bluuebunny/crossref_metadata_2025 huggingface dataset:
Sol–gel behavior and image formation in poly(glycidyl methacrylate) and its copolymers with ethyl acrylate
Authors: Feit E. D. , Wurtz M. E. , Kammlott G. W.
Published: July 2002

For the formation of rather large features (≳5 μm on a side) in poly(glycidyl methacrylate) and its copolymers with ethyl acrylate, the lithographic response can be characterized by a gel-point energy and a contrast. The gel-point occurs at 6.8 eV absorbed energy per molecule of homopolymer or of its effective equivalent in the copolymer. The contrasts of these negative resists are dynamic properties, but depend in part on the polymer’s dispersitivity, at least to a value 2.3. As features are progressively reduced, the gel-point advances to higher dose and the contrast increases. These new parameters correlate poorly with the molecular parameters of the polymers; they depend instead on developer-induced swelling of the image, on competitive wetting of the support layer by the developer and the polymer and on gel-rupture by forced development.
Electron-beam broadening effects caused by discreteness of space charge
Authors: Groves T. , Hammond D. L. , Kuo H.
Published: July 2002

The importance of mutual repulsion of beam electrons is investigated for conditions relevant to electron beam lithography. A Monte Carlo calculation has been performed which evaluates the spot size for a given beam current and column geometry. The results show that the beam broadening due to mutual repulsion significantly limits the current density obtainable. Experimental results are presented, and the agreement with theory is excellent. For a column 21.5 cm long operating with a 3 mrad aperture semi-angle, the spot size was measured as .14 μm at 100 nA, and .30 μm at 1 μA of beam current. The theory predicts that an optimum spot size exists that is proportional to the 1/4 power of the lens spherical aberration coefficient, and to the 3/4 power of the beam current.
Practical way to calculate molecular conductances of pipes with constant annular cross section
Authors: Onusic H.
Published: July 2002

Knudsen’s formula is used to calculate molecular conductances of pipes with constant annular cross-section. (AIP)
Design for an inexpensive externally operable variable aperture to be used in UHV
Authors: Homan F.
Published: July 2002

A design for an inexpensive externally operable variable aperture to be used in ultrahigh vacuum system is described. (AIP)
Study of a new method to control precisely the evaporation rate of titanium sublimation pumps
Authors: Strubin P.
Published: July 2002

Conventional ways of activating a sublimation pump, consisting of feeding a constant current or voltage to a titanium filament, do not give a control of the sublimation rate over the lifetime of the filament. The new method proposed here relies on the fact that a hot filament emits electrons. The temperature of the filament, and hence the sublimation rate, can be stabilized by regulating the electron emission, e.g., as is done for ionization gauges. Well controlled temperature and sublimation rates lead to an increase of the lifetime of the filament. This report presents the results of calculated temperatures and sublimation rates with and without emission stabilization. Experimental values are compared with the calculated ones.
Etching and film formation in CF3Br plasmas: some qualitative observations and their general implications
Authors: Flamm D. L. , Cowan P. L. , Golovchenko J. A.
Published: July 2002

CF3Br plasma discharges exhibit a number of unusual phenomena. Under some conditions silicon and SiO2 can be concurrently etched, while in other circumstances only SiO2 is etched and silicon is coated with a uniform, cross-linked fluorocarbon film. Single crystal n-doped silicon is etched more rapidly than undoped silicon. The fluorocarbon polymer overlays a monolayer of bromine at the silicon interface, but negligible bromine is incorporated into the fluorocarbon. Some chemical mechanisms which account for these observations are presented.
Photodesorption studies of CO and CO2 from the solar absorber black chrome
Authors: Zajac G. , Ignatiev A. , Smith G. B.
Published: July 2002

Photodesorption of CO and CO2 has been studied on black chrome solar absorbing coatings. It is shown that the desorption is a quantum effect with possible basis in the photo-hole model applicable to semiconductor surfaces, but with yields similar to that reported for photodesorption from metals.
A new theory of electronic surface states
Authors: Lee D. H. , Joannopoulos J. D.
Published: July 2002

Presently, state-of-the-art theoretical studies of elementary excitations at surfaces involve large, time consuming, and often cumbersome calculations, which are not very practical for surfaces with low symmetry. A new method has just been developed that directly yields more information and is computationally faster (by an order of magnitude) than any current method available. This method will allow calculations of complicated surface systems not previously possible. The ingredients of the theory are any semi-infinite system of atomic layers, any set of basis functions spanning each layer, and one single matrix which depends only on the interaction matrix elements among the atomic layers. What is remarkable is that the eigenanalysis of this matrix gives a complete quantum mechanical description of the elementary excitations of the system. Therefore with just a diagonalization of this matrix, the knowledge of its eigenvalues and eigenvectors give directly (1) the projected bandstructure (without having to diagonalize the bulk Hamiltonian for a large number of K⊥’s); (2) the bona fide surface bands with high accuracy (without the need of a bulk Green function first); (3) the surface Green function as a function of energy E+iδ, in the correct limit that δ→o! (without the need to solve slowly convergent iterative equations for complex energies); and (4) the precise decay length, orbital character, and symmetry of the wave function. The method, although versatile, is also conceptually simple. All the properties mentioned above are obtained from a simple set of rules. A tutorial discussion of the formalism will be presented along with several instructive examples illustrating its use.
Comparison of CF4/O2 and CF2Cl2/O2 plasmas used for the reactive ion etching of single crystal silicon
Authors: Paraszczak J. , Hatzakis M.
Published: July 2002

The etching of single crystal silicon in CF4/O2 and CF2Cl2/O2 gas mixtures in a 13.56 MHz plasma generated in a radial flow reactor at 0.4 W/cm2 and 30 mTorr total pressure has been studied. It was found that for an unloaded reactor, the addition of oxygen to a CF4 plasma increased the lateral and vertical etch rate of single crystal silicon. The maximum lateral etch rate coincided with the maximum vertical etch rate of silicon which occured at 30% of added oxygen (by flow). Simultaneously, the sputtering of gold, used to define the silicon pattern, increased with the proportion of added oxygen. The effect of reactor loading with silicon was also studied, and found to affect the etching anisotropy in a CF4/O2 plasma. Under similar conditions of flow, pressure and power, the CF2Cl2/O2 plasma was found to produce a reduced self bias voltage at the cathode. The etch rate of the gold mask was found to vary with added oxygen in this plasma, as did the silicon etch rate. In general, the anisotropy of etching silicon in the CF2Cl2/O2 plasma was found to be much higher than that in the CF4/O2 plasma at the same oxygen concentration. Possible mechanisms accounting for this difference in anisotropy will be explored.
A simple method to minimize charging and thermal damage during AES analysis of paper and paper coatings
Authors: Clark A. H. , Michalka T. L. , Maley J.
Published: July 2002

A simple technique which allows Auger electron spectroscopy analysis of paper and paper coatings to be performed is described which minimizes charging and thermal damage.
Annealing of implantation damage in integrated-circuit devices using an incoherent light source
Authors: Powell R. A. , Fulks R. T. , Kamins T. I.
Published: July 2002

A stationary large-diameter incoherent light beam has been used to anneal ion-implantation damage (75As:185 keV, 1×1016 cm−2) in an array of n+-p junction diodes fabricated on 3 in. Si (100) wafers. The best diodes produced in this way had device characteristics comparable to those of furnace-annealed diodes (∠0.5;1 nA/cm2 leakage current at 5 V reverse bias). The variation of reverse leakage current across a 3 in. wafer (∠0.5–20 nA/cm2) was much greater than for furnace-annealed diodes and may be caused by nonuniformities in the incoherent light beam. Tests on MOS capacitors showed that when incoherent light annealing was performed with the sample in moderate vacuum (≲10−2 Torr), damage introduced at the Si–SiO2 interface was negligible.
Effect of substrate temperature during growth on defect density in bismuth thin films
Authors: Damodara Das V. , Jayaprakash N.
Published: July 2002

Bismuth thin films of thickness 720±10 Å have been vacuum deposited on clean glass substrates held at various substrate temperatures ranging between 30 ° and 185 °C at a constant deposition rate 0f 3±0.3 Å s. Immediately after the formation of the films, the heat treatment was given to the films in situ. For each film, the resistance was recorded as a function of temperature during the heat treatment. It has been found that the resistance of the films decreases when the temperature increases during the first cycle of heating, and this has been attributed to the removal of defects. During the first cycle of cooling and the next cycle of heating and cooling, it is observed that the resistance first decreases when the temperature increases and then increases as the temperature increases. Using Vand’s theory, from the resistance–temperature data during heat treatment, both defect density and the initial lattice distortion energy spectra have been evaluated. It is observed from these spectra that as the substrate temperature increases the defect density decreases. It is also found that the defects have a mild preference for energies in the range between 1.30 and 1.35 eV. The results have been interpreted on the basis of the influence of the substrate temperature on the nucleation and growth phenomena in thin films.
Spectroscopic techniques for the study of solid–liquid interfaces
Authors: Yeager E. , Homa A. , Cahan B. D. , Scherson D.
Published: July 2002

Substantial progress has been made with in-situ use of optical reflectance spectroscopy and Raman spectroscopy in studies of the properties of solid–liquid interfaces and particularly metal– and semiconductor–electrolyte interfaces. Such techniques provide molecular level information concerning adsorbed species and various thin films not available from electrochemical and other measurements. Ex-situ techniques including electron and ion spectroscopies also have proved helpful in understanding the properties of solid–liquid interfaces but special techniques are necessary to minimize changes in the surface structure of the solid during the transfer between the liquid and ultrahigh vacuum environment. Some of the results obtained recently by various workers using such transfer techniques in the study of adsorption on single crystals at electrochemical interfaces will be reviewed. For adsorption on some solid surfaces there is parallel behavior in the liquid and vacuum environments and comparison affords insight as to the interactions of the adsorbed species at solid–liquid interfaces.
Plasma beam studies of Si and Al etching mechanisms
Authors: Smith Donald L. , Saviano Paul G.
Published: July 2002

The study of plasma etching mechanisms is made difficult by the presence of the plasma at the etching surface. However, etching in a plasma beam extracted from such a plasma allows: (1) mass analysis of beam species and of directly desorbing etching product, (2) simultaneous XPS analysis of the etching surface, and (3) ion acceleration or retardation without changing neutral species composition. The present beam was extracted from the unpowered electrode of a source operating under typical plasma etching conditions of 50 Pa and 27 MHz. Appearance potential mass spectroscopy was used to distinguish spectrometer fragmentation products from plasma radicals. Cl2 plasma beam reaction with undoped Si and with oxidized Al occurs only with ion bombardment, whereas with clean Al it occurs without ions, leading to isotropic (undercut) etching. Surface oxygen depletion and the onset of Al2Cl6 desorption were simultaneously monitored during the ’’initiation’’ phase of Al etching. Anisotropic Al etching in chlorocarbon plasmas is believed to be dependent on sidewall etching inhibition by chlorocarbon deposits. Mass analysis of beams from Cl2 plasmas containing 20% CCl4, CHCl3, or CH3Cl showed a similar product distribution of CmCln species in all three cases and, in the latter two cases, almost complete abstraction of H by Cl atoms to form HCl. The HCl neither enhances nor inhibits the reaction of Al with Cl2 plasma, nor does it react with clean Al in the absence of ion bombardment.
Electronic states of ideal Ge–Al interfaces
Authors: Batra Inder P. , Herman Frank
Published: July 2002

The structural complexity normally present at interfaces between metals and semiconductors poses a major obstacle to understanding the electronic structure of such interfaces, particularly the Fermi-level pinning. The difficulty arises from different lattice periodicities and bonding characteristics. One way of avoiding lattice incommensurability is to represent the metal by a jellium model, but this idealization leaves out structural effects and deemphasizes the role of local chemical bonds between metal and semiconductor atoms. We have studied the electronic structure of Ge–Al interfaces using a model which includes these effects and at the same times bypasses the lattice incommensurability problem. In particular, we constructed a lattice-matched interface model using an ideal (001)Ge substrate and a 45° rotated (001)Al film. We used a repeating slab geometry and considered geometrical configurations which included on-top, interstitial, and Al vacancy models. Using the self-consistent pseudopotential approach, the electronic energy levels, total energies, local densities of states, and work function were calculated in all cases. On the basis of these calculations, it is concluded that Al atoms remove the surface states of pure Ge and replace them by metal-like states. The tails of the metal wave functions in the interfacial region are indeed responsible for pinning the Fermi level as originally proposed by Heine. These results go beyond earlier calculations, enabling us to discuss the effect of the discrete lattice on the electronic structure at the interface.
On the determination of the energy band offsets in Hg1−xCdxTe heterojunctions
Authors: Casselman T. N. , Sher A. , Silberman J. , Spicer W. E. , Chen A.-B.
Published: July 2002

The energy band offsets play a critical role in the electrical properties of heterojunctions. We raise here serious questions regarding the precision and accuracy of the measured and calculated energy band offsets in HgCdTe heterostructures. We argue that the Anderson model prescription for the offsets is not useful and is not correct for abrupt heterojunctions when narrow band gap semiconductors are the constituents of the heterostructure. New values for the photoionization energy of Hg1−xCdxTe are given and the reliability of these values for determining the offset is discussed.
Simultaneous electron and x-ray analysis and its application in corrosion science
Authors: Castle J. E.
Published: July 2002

The new generation of scanning Auger electron microscopes permit the acquisition, simultaneously, of both the Auger electron and the x-ray signals excited from a given picture element. Such an instrument is effectively an electron probe microanalyzer working in two depth planes. Addition of an x-ray source provides an XPS facility which is in turn complemented by the simultaneously acquired XRF spectrum. The use of such a dual signal instrument can be well-illustrated by its application in electrochemistry and corrosion science. In pitting corrosion the high spatial resolution and low beam currents permit identification of anodic and cathodic areas by measurement of the anion/cation ratios, whereas the EDXA analysis locates the pitting with depleted zones in the substrate alloy: In marine corrosion, XRF permits the total assay of all mineral layers while XPS enables determination of the layer by layer composition. These and other similar studies are described with the aim of illustrating the way in which a surface analysis is enhanced by knowledge at all times of the underlying composition.
Characteristics of the material improvement process for silicon on sapphire by solid phase epitaxial regrowth
Authors: Richmond Eliezer Dovid , Knudson Alvin R. , Magee T. J. , Kawayoshi H. , Leung C.
Published: July 2002

A comprehensive matrix study of the material modification of chemical vapor deposited silicon on sapphire has been made. The modification is effected by applying twice the process of ion implantation with 28 Si+ ions followed by solid phase epitaxial regrowth (SPEG) induced by furnace annealing. The matrix consisted of varying the fluences for the first and second implantation from just above the critical fluence for forming an amorphous phase to 7×1015cm−2. The defect reduction was studied for each sample of the matrix. This study has shown that the density of defects at the silicon/sapphire interface and silicon surface depends on both implantations. The deposited damage energy peak position is estimated to occur at a depth ∼Rp for a substrate temperature of 330 K during implantation. The position of the damage peak affects the estimated damage at the silicon sapphire interface which controls the aluminum outdiffusion. The major reduction in the defect density has already occurred for fluences of the two implantations such that the amorphous regions terminate within about 200 Å of each other. Using the width of the random yield portion of the channeled spectra for MeV 3He+ ions overestimates the width of the amorphous layer.
Use of surface behavior diagrams to study hydration/corrosion of aluminum and steel surfaces
Authors: Davis G. D. , Ahearn J. S. , Venables J. D.
Published: July 2002

The use of surface behavior diagrams can frequently enhance the understanding of quantitative x-ray photoelectron and Auger electron spectroscopic measurements. These diagrams have been used to study (1) the adsorption of nitrilotris methylene phosphonic acid (NTMP), a hydration inhibitor, from aqueous solutions onto aluminum surfaces prepared for adhesive bonding and onto steel surfaces, and (2) the subsequent hydration/corrosion of the oxide or metal when exposed to moist environments. The results show that NTMP coverage increases with inhibitor solution concentration until saturation (monolayer) coverage is achieved. At the same time, NTMP displaces the water initially adsorbed on the oxidized surface. Subsequent exposure of the treated surfaces to moisture results in a slow dissolution of the NTMP, exposing amorphous Al2O3 or Fe2O3, which rapidly hydrates to AlOOH or FeOOH. Further hydration of the aluminum samples occurs as Al(OH)3 grows on the surface. These findings indicate that the dissolution of the adsorbed inhibitor is the rate-limiting step in the hydration/corrosion of oxide-covered metal surfaces which, in the case of aluminum, is the mechanism of failure of adhesive bonds in a humid environment.
Pumping of inert gases by electron-impact ionization sources and associated memory effects
Authors: Willis R. D. , Allman S. L. , Chen C. H. , Alton G. D. , Hurst G. S.
Published: July 2002

Radioactive 85Kr was used in a closed volume under moderate vacuum to investigate mechanisms by which inert gas atoms are pumped in an electron-impact ionization source of a simple diode design. Results are consistent with a mechanism involving the conversion of inert gas ions into energetic neutrals at the cathode followed by reflection and trapping of some of the neutral atoms in nearby surfaces. We discuss the possibility that the source pumping speed is enhanced due to burial of the trapped neutrals by material released in low-energy sputtering processes. The implications of these results for reducing memory effects in UHV mass spectrometer systems are discussed.
Time-resolved optical diagnostics of radio frequency plasmas
Authors: Gottscho Richard A. , Mandich Mary L.
Published: July 2002

Radio frequency plasmas are widely employed in the microelectronics industry because they provide a means for anisotropic fabrication of microscopic patterns under low temperature conditions. Many of the engineering aspects of plasma processing can be understood in terms of a modified chemical vapor transport theory. In this theory, the transport of reactants and products is driven by gradients in concentration and temperature. Ion transport is driven by gradients in electric potential or sheath fields. The acceleration of ions along the field lines leads to enhanced neutral chemistry along the surface normal (the so-called ion-neutral synergism). While the transport theories as they exist now are useful frameworks from which to design or optimize processes, they address neither the microscopic origin of synergistic effects nor the effects of time-varying gradients, which may be particularly important in rf plasmas. The subject of this paper is the use of time-resolved optical diagnostics to measure time-varying plasma phenomena. In particular, we discuss how laser-induced fluorescence (LIF) spectroscopy can be used to measure plasma formation and decay rates, concentration gradients, and electric fields. The effects of the time-varying applied potential are monitored by firing the laser synchronously with the field. These time-resolved measurements lead to a better understanding of previous, time-averaged, measurements of ion densities and sheath thicknesses as a function of frequency and provide definite limits for the applicability of steady state plasma transport theories.
Thin films for optical data storage
Authors: Lee Wen-yaung
Published: July 2002

The most important requirements of thin film media for optical data storage applications as well as the most commonly studied write-once and erasable optical recording media were reviewed. Detailed optical properties and degradation characteristics of thin Te films were presented to illustrate some of these requirements. Both of these two material properties were shown to be strongly influenced by the film thickness, film deposition rate, and substrate due to their influence on the microstructure of the film. The degradation mechanism and the effects of humidity and temperature on the degradation rate of thin Te films were also reported and discussed.
Preparation and characterization of fcc MoxNb1−x(NyC1−y)z thin films
Authors: Qadri S. B. , Fuller W. W. , Kihlstrom K. E. , Simon R. W. , Skelton E. F. , VanVechten D. , Wolf S. A.
Published: July 2002

In an attempt to make the predicted high superconducting transition temperature (Tc) form of MoN, we have prepared and characterized thin films of MoxNb1−x(NyC1−y)z, where 0.93>x>0, 1.0>y>0.8, and 1.0>z>5. These films were reactively rf sputtered in a UHV system using an evenly split Nb–Mo split target and mixtures of argon, nitrogen, and cyanogen gases. Films prepared at elevated substrate temperatures (600–800 °C) were all of single phase NaCl (or Bl) structure with varying amounts of nitrogen deficiencies. Both the lattice parameter and Tc were found to be smoothly varying functions of composition and to correlate with deviations from stoichiometry of the molybdenum-rich samples. Ion implantation of nitrogen into several of the more nitrogen deficient films resulted in both the lattice parameter and the Tc moving toward the values expected for stoichiometric films of the same Mo content. Results of postdeposition plasma nitriding and thermal ammonia annealing are also discussed.
A comparison of sputtered Ni/Cr interface depth resolution as obtained by the quartz crystal microbalance mass-loss method and Auger spectroscopy
Authors: Navinšek B. , Panjan P. , Žabkar A. , Fine J.
Published: July 2002

Sputter depth profiles of interfaces have been obtained by monitoring the change in sputtering rate as a function of sputtered depth. One very sensitive technique which we use to measure sputtering rates is the quartz crystal microbalance mass-loss method; it has been used to characterize Ni/Cr interfaces sputtered with a mass-analyzed 6 to 12 keV argon ion beam. Interface widths or interface depth resolution (90 to 10% points) obtained from the mass-loss depth profile data are compared to Auger sputter depth profile widths (1 to 4.5 keV argon ions) obtained on similar Ni/Cr thin-film materials. The somewhat narrower widths found with the mass-loss method indicate that this type of depth profiling can be used to characterize interface structures and to estimate the abruptness of an original, unsputtered interface.
Sb overlayers on (110) surfaces of III–V semiconductors: A new type of chemical bond
Authors: Mailhiot C. , Duke C. B. , Chadi D. J.
Published: July 2002

An elastic low-energy electron diffraction (ELEED) structure analysis of ordered p(1×1) monolayers of Sb on GaAs(110) reveals a puzzle: the Sb atoms form chains within which the bonding is purely p2 but which exhibit tetrahedral (sp3) coordination with the substrate. No bulk or molecular compounds with such properties are known. Using a tight-binding total-energy minimization model we predict the atomic geometries of saturated Sb monolayers on the (110) surfaces of GaP, GaAs, GaSb, InP, InAs, and InSb. The known structure for Sb on GaAs(110) is obtained and the prediction for InP(110) is verified by a preliminary ELEED intensity analysis. The same model is used to predict the surface state eigenvalue spectra and to analyze the nature of the Sb-substrate bonds. The bonds are hybrids between pi states in the Sb chain and sp3 substrate dangling bond orbitals. Their existence is a unique surface phenomenon caused by the structural constraint of epitaxial growth of the Sb monolayer.
Electronic structure of ternary semimagnetic semiconductors
Authors: Franciosi A. , Chang S. , Caprile C. , Reifenberger R. , Debska U.
Published: July 2002

The electronic structure of the ternary semiconductor alloys Hg1−xMnxSe, Cd1−xMnxSe, and Zn1−xMnxSe was examined through synchrotron radiation photoemission studies of single crystals cleaved in situ for 0≤x≤20%. Comparison with the parent binary compounds HgSe, CdSe, and ZnSe and resonant photoemission at the 3p–3d transition energy indicate that a Mn-derived density of states feature appears 3.5±0.1 eV below the valence band maximum and exhibits elemental 3d character with no evidence of important hybridization effects. The constant binding energy of the Mn 3d states in all of the compounds explored forces a reevaluation of existing models of bonding in ternary magnetic semiconductors.
Chromium nitride films synthesized by radio-frequency reactive ion plating
Authors: Kashiwagi K. , Kobayashi K. , Masuyama A. , Murayama Y.
Published: July 2002

Cr–N films were synthesized successfully on the stainless steel and the high-speed steel plates by rf reactive ion plating. Cr was evaporated from an electron beam evaporator into the N2 or NH3 gas plasma at the pressure of the order of 10–2 Pa. The optimum net rf power to synthesize was about 200 W. The amount of Cr and N atoms contained in the synthesized layer were measured by Auger electron spectroscopy. The crystal structure of the layer was analyzed by x-ray diffraction and reflection high energy electron diffraction. The resulting Cr–N films formed by rf reactive ion plating had two types of crystal structures composed of β-Cr2N and CrN, and the type of structure was able to be controlled easily by the ion plating conditions. The structure of the hardest film was found to be β-Cr2N (hexagonal structure) by micro-Vickers method. All the films synethsized under the various conditions by the method were observed to be composed of small size grains with microcracks.
A Fourier transform method of decomposing overlapping spectral peaks of similar line shape into individual components
Authors: Slavin A. J. , Underhill P. R. , Young M. B.
Published: July 2002

In various spectroscopies composite curves are often encountered which are linear superpositions of overlapping, chemically shifted spectral peaks. Separation of such a composite into its individual components allows the identification of the chemical complex by its chemical shift, and the determination of the amount of the element in the complex from the strength of the spectral component. This paper describes a Fourier transform method for the decomposition of such a composite spectrum which is applicable under two conditions which are often satisfied in practice: all the chemically shifted peaks of the element of interest must have very similar line shapes, and one of these peaks must be observable in isolation to provide a reference line shape. Even in the event of line broadening this technique should give good starting parameters for the number and positions of the components. Applications to XPS and Auger spectroscopy are discussed.
Surface band dispersion of Ge(111)c(2×8) and Ge(111):As 1×1
Authors: Bringans R. D. , Uhrberg R. I. G. , Bachrach R. Z. , Northrup John E.
Published: July 2002

The surface band dispersions of Ge(111)c(2×8) and Ge(111):As 1×1 were determined experimentally using angle resolved photoemission. The clean c(2×8) reconstructed Ge(111) surface exhibited a surface dispersion with a 2×2 periodicity, suggesting that the surface has a 2×2 local geometry. Addition of As to the surface converted the surface symmetry to 1×1 and produced a strongly dispersing surface state with a bandwidth of 1.7 eV. Energy minimization calculations based on the first principles pseudopotential method and the local density approximation were used to determine the structure of Ge(111):As 1×1. The dispersion of the lone pair band was calculated and compared to the experimental results. The agreement found between experiment and theory allows us to conclude that arsenic interaction with the Ge(111) surface results in the replacement of the outer Ge layer with a single As layer.
Secondary ion mass spectroscopy analysis of moisture penetration of dielectric films
Authors: Barocela Edward
Published: July 2002

The permeability of optical coatings to water was studied by using secondary ion mass spectroscopy to sputter coated samples exposed to water enriched with the stable isotope 18O. The secondary-ion intensities of 16O and 18O were compared both inside the coating material and in the underlying substrate material to determine the degree of penetration. This method was used to compare permeability of Al2O3 coatings deposited at varying temperatures, and to verify that a material that is permeable to liquid water is also permeable to water vapor. Si3N4 and SiO2 were also examined with this technique. Both were found to be permeable. In addition, SiO2 showed a high concentration of 18O in the coating material itself, indicating that this method can distinguish between absorption of water in the coating and permeability to water of the coating. This technique can be used to evaluate the effects of material composition and deposition conditions on the permeability of thin films.
Electron energy loss spectroscopy and work function measurements on Sb/GaAs(110): Example of an unpinned interface?
Authors: Li K. , Kahn A.
Published: July 2002

Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), and contact potential difference (CPD) measurements are performed on Sb overlayers deposited on room temperature and low temperature n-type GaAs(110) surfaces. In agreement with earlier LEED results, AES indicates a Stranski–Krastanov growth mode at room temperature with 3-D nucleation on top of an atomically ordered Sb monolayer (ML). The changes in the GaAs surface exciton amplitude and surface plasmon energy also show the growth of an epitaxial monolayer which suppresses the substrate surface relaxation. Work function and surface photovoltage measurements performed during deposition of Sb on the room temperature substrate show that the band bending at the GaAs(110) surface first increases as a function of Sb coverage and then decreases to a very small value when the coverage reaches 1 ML. This suggests that, with a 1 ML coverage, the density of deep acceptor states due to Sb induced defects, atomic reconstruction or chemical bonds, is considerably smaller than the density of states which pin the Fermi level at other metal–GaAs interfaces.
Characterization of reactively magnetron sputtered hydrogenated amorphous silicon films
Authors: Rübel H. , Schröder B. , Geiger J.
Published: July 2002

This work gives the results of a systematic study of the infrared absorption and photoelectric conductivity of hydrogenated amorphous silicon (a-Si:H) prepared by rf planar magnetron sputtering. The substrate temperature TS was varied between 300 and 700 K and the hydrogen partial pressure during the sputtering process between pH=1×10−4 and 1×10−3 Torr, the argon pressure was kept constant at 2.5×10−3 Torr. Characteristic changes in the hydrogen bonding configuration and in the electrical conductivity were observed. A sufficiently passivated and chemical homogeneous a-Si:H material which contains an amount of 10–15 at. % hydrogen mainly in dispersed SiH bonding configurations, could be obtained at elevated substrate temperatures between 500 and 600 K and an adequate hydrogen partial pressure. The AM1 photoconductivity σph(AM1) reaches values around 10−4 (Ω cm)−1. The ratio of the photoconductivity to the dark conductivity σph(AM1)/σD, which served as a measure for the quality of the films, was 104 to 105 at TS=590 K and corresponded to a minimum value of the density of states near the midgap of the films. At lower substrate temperatures a rather inhomogeneous material grew with high hydrogen content. Both the photoconductivity and dark conductivity were very low. At temperatures above 600 K hydrogen clustering at inner surfaces occurred and the photoconductivity decreased by several orders of magnitude.
The influence of gallium ingot cleaning procedures on the carbon impurity level in molecular beam epitaxy GaAs
Authors: Chambers F. A. , Chumbley P. E. , Meese J. M. , Vojak B. A. , Zajac G. W. , Fleisch T. H.
Published: July 2002

Carbon impurity in molecular beam epitaxy (MBE) grown GaAs was traced to contamination of the gallium source. The gallium was contaminated by abrasion of the polyethylene wrapping around the ingots during shipment. A cleaning procedure for removing the surface contamination of the ingots is described and results are given that show a reduced carbon impurity level in GaAs samples grown after gallium cleaning.
Unified model of ideal metal–semiconductor contacts and semiconductor heterojunctions
Authors: Mailhiot C. , Duke C. B.
Published: July 2002

A many-electron model is proposed to describe the electronic structure of metal–semiconductor interfaces and semiconductor heterojunctions. The model is utilized to calculate the self-consistent one-electron potential in the vicinity of the interface. The boundary conditions ensuring thermal, mechanical, and electron-transfer equilibrium are imposed explicitly. Numerical calculations predict the validity of Schottky’s phenomenological boundary condition for metal–semiconductor contacts, the applicability of the electron affinity rule for semiconductor heterojunctions, and systematic correlations between Schottky barrier heights and heterojunction energy band offsets. The latter three results emerge as consequences of the rigorous and complete imposition of thermodynamic equilibrium boundary conditions and of the use of a model of the ion-core charge density in which atomic reconstructions at the interface relative to the vacuum surfaces are not considered explicitly.
Plasma-assisted low temperature epitaxy
Authors: Pande K. P. , Aina O.
Published: July 2002

Plasma-assisted low temperature (≤430 °C) epitaxy of GaAs layers is reported. The layers were grown at low pressures using metalorganic sources in an isolated plasma-enhanced chemical vapor deposition system. The epitaxial layers show excellent surface morphology when grown at ∼400 °C. Mobility values of 5500 cm2/V s at room temperature and 28 500 cm2/V s at 77 K were obtained for these layers. Photoluminescence spectra showed sharp and well-resolved exciton emissions. A deep electron trap at 0.63 eV with a concentration of 5×1013 cm−3 was observed in the plasma grown layers. Schottky barrier devices fabricated on epi-n/n+ substrates showed nearly ideal I–V characteristics.
Ultraviolet induced metal-organic chemical vapor deposition growth of GaAs
Authors: Balk P. , Heinecke H. , Pütz N. , Plass C. , Lüth H.
Published: July 2002

Ultraviolet stimulated growth of GaAs from AsH3 and Ga(CH3)3 or Ga(C2H5)3 was studied using 254 nm irradiation, which dissociates all three reactants. Growth from the ethyl compound takes place at a temperature ∼150° lower than that using the methyl compound, indicating the important role of the alkyl in the rate-determining step. The temperatures at which UV irradiation affects the rate show a similar difference. The radiation effects on rate and morphology are more pronounced for the methyl than for the ethyl compound at the temperatures investigated (783–923, and 653 K, respectively). Some important implications of the data for the kinetics of the process are discussed.
Anomalous structures in thin In–Ge films
Authors: Kapitulnik A. , Lereah Y. , Deutscher G. , Voss R. F.
Published: July 2002

We present structural studies of coevaporated thin In–Ge films. At high thicknesses they show random percolation structure. At low thicknesses no sharp percolation threshold is observed. Annealing induces the appearance of orientational clusters which span to a length scale of ∼300 times the crystallite size. Their fractal dimension was found to be 1.65±0.1.
Negative resist behavior of obliquely deposited Ge–Sb–Se thin films
Authors: Kumar Ajay , Malhotra L. K. , Chopra K. L.
Published: July 2002

The photon and electron-beam induced changes in the selective solubility of Ge–Se–Sb films have been investigated to show that these films can be used as a negative resist. The formation of Sb2Se3 on the top of the film surface after exposure, as confirmed by x-ray diffraction and x-ray photoelectron spectroscopy studies, gives rise to a negative tone behavior. The addition of 10 at. % of Sb in the GeSe3 films saturates the change in the chemical solubility after photo- and electron-beam exposures. The chemical solubility difference can be increased by increasing the angle of deposition and is maximum at 80°. Contrast (γ) and sensitivity (s) values of 2.85 and ∼1020 photons/cm2, respectively, for photons, and 4.2 and 6×10−5 C/cm2 for electrons have been obtained.
Mercury cadmium telluride n-isotype heterojunctions grown i
n s
i
t
u by molecular-beam epitaxy
Authors: Boukerche M. , Sou I. K. , DeSouza M. , Yoo S. S. , Faurie J. P.
Published: July 2002

Electrical characterizations of the first n–N HgCdTe heterojunctions grown in situ by molecular-beam epitaxy are reported. The cadmium concentrations of the two materials are 0.18 for the bottom layer and 0.26 for the top. The measurements by Hall, IV, CV, and spectral responsivity are consistent with the existence of a conduction-band barrier at the interface behaving as an insulator at low temperature. We suggest that transient effusion cell fluxes occurring during shutter sequencing created such barriers at the heterojunction interfaces during the growth. The high R0A (600 Ω×cm2) measured suggests that this effect might be of interest for future heterojunction gate field-effect transistor investigations.
Two-photon photoelectron spectroscopy of Pd(111)
Authors: Kubiak Glenn D.
Published: July 2002

Angle-resolved two-photon photoelectron spectroscopy has been used to study the normally unoccupied band structure of Pd(111). Photoemission features are studied as functions of excitation photon energy hν, polarization state, and emission angle. Below hν=4.9 eV, photoemission is dominated by excitation of bulk states near the Fermi level of Λ3 symmetry through Λ1 excited states between the Fermi and vacuum level. The kinetic energy of the corresponding spectral peak is found to increase according to 2 hν−Φ, where Φ is the Pd(111) work function. At hν=4.9 eV, a surface state with 0.65±0.1 eV binding energy is found. The dispersion of this state with k∥ is well-described by an effective mass m* equal to the free electron mass me. This state is assigned as the n=1 member of the image potential series of states. The relationship between the dispersion of this state and its position in the (111) projected bulk band gap is discussed.
Crystallographic target effects in magnetron sputtering
Authors: Wickersham C. E.
Published: July 2002

Film uniformity and sputter deposition rate are found to depend upon the crystallographic orientation of the sputtering target. Improvements in film uniformity from ±10% to ±5% on a 125-mm silicon wafer are achieved by controlling the target metal working process so that an optimum crystallographic orientation is maintained in the target. A Monte Carlo computer simulation of the crystallographic effect is described and used to calculate coating uniformity curves for a uniformly emitting disk and a conical magnetron sputtering source. These results are compared to experimental data obtained with a conical magnetron sputter deposition source equipped with aluminum alloy targets fabricated with radically different crystallographic orientations. Excellent agreement is obtained between the Monte Carlo simulations based upon the measured target crystallographic orientation and the measured uniformity curves. The target crystallographic orientation is established using direct x-ray pole figures.
Raman scattering in GaSb/AlSb strained-layer lattices
Authors: Schwartz G. P. , Gualtieri G. J. , Sunder W. A. , Farrow L. A. , Aspnes D. E. , Studna A. A.
Published: July 2002

Raman scattering has been used to study the folded acoustic mode structure in strain-layer GaSb/AlSb lattices with periodic, quasiperiodic, or random-layer sequencing. In periodic lattices we report on the effects of isolating the strain in a selective layer and on (111)- vs (001)-oriented growth. The quasiperiodic lattice was grown with Fibonacci layer sequencing and its reduced Raman spectrum displayed numerous dips which are associated with gaps in the one-dimensional vibrational density of states. The lattice grown with nominally random layer sequencing also displayed structure in its reduced Raman spectrum. This structure results from the finite lattice size, the finite sampling depth, and any local bias in the random sequencing.
A description of the high-pressure ammonia synthesis reaction based on surface science
Authors: Stoltze P. , No/rskov J. K.
Published: July 2002

A kinetic description is given of the high-pressure ammonia synthesis reaction over a Fe based catalyst. The description is based on a statistical mechanical treatment of the competitive adsorption of the reactants, intermediates, and product on identical surface sites. The kinetic expression with parameters taken from ultrahigh vacuum for the reactants on single-crystal surfaces is able to describe the observed ammonia kinetics for an industrial catalyst over a very broad range of conditions (1–300 atm, 375–500 °C). This provides a direct verification that the ‘‘pressure gap’’ between surface science and heterogeneous catalysis can be bridged. Further, the present description gives a detailed picture of the working catalyst. Variations in surface coverages down through the reactor bed and the origin of the activation enthalpy of the overall reaction are discussed. We also show that the microscopic picture of poisoning and promotion whereby the effect of surface impurities is to stabilize or destabilize the adsorbing molecules can account quantitatively for the observed promotion and poisoning of the high-pressure ammonia synthesis by potassium and water, respectively.
Systematics of metal contacts to Hg1−xCdxTe
Authors: Friedman D. J. , Carey G. P. , Lindau I. , Spicer W. E.
Published: July 2002

We discuss the morphology and band bending properties of metal contacts to the surface of Hg1−xCdxTe (MCT) cleaved in ultrahigh vacuum. The dependence of these interface properties upon the overlay metal is seen to correlate with the bulk thermodynamic and doping properties of the overlayer metal with MCT.
HgTe–CdTe superlattices and Hg1−xCdxTe grown by low-temperature metalorganic chemical vapor deposition
Authors: Lu P.-Y. , Williams L. M. , Wang C.-H. , Chu S. N. G.
Published: July 2002

We report low-temperature metalorganic chemical deposition of Hg1−xCdxTe and HgTe–CdTe superlattices using a thermal precracking technique. The success of the precracking technique depends on decomposing the incoming metalorganic reactants at a high temperature while allowing the epitaxial growth to occur at a lower temperature. The precracking technique enables one to grow Hg1−xCdxTe and HgTe–CdTe superlattices at temperatures of 200 to 250 °C. The Hg1−xCdxTe epilayers show infrared transmission with sharp cutoffs. The as-grown Hg0.7Cd0.3Te is n type and has a room-temperature mobility of 12 200 cm2/V s with carrier concentration of 2.7×1017 cm−3 and 77 K mobility of 27 000 cm2/V s with carrier concentration of 1.0×1017 cm−3. For the HgTe–CdTe superlattices, HgTe layer thicknesses as small as 60 Å were obtained. All the samples examined using transmission electron microscopy had sharp HgTe–CdTe interfaces. The HgTe–CdTe superlattice structures also exhibited sharp infrared transmission cutoffs. In addition, compared to the band gap of Hg1−xCdxTe alloy with the same composition, a narrowing of the superlattice band gap was observed as predicted by theoretical studies.
Computer design and analysis of vacuum systems
Authors: Santeler Donald J.
Published: July 2002

A computer program has been developed for an IBM compatible personal computer to assist in the design and analysis of vacuum systems. The program has a selection of 12 major schematics with several thousand minor variants incorporating diffusion, turbomolecular, cryogenic, ion, mechanical, and sorption pumps as well as circular tubes, bends, valves, traps, and purge gas connections. The gas throughput versus the inlet pressure of the pump is presented on a log–log graphical display. The conductance of each series component is sequentially added to the graph to obtain the net system behavior Q(P). The component conductances may be calculated either from the inlet area and the transmission probability or from the tube length and the diameter. The gas-flow calculations are valid for orifices, short tubes, and long tubes throughout the entire pressure range from molecular through viscous to choked and nonchoked exit flows. The roughing-pump and high-vacuum-pump characteristic curves are numerically integrated to provide a graphical presentation of the system pumpdown. Outgassing data for different materials is then combined to produce a graph of the net system ‘‘outgassing pressure.’’ Computer routines are provided for differentiating a real pumpdown curve for system analysis. The computer program is included with the American Vacuum Society course, ‘‘Advanced Vacuum System Design and Analysis,’’ or it may be purchased from Process Applications, Inc.