High resolution soft x-ray photoelectron spectroscopy with synchrotron radiation is used to study the interfaces of SiO 2 /Si(111), SiO 2 /Si(100), Si͑111͒/Si 3 N 4 , and SiO 2 /Si 3 N 4 for device-quality ultrathin gate oxides and nitrides. The thin oxides and nitrides were grown by remote plasma deposition at a temperature of 300°C. Aftergrowth samples were further processed by rapid thermal annealing for 30 s at various temperatures from 700 to 950°C. The Si͑111͒/Si 3 N 4 samples were air exposed and formed a thin ϳ6 Å SiO 2 layer with a Si(2p) core-level shift of 3.9 eV, thus allowing us to study both the Si͑111͒/Si 3 N 4 and SiO 2 /Si 3 N 4 interfaces with a single type of sample. We obtain band offsets of 4.54Ϯ0.06 eV for SiO 2 /Si(111) and 4.35Ϯ0.06 eV for SiO 2 /Si(100) with film thicknesses in the range 8-12 Å. The Si͑111͒/Si 3 N 4 nitrides show 1.78Ϯ0.09 eV valence-band offset for 15-21 Å films. This value agrees using the additivity relationship with our independent photoemission measurements of the nitride-oxide valence-band offset of 2.66Ϯ0.14 eV. However, we measure a substantially larger SiO 2 /Si 3 N 4 ⌬E V value of 3.05 eV for thicker ͑ϳ60 Å͒ films, and this indicates substantial differences in core-hole screening for films of different thickness due to additional silicon substrate screening in the thinner ͑15-21 Å͒ films.

Muscovite mica is an important mineral that has become a standard substrate, due to its easy cleavage along the {001} planes, revealing a very flat surface that is compatible with many biological materials. Here we study mica surfaces by dynamic atomic force microscopy (AFM) operated in the non-contact mode (NC-AFM) under ultra-high vacuum (UHV) conditions. Surfaces produced by cleaving in UHV cannot be imaged with NC-AFM due to large surface charges; however, cleavage in air yields much less surface charge and allows for NC-AFM imaging. We present highly resolved NC-AFM images of air-cleaved mica surfaces revealing a rough morphology originating from a high density of nanometre-sized particles. Among these particles, we find regularly shaped structures indicating the growth of crystallites on the surface. The contamination layer cannot be removed by degassing in UHV; even prolonged heating at a temperature of 560 K under UHV conditions does not yield an atomically flat surface.

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Air-cleaved mica surfaces exhibit a high density of nanometer or micrometer size particles that have been ascribed to potassium carbonate formed as a reaction product of carbonaceous gases with potassium ions. Unambiguous evidence for this assignment has, however, never been presented. We study air-cleaved mica surfaces by high-resolution noncontact atomic force microscopy (NC-AFM) in ultrahigh vacuum to reveal the detailed structure of such precipitates on the surface. Among a large number of irregularly shaped surface structures, we find f lat, hexagonally shaped islands exhibiting two different patterns on their surfaces, namely a rectangular atomic corrugation pattern and a hexagonal moir e structure. The unit cell of the rectangular pattern corresponds to the dimensions of the potassium carbonate bulk structure and is found on high crystallites. The moir e structure solely appears on very f lat islands and is caused by the interference of the potassium carbonate lattice periodicity and the lattice periodicity of the underlying mica substrate. Both results strongly point to the presence of potassium carbonate crystallites on air-cleaved mica surfaces.

Device-grade ultrathin ͑9-22 Å͒ films of silicon dioxide, prepared from crystalline silicon by remote-plasma oxidation, are studied by soft x-ray photoelectron spectroscopy ͑SXPS͒. The 2p core-level spectra for silicon show evidence of five distinct states of Si, attributable to the five oxidation states of silicon between Si 0 ͑the Si substrate͒ and Si 4ϩ ͑the thin SiO 2 film͒. The relative binding energy shifts for peaks Si 1ϩ through Si 4ϩ ͑with respect to Si 0 ͒ are in agreement with earlier work. The relatively weaker signals found for the three intermediate states (I 1 , I 2 , and I 3 ͒ are attributed to silicon atoms at the abrupt interface between the thin SiO 2 film and substrate. Estimates of the interface state density from these interface signals agree with the values reported earlier of ϳ2 monolayers ͑ML͒. The position and intensity of the five peaks are measured as a function of post-growth annealing temperature, crystal orientation, and exposure to He/N 2 plasma. We find that annealing produces more abrupt interfaces ͑by reducing the suboxide interface state density͒, but never more abrupt than ϳ1.5 monolayers. We observe a 15%-20% drop in the interface thickness ͑in particular the ''Si 2ϩ '' peak intensity͒ with increasing annealing temperature. Somewhat different behavior is observed with small amounts of nitrogen in the SiO 2 film where an apparent increase in interface state density is seen. A quantitative analysis is presented which explores the effects of these sample preparation parameters in terms of relative interface state density and modeling of the SXPS data.

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Electron-stimulated desorption of the (100)KBr surface has been investigated in vacuum with noncontact atomic force microscopy and mass spectroscopy. It has been found that both desorption components (K and Br) show oscillatory dependence on the electron dose with the oscillation amplitude decaying gradually. These results correspond with periodically varying, as a result of a layer-by-layer desorption, surface topography. It is proposed that the surface terrace edges act as traps for excited F centers diffusing in the crystal. The oscillating density of terrace edges varies surface recombination/reflection rates for the F centers and modulates the balance between surface and bulk deexcitation of the crystal.

We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with {211} faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W ''float'' on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd͞W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C 2H2 over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H 2 from C2H2 dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 ؋ 4)͞W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 ؋ 4) periodicity, having broad planar terraces (Ϸ30 nm in width). Fractional monolayers of vapordeposited Cu grow as threedimensional clusters on the S(4 ؋ 4) surface over a wide coverage range. At low Cu coverage (< 0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.A n important issue in surface chemistry and catalysis is how surface structures and features with nanometer dimensions affect reactivity in heterogeneous systems (1-3). The focus of our work has been on several aspects of nanoscale phenomena that influence surface chemistry, including faceting of metallic and model bimetallic catalyst surfaces, and nucleation of subnanometer metallic clusters on sulfided surfaces. We study atomically rough substrates [bcc (111) surface of W, and fcc (210) surface of Ir] that are morphologically unstable, that is, the initially planar substrate becomes covered with nanoscale facets when covered with monolayer films of gases or other metals, and heated to elevated temperature. Major objectives of this work have been (i) to determine how the surface transition from planar to faceted affects the surface reactivity of metallic and bimetallic systems, and (ii) to characterize the nucleation and growth of metals on sulfided W surfaces. The three main components of this effort are surface structure, surface chemistry, and surface electronic properties.The importance of bimetallic catalysts based on Pt-group metals has been increasing in recent decades (4, 5). These catalysts display important advantages over classical reforming catalysts, including better stability, as well as improved activity and selectivity. In particular, refractory metals (W, Mo, Re, . . .) in combination with ...