bluuebunny/crossref_metadata_2025 huggingface dataset:
Rarefied gas flow through a long tube at arbitrary pressure and temperature drops
Authors: Sharipov Felix
Published: July 2002

The mass flow rate of a rarefied gas through a long tube caused by large drops of the pressure and temperature has been calculated. The temperature ratio T2/T1=3.8, which is usually realized in practice, and the three values of the pressure ratio (P2/P1=3.8, 10, and 100) have been taken. It has been found that the application of the linear theory based on the average rarefaction parameter to the nonisothermal gas flow reveals the error to be within 10%.
Wide band gap materials for field emission devices
Authors: Zhirnov V. V. , Wojak G. J. , Choi W. B. , Cuomo J. J. , Hren J. J.
Published: July 2002

An analysis of wide band gap materials from the point of view of their application in cold emission devices is presented, and criteria of material choice for device application are discussed. Not only material but also technological parameters are taken into consideration. Among the material parameters, the following were found to be the most important; electron affinity, dielectric constant, thermal conductivity, melting point, chemical and physical robustness. The major technological parameter is compatibility of the material deposition process with commercially available facilities and other steps of cathode fabrication. It was shown that wide band gap materials are most effective for emission if deposited on sharp conductive tips. Experimental results from diamond, AlN, c-BN, and SiO2 field emitters are presented and some possible mechanisms explaining their I–V characteristics are discussed.
Etching of GaAs/AlGaAs by bisdimethylaminochlorarsine
Authors: Okamoto Naoya , Tanaka Hitoshi
Published: July 2002

We report on the etching characteristics of GaAs and AlGaAs by gas source molecular beam epitaxy (GSMBE) using a new precursor bisdimethylaminochloroarsine (BDMAAsCl). The etching rate of GaAs is linearly dependent on the BDMAAsCl flow rate. The activation energy of GaAs etching was 0.25 eV. However, Al0.28Ga0.72As etching shows the negative activation energy of −0.29 eV at the substrate temperature below 500 °C. The etchings of GaAs and AlGaAs are probably attributed to the formation of GaCl and AlCl2 or AlCl3, respectively. Furthermore, we demonstrated that the BDMAAsCl etching remarkably reduced the interfacial impurity (carbon, oxygen, silicon) density of GSMBE-grown GaAs epilayers/epiready substrates by one order of magnitude compared with trisdimethylaminoarsine cleaning only.
Fabrication and domain imaging of iron magnetic nanowire arrays
Authors: Tulchinsky D. A. , Kelley M. H. , McClelland J. J. , Gupta R. , Celotta R. J.
Published: July 2002

Arrays of magnetic nanowires are fabricated by using a corrugated surface, produced by chromium atoms laser-focused in a one-dimensional standing wave, as a shadow mask for an iron evaporator. The deposited iron forms a periodic array consisting of thousands of 20–40 nm high ferromagnetic lines with width of ∼100 nm, spaced every 213 nm, and having aspect ratios (width:length) of 1:1500. Images of the magnetic domain structures of these nanowires are obtained by scanning electron microscopy with polarization analysis. Elongated domains ∼100 nm wide and ∼16 μm long are observed.
Turbomolecular pump design for high pressure operation
Authors: Spagnol M. , Cerruti R. , Helmer J.
Published: July 2002

A simple model of compressible Couette–Poiseuille flow is developed to describe the pressure distribution in a single Gaede stage, in viscous flow. From this a scaling law to optimize the stage design has been derived. Both model and scaling law are verified using experimental measurements of different channel geometries. A practical limit to high pressure operation is set by the power consumption of the motor. A program has been developed to optimize the design of a multistage Gaede pump and the results of it suggests that a pump with an axial height of only 3 cm can exhaust at pressures up to 100 mbar.
Novel electronic and magnetic properties of ultrathin chromium oxide films grown on Pt(111)
Authors: Robbert P. S. , Geisler H. , Ventrice C. A. , van Ek J. , Chaturvedi S. , Rodriguez J. A. , Kuhn M. , Diebold U.
Published: July 2002

The growth of epitaxial metal–oxide films on lattice-mismatched metal substrates often results in the formation of unique overlayer structures. In particular, epitaxial chromium oxide films grown on Pt(111) exhibit a p(2×2) symmetry through the first two monolayers of growth which is followed by a (√3×√3)R30° phase that is attributed to the growth of a Cr2O3(0001) overlayer. Ultraviolet photoelectron spectroscopy measurements have been performed on the CrOx/Pt(111) system. The electronic structures of CrO2, Cr2O3, and Cr3O4 were calculated using the linear muffin-tin orbital method in the atomic sphere approximation. Comparison of the photoemission valence band spectra with the calculated density of states indicates that the CrOx initially grows in a cubic spinel Cr3O4 structure. Beyond ∼0.2 monolayers, the metallic behavior of the CrOx overlayer begins a transformation to an insulating state. The measured valence emission for the p(2×2) phase beyond ∼0.2 monolayers is more consistent with either a γ-Cr2O3(111) overlayer or possibly a reconstructed Cr2O3(0001) overlayer.
Microstructural characterization of ion assisted SiO2 thin films by visible and infrared ellipsometry
Authors: Brunet-Bruneau A. , Souche D. , Fisson S. , Nguyen Van V. , Vuye G. , Abeles F. , Rivory J.
Published: July 2002

The complex dielectric function of SiO2 films deposited on silicon substrates by electron-beam evaporation under ion bombardment is determined by infrared ellipsometry between 700 and 5000 cm−1. Analysis of the absorption band associated with the Si–O–Si stretching mode at about 1040 cm−1, the water absorption band at about 3300 cm−1, and knowledge of the visible refractive index determined by visible ellipsometry (in situ and after venting to atmosphere) enable us to model the complex structure of the films, as a densified silica matrix with pores. Depending on the ion assistance conditions, the pores are either mostly connected (allowing water penetration) or largely isolated (no water adsorption). The refractive index of the silica matrix, the pore volume fraction and the pore water filling rate are evaluated as a function of the assistance conditions.
Consequences of three-dimensional physical and electromagnetic structures on dust particle trapping in high plasma density material processing discharges
Authors: Hwang Helen H. , Keiter Eric R. , Kushner Mark J.
Published: July 2002

Plasma processing discharges are typically designed with the goal of having uniform reactant fluxes to the substrate and a minimum of dust particle contamination of the wafer. It is not uncommon, however, that reactors have three-dimensional (3D) structures such as antennas (or coils), gas injection nozzles, sub- or super-wafer topography and single-sided pump ports. These structures can contribute to azimuthal asymmetries in reactant fluxes. These structures may also produce dust particle traps. In this paper, a 3D plasma equipment model is applied to investigate the impact of these structures on reactant fluxes and their influence on dust particle trapping in inductively coupled radio frequency discharges under conditions where trapping is not typically obtained. We find that 3D structures, such as injection nozzles, perturb the plasma potential and ion fluxes to distances well beyond their geometrical boundaries. These perturbations are sufficient to create dust particle traps. Electromagnetic asymmetries caused by coils which have poor impedance matching may also produce sufficient azimuthal asymmetries in ion fluxes that dust particle traps are generated.
Acquisition of clean ultrahigh vacuum using chemical treatment
Authors: Tatenuma K. , Uchida K. , Itoh T. , Momose T. , Ishimaru H.
Published: July 2002

The purpose of this study is to attain a state of ultrahigh vacuum (UHV) under a mild baking condition using chemical treatment. The vacuum test involved the study of an aluminum alloy chamber 8.5 m long, 2 m2 in surface area and 27 l in volume with an elliptical aperture for the Recycler Ring at Fermilab. After the chamber was exposed to enough air of 95% relative humidity and evacuated to 10−5 Torr, dichloropropane (DCP) gas of 1 Torr was injected into the mildly heated chamber (at 80 °C) and evacuated. This process was repeated between three and four times, depending on the type of test performed. After the first chemical treatment (test-1 DCP), the ultimate pressures were 7×10−10 Torr at the left end and 9×10−10 Torr at the right end, an only slight difference. When test-1 DCP reached the ultimate lowest pressure, the chemical retreatment (test-2 DCP) was applied under the mild baking condition on the entire inner surface of all components: chamber, Bayard-Alpert gauges, a quadrupole mass spectrometer, an ion pump, and a turbomolecular pump. Test-2 DCP showed that the ultimate pressure at the left end near the main pump was less than 1×10−11, which was under the detectable limit of the gauge. On the other hand, in the case of a reference test (test-1 ref) conducted under the same condition as test-1 DCP but without the chemical treatment, the ultimate pressures were twice as high at the left end and 80 times higher at the right end than the pressures of test-1 DCP. Therefore, it was assumed that the lower outgassing rate resulted from the chemical treatment. In addition, it was suggested that DCP has a potential to promote water evacuation by breaking a weak intermolecular hydrogen bond of adsorbed multilayer water which is preferentially dissociated rather than a strong intramolecular oxygen-hydrogen bond of water.
Structural characteristics of AlN films deposited by pulsed laser deposition and reactive magnetron sputtering: A comparative study
Authors: Jagannadham K. , Sharma A. K. , Wei Q. , Kalyanraman R. , Narayan J.
Published: July 2002

Aluminum nitride films have been deposited on Si(111) substrates at different substrate temperatures using two techniques; pulsed laser deposition or reactive magnetron sputtering. The films deposited by either of the techniques have been characterized by x-ray diffraction and transmission electron microscopy to determine the crystalline quality, grain size, and epitaxial growth relation with respect to the substrate. The bonding characteristics and the residual stresses present in the films have been evaluated using Raman and Fourier transform infrared spectroscopy. Secondary ion mass spectrometry has been performed to determine the nitrogen stoichiometry and the presence of impurities such as oxygen and silicon. The adhesion strength of the AlN films to the silicon substrate and the wear resistance have been determined by scratch test and a specially designed microscopic wear test. A comparison of the different characteristic features associated with the AlN films deposited by pulsed laser deposition or magnetron sputtering is presented with particular emphasis to electronic and tribological applications.
Doping effects and reversibility studies on gas-exposed α-sexithiophene thin films
Authors: Kendrick C. , Semancik S.
Published: July 2002

The electronic effects produced by controlled gas exposures on α-sexithiophene thin films have been investigated using x-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Incremental exposures of α6T films to NO2, O2, NH3 in N2, and water vapor in N2 were performed in ultrahigh vacuum, while repeated doses of O2, H2 in N2, and methanol vapor in N2 were performed ex situ at ambient pressure. In both conditions XPS spectra of gas-exposed films showed no evidence of chemical changes. However, the features in both XPS and UPS spectra were observed to shift as a function of gas dosage, with the magnitude dependent on each particular gas. These effects could be reversed by heating the films to temperatures around 100 K. This behavior is interpreted in terms of doping by weakly bonded gas species within the near-surface region of the α6T films. Greater doping effects were observed for films dosed at ambient pressure. We discuss possible gas adsorption models that may explain the differing gas sensitivities and their dependences on pressure, temperature, and exposure time. Finally, temperature programmed desorption was used to study the reversibility and resistance to reaction of α6T films during this gas adsorption/desorption process. The films were found to be chemically stable to gas exposure and subsequent thermal desorption in the dosage ranges explored.
Seeded pulsed supersonic molecular beam growth of silicon carbide thin films
Authors: Jamison K.D. , Kempel M.L. , Ballarotto V. W. , Kordesch M.E.
Published: July 2002

SiC thin films have been grown on Si(100) and 6H SiC substrates using a dual pulsed supersonic molecular beam system. Silane and hydrocarbon precursors were seeded in helium and hydrogen molecular beams to increase the translational kinetic energy of the arriving reactants. Energies between 2 and 4.8 eV were used for single source growth, the hydrocarbon and silane source energy ranged from 0.3 to 1.8 eV. High quality, single polytype homoepitaxial films were grown on 6H SiC at 1530 K; SiC films were grown on Si(100) substrates at 1400 K. Generally, better morphology and growth rate (0.8 μm/h) was achieved with silane and hydrocarbon source material seeded in a hydrogen carrier gas rather than a He carrier gas. Pulsed seeded supersonic molecular beam grown SiC was imaged using 5 and 10 eV photons in a photoelectron emission microscope. Contrast effects that could potentially be used to determine film quality were observed.
Energy distribution of ions bombarding biased electrodes in high density plasma reactors
Authors: Edelberg Erik A. , Perry Andrew , Benjamin Neil , Aydil Eray S.
Published: July 2002

A compact retarding field ion energy analyzer has been designed and built to measure the energy distribution of ions bombarding the wafer surfaces placed on radio frequency (rf) biased electrodes in high-density plasma reactors. The analyzer was used to measure the energy distribution of ions impinging on the rf-biased electrostatic chuck in a high-density transformer coupled plasma (TCP) reactor. The effects of TCP power, rf bias, gas composition, and ion mass on the ion energy distributions (IEDs) were demonstrated through Ar, Ne, Ar/Ne, O2 and CF4/O2 discharges. In the operating range studied, the average ion energy increased linearly with increasing rf bias while the ion flux remained constant indicating that independent control of ion flux and energy was achieved in the TCP reactor. Bimodal ion energy distributions resulting from ion energy modulation in the sheath were observed and multiple peaks in the IEDs measured in gas mixtures were identified as ions with different masses falling through the sheath.
Ex situ characterization of phase transformations and associated microstructures in polycrystalline thin films
Authors: Barmak K. , Rickman J. M. , Michaelsen C. , Ristau R. A. , Kim J. , Lucadamo G. A. , Carpenter D. T. , Tong W. S.
Published: July 2002

This article presents new ex situ techniques, as well as novel applications of conventional methods, to investigate phase transformations and associated microstructures in polycrystalline thin films. Specifically, we review the results of a dark-field transmission electron microscopy study of the disorder-order transformation thermodynamics and kinetics in CoPt and FePt thin films and, in addition, discuss the applications of differential scanning calorimetry to the interrogation of solid-state reactions in Nb/Al, Ni/Al, and Ti/Al systems. These investigations have naturally prompted the development of methods to analyze and better understand our experimental data. Thus, we also outline a new, automated image processing algorithm for transmission electron micrographs and discuss its implementation to interpret images of thin Al film microstructures. Further, as computer simulation has been employed to highlight important nucleation and growth parameters, we briefly summarize the results of these investigations.
Dynamics of the m=1 diocotron mode in the electron diffusion gauge experiment
Authors: Chao Edward H. , Paul Stephen F. , Davidson Ronald C.
Published: July 2002

The evolution of the m=1 diocotron mode is studied experimentally over a wide range of system parameters for a pure electron plasma confined in a Malmberg–Penning trap. The frequency of the m=1 diocotron mode is monitored and found to be higher than that predicted by Levy [Phys. Fluids 11, 920 (1968)] for an infinite-length plasma column. However, the experimental measurements of the m=1 mode frequency reported here are found to be in excellent agreement with the theoretical predictions for a finite-length plasma column. The growth and damping rates of the m=1 diocotron mode are also carefully measured and compared with theoretical predictions. Probably the strongest factor affecting the stability of the m=1 diocotron mode in a nonneutral plasma with a monotonically decreasing radial density profile is resistive-wall destabilization. The measured resistive growth rates are found to agree with theory, at least for moderate values of resistance. Even after minimizing the resistive-wall instability, and maintaining the background gas pressure sufficiently low that pressure effects on the m=1 diocotron mode are negligible, it is observed experimentally that there is an “anomalous” damping of the m=1 diocotron mode that depends on the magnetic field, B, and the line density, NL, through the ratio NL/B, which is proportional to the oscillation frequency of the m=1 diocotron mode. Although the cause of this damping is not understood from first principles, its effects can be minimized by tuning the frequency of the diocotron mode. Finally, although electron collisions with background neutral atoms had been expected to result in an unstable m=1 diocotron mode, it was observed that increasing the background gas pressure causes a damping of the m=1 mode.
Ultrathin Cu films on Si(111): Schottky barrier formation and sensor applications
Authors: Nienhaus H. , Bergh H. S. , Gergen B. , Majumdar A. , Weinberg W. H. , McFarland E. W.
Published: July 2002

Ultrathin Cu films were evaporated on Si(111) surfaces at substrate temperatures of 175 K. By use of a microfabricated device structure, zero-force electrical contacts were formed on the thin Cu layers during evaporation. They allowed current/voltage measurements of diodes with Cu films between 40 and 60 Å . Although the rectifier properties are improved with increasing thickness, the 60 Å diode still exhibits a large inhomogeneous interface with a low barrier height of 0.47 eV and an ideality factor of 2.1. Annealing the diode to room temperature leads to significant changes in the barrier height which increases to 0.65 eV and the ideality factor which decreases to unity, suggesting a modification of the interface. The annealed thin-metal diodes may be used as atomic hydrogen sensors. A chemicurrent is observed in the diode when exposed to H atoms. The current is based on a nonadiabatic electron–hole pair creation which occurs during exothermic adsorption of hydrogen on Cu surfaces.
Implications of contact mechanics models for mechanical properties measurements using scanning force microscopy
Authors: Unertl W. N.
Published: July 2002

Considerable effort is aimed at using the scanning force microscope (SFM) to measure the mechanical properties of surfaces with nanometer-scale resolution. The properties of interest include the Young and shear moduli, shear strength, and work of adhesion. The most widely used approach is to extract these properties from the SFM data by simply scaling the results of the macroscopic continuum mechanics theory of contact to the dimensions and forces of a SFM contact. This article provides a contemporary overview of contact mechanics from the perspective of SFM. The discussion is limited to nonsliding contacts between linear elastic materials. One of its main goals is to emphasize the assumptions underlying and restricting the application of the most commonly used models and their implications for SFM measurements.
Mass spectrometric detection of reactive neutral species: Beam-to-background ratio
Authors: Singh Harmeet , Coburn John W. , Graves David B.
Published: July 2002

Mass spectrometry and appearance potential mass spectrometry (APMS) have recently gained importance for detection and quantitative measurements of reactive radical species in plasmas using line-of-sight sampling of reactive species. In this work, we have characterized the contributions to the mass spectrometer signal from the line-of-sight “beam” component and the background component of the species in the ionizer of the mass spectrometer. The beam signal is proportional to the number density of the species in the plasma, while the background component of the signal depends on various factors like the vacuum system design and pump speeds. Single differential pumping of the mass spectrometer is found to be inadequate as the background signal dominates the beam signal for radical and stable neutral species. The beam to background ratio for CFx (x=1–3) radicals is smaller than 0.25 and the large background signals of the species of interest necessitates implementation of modulated beam mass spectrometry using a mechanical chopper in the beam path. The uncertainty in the beam component measurement is found to be as large as ±180%. High beam-to-background signal ratio is achieved using three stages of differential pumping, and this vastly reduces the uncertainty in the beam component measurement to less than ±10%.
Growth and characterization of Ba0.6Sr0.4TiO3 thin films on Si with Pt electrodes
Authors: Kinder L. , Zhang X. F. , Grigorov I. L. , Kwon C. , Jia Q. X. , Luo L. , Zhao J.
Published: July 2002

Both metalorganic chemical vapor deposition and pulsed laser deposition have been used to grow Ba0.6Sr0.4TiO3 (BST) thin films on Si with Pt electrodes. The smoother Pt electrodes allow the BST to grow with greater crystallinity. Thin film Pt/BST/Pt capacitors with a dielectric thickness of around 170 nm show dielectric constants over 400 and dielectric losses in the range of 0.01–0.03 at 10 kHz. The electric/dielectric properties of the BST films are further improved by first depositing a 27 nm BST seed layer by metalorganic chemical vapor deposition followed by a 145 nm BST layer deposited by pulsed laser deposition.
Precursor chemistry and film growth with (methylcyclopentadienyl) (1,5-cyclooctadiene)iridium
Authors: Sun Y.-M. , Yan X.-M. , Mettlach N. , Endle J. P. , Kirsch P. D. , Ekerdt J. G. , Madhukar S. , Hance R. L. , White J. M.
Published: July 2002

We have investigated the chemistry of the iridium precursor ((methylcyclopentadienyl) (1,5-cyclooctadiene))iridium (MeCpIrCOD) and have utilized the precursor for chemical vapor deposition (CVD) of iridium films. The vapor pressure of the precursor is ∼80 and ∼280 mTorr at 80 and 120 °C, respectively. The precursor slowly dimerized at elevated temperatures (>60 °C). Pyrolysis studies revealed that the compound decomposes by breaking the methylcyclopentadienyl–Ir and cyclooctadiene–Ir bonds nearly simultaneously at temperatures above 400 °C. Iridium films grown at substrate temperatures between 250 and 400 °C were characterized by in situ x-ray photoelectron spectroscopy, x-ray diffraction, and scanning electron microscopy. Pure CVD iridium films were obtained on various substrates by codosing MeCpIrCOD with oxygen or hydrogen. Without oxygen, the metal films required higher growth temperatures and contain ∼87% carbon. Oxygen also affected the film deposition rate and lowered growth temperature. X-ray diffraction analysis indicated that films grown below 270 °C are randomly oriented, while films grown at 350 °C favor the (200) orientation. Excellent step coverage has been achieved on Si3N4 and other substrates. The effective activation energy for Ir film growth, with oxygen present, is 71 kJ/mol.
Multicolor-emitting thin-film electroluminescent devices using Ga2O3 phosphors co-doped with Mn and Cr
Authors: Minami Tadatsugu , Nakatani Toshikuni , Miyata Toshihiro
Published: July 2002

High-luminance multicolor-emitting thin-film electroluminescent (TFEL) devices have been developed using a new Mn- and Cr-co-doped Ga2O3 phosphor with variations of both Mn and Cr from 0 to a content of 20 at. %. Ga2O3:Mn,Cr TFEL devices driven at 1 kHz exhibited a change from green to red in emission color, maintaining luminances above 100 cd/m2, when the co-doped Cr content was varied from 0 to 20 at. % under a constant Mn content of 0.3 at. % doping. In addition, the emission color of a Ga2O3:Mn,Cr TFEL device co-doped with a Mn content of 0.3 at. % and a Cr content of 3 at. % changed from green to red with an increase of the applied voltage.
Generation of positively charged particles at an anode and transport to device wafers in a real radio frequency plasma etching chamber for tungsten etch-back process
Authors: Moriya Tsuyoshi , Ito Natsuko , Uesugi Fumihiko , Hayashi Yuji , Okamura Koji
Published: July 2002

The transport path of particles, that have flaked off a grounded anode of a real parallel-plate radio frequency (rf) plasma etching equipment for tungsten etch-back processing to device wafers on the cathode, is measured by using an in situ monitoring system that measures laser light scattered by the particles. A few particles appear constantly only near the grounded anode while rf power is being supplied. These particles have parabolic trajectories that open upward. Few particles are observed near the wafer. On the other hand, at the timing of the turning off of rf power, many particles appear near the anode. Their trajectories from the anode to the chamber wall are sharply curved. Near the wafer, which has a negative self-bias voltage, almost all the particles’ trajectories are from the chamber wall to the wafer. Therefore, the particles move from the anode to the device wafer on the cathode and keep away from the after-glow plasma as well as from the bulk plasma. These particles are then attracted to the wafer by its negative self-bias voltage. These phenomena indicate that the particles generated at the anode have positive charges.
Dry vacuum pumps: A method for the evaluation of the degree of dry
Authors: Davis R. P. , Abreu R. A. , Chew A. D.
Published: July 2002

The drive towards dry vacuum pumping has occurred across the spectrum of vacuum applications from semiconductor manufacture to industrial processing. This brings with it a need to systematically evaluate and quantify the degree of cleanliness characteristic of any particular pump; currently, there is no universally accepted method to perform this function. A methodology developed for repeatable measurements of pump cleanliness will be discussed. It utilizes residual gas analysis with carefully controlled pump conditions. This facilitates direct comparisons of the degree of cleanliness between pumps of the same and those of different design. Additionally, it allows for the assessment of methods (either in pump design or use) introduced to improve cleanliness.
Interaction of HCOOH with stoichiometric and reduced SrTiO3(100) surfaces
Authors: Wang Li-Qiong , Ferris K. F. , Herman G. S. , Engelhard M. H.
Published: July 2002

Interaction of formic acid with stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces has been investigated using temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). Formic acid was dissociated to form formate and a surface proton below 250 K on both stoichiometric and reduced SrTiO3(100) surfaces. Formate was decomposed primarily through dehydration to produce CO and H2O, instead of through dehydrogenation to produce CO2 and H2, on both surfaces. Formaldehyde produced from decomposition of formate was also observed on both surfaces. On stoichiometric surfaces, formaldehyde was produced through bimolecular coupling of two formates on low-coordination Ti cation sites. However, on the reduced surface, formaldehyde formation involves the reduction of surface formates through the oxidation of reduced Ti cations. XPS results show that surface defects on reduced SrTiO3(100) surfaces were reoxidized significantly upon exposure to 30 L HCOOH at 300 K, in contrast to defects on reduced TiO2(110) surfaces where no reduction in defect intensity was observed under identical conditions. The TPD peak of formaldehyde on a reduced SrTiO3(100) surface is shifted to lower temperature and is significantly broader (down to below 300 K) compared to on the stoichiometric surface. The adsorption and decomposition of HCOOH on SrTiO3(100) surfaces are compared with those on TiO2 surfaces.
Model for detection of immobilized superparamagnetic nanosphere assay labels using giant magnetoresistive sensors
Authors: Tondra Mark , Porter Marc , Lipert Robert J.
Published: July 2002

Commercially available superparamagnetic nanospheres are commonly used in a wide range of biological applications, particularly in magnetically assisted separations. A new and potentially significant technology involves the use of these particles as labels in magnetoresistive assay applications. In these assays, magnetic bead labels are used like fluorescent labels except that the beads are excited and detected with magnetic fields rather than with photons. A major advantage of this technique is that the means for excitation and detection are easily integrable on a silicon circuit. A preliminary study of this technique demonstrated its basic feasibility, and projected a sensitivity of better than 10−12 molar [Baselt et al., Biosensors Bioelectronic 13, 731 (1998)]. In this article we examine the theoretical signal to noise ratio of this type of assay for the special case of a single magnetic bead being detected by a single giant magnetoresistive (GMR) detector. Assuming experimentally observed and reasonable parameters for the magnetic label and the sensitivity of the GMR detector, the signal to noise ratio is calculated to be greater than 5000:1 for detection of a single 1 μm diameter magnetic microsphere immobilized on the surface of a 1 μm×1 μm GMR sensor. Based on this large signal to noise ratio, the detection format should be applicable to more complicated assays where linear quantification is required or to assays requiring significantly smaller beads. Detection of microsphere labels approaching 10 nm may be possible upon further technological advances.
Contact lithography at 157 nm with an F2 excimer laser
Authors: Craighead H. G. , White J. C. , Howard R. E. , Jackel L. D. , Behringer R. E. , Sweeney J. E. , Epworth R. W.
Published: July 2002

We report the first use of an F2 excimer laser and a novel mask technology for high resolution photolithography at 157 nm. With a contact lithography technique resist lines as narrow as 0.15 μm have been made. Because of the short wavelength involved, conventional mask technology using quartz substrates could not be employed. Alkaline-earth halide substrates (e.g., CaF2) that have high transmittance at 157 nm were used as a base for the mask production. Resolution test masks were prepared using e-beam lithography and reactive ion etching to pattern a polyimide film on the substrates. The development of lithographic techniques at this wavelength is significant since the 157 nm radiation is currently the deepest VUV radiation available with high energy flux from a readily obtained commercial laser.
Electron densities in InAs–AlSb quantum wells
Authors: Chang Chin-An , Chang L. L. , Mendez E. E. , Christie M. S. , Esaki L.
Published: July 2002

Both single wells and superlattices of InAs–AlSb have been grown by molecular beam epitaxy on GaAs substrates. The measured electron densities were found to depend on the processes used to define the sample geometry. While freshly cleaved samples show increasing electron densities and mobilities with the InAs layer thickness, etched samples give rise to much higher densities with no apparent dependence on the thickness. The discrepancies are believed to be related to the hygroscopic nature of AlSb.
Influence of growth conditions and alloy composition on deep electron traps of n-AlxGa1−xAs grown by MBE
Authors: Yamanaka K. , Naritsuka S. , Mannoh M. , Yuasa T. , Nomura Y. , Mihara M. , Ishii M.
Published: July 2002

Two dominant deep electron traps, named ME3 and ME6, in Si-doped AlxGa1−xAs grown by MBE have been studied by Deep Level Transient Spectroscopy (DLTS) and photoluminescence measurements. The change of the two trap concentrations with growth conditions and alloy composition, and the effects of them on photoluminescence intensity are described. The ME3 concentration shows strong alloy composition dependence and has a maximum nearly equal to the doping concentration at x of about 0.4. The ME3 concentration depends linearly on the doping concentration. The ME6 concentration depends strongly on the growth temperature above 720 °C. Group V/III beam flux ratio dependence of ME6 is less than the growth temperature dependence. The growth temperature and beam flux ratio dependences of ME3 are opposite to those of ME6. ME3 has no effect on photoluminescence intensity at room temperature, whereas ME6 reduces the photoluminescence intensity.
A pure metal polycide metal-oxide-semiconductor gate technology
Authors: Sakiyama Keizo , Yamauchi Yoshimitsu , Matsuda Kenzo
Published: July 2002

A pure metal (molybdenum) polycide MOS gate with a trilevel gate structure (Mo/thin MoSix/polysilicon) has been studied and different MoSix thicknesses ranging from 0 to 40 nm were examined. Even at high temperatures there is no interfacial reaction between the Mo and polysilicon nor any deterioration of the dielectric strength of MOS capacitors when this trilevel gate structure is used with an inter-MoSix layer thickness of around 15 nm. This MOS structure offers stable MOS characteristics and low resistive molybdenum interconnections in VLSI process technology.
Surface studies of GaAs(111) and GaAs(1̄1̄1̄) using high-resolution electron energy loss spectroscopy, x-ray photoelectron spectroscopy, and low-energy electron diffraction
Authors: Frankel D. J. , Anderson J. , Lapeyre G. J. , Farrell H. H.
Published: July 2002

High-resolution electron energy loss spectroscopy (HREELS), in conjunction with x-ray photoelectron spectroscopy (XPS) and low-energy electron diffraction (LEED), is applied to the study of the sputter/annealed (111) and (1̄1̄1̄) surfaces of GaAs. The HREELS results from the clean (111) (2×2) surface show the Fuchs–Kleiwer surface optical phonon energy (∼36 meV) to be essentially the same as that reported for the (110) and (100) surfaces. Following atomic hydrogen exposure, a strong Ga–H stretching mode is observed which shifts from ∼226 to ∼234 meV upon going from light hydrogen exposures to saturation. A small peak at 268 meV is attributed to a combination of a Ga–H plus phonon double loss, and a very weak As–H stretch (less than 4% of the Ga–H mode). We interpret the results in terms of the Ga vacancy model for the (111) surface [Phys. Rev. Lett. 52, 1693 (1984)]. For the (1̄1̄1̄) surface, the Ga–H stretch mode loss is again predominant, with only a small peak (approximately 10% of the Ga–H) due to the As–H, consistent with the measurements of Ranke and Jacobi [Surf. Sci. 63, 33 (1977)] who found from Auger measurements that this sputter/annealed surface is depleted of As.
Photoresponse of GaAs/AlAs heterostructures under external bias
Authors: Schlesinger T. E. , Zur A. , McGill T. C. , Burnham R. D.
Published: July 2002

Results of a photoresponse technique which has been employed to study the photovoltaic properties of GaAs/AlAs heterostructures are presented. The structures consist of a thin layer of AlAs (50 or 200 Å thick) sandwiched between layers of GaAs which were several microns thick. The experimental procedure consisted of illuminating these structures with chopped light while also applying a constant dc bias across the samples. The resulting photovoltage across the sample was then measured as a function of the wavelength of the incident light. It has been found that the application of a dc bias can increase (by several orders of magnitude in some cases) the magnitude of the photovoltage signal compared with that observed when no dc bias is applied. The dc bias was found to shift the photovoltage spectrum to slightly longer wavelengths.
Selective area MBE technique for GaAs coplanar diodes and GaAs SOI applications
Authors: Aizhen Li , Shen Dexin , Qiu Jianhua , Yang Yuifei
Published: July 2002

The selective eptiaxy of GaAs multilayer structures through windows in SiO2 deposited on (001) oriented semi−insulating GaAs substrates for GaAs coplanar diodes, has been studied. It is shown that excellent flat, smooth, and straight boundaries of windows can be obtained by use of sidewall orientation with [1̄01] or [101] directions. GaAs coplanar diodes were fabricated by this technique. The characteristics of the polycrystalline GaAs grown on SiO2 (001)GaAs, SiO2-(100)Si, and SiO2-(111)Ge substrates have been examined by TEM, SEM, and x-ray diffraction. The results obtained show grain size of 1.0 to 3.3 μm can be achieved. The preferred orientation of the grains were [111], [400] and [800] for polycrystalline GaAs SOI on SiO2-(001)GaAs and [111] for polycrystalline GaAs SOI on SiO2-(100)Si and SiO2-(111)Ge.
Molecular beam epitaxy beam flux modeling
Authors: Curless Jay A.
Published: July 2002

A computer model for the effusion process commonly utilized in molecular beam epitaxy (MBE) is presented. The model is capable of simulating effusion sources of both conical and cylindrical design. The results of the model, for one MBE setup, shows that the gallium flux incident on a 4×4cm area has greater than a 30% variation and the gallium to arsenic ratio incident upon the same region varies by a factor of 2. A gallium arsenide (GaAs) layer was then grown under approximately the same conditions as used in the simulation. Observation of the layer’s thickness and morphology show reasonable agreement with the computer model’s results.
The significance of reactive ions and reactive neutrals in ion-beam-assisted etching
Authors: Chinn J. D. , Wolf E. D.
Published: July 2002

Using a custom designed beam analyzer system, the ionic species from a broad beam ion source have been measured under various etching conditions. In broad beam ion-beam-assisted etching (IBAE), it was observed that up to 25% of the ionic species are from the reactive neutral component backdiffusing into the ion source through the extraction grids depending on the ambient conditions. With the use of the constructed beam analyzer, known ionic species were also used for reactive sputtering and IBAE. The ionic species from Ar, Kr, Xe, Cl2, and CCl4 gases were directed at a Si substrate at 1 keV both with and without the presence of a chemically reactive neutral component of Cl2. In reactive sputtering, the etch yields were primarily dependent on the reactivity of the ionic species. However, in IBAE the etching mechanisms are more dramatically affected by the reactive neutral component on the surface than by the ion beam. That is, the etching component of the ion beam in IBAE is dominated more by its mass and acceleration voltage than by the reactivity of the species.
Comparison of low temperature and high temperature refractory metal/silicides self-aligned gate on GaAs
Authors: Kwok S. P.
Published: July 2002

Since the first reported use of TiW gate as a stopping mask for n+ implantation for GaAs self-aligned gate field effect transistors (SAGFET) in 1981, refractory metal compounds and silicides have found a widespread use in the GaAs SAGFET technology. However, the technique requires that the refractory metal (silicide)/GaAs interface be subjected to a high temperature postimplantation annealing, typically 750–900 °C. The mechanical, metallurgical, and most sensitively the electrical stabilities and reproducibility of the refractory metal (silicide)/GaAs Schottky barrier characteristics have become the major issues confronting the high temperature SAGFET technology. The use of a dielectric substitutional gate, which allows the refractory metal gate to be formed at a low temperature, typically room temperature to 400 °C, for fabrication of SAGFET was reported in 1983. With the advance of well controlled dry etching and photoresist planarization techniques, the low temperature SAGFET process becomes a potentially viable alternate technology. In this paper, comparisons of the two technologies and associated device performances are made. The basic issues of refractory metal/GaAs and dielectric/GaAs interface reactions, stress and impurity incorporation at high annealing temperatures are discussed.
Oxygen chemisorption on GaAs(110): Surface or subsurface growth?
Authors: Bertness K. A. , Friedman D. J. , Mahowald P. H. , Yeh J. J. , Wahi A. K. , Lindau I. , Spicer W. E.
Published: July 2002

Recently there has been a resurgence of interest in the question of whether initial oxidation of GaAs(110) produces a layer of more or less uniform thickness from the first monolayer on up or whether the oxide grows nonuniformly in depth with substantial subsurface oxidation. We present here new synchrotron radiation photoemission results which are inconsistent with subsurface oxide growth and support a surface growth model for exposures up to 1014 L (langmuirs). Our primary evidence for surface growth comes from using the tunability of synchrotron radiation to vary the escape depth of photoemitted electrons and thus change the relative contributions of the surface and bulk atoms. Specifically, we have examined the fractional shifted area of the As 3d core level peak corresponding to oxidized As at several photon energies for a GaAs(110) surface exposed to the equivalent of 1013 L of molecular oxygen. Because the chemical shift in the As 3d is large (3 eV), the oxidized peak is well-resolved from the unoxidized peak, and consequently our results do not depend on elaborate curve fitting routines. Our data are found to be consistent only with oxygen chemisorption taking place in the first and possibly second atomic layers, clearly inconsistent with any kind of subsurface growth model. We also present a calculation of the scattering length curve for GaAs in the kinetic energy range from 15 to 300 eV based on our data.
Two-level model of heterojunction band offsets
Authors: Kane E. O.
Published: July 2002

We introduce a new method for studying band offsets in heterostructure junctions. We ‘‘break’’ the junction bond and use the ‘‘broken’’ states as reference states. If only valence states plus broken bond states are allowed to interact they expel an ‘‘antibonding’’ state from the valence manifold. The charge transfer ρanti is the negative of the antibonding charge. The conduction–valence interactions give a further small charge transfer ρpert which can be calculated by perturbation theory. We develop a rigorous two-level model for calculating ρanti. When the hybrid levels of this model match exactly, ρanti=0.
The effect of acceleration voltage on linewidth control with a variable-shaped electron beam system
Authors: Murai Fumio , Okazaki Shinji , Saito Norio , Dan Masahiro
Published: July 2002

Variable-shaped electron beam systems offer high throughput in electron beam lithography. However, excessive heating of resist on low thermal conductivity substrates and the proximity effect act to degrade the linewidth of photomasks. The effect of acceleration voltage (15, 20, and 30 kV) and electron beam size (0.5–4 μm⧠) linewidth was investigated in positive (RE5000P) and negative (CMS) resist. An acceleration voltage of 20 kV is seen to be suitable for mask fabrication in patterns with features larger than 1 μm from the viewpoint of linewidth accuracy and pattern quality.
Effects of Ar+ angle of incidence on the etching of Si with Cl2 and low-energy Ar+ ions
Authors: van Zwol J. , van Laar J. , Kolfschoten A. W. , Dieleman J.
Published: July 2002

Experiments have been performed in a beam apparatus where an Ar+ beam and a Cl2 beam are directed simultaneously at a Si target. The apparatus allows independent variation of the ion angle-of-incidence and the detection angle. Time-of-flight (TOF) distributions of the reaction product SiCl are measured for various Cl2 to Ar+ flux ratios and various angles-of-incidence. These distributions can for the larger part well be fitted with collision cascade distributions, showing that at the parameter values chosen, the etch process in Si(Cl2,Ar+) is a chemically enhanced physical sputtering process. The observed variations of the binding energy U0 obtained from the TOF distributions support this mechanism. A discussion in relation to similar studies on the etch rate and the composition of the surface and the altered layer leads to a consistent picture in support of the above model.
Dry etching of via connections for GaAs monolithic microwave integrated circuits fabrication
Authors: Salimian S. , Cooper C. B. , Day M. E.
Published: July 2002

Through-the-wafer low-impedance via connections can improve characteristics of GaAs monolithic microwave integrated circuits (MMICs). A reactive ion etch process has been developed to etch via holes through 100-μm-thick substrates using SiCl4/Cl2 mixtures in a single-wafer, load-locked reactive ion etcher. The process satisfies the requirements of the via etch with regard to anisotropy, selectivity, profile, and surface morphology. In addition, the single-wafer system has low-overhead cycle times and a wide-process latitude. This process was used to fabricate a MMIC distributed amplifier with significant improvements in gain performance.
Intrinsic SiO2 film stress measurements on thermally oxidized Si
Authors: Kobeda E. , Irene E. A.
Published: July 2002

We have investigated the effects of varying Si oxidation conditions on intrinsic film stress for SiO2 films formed on Si. This study includes stress measurements on four Si orientations: (100), (110), (111), and (311); at oxidation temperatures ranging from 700 to 1100 °C; wet (H2O) versus dry (O2) oxidations for (100) and (111) surfaces; and the effects of postoxidation annealing on stress. We find the following orientation dependence for intrinsic stress measured as a function of oxidation temperature: (110)≥(311)≥(100)>(111); a reduction in stress for wet versus dry studies; and an even larger reduction for postoxidation anneals. A recently proposed step model accounts for the differences in stress between the (111) and the remaining Si orientations. A number of Si oxidation models based on intrinsic stress are compared in their ability to describe observed kinetic behavior, and we conclude that within the Deal–Grove oxidation model, the linear rate constant is strongly influenced by stress in the initial regime while stress is also likely to be important for thicker films.
Fabrication of ultrahigh resolution structures in compound semiconductor heterostructures
Authors: Behringer R. E. , Mankiewich P. M. , Howard R. E.
Published: July 2002

Techniques have been developed to make silicon metal-oxide-semiconductor field-effect transistor (MOSFETs) with minimum dimensions as small as 25 nm for fundamental electron transport studies. These devices have been used to study a variety of quantum conduction and electron scattering effects. At a given size scale, these quantum effects can be increased by replacing silicon in these devices with a high mobility, low electron mass semiconductor like GaAs. To make such devices, we have extended our technology to the fabrication of III–V semiconductor nanostructures. As an example of this, we describe the fabrication of 0.1 μm rings in modulation-doped GaAs using electron beam lithography and reactive-ion etching.