bluuebunny/crossref_metadata_2025 huggingface dataset:
The Ag/(Hg,Cd)Te and Al/(Hg,Cd)Te interfaces
Authors: Friedman D. J. , Carey G. P. , Shih C. K. , Lindau I. , Spicer W. E. , Wilson J. A.
Published: July 2002

The interfaces of UHV-cleaved Hg0.77Cd0.23Te with Ag and with Al have been studied with x-ray and ultraviolet photoelectron spectroscopy at coverages from submonolayer to tens of monolayers. These two metals were chosen because they lie at the extremes of chemical reactivity with Te: Al is very reactive, while Ag is nonreactive. As expected, little chemical reaction is seen for the Ag/(Hg,Cd)Te interface. The Ag diffuses 102–103 Å into the bulk, displacing Hg in the process. No such deep diffusion is seen for Al. For the Al/(Hg,Cd)Te interface, the first 3–5 ML are incorporated into an Al–Te reacted layer, with further Al deposition being incorporated as metallic Al. A significant depletion of Hg in the first few semiconductor layers occurs even at submonolayer coverages. Our results are compared with other work on metal/(Hg,Cd)Te and related interfaces.
Gate-controlled Hg1−xCdxTe photodiodes passivated with native sulfides
Authors: Nemirovsky Y. , Adar R. , Kornfeld A. , Kidron I.
Published: July 2002

The properties of ion-implanted junctions in Hg1−xCdxTe with x=0.22, passivated by a novel native sulfide technique, are described. The junctions are typically operated at 77 K, with 50% responsivity at a wavelength of 10.5 μm. To study the effects of the passivated surface on the diode properties, special gate-controlled diodes were processed, including both nickel and titanium gate metals. In addition, a novel enhancement mode metal–insulator–semiconductor field-effect transistor (MISFET) is presented. The measured dc properties and noise current spectral densities of various diodes as a function of reverse bias and gate control voltage show that close to background limited devices may be realized with the above surface passivation. In particular, such diodes exhibit good 1/f noise performance at low frequency, with reverse bias of close to 200 mV, without requiring a special gate electrode.
Electrical properties of molecular beam epitaxy produced HgCdTe layers doped during growth
Authors: Boukerche M. , Wijewarnasuriya P. S. , Reno J. , Sou I. K. , Faurie J. P.
Published: July 2002

Electrical characterizations of n- and p-type mercury cadmium telluride epitaxial layers intentionally doped during their growth by the molecular beam epitaxy technique are reported. The doping by stoichiometry adjustment can produce good mobility material of both n or p semiconductor types. However, p-type material is feasible only for cadmium fraction (x) greater than 0.2, whereas the n-type conduction is possible only for x less than 0.3. For the first time in situ doping of HgCdTe by indium is reported. It is incorporated in the material during the growth and behaves as a n-type dopant. Carrier concentrations up to 1018 cm−3 have been obtained. An indium doped layer having a cadmium fraction of 0.55 was also found to be n-type.
Hall and electroreflectance studies of the effects of doping in mercury–cadmium telluride grown by molecular-beam epitaxy
Authors: Raccah Paul M. , Garland J. W. , Zhang Z. , Chu Amy H. M. , Reno J. , Sou I. K. , Boukerche M. , Faurie J. P.
Published: July 2002

Hall and electrolyte electroreflectance (EER) measurements have been performed on a series of mercury–cadmium telluride samples grown by molecular-beam heteroepitaxy on semi-insulating GaAs substrates and varying from p type to heavily doped n type. Our results show for as-grown, n-type samples grown under nearly ideal conditions that the mobilities are high and that the n character is not primarily associated with defects. Our Hall results show that as one departs from nearly ideal growth conditions in order to obtain higher n-type doping levels, the mobility decreases precipitously. An analysis of the temperature dependence of the mobility suggests that this decrease is associated with a rapid increase in both the number of ionized impurities and the number of neutral scattering centers. Our EER results show that the initial decrease of the mobility is associated with the formation of inhomogeneous strains and, to a lesser extent, both point defects and two-dimensional structural faults. They also show that the final precipitous decline in the high-temperature mobility is associated with a decrease in the density of inhomogeneous strains accompanied by the formation of many more two-dimensional structural faults, although they are not sensitive to the high degree of compensation which is largely responsible for the precipitous decline in the low-temperature mobility.
The organometallic heteroepitaxy of CdTe and HgCdTe on GaAs substrates
Authors: Bhat Ishwara B. , Taskar Nikhil R. , Ghandhi Sorab K.
Published: July 2002

Heteroepitaxial films of CdTe and Hg1−xCdxTe have been grown on (100)GaAs substrates by organometallic vapor phase epitaxy. It is shown that high quality CdTe films can be grown on GaAs substrates over the temperature range from 350 to 440 °C. The effects of the reactant pressures on the growth rate of CdTe are also discussed. HgCdTe layers were grown, on GaAs substrates with a CdTe buffer layer, at 415 °C. Transport measurements made on these layers indicate that they are of high quality with a 80 K mobility of over 2×105 cm2/V s for layers of composition x≂0.2. Layers have been grown with different buffer layer thicknesses. It is shown that a CdTe buffer layer of at least 2–3 μ is necessary to fully accommodate the misfit dislocation at the CdTe–GaAs interface.
An automated method for high dynamic range secondary ion image depth profiling
Authors: Bryan S. R. , Linton R. W. , Griffis D. P.
Published: July 2002

A variable parameter mode of digital image acquisition was developed for secondary ion image depth profiling (IDP). The new method substantially extends the dynamic range of the IDP technique by automatically optimizing the detector gain and acquisition time for each individual image. Postacquisition processing of IDP’s allows generation of local area depth profiles (LADP’s) from areas ranging in size from 1 μm2 to the entire image field. Detection limits of LADP’s using the entire image field are within a factor of 2–3 of those obtained using conventional pulse-counting electron multiplier detection. The lowest detectable signal is limited by camera noise to approximately ten secondary ions per second. A LADP from a 1 μm2 area with a dynamic range of four orders of magnitude was achieved using the variable parameter mode.
Gas dynamics in residual gas analyzer calibration
Authors: Santeler Donald J.
Published: July 2002

Residual gas analyzers are used for measuring partial flow rates as well as for measuring partial pressures. The required calibration may also be obtained with either known flow rates or known pressures. The calibration and application procedures are straightforward when both are of the same type; however, substantial errors may occur if the two types are mixed. This report develops the basic equations required to convert between partial pressure calibrations and partial flow rate calibrations. It also discusses the question of fractionating and nonfractionating gas flow in various gas inlet and pumping systems.
Characterization of a Ti vacuum arc and the structure of deposited Ti and TiN films
Authors: Martin P. J. , Netterfield R. P. , McKenzie D. R. , Falconer I. S. , Pacey C. G. , Tomas P. , Sainty W. G.
Published: July 2002

The energy of Ti neutrals and Ti+ ions was determined in the region of the cathode spot of a Ti vacuum arc by measurement of the Doppler broadened profiles of Ti(I) and Ti(II) emission lines using Fabry-Perot interferometry. Energy analysis of Ti+ and Ti++ was carried out at a distance from the cathode using an electrostatic analyzer. The emission lines showed that the neutrals had energies corresponding to approximately 0.4 eV while the Ti+ ions had a most probable energy of 3 eV. The energy analyzer showed that remote from the cathode spot Ti+ had a most probable energy of 80 eV. This large difference is explained in terms of a simple model for the arc which includes a potential ‘‘hump’’ near the cathode. The charge fraction of the evaporant was measured to be approximately 0.5, and the effect of nitrogen background gas on the energy distribution was observed. Deposited films of Ti and TiN were prepared and the influence of substrate bias investigated. It was found that under high negative bias the Ti films exhibited a preferred 〈110〉 orientation consistent with previous studies of ion-beam-induced textures.
Calibration of oxygen Auger signal from single-crystal ZnO surfaces
Authors: Weisz S. Z. , Resto O. , Yaron G. , Many A. , Goldstein Y.
Published: July 2002

The Auger spectra of the polar and prism surfaces of single-crystal ZnO were measured. The sensitivity factors derived from the differentiated spectra are 0.26 and 0.25 for a beam energy of 3 and 5 keV, respectively. These values agree fairly well with the theoretical calculations of Mroczkowski and Lichtman.
Secondary ion mass spectrometry profiling of shallow, implanted layers using quadrupole and magnetic sector instruments
Authors: Vandervorst W. , Shepherd F. R.
Published: July 2002

The analysis of shallow, steeply varying profiles, using secondary ion mass spectrometry with quadrupole and magnetic sector instruments, is described. The problems in generating a reliable profile, free of transient effects, and ion beam induced displacements and broadening effects, in both types of instruments are discussed. The most important conclusion is that one should be capable of varying the primary beam energy, primary mass, the angle of incidence, and the polarity of the secondary ions independently of each other. We also report the use of special experimental conditions which circumvent some of the problems posed in magnetic sector instruments; low-energy Cs beams (down to 1 keV) are used in combination with sensitive detection of positive secondary ions. Finally, it is shown that for a 5-kV As+ implanted Si specimen all the projected ranges and decay lengths of the profiles, measured with different primary masses, angles of incidence, and primary energies, can be correlated in a universal curve which has the penetration depth of the primary ion as the basic parameter. The effect of the ambient pressure on the results is also described.
Carbon segregation to the low-index surfaces of an Fe–10 at. % Si single crystal
Authors: Viljoen P. E. , du Plessis J. , Bezuidenhout F.
Published: July 2002

Several mechanisms for carbon segregation to iron and iron alloy surfaces, specifically Fe–Si alloy single crystals, have been reported. This paper reports on measurements of carbon segregation to the (111) surface of an Fe–10 at. % Si crystal as measured with Auger electron spectroscopy in the temperature range 573–690 K. Three distinct consecutive processes have been identified in support of reported results on the (100) and (110) surfaces. The first is bulk to surface diffusion until the surface coverage reaches a value of 12–14 at. %. The activation energy for this process is 41±5 kJ/mol. The second process is associated with the growth of two-dimensional graphite islands on the surface until a surface coverage of about 45% is reached. This coverage is described as a ‘‘stable monolayer.’’ Further increase in the carbon concentration is associated with a further two-dimensional growth. The rate constants for the last two processes were determined by a fit of the Avrami equation to the data. The activation energies, determined from the Arrhenius plots, are, respectively, 115±10 and 147±10 kJ/mol. The results also indicate that the segregated graphite layer has some epitaxial relation to the substrate.
HgTe–CdTe heterojunction valence-band discontinuity
Authors: Faurie Jean-Pierre , Hsu C. , Duc Tran Minh
Published: July 2002

In this paper we present experimental data related to the determination of the valence-band offset between HgTe and CdTe. Magneto-optical and resonant Raman scattering data are consistent with a small valence-band offset, i.e., Λ<100 meV. From our x-ray photoelectron spectroscopy (XPS) measurements carried out on CdTe//HgTe(1̄1̄1̄)B and HgTe//CdTe(1̄1̄1̄)B we found Λ=0.36±0.06 eV in very good agreement with data previously reported. We discuss the different assumptions made in the determination of Λ by XPS. The reason for such a discrepancy is not understood at the present time even though we propose two factors which could contribute to such a difference.
Interdiffusion and related defect mechanisms in the HgTe–CdTe system
Authors: Tang M. F. S. , Stevenson D. A.
Published: July 2002

Interdiffusion coefficients (D) were measured from 300 to 600 °C using an angle lapping technique and electron microprobe analysis. The influence of the binary composition, temperature, and Hg overpressure was studied. There is a significant difference in the behavior of D in the higher and lower temperature regions. At higher temperatures (T≥450 °C), D is a strong function of composition and temperature and is insensitive to the Hg overpressure. At lower temperatures (T<450 °C), we observe the following trends with decreasing temperature: (i) the activation energy decreases, (ii) D is less dependent on X (X is the CdTe mole fraction), and (iii) D depends on Hg overpressure. Based on an analysis of our interdiffusion data, we propose a dual mechanism for higher temperatures with a vacancy and an interstitial mechanism dominating at low and high X values, respectively, and an interstitial mechanism at lower temperatures.
Extended x-ray absorption fine-structure studies of semiconductor structure
Authors: Bunker Bruce A.
Published: July 2002

New advances and applications of extended x-ray absorption fine-structure spectroscopy are discussed, including (a) the investigation of the site and local bond lengths of Fe impurities implanted into Si, where it is shown that the first coordination shell expands about the impurity atom while the second shell contracts; (b) the determination that the local Mn–Se and Zn–Se bond lengths in the diluted magnetic semiconductor Zn1−xMnxSe are constant within 0.01 Å even though the lattice constant as determined by x-ray diffraction varies by ∼0.1 Å; (c) the demonstration that the ferroelectric phase transition in Pb1−xGexTe has an order–disorder as well as displacive character; and (d) preliminary results showing the feasibility of nondestructive measurement of a ‘‘buried’’ Al/GaAs interface.
Electronic and transport properties of HgCdTe and HgZnTe
Authors: Berding M. A. , Krishnamurthy S. , Sher A. , Chen A.-B.
Published: July 2002

Band structures are calculated for HgTe, CdTe, and ZnTe. A near band-edge approximation to the coherent potential approximation is used to compute the alloy band-gap variation and electron and hole effective masses in HgZnTe and HgCdTe. Both diagonal and off-diagonal disorder are included. The alloy-scattering contributions to the electron and hole mobilities are calculated and found to be unimportant for electrons but significant for hole transport. The consequences for hole mobility are found to be significant if anion disorder is present because of a valence-band offset.
Initial stages of growth during boron carbide chemical vapor deposition
Authors: Jansson Ulf , Carlsson Jan-Otto , Stridh Bengt
Published: July 2002

The initial stages of growth of boron carbides from a BCl3/H2/hydrocarbon vapor on molybdenum were investigated. The initial growth of boron carbide was strongly dependent on the type and concentration of hydrocarbon in the vapor. From the results of the experiments it was concluded that an ‘‘overlayer’’ of adsorbed hydrocarbons was formed on the surface. The overlayer inhibited some reactions on the surface and affected the growth rate, the morphologies of the nuclei, as well as the growth rates of molybdenum borides.
Ion beam mixing of Al with GaAs
Authors: Fastow R. , Eizenberg M. , Kalish R. , Richter V. , Brener R. , Peisach M.
Published: July 2002

Ion beam mixing of thin aluminum films deposited on GaAs substrates was analyzed using Auger electron spectroscopy and transmission electron microscopy. The formation of an AlAs region near the interface was observed after room-temperature implantation. High-temperature implantation (at 400 °C) resulted in an Al–GaAs layer, which was identified as AlxGa1−xAs. The observed reactions resulted from arsenic outdiffusion and subsequent compound formation during implantation.
Study of polycrystalline boron and boron-containing metallic glasses by surface extended energy-loss fine structure
Authors: Fei L. , Zheng Y. , Zhang Q. J. , Hua Z. Y.
Published: July 2002

The technique of surface extended energy-loss fine structure (SEELFS) can be used to obtain detailed local surface structural information of solids, especially for light elements. Polycrystalline boron, Fe80B20, and Fe78.5Si5B16.5 were investigated from the oscillating structures up to 300 eV above the boron K edge. By using a program developed in this lab, the radial distribution functions around the excited site were obtained by analyzing the SEELFS data. After phase-shift correction, the nearest-neighbor bond lengths of surface atoms were found: 1.57 Å for polycrystalline boron, 2.14 Å for Fe80B20, and 2.12 Å for Fe78.5Si5B16.5. Factors affecting the calculation were discussed and the total error has been estimated as 0.05–0.08 Å for central atoms with low atomic number.
Theoretical calculations for the dimer–adatom–stacking-fault model of Si(111)-7×7 surface
Authors: Qian Guo-Xin , Chadi D. J.
Published: July 2002

Calculations were performed for the Si(111)-7×7 surface reconstruction model proposed by Takayanagi et al. The structure contains dimers, adatoms, and stacking faults (DAS) in the surface layer. The DAS model has 19 dangling bonds and 12 ‘‘top site’’ adatoms per 7×7 unit cell. In the tight-binding based energy-minimization calculation, Hellmann-Feynman forces were used to relax adatoms and three layers of a five-layer 7×7 slab configuration. The surface energy is found to be at least 0.40 eV per 1×1 cell lower than that of an ideal and unrelaxed Si(111) surface, as compared to 0.36 eV for the π-bonded chain structure for the 2×1 cleaved surface. This is the lowest surface energy calculated for the Si(111) surface. Atomic configurations were obtained for the completely relaxed system. The calculated surface electronic density of states and the atomic origin of the surface electronic structure are in good agreement with experimental data.
Comparison of the synchrotron radiation induced gas desorption in aluminum vacuum chambers after chemical and argon glow discharge cleaning
Authors: Mathewson A. G. , Alge E. , Gröbner O. , Souchet R. , Strubin P.
Published: July 2002

In a dedicated synchrotron radiation beam line at the DCI storage ring at Orsay, France a series of measurements were made to evaluate the effect of argon glow discharge cleaning either as a pretreatment in the laboratory, or in situ, following the installation and pumpdown of the vacuum system in the LEP (Large Electron Positron storage ring) machine. On exposure to synchrotron radiation the gases desorbed were H2, CH4, CO, and CO2 with Ar in the glow discharge cleaned chambers. With continued exposure the gas desorption decreased with time except that the Ar showed a definite reluctance to clean up. When applied to LEP conditions, chemical cleaning only gave the required beam-gas lifetime after the shortest exposure. The worst overall situation results from the in situ Ar discharge cleaning due to the low pumping speed for Ar in the LEP machine.
Properties of silicon and aluminum oxide thin films deposited by dual ion beam sputtering
Authors: Emiliani G. , Scaglione S.
Published: July 2002

Silicon dioxide and aluminum oxide thin films have been deposited by dual ion beam sputtering. Their properties have been investigated in order to point out the advantages of this technique. A bombardment of growing films has been performed by an Ar++O+2 ion beam with different energy and current values. The obtained films have correct stoichiometry, high density, and good mechanical properties.
Insulator interface effects in sputter-deposited NbN/MgO/NbN (superconductor–insulator–superconductor) tunnel junctions
Authors: Thakoor S. , Leduc H. G. , Stern J. A. , Thakoor A. P. , Khanna S. K.
Published: July 2002

All refractory, NbN/MgO/NbN (superconductor–insulator–superconductor) tunnel junctions have been fabricated by in situ sputter deposition. The influence of MgO thickness (0.8–6.0 nm) deposited under different sputtering ambients at various deposition rates on current–voltage (I–V) characteristics of small-area (30×30 μm) tunnel junctions is studied. The NbN/MgO/NbN trilayer is deposited in situ by dc reactive magnetron (NbN), and rf magnetron (MgO) sputtering, followed by thermal evaporation of a protective Au cap. Subsequent photolithography, reactive ion etching, planarization, and top contact (Pb/Ag) deposition completes the junction structure. Normal resistance of the junctions with MgO deposited in Ar or Ar and N2 mixture shows good exponential dependence on the MgO thickness indicating formation of a pin-hole-free uniform barrier layer. Further, a postdeposition in situ oxygen plasma treatment of the MgO layer increases the junction resistance sharply, and reduces the subgap leakage. A possible enrichment of the MgO layer stoichiometry by the oxygen plasma treatment is suggested. A sumgap as high as 5.7 mV is observed for such a junction.
Near-eutectic Si–Cr alloy formation with electron-beam and laser pulses
Authors: D’Anna E. , Leggieri G. , Luches A. , Majni G. , Ottaviani G.
Published: July 2002

A study is presented of amorphous Cr–Si alloy formation with intense electron-beam and laser pulses. Two different kinds of samples were used: the first consisted of a 100-nm Cr film deposited on a 200-nm Si layer; the second was of a 60-nm Cr film on a 700-nm Si layer. The substrates were SiO2. Formation of Cr–Si alloys, with a composition near the Cr45Si55 and Cr13Si87 eutectics in the first and second sample, respectively, was observed in a narrow range of energy fluences. The results are discussed with the aid of transient temperature profiles of the samples, obtained by computer simulations.
Structural and electronic effects of the solid-state amorphization and recrystallization of Cu–Ho thin films
Authors: Venkert A. , Shamir N. , Talianker M. , Atzmony U. , Dariel M. P.
Published: July 2002

Amorphous alloys can be produced as the outcome of diffusion reaction between evaporated thin crystalline films. The Cu–Ho system is a favorable one for such a reaction to occur. The amorphization and the recrystallization of Cu–Ho multilayers has been previously observed using transmission electron microscopy. In the present study the various stages of the interdiffusion processes in this multilayer system were investigated by means of electron spectroscopy techniques. Auger electron spectroscopy depth profiling showed the evolution of the mixing process from the multilayer structure to its completion. The mixing process was accompanied by a shift without broadening of the x-ray photoelectron spectroscopy Ho 4d line. This shift is attributed to a charge transfer from the Ho ion. The lack of line broadening indicated that the mixing was completed down to the atomic level.
Metal penetration and dopant redistribution beneath alloyed Ohmic contacts to n-GaAs
Authors: Shappirio J. R. , Lareau R. T. , Lux R. A. , Finnegan J. J. , Smith D. D. , Heath L. S. , Taysing-Lara M.
Published: July 2002

In order to estimate the depth of penetration of Ohmic metals into n-GaAs during the alloying process, marker layer samples fabricated by molecular-beam epitaxy were utilized. The samples consisted of n-GaAs substrates onto which were grown a 500-nm undoped GaAs buffer layer, followed by 20 layers, each 50 nm thick, alternately doped with Si at levels of 1×1016 and 1×1018 atoms/cm3. Ohmic metals consisting of Ni/Ge/Au/Ni/Au or Ni/Ge/Au/TiB2/Au were evaporated onto the marker layer samples and alloyed on a strip heater. Secondary ion mass spectroscopy (SIMS) depth profiling showed that metal penetration extended as much as 0.5 μm into the GaAs. In addition, significant redistribution of the marker layer Si was observed, and is assumed to result from a vacancy enhanced diffusion process. In a SIMS depth profile performed from the back side of the wafer no dispersion of the metal depth penetration was observed, suggesting that differences in penetration depth of the metals observed in front side profiles are the result of ion beam mixing, surface roughness, and/or inhomogeneities within the alloyed region.
Formation of self-aligned TiSi2 for very large scale integrated contacts and interconnects
Authors: Ho V. Q. , Poulin D.
Published: July 2002

In this paper, we present the results of a study on self-aligned TiSi2 formation. TiSi2 was formed by a rapid thermal anneal (RTA) of a sputtered Ti film on crystalline Si and on phosphorus doped poly-Si at a temperature from 600 to 1100 °C. A film resistivity of 16 μΩ cm has been achieved after an RTA at 800 °C for 10 s. Auger electron spectroscopy analysis indicated that a stoichiometric TiSi2 below a surface TiNxOy layer was formed by RTA in N2. The Si consumption during RTA in N2 was less than in Ar ambient due to the TiNxOy formation. In addition, RTA in N2 resulted in less lateral silicide growth than in Ar. Ti film started to react with the underlying SiO2 layer at 600 °C for 30 s and titanium oxide and titanium silicide were observed after RTA at 800 °C for 30 s. The TiSi2 formation on POCl3 doped poly-Si and moderately doped crystalline Si exhibited the same characteristics as on bare Si. Secondary ion mass spectroscopy analysis indicated that As+ and B+ did not pile up at the TiSi2/Si interface. From the measurements of lateral silicide growth lengths after stripping the ‘‘unreacted Ti’’ on oxide, a process window of RTA temperature and time for forming self-aligned silicide was determined. Good I–V characteristics of silicided diodes and salicide metal–oxide semiconductor field-effect transistors have been obtained.
X-ray photoelectron spectroscopy and Auger studies of selected Zr/Ni intermetallic alloy catalysts
Authors: Wright R. B. , Hankins M. R. , Owens M. S. , Cocke D. L.
Published: July 2002

Various intermetallic alloys of Zr/Ni have been studied with regard to their surface composition using Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) as functions of elevated temperature exposure to hydrogen and oxygen gas. The intermetallic alloys (ZrNi5, Zr2Ni7, ZrNi3, ZrNi, and Zr2Ni) were prepared by arc melting techniques, and were studied either as powders or as slices of the arc melted sample button. The elevated temperature treatments were chosen to duplicate the sample treatment these alloys received prior to a recently reported catalytic hydrogenation study. The alloys were sequentially analyzed with XPS and/or Auger techniques after the following representative treatment cycle: as-prepared; 400 °C H2; 400 °C O2; and a final 400 °C H2 exposure. In general, the as-prepared alloys are depleted in nickel at the surface (as compared to the bulk), and the alloys have been decomposed into a mixed Ni (or NiO) and ZrO2 surface layer residing on the bulk alloy. After the elevated temperature H2 treatment, the NiO (if present) is reduced to Ni(>200 °C) and additional ZrO2 is formed. Elevated temperature O2 treatment causes additional Ni to segregate to the surface and form NiO (≂200 °C), which is reduced to Ni after the final 400 °C H2 treatment. After this general sequence of treatments, two alloys (ZrNi5 and Zr2Ni) were found to have greater Ni surface concentrations than their corresponding bulk composition. These results follow the measured catalytic hydrogenation of ethylene to ethane activities, which indicate that the higher the surface nickel concentration the greater the catalytic hydrogenation activity.
High-resolution low-energy electron reflection from W(100) using the electron energy-loss spectrometer: A step towards quantitative analysis of surface vibrational spectra
Authors: Woods J. P. , Erskine J. L.
Published: July 2002

High-resolution low-energy electron reflection measurements and electron energy-loss measurements for clean W(100) and for the hydrogen saturated phase (β1) on W(100) are reported. The ability to perform both low-energy reflectance, low-energy electron diffraction, and high-resolution electron energy-loss experiments on the same crystal using the same electron optics permits several novel experiments. For example, it is possible to quantitatively test the dipole scattering mechanism and examine dipole and impact loss cross sections under precisely defined scattering conditions (i.e., under diffracted beam emergence conditions or at energies corresponding to reflectance resonance conditions). Under certain scattering conditions, the ‘‘dipole’’ scattering selection rule is shown to break down. Suitable modifications of the electron optics control electronics also permit direct measurements of the cross-section energy dependence of vibrational losses. These new features represent an important step towards quantitative applications of vibrational spectroscopy based on comparing electron energy-loss spectroscopy signals from chemisorbed species on different crystal surfaces.
Composite thin-film production by ion bombardment
Authors: Carr W. , Seidl M. , Tompa G. S. , Souzis A.
Published: July 2002

Composite thin-film production by low-energy cesium ion bombardment was investigated. An ion beam impinging on a target produces a steady-state film, with a total dose of less than 1016 ions/cm2. The fractional concentration of the beam species depends on the incident energy. When the incident energy is sufficiently low, the surface binding energy is greater than the recoil energy and an overlayer forms. An analytical model describing the steady-state film, including sputtering, backscattering, implantation, and diffusion is given. In the absence of diffusion the surface concentration depends on the ratio of the implanted fraction to the target sputtering yield. When diffusion is important, the depth profile becomes exponential and the surface concentration is reduced.
Design studies of distributed ion pumps
Authors: Chou T. S.
Published: July 2002

To upgrade the existing vacuum system, a new type of diode distributed ion pump (DIP) has been tested and developed for the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The new pumps consist of two titanium cathodes as before, but have five parallel 304 stainless-steel plates with 16-mm-diam holes to replace the 11-mm-diam cylinders of the old ‘‘egg crate’’ anode structure. They have pumping speeds for nitrogen of several tens of liters per seconds per meter at 10−9 Torr as is required by the NSLS for good lifetime operation. We first survey previous work on DIP and then present the results of our study on new pumps with different cell sizes and two different anode materials (aluminum and stainless steel). The pumping characteristics of the new pumps are much superior to the old types with respect to ultimate pressure and easier conditioning.
Evidence for the neutralization of boron in silicon using surface analysis techniques
Authors: Kazmerski L. L. , Nelson A. J. , Dhere R. G.
Published: July 2002

Interactions between the shallow acceptor boron and hydrogen in single crystal, polycrystalline, and amorphous Si are investigated. Low-temperature secondary ion mass spectrometry depth-compositional profiles indicate a definite interaction between the boron concentration and the hydrogen penetration in single crystals and at grain boundaries. The bonding of the H is identified to be directly to the Si rather than to the B, and is confirmed by infrared measurements. Electrical neutralization of the B by hydroxyl-group bonding is also reported at oxygen-rich Si grain boundaries. No similar relationships between P concentration and H penetration are observed. In amorphous Si material, the B-doping level has only a limited effect on the hydrogen penetration which seems to be controlled instead by structural diffusion mechanisms.
Conductivity and mobility in very thin epitaxial NiSi2 layers
Authors: Henzler M. , Adamski C. , Rönner K.
Published: July 2002

Epitaxial NiSi2 layers (type B) of increasing thickness (1.5–10 nm) have been produced on Si(111)7×7. The low-energy electron diffraction pattern shows silicide formation with at least 3 monolayers (ML) of Ni. Point defects in the film and steps at its surface are derived from spot profile. The conductance of the film is described by a constant bulk mobility with the first two monolayers of Ni not contributing to the metallic conductivity. Due to protection with SiO no change was observed after transfer through air into a liquid He cryostat. The resistance decreases with T to a residual value with a slight increase for T<30 K. Weak localization is also seen from transversal magnetoconductance, as derived from temperature and field strength dependence at T<40 K. Starting with 3 ML of Ni it is possible to produce thin, coherent, homogeneous, defect-free silicide layers with high mobility and well-defined electrical parameters.
The sources for contaminants in the trace analysis of carbon in GaAs by secondary ion mass spectrometry
Authors: Kobayashi Junji , Nakajima Masato , Ishida Koichi
Published: July 2002

We have studied the sources for contaminants in the trace analysis of carbon in GaAs by secondary ion mass spectrometry (SIMS). The results suggest that the detectivity of carbon is mainly limited by the ionization of carbon-containing molecules in the residual gases by the primary ion beam and/or the memory effect rather than the deposition of these molecules onto the sample surface. The model for the quantitative analysis of carbon in GaAs by SIMS is presented. The detection limit of carbon is (1–2)×1015 cm−3. The possibility is also presented to achieve the detection limit of middle 1014 cm−3 by SIMS even at the conventional vacuum pressure (∼1×10−9 Torr).
A reentrant bakable all-metal feedthrough vacuum seal
Authors: Edmonds P. H. , Shipley W. D.
Published: July 2002

The design, construction, and testing of a reentrant metal seal are described. The principal advantages of this seal are that it occupies a minimum cross-section diameter, requires no special alignment of the components, does not need a large rotational torque to seal, and has a large margin of sealing force. Jackscrews are used to apply the sealing force.
Effects of random impurities on two-dimensional island growth in a chemisorbed overlayer
Authors: Zuo J.-K. , Wang G.-C.
Published: July 2002

Adsorption of oxygen on a W(112) surface leads to (2×1) superlattice structure. At room temperature, two-dimensional (2D) oxygen islands of (2×1) structure are created at very low coverage. As the coverage increases, the islands grow and coalesce with the possibility of forming either antiphase boundaries or larger islands. Using high-resolution low-energy electron diffraction, we have examined the effects of preadsorbed nitrogen (<5% monolayer) on the island in both equilibrium state and dynamical growth by monitoring the angular profile and peak intensity of the superlattice beam. We found that nitrogen atoms distributed randomly on the surface, which not only limited the size of the 2D oxygen islands but also slowed down the dynamics of the island growth. The implication of these findings to recent intense theoretical discussions on low-dimension random-field Ising model is presented.
Mechanism and consequences of surface reconstruction on the cleavage faces of wurtzite-structure compound semiconductors
Authors: Duke C. B. , Wang Y. R.
Published: July 2002

The atomic geometries and electronic structures of (101̄0) and (112̄0) cleavage faces of wurtzite-structure semiconductors are predicted using a sp3 tight-binding model. The surface reconstructions are caused by the lowering in energy of a surface state from near midgap to the top of the valence band. The atomic relaxations are roughly analogous to those on zinc-blende (110) with top-layer anions relaxed outward from the surface and top-layer cations inward. Each surface exhibits distinctive surface relaxations, however, which vary from one material to another via a linear scaling of the independent structural variables with the bulk lattice constant. Several bands of surface bound states and surface resonances are predicted for each surface.
Two-dimensional energy bands for Cr/Au(001)
Authors: O’Neill D. G. , Weaver J. H.
Published: July 2002

The two-dimensional electronic structure of 1 ML Cr/Au(001) along Γ̄–Σ̄–M̄ was investigated with angle-resolved photoemission. Emission from a dx2−y2 (Γ̄3)-derived band is observed near 4.5-eV binding energy. At Γ̄, a dxz, yz or Γ̄5 state is detected at 0.8 eV which splits along Γ̄–Σ̄–M̄ into its odd (Σ̄2) and even (Σ̄1) components. The odd dxz, yz-derived component disperses from 0.8 to 1.2 eV. In addition, emission from a second dx2−y2-derived band is observed at 0.5 eV. Cr induces a loss of photoemission intensity in the Au d band that is attributed to Cr–Au d–d hybridization.
Surface structure analysis using reflection high-energy electron diffraction
Authors: Jamison K. D. , Zhou D. N. , Cohen P. I. , Zhao T. C. , Tong S. Y.
Published: July 2002

For the reconstructions present during epitaxial growth, the diffraction experiment of choice is reflection high-energy electron diffraction (RHEED). In the RHEED geometry, the momentum transfer normal to the surface is small and can be varied by changing the angle of incidence at constant incident energy. The resulting rocking curve, or diffracted intensity versus angle of incidence, is comparable to a low-energy electron diffraction IV profile. We present rocking curve results for the 〈001〉 and 〈11̄0〉 principle azimuths for 10-keV diffraction experiments from GaAs(110). This known surface structure is the best tested structure to develop the analysis. The measurements are compared to a fully convergent dynamical N-beam RHEED calculation which includes as many propagating and evanescent beams as needed. An important issue is to determine what extent the diffraction is dominated by bulk features and single-layer resonances. This is addressed by selectively eliminating beams in the analysis and comparing beam emergence predictions to experimental observations.
Microprocessor controlled ultrahigh vacuum evaporations of critical thin films, multilayer structures, and coevaporated materials
Authors: Fischer H. , Grob M. , Peter G. , Koprio J. A.
Published: July 2002

The use of a quadrupole mass spectrometer as a rate monitor is described for a fast and accurate deposition control down to very low evaporation rates in an ultrahigh vacuum plant. In several applications of this concept, a rate control down to 0.1 Å/s in single-source and coevaporation experiments could be documented for different materials and material combinations. The feasibility of operation at oxygen gas pressures of up to 4.5×10−5 mbar is shown for yttrium/copper coevaporations. Multilayer structures were deposited under microprocessor control with the same rate control method used.
Performance testing of carbon–carbon composites as plasma-facing materials for magnetic fusion reactors
Authors: Causey R. A. , Hsu W. L. , Pontau A. E. , Croessmann C. D. , Ulrickson M. A.
Published: July 2002

This paper describes the results of a study of the properties affecting the performance of carbon–carbon composites as plasma-facing materials in magnetic fusion reactors. A composite has already been chosen for the protective limiter of the rf antenna in the Tokamak Fusion Test Reactor and composites are being considered for divertor applications in the Compact Ignition Tokamak. In direct comparison with results for POCO AXF-5Q graphite, the composites were able to survive more severe high heat flux conditions, released less water vapor and gases during the thermal outgassing tests, and retained less tritium during exposure to tritium gas and plasma.
Deposition of device quality silicon dioxide thin films by remote plasma enhanced chemical vapor deposition
Authors: Kim Sang S. , Tsu D. V. , Lucovsky G.
Published: July 2002

A multichamber system specifically designed for growing Si-based dielectric films on processed and characterized semiconductor surfaces is described. The system consists of a semiconductor surface preparation chamber, an in situ surface analysis chamber, a dielectric deposition chamber, and two load-lock sample introduction chambers. Device quality silicon dioxide thin films have been grown on Si substrates. The electrical and structural properties of these films are discussed.
Short-range order in submonolayer Ni on GaAs(110) by x-ray photoelectron forward scattering
Authors: Egelhoff W. F. , Steigerwald D. A. , Rowe J. E. , Bussing T. D.
Published: July 2002

Angle-resolved x-ray photoelectron spectroscopy has been used to examine the structural arrangement of atoms in submonolayer coverages of Ni on GaAs(110). Insights into the short-range order around a lattice atom are possible because the outgoing photoelectron diffracts off overlying lattice atoms. This diffraction contains a strong component of forward scattering that yields enhanced intensities along axes connnecting the emitting atom to its overlying nearest and next nearest neighbors. We have found that these enhanced intensities are observed in the Ni 2p core level for 0.5 monolayer Ni on GaAs(110) after annealing to 300 °C but are much weaker before annealing. The angles at which these enhanced intensities appear suggest that upon annealing the Ni atoms replace Ga and As atoms in the third layer from the surface and perhaps in the second layer. Replacement may also occur in the top layer but this would produce no forward scattering signature. Thus our structural results are consistent with this being a reactive interface.
Angular dispersion of Fuchs–Kliewer modes on GaAs(100) by digital high-resolution electron energy-loss spectroscopy
Authors: Black D. S. , Seo J. M. , Holloway P. H. , Rowe J. E.
Published: July 2002

High-resolution electron energy-loss spectroscopy has been used to study surface phonon and surface plasmon excitations of GaAs (100). The angular dispersion (energy loss versus electron beam incidence angle) of Fuchs–Kliewer surface optical phonons and free-carrier surface plasmons of GaAs(100) have been determined with an experimental resolution of 10 meV. Theoretical dispersion relations were calculated for these samples using the Lindhard dielectric function and assuming that only the surface loss function is measured. Phonon loss and gain peaks were observed which decreased in intensity with increasing momentum parallel to the surface q∥, as expected for dipole selection rules. The plasmon loss features seemed to decrease less rapidly with q∥ than the phonon peaks. Experimental peaks positions are described reasonably well by the two coupled modes that result from the theoretical model with an effective surface carrier density of 6×1018 cm−3 which is (1)/(3) that of the bulk.
Double-crystal x-ray topography and rocking curve studies of epitaxially grown ZnSe
Authors: Qadri S. B. , Jonker B. T. , Prinz G. A. , Krebs J. J.
Published: July 2002

X-ray double-crystal rocking curve and topography measurements have been used to obtain a detailed evaluation of the crystalline quality of ZnSe(001) films grown by molecular-beam epitaxy on GaAs(001). Rocking curves were measured for three incident x-ray wavelengths to identify and determine the contribution of specific physical defects to the linewidth through their wavelength dependence. X-ray topography was used to obtain a very direct, nondestructive depth-resolved record of the physical structure of both epilayer and substrate through the use of various asymmetric reflections having different penetration depths for dynamical scattering. We find that the GaAs substrate is strained by the misfit to the ZnSe epilayer. The epilayers exhibit a fine microstructure, consisting of slightly misoriented mosaic blocks, which is very uniform both laterally and throughout the thickness of the film. The rocking curve linewidth for the epilayer is dominated by the lattice tilt between these blocks. The resultant dislocation density as estimated from the linewidth is on the order of 1.4×108 cm−2.
Diffusion model for Ohmic contacts to GaAs
Authors: Kulkarni A. K. , Lai C.
Published: July 2002

A diffusion model based on gallium vacancy dependent diffusion assumes grain boundary diffusion of gallium in gold as the principal mechanism of diffusion. This diffusion model is tested with the experimental results on AuGe Ohmic contacts to GaAs. The grain boundary diffusion model is modified since the annealing temperature is above the AuGe eutectic point. The diffusion is better characterized by the process of capture by traps (vacancies and dislocations) created during heat treatment. The diffusion parameters for gallium diffusion in gold, and for gold and germanium diffusion in GaAs are reported. The germanium incorporation into the defect lattice structure of GaAs results in donor (Ge+Ga), acceptor (Ge−As), neutral pair (Ge+Ga–Ge−As), and acceptor complex states. The increase of the contact resistance beyond the critical annealing temperature is related to the saturation of the gold layer with gallium and excessive arsenic loss.
Enhancement of silicon nitridation by alkali metals
Authors: Soukiassian P. , Bakshi M. H. , Starnberg H. I. , Hurych Z. , Gentle T. M. , Schuette K. P.
Published: July 2002

We have studied alkali metal (Na, K) promoted nitridation of the Si(100)2×1 surface by core level photoemission spectroscopy using synchrotron radiation. Work function changes during nitridation were also measured. Exposure to N2 of a surface covered by a monolayer of sodium or potassium at room temperature resulted in the formation of Si–N bonds, although the nitrogen at this initial stage of nitridation does not seem to interact as strongly with the substrate as oxygen does in the corresponding stage of alkali metal promoted oxidation. By a brief annealing at moderate temperature sodium was completely removed, and a Si3N4–Si interface was formed at a much lower temperature than without the alkali metal. In contrast, for the same nitrogen exposure on a Si(100) surface modified by potassium, only a very small amount of Si3N4 was formed. Potassium was subsequently desorbed at 600 °C. An exposure beyond 10 000 L of molecular nitrogen was necessary to obtain a significant amount of Si3N4.
Fermi-level pinning and heavily doped overlayers
Authors: Mahowald P. H. , Spicer W. E.
Published: July 2002

Can a pinned Fermi level at a semiconductor surface be moved by deposition of heavily doped overlayers? We calculate the position of the Fermi energy in the gap for structures composed of a bulk semiconductor substrate and a heavily n-type doped Ge overlayer. This structure is the basis for the new Ohmic contact for n-GaAs proposed by Waldrop and Grant [Appl. Phys. Lett. 50, 250 (1987)], and relevant to our Ge/InP data where the Ge overlayer is naturally doped by intermixed P from the substrate. Using the defect model, we find that 10 Å of heavily doped Ge can move the Fermi energy towards the conduction-band minimum, even if it is initially pinned by induced defects. This is due to the electrons from the heavily doped overlayer filling all available interface defect levels. Modeling of a final metallic overlayer to complete the Ohmic contact is also discussed.