bluuebunny/crossref_metadata_2025 huggingface dataset:
Characteristics and limits of a dynamic expansion system
Authors: Calcatelli A. , Rumiano G.
Published: July 2002

Since in practice dynamic systems do not completely satisfy prerequisites regarding, e.g., isotropy, molecular behavior, and temperature stability, two spinning rotors were employed to study the behavior of effective pumping speed and conductance vs pressure, so that operating pressure limits could be defined, mainly as regards the upper one. The study evidenced a transition range starting between 4.6×10−2 and 1.2×10−1 Pa and depending on conductance geometry. Conductance values below the transition range were then determined and their dependence on the position of the conductance-control screw was derived. Pumping speed of the turbomolecular pump and overall effective pumping speed were experimentally determined. Investigations on spinning rotor and ion gage reading behavior with varying gas flow or conductance values provided an overall check of the system and a definition of its actual operating range.
History of the modern cryopump
Authors: Bridwell M. C. , Rodes J. G.
Published: July 2002

Fast clean pumpdown to 10−7 Torr range from high crossover pressure was a luxury a decade ago. Today the manufacturing/equipment engineer has at hand reliable, economical, and maintainable cryopumps alternative to conventional high vacuum pumps. This paper traces the nonliquid cryogen, mechanical refrigeration supported pump package from its inception to acceptance within IBM.
Electron stimulated desorption and the role of backscattered electrons
Authors: Hutson F. L. , Ramaker D. E. , Bermudez V. M. , Hoffbauer M. A.
Published: July 2002

Electron stimulated desorption (ESD) yields of ions and neutrals from surfaces are expected to contain large contributions due to backscattered electrons from the bulk. A procedure for deconvoluting these effects from experimental yield data has been developed, which includes contributions from both true secondaries and backscattered primaries. The procedure allows for changes in the backscattered yield with primary energy which is particularly necessary at low primary energies (10–40 eV) where most ESD thresholds occur. This method is applied to recently published excited OH neutral (OH*) yield data from TiO2. Although this deconvolution procedure reveals significant contributions from backscattered electrons, more importantly it also indicates that a significant yield arises from a direct nonresonant excitation mechanism and not from secondary electrons. The resonant portion of the OH* yield curve from TiO2 is found to be similar to published O− ESD data from O/W and O/Mo.
High resolution SIMS and neutron depth profiling of boron through oxide–silicon interfaces
Authors: Vandervorst W. , Shepherd F. R. , Downing R. G.
Published: July 2002

SIMS and neutron depth profiling (NDP) have been used to study the distribution 10B+ implanted into silicon through thermal oxide, and its segregation at the oxide–silicon interface on annealing. Using the reaction 10B (n,∝)7 Li, and a grazing exit geometry, the depth resolution in the ∝- particle spectrum in NDP is ∼10 nm. Better depth resolution is obtained in the calibrated SIMS profiles. For a wafer implanted with 70 keV 10B+ through 194 nm SiO2 to a dose of 1×1016 cm−2, the segregation coefficient after annealing at 1000 °C for 30 min was ∼3 from NDP. The segregated B was resolved better by SIMS and segregation coefficient of 10 was indicated from the SIMS depth profile. Analysis of the B profile in the Si surface after chemically stripping the SiO2 indicated a value ≥50. Higher depth resolution with NDP should be achievable by using a more grazing exit geometry, but higher flux neutron sources are required to maintain reasonable counting times.
Surface analysis of contaminated GaAs: Comparison of new laser-based techniques with SIMS
Authors: Becker C. H. , Gillen K. T.
Published: July 2002

A method of surface analysis by laser ionization (SALI) has been developed recently with an elemental sensitivity comparable to secondary ion mass spectrometry (SIMS). This mass spectrometric method uses intense UV laser general postionization following desorption or sputtering. The method has been applied to the analysis of a clean and contaminated GaAs(100) sample. Using an Ar+ beam for sputtering, direct comparison with SIMS shows the much more quantitative nature of SALI spectra. A 1.06-μm laser beam also was used to remove material for analysis, and comparisons are again made between desorbed neutral and ion species. Several potential uses of SALI are described for chemical analysis and for investigations of fundamental desorption mechanisms.
The Doublet III Big Dee Project
Authors: Davis L. G. , Luxon J. L.
Published: July 2002

The Doublet III tokamak is presently being reconfigured into a new larger dee-shaped plasma configuration. Experiments will begin in 1986 with a goal of high current, high beta plasma operation at moderate magnetic field. The existing toroidal field coil, Ohmic heating coil, and innermost plasma shaping coils will be retained. A new water-cooled vacuum vessel is being fabricated using a corrugated Inconel sandwich wall construction. Six new water-cooled copper poloidal field coils are also being fabricated. The resultant device along with additional power supplies will provide a capability for plasma currents of 3.5 MA for 1.5 s during the first phase of operations; the tokamak systems are designed for 5 MA operation with additional power systems. The four existing 80 keV, 3 MW neutral beam lines are being modified for optimum torus access and 0.7 s operation. These injectors will be upgraded to allow 5 s operation with new sources in 1987. The device has been designed to accommodate an additional 20 MW of ICRH and ECH power in the future. Limiters and vessel wall protection will be provided for initial operation with up to 40 MJ of input energy. Future installation of additional thermal armor will allow operation with up to 200 MJ of input energy over a 10 s period. Most of the existing diagnostics will be modified as required and reinstalled on the new vessel.
Initial oxidation of Zn(0001) surfaces: H2 adsorption and XPS studies
Authors: Chan L. , Griffin G. L.
Published: July 2002

H2 adsorption capacity measurements have been combined with XPS intensity measurements to study the kinetics of O2 uptake on Zn(0001) surfaces. On clean Zn(0001) the temperature programmed desorption spectrum of H2 shows half-order desorption kinetics and an apparent desorption energy of 11 kcal/mole. As the surface is oxidized by progressive dosing with O2, two features are noted: The H2 desorption spectrum characteristic of clean Zn(0001) is attenuated in a nonlinear fashion and a new H2 desorption state with an activation energy of 18 kcal/mole is observed for surfaces with less than saturation oxygen coverage. XPS results indicate that H2 adsorption is not completely blocked until an oxygen coverage corresponding to approximately 3 monolayers is achieved. Both H2 uptake and XPS intensity measurements also indicate that the oxygen sticking coefficient increases with oxygen coverage up to about one half of the O2 saturation coverage. These results indicate that the initial oxidation of Zn surfaces involves the formation and growth of multilayer oxide islands, where oxygen incorporation at the perimeter of the islands is the rate limiting step.
Dipole active surface vibrational resonances on clean and hydrogen covered Ni(110)
Authors: Stroscio Joseph A. , Persson M. , Bare Simon R. , Ho W.
Published: July 2002

A vibrational resonance of the surface metal atoms on the clean Ni(110) surface is observed by electron energy loss spectroscopy. The dipole coupling arises from the incomplete screening of the moving ion cores by the conduction electrons in the surface region. Surface lattice dynamical calculations are in excellent agreement with the measured spectra for the clean surface and show that there are no significant deviations in the surface force constant from the bulk value. This resonance results from a pseudo band gap in the longitudinal bulk phonon dispersion relation along the [110] direction. On the (1×2) reconstructed surface induced by hydrogen chemisorption, a dramatic decrease in intensity of the resonance is observed.
Observation of a grain boundary phase transformation induced by solute segregation
Authors: Sickafus K. , Sass S. L.
Published: July 2002

Comparisons were made of the defect structures present in low angle Fe twist boundaries in the absence and in the presence of Au solute. Samples for this study were prepared by sintering together epitaxial Fe–Au dilute alloy thin films, grown by electron beam vapor deposition. Rutherford backscattering spectrometry (RBS) measurements showed that the Au solute was concentrated in the vicinity of the grain boundaries in this study. Transmission electron microscopy observations showed that the Au solute segregated at the boundary induces a transformation in the grain boundary dislocation (GBD) structure from a 〈110〉-oriented GBD network, to a 〈100〉-oriented GBD network. Analytical electron microscopy showed that the grain boundary regions containing the 〈100〉-GBD networks were substantially richer in Au than the regions containing the 〈110〉-GBD networks. These results are evidence for the occurrence of a two-dimensional phase transformation at the grain boundary.
Decomposition of ethylene on small Pd particles
Authors: Durrer W. G. , Poppa H. , Dickinson J. T. , Park C.
Published: July 2002

New results have been obtained which contribute to the understanding of hydrocarbon reactions on the surface of highly dispersed metal systems. Small particles of Pd were grown by electron beam evaporation on cleavage planes of high purity natural mica under ultrahigh vacuum conditions. Samples were subsequently characterized by transmission electron microscopy. Average particle sizes ranged from about 1 to 10 nm diameter. The chemisorption and decomposition of C2H4 on the Pd particles was studied using Auger electron spectroscopy and flash thermal desorption. We show that (a) C2H4 decomposes on Pd particles at room temperature, (b) specific surface sites are causing decomposition, and (c) the proportion of such active sites is significantly greater for the smaller metal particles. This enhanced reactivity may be due to an increase in the density of step, corner, and edge sites with a decrease in particle size.
Deposition and characterization of CdSe1−xTex thin films
Authors: Russak Michael A.
Published: July 2002

Photovoltaic activity in CdSeTe thin films is discussed (AIP).
A versatile reactor for ultrahigh vacuum surface modification studies based on a reusable metal sealing system
Authors: Hayes Todd R. , Evans John F. , Rusch Thomas W.
Published: July 2002

A versatile reactor is described which enables nonequilibrium plasma and/or vapor phase chemical modification of surfaces in an inert reaction volume, and features sample transfer to a surface analytical chamber through high vacuum. Separate, specialized reaction chambers may be interchanged to optimally perform a wide variety of experiments such as high temperature catalysis or plasma deposition studies. The reactor body and the reaction chamber designed for plasma and chemical surface modification are described, with emphasis on the considerations involved in their design. Data from two types of experiments which demonstrate the capabilities of the reactor, covalent modification of SiO2 with organosilyl species, and O2 plasma etching of polystyrene, are presented and interpreted.
Molecular beam epitaxy growth and applications of epitaxial fluoride films
Authors: Schowalter L. J. , Fathauer R. W.
Published: July 2002

The growth of an epitaxial insulator on a semiconductor substrate and its subsequent overgrowth with an epitaxial semiconductor has a number of important applications, including the construction of three-dimensional devices, better isolation of devices in VLSI circuits, improved gate insulators, and optoelectronic devices. In addition, it presents a unique opportunity to study an insulator/semiconductor interface under controlled conditions. Many of the fluoride compounds are excellent insulators and can be grown epitaxially by MBE on many common semiconductors. Thus, epitaxial fluoride growth is being extensively studied at a number of laboratories. In our laboratory, the growth of epitaxial CaF2 on Si substrates and subsequent overgrowth with Si or Ge has been studied. While epitaxial growth of CaF2 (which has an fcc lattice structure as does Si) can be obtained on (100), (110), and (111) oriented Si substrates, the best quality crystal growth and surface morphology is obtained on (111) substrates, as the (111)CaF2 surface has the lowest free energy. Epitaxial Si overgrowth of the (111)CaF2 layers generally results in a high density of planar defects near the Si/CaF2 interface. However, epitaxial Ge overlayers have been grown with much lower defect densities.
The role of grain boundaries in the surface segregation of tin in tin–lead alloys
Authors: Opila R. L.
Published: July 2002

Lead alloyed with low levels of tin has been shown to exhibit surface segregation of tin, especially in the presence of oxygen. In lead alloyed with 3 atm % tin, the grain boundaries are shown to be significant sources of tin with scanning Auger microprobe techniques. The kinetics of the segregation process have been determined for both grain boundary and boundary-free regions at room temperatures. The subsequent surface diffusion of tin from the highly enriched grain boundaries has also been studied. The rate of tin surface segregation is enhanced considerably by exposing the clean surface to oxygen. While oxygen is found to be immobile at room temperature, diffusion of tin from the highly enriched grain boundary is greatly increased.
Atomic and electronic structure of the (311) surfaces of GaAs
Authors: Duke C. B. , Mailhiot C. , Paton A. , Kahn A. , Stiles K.
Published: July 2002

The polar (311) surfaces of GaAs exhibit four inequivalent bulk (1×1) terminations each of which exhibits a metallic electronic character associated with partly occupied surface states. Experimentally prepared surfaces also display a (1×1) symmetry, but the tendency for surfaces to reconstruct towards an insulating ground state suggests adatom surface structure models which yield the saturation of the surface dangling bonds. We examine two Ga–adatom models on an otherwise clean As-terminated (311) surface. Empirical total-energy minimization calculations reveal that the As-terminated surfaces exhibit two possible minimum-energy adatom structures with the top-layer (adatom) Ga in a bridge or hollow site. In the bridge-site configuration, the Ga species occupy twofold sites on top of a relaxed As-terminated surface. In the hollow-site configuration, the Ga occupy threefold sites slightly above a nearly unrelaxed surface. Both correspond to simple saturated-bond models of the surface chemical bonding but only the threefold structure is an insulator. Low-energy electron diffraction (LEED) intensities for fifteen beams have been measured at T=300 K for the experimentally inequivalent A and B surfaces. Comparison of calculated LEED intensities with the data for the B surfaces reveals the existence of two minimum-Rx regions of surface structures. Previously proposed structures associated with metallic surface states are not compatible with the experimental LEED intensities for either surface.
Infrared spectroscopic study of SiOx films produced by plasma enhanced chemical vapor deposition
Authors: Pai P. G. , Chao S. S. , Takagi Y. , Lucovsky G.
Published: July 2002

We have studied the local atomic structure of silicon suboxide (SiOx, x<2) thin films using infrared (IR) spectroscopy. The films were prepared by plasma enhanced chemical vapor deposition (PECVD) of silane (SiH4) and nitrous oxide (N2O) mixtures, which were then diluted with He. The IR spectra were found to vary significantly with the degree of He dilution. Films grown with no He showed SiN, NH, and SiH bonding groups in addition to the three characteristic vibrations of the Si–O–Si linkage. The addition of He reduced the strength of the SiN, NH, and SiH absorption bands, and resulted in systematic increases in the frequency of the Si–O–Si asymmetric stretching vibration. The frequency of this Si–O–Si stretching vibration scales linearly with the oxygen concentration from approximately 940 cm−1 in oxygen doped amorphous silicon to 1075 cm−1 in stoichiometric noncrystalline SiO2. A deposition temperature of 350 °C and a He dilution of 50% gave a film composition close to SiO1.9. We propose a model for the deposition process that emphasizes the role of the He dilution.
Diluted magnetic semiconductors: Issues and opportunities
Authors: Furdyna J. K.
Published: July 2002

We review the crystallography, band structure, and optical, electrical, and magnetic properties of diluted magnetic semiconductors (DMS), with emphasis on ternary alloys of the type AII1−xMnxBVI (e.g., Hg1−xMnxTe). Considerable attention in the review is given to spectacular novel effects which arise from the selective enhancement of spin-dependent properties due to exchange interaction between the localized magnetic moments (Mn+2) and band electrons. We then identify the major issues currently facing the field of DMS, and we discuss the major opportunities which these materials hold in store, both in the area of further scientific discoveries and in technological applications. In particular, we stress the need for a systematic concerted effort in the area of controlling impurities and defects in DMS, and we argue that exchange interaction in these materials offers an additional tool for studies in this area. We then describe, as examples of important future opportunities, the emerging field of Fe-based DMS (e.g., Hg1−xFexSe), and the rapidly evolving area of DMS superlattices and heterostructures. In connection with the latter, we give special attention to the unique features arising from the exchange interaction in the quantum well situation in alternating DMS/non-DMS structures.
Variable magnetic field Hall effect measurements and analyses of high purity, Hg vacancy (p-type) HgCdTe
Authors: Gold M. C. , Nelson D. A.
Published: July 2002

This paper reports on complete Hall characterization of Hg vacancy ( p-type) HgCdTe with very high purity and compositional uniformity. The x-values of the Hg1−xCdxTe samples ranged from 0.180 to 0.300 with carrier concentrations of 6×1015 to 3×1017. Each sample was of very uniform composition with Δx<0.0015, as verified from room temperature infrared transmission measurements. This characterization couples variable magnetic field (up to 150 000 G) and temperature (from 15 to 160 K) data with an analysis procedure which includes heavy holes, light holes, and electrons. The measurements and analyses yield unambiguous determination of the two Hg vacancy ionization energies, one shallow level at 12–14 meV for x=0.216 HgCdTe and one deeper level at 40% of the band gap measured from the valence band edge. In addition, not only can the majority carrier (heavy holes) temperature dependent concentrations and mobilities be determined by this technique, but both the minority carrier (electron) and the light hole concentrations and mobilities have been determined as a function of temperature. At magnetic field strengths of 40 000 G and above, the heavy hole concentration was observed to decrease with increasing magnetic field. This occurred at all temperatures (15 to 160 K) investigated. This effect is commonly referred to as magnetic ‘‘freeze out’’ and is reported here for HgCdTe for the first time.
Reclamation of vacuum pump fluids
Authors: Whitman Christine B.
Published: July 2002

Several types of spent vacuum pump fluids were reconditioned by molecular distillation and filtration, which subsequently performed equal to or better than new fluids. The reclaimed fluids were analyzed for and found to be free of suspected trace contaminants, particularly those reaction products generated during process runs. The quality of different reclaimed fluids versus yield and the resulting economics for different fluids exposed to various processes were compared. Reclamation was found to produce fluids of a quality equal to new fluids, and to be economical. Pros and cons for in-house reclamation programs were reviewed. Recommended procedures for spent fluid collection were proposed. A cost analysis of a typical on-site reclamation program was reviewed. A cost comparison was made for disposal of waste vacuum pump fluid versus a reclamation program using an outside contractor. Alternate reclamation procedures were described.
Thin-film temperature sensors deposited by radio frequency cathodic sputtering
Authors: Godefroy J. C. , Gageant C. , François D. , Portat M.
Published: July 2002

In order to gain better knowledge on the heat exchanges occurring in the turbines of aircraft motors, ONERA is presently developing thin-film heat sensors capable of operating up to a maximum temperature of 1100 °C. These high-temperature sensors are surface thermometers and fluxmeters of the gradient type. They consist of metal films forming thermocouples and insulating films such as alumina or zircon stabilized by yttrium oxide. These layers are deposited by radio frequency cathodic sputtering on substrates such as IN100, DS200, and CMXS2 superalloys which are either aluminized or coated with a NiCoCrAlY alloy. The present state of development of the thermometers is described herein as well as the first results obtained.
Characterization of charcoals for helium cryopumping in fusion devices
Authors: Sedgley D. W. , Tobin A. G. , Batzer T. H. , Call W. R.
Published: July 2002

The capability of charcoal as a sorbent for helium at cryogenic temperatures depends upon charcoal characteristics that are not well understood. Previous work by the authors has indicated that the charcoals’ pumping capability for helium depends as much on their source as on their particle size distributions. To develop a correlation between the physical characteristics of charcoal and helium pumping performance, different charcoals based on wood, coal, coconut, and a petroleum by-product were obtained from commercial sources. They were bonded to an aluminum substrate, and cooled to liquid-helium temperatures in a vacuum chamber. The helium pumping speed at constant throughput versus quantity of helium absorbed was measured for each charcoal grade. Porosimetry measurements on each charcoal grade using nitrogen as the sorbent gas were made that included total surface area, adsorption and desorption isotherms, and pore area and pore volume distributions. Significant differences in helium pumping performance and in pore size distribution were observed. Comparisons are made between helium pumping performance and charcoal characteristics and a possible correlation is identified.
Plasma surface interactions in compact ignition devices
Authors: Ulrickson M. , Cecchi J. L. , Lipschultz B.
Published: July 2002

Owing to their small size compact ignition devices offer the potential of studying the physics of ignited plasma with only a relatively small capital outlay. The penalty for the small size is that the limiter/divertor heat loads are high due to the high energy density and the small surface area of these devices. The large amount of stored energy also implies that disruption effects will be very severe. Since the distance between the divertor and the plasma is minimized, careful attention must be paid to impurity generation. Hence erosion becomes a critical issue. Another important issue is that of particle handling, since the machine operates at high density. These issues of heat loads, disruptions, impurity generation, and particle handling will be discussed in this paper.
The design and operation of a new extractor gauge
Authors: Tang Zheng-qing , Chen Hong-yuan , Lu Zu-hong
Published: July 2002

A new type of extractor gauge with a cylindrical reflector and a squirrel-cage grid shaped as a frustum of a cone is presented. The trajectories of electrons and ions are evaluated by numerical method, while the extraction and collection efficiency of the new gauge, as well as its x-ray current, have been studied experimentally. These results show that the new geometry with appropriate applied voltage of the gauge are favorable for enhancing the sensitivity and reducing the x-ray current. It has been found that self-modulation and ‘‘deep modulation’’ can be used to determine the x-ray current of an extractor gauge without an additional modulator electrode. An interesting result of deep modulation is that the value of modulation factor k (defined as the ratio of the collector current at modulation to normal operating conditions at high vacuum, 10−6–10−8 Torr) of 10−4 has been achieved, so that the x-ray current can be measured at a pressure 2–3 orders of magnitude higher than the x-ray limit by deep modulation. As a result of this improvement in design and operation of the extractor gauge, both of the x-ray limits estimated by comparing with a normal Bayard–Alpert gauge in geometry factor and measured by using the deep modulation are less than 1×10−13 Torr (equivalent for N2), and the sensitivity is about 25 Torr−1.
The use of turbomolecular pumps in television tube production
Authors: Goetz D. , Schaefer G. , Rufus J.
Published: July 2002

In the production process of television tubes vacuum pumps are required for the aluminum coating process and for the evacuation of the assembled tube before melting off. The general vacuum and pump down characteristics are discussed. It is described how the use of turbomolecular pumps can improve the product quality and the efficiency of the process compared to diffusion pumps which have been an industry standard for this application until today. Harsh industrial operating conditions like tube implosions, external shocks due to the intermittent movement of the pump carts, and high environmental temperatures have to be considered. Turbomolecular pumps had to be optimized for this application over many years. Typical setups of such pump carts are presented.
Preparation of cubic boron nitride film by activated reactive evaporation with a gas activation nozzle
Authors: Inagawa K. , Watanabe K. , Ohsone H. , Saitoh K. , Itoh A.
Published: July 2002

The preparation of a cubic boron nitride film by physical vapor deposition processes has been investigated. The electron beam guns of a hollow cathode discharge type and a conventional high-voltage type were used for the boron evaporation source. The stoichiometric boron nitride film could be obtained by using a gas activation nozzle. It was necessary to apply the rf bias potential to the substrate to accelerate the formation of the cubic BN phase. It was found that mixing argon gas to the reactant gas N2 assists the formation of cubic phase. The film structure was characterized by the infrared absorption spectrum and transmission electron diffraction observation. The microhardness of the cubic boron nitride film obtained was estimated to be about 4000 kg/mm2.
Residual currents in several commercial ultrahigh vacuum Bayard–Alpert gauges
Authors: Filippelli Albert R.
Published: July 2002

Residual currents have been determined for several commercial UHV Bayard–Alpert gauges, including two modulated gauges, by comparison against an extractor gauge. Residual currents in the modulated gauges were also determined by the modulation method (Redhead mode I) and these results are compared with those of the first method. Residual current modulation factors were estimated by combining the results of these two methods. A significant residual current modulation factor (1.33) was found for one of the modulated Bayard–Alpert gauges. One notable aspect of this work is the finding that residual current values can differ by more than a factor of 10 among nominally identical gauges. Another interesting and somewhat surprising feature of these measurements, which were performed in H2 at pressures in the range 4×10−9 to 4×10−5 Pa, is that no hysteresis was detected in the gauges’ response to large, rapid decreases in the H2 pressure.
Secondary electrons in ion gauges
Authors: Grosse G. , Harten U. , Jitschin W. , Gentsch H.
Published: July 2002

The secondary electron emission induced by the electron impact on grid materials of ionization gauges has been studied. It makes up for about 10% of the sensitivity of the gauge. Changes in the chemical composition of the grid result in changes of the secondary electron yield and thus in changes of the gauge sensitivity amounting up to several percent. Gold and carbon constitute especially stable surfaces.
A miniature deflected-ion-beam ionization gauge with the channel electron multiplier
Authors: Li Wangkui , Zhang Zhou
Published: July 2002

A miniature deflected-ion-beam ionization gauge with a channel electron multiplier (CEM) has been developed. In the gauge, the ionization region is a cylinder shell between the two anode cylinders and the CEM is mounted on the axis in the inner anode. The ions extracted from the shell are deflected to 180° towards the axis. The construction of the gauge is compact and exquisite. The gauge’s sensitivity is 1.9×1016 counts/s Pa A. The x-ray background corresponds to a pressure in the 10−11 Pa range.
Nonlinear optical response of [111] growth axis strained-layer superlattices
Authors: Smith D. L. , Mailhiot C.
Published: July 2002

The presence of internal strains in [111] growth axis strained-layer III–V superlattices leads to the generation of large polarization fields (exceeding 100 kV/cm) because the constituent semiconductors are piezoelectric. These strain-induced polarization fields produce a substantial reduction of the superlattice band gap and cause a spatial separation of the electrons and holes in the confining layers in such a way as to screen the internal fields. When free carriers are present to screen the strain-induced piezoelectric fields, the electronic structure (energy band gap, in-plane effective masses) and the optical response (absorption edge, optical constants) of the strained-layer superlattice depend on the density of free carriers. In the case where the electrons and holes are photogenerated, the superlattice photoabsorption profile can be modified by variations of the intensity of the optical excitation. The nonlinear optical response of strained-layer superlattices owing to free-carrier screening of strain-generated internal fields is illustrated here for the case of Ga1–xInxAs–Al1–yInyAs strained-layer superlattices.
X-ray photoelectron forward scattering studies of epitaxial overlayers and sandwich structures of Au and Ag on Ni(100)
Authors: Egelhoff William F.
Published: July 2002

Angle-resolved x-ray photoelectron spectroscopy (XPS) has been used to study the short-range order (SRO) in epitaxial layers (1–73 monolayers) of Ag and Au on Ni(100). The basis for this technique is the enhancement of core-level intensities which occur along nearest-neighbor axes in the lattice due to photoelectron forward scattering. The XPS results indicate the Ag and Au films are very different in structure. Ag films exhibit conventional (111) surfaces, whereas the Au films have a SRO closely resembling a (100) surface. Surprisingly, the Au films exhibit a clear, sharp, hexagonal low-energy electron diffraction (LEED) pattern indistinguishable from the Ag(111) LEED pattern at beam energies below 200 eV. Above 300 eV, the Au films exhibit additional spots, which would seem to imply that deeper Au layers have a (100) structure and are in registry with the Ni(100). However, such registry would require an unrealistically large compression of the Au. Therefore, this structure cannot be considered fully ‘‘solved.’’ However, it is clear the Au films are (100)-like in the bulk and likely that they have a hexagonal monolayer at the surface. This is somewhat like the pure Au(100) surface. Thus the long-range order of the surface monolayer dominates in LEED below 200 eV, while the SRO of the bulk dominates in XPS. These results demonstrate the importance of XPS forward scattering as a probe of SRO in epitaxial films.
Medium-energy ion scattering studies of relaxation at metal surfaces
Authors: Yalisove S. M. , Graham W. R.
Published: July 2002

The creation of a surface leads to the rearrangement of the positions of the atoms composing the outerlayers of the crystal on a magnitude of hundredths of angstroms to angstroms. Medium-energy ion scattering combined with channeling and blocking (MEIS–CB) has become one of the most accurate methods to measure the three-dimensional (3D) structure of these relaxed and often reconstructed surfaces. The Ni(110) and NiAl(110) surfaces are used to demonstrate the effectiveness and accuracy of MEIS–CB as a method of 3D structural characterization of metal and alloy surfaces. Other recent MEIS–CB studies of metal relaxation are reviewed.
The effects of oxygen contamination on the properties of reactively sputtered indium nitride films
Authors: Westra K. L. , Lawson R. P. W. , Brett M. J.
Published: July 2002

Thin films of InN, a direct band gap semiconductor, were deposited by dc planar magnetron sputtering of an indium target in a pure nitrogen atmosphere. Rutherford backscattering analysis of the film stoichiometry showed oxygen contamination of at least 11 at. % throughout the film thickness. In spite of this bulk contamination, the crystal structure showed no evidence of In2O3 or of an indium oxynitride. The band gap also was indicative of InN (1.89 eV), not In2O3 (3.6 eV). We propose that the oxygen forms an amorphous indium oxynitride that exists in an InN polycrystalline matrix, and that this amorphous oxynitride decreases the electron mobility of the films.
Tunneling through localized barriers with application to scanning tunneling microscopy: New scattering theoretic approach and results
Authors: Lucas A. A. , Morawitz H. , Henry G. R. , Vigneron J.-P. , Lambin Ph. , Cutler P. H. , Feuchtwang T. E.
Published: July 2002

The general scattering theoretic technique of localized Green functions is applied to the calculation of one-electron elastic tunneling current through nonseparable, microscopically localized barriers. The new technique is an exact theory of tunneling, not restricted, as the transfer-Hamiltonian method of Bardeen, to weakly coupled electrodes. For illustration purpose and application to scanning tunneling microscopy (STM), we consider a model hemisphere–plane junction for which the multiple-image barrier is taken into account for the first time. The full Green function is calculated at the points of a fine grid covering the tunnel region from the solution of Dyson’s equation which treats the localized tunnel barrier to all orders of perturbation of the planar barrier Green function. The tunnel current is obtained at the grid points from the Lippman–Schwinger equation, starting from the solution of the strictly planar, separable junction. The principal results of the calculation are presented as a current distribution at the planar electrode from which the lateral resolution of the model STM can be discussed. We find that the tunnel current intensity is already reduced by a factor of 2 at a distance from the tip axis equal to the tip radius. This explains, from a calculation based on first principles, the observed lateral resolution of the microscope.
The copper–silicon interface: Composition and interdiffusion
Authors: Corn Stewart H. , Falconer John L. , Czanderna A. W.
Published: July 2002

The interdiffusion of Cu and Si was studied using ion scattering spectrometry (ISS) and x-ray photoelectron spectroscopy (XPS). The samples were prepared in very high vacuum (VHV) by evaporating Cu onto Ar+-cleaned and -annealed Si(111) surfaces. The surfaces were transferred directly from VHV to UHV and analyzed by ISS and XPS. Diffusion was studied at 295 K, and then at temperatures up to 825 K. A comparison of data taken at the Cu–Si interface was made with Cu3 Si. Copper and Si interdiffuse rapidly at 295 K and above; diffusion is completed in <60 min as temperatures are increased up to 825 K, depending on Cu overlayer thicknesses of 2 or 20 nm. In contrast to previously published results, our ISS and XPS data did not provide evidence for the formation of Cu3 Si at the Cu–Si interface under the range of Cu coverage (2–20 nm) and temperatures (295–825 K) studied. The Cu 2p3/2 peak shifted 0.5 eV to lower kinetic energy for Cu–Si interfaces; this peak does not shift for Cu3 Si. Furthermore, the compositions of the Cu–Si interfaces and Cu3 Si differed. Oxide-covered Si surfaces were used in some investigations for comparative purposes, and the effect of O2 exposure to the Cu–Si interface was also studied. Oxygen contamination on Si substrates was found to retard the interdiffusion process. The Cu–Si interface reacts with O2 to form oxides of Si; no Cu oxides formed.
Some characteristics of an inverted magnetron cold cathode ionization gauge with dual feedthroughs
Authors: Peacock N. T. , Peacock R. N.
Published: July 2002

Single feedthrough cold cathode ionization gauges often suffer from spurious currents due to insulator leakage and field emission which mask the small pressure dependent ionization currents. This paper describes an inverted magnetron gauge tube designed to reduce these problems by using separate feedthroughs for the anode high voltage and the cathode current. The cathode is insulated from the grounded metal envelope. This gauge is useful for pressure measurement from 10−4 to 10−10 Torr in the constant voltage mode and from 10−2 to 10−6 Torr in the constant current mode. The characteristic curves typically show a change of slope at ∼10−9 Torr. When used at a magnetic field of 1200 G the slope above the inflection point is 1.08±0.02, while below it the slope varies from 1.4–1.9. It was found that operating the gauge for some time at a pressure of 10−4 Torr does not change the curve above the inflection point, but can alter the lower portion temporarily.
A comparative study of the electrical properties of epitaxial fluorides
Authors: McMullin P. G. , Sinharoy S.
Published: July 2002

Epitaxial fluorides on semiconductors have potential applications in three-dimensional integrated circuits and other very high-speed devices. Most of the attention in this technology has been focused on attempts to grow good quality epifluoride/semiconductor and semiconductor/epifluoride/semiconductor structures and characterize their crystalline and interfacial structural quality. However, the ultimate test of the usefulness of the epitaxial fluorides will be provided by their electrical properties. We have measured the detailed electrical properties of the CaF2/Si (100), LaF3/Si (111), CeF3/Si (111), PrF3/Si (111), NdF3/Si (111), and the (CaxSr1−x)F2/GaAs (100) structures grown in this laboratory by molecular-beam epitaxy. The results are compared with each other and also with those reported by other laboratories when available.
Thermal oxidation of gallium arsenide
Authors: Monteiro Othon R. , Evans James W.
Published: July 2002

Here we present some results of transmission electron microscopy and secondary ion mass spectroscopy of thermally oxidized gallium arsenide with different types of dopants. At temperatures below 400 °C an amorphous oxide is formed. Oxidation at temperatures between 500 and 600 °C initially produces an epitaxial film of γ-Ga2O3. As the reaction proceeds, this film becomes polycrystalline and then transforms to β-Ga2O3. This film contains small crystallites of As2O5 and As2O3 in the case of the chromium doped samples, whereas only the former was detected in the case of silicon and tellurium doped samples. Elemental arsenic was always found at the interface between the oxide and GaAs. Chromium doped gallium also exhibited a slower oxidation kinetics than the other materials.
Recommended procedures for measuring the performance of positive-displacement mechanical vacuum pumps
Authors: Kendall B. R. F.
Published: July 2002

It is essential that uniform and realistic test procedures be used for measuring the performance of vacuum apparatus. This paper recommends procedures for measuring the pumping speeds and ultimate pressures of positive-displacement mechanical vacuum pumps. It replaces outdated material originally published in three separate sections between 1963 and 1967. Changes include emphasis on more modern pressure gauges such as the capacitance manometer and spinning rotor gauge, an attempt to harmonize procedures with existing international standards, a shift to Système International units of pressure, and minor rewording to include dry vacuum pumps. Minor changes have been made in test dome geometry to reduce sources of error. Material has been added concerning some of the causes of pumping speed variations during continuous operation of a given pump. Other new material covers the problems of measuring water vapor tolerance and water vapor pumping capacity. Practical details which minimize measuring errors are discussed.
Surface atomic geometry and electronic structure of II–VI cleavage faces
Authors: Duke C. B. , Wang Y. R.
Published: July 2002

Recent models of the electronic structure and surface atomic geometries on wurtzite-structure CdS and CdSe have been extended to encompass zinc-blende structure CdTe. The driving force for the surface reconstruction of the cleavage faces of all these materials is the lowering in energy of a band of anion dangling bond surface states accompanied by their acquisition of back-bonding character. The resulting reconstructed surface is characterized by the eigenvalue spectrum associated with this band of states as well as the atomic geometries. The predicted geometries for CdTe(110), CdSe(101̄0), and CdSe(112̄0) are in good correspondence with low-energy electron diffraction (LEED) and low-energy positron diffraction intensity analyses. The calculated surface state eigenvalue spectra are compared with recent angle-resolved photoemission spectra (ARPES) for CdTe(110) and CdSe(101̄0). The main features of the ARPES measurements are reproduced by the calculated eigenvalue spectra. The predicted atomic geometry for CdTe(110) corresponds well both with prior predictions for ZnSe(110) and ZnS(110), and with LEED experimental surface structures indicating that these surfaces exhibit a universal structure in which the independent surface structural parameters scale linearly with the bulk lattice constant.
Full potential linear muffin-tin orbital study of interface behavior in Ti–C superlattices
Authors: Price David L. , Cooper Bernard R.
Published: July 2002

We have used total energy calculations to study the crystallographic structure and electronic behavior at the interface in Ti–C superlattices. This involved examining a series of structures in the Ti–C (and to a lesser extent, W–C) system, proceeding from the ground-state NaCl structure to superlattices. In particular we present total energy calculations for an arrangement of atoms designed to allow direct investigation of the competition between Ti–C bonds and C–C bonds. We conclude that the equilibrium structure is dominated by C–C bonding and find that the interface has a graphitelike atomic arrangement rather than a carbidelike arrangement. This carries important implications for the diffusion and bonding at the interface. These total energy calculations have been completed using a recently developed, self-consistent linear combination of muffin-tin orbitals electronic structure method. This is a full-potential, all-electron, variation on standard linear muffin-tin orbital electronic structure methods and, along with careful self-consistent determination of the parameters involved, allows accurate total energy calculations of the type of low-symmetry systems involved in this study.
Low-coverage metal-induced structural changes in the substrate at metal/InP(110) interfaces determined by photoemission extended x-ray absorption fine structure
Authors: Choudhary K. M. , Mangat P. S. , Kilday D. , Margaritondo G. , Soukiassian P. , Starnberg H. I. , Hurych Z.
Published: July 2002

Photoemission extended x-ray absorption fine-structure (PEXAFS) studies of cleaved InP(110) surface and metal/InP(110) interfaces for low coverages of metals are described. Using P 2p PEXAFS for InP(110) and InP(110)+1-Å Al, low-coverage metal(cluster)-induced structural changes in the substrate are derived. The data were analyzed by conventional Fourier analysis procedures using the theoretical backscattering phase function of McKale et al. plus absorber phase function of Teo and Lee. The structural changes include removal of relaxation of the surface unit mesh in combination with reconstruction of in-plane P–In bonds within the surface unit mesh (change in the basis). This surface structure should give a (1×1) low-energy electron diffraction pattern. A similar observation was recently made by Choudhary et al. for (1)/(8) monolayer Na/InP(110), in which case the low-coverage alkali-metal-induced removal of relaxation of the surface unit mesh was found as in the case of InP(110)+1-Å Al, but the change in the basis was determined to be different.
Surface studies of amorphous W75Si25 oxidation
Authors: Christensen T. M.
Published: July 2002

The surface composition and oxide growth kinetics of amorphous W75Si25 sputter-deposited thin films were examined using x-ray photoelectron spectroscopy (XPS), ellipsometry, and thermogravimetric analysis (TGA). These films have been proposed as metallization layers on compound semiconductors for high-temperature applications. The 1-μm-thick films were oxidized in air at temperatures of 30 to 600 °C for times of 10 min to 30 h. Angle resolved XPS measurements indicated that the surface region consisted of a thin layer (∼16 Å) of primarily SiO2 above a layer of mainly WO3 for oxidation temperatures up to 425 °C. At oxidation temperatures between 150 and 425 °C the WO3 signal from the surface region was enhanced. In this range the SiO2 layer was thinner resulting in a larger detected signal from the underlying WO3 layer. For oxidation temperatures above 450 °C, the two oxides appeared to coexist although the surface region was enhanced in SiO2. Ellipsometry and TGA measurements did not indicate a change in oxide growth kinetics in the temperature range 150–550 °C. The kinetics were parabolic and displayed an Arrhenius-type temperature dependence with rates similar to W oxidation.
Grain boundary diffusion of phosphorous in silicon
Authors: Holloway P. H. , Abrantes T.
Published: July 2002

Grain boundary diffusion of phosphorous in Wacker SILSO polycrystalline Si has been studied at temperatures between 750 and 1050 °C using a groove and stain technique. It is shown that the Fisher model of grain boundary diffusion describes the data accurately assuming an infinite source (Whipple solution) aided by Le Claire’s graphical solution of the model. The bulk DB and grain boundary D′ diffusion coefficients were best described by DB=1.6×1015 cm2/s exp(−2.09eV/kT) and D′=4.0×10−5 cm2/s exp(−1.4eV/kT), respectively, where a boundary width of 5 Å was assumed for D′. Proper procedures for reducing the data are illustrated including an important correction to penetration depth made necessary because of the inability of optical microscopy to detect very narrow diffusion regions near a grain boundary.