bluuebunny/crossref_metadata_2025 huggingface dataset:
A model for the saturated water bilayer on Ru(001) based on a comparison of experimental and calculated LEED patterns
Authors: Williams Ellen D. , Doering Dale L.
Published: July 2002

A complex coverage- and temperature-dependent LEED pattern has been observed for water adsorbed on clean Ru(001). This pattern consists of the (1×1)-Ru spots, ( 7/8 × 7/8 ) R 30°-H2O spots and a water-induced halo around each integral order spot at low temperatures (80 to 150 K). At higher temperatures, the integral halo split into a distinct hexagon of satellite spots oriented in the 7/8 directions and the 7/8 spots formed streaks in a threefold symmetric pattern. The radius of the integral halo (hexagon) increased continuously with increasing water coverage to a maximum 20%–25% of the radius of the 7/8 beam above three bilayers coverage. This pattern has been modeled in terms of an antiphase domain structure of two-dimensional, hydrogen-bonded bilayer domains. A computer simulation of the LEED patterns arising from this model structure gives good agreement with the experimental results. Other considerations in the model determination were based on simple water bilayer island models which were developed to explain experimental thermal desorption and electron stimulated desorption ion angular distribution data.
Investigations on solid state reactions between tantalum thin films and oxidized silicon crystals
Authors: Chen Jiann-Ruey , Liauh Her-Rern , Liu Yuen-Chung , Yeh Fon-Shan
Published: July 2002

Solid state reactions on Ta/SiO2/Si systems were investigated by Rutherford backscattering spectrometry (RBS) techniques and x-ray diffractometry analysis. It is found that a β-Ta2O5 layer is formed on the Ta/SiO2 interface with no silicide formation observed when samples are annealed at temperatures from 650 to 950 °C. The formation of tantalum silicides is possible at higher annealing temperatures. When samples are annealed at 1100 °C for 60 min, only the formation of the hexagonal Ta/SiO2 phase is observed.
The nature of the metal–CO interaction and bonding
Authors: Bagus P. S. , Nelin C. J. , Bauschlicher C. W.
Published: July 2002

The adsorption of CO on metal surfaces is represented by molecular orbital cluster models of CO at an on top site and adsorbed normal to the surface carbon end down. Ab initio SCF and MCSCF calculations are performed for several clusters. The new constrained space orbital variation CSOV approach is used to analyze the bonding and to compare CO adsorption on Al, representative of sp metals, with that on Cu, representative of transition metals. There is a large repulsion between the superposed free CO and metal charge distributions which is considerably smaller for Cu than for Al because there are fewer valence σ electrons for Cu than for Al. The CSOV analysis shows that the metal to CO π donation is much more important than the CO to metal σ donation. It is also shown that for Cu, the dπ contribution to the metal π donation is larger than the valence 4 pπ contribution. The dπ donation is compared between Fe, Ni, and Cu and this donation and the metal–CO interaction are found to be different in the order Fe>Ni>Cu.
Wear improvement of surgical titanium alloys by ion implantation
Authors: Sioshansi Piran , Oliver Richard W. , Matthews Frank D.
Published: July 2002

Titanium based alloys are being considered as orthopaedic implants for a variety of reasons such as excellent corrosion resistance, high fatigue strength, low modulus of elasticity, and most importantly, excellent biocompatibility. The only questionable property of these alloys is their wear resistance. While Ti–6Al–4V has recently been introduced for construction of the femoral component in knee-joint systems, the questionable wear performance of the Ti–6Al–4V is a barrier for a more widespread acceptance of the alloy for this application. The ion implantation process as shown in this study is being considered as an effective method for improving the wear performance of titanium based alloys for orthopaedic implants. Ion beams of carbon and nitrogen have been used to implant the surface of Ti–6Al–4V hemispherical samples. The samples were tested in a pin-on-disk machine against ultrahigh molecular weight polyethylene (UHMWPE) with loads representative of total joint replacements. Deionized water and Ringer’s solution have been used as lubricants for the wear tests. The wear data of the UHMWPE and implanted samples are presented. The results show that the implantation process is effective in reducing the wear associated with the Ti–6Al–4V alloy used for prosthesis purposes.
Endpoint detection in plasma etching
Authors: Roland James P. , Marcoux Paul. J. , Ray Gary W. , Rankin Glenn H.
Published: July 2002

Plasma etching is a main stream technology in integrated circuit fabrication. The general trend towards fine linewidths and decreasing thickness of underlying layers (etch stops) results in an increasing dependence on highly controlled plasma etch processes. Such tight control requires in situ process monitoring, particularly, for determination of the etch endpoint. This paper reviews the various methods for endpoint detection. A novel application of interferometric principles for very precise etch endpoint determination in high density circuits is described. A comparison of several methods used simultaneously in a plasma planarization process is presented. The etch endpoint can also be determined by monitoring the pressure change in the chamber upon completion of the etch. An application of this method to dielectric etching is demonstrated.
UV/ozone cleaning of surfaces
Authors: Vig John R.
Published: July 2002

The ultraviolet (UV)/ozone surface cleaning method is reviewed. The UV/ozone cleaning procedure is an effective method of removing a variety of contaminants from surfaces. It is a simple-to-use dry process which is inexpensive to set up and operate. It can rapidly produce clean surfaces, in air or in a vacuum system, at ambient temperatures. By placing properly precleaned surfaces within a few millimeters of an ozone producing UV source, the process can produce clean surfaces in less than 1 min. The technique is capable of producing near-atomically clean surfaces, as evidenced by Auger electron spectroscopy, ESCA, and ISS/SIMS studies. Topics discussed include: the variables of the process,the types of surfaces which have been successfully cleaned, the contaminants which can be removed, the construction of a UV/ozone cleaning facility, the mechanism of the process, UV/ozone cleaning in vacuum systems, rate enhancement techniques, safety considerations, effects of UV/ozone other than cleaning, and applications.
Desorption rate limited structural transitions of CO on Ni(110) studied by time-resolved electron energy loss spectroscopy
Authors: Gurney Bruce A. , Ho W.
Published: July 2002

Time-resolved electron energy loss spectroscopy (TREELS) was used to monitor the transitions between the terminal, short bridge, and (2×1)p1g1 saturation overlayer sites of CO on Ni(110). Three coverage regimes (each corresponding to the dominance of one adsorption site) were observed in the CO stretch corrected relative intensity measured continuously during adsorption. The populations of the three species and their relative dynamic dipole moments were derived from a simple model. The reduction in EEL cross section in the p1g1 phase is consistent with a tilt of the CO molecule by as much as 55° from the surface normal. Kinetics of the transformation from the p1g1 to mostly bridge-bonded phase was obtained by monitoring (1) changes in EEL intensities induced by a linear temperature ramp, and (2) the isothermal nonequilibrium relaxation of CO following a rapid saturation exposure from a pulsed gas doser.
Potential barriers in HgCdTe heterojunctions
Authors: Bratt P. R. , Casselman T. N.
Published: July 2002

The graded energy gap heterojunction in HgCdTe is analyzed using a highly accurate computer model that solves the one-dimensional semiconductor equations. Results are shown in the form of energy band diagrams which illustrate several important features expected to be encountered in these heterojunctions. In particular, our calculations demonstrate the existence of potential energy barriers to minority carrier transport in the conduction band of wide-gap-n on narrow-gap-p structures and in the valence band of wide-gap-p on narrow-gap-n structures. Also demonstrated are cases where these barriers are eliminated. The physics of barrier formation is discussed in terms of the internal force fields due to the electrostatic potential gradient in the depletion region of the p-n junction and the gradient of electron affinity in the graded gap region of the heterojunction. Results obtained from computer simulations were found to be in qualitative agreement with previous experimental data on HgCdTe heterojunctions.
Epitaxial growth of superconducting niobium thin films by ultrahigh vacuum evaporation
Authors: Wolf S. A. , Qadri S. B. , Claassen J. H. , Francavilla T. L. , Dalrymple B. J.
Published: July 2002

A series of thin films of niobium have been grown on various orientations of single crystal sapphire by e-beam evaporation in an ultrahigh vacuum molecular beam epitaxy system. The films were grown at temperatures between 600 and 950 °C and at growth rates from 0.1 to 10 Å/s. The films were single-crystal as determined by both in situ reflection high energy electron diffraction patterns and by Read photographs. The best films had rocking curve widths of the order of 1/10 of a degree. The superconducting transition temperature was measured as a function of film thickness and crystal orientation and ranged up to 9.35 K for a thick sample grown on [11̄02] sapphire.
Prediction of flow rates through an orifice at pressures corresponding to the transition between molecular and isentropic flow
Authors: DeMuth S. F. , Watson J. S.
Published: July 2002

A model of compressible flow through an orifice, in the region of transition from free molecular to isentropic expansion flow, has been developed and tested for accuracy. The transitional or slip regime is defined as the conditions where molecular interactions are too many for free molecular flow modeling, yet not great enough for isentropic expansion flow modeling. Due to a lack of literature establishing a well-accepted model for predicting transitional flow, it was felt such work would be beneficial. The model is nonlinear and cannot be satisfactorily linearized for a linear regression analysis. Consequently, a computer routine was developed which minimized the sum of the squares of the residual flow for the nonlinear model. The results indicate an average accuracy within 15% of the measured flow throughout the range of test conditions. Furthermore, the results of the regression analysis indicate that the transitional regime lies between Knudsen numbers of approximately 2 and 45.
The scattering of thermal helium atoms from Ag(110): A semiclassical Gaussian wave-packet study
Authors: Capozzi C. , Francis S. M. , Roscoe D. , Richardson N. V. , Holloway S.
Published: July 2002

The semiclassical Gaussian wave-packet scattering method for diffractive atom scattering has been employed for the first time to the study of He atom scattering from a metal surface. New experimental results for the scattering of 70- and 140-meV He beams from a clean Ag(110) single-crystal surface have been analyzed using various functional forms for the He–Ag potential. The semiclassical wave-packet approach is well suited to systems of this type, which exhibit both a high corrugation and a nonsinusoidal shape function. It is shown that a good initial estimate of the surface corrugation function for a given energy may be obtained by comparing the angular position of a sharp minimum in the specular beam intensity with a simple hard-wall analysis.
Recent developments in turbomolecular pump rotor balancing techniques
Authors: Rava Edo , Rava Enrico , Marzio Marco
Published: July 2002

Due to the necessarily high rotational speeds, the rotors of turbomolecular pumps require precise balancing, i.e., achieving coincidence of inertial and rotational axes. The importance of a balanced rotor and the effect of unbalance on the performance of the pump is discussed. The detection of unbalance and the measurements of amplitude and phase of vibrations are considered. Selection of vibration detecting transducers, stroboscopic and digital instrumentation, and data acquisition computers for the printing of results will be covered. Unbalance can be corrected by different methods: addition, removal, or displacement of masses. A four-leafed balancing device is described and a computer-assisted balancing method is illustrated utilizing a transfer function associating unbalance and vibration. In addition, the following subjects are considered: deformation problems, flexible rotors, nonlinear behavior with increasing rotational speeds, temperature drift, and artificial aging.
The Si/GaAs(110) heterojunction discontinuity: Amorphous versus crystalline overlayers
Authors: List R. S. , Woicik J. , Mahowald P. H. , Lindau I. , Spicer W. E.
Published: July 2002

We have studied both the electrical and structural properties of the Si/GaAs(110) interface in an attempt to understand the factors affecting the valence-band discontinuity, ΔEv, and to gain insight into the fundamental properties of the amorphous silicon overlayers. In order to differentiate between effects due to intrinsic material properties and those due to extrinsic defects, we grew the interfaces at three different temperatures and performed postgrowth anneals in ultrahigh vacuum. Such growths resulted in overlayers with varied crystalline perfection. In situ ultraviolet and x-ray photoemission spectroscopies, low-energy electron diffraction, and surface extended x-ray absorption fine-structure measurements were obtained for both amorphous and crystalline overlayers. We found a valence-band offset of 0.23±0.1 eV for both amorphous and crystalline Si on GaAs provided that the amorphous Si had either been annealed to or grown at sufficiently high temperatures, i.e., 375 and 300 °C, respectively. For the 20 °C grown unannealed Si, the valence-band maximum was diffuse and shifted to higher kinetic energy with respect to the GaAs substrate core levels resulting in a larger apparent offset of 0.55 eV. These effects were most probably due to dangling bond states in the Si overlayer.
Vibrational relaxation of adsorbed molecules: Comparison with relaxation rates of model compounds
Authors: Cavanagh R. R. , Casassa M. P. , Heilweil E. J. , Stephenson J. C.
Published: July 2002

Time-resolved measurements of vibrational relaxation rates are reviewed. Depopulation of the vibrationally excited OH oscillator (on high surface area SiO2, ZSM-5, and ZnO) was found to occur on a time scale of 200 ps or less, depending on the substrate. The temperature dependence of both the OH and OD vibrational relaxation rates on SiO2 was found to follow simple multiphonon relaxation models. However, the relative rates for the two isotopes did not scale as predicted by the same theoretical models. Measurements of isotopically substituted silanols and alcohols [i.e., R3SiOH(D) and R3COH(D) dilute in CCl4] also find OD decay times comparable to those of OH. Based on results from these model compounds, the observed relaxation rates for OH/SiO2 appear to be dominated by processes which proceed through a SiOH bending mode.
Spatial mapping of electrically active defects in HgCdTe using laser beam-induced current
Authors: Bajaj J. , Bubulac L. O. , Newman P. R. , Tennant W. E. , Raccah P. M.
Published: July 2002

A high-resolution and nondestructive optical characterization technique called laser beam-induced current (LBIC) has been developed and utilized to obtain maps of electrically active defects in liquid phase epitaxy HgCdTe. The LBIC technique is also suitable for studying the p–n junction detector elements in an array nondestructively, without requiring any electrical contacts to individual detector elements.
Effect of resputtering on composition of WSix films deposited by multilayer sputtering
Authors: Bruce R. , Eicher S. , Westwood W. D.
Published: July 2002

WSix films for use as gates in a GaAs-based self-aligned gate field-effect transistor (FET) process were formed by sequential sputter deposition of multiple layers of W and Si. The power applied to the W- and Si-opposed planar magnetron targets could be individually controlled and was fixed at 530 and 500 W, respectively. The thickness of the W and Si layers decreased from ∼6 to 0.3 nm as the rotation speed of the substrate carousel was increased from 2 to 30 rpm. However, the W and Si thicknesses decreased at different rates. From Rutherford backscattering spectrometry measurements of films deposited on carbon-coated substrates, the films became depleted in Si as the rotation rate increased; the composition varied from WSi0.58 at 2 rpm to WSi0.16 at 30 rpm. This large change in composition for constant sputtering conditions can be explained by the resputtering of the thin Si layers by energetic neutral Ar atoms reflected from the W target. Films with a Si/W ratio of 0.16 showed good thermal stability and were excellent Schottky barriers after rapid thermal annealing at 900 °C for 6 s. FET’s with 0.6 μm-long W0.16Si gates gave fT≥20 GHz.
Photodesorption from CdS(0001) exposed to NO and CO
Authors: Ekwelundu E. C. , Ignatiev A.
Published: July 2002

Photodesorption from the (0001) surface of CdS has been studied with low-energy photons of wavelength 200–850 nm. The clean, unannealed surface was exposed to high doses (∼108 L) of CO and NO at room temperature. CO and CO2 were observed to desorb from the surface exposed to CO while NO was the only desorbing specie from the NO/CdS system. The measured photodesorption threshold energy for all species is ∼1.77 eV, which is much less than the CdS energy gap and is approximately the energy of surface states within the band gap of CdS. A second threshold is observed at the direct gap of CdS. The desorption signal is found to be proportional to the incident photon flux, with the calculated quantum efficiency (yield) of ∼10−5 molecules/photon at λ=300 nm. The desorption mechanism for this system agrees with past work on silicon where photohole creation in both the surface states and bulk valence bands was prominent.
The effect of a thin ultraviolet grown oxide on metal–GaAs contacts
Authors: Schmidt M. T. , Podlesnik D. V. , Evans H. L. , Yu C. F. , Yang E. S. , Osgood R. M.
Published: July 2002

Photochemistry has been used to grow very thin oxide layers (3–10 Å) on GaAs(100) surfaces. Some of these oxides have been annealed to produce Ga-rich oxide surface layers. Electrical characterization of metal contacts deposited on these surfaces show a much greater variation of Schottky barrier heights for the oxidized surfaces, especially for the Ga-rich oxide surfaces. The observed variation is ∼25% of the GaAs band gap for Ga-rich oxide surfaces, compared to ∼8% for clean surfaces. The change in barrier heights is toward the ideal Schottky limit for contacts without interface states. This behavior suggests that the oxidized surface has reduced the density of interface states present in the metal–GaAs contacts.
Surface hydrogen content and passivation of silicon deposited by plasma induced chemical vapor deposition from silane and the implications for the reaction mechanism
Authors: Vepřek S. , Sarrott F. -A. , Rambert S. , Taglauer E.
Published: July 2002

A systematic study of the enhanced concentration of hydrogen in the near surface layer of silicon deposited from silane by plasma induced chemical vapor deposition is presented as a function of temperature and ion bombardment of the surface during growth. The passivation effect of the chemisorbed hydrogen towards oxidation of the surface is documented, and a possible explanation is suggested in terms of a donatorlike effect of that hydrogen to underlying silicon. It is shown that the rate limiting step in the composite reaction mechanism of the deposition of high-quality silicon films is the ion bombardment induced dehydrogenation of the surface of the growing film. A critical value of substrate bias of −100 eV with respect to the floating potential at which radiation induced damage of the growing film commences is found experimentally. The corresponding ion impact energy agrees very well with Monte Carlo calculations using the TRIM code.
Infrared spectroscopic study of Cr and Cu metallization of polyimide
Authors: Dunn D. S. , Grant J. L.
Published: July 2002

Infrared spectra of polyimide films [pyromellitic dianhydride–oxydianiline (PMDA–ODA)]metallized with 1 nm of Cr or Cu are used to describe the nature of metal–polyimide bonds formed at the interface. Cr and Cu cause similar changes in the polyimide spectrum though larger changes were observed for the more reactive Cr. The spectral evidence indicates that the metals react predominantly with the PMDA part of the polyimide with only minor interactions with the ODA segment. The results confirm previous observations which suggest that the carbonyl groups on the polymer surface are the primary reaction sites; no complex formation between the metal atoms and the aromatic rings in the polymer chain was observed. These conclusions are supported by infrared studies of Cr or Cu metallized poly(ethylene terephthalate) thin films.
Structure of Cs on GaAs(110) as determined by scanning tunneling microscopy
Authors: First P. N. , Dragoset R. A. , Stroscio Joseph A. , Celotta R. J. , Feenstra R. M.
Published: July 2002

Submonolayer coverages of Cs adsorbed at room temperature on the GaAs(110) surface are examined with scanning tunneling microscopy. Linear chains, formed by two adjoining rows of Cs atoms, are observed along the [11̄0] direction for coverages as low as 0.03 monolayer. The one-dimensional Cs chains are observed to be several hundred Å long, and are seen in images of both the occupied and unoccupied electronic states. At higher coverages, approaching 0.15 monolayer, stable linear structures consisting of three neighboring Cs rows have been found.
Infrared absorption and thermal evolution study of hydrogen bonding in a-SiH
Authors: Maley N. , Myers A. , Pinarbasi M. , Leet D. , Abelson J. R. , Thornton J. A.
Published: July 2002

Hydrogenated amorphous silicon (a-SiH) films were prepared by reactive dc magnetron sputtering under a wide range of conditions. The amount of hydrogen in the films CH and the nature of the Si–H bond were studied using infrared (IR) absorption and thermal evolution of H2. Hydrogen content varied between 2 and 40 at. %. The trends in the IR spectra are qualitatively similar to those previously reported. For low CH the thermal evolution spectra show a single peak centered around 600 °C, and with increasing CH, two additional peaks at ∼500 and 400 °C. A comparison of the IR and evolution spectra of the sputtered films yields an oscillator strength of 2.0×1019/cm2 for the 640 cm−1 wagging mode. This is 25% higher than the value previously reported for glow discharge and rf sputtered films. The hydrogen evolving from the 400 and 500 °C peaks correlates well with the integrated absorption of the 2100 cm−1 stretching mode. However, no good correlation is found between the high-temperature evolution peak and the 2000 cm−1 stretching mode. Our results enable us to define a CH independent oscillator strength for the 2100 cm−1 mode, but not for the 2000 cm−1 mode.
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u scanning tunneling microscopy observation of surface roughness of electrochemically modified Ag under potentiostatic control in conjunction with surface-enhanced Raman scattering
Authors: Sakamaki K. , Itoh K. , Fujishima A. , Gohshi Y.
Published: July 2002

A morphological transformation of Ag electrode in 0.1 mol dm −3 KCl has been studied by in situ STM observation by controlling the electrode potential during a single electrochemical oxidation–reduction cycle (ORC) treatment in conjunction with SERS. Prior to the oxidation the Ag electrode surface has ellipsoidal structures. After the single oxidation treatment electrochemical formation of AgCl clusters and adatoms occurred at the edge of ellipsoidal structures. These species were stripped by the reduction processes. A single ORC treated Ag surface has roughness on an atomic scale due to the presence of residual AgCl clusters, adatoms, and adsorbed chloride ions. The ORC treated surface has roughness characteristics more suitable for observing the SERS effect than the electrochemically untreated surface.
Auger and x-ray electron spectroscopy studies of hBN, cBN, and N+2 ion irradiation of boron and boron nitride
Authors: Trehan R. , Lifshitz Y. , Rabalais J. W.
Published: July 2002

Boron and nitrogen KVV Auger electron spectroscopy (AES) and core level x-ray photoelectron spectroscopy (XPS) features are shown to be conspicuously different for the cubic and hexagonal phases of boron nitride, i.e., cBN and hBN, respectively. The origin and assignments of these features are discussed in terms of the valence band density of states and plasmon losses. Application of these results to the identification of the boron nitride phase formed by N+2 irradiation of boron and N+2 or Ar+ irradiation of cubic boron nitride are presented.
A new Fourier transform infrared reflection-absorption spectrometer and effective background subtraction method
Authors: Xu Zhi , Yates J. T.
Published: July 2002

A new method to obtain high quality Fourier transform infrared reflection absorption spectra (FT-IRAS) from adsorbates on single crystal surfaces is presented. This new method can effectively cancel absorptions in the spectrometer due to atmospheric gases, and also effectively compensates for long term drifts in the background spectrum. The apparatus design and the background subtraction procedure are shown along with sample FT-IRAS measurements.
Adsorption of boron on Si(111): Physics, chemistry, and atomic-scale electronic devices
Authors: Avouris Ph. , Lyo In-Whan , Bozso F. , Kaxiras E.
Published: July 2002

We have used scanning tunneling microscopy, atom-resolved tunneling spectroscopy, and photoemission to investigate the interaction of B with Si(111). Using decaborane as the source of B, we have followed the structural and electronic modifications of the surface as a function of the annealing temperature. In the stable B/Si(111)- 7/8 × 7/8 surface, B occupies a novel configuration where it substitutes for a Si atom in the 3rd atomic layer directly below a Si adatom. Because of a Si-to-B charge transfer, the top Si adatom layer has no occupied dangling-bond states and is insulating. As a result, the chemical properties of Si adatoms on the B/Si(111)- 7/8 × 7/8 surface are very different from those of the adatoms on the Si(111)-7×7 surface. We find evidence for doping effects on chemistry that involve short-range direct dopant-reactive site interactions. Finally, we report on the electrical characteristics of localized defect sites on the B-doped Si surface. We found that I–V curves over such sites may show regions of negative differential resistance and that this behavior is localized in areas of atomic dimensions (∼1 nm). We propose a model according to which the development of negative resistance requires the existance of narrow peaks at appropriate energies in the density of states spectra of both sample and tip.
Scanning tunneling microscopy and spectroscopy of thin metal films on the GaAs(110) surface
Authors: Shih C. K. , Feenstra R. M. , Mårtensson P.
Published: July 2002

The scanning tunneling microscope is used to study the properties of metal films with thicknesses in the range 1–10 monolayers (ML). Results are presented for two materials—antimony and tin, on the GaAs(110) surface. The acquisition and analysis methods used in the spectroscopic measurements are described in detail. For Sb films, a layered structure is observed. Very low conductivity is found at the Fermi-level, indicating semimetallic behavior. For Sn films, a layered structure with semimetallic nature is observed below 2 ML. It is found that the existence of an adatom layer is important for layer by layer growth of the Sn overlayer. At Sn coverages above 3 ML, a structural phase transition is observed, accompanied by an electronic transition to a metallic state.
Relative sensitivity factors for positive atomic and molecular ions sputtered from Si and GaAs
Authors: Stevie F. A. , Wilson R. G.
Published: July 2002

Secondary ion mass spectrometry (SIMS) relative sensitivity factors (RSFs) are tabulated for atomic and molecular ions sputtered from Si and GaAs matrices using oxygen primary ions and positive secondary ions. The RSFs are inversely proportional to the secondary ion yields. RSFs are plotted versus atomic number to establish patterns and trends. The data for molecular ions aid in the resolution of certain mass interferences and also have implications for the SIMS ionization process.
Photoemission study of the ZnSe/GaAs (100) interface: Composition and band offset
Authors: Colbow K. M. , Gao Y. , Tiedje T. , Dahn J. R. , Eberhardt W.
Published: July 2002

The interface between ZnSe and GaAs (100) has been studied as a function of annealing temperature by high-resolution photoemission spectroscopy using synchrotron radiation. From an analysis of chemical shifts and relative intensities of the atomic core levels, we find an interfacial Ga2Se3 layer forms with loss of Zn and As at the interface. The valence band offset between ZnSe and GaAs is found to be 1.25±0.07 eV, from photoemission measurements of the top of the valence band.
Ammonia decomposition on silicon surfaces studied using transmission Fourier transform infrared spectroscopy
Authors: Dillon A. C. , Gupta P. , Robinson M. B. , Bracker A. S. , George S. M.
Published: July 2002

Fourier transform infrared (FTIR) transmission spectroscopy was used to monitor the decomposition of NH3 and ND3 on silicon surfaces. Experiments were performed in situ in an ultrahigh vacuum (UHV) chamber using high surface area porous silicon samples. The FTIR spectra revealed that NH3 (ND3) dissociatively adsorbs at 300 K to form SiH (SiD) and SiNH2 (SiND2) surface species. A comparison of the vibrational absorbances for the SiNH2 and Si2NH surface species indicated that the Si2NH species could account for ≤8% of the surface coverage at 300 K. The infrared absorbances of the SiN–H2 (SiN–D2 ) scissors mode at 1534 cm−1 (1157 cm−1 ), the Si–H (Si–D) stretch at 2077 cm−1 (1510 cm−1 ) and the Si3 –N vibrational modes at 930 and 750 cm−1 were employed to monitor the decomposition of the SiNH2 (SiND2) surface species. As the silicon surface was annealed to 700 K, the FTIR spectra revealed that the SiNH2 (SiND2) surface species gradually decomposed to produce Si3N species and additional SiH species. Above 680 K, the SiH surface species decreased concurrently with the desorption of H2 from the porous silicon surface. The uptake of surface species at various adsorption temperatures was also monitored as a function of NH3 (ND3) exposure time. These studies revealed that similar results were obtained after NH3 (ND3) was adsorbed to saturation coverage at a particular adsorption temperature and after the surface was annealed to the same temperature following a saturation NH3 (ND3) exposure at 300 K.
Ion channeling study of nickel metal growth on Ge(111) at room temperature
Authors: Smith G. A. , Luo Li , Gibson W. M.
Published: July 2002

The room temperature growth of Ni on Ge(111) was studied by MeV ion channeling and Auger electron spectrometry in an ultrahigh vacuum environment. For Ni coverages of up to 3 monolayers (ML), Ni reacts with the Ge substrate and effectively displaces 3 ML of Ge atoms from their original lattice positions forming a possible interfacial layer of NiGe. Further deposition of Ni results in a continuous, uniform metal overlayer. These room temperature results are very similar to those found for Ni and Co on Si(111). The similarities and differences between Ni on Ge and Ni on Si are also discussed.
Ferroelectric thin films for electronic applications
Authors: Haertling Gene H.
Published: July 2002

This is a review paper on the history of ferroelectric thin films, current practices in their fabrication, characteristics of interest which relate to their use, and the application of these films to memory and electrooptic devices.
Structures of selected boranes and carboranes
Authors: Lee Sunwoo , Dowben P. A. , Wen A. T. , Hitchcock A. P. , Glass John A. , Spencer James T.
Published: July 2002

Nido-pentaborane(9) (B5H9), 1, nido-2,3-diethyl-2,3-dicarbahexaborane(8) [(C2H5)2C2B4H6], 2, nido-decaborane(14) (B10H14), 3, closo-1,2-dicarbadodecaborane(12) (H2C2B10H10), 4, can be used as possible source compounds for boron and boron carbide thin film deposition. Inner shell electron energy-loss spectroscopy (ISEELS) studies of the boron 1s and carbon 1s core excitations of gas phase species have been undertaken so as to characterize these molecular precursors at solid surfaces. The near edge structure of ISEELS provides a good ‘‘fingerprint’’ for the identification of these molecular species. A comparison is made between calculated [modified neglect of differential overlap (MNDO)] bond lengths for molecular clusters and the x-ray or electron diffraction bond lengths, bond lengths determined from extended energy-loss fine structure.
Low-resistivity p-type ZnSe:N grown by molecular beam epitaxy using a nitrogen free-radical source
Authors: Park Robert M.
Published: July 2002

The provision of stable, low-resistivity p-type ZnSe epitaxial material, a necessary ingredient for the fabrication of ZnSe-based short-wavelength (blue/green) diode lasers, has been an elusive goal for many years. However, a recently developed doping technique which has been employed to incorporate nitrogen impurities into ZnSe during molecular beam epitaxial growth shows excellent promise in this regard. The new doping technique involves the use of an rf plasma discharge, nitrogen free-radical source which supplies highly reactive nitrogen species to the growing ZnSe surface. As will be described in this paper, the remote (to the substrate) discharge plasma contains a significant concentration of ground state N atoms as evidenced by molecular emission spectroscopy studies of the nitrogen plasma itself and it is postulated that the fraction of N(4S) atoms which emanate from the source are the reactive species responsible for enhanced nitrogen doping of the ZnSe epilayers. To date, ZnSe:N epilayers have been grown at the University of Florida having resistivities as low as 0.75 Ω cm using the nitrogen free-radical doping approach.
Surface studies of (0001) Al2O3 and the growth of thin films of Cu on Al2O3
Authors: Varma Shikha , Chottiner Gary S. , Arbab Mehran
Published: July 2002

Angle resolved x-ray photoemission spectroscopy has been employed to examine the (1×1) unreconstructed and the (√31×√31)R±9° reconstructed surfaces of (0001) Al2O3 and the bonding of copper to this substrate. Clean (0001) Al2O3 is found to contain at least two chemically distinct species of oxygen, with very subtle differences between the (1×1) and the reconstructed faces. Copper initially grows as uniform layers on (1×1) Al2O3 but forms clusters after 2–3 atomic layers of metal are deposited at room temperature. The data indicate that copper is donating charge to surface oxygen with Cu–O bonds forming at the interface. However, the thickness of the reaction layer is limited to one atomic layer.
Integration of scanning tunneling microscope nanolithography and electronics device processing
Authors: Dagata J. A. , Tseng W. , Bennett J. , Dobisz E. A. , Schneir J. , Harary H. H.
Published: July 2002

The emerging field of nanoelectronics demands innovative methods to fabricate nanometer-scale structures. Such structures will play a critical role in the quantum-effect device physics of future highly integrated circuit architectures. An integrated approach to compound semiconductor nanostructure fabrication based on scanning tunneling microscope (STM) nanolithography, molecular-beam epitaxy, and reactive ion etching techniques is described. The critical elements of this approach, which have been demonstrated recently, are reviewed. Prospects for the coevolutionary development of nanoelectronics and STM-based fabrication and characterization are considered.
Nucleation and growth of thin metal films on clean and modified metal substrates studied by scanning tunneling microscopy
Authors: Hwang R. Q. , Günther C. , Schröder J. , Günther S. , Kopatzki E. , Behm R. J.
Published: July 2002

The nucleation and growth of several metal/metal thin film systems have been studied and compared using scanning tunneling microscopy (STM). Homogeneous and heterogeneous nucleation have been observed and the effect of surface inhomogeneities on the nucleation behavior has been studied. Systematic measurements of the film morphology as a function of coverage and annealing temperature have allowed different two- and three-dimensional growth mechanisms to be studied. Both kinetic and thermodynamic factors are found to determine these growth processes and in some cases could be separately identified. In addition, the effect of chemically modifying the substrate by oxygen preadsorption has been studied and compared to the clean substrate systems. A variety of changes in the growth behavior were found to occur, which can be understood in terms of changes in both the kinetics and thermodynamics due to the oxygen adlayer.
Thermal decomposition of SnS2 and SnSe2: Novel molecular-beam epitaxy sources for sulfur and selenium
Authors: Shimada Toshihiro , Ohuchi Fumio S. , Parkinson Bruce A.
Published: July 2002

New sources of selenium and sulfur for UHV preparation of thin films are described. These sources, utilizing the high temperature thermal decomposition of SnSe2 and SnS2, are bakeable to at least 300 °C providing much more convenience than element Se and S sources. The thermal decomposition is studied with thermal gravitometry, mass spectrometry, and x-ray photoemission spectroscopy. The utility of the sources is demonstrated by the epitaxial growth of TiSe2 on MoS2.
Alternating current scanning tunneling spectroscopy of self-assembled monolayers on gold
Authors: Stranick S. J. , Weiss P. S. , Parikh A. N. , Allara D. L.
Published: July 2002

We demonstrate the spectroscopic capabilities of an ac scanning tunneling microscope. This ac scanning tunneling microscope has a bias voltage modulation frequency tunable over the range dc–20 GHz and is thus capable of recording images and local spectra of insulating as well as conducting substrates. In this article we detail the spectral sensitivity and the tip-sample separation dependence of the spectra recorded by simultaneously tuning the modulation and detection frequencies. Spectra of an evaporated gold film and of self-assembled monolayers of CH3(CH2)15SH and of CH3O2C(CH2)15SH on gold are presented.
Preparation and structure of 1–8 monolayer thick epitaxial iron oxide films grown on Pt(111)
Authors: Weiss Werner , Somorjai Gabor A.
Published: July 2002

Well ordered epitaxial iron oxide layers with FeO stoichiometry were grown onto Pt(111) single crystal substrates and characterized afterwards with low-energy electron diffraction (LEED) and Auger electron spectroscopy. They were prepared by first depositing iron from the vapor onto the substrate and then oxidizing it in 10−6 Torr of oxygen. At 1 monolayer coverage, a very stable structure with a hexagonal unit cell 15% larger than the platinum (1×1) substrate unit cell is formed, exhibiting a threefold symmetric LEED pattern. This pattern is consistent with a (8×8) superstructure, as 8 FeO(111)-(1×1) lattice spacings fit onto nine lattice spacings on the Pt(111)-(1×1) substrate surface due to the lattice mismatch. Above 2 monolayer coverage a hexagonal (2×2) structure with respect to the FeO(111)-(1×1) unit cell forms, which shows a sixfold symmetric LEED pattern revealing a lattice constant which is in agreement with that of the sodium chloride structure of bulk FeO. LEED beam intensity spectra as a function of the electron energy of these two LEED structures have been measured and are discussed with respect to their symmetry and their changes as a function of the overlayer thickness. The step structure on the surface of an 8 monolayer thick iron oxide film with a FeO(111)-(2×2) LEED pattern was studied by analyzing the LEED beam profiles. We observe steps one iron–oxygen bilayer high as well as one Pt(111) layer distance high. This indicates that only one type of atom termination exists on the FeO(111)-(2×2) surface.
Chemistry and morphology of Cr/BPDA-PDA interface formation
Authors: Anderson Steven G. , Leu Jihperng , Silverman B. David , Ho Paul S.
Published: July 2002

X-ray photoelectron spectroscopy has been used to investigate the interaction between Cr and BPDA-PDA polyimide for Cr coverages up to 18 Å. Detailed core level analysis reveals that the spectral changes induced by Cr deposition are a consequence of both the chemistry and the redistribution of reaction products in the overlayer. Two distinct N bonding configurations are readily resolved; one formed at lowest coverage remains localized at the buried interface, whereas the second becomes intermixed in the overlayer. Uniform overlayer growth is observed above ∼3.5 Å. The data for low Cr coverages are compared to molecular orbital calculations for two model bonding configurations. Agreement between theory and experiment for the C 1s core level is better for a model compound with an O atom removed from the polymer backbone than for one where Cr is bound to a benzene ring. However, the quantitative differences suggest that a model should be considered where Cr interacts with carbonyl groups and additional reaction sites.
Carbon content of silicon oxide films deposited by room temperature plasma enhanced chemical vapor deposition of hexamethyldisiloxane and oxygen
Authors: Theil J. A. , Brace J. G. , Knoll R. W.
Published: July 2002

Hexamethyldisiloxane is used as a silicon-source gas for plasma enhanced chemical vapor deposition (PECVD) growth of silicon oxide films for passivation layers in microelectronics. In order to produce low permeability films, it is necessary to minimize the carbon content. Films were deposited by a room temperature PECVD process in a parallel plate reactor, and were characterized by infrared spectroscopy, ellipsometry, microbalance, and elastic recoil detection (ERD). The infrared-active carbon groups appear in Si–CH3, CHx, C=O, and SiC bands. A double band at 2343 cm−1 has also been seen to track these bands. Deposition rate data shows linear behavior as a function of the Hexamethyldisiloxane (HMDSO) concentration in the reactor. The carbon within the films ranges from 36% to <1% of the amount of carbon assisted with HMDSO required to grow the films, depending upon the oxygen flow rate. By comparing the ERD composition data to the infrared spectral data, a carbon content correlation value has been found, and a detailed study of the data reveals evidence of C–C bond existence within films having the greatest total carbon content. Finally, it appears that the 2343 cm−1 band corresponds to CO2 rotational bands, in which the distribution of rotational states is evidence of hindered molecular motion.
Attaining an ultralow outgassing rate of 10−12 Pa m3 s−1 m−2 from an oxygen-free high conductivity copper chamber with beryllium–copper–alloy flanges
Authors: Watanabe Fumio , Koyatsu Yasuki , Miki Hirofumi
Published: July 2002

A vacuum sealing technique using beryllium–copper (BeCu)–alloy Conflat flanges with an oxygen-free copper gasket has been developed, and an outgassing rate from a vacuum cast oxygen-free high conductivity (OFHC) copper chamber has been measured using the throughput method. The outside surface of the test chamber was protected against oxidation during bakeout by an electroless nickel plating (Ni–P alloy). The outgassing rate from the electropolished internal OFHC copper surface of the test chamber was about 4×10−8 Pa m3 s−1 m−2 nitrogen equivalency after 10 h evacuation at 20 °C, and reached a value of 2.9×10−11 Pa m3 s−1 m−2 nitrogen equivalency after bakeout at 100 °C for 24 h. After a 300 °C additional bakeout for 48 h, the outgassing rate decreased to the ultralow level of 6.2×10−13 Pa m3 s−1 m−2 nitrogen equivalency. This value is a hydrogen equivalency of 5.4×10−12 Pa m3 s−1 m−2, which is one of the lowest outgassing rates ever attained using any material and the throughput method.
X-ray photoelectron spectroscopy and transmission electron microscopy studies of the NiAl/Al2O3 interfacial chemical compatibility
Authors: Silvain J. F. , Bihr J. C. , Alnot M. , Lambert J. , Ehrhardt J. J.
Published: July 2002

β-NiAl thin films were prepared by physical vapor deposition (PVD) either on thin PVD Al2O3 films on Si substrates or on sapphire single crystals. These samples were characterized by x-ray diffraction and electron spectroscopies [Auger electron spectroscopy and x-ray photoelectron spectroscopy (XPS)]. Crystalline films were obtained after deposition at room temperature. XPS depth profiles and transmission electron microscopy cross sections show that the NiAl–Al2O3 interface is sharp (1 nm) and that NiAl oxidation occurs. Ni remains chemically unaffected by the presence of oxygen while the formation of Al3+ compounds is discussed. Within the film Al and Ni appear in a single chemical environment characterized by binding energies close to those of pure intermetallic compounds. Annealing of these wafers at 600 °C induces surface oxidation and, particularly, the formation of an Al2O3 protective layer.
Low-energy (5<E
i<100 eV), high-brightness, ultrahigh vacuum ion source for primary ion beam deposition: Applications for Al and Ge
Authors: Kim Y.-W. , Petrov I. , Ito H. , Greene J. E.
Published: July 2002

The design, operation, and use of a double-grid, electron-impact, ultrahigh-vacuum compatible, low-energy ion source capable of operating with relatively low-vapor-pressure solid source materials such as Al and Ge are described. The source can run at cell temperatures up to 1450 °C and deliver ion current densities Ji≳30 mA cm−2 (corresponding to 1.9×1017 cm−2 s−1) at source-to-substrate separations d of 2 cm, and Ji≳100 μA cm−2 (6.3×1014 cm−2 s−1) at d=40 cm with beam energies which can be varied from 5 to 100 eV. The uniformity in the ion current density from a 2.6-cm-diam source over 6-cm-diam substrates is better than 97%. A postextraction magnetic filter was used to separate the ion and neutral components and provide a pure ion beam. Initial nucleation and growth experiments on amorphous carbon substrates at 30 °C using thermal and accelerated (EAl+=35 and 75 eV) Al+ beams show that the average island size in layers with thicknesses between 2 and 6 nm increases by factors ranging from 3 to 14 with increasing ion energy. Layers deposited with thermal Al have an island size distribution which is near-Gaussian while Al islands formed from ion beams exhibit a bimodal size distribution.
Nitrogen-atom incorporation at Si–SiO2 interfaces by a low-temperature (300 °C), pre-deposition, remote-plasma oxidation using N2O
Authors: Lee David R. , Lucovsky Gerald , Denker Mark S. , Magee Charles
Published: July 2002

Nitrogen-atom incorporation at Si–SiO2 interfaces has been achieved using a N2O remote-plasma-assisted oxidation at 300 °C prior to remote plasma enhanced chemical vapor deposition of thin SiO2 gate dielectric films. The N2O remote plasma pre-deposition treatment removed residual hydrocarbon contamination, oxidized the Si surface, and incorporated nitrogen atoms at the Si–SiO2 interface. The amount of N incorporated at the Si–SiO2 interface was similar to amounts reported in the literature for oxides nitrided at much higher temperatures (800–1200 °C), where improved diffusion barrier properties and improved resistance to hot-carrier degradation were attributed to the presence of this N. The N2O remote plasma pre-deposition treatment was compared to previously developed, in situ H2 and O2 remote plasma surface treatments by Auger electron spectroscopy and secondary ion mass spectrometry measurements. Metal–oxide–semiconductor (MOS) capacitors were also fabricated to compare the electrical properties of interfaces formed with these different techniques. MOS C–V tests revealed that the interface formed by the N2O remote plasma-assisted oxidation had a low density of interface trapping states, midgap Dit=1×1010 cm−2 eV−1.