites. These differences are manifested in the entropy of activation. With changes in the entropy of activation, the preexponential term changes, producing the variation in mobilities which is observed.