bluuebunny/crossref_metadata_2025 huggingface dataset:
Epitaxial photochemical deposition of II–VI semiconductors
Authors: Irvine S. J. C. , Mullin J. B.
Published: July 2002

The materials problems in the preparation of II–VI semiconductors are outlined, and the benefits of epitaxial growth at low temperatures are reviewed. The majority of the II–VI compounds have been prepared by pyrolytic metal–organic vapor phase epitaxy but relatively few have so far been produced by photolytic dissociation. ZnO has been deposited at room temperature using an excimer laser; improved electrical characteristics are claimed for the laser deposition. The majority of the effort on photodeposition has been with the infrared detector material, cadmium mercury telluride (CMT) and the related binaries, CdTe and HgTe. Good-quality epitaxial growth has been reported at temperatures as low as 250 °C for CMT and below 200 °C for HgTe. These growth temperatures enable the growth of more abrupt interfaces and lower metal vacancy concentrations.
An analysis of the thermally induced formation of a uniform liquid layer of ternary deuterium–tritium mixture inside a cryogenic spherical shell inertial confinement fusion target
Authors: Varadarajan V. , Kim K. , Bernat T. P.
Published: July 2002

The values of the externally applied thermal gradients that give rise to uniform liquid layers of a ternary deuterium–tritium mixture inside a cryogenic spherical shell inertial confinement fusion target are calculated using a model recently developed by the authors. It is shown that the surface tension gradients induced by the component separation at the liquid–vapor interface pull the liquid upward, thus counteracting the gravity-induced fuel sagging and forming dynamically stable uniform liquid layers. The governing equations are the equations of continuity, momentum, energy, and mass diffusion–convection, which are solved using finite-difference methods. The solutions indicate that one needs fairly large positive thermal gradients, obtained by keeping the top of the target warmer than the bottom, in order to create uniform liquid layers on the inner surface of the target.
Graphitization of amorphous diamondlike carbon films by ion bombardment
Authors: González-Hernández J. , Asomoza R. , Reyes-Mena A. , Rickards C. J. , Chao S. S. , Pawlik D.
Published: July 2002

Amorphous hydrogenated carbon films prepared by glow discharge decomposition of CH4 at a power density of 0.4 W cm−2 were bombarded with hydrogen and nitrogen ions of energies between 100 and 600 keV. The structural changes due to various ion doses were analyzed using Raman, photoluminescence, and infrared spectroscopy. Nitrogen bombardment produces essentially a hydrogen depletion in the whole dose interval as shown by the IR spectra; as a consequence there is a drop of several orders of magnitude in the integrated photoluminescence intensity, and carbon–carbon double bonds, characteristic of a graphiticlike structure, appear in the Raman spectra. The same general effects are produced under proton bombardment, but their intensities change more gradually with the ion dose, in agreement with a vacancy production mechanism.
Tungsten–rhenium alloys as diffusion barriers between aluminum and silicon
Authors: Hörnström S. E. , Lin T. , Thomas O. , Fryer P. M. , Harper J. M. E.
Published: July 2002

Tungsten–rhenium alloys have been investigated as possible diffusion barriers between aluminum and silicon for high-temperature metallization of microelectronic devices. Al/W–Re/Si structures with the alloy compositions W–30 wt. % Re, W–61 wt. % Re, and W–72 wt. % Re were prepared by electron beam evaporation of W–Re and Al on Si wafers. Thin W–Re films, without Al, were also deposited on Si3N4 windows. The samples were annealed at different temperatures and characterized using Rutherford backscattering spectroscopy, x-ray diffraction, and transmission, and scanning electron microscopy (TEM and SEM). The W–30% Re sample was crystalline with a β-W structure after deposition, while the W–61% Re and the W–72% Re samples were amorphous and crystallized into the α-Mn structure (χ phase) at a temperature of 700 and 600 °C, respectively. All Al/W–Re/Si structures were stable against Al penetration up to 500 °C. At an annealing temperature of 550 °C signs of a reaction leading to the formation of (W,Re)–Al12 could be observed. At 600 °C the barrier layers failed completely due to a reaction between W, Re, Al, and the Si substrate. SEM analyses of the bare Si substrate after the metal layer had been chemically removed showed that etch pits were formed after annealing the diffusion barrier structure at 550 °C for 30 min. The failure of the diffusion barriers is most likely due to local penetration of Al through defects in the films.
Effects of initial surface composition on the interface chemistry of gold on cubic SiC(100)
Authors: Parrill T. M. , Chung Y. W.
Published: July 2002

Clean cubic β-SiC(100) surfaces of four surface compositions ranging from Si rich to Si deficient were prepared by annealing samples both in a Si flux and in ultrahigh vacuum. Gold films of thicknesses ∼1–35 Å were deposited on each of the clean SiC surfaces and the interface studied using Auger and x-ray photoemission. On Si-terminated surfaces, Au–Si alloy forms, and some Si segregates to the surface of the Au film in proportion to its initial surface concentration. On Si-deficient surfaces, however, Au clusters appear, and no Au–Si interaction is evident. Results indicate that morphology and chemical interaction for this system depend on the composition of the top one to two SiC atomic layers, not bulk properties.
Angular resolved surface-plasmon loss from Si(111) surfaces
Authors: Seo J. M. , Black D. S. , Holloway P. H. , Rowe J. E.
Published: July 2002

The dispersion of plasmon-loss peaks measured with angle resolved electron energy-loss spectroscopy gives evidence of increased surface carrier concentration from B-doped (1015–1016 cm−3) Si(111) surfaces which were Ar sputter cleaned and annealed surfaces at temperatures between 800 and 1250 °C. The plasmon loss dispersion curve (ΔE vs Δk∥) agreed with the calculated plasmon energy and cutoff wave vector for an effective carrier concentration ∼10–50 times greater than the bulk doping density. Annealing at temperatures, T>950 °C reduced the surface plasmon peak presumably by annealing defects, and allowed detection of a surface-phonon loss peak at 22 to 30 meV probably associated with the adatoms of the 7×7 reconstructed surface.
Surface modification in the nanometer range by the scanning tunneling microscope
Authors: Staufer U. , Wiesendanger R. , Eng L. , Rosenthaler L. , Hidber H.-R. , Güntherodt H.-J. , Garcia N.
Published: July 2002

Ar+ ion milling reduces the roughness of the surface of a glassy Rh25Zr75 sample to a rms value of <1 Å as proven by studies with the scanning tunneling microscope (STM). Therefore, such metallic glasses provide ideal substrates for nanometer-scale structure fabrication. Here we present a new method to modify, in a reproducible way, the surface of metallic glasses by the STM. The resulting nanometer-scale structures are shown by the STM as well. The high local current density in the STM can raise the temperature in a very small volume near the surface of the metallic glass. An increase in temperature and electric field can lead either to enhanced diffusion of one component or to local crystallization of the glassy state or even to local melting of the surface. Under the influence of the high electric field between the sample and the tip the fluid forms a meniscus similar to a Taylor cone. The nanometer-scale structure is then created upon cooling. It is also possible to similarly produce line-shaped structures by moving the tip across the surface during the heating process. The physics of the formation of these hillocks and lines is discussed.
Application of the scanning tunneling microscope to insulating surfaces
Authors: Jaklevic R. C. , Elie L. , Shen Weidian , Chen J. T.
Published: July 2002

We will show the results of experiments in which very thin Au coatings are applied to insulating surfaces to allow the scanning tunneling microscope (STM) to produce topographic images of these surfaces. Results from a variety of materials including fused silica, salt crystals, oxidized metals, silicon, plastic, and a ceramic high-Tc superconductor will be shown. The Au films are made by a low-temperature deposition method which allows for formation of very thin continuous films. Gold coatings are applied in a vacuum system separate from the STM itself. The STM is operated in air or moderate vacuum, thus allowing for rapid and routine sample turnaround. Lateral resolution of ∼10 Å is obtained without the appearance of structure contributed by the Au coatings from 20- to over 100-Å thickness. Good-quality reproducible images of cleavage steps and other features on surfaces are obtained over areas limited by the piezoelectric drive elements.
An energy dispersion spectroscopy technique to measure titanium silicide lateral diffusion
Authors: La Via F. , Spinella C. , Rimini E. , La Mantia A. , Ferla G.
Published: July 2002

Energy dispersion spectroscopy has been used to measure the amount of titanium silicide formed on wedged SiO2 layers. The best experimental conditions for the measurements were determined using a Monte Carlo calculation for the x-ray yields and for the evaluation of the lateral resolution. On the basis of these calculations we have chosen during measurements a step displacement of the electron beam of ∼40 nm. With this technique lateral extension of titanium silicide ranging from 120 to 520 nm was measured. The method has been applied to the analysis of lateral diffusion in titanium silicide obtained by annealing either in a furnace or in a rapid thermal apparatus.
Substrate effect on the electrical properties of a-Si:H thin films and its modification by diffusion-blocking interlayers
Authors: Bregman J. , Gordon J. , Shapira Yoram , Fortunato E. , Martins R. , Guimaraes L.
Published: July 2002

Electrical dark conductivity (σd) and surface composition of undoped and doped a-Si:H thin films have been investigated, using coplanar I–V measurements as a function of temperature and Auger electron spectroscopy (AES). The films were prepared by rf glow discharge deposition on standard soda-lime glass and on alkali-free glass substrates. Comparing these two sets of substrates for undoped films, we find that σd of the films deposited on soda-lime glass substrates at room temperature is higher by more than two orders of magnitude, their activation energy is lower by about a factor of 3, and their photosensitivity (σph/σd) is lower by two orders of magnitude than that of the films deposited on alkali-free glass substrates. We suggest that Na ions, leached from the glass into the a-Si:H overlayer play a significant role in determining the film conductivity by creating electrically active donorlike states. This conclusion is supported by similar measurements on p- and n-type a-Si:H films on the same substrates and by AES results. Films of a-Si:H, grown on thin a-Si:C:H interlayers on soda-lime glass, showed very low Na concentrations and low dark conductivities as found by AES and electrical measurements, respectively. The role of the a-Si:C:H interlayers as diffusion barriers is discussed.
Calculation and measurement of the volume flow rate dependence of turbomolecular pumps on the type of gas
Authors: Reich G.
Published: July 2002

Turbomolecular pumps are finding more and more applications in helium leak detectors and in equipment for mass spectrometric gas analysis. The knowledge of the volume flow rate for different gases is of considerable significance not only in such applications. Based on known formulas a simple expression is derived for the volume flow rate as a function of the relative molecular mass Mr of the gas, the circumferential velocity u, and the maximum compression ratio of the (multi-)stage turbomolecular pump and the volume flow rate of the backing pump. It is shown that the volume flow rate increases in the range of the lighter gases by u2(Mr)1/2 and decreases for the heavier gases by 1/(Mr)1/2. These calculations have been confirmed for the gases H2, He, Ne, N2, Ar, Kr, and Xe. The measurement of the volume flow rates was performed with a test gas mixture and a mass spectrometer, according to the method of determining the pressure drop across a known conductance (DIN 28429).
Selection criteria for oil-free vacuum pumps
Authors: Duval Pierre
Published: July 2002

Some high-vacuum pumps without oil in the swept pumping volume are now available for the evacuation and process pumping of vacuum systems. These pumps are from different principles, i.e., multistage roots blowers, screw compressor, claw pump, dry piston, or molecular drag pump. The selection of the right pump for a given application must take into consideration the different parameters specific to the application or process: contamination by dust and/or oil backstreaming, corrosion resistance, safety, and cost of operation and maintenance. I will compare the basic performances and characteristics of the existing oil-free vacuum pumps, with emphasis on cleanliness and compatibility to a process as well as the estimation of their cost of operation. I will also examine how to pump large flows of light gases, i.e., hydrogen or helium, with the multistage oil-free vacuum pumps.
(1×1) rippled relaxation of (100) transition-metal carbide surfaces
Authors: Gruzalski G. R. , Zehner D. M. , Noonan J. R. , Davis H. L. , DiDio R. A. , Müller K.
Published: July 2002

An investigation of (100) surfaces of TaC and HfC was performed using low-energy electron diffraction (LEED) and angle-resolved ultraviolet photoemission spectroscopy with synchrotron radiation. Detailed LEED I–V analyses show that, on both surfaces, carbon atoms displace outward and metal atoms inward, giving rise to (1×1) rippled relaxations, with the magnitude of the ripple being larger on TaC(100). Photoemission spectra of metal 4f levels from these surfaces show no obvious surface core-level shifts. Using x-ray photoemission spectroscopy (XPS) and Auger electron spectroscopy (AES), changes in (bulk) core-level binding energies, valence-band structures, and local carbon environments were determined for TaCx (0.5<x<1.0) and HfCx (0.6<x<1.0). From these bulk studies, it was inferred that, as x decreases, added negative charge accumulates in the vicinity of metal atoms while some charge transfers away from remaining carbon atoms. Insight obtained from the XPS and AES investigations is used in this paper to discuss the charge distribution at the (100) surfaces of TaC and HfC. It is concluded that carbon atoms in the outermost layer move outward to allow their compressed s,p electrons to expand to a state of lower kinetic energy, and metal atoms move inward because of added charge accumulating in their vicinity. We propose that the larger ripple on TaC(100) results form a larger accumulation of charge near the metal atoms on that surface.
Reactive atom–surface scattering: The adsorption and reaction of atomic oxygen on the Si(100) surface
Authors: Engstrom J. R. , Nelson Mark M. , Engel Thomas
Published: July 2002

The adsorption and reaction of atomic oxygen on the Si(100) surface has been examined by employing supersonic beam techniques, x-ray photoelectron spectroscopy, and mass spectrometry. Atomic oxygen adsorbs with a unit probability of adsorption on the clean Si(100) surface, independent of incident translational energy, incident angle, and surface temperature. The probability of adsorption decreases monotonically with increasing coverage. At surface temperatures above ∼1000 K, the adsorption of atomic oxygen results in the gasification of the substrate, producing SiO(g). In comparison to molecular oxygen for this reaction, where two surface intermediates were implicated, only one surface intermediate is formed from the reaction of atomic oxygen with the Si surface. We suggest this is due to the ability of atomic oxygen to attach to the dangling bond states present on the Si(100) surface.
Transmission characteristics of a cylindrical energy analyzer
Authors: Craig J. H. , Durrer W. G.
Published: July 2002

A ‘‘Bessel box’’ energy filter was constructed, adding design improvement features of increased length, a central grid, and a conical front end cap. The priorities of compactness, simplicity, and low cost were maintained relative to more complex models. Performance characteristics were determined in an ultrahigh vacuum system equipped for electron stimulated desorption work and thermal desorption experiments.
The electrical characterization of whole wafer silicon on sapphire for process control
Authors: McLeod Donald , Shields D. A. , Maurits J. E. A. , Forbes D. H.
Published: July 2002

Electrical measurements were made on silicon on sapphire (SOS) before processing to the device stage. The objective was to determine process conditions for producing high-resistivity, high-mobility SOS. The phenomenological approach was used to study variations in electrical properties across a wafer. Conductivity-type measurements were made where large variations in the resistivity occurred. A p-type autodoping pattern was observed and traced to aluminum diffusion into the episilicon layer from sapphire substrate defect areas which were identified by scanning electron microscopy. Process parameters were adjusted to eliminate patterned autodoping. The average of point-to-point resistivity parameter values was then equal to the value determined for the whole wafer using peripheral contacts. The resistivity parameter was influenced both by carrier concentration and microstructure. The Hall mobility was highest for material where patterned autodoping was absent. If the silicon deposition temperature was lowered too much, the Hall mobility decreased significantly. Resistivity parameter mapping and mobility measurements were necessary to fix epigrowth parameters.
Atomic force sensors constructed from carbon and quartz fibers
Authors: Schmidt H. , Heil J. , Wesner J. , Grill W.
Published: July 2002

A simple design of force sensors constructed from carbon and fused quartz fibers is presented. The handmade force sensors exhibit characteristics similar to those of cantilevers produced by micromechanic techniques. Atomically resolved force maps of the c surface of graphite and boron nitride and a topographic image of mica have been obtained using an atomic force microscope equipped with a fiber sensor.
Room temperature pumping characteristics for gas mixtures of a Zr–Al nonevaporable getter
Authors: Benvenuti C. , Francia F.
Published: July 2002

The main pumping for the vacuum system of the large electron positron (LEP) collider, which has been commissioned at CERN in the Geneva area, is achieved by a linear getter pump. In distinction with the usual operating conditions of nonevaporable getter (NEG) pumps, which consist in maintaining the getter hot to continuously diffuse into the getter bulk the gases adsorbed on its surface, the LEP pump will be kept at ambient temperature and heated only intermittently. This difference has required a study on the behavior of a NEG pump under these unusual operating conditions, which result in a progressive reduction of pumping speed with increasing quantity of the gas adsorbed. It was found that the pumping speed reduction may be described in terms of a progressive saturation of the NEG surface inside the cracks and porosities which are present in the getter film thickness. The formulation developed permits one to reconstruct the pumping curves of a NEG, provided that a few basic geometrical and physical quantities are known. This description, which had been applied to individual gases in a previous paper, is extended here to the case in which mixtures of different gases are pumped.
Growth of multidimensional superlattices using step array templates: Evolution of the terrace size distribution
Authors: Gossmann H.-J. , Sinden F. W. , Feldman L. C.
Published: July 2002

The process of step-mediated growth in molecular beam epitaxy, where deposited atoms move along surface terraces until they eventually bind at a surface step, is currently being explored as a mechanism for forming two-dimensional periodic structures. To succeed, such schemes require a periodic step distribution, i.e., uniform terrace lengths. A model, based on step-mediated growth, is presented, which yields an analytical derivation of the approach to uniform terrace lengths on a stepped surface, given a terrace length distribution of finite width at the outset. The results show that the approach to uniform terrace lengths with increasing deposition is quite slow. The width of the terrace length distribution varies approximately as the inverse fourth root of the deposited coverage. This will only occur if the atoms attach themselves predominately at the up-step of each terrace. Otherwise, the width of the terrace length distribution will grow without bounds.
Modification of tantalum surfaces by scanning tunneling microscopy in an electrochemical cell
Authors: Thundat T. , Nagahara L. A. , Oden P. I. , Lindsay S. M. , George M. A. , Glaunsinger W. S.
Published: July 2002

The morphological evolution of tantalum surfaces induced by scanning in an electrochemical cell have been investigated by scanning tunneling microscopy (STM). Repeated scanning over the same region was found to create a surface with a reduced peak-to-peak corrugation. This phenomenon was observed for negative tip biases only, and was also found to occur at a slower rate while scanning in air. X-ray photoelectron spectroscopy shows layers of Ta2O5 deposited anodically on samples imaged with STM.
Chemical reaction dynamics of F atom reaction with the dimer reconstructed Si{100}(2×1) surface
Authors: Schoolcraft T. A. , Garrison B. J.
Published: July 2002

We have calculated the initial sticking probability S0 for near thermal atomic fluorine on the clean dimer reconstructed Si{100}(2×1) surface to be near unity using molecular dynamics simulations. The dependence of the sticking probability on coverage decreases to approximately zero for the fully fluorinated surface. The sticking probability of an F atom on a fully fluorinated silicon surface increases when the kinetic energy of the fluorine atom is above 0.50 eV.
Evaluation of the electron attenuation length for the determination of the lubricant overlayer thickness on magnetic disks with x-ray photoelectron spectroscopy
Authors: Nishimori Ken-ichi , Tanaka Kohichi , Sato Kazunori
Published: July 2002

The electron attenuation length (AL) of the C 1s photoelectrons in a fluorocarbon lubricant overlayer λfc on sputter-carbon coated magnetic disks has been experimentally determined. The C 1s intensity ratio of the carbon substrate signal with overlayer thickness to the –CF2– signal gives a value of λfc, 2.88±0.68 nm. The result is discussed in a light of the problem associated with an accurate determination of the lubricant overlayer thickness with x-ray photoelectron spectroscopy.
Radio frequency sheaths—adjustable waveform model
Authors: Horwitz Chris M. , Puzzer Tom
Published: July 2002

An approximate analytic description of radio frequency (rf) discharge sheaths is given here. This description employs simple power-law equations, obtaining results for a family of sheath voltage waveforms. Both low-pressure (‘‘free space’’) and high-pressure (‘‘collisional’’) regimes are analyzed, yielding power-law dependences of voltages, fields, and charges on distance into the sheath. The resulting sheath model yields asymmetric discharge sheath heights 1.1–1.3 times the corresponding dc sheath model. This family of sheath voltage waveforms is shown to be consistent with experimental sheath voltage waveforms, through experimental measurements of particle arrival times at rf sputtering targets with various electrode area ratios and chamber pressures. The imposition of a nonsinusoidal waveform on the sheath has interesting implications for voltages and ion flows in the remaining regions of the discharge. Ions may be accelerated from the ‘‘presheath’’ region both toward and away from the sheath around a negatively biased target. This is in contrast with dc sheaths, where ions are accelerated only toward the more negative electrode. Thus, ion bombardment of both the target and the counterelectrode is indicated by this model of asymmetric rf discharges. Final sections of this paper consider the velocity of the glow electron wavefront in the sheath, and the close correspondence between these power-law solutions and sine waveforms.
Stimulated emission at 2.8 μm from Hg-based quantum well structures grown by photoassisted molecular beam epitaxy
Authors: Giles N. C. , Yang Z. , Han Jeong W. , Cook J. W. , Schetzina J. F.
Published: July 2002

We report the first observation of stimulated emission from Hg-based quantum well structures in which the active region is a HgCdTe superlattice. The laser structures were grown on (100) CdZnTe substrates by photoassisted molecular beam epitaxy. Cleaved laser cavities were optically pumped using the 1.06 μm output from a continuous wave Nd:YAG laser. Stimulated emission cavity modes were seen at cw laser power densities as low as 3.4 kW/cm2 and at temperatures ≥60 K.
Metal overlayers on organic functional groups of self-organized molecular assemblies. 1. X-ray photoelectron spectroscopy of interactions of Cu/COOH on 11-mercaptoundecanoic acid
Authors: Czanderna A. W. , King David E. , Spaulding D.
Published: July 2002

X-ray photoelectron spectroscopy spectra were recorded after sequential deposition of submonolayer amounts of copper on an organized molecular assembly (OMA) of HS(CH2)10COOH 11-mercaptoundecanoic acid (MUA) on 80-nm-thick gold films on silicon substrates. The self-assembly process results in stable monolayers with the thiol group attached to the gold film and the COOH group as the outer monolayer. Copper deposited in vacuum interacts with the surface of the MUA OMA to produce a unidentate copper complex with the single bonded oxygen atom of the acid monolayer. The data do not support forming a bidentate copper complex or chemical interaction of the double bonded oxygen atom of the acid group with copper. The maximum charge residing on the copper atom of the complex is +1 with one charged copper atom on the surface per MUA molecule of the OMA.
Outgassing characteristics and microstructure of a ‘‘vacuum fired’’ (1050 °C) stainless steel surface
Authors: Yoshimura Nagamitsu , Hirano Haruo , Sato Tomoshige , Ando Ichiro , Adachi Sachiko
Published: July 2002

‘‘Vacuum fired’’ (1050 °C) type 304 stainless steel (SS304) surfaces were investigated with scanning electron microscopy and Auger electron spectroscopy. The grain boundaries of the surface were vague and shallow, which occurred due to elemental diffusion at high temperature in vacuum. The newly formed oxide layer of the vacuum fired surface was much thinner, and could be said to be finer in microstructure than the native layer of an ‘‘as-received’’ surface. A vacuum fired (1050 °C) SS304 chamber was evacuated by a sputter ion pump whose vessel was pretreated by vacuum firing (1050 °C). An extremely high vacuum of 1.5×10−9 Pa (N2 equivalent pressure) was indicated by an extractor ionization gauge after a mild bake (170 °C) following an air exposure. The outgassing rate of the chamber wall at the elapsed time of one day after a mild bake (170 °C) was roughly estimated as low as 2×10−11 Pa ℓ/s cm−2 (N2 equivalent value) by an orifice method. Vacuum firing has the effects of degassing the gas molecules soluted in the interior of stainless steel wall, and of forming a new, thin, and fine (in microstructure) oxide layer on the surface.
Beam steering laser assisted deposition system for high-T
c superconducting thin film devices
Authors: Pham Thien T. , Chen K. Wendell
Published: July 2002

We present the design and construction of a beam steering laser-assisted deposition system (LAD) for high quality epitaxial YBaCuO superconducting thin film production suitable for commercial application. Deposition of single layer or multilayer YBaCuO superconducting thin film on large and complex surfaced substrate is now feasible. Expitaxial and polycrystaline films with onsets at 90 K having 6 K transition widths have been produced. Dome-shaped magnetic shield enclosures, microwave cavity resonant in the TM010 mode, and short dipole antennas have been fabricated. The advantages of the laser ablation deposition method [J. T. Cheung and D. T. Chueng, J. Vac. Sci. Technol. 21, 182 (1982)] are its simplicity and cleanliness. Laser ablation deposition is a viable method to produce high quality thin film of the Tl-based compound [S. H. Liou and K. D. Aylesworth, Appl. Phys. Lett. 54, 760 (1989)] by using a small target in a sealed environment which is important in handling toxic material of a Tl-based compounds.
Metal/metal-oxide interfaces: A surface science approach to the study of adhesion
Authors: Peden C. H. F. , Kidd K. B. , Shinn N. D.
Published: July 2002

Metal-oxide/metal interfaces play an important role, for example, in the joining of an oxide ceramic to a metal for sealing applications. In order to probe the chemical and physical properties of such an interface, we have performed Auger electron spectroscopic (AES) and temperature programed desorption (TPD) experiments on a model system composed of very thin films of Cr, Fe, Ni, or Cu evaporated onto a very thin thermally grown oxide on a W single crystal. Monolayer films of Fe and Cr were found (by AES) to completely wet the oxide surface upon deposition, and were stable up to temperatures at which the films desorbed (≊1300 K). In contrast, monolayer Ni and Cu films formed three-dimensional islands exposing the oxidized W surface either upon annealing (Ni) or even upon room-temperature deposition (Cu). The relative interfacial interaction between the overlayer metal and the oxide, as assessed by TPD, increases in the series Cu<Ni<Fe<Cr. This trend follows the heats of formation of the various oxides of these metals.
Excimer laser cleaning and processing of Si(100) substrates in ultrahigh vacuum and reactive gases
Authors: Watanabe J. K. , Gibson U. J.
Published: July 2002

We have examined the effect of KrF excimer laser irradiation (248 nm, 20–25 ns) on Si(100) substrates under ultrahigh vacuum conditions and in controlled partial pressures of reactive gases. Surface compositions were determined before and after irradiation using in situ angle resolved x-ray photoelectron spectroscopy. We varied peak power, pulse energy, and halocarbon (CF4) or carbon monoxide (CO) pressures to investigate their effects on oxygen and carbon removal. Energy densities near 500 mJ/cm2 were sufficient to eliminate the carbon contamination; higher energies, which depended on the silicon preparation conditions, were required for oxygen removal. Samples were irradiated by pulses at 480–740 mJ/cm2 in molecular oxygen (O2) environment after cleaning to determine pulse energy and gas pressure requirements for oxide regrowth.
Transmission electron microscopy and vibrational spectroscopy studies of undoped and doped Si,H and Si,C:H films
Authors: Chen Y. L. , Wang C. , Lucovsky G. , Maher D. M. , Nemanich R. J.
Published: July 2002

Films of undoped and doped Si:H and Si,C:H alloys were deposited by remote plasma-enhanced chemical vapor deposition onto thermally grown silicon oxide layers at a substrate temperature of 250 °C. The microstructure of the films, including the degree of crystallinity, and the distribution of carbon within the Si,C alloys films were characterized by transmission electron microscopy, Raman scattering, and infrared absorption spectroscopy. The degree of crystallinity depends on both the doping level and on the presence of carbon. For two-phase μc-Si or μc-Si,C alloy films, the results indicate that (i) the crystallites are Si, and (ii) the amorphous encapsulating materials are a-Si:H for μc-Si,H, and a-Si,C:H for the μc-Si,C:H alloys. A relationship between microstructure, doping levels, and the measured dark conductivity is discussed.
Sub-centimeter micromachined electron microscope
Authors: Feinerman A. D. , Crewe D. A. , Perng D. C. , Shoaf S. E. , Crewe A. V.
Published: July 2002

A new approach for fabricating macroscopic (∼10×10×10 mm3) structures with micrometer accuracy has been developed. This approach combines the precision of semiconductor processing and fiber optic technologies. A (100) silicon wafer is anisotropically etched to create four orthogonal v-grooves and an aperture on each 10×12 mm die. Precision 308 μm optical fibers are sandwiched between the die to align the v-grooves. The fiber is then anodically bonded to the die above and below it. This procedure is repeated to create thick structures and a stack of 5 or 6 die will be used to create a miniature scanning electron microscope (MSEM). Two die in the structure will have a segmented electrode to deflect the beam and correct for astigmatism. The entire structure is ultrahigh vacuum compatible. The performance of a SEM improves as its length is reduced and a sub-cm 2 keV MSEM with a field emission source should have approximately 1 nm resolution. A low-voltage high-resolution MSEM would be useful for the examination of biological specimens and semiconductors with a minimum of damage. The first MSEM will be tested with existing 6 μm thermionic sources. In the future a micromachined field emission source will be used. The stacking technology presented in this paper can produce an array of MSEMs 1–30 mm in length with a 1 mm or larger period. A key question being addressed by this research is the optimum size for a low-voltage MSEM which will be determined by the required spatial resolution, field of view, and working distance.
Faceting, reconstruction, and defect microstructure at ceramic surfaces revealed by atomic force microscopy
Authors: Antonik M. D. , Lad R. J.
Published: July 2002

Atomic force microscopy has been used to directly observe the surface microstructure and morphology of several single crystal ceramics which were either cleaved in air or processed in ultrahigh vacuum. Atomic and/or multiatomic height steps and atomically flat terraces were observed on cleaved Cr2O3 (101̄2) and NiO (100) surfaces and on polished and annealed Al2O3 (101̄2). Examples of faceting, reconstruction, and Ar+ ion induced damage were revealed on the (110), (100), and (001) surfaces of TiO2. Tip degradation and artifacts were evident during imaging of VC0.75 (100) surfaces. The atomic force microscopy results indicate that a wealth of nanometer scale features exists even on carefully prepared single crystal ceramic surfaces.
Monitoring of the aluminum nitride sputtering deposition by soft x-ray emission spectroscopy
Authors: Legrand P. B. , Dauchot J. P. , Hecq M.
Published: July 2002

The deposition of aluminum nitride, by dc diode reactive sputtering has been studied by in situ soft x-ray emission spectroscopy. The deposition chamber is coupled with a vacuum x-ray computerized spectrometer. The x-ray emission from the film is excited by high energy secondary electrons from the electric discharge. The Al cathode is sputtered at constant current and voltage mode. Films are deposited at different argon/nitrogen gas mixtures, in the bias mode. For each cycle, x-ray intensities, deposition rate at the quartz microbalance and total pressure are recorded. It has been found that the Al Kα and N Kα x-ray intensity ratio decreases with increasing nitrogen flow rate. At partial nitrogen flow rates of 8.5%, a sharp decrease appears, followed by a plateau, where the film has the bulk stoichiometry. Films have been characterized for ultraviolet-visible absorption. Optical band gap and refractive index have been calculated. The fine structure of the Al Kβ line has been studied. A shift in the maximum intensity position is coupled with a satellite line toward the low energy side, both appear before the transition. These effects are characteristic of the chemical bonding between Al and N. X-ray diffractograms show a preferential orientation depending on the gas mixture composition, but the peaks are small and broad.
Barrier-limited transport in μc-Si and μc-Si,C thin films prepared by remote plasma-enhanced chemical-vapor deposition
Authors: Lucovsky G. , Wang C. , Chen Y. L.
Published: July 2002

Systematic variations of room-temperature dark conductivities and dark conductivity activation energies for n- and p-type μc-Si and μc-Si,C thin films with optical band gaps between 1.9 and 2.2 eV and deposited by remote plasma-enhanced chemical-vapor deposition are interpreted in terms of a band alignment model. This leads to an observation that the maximum attainable dark conductivities of these microcrystalline thin films are limited by either thermally assisted transport through, or over interfacial potential barriers between Si crystallites, c-Si, and the encapsulating amorphous materials: a-Si:H and a-Si,C:H, respectively. As the doping is increased in n- or p-type μc-Si, there is a transition from thermal emission limited to thermally assisted tunneling transport. For all levels of doping so-far achieved in the μc-Si,C alloys, the transport is determined by thermionic emission over interfacial barriers at the c-Si/a-Si, C:H interface.
Pulsed laser irradiation at 532 nm of In and Ga adsorbed on Si(100): Desportion, incorporation, and damage
Authors: Strupp Paul G. , Alstrin April L. , Korte Brenda J. , Leone Stephen R.
Published: July 2002

Laser-induced desorption (LID) of In and Ga from Si(100) under ultrahigh vacuum conditions is investigated. The frequency-doubled 532 nm, 2–6 ns output of a Nd:YAG laser is focused to 0.14±0.03 J/cm2 on the Si surface to induce desorption. Desorbed In or Ga atoms are detected by laser-induced fluorescence initiated by a second pulsed laser propagating in front of the surface. LID occurs by thermal desorption with approximate desorption energies and preexponential factors in agreement with the literature values obtained previously by isothermal desorption measurements. Experiments at higher coverages suggest that desorption occurs predominantly from the two-dimensional (2D) adlayer with little desorption occurring from the adsorbate islands directly into the vacuum. The 2D layer is resupplied by either diffusion out of adsorbate islands or by diffusion of incorporated adsorbate out of the bulk. Adsorbate-enhanced laser-induced surface damage is also observed; only 0.2 monolayer of In reduces the number of laser pulses required to observe damage by greater than a factor of 30.
Stress stabilization of β-tantalum and its crystal structure
Authors: Kondo Kazuaki , Nakabayashi Masaaki , Kawakami Ken’ichi , Chijimatsu Tatsuo , Nakaishi Masafumi , Yamada Masao , Yamabe Masaki , Sugishima Kenji
Published: July 2002

We studied the relationship between Ta crystal structures and the stress stability of Ta under heating. The stress in Ta which was sputter deposited on SiC was unstable and changed more than 2×109 dyn/cm2 to the compressive side during heating at 200 °C for 30 min in air. In contrast, the stress in Ta which was sputter deposited on SiC whose surface was modified by Ar sputtering was very stable, and the stress change was less than 1.5×108 dyn/cm2 even after 6 h of heating at 200 °C. The x-ray diffraction patterns of the Ta revealed that stable Ta was strongly (002) oriented β-Ta, and that unstable Ta was randomly oriented β-Ta with some α-Ta. We found that amorphizing the SiC surface or inserting a thin amorphous interlayer enhanced growth of strongly (002) oriented β-Ta.
Selective deposition of copper by chemical vapor deposition using Cu(HFA)2
Authors: Kim Do-Heyoung , Wentorf Robert H. , Gill William N.
Published: July 2002

The selectivity of chemical vapor deposition copper for metal or metallic substrates in preference to silicon dioxide was investigated systematically using copper (II) hexafluoroacetylacetonate, Cu(HFA)2, with pure hydrogen or hydrogen/argon mixtures (1:3) in a cold wall type vertical flow reactor as a function of measured total pressure (2–10 Torr), deposition temperature (310–390 °C), and inlet precursor mole fraction (0.008–0.09). The deposition temperature was found to be the most important parameter. Temperatures of 310–360 °C resulted in selective deposition under the above conditions. Auger electron spectroscopy depth profiles indicate the composition of the films. Some local nucleation of copper due to the surface imperfections on silicon oxide was observed. These studies help to clarify some discordant results reported in the literature by determining to a good approximation the conditions of temperature, pressure and gas phase composition under which selective deposition of copper was observed.
Low-energy, ion-enhanced etching of III–V’s for nanodevice applications
Authors: Pearton S. J.
Published: July 2002

High-density (≤5×1011 cm−3) magnetically enhanced discharges operated at low pressure (1 mTorr) with low additional rf-induced dc bias (≤−100 V) on the sample enable self-aligned dry etch fabrication of a wide variety of III–V devices and circuits for lightwave digital and microwave applications. In many cases the ohmic metal contacts are used as the etch masks in order to minimize parasitic resistances and capacitances resulting from the lateral separation of these contacts. Applications range from formation of shallow mesas (≤400 Å) on high electron mobility transistors to etching of through-wafer vias (∼100 μm). The chemistries employed for these fabrication steps are reviewed, together with examples of processing sequences for heterojunction bipolar transistors and novel microdisk lasers that may form the basis of future electronic and microphotonic circuits.
History and current status of vacuum metallurgy
Authors: Bunshah Rointan F.
Published: July 2002

This paper deals with the processing of metals and alloys from the liquid and vapor phases. The history and current status is presented. These developments are an accurate reflection of the progress of metallurgy and material processing without which we would be unable to meet the needs of modern technology in its many diverse aspects.
Sensitivity of second harmonic generation to space charge effects at Si(111)/electrolyte and Si(111)/SiO2/electrolyte interfaces
Authors: Fischer P. R. , Daschbach J. L. , Gragson D. E. , Richmond G. L.
Published: July 2002

The potential dependence in the surface second harmonic response from hydrogen terminated n-Si(111) and oxidized n-Si(111) surfaces has been examined in aqueous NH4F and H2SO4 solutions. The relative phase of the nonlinear response as measured by rotational anisotropy experiments is found to be highly sensitive to the presence of the oxide and the field applied across the Si(111)/oxide/electrolyte interface. These observations are attributed to field effects within the space–charge region of the semiconductor which vary with the presence and thickness of the insulating oxide layer on the Si(111) surface.
Estimation of indium-to-germanium and gallium-to-germanium sputtering yield ratios using cosputtering deposition
Authors: Comedi D. , Fajardo F. , Chambouleyron I. , Tabacniks M.
Published: July 2002

The relative concentration of In and Ga atoms incorporated to hydrogenated amorphous germanium (a-Ge:H) films by the cosputtering of solid In and Ga sources together with a crystalline Ge target in a H-containing Ar plasma has been measured by means of Rutherford backscattering spectrometry and particle-induced x-ray emission. Both the experimental In-to-Ge and Ga-to-Ge relative concentrations were found to linearly scale with the respective In/Ge and Ga/Ge sputtered area ratios. From the slopes of these linear dependencies, In-to-Ge and Ga-to-Ge sputtering yield ratios were estimated. The results were found to be consistent with published theoretical sputtering yield values, but to disagree with experimental sputtering yield results previously reported for the case of the In-to-Ge yield ratio. For the case of Ga, a sputtering yield value of 1.6 atoms/ion was deduced from the present results, which gives first experimental account of Ga sputtering by Ar ions.
Modeling the pump-down of a reversibly adsorbed phase. I. Monolayer and submonolayer initial coverage
Authors: Redhead P. A.
Published: July 2002

An analysis is presented of the pressure versus time characteristics during the pump-down of a vacuum system with a reversibly adsorbed phase, e.g., water at room temperature, using the conservation of mass equation, where the quantity adsorbed is derived from a suitable adsorption isotherm. This quasistatic approach is suitable for cases of reversible adsorption and takes readsorption into account. Equations for the pressure versus time characteristics are derived for several isotherms, Henry’s law, Langmuir isotherm, Freundlich isotherm, Temkin isotherm, and an extended form of the Temkin isotherm. The p(t) characteristics resulting from these different isotherms are compared and their usefulness considered. For monolayer adsorption the extended Temkin isotherm gives p=kt−γ (where γ is about 1) in good agreement with recent experimental data on the pump-down of water sorbed on stainless steel when reasonable values of the parameters are assumed. For initial coverage less than a monolayer the extended Temkin isotherm shows that γ tends to decrease as the initial coverage is reduced. It is shown that outgassing rates, measured by pumping a system through an orifice of known conductance, may be considerably lower than the true outgassing rate if the sticking probability is not very small, an estimate of this effect is made in terms of the sticking probability.
A sensitive method for measuring adsorbed carbon on palladium surfaces: Titration by NO
Authors: Ramsier R. D. , Lee K.-W. , Yates J. T.
Published: July 2002

Small quantities of adsorbed C have been deposited on Pd(112) by the dissociative adsorption of C2H4. It is demonstrated that although Auger electron spectroscopy is insensitive to the presence of this C(ads), temperature programmed desorption (TPD) can be used to determine the cleanliness of the surface. TPD spectra following the 300 K adsorption of 15NO on C-covered Pd(112) indicate that reactive chemistry occurs during heating, as evidenced by the production of high temperature 15N2 (≊590 K) and CO (≊520 K) thermal desorption features. The results of this study are used to formulate an improved method for the removal of C from Pd surfaces in ultrahigh vacuum.
Atomic layer molecular beam epitaxy growth of GaAs1−xPx layers: Study of P2 incorporation by the reflectance difference technique
Authors: González Y. , González L. , Briones F.
Published: July 2002

A study of the P2 incorporation process onto the GaAs1−xPx(001) surface by the reflectance difference technique is presented. The growth technique is atomic layer molecular beam epitaxy (ALMBE), using successive pulses of P2 and As4 on a Ga rich intermediate surface. Experimental results on the dependence of the efficiency of phosphorus incorporation on P2 dosage and Ga surface coverage show that the saturation of P2 incorporation is limited by the Ga surface coverage and that the efficiency of phosphorus incorporation is larger for a Ga surface coverage above 0.12Ns, where Ns=6.25×1014 sites cm−2 for an ideal (001) surface. These results allow one to establish the appropriate experimental conditions to grow GaAs1−xPx layers by ALMBE with accurate control of the phosphorus content.
Characterization of molecular beam epitaxy grown CdF2 layers by x-ray diffraction and CaF2:Sm photoluminescence probe
Authors: Sokolov N. S. , Faleev N. N. , Gastev S. V. , Yakovlev N. L. , Izumi A. , Tsutsui K.
Published: July 2002

For studies of crystal quality and strain relaxation in heterostructures with epitaxial CdF2 and CaF2 layers on Si(111), differential double crystal x-ray diffractometry and impurity photoluminescence techniques were used. The crystalline quality of molecular beam epitaxy grown CdF2 layers can be high and comparable with that of III–V compound pseudomorphic heterostructures. The best results were obtained for CdF2 layers grown on CaF2 buffer layers which were coherent with the Si substrate.