bluuebunny/crossref_metadata_2025 huggingface dataset:
Radio frequency sheaths—modeling and experiment
Authors: Horwitz Chris M. , Puzzer T. , Smith A. M.
Published: July 2002

Measurements of ion flux and energy at a radio frequency (rf) target are given here for Ar sputtering discharges at 7 and 27 MHz, and at pressures between 1–20 Pa. These measurements allow comparison with models of the rf discharge sheath. In turn, these models can be used to correlate external observables, such as sheath voltage and height, with microscopic quantities such as sputtering rate. A numerical model of the rf sheath based upon experimentally determined glow voltage waveforms is used to calculate sheath heights, which are compared with experimental measurements. The rf sheath is modeled as a monotonic structure, permitting a simple description of ion flow in the sheath. This is an approximation because the target ion energy distribution contains a series of discrete energy peaks, superimposed upon a background caused by ion-neutral collisions. However, at high frequencies or at high pressures these peaks decrease in size relative to the background. The method used here accounts accurately for ion collision behavior in the normal sputtering regime, in between the ‘‘free-space’’ and the ‘‘collisional’’ pressure extremes. Sheath voltage waveforms are determined from the observed discharge glow voltage, and yield close agreement between theoretical and experimental sheath heights. In addition, sheath voltages are close to being self-consistent. Sine-derived waveforms are shown to yield similar self-consistency, and are appropriate at high-target voltages. The rf sheath heights at low pressure are about 1.2 times larger than would be predicted from the free-space dc sheath model, for a given ion current density and average sheath voltage. In summary, this paper shows that: (i) rf sheath voltage waveforms are distinctly nonsinusoidal, especially at low voltages; and (ii) the major features of the target ion flux can be deduced from knowledge of the discharge pressure, voltage, and sheath height.
Interdiffusion and conversion of InP/In0.53Ga0.47As superlattices induced by p-type dopants
Authors: Schwarz S. A. , Hwang D. M. , Mei P. , Schwartz C. L. , Werner J. , Stoffel N. G. , Bhat R. , Chen C. Y. , Ravi T. S. , Koza M.
Published: July 2002

Zn diffusion into an unstrained InP/In0.53Ga0.47As superlattice has been observed to result in the formation, first, of a strained layer In1−xGaxP/In1−xGaxAs supperlattice due to the selective diffusion of In and Ga, then a Zn3P2/In1−xGaxAs superlattice due to ‘‘kickout’’ of the cations, and finally a Zn3P2/Zn3As2 superlattice. The lateral confinement of Zn induced interdiffusion and conversion is examined by diffusing through a 3 μm period Si3N4 grating. The effects of Cd diffusion and Be ion implantation are also examined. Organometallic chemical vapor deposited InP/In0.53Ga0.47As superlattices were diffused at 600 or 650 °C in sealed ampoules with Zn3As2 or Cd3P2 powder as the dopant source. Samples were examined by secondary ion mass spectrometry and analytical electron microscopy. In the grating experiment, cation homogenization resulted in the formation of a uniform unconfined strained layer superlattice with superior surface quality in the regions protected by Si3N4. Zn3P2 superlattice conversion was confined within the grating window regions; however, a thick unconfined Zn3P2 layer was observed to form below the superlattice. A Ga concentration spike was present between this thick Zn3P2 layer and the underlying substrate. Cd was less efficient at intermixing the superlattice, owing largely to its relatively low solubility and diffusion coefficient. Partial intermixing was induced by Be implantation, with Be segregation into the InP layers observed near the implant peak.
Thin SiC films as GaAs field effect transistor insulators
Authors: Sullivan J. R. , Soukup R. J.
Published: July 2002

A thorough investigation of the electrical properties of the interface between sputter deposited SiC and single crystal GaAs has been performed. Using a previously developed modeling system, element values for the circuit model were obtained. From these element values, physical parameters have been calculated. Correlation of the physical parameters with the deposition procedures was made in order to determine the optimal deposition parameters. The results indicate that SiC does not have a high enough resistivity to be a field effect transistor (FET) gate insulator. However, good adhesion and other film properties indicate that SiC may make a good ‘‘barrier layer’’ for other possible insulators. An attempt to fabricate pseudomorphic films of SiC on GaAs was made in order to study the more complex metal–insulator–SiC–GaAs system for potential FET application.
Si2Cl6 and Si2H6 adsorption on CaF2/Si(111)
Authors: Terminello L. J. , Himpsel F. J. , McFeely F. R. , McLean A. B.
Published: July 2002

Adsorption of Si2Cl6 and Si2H6 on epitaxially grown CaF2/Si(111) was investigated by core level and valence band photoemission. CaF2 surfaces that were first photoactivated with intense synchrotron radiation showed enhanced molecular adsorption indicating that patterned deposition of silicon may occur. Attempts to chemical vapor deposit Si with these molecular precursors under elevated temperatures revealed that Si2H6 was able to sustain Si growth on the CaF2 epitaxy, while Si2Cl6 adsorption terminated after ∼1 monolayer of deposition. Possible explanations for this behavior are presented.
Nucleation controlled phase selection in vanadium/amorphous-silicon multilayer thin films
Authors: Clevenger L. A. , Thompson C. V. , de Avillez R. R. , Ma E.
Published: July 2002

Cross-sectional transmission electron microscopy, isothermal and constant-heating-rate calorimetry, and thin film x-ray diffraction have been used to investigate amorphous and crystalline silicide phase formation in vanadium/amorphous-silicon multilayer thin films. The atomic concentration ratio of the films was one V atom to two Si atoms and the modulation period was either 14 or 50 nm. The first silicide phase to form at the vanadium/amorphous-silicon interface was amorphous-vanadium–silicide. Heating to temperatures above 750 K caused crystalline VSi2 to form at the amorphous-vanadium–silicide/amorphous-silicon interface. Analysis of cross-sectional transmission electron microscopy and both isothermal and constant-scan-rate calorimetric data suggest that nucleation barriers control the formation of crystalline VSi2.
Photoemission study of the adsorption of nitric oxide on gallium arsenide (110) at low temperature
Authors: Bermudez V. M. , Williams R. T. , Williams G. P. , Rowe M. W. , Liu H. , Wu A. , Sadeghi H. R. , Rife J. C.
Published: July 2002

Photoelectron spectroscopy with synchrotron radiation has been used to study changes in the region of the valence band and Ga and As 3d core levels of GaAs (110) resulting from exposure to nitric oxide (NO) at substrate temperatures of 40–140 K. Up to about 60 K, NO physisorbs. Thermal and photochemical effects have been observed during annealing of the adsorbed layer or irradiation by the monochromatized synchrotron radiation beam. At about 70 K, a distinct molecular species forms along with adsorbed O. This species, which desorbs and/or dissociates above about 90 K, is identified as nitrous oxide (N2O ) on the basis of comparison with similar data for N2O condensed on GaAs. Pre-adsorbed O inhibits N2O formation, suggesting that the GaAs surface participates actively in the process. Above about 100 K, only O adsorption is observed, occurring by a mechanism different from that leading to O adsorption at lower temperature.
Dry etching of TiN/Al(Cu)/Si for very large scale integrated local interconnections
Authors: Hu C.-K. , Mazzeo N. , Basavaiah S. , Small M. B. , Choi K. W.
Published: July 2002

Patterning Ti/TiN/Ti/Al(2 wt. % Cu)/Si interconnection lines over 700 nm topographic steps in SiO2 with an angle of 84° is reported using a reactive ion etching. The etching gas is a mixture of BCl3/Cl2/N2/CHCl3 and the etching process uses a combination of high and low reactor pressures in a single wafer etching tool. No metal rails along the sidewall of topographic features were detected using an electrical test site. The etched metal profile shows a near vertical metal sidewall with negligible undercut. The metal linewidths are 0.7 to 1.8 μm wide with the variation of about 0.1 μm near the steps, due to resist linewidth variations over the topographic step. It also demonstrated that the Al(Cu) line can be a good landing pad for contact hole dielectric etching.
Current approaches to p
n junctions in wider band gap II–VI semiconductors
Authors: McCaldin J. O.
Published: July 2002

Long-standing problems to make junctions in wider band gap semiconductors, especially II–VIs, are being restudied today by new low-temperature epitaxial growth methods, which may lead to current-injecting devices suited to light emission. This paper reviews various approaches briefly, with particular emphasis on heterojunctions and methods to control dopants. A few of the many possible heterojunctions are favored by small offset barriers, but are not without other problems. New dopants, besides the much-studied Li, are being introduced in new ways, in the effort to attain reproducibility and stability.
Material characteristics of metalorganic chemical vapor deposition Hg1−xCdxTe/GaAs/Si
Authors: Edwall D. D. , Bajaj J. , Gertner E. R.
Published: July 2002

Layers of epitaxial Hg1−xCdxTe have been grown by metalorganic chemical vapor deposition (MOCVD) on GaAs/Si substrates. Data is presented on surface morphology, compositional uniformity, double crystal x-ray diffraction, chemical defect etching, laser beam induced current imaging, and Hall effect. Compositional uniformity is 6% for 3 in. diam areas and 0.6% for the interior 2 in. diam area. The material characteristics of these layers are in the range suitable for fabrication of infrared detectors. The material characteristics are compared with those of similar layers grown on bulk GaAs substrates.
The growth of CdHgTe on GaAs and fabrication of high-quality photodiodes
Authors: Smith L. M. , Byrne C. F. , Patel D. , Knowles P. , Thompson J. , Jenkin G. T. , Nguyen Duy T. , Durand A. , Bourdillot M.
Published: July 2002

The use of metal organic vapor phase epitaxy (MOVPE) for the growth of device quality CdHgTe (MCT) on GaAs has been studied extensively. This is the first report of diode formation by mercury diffusion in the MOVPE grown material. It is also the first report of medium-wavelength infrared (MWIR) photodiodes made in the material. The effect of altering the cadmium to tellurium alkyl ratios on the growth of a CdTe buffer on (100) and (111)GaAs was investigated. For every alkyl ratio used, it was possible to grow (111) or (100)CdTe on (100)GaAs. (100)CdTe/(100)GaAs samples exhibited very sharp, well-defined photoluminescence spectra and x-ray rocking curve full width half-maximum (FWHM) peaks of <90 arc s were found for layers of 6–10 μm thick. Generally the (111)CdTe/(111)GaAs layers were dominated by twins, but the growth of untwinned (111)CdTe and MCT is reported. CdTe, high x MCT, and combinations of CdTe and HgTe layers were used as buffers for (100)MCT growth to reduce the hillock density. The combinations of CdTe and HgTe layers gave rise to the lowest hillock density (at best 30–100/cm2) over a large area (25×25 mm2). FWHMs of 80–90 arc s are routinely achieved for (100)MCT layers. Linear diode arrays have been fabricated with 77 K R0A values typically of 3×103 Ω cm2 for 7.5 μm cutoff. The detectivity of a 5.5 μm cutoff diode was 1.5×1011 cm Hz1/2 W−1 at 77 K, and 1.3×1010 cm Hz1/2 W−1 at 193 K. This is comparable to the performance obtained with the best traveling heater method devices.
Characteristics of newly developed scanning tunneling microscopy/spectroscopy at low temperatures
Authors: Endo T. , Yamada H. , Sumomogi T. , Kuwahara K. , Fujita T. , Morita S.
Published: July 2002

A ceramics tunneling unit and a cryostat are newly designed for variable-temperature experiments of scanning tunneling microscopy/spectroscopy (STM/STS) at low temperatures. The unit has high symmetry and rigidity. Coarse adjustment of sample-tip position is made using a ceramics differential screw and a stacked piezoelectric transducer, attached to the unit. The unit is succesfully tested at room and low temperatures in He gas, not directly being immersed into cryogenes. The stability is suitable also for STS measurements using a lock-in amplifier.
Determination of surface atomic positions by scanning tunneling microscope observations
Authors: Becker Russell , Vickers James
Published: July 2002

The scanning tunneling microscope (STM) is used to study the clean surface of Ge(111)-1×1:As. The surface is found to consist of hexagonal incommensurate 1×1 domains having a lateral extent of ∼ 150 Å. The domain boundaries are determined to be one double-layers deep ‘‘trenches’’ in the surface, terminated with arsenic. Measurement of the occupied sample state tunneling images indicates the domains are laterally contracted by 0.7% due to the substitution of arsenic for germanium in the top layer. The geometric structure of the domain boundaries is discussed in the light of simulated filled state tunneling images for several possible domain terminations. The reliability of the STM for these kinds of measurements is discussed.
X-ray photoelectron spectroscopy investigation of fluorocarbon lubricant films
Authors: Noël S. , Boyer L. , Bodin C.
Published: July 2002

Fluorocarbon lubricants are of great interest in many applications where they are used in very thin layers. Control of the lubricant presence is easily performed by a surface analysis technique such as x-ray photoelectron spectroscopy. However, the interpretation of the spectra is not straightforward. A simple electronegativity description of the charge shifts is developed from the group electronegativity and the Sanderson equalization approaches. It is applied to two types of high molecular weight molecules, and allows us to assign the experimental photoelectron spectra of the two fluorocarbon lubricants under study.
Theory of free-molecular flow under the drag conditions
Authors: Liu N. , Zhu Y. , Wang X. Z. , Pang S. J.
Published: July 2002

In the present article, starting from microscopic viewpoint, a new theory of rarefied gas dynamics in the range of free molecular flow has been studied. In this theory, the matrix-probability method Y. Wu, Rarefied Gas Dynamics, edited by L. Talbot (Academic, New York, 1961), p. 141; J. Vac. Sci. Instrum. 42, 876 (1965) has been employed to calculate transmission probability and a three-dimension dynamic model has been suggested which can satisfy any complicated boundary conditions theoretically. Finally, the reasons why the transmission probability of gas molecules through a moving channel tends to a constant value (which is smaller than 1) rather than unity while the drag speed increases infinitely have been explained.
Recommended practices for the calibration and use of capacitance diaphragm gages as transfer standards
Authors: Hyland Richard W. , Shaffer Richard L.
Published: July 2002

Capacitance diaphragm gages which were previously known as capacitance manometers, have become a ubiquitous part of systems used in the vacuum community because of their ease of use, compatibility with most gases, and potential accuracy and resolution. They are used not only as pressure measuring devices, but also as transfer standards against which other pressure instruments are calibrated. There are a variety of precautions that must be considered when using these instruments. The American Vacuum Society is therefore presenting this Recommended Practice as a guide to their proper calibration and use as transfer standards. The document covers suggested plumbing configurations, laboratory practices, calibration methods, and reports.
Control of Ge homojunction band offsets via ultrathin Ga–As dipole layers
Authors: McKinley J. T. , Hwu Y. , Koltenbah B. E. C. , Margaritondo G. , Baroni S. , Resta R.
Published: July 2002

We have created 0.35–0.45 eV band offsets at Ge homojunctions using Ga–As dipole intralayers, with the Ge valence band edge on the As side of the junction at lower energy. This is, to our knowledge, the first time that intralayer control of band discontinuities is extended to homojunctions, thereby expanding the potential domain of band gap engineering. Because these offsets occur over just a few atomic spacings, they rival heterojunction band edge discontinuities in breadth. The offsets were measured with synchrotron-radiation photoemission spectroscopy. Similar band offset magnitudes occur for both ‘‘Ga-first’’ and ‘‘As-first’’ growth sequences, consistent with a truly dipolar effect. Both cleaved Ge(111) and thick ≊50 Å Ge films deposited on cleaved Ge(111) were used as substrates, obtaining consistent results. The sign and magnitude of the effect is in agreement with a ‘‘theoretical alchemy’’ model. Experimental evidence of microdiffusion is discussed.
Determination and observation of electronic defect levels in CuInSe2 crystals and thin films
Authors: Abou-Elfotouh Fouad A. , Kazmerski L. L. , Moutinho H. R. , Wissel J. M. , Dhere R. G. , Nelson A. J. , Bakry A. M.
Published: July 2002

The effects of heat and surface treatments used in the fabrication of CuInSe2 (CIS) single-crystal and thin-film solar cells are investigated, and the associated dominant defect states are identified using high-resolution photoluminescence (PL) and deep-level transient spectroscopy (DLTS) measurements. The results are correlated with junction electrical characteristics, and comparisons between thin films and single crystals are established. For the first time, direct evidence for several major defect types (Cu vacancies, Cu at In sites, and In at Cu sites) responsible for the majority-carrier type of the CIS is presented based upon spectroscopic atomic imaging of the n- and p-type semiconductor surfaces. Radiative recombination levels of extrinsic origin resulting from the surface processing (including polishing, etching, and annealing) were detected in the near surface region of the CIS single crystals, but not in the thin-film material having the same composition. The energy locations and depth of these states are responsible for the formation of an interfacial layer that is confirmed from frequency-dependent capacitance–voltage (C–V) data. The DLTS measurements on single-crystal and thin-film heterostructure {i.e., [Cd,Zn]S/CIS} devices (of nearly identical chemical compositions) have also confirmed the presence of a dominant trap level in the 270-meV region above the valance band. However, the trap density is some two orders of magnitude lower in the thin-film device, giving evidence that this processing-related defect level is responsible for the inferior performance of the single-crystal solar cells.
A modeling study on the early stages of GaAs deposition on the GaAs(1̄00) surface
Authors: Halicioglu T. , Tiller W. A.
Published: July 2002

A computer simulation technique based on a molecular dynamics procedure was used in this study to analyze the early stages of the homoepitaxy process taking place in gas-phase deposition of GaAs on the GaAs(1̄00) surface. Simulation calculations were carried out employing a model potential energy function comprising two- and three-body interactions. The relaxed GaAs(1̄00) surface exhibited the usual formation of As dimerization on the top layer. For a Ga adatom, two energetically favorable binding sites were found on this reconstructed surface. Binding energies for clusters of Gan were calculated as a function of n (up to n=6) utilizing these sites. To analyze the very early stages of the deposition process, As2 molecules were deposited in the vicinity, as well as on top of, adsorbed Ga clusters having different configurations. Simulation results indicate that an As2 molecule adsorbed on top of a particular Ga3 cluster (in a specific configuration) splits readily into its individual atoms to form 2GaAs+Ga while As2 adsorption on the other stable, but less energetic Ga3, leaves the As2 molecule in its molecular state. Adsorption of As2 on already deposited Ga2 or Ga1 does not spontaneously produce As2 dissociation. Similarly, no As2 dissociation was found when an As2 molecule was deposited in the vicinity of a Gan cluster on the terrace sites of a reconstructed GaAs(1̄00) surface.
Thin-film development and methods for compact disk and laser disk manufacturing
Authors: Schulz S. , Lacher R. , Reineck S. R. , Seiler R. , Marcantonio J.
Published: July 2002

The outstanding invention of compact disks (CDs) has created huge demands for new consumer products worldwide. The contribution of thin-film development is imperative for CD manufacturing either using evaporation or sputtering technologies. The fast interactions of ‘‘market pull’’ and ‘‘technology push’’ have permanently improved product quality and reduced product costs. Today the fifth generation of machines in a period of 11 yr is supplied to the market for depositing aluminum as reflective layer on CD. Off-line CD manufacturing processes switched to fully automated in-line production. The aim for the future is the laser disk to repeat the CD-market success shortly. Again thin-film technology will be the important key for excellent product quality and lowest product costs.
Analysis of ion scattering by thin SiO2 layers in boron implants through SiO2 into silicon
Authors: Park Changhae , Klein Kevin M. , Yang Shyh-Horng , Tasch Al F. , Simonton Robert B. , Lux Gayle E.
Published: July 2002

The effects of ion scattering by a silicon dioxide layer on boron distribution profiles implanted through the oxide layer into single-crystal silicon have been studied. The intensity of ion scattering and the degree of randomization of the directions of the implanted ions have been investigated through observations of a series of boron profiles measured by secondary ion mass spectroscopy (SIMS) analysis. The effectiveness of the oxide layer in randomizing the directions of implanted ions is found to be strongly dependent on the correlation between the ion energy and the oxide thickness. It is also shown by SIMS anaysis that even the total randomization of the direction of the ions does not completely eliminate ion channeling. This study reveals an unexpected effect of ion scattering by screen oxide layers on implant profiles: ion scattering by the oxide layer can cause enhanced channeling and deeper profile depth.
Differential method for measuring coverage-dependent adsorption probabilities
Authors: Glines Andrew M. , DeAngelis Michael A. , Anton A. Brad
Published: July 2002

King and Wells have described a convenient technique for measuring adsorption probabilities in ultrahigh vacuum systems equipped with a collimated source and a mass spectrometer for detecting the adsorbate’s ambient partial pressure [Surf. Sci. 29, 454 (1972)]. We describe a method based on similar principles for systems equipped instead with a leak valve for back-filling the chamber and a differentially pumped mass spectrometer with line-of-sight to the adsorbent. The method is demonstrated with measurements of the coverage- and temperature-dependent probability of CO adsorption on Ni(110).
Electrical characteristics of plasma-deposited diamondlike carbon/silicon metal–insulator–semiconductor structures
Authors: Seth Jayshree , Chaudhry M. I. , Babu S. V.
Published: July 2002

Diamondlike carbon (DLC) films were deposited on n-type silicon substrates by the plasma decomposition of butadiene and two binary mixtures of butadiene with hydrogen or argon. The ac conductance and capacitance characteristics for the DLC/silicon metal–insulator–semiconductor (MIS) structures were measured over a range of frequencies. The electrical characteristics of the DLC film deposited from a pure butadiene plasma were strongly influenced by carrier tunneling into the traps located in the film. The films exhibited large flatband voltage and hysterisis. However, in the films deposited with both hydrogen and argon, the traps on the insulator side were reduced and the effects due to film charges and hysteretic instability could be eliminated. The addition of argon, however, increased the density of interface states. The presence of hydrogen in the deposition gas mixture improved the interfacial characteristics significantly. The density of interface states for DLC/Si MIS with the insulating DLC layer deposited from a butadiene/hydrogen plasma was estimated to be 4×1010 eV−1 cm−2.
Measurement of x-ray currents in Bayard–Alpert type gauges
Authors: Peacock N. T.
Published: July 2002

The collector current in a hot cathode gauge at low pressure is the sum of the pressure dependent ion current and a nonpressure dependent residual current. A portion of this residual term is photocurrent caused by soft x-ray production within the gauge. At ultrahigh vacuum, the photocurrent can be comparable to the ion current. For accurate measurements, it is necessary to determine the pressure equivalent of the photocurrent and subtract it from the indicated pressure. Several methods have been used to determine the x-ray limit of a gauge. Using the variation of electron energy method, also known as the Alpert method, and the comparison method, the x-ray limits were found for a commercial Bayard–Alpert (BA) gauge and several experimental nude BA gauges. X-ray limits from both methods were in good agreement for gauges with tungsten filaments. In gauges with oxide coated cathodes, agreement varied and the residual current was noted to depend upon operating history. The x-ray limit for the commercial BA gauge was found to be 3.7×10−10 Torr at 1.0 mA grid current and 2.8×10−10 Torr at 10.0 mA. For the experimental gauges, x-ray limits were about 1.5×10−10 Torr.
Design of the Burning Plasma Experiment Vacuum Vessel
Authors: Ulrickson M. , Bialek J. , Goldston R. , Reiersen W. , Dinkevich S. , Fragetta W. , Rocco R. , Warren J. , Langley R. , Neilson G. H. , Nelson B. , Haines J. , Mantz H. , McSmith M.
Published: July 2002

The Burning Plasma Experiment (BPX) is being designed to determine the physics behavior of self-heated fusion plasmas and demonstrate the production of substantial amounts of fusion power. The machine is designed to operate with a plasma current of 11.8 MA and a toroidal field of 9 T. The machine will have a major radius of 2.59 m and will have a fusion power of 100–500 MW. The interior surface area of the vessel is about 150 m2. The high power density on the divertor plates requires the use of pyrolytic graphite. Most of the remainder of the metallic surface of the vessel is covered with either carbon fiber composite or graphite tiles. The operating temperature of the vacuum vessel is chosen to be 350 °C because of the good experience of JET and JT-60 with graphite tiles and operation at similar temperatures. The high plasma current, large stored energy, and large toroidal field give rise to large forces on the vacuum vessel during plasma current disruptions. These forces, together with the operating temperature, require the use of Inconel for the vacuum vessel structure. Even with the high strength of Inconel, thicknesses of up to 90 mm are required to have acceptable stress in the vacuum vessel shell. The vacuum pumping speed on BPX is limited to about 7000 ℓ/s (N2) by the size of the access ports on the vessel and by remote handling considerations. The vacuum vessel can be baked to temperatures up to 550 °C without significant changes in the Inconel material properties. The requirements for outgassing of the vacuum vessel and the plasma facing components are discussed.
Potassium adsorption on the Si(100)(2×1) surface studied by Si and K core level photoemission and photoabsorption spectroscopy
Authors: Ma Y. , Rudolf P. , Chen C. T. , Sette F.
Published: July 2002

The questions of K–Si bonding and the K adsorption site of the K/Si(100)(2×1) surface system are studied by Si and K 2p core level photoabsorption and photoemission spectroscopy as a function of K coverage and incident x-ray polarization. An unoccupied K 4s derived surface state is found in the K L-edge spectra that is coverage as well as polarization dependent. It is also found that the π* state of the surface Si dimer is eliminated by the potassium adsorption, indicating that there is some degree of charge transfer from K to the substrate. The K–Si bond is thus characterized as of mixed covalent/ionic type. The polarization dependence suggests that the Si–K bond is inclined towards the surface plane.
Surface structure, bonding, and dynamics: Universality of zinc blende (110) potential energy surfaces
Authors: Godin T. J. , LaFemina John P. , Duke C. B.
Published: July 2002

Using a tight-binding, total energy (TBTE) model we examine the hypothesis that the potential energy surfaces (PES) describing the (110) cleavage faces of the tetrahedrally coordinated zinc blende structure compound semiconductors exhibit a common ‘‘universal’’ form if expressed in terms of suitably scaled parameters. TBTE calculations on both III–V and II–VI compounds reveal a linear scaling with bulk lattice constant of the geometric parameters of the reconstructed surfaces. This scaling is analogous to that found using low-energy, electron-diffraction surface-structure determination. The surface atomic force constants (found from a TBTE calculation) also scale monotonically with the lattice constant. Using TBTE models proposed previously for GaP, GaAs, GaSb, InP, InSb, and ZnSe, we find that the force constants scale as the inverse square of the bulk lattice constant. These results suggest that if distances are measured in units of the bulk lattice constant, the PES may be a universal function for the cleavage surfaces of zinc blende structure compound semiconductors, on average, with small fluctuations from this average occurring in individual materials.
Ion-assisted sputtering of tungsten oxide solar-control films
Authors: Rubin M.
Published: July 2002

Spectrally selective solar control films can greatly reduce solar heat gain from the infrared while admitting a high fraction of visible daylight. Single-crystal tungsten bronzes of general formula MxWO3, where M is a metal, can have sharp spectral selectivity, but this property is difficult to obtain in thin films. Reversible electrochromic coloration caused by insertion of alkali metal or hydrogen ions has been extensively studied in WO3. Polycrystalline films were made by reactive magnetron sputtering of WO3 together with substrate heating and/or ion bombardment. Bronze forming elements were introduced after deposition either by electrochemical injection of H+ or Li+ or by ion implantation of Ag and Cu. During deposition, HxWO3 or substoichiometric WOx could be formed by controlling the sputtering atmosphere. Films treated by ion bombardment had optical properties approaching those of single crystals.
Growth of GexSi1−x/Si heteroepitaxial films by remote plasma chemical vapor deposition
Authors: Qian R. , Kinosky D. , Hsu T. , Irby J. , Mahajan A. , Thomas S. , Anthony B. , Banerjee S. , Tasch A. , Rabenberg L. , Magee C.
Published: July 2002

GexSi1−x/Si heteroepitaxial thin films have been grown using the low-temperature remote plasma-enhanced chemical vapor deposition (RPCVD) approach, in which the substrate is kept remote from the glow discharge, and an Ar plasma is employed to indirectly activate the reactant gases (SiH4 and GeH4) and drive the chemical deposition reactions. Secondary ion mass spectroscopy (SIMS), plan-view and cross-sectional transmission electron microscopy (TEM), and in situ reflection high-energy electron diffraction (RHEED) have been employed to analyze the films with different Ge mole fractions and thicknesses. Abrupt Si/GexSi1−x heterointerfaces with the Ge concentration changing by 10× in about 30 Å (SIMS resolution limit) have been achieved. Commensurate growth has been observed for layers whose thicknesses are below the critical layer thicknesses (CLTs). Crystalline GexSi1−x/Si films with high mole fractions of Ge (up to 60%), which are thicker than the CLTs, show relaxation of misfit strain. This results in more islandlike growth, accompanied by interfacial corrugation and inhomogeneous strain in the thin films, as observed by cross-sectional TEM analysis. Plan-view TEM also shows Moiré fringes formed by the overlap between the relaxed GexSi1−x films and the Si substrate. The spacing between the fringes has been used to estimate the relaxed lattice constant, which agrees very well with the value predicted by Vegard’s law. A detailed study of the GexSi1−x growth parameter space has also been conducted. Effects of various growth variables have been investigated. Stable control over growth rate and germanium incorporation rate in RPCVD has been demonstrated.
Hard synchroton radiation and gas desorption processes at a copper absorber
Authors: Trickett B. A. , Schmied D. , Williams E. M.
Published: July 2002

Developments with high energy synchrotron radiation machines, such as the European Synchrotron Radiation Facility (ESRF) under construction at Grenoble, present the particular problem of the absorption of radiation at localities in the region of bending magnets where the power density can attain levels of around 400 W/mm2 at a critical energy of about 20 keV. In the present article we describe an investigation of the gas desorption and of photocurrent generation in a test facility at the Wiggler beam line at LURE, Orsay, which was designed to simulate and explore the conditions of operation at the local absorber sites in the ESRF. The experiment allowed, in particular, for varying angles of incidence of the radiation on an absorber and for surveys of photocurrents both from the absorber and over the surface of the vacuum enclosure. The observations of gas desorption are shown to follow most closely a model of photon induced desorption in which a significant role is assigned to fluorescence radiation from the absorber.
Detection of reactions and changes in thin film morphology using stress measurements
Authors: Gardner Donald S. , Longworth Hai P. , Flinn Paul A.
Published: July 2002

Mechanical stresses in thin films arise from thermal expansion differences

Mechanical stresses in thin films arise from thermal expansion differences between the film and substrate, structural changes within the films, and intrinsic stresses from the deposition process. These stresses can lead to open circuit failures from voiding in interconnections, short circuits from hillocks, or delamination. Measurement of this stress in situ as a function of temperature have proven to be an easy and powerful technique for obtaining information on the temperature, time, rate, and extent of formation of both solid-state reactions and changes in morphology and structure. This information is helpful in obtaining an understanding and finding solutions for stress-induced voiding. A theoretical model is developed for the stress change when precipitation occurs from elements in solid solution. The stress during thermal cycling was measured for aluminum and copper on oxidized silicon substrates and for several aluminum alloys and layered films consisting of Si, Cu, Ti, W, Ta, V, Cu–Cr, and/or TiSi2. Correlating these changes using hot-stage transmission electron microscopy and x-ray diffraction has demonstrated that some stress changes do correspond to reactions and structural changes. In most multielement films, there were changes in stress that correlated to phase changes. Morphological changes such as during secondary grain growth of ultralarge grains (50–100 μm) in Al/Cu/Cr films can also be detected using stress measurements. Finally, pure copper and Mo/Cu films exhibited several differences in stress behavior as compared to aluminum.
Predicting intrawafer film thickness uniformity in an ultralow pressure chemical vapor deposition reactor
Authors: Raupp Gregory B. , Levedakis Dimitris A. , Cale Timothy S.
Published: July 2002

We present a reaction engineering analysis of a multiple wafer-in-tube ultrahigh vacuum chemical vapor deposition reactor which allows an estimate of wafer throughput for a reactor of fixed geometry and a given deposition chemistry with specified film thickness uniformity constraints. The model employs a description of ballistic transport and reaction based on the pseudosteady approximation to the Boltzmann equation in the limit of pure molecular flow. The model representation takes the form of an integral equation for the flux of each reactant or intermediate species to the wafer surfaces. Expressions for the reactive sticking coefficients (RSC) for each species must be incorporated in the term which represents reemission from a wafer surface. In our model we use a published expression for the RSC of silane as a function of flux and wafer temperature developed from molecular beam measurements. Numerical solution of the resulting integral equation using Gauss–Legendre quadrature yields quantitative estimates of intrawafer film thickness uniformities for epitaxial silicon deposition from silane for specified process conditions and wafer radius:wafer separation. For given reactor dimensions and specified uniformity, throughputs can then be estimated.
Adsorption of antimony on Au(111) at room temperature
Authors: Ma P. , Slavin A. J.
Published: July 2002

This paper describes the growth of the film formed by the evaporation of antimony on to a Au(111) surface at room temperature. It has been studied using Auger electron spectroscopy, electron energy-loss spectroscopy, low-energy electron diffraction, and work function measurements. The first layer forms a (2√3×2√3)R30° structure, probably with AuSb2 stoichiometry. Subsequent layers are of pure Sb, with a layer-by-layer growth mode, and are disordered. The atomic density of Sb is approximately the same in the first layer and the subsequent Sb layers, and close to that for pure Sb. This work is compared to the formation of bimetallic layers of Pb, Bi, and Sn on Au(111). Also presented is a method for determining changes in the transmission probability of an Auger electron as it passes from one layer to the next.
Electron-beam rocket acceleration of hydrogen pellets
Authors: Tsai C. C. , Foster C. A. , Milora S. L. , Schechter D. E. , Whealton J. H.
Published: July 2002

A proof-of-principle device for characterizing electron-beam rocket pellet acceleration has been developed and operated during the last few years. Experimental data have been collected for thousands of accelerated hydrogen pellets under a variety of beam conditions. One intact hydrogen pellet was accelerated to a speed of 578 m/s by an electron beam of 10 kV, 0.8 A, and 1 ms. The collected data reveal the significant finding that the measured burn velocity of bare hydrogen pellets increases with the square of the beam voltage in a way that is qualitatively consistent with the theoretical prediction based on the neutral gas shielding (NGS) model. The measured burn velocity increases with the beam current or power and then saturates at values two to three times greater than that predicted by the NGS model. The discrepancy may result from low pellet strength and large beam-pellet interaction areas. Moreover, this feature may be the cause of the low measured exhaust velocity, which often exceeds the sonic velocity of the ablated gas. Consistent with the NGS model, the measured exhaust velocity increases in direct proportion to the beam current and in inverse proportion to the beam voltage. To alleviate the pellet strength problem, experiments have been performed with the hydrogen ice contained in a lightweight rocket casing or shell. Pellets in such sabots have the potential to withstand higher beam powers and achieve higher thrust-coupling efficiency. Some experimental results are reported and ways of accelerating pellets to higher velocity are discussed.
Excimer laser reactive ablation: An efficient approach for the deposition of high quality TiN films
Authors: Mihailescu I. N. , Chitica N. , Teodorescu V. S. , De Giorgi M. Luisa , Leggieri G. , Luches A. , Martino M. , Perrone A. , Dubreuil B.
Published: July 2002

The successful deposition of TiN films on Si wafers by excimer (λ=308 nm) multipulse laser ablation of Ti in a low pressure N2 jet is reported in this article. The films are particularly pure, entirely consisting of polycrystalline fcc TiN. One difficulty still seems to be related to the presence of small droplets probably ejected in a liquid phase. According to the analyses, TiN is formed on Ti target with small contributions only, if any, of the next transit through gas and final deposition onto the Si wafer support.
Infrared study of electron-beam-induced reactions in Langmuir–Blodgett films of stearic acid
Authors: Yano Fumiko , Burrows Veronica A. , Kozicki Michael N. , Ryan Joseph
Published: July 2002

To investigate the mechanisms of carbon enhanced vapor etching of silicon oxide, electron-beam-induced reactions in stearic acid Langmuir–Blodgett (LB) films were studied as a model system. Stearic acid LB films were deposited on silicon, and characterized using multiple internal reflection infrared spectroscopy. Spectra of these films exposed to an electron beam show changes in both peak shape and intensity. The changes are shown to result from both electron-beam-induced evaporation, and from carbon–carbon bond dissociation within the films.
Electron desorption study of HF etched Si(100)
Authors: Craig J. H. , Cariss C. , Craig M. J.
Published: July 2002

The electron stimulated desorption signature of a HF etched Si(100) surface has been examined in this study. Hydrogen passivation results in strong H+ and H− desorption signals. Both F+ and F− species are electronically desorbed, although no fluorine is observed in Auger electron spectroscopy. Low current density argon ion sputtering results in significant changes in the H+ and H− energy distributions which suggest an ion induced change in hydrogen adsorbate structure.
Using characterized variable reservoir helium permeation leaks to generate low flows
Authors: Tison S. A. , Mohan Pardeep
Published: July 2002

Fixed reservoir helium permeation leaks are frequently used to generate known flows for calibration of leak detectors. Determination of the flow from helium permeation leaks becomes difficult in the low flow range (10−12 mol/s, 2×10−8 std. cc/s 273.15 K, and below) which is of high interest to industrial and government users. It has been shown that the flow of a helium permeation leak varies linearly with the helium concentration within the leak reservoir. This relationship was established over a flow range from 10−11 to 10−12 mol/s for a particular variable reservoir leak artifact by measuring the flow generated when the reservoir concentration was varied over a range from 4.6×10−1 to 4×10−2 mol/ℓ. Using this relationship, flows in the 10−13–10−14 mol/s region were calculated from measured helium concentrations in the leak artifact reservoir and then compared with the NIST primary leak standard. The results showed agreement, to within experimental error between the two techniques for generating low flows. This technique of generating known low flows by varying the helium concentration in the reservoirs of previously characterized helium permeation leak artifacts has the potential of significantly extending low flow measurements beyond current capabilities.
Al deposition on Fe: Formation of an iron aluminide surface alloy
Authors: Schulze Roland K. , Taylor Thomas N. , Paffett Mark T.
Published: July 2002

An investigation has been made of the formation of a surface iron-aluminum alloy through aluminum adsorption (and subsequent reaction) on Fe(100) single-crystal and polycrystalline Fe surfaces. On the Fe(100) surface, Auger electron spectroscopy and low-energy ion scattering spectroscopy (LEISS) studies indicate that at low Al coverages (<1 ML), and a very low Al deposition rate (<0.03 ML/min), a surface alloy grows uniformly at 25 °C to yield an aluminide with an approximate average stoichiometry of FeAl3. At higher Al exposures the surface becomes more aluminum rich. This Al enrichment is due to a kinetic limitation in the formation of the surface alloy. Heating the surface (300 °C during or after Al deposition) partially overcomes this kinetic limitation, and the topmost surface layer changes to a stable stoichiometry with some reduction in the relative aluminum concentration. Low-energy electron diffraction observations made during the Al deposition (or subsequent heating experiments) gave no indication of long-range order, suggesting the formation of a disordered surface alloy. LEISS experiments show that at 25 °C, the final stoichiometric configuration of the surface is highly dependent on the average arrival rate of Al at the surface. A higher deposition rate ultimately leads to a more aluminum-rich surface than for an equivalent dose at a lower rate. This may be rationalized by noting that at higher deposition rates the Al atoms are more likely to encounter other Al atoms at the surface and may proceed to form two-dimensional microclusters of Al instead of reacting to form a surface or subsurface aluminide alloy. The thermodynamic implications of these observations in the context of the formation of intermetallic alloy phases seen here will be discussed.
Cold beam tube photodesorption and related experiments for the Superconducting Super Collider Laboratory 20 TeV proton collider
Authors: Anashin V. V. , Derevyankin G. , Dudnikov V. G. , Malyshev O. B. , Osipov V. N. , Foerster C. L. , Jacobsen F. M. , Ruckman M. W. , Strongin M. , Kersevan R. , Maslennikov I. L. , Turner W. C. , Lanford W. A.
Published: July 2002

The next generation of proton colliders that has been contemplated—the Superconducting Super Collider (SSC) in the U.S. and the Large Hadron Collider (LHC) at CERN—will be the first to encounter significant intensities of synchrotron radiation within the cold bore tube of superconducting magnets. Aside from removal of the synchrotron radiation heat load, the main problem encountered is how to deal with the magnitudes of photodesorbed gases. Choosing the beam tube to coincide with the cryosorbing magnet bore tube has the advantages of simplicity and, in principle, of providing very high pumping speed. Tightly bound H, C, and O in the near-surface layer (∼100 Å) are converted by photodesorption to a steadily increasing surface density of physisorbed molecules. However, the effective pumping by the bore tube is greatly reduced by the photodesorption of relatively weakly bound physisorbed molecules. In addition, the saturation vapor density of H2 at the ∼4.2 K temperature of the SSC cryostats exceeds, by a factor of fifty, the upper bound allowed by the nuclear scattering deposition of energy in the magnet cryostats. Consequently, accumulation of a monolayer of physisorbed H2 must be avoided even locally. An alternative approach is to install a coaxial perforated tube or liner inside the magnet bore tube which allows the photodesorbed gases to be pumped out of the view of the synchrotron radiation photons. The purpose of the work described in this paper is to develop a methodology that will allow prediction of the SSC beam tube vacuum for simple, 4.2 K beam tubes and for distributed pump or liner configurations—and to provide the technical data required for choosing among the alternative possibilities.

The first photodesorption experiments have been completed on the VEPP2M storage ring at the Budker Institute of Nuclear Physics (BINP). Additional photodesorption experiments are underway at BINP and are being planned for a beamline at the UV ring of the Brookhaven National Laboratory National Synchrotron Light Source (BNL NSLS). Related experiments at BNL measuring molecular sticking coefficients and at the State University of New York-Albany (SUNY-Albany) measuring the depth profile of hydrogen on beam tube surfaces are also beginning to yield data. New ideas for directly measuring molecular density inside a cryosorbing beam tube are under development—neutralization of H− and H+ beams at BINP and positron annihilation at BNL. A status report of these activities is given in this paper.