Under Ultrahigh Vacuum Conditions Research Articles

Comparative Study of the Tribological and Oxidative Properties of AlPdMn Quasicrystals and Their Cubic Approximants

AbstractAn experimental comparison has been made between the properties of the surfaces of an Al70Pd21Mn9 quasicrystal and its Al48Pd42Mn10 approximant. The Al70Pd21Mn9 sample was a single grain icosahedral quasicrystal cut to expose its five-fold symmetric (000001) surface. The approximant was polycrystalline β-phase Al48Pd42Mn10, which has a CsCl-type cubic structure. Surfaces of both were prepared under ultra-high vacuum (UHV) conditions and then used for comparative measurements of their frictional properties and oxidation rates. Both materials are oxidized by reaction with O2 to form a thin film of aluminum oxide that ultimately passivates their surfaces. The interesting difference between the two is that the rate of oxidation of the approximant is significantly higher than that of the quasicrystal in spite of the fact that the bulk Al concentration of the approximant is lower than that of the quasicrystal. Friction measurements were made under UHV conditions between pairs of quasicrystals and pairs of approximants whose surfaces were either clean or oxidized to varying degrees. The friction between pairs of the approximant surfaces is significantly higher than that measured between the quasicrystal surfaces under all conditions of surface oxidation.

Reactions of N 2O 4 with ice at low temperatures on the Au(111) surface

Reactions of N 2O 4, formed by condensation of NO 2 gas, with adsorbed water films as models for ice surfaces were studied on a Au(111) substrate at low temperatures under ultrahigh vacuum (UHV) conditions. Two thermal reaction paths were found, primarily by using infrared reflection–absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) techniques. One path evolved gas phase HONO (nitrous acid) and HNO 3 (nitric acid) below 150 K, independent of both the crystallinity of the ice film and the exposure of NO 2. In contrast, another reaction pathway depended strongly on these variables and was conveniently monitored by the formation of oxygen adatoms on the Au(111) substrate surface. The latter reactions only occurred following multilayer adsorption of NO 2 (present as N 2O 4) and only if the adsorption was on amorphous ice clusters and not crystalline ice. The extent of these reactions was proportional to the concentration of ‘free-OH’ groups on the ice film, indicating that water molecules with only two hydrogen bonds were required to initiate these reactions by strongly hydrating N 2O 4. Following adsorption of N 2O 4 multilayers on amorphous ice, we propose that isomerization of O 2N–NO 2 (D 2h–N 2O 4) to ONO–NO 2 (nitrite–N 2O 4) occurs upon heating to 130–185 K. Further heating to 200–260 K leads to formation of NO +NO − 3 (nitrosonium nitrate). This compound is stable on the Au(111) surface up to at least 275 K. Decomposition of NO +NO − 3 on the Au(111) surface at 275–400 K evolves gas-phase NO 2, and possibly NO (nitric oxide), and also produces oxygen adatoms that recombine and thermally desorb as O 2 at about 520 K. These investigations provide new data on reactions of nitrogen oxides and condensed phases of water, which are of general importance for several technologies and improved understanding of atmospheric chemistry. In addition, new details are revealed concerning a novel method for producing oxygen adatoms on Au(111) surfaces under UHV conditions.

Oxidation of Pt(111) by ozone (O 3) under UHV conditions

Ozone (O 3) is a reactive oxidant, creating high effective oxygen pressures under ultra-high vacuum (UHV) conditions. Large oxygen concentrations up to Θ O=2.4 ML were produced by ozone exposure on Pt(111) at 300 K and analyzed using temperature-programmed desorption (TPD), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). O 2 desorption occurs in broad TPD peaks that shift from 814 to 558 K as the oxygen coverage increases from 0.03 to 0.95 ML. For coverages of Θ O≥1.2 ML, “undercutting” of the desorption curves occurs, along with a narrowing and shift to higher temperature, such that O 2 desorption occurs in a sharp (23 K wide) peak at 708 K at Θ O=2.4 ML. LEED shows a (2×2) pattern for Θ O near 0.25 ML, but then only a (1×1) pattern between 0.3 and 1.2 ML. For oxygen coverages larger than 1.2 ML, the Pt(111) surface is disordered. Using the leading edge method, we estimate that the desorption activation energy decreases from 45 to 20 kcal mol −1 with increased oxygen concentration for 0< Θ O<1.2 ML, and thereafter increases with increasing concentration to 38 kcal mol −1 at Θ O=2.4 ML. We interpret the O 2 desorption curves for Θ O>1.2 ML as arising from decomposition of particles of platinum oxide of increasing size near the surface. A very weakly bound state of oxygen also exists at high Θ O and desorbs at 390 K. Oxidized platinum is reduced upon heating to Pt metal in the temperature range of 674–708 K in UHV.

Description of a dust particle detection system and measurements of particulate contamination from shock, gate valve, and ion pump under ultrahigh vacuum conditions

Dust particle contamination is known to be responsible for reduced quality and yield in microelectronic processing. However it may also limit the operation of particle accelerators as a result of beam lifetime reduction or enhanced field emission in radio-frequency accelerating cavities. Intrinsic dust contamination from sources such as valves or ion pumps has not yet been studied due to the inability of commercial particle counters to be able to detect across large cross sections under ultrahigh vacuum (UHV) conditions. This motivated the development of the dust particle detector described here which is able to quantify, in situ, the level of contamination on a representative part of a vacuum vessel. This system operates under UHV conditions and measures flashes of scattered light from free falling dust particles as they cross a thin laser light sheet across a 100 mm diam vacuum vessel. A calibration using microspheres of known diameter has allowed estimation of the particle size from the scattered signal amplitude. Measurements of particulate contamination generated by shocks onto the vessel walls are presented and determination of the height of origin of dust particles from their transit time across the irradiation sheet is discussed. Measurements of dust particle release right to operation of an all-metal gate valve are also presented in the form of time resolved measurements of dust occurrence during the open/close cycles of the valve, as well as histograms of the particle size distribution. A partial self-cleaning effect is witnessed during the first 10 operation cycles following valve installation. The operation of an ion pump has also been investigated and revealed that, in our conditions, particles were released only at pump startup.

Growth and morphology of Rh deposits on an alumina film under UHV conditions and under the influence of CO

Adsorption of certain gases may alter the structure of small metal particles deposited on a surface. If the particles are prepared by vapour deposition, the morphology may also be changed by the presence of these gases during growth. We have investigated the growth and morphology of Rh under ultrahigh vacuum (UHV) conditions and under the influence of CO using a thin, well-ordered Al 2O 3 film as substrate. First, the growth modes at UHV pressures were characterized with the help of spot profile analysis low energy electron diffraction (SPA-LEED) and scanning tunnelling microscopy at two substrate temperatures: 90 and 300 K. At 300 K, we observe the decoration of domain boundaries between different antiphase domains of the film. The particles are mainly disordered, only a small fraction is crystalline. Keeping the substrate temperature at 90 K, smaller (disordered) aggregates form, which are mostly located inside the domains. If the deposits are exposed to CO, in both cases structural changes are detectable: SPA-LEED results point to a spreading of the particles, resulting in an expansion of their surface area. Exposing the sample to CO during deposition, on the other hand, leads to the formation of a carbonyl-like species in the case of large amounts of Rh. Its decomposition gives rise to a characteristic peak in the thermal desorption spectrum.

Correlation Between Nanoscale Structural, Electronic, and Magnetic Properties of Thin Films by Scanning-Probe Microscopy and Spectroscopy

Heteroepitaxial pseudomorphic thin-film growth allows the stabilization of solids in structures—that is, with crystalline symmetries and lattice constants that are far from those of the corresponding bulk material. This facilitates the tailoring of materials properties that do not exist in nature. Usually a strong correlation is found between the atomic, electronic, and magnetic structure of thin films, which leads to a drastic dependence of their physical properties on the details of growth conditions such as substrate temperature, evaporation rate, and background pressure. To learn more about the fundamental relationship between structural, electronic, and magnetic properties of thin films as well as to improve the reproducibility of their physical properties in view of technical applications, it is mandatory to apply experimental techniques with sufficient, ultimately atomic spatial resolution combined with high sensitivity. The class of scanning-probe microscopies and spectroscopies is ideally suited for that purpose as schematically illustrated in Figure 1. Scanning tunneling spectroscopy (STS) establishes the correlation between atomic structure and local electronic properties, while spin-polarized scanning tunneling spectroscopy (SPSTS) relates local electronic and magnetic properties. Additionally the correlationbetween topographical and magnetic structure can be addressed by magnetic force microscopy (MFM) though the spatial resolution is limited to about 10–50 nm. Since magnetic thin films are usually sensitive to oxidation, their preparation and characterization have to be performed in situ under ultrahigh vacuum (UHV) conditions in order to achieve reproducible experimental conditions. While STS is now routinely applied in UHV, it has only been recently that the first MFM studies under UHV conditions have been reported. In the following, representative examples of the application of STS, SPSTS, and MFM in ultrathin Fe, Gd, and Co films will be presented.

Real-time diagnostics of II–VI molecular beam epitaxy by spectral ellipsometry

Spectral ellipsometry has been applied to in situ diagnostics of Hg1−xCdxTe molecular beam epitaxial growth. Spectral ellipsometry provides surface, composition, and film thickness properties of thin film electronic materials in real-time during growth and processing. This study is discussed in two parts. In the first part, the surface state properties of Hg1−xCdxTe and the substrate material Cd1−xZnxTe were studied under ultrahigh vacuum (UHV) conditions. By measuring the spectral ellipsometric quantities Ψ and Δ versus temperature under UHV conditions, the presence and desorption temperatures of surface species have been determined. Tellurium stabilized Cd1−xZnxTe surfaces were not observed to exhibit any apparent surface changes during heating under these conditions. Oxidized Cd1−xZnxTe surfaces, however, exhibit discontinuities in Ψ and Δ indica-tive of desorption of a surface species in the 300 °C range. Tellurium stabilized Hg1−xCdxTe surfaces also exhibit discontinuities in Ψ and Δ occurring at about 177 °C. In the second part of the study, dielectric functions at the growth temperature, 180 °C, were determined for Hg1−xCdxTe alloys and Cd0.96Zn0.04Te. Film compositions were monitored in situ and in real-time during Hg1−xCdxTe epitaxial growth based on this knowledge of growth temperature dielectric func-tions. An experimental analysis of measurement factors illustrates that compositional sensitivity for in situ Hg1−xCdxTe measurement is limited by uncertainties in the angle and plane of inci-dence. Simulations show an ultimate multiwavelength sensitivity of significantly better than ±0.0008 is possible for in situ composition measurements.

Focused ion-beam line profiles: A study of some factors affecting beam broadening

The current–density profile of a focused ion beam (FIB) has a central peak accompanied by broader ‘‘wings’’ that, while unimportant in lithographic applications, can lead to unwanted effects during an implantation operation. The origin of the wings, and hence the best way to minimize them, is not clear and needs further study. We have measured the line profiles of several of the ions available in our FIB machine as a function of a number of variables, under ultrahigh vacuum (UHV) conditions. No effects are observed from changes in emission current or deliberate defocusing of the objective lens. There are some changes with beam aperture and/or current, but the biggest differences seem to be associated with a change of source type and hence, possibly, with a change in the source/extractor configuration or in the alloy and the emission process. The wing amplitudes are appreciably lower than many previously observed, and their profiles, at least for the lighter ions studied (Be++, Be+, and B+), are Gaussian rather than exponential. It seems possible that our UHV conditions may have eliminated a scattering mechanism responsible for the larger, exponential wings previously observed. The corresponding beam and rectangle-edge profiles have been calculated.

Chemisorption of high coverages of atomic oxygen on the Pt(111), Pd(111), and Au(111) surfaces

Studies of the interaction of oxygen with some transition metal surfaces are limited by activation barriers to adsorption and reaction of O2. These barriers can be surmounted by using oxidants more powerful than O2 to chemisorb oxygen on these surfaces under ultrahigh vacuum (UHV) conditions, but at high ‘‘effective’’ pressures of O2. We report here on the use of very reactive molecules, such as nitrogen dioxide (NO2) and ozone (O3) to study the interaction of high concentrations of oxygen atoms on the Pt(111), Pd(111), and Au(111) single crystal surfaces. Chemisorbed oxygen adatom coverages of Θ0≤0.75 monolayer (ML) on Pt(111), Θ0≤1.4 ML on Pd(111), and Θ0≤0.80 ML on Au(111) surfaces have been characterized. On the Pd(111) surface, higher concentrations (Θ0≤3.1 ML) can be formed and the initial stages of oxidation, including migration of oxygen into the subsurface region and formation of Pd oxide can be studied. This method of cleanly forming high coverages of oxygen on metal surfaces allows for new insight into the nature of oxygen–metal interactions. For example, in the case of Pt(111) and Au(111), temperature programmed desorption (TPD) can be used to determine kinetic parameters of O2 adsorption and desorption as a function of oxygen coverage and upper limits for the metal–oxygen bond strengths over a large range of oxygen coverages can be determined from the activation energy of O2 desorption. Also, the kinetics of oxidation reactions can be studied over a much larger range of oxygen coverage than previously carried out under UHV conditions on surfaces that are relatively unreactive to O2 chemisorption, such as Pt(111) and Au(111). We report preliminary results concerning transient kinetic studies of CO oxidation over Pt(111) and Au(111) surfaces.

Electron-stimulated damage processes in oxides under ultra-high vacuum (UHV) conditions

HREM has recently entered the surface science arena as a complementary technique for studies of electron-stimulated damage of oxide surfaces. The primary advantage offered is the ability to observe in-situ structural changes of the surface, as has been exemplified in the literature for NiO, WO3, V2O5, and TiO2, among other oxide crystals. One major drawback has been the inability to perform experiments under UHV conditions comparable to those used in conventional surface science techniques. Previous HREM reports have not considered the effect of the microscope environment in surface radiation damage.In this study, we compared the surface radiation damage effects of oxides under UHV conditions (l0-10 Torr in a Hitachi UHV-H9000 instrument) to damage effects observed in conventional instruments (operating at approximately 10-7 Torr). A number of oxides were investigated, including NiO, V2O5 , WO3 , MoO3 , TiO2, and Ta2O5. Our results indicate that although the same basic damage mechanisms are operative under both vacuum conditions, subsequent surface alteration due to reoxidation or reaction with surface contamination occurs under non-UHV conditions. WO3, for example, underwent surface-initiated reduction to metallic W under UHV conditions (Fig. 1, 2 ).

Angular distribution of Ag atoms sputtered with A 5 keV Ar + ion beam

A polycrystalline, high-purity Ag target was sputtered with a normally incident, 5 keV Ar + ion beam under ultrahigh vacuum (UHV) conditions. Sputtered material was collected on a cylindrically shaped Al foil and subsequently analysed in an Auger electron spectrometer. The obtained results show that sputtering under UHV conditions leads to a cosine-like distribution. The distribution taken after prolonged ion bombardment contained a hump at angles of 30–45° superimposed on the general form which we ascribe to an ion beam-induced texturing effect. The results are compared with recent investigations of other authors. The influence of the surface conditions of both the target and the foil used for collection of the sputtered material on the angular distribution measurements has also been investigated.