Initial oxidation of Ni(1 1 1) observed by electron emission microscopy: PEEM and MEEM

The initial oxidation of Ni(1 1 1) in the temperature range of 550-700 K has been monitored by photoelectron emission microscopy (PEEM) and metastable-atom electron emission microscopy (MEEM). The PEEM and MEEM images show uniform patterns for the chemisorbed overlayer, reflecting the electronic homogeneity as seen at the μm scale. During the nucleation and lateral growth of oxide, however, the μm-scale pattern due to the formation of oxide domains appears and its evolution depends strongly on the substrate temperature and dose pressure of gaseous O₂. Our data indicate that the high-temperature oxidation is regarded as a successive multi-nucleation process in a reaction-diffusion field.     

Ultrasmall Ge islands with low diameter-to-height aspect ratio on Si(1 0 0)-(2 × 1) surfaces

Scanning tunneling microscopy (STM) and high resolution cross-sectional transmission electron microscopy (XTEM) studies have been used to investigate the formation of Ge nanocrystals grown on Si(1 0 0)-(2 × 1) surfaces by molecular beam epitaxy (MBE). We observe relatively high density of Ge islands where small ‘pyramids’, small ‘domes’ and facetted ‘domes’ of various sizes co-exist in the film. As revealed from XTEM images, a large fraction of islands, especially dome-shaped Ge islands have been found to have an aspect ratio of ∼1 (diameter):1 (height). Observation of truncated-sphere-shaped Ge islands with a narrow neck contact with the wetting layer is reported.     

RHEED wave functions and the effect of reconstruction on secondary electron emission from the Si(1 0 0) surface

The effect of surface reconstruction on contrast in scanning electron microscopy of the Si(1 0 0)-2 × 1 surface is investigated. A theory of the initial secondary production rate is developed and an upper bound on the rate is shown to depend on the product of the integrated intensities of the initial and final RHEED states. These states are calculated with a reflection matrix method and their depth dependence is investigated. The results are used to analyse scanning electron microscopy contrast in images of 1 × 2- and 2 × 1-regions of the Si(1 0 0)-2 × 1 surface reported by Watanabe et al. The calculated integrated intensities are consistent with the experimental images and with the experimentally observed dependence of the contrast on the azimuth of the incident electron beam. This supports the idea that the observed contrast is caused by the effect of surface reconstruction on the RHEED states.     

Morphology and electronic structure of bcc Co(1 1 0) and fcc/hcp Co(1 1 1) on Fe(1 1 0) investigated by STM and STS

We report on the growth of ultrathin epitaxial Co films on Fe(1 1 0) examined by scanning tunneling microscopy and spectroscopy (STM and STS). At room temperature Co forms pseudomorphic, ideally ordered body-centered cubic (bcc) layers for the first two monolayers as confirmed by atomically resolved STM images. This is in contrast to the related case of Co/Cr(1 1 0) where a superstructure occurs in the second layer. The third monolayer forms a close-packed structure and causes a transformation of the buried second monolayer into a close-packed structure. The Fe(1 1 0) substrate strongly influences the electronic structure of the first Co monolayer as concluded from the dI/dU spectra. This influence is less important for the second monolayer. The measured local density-of-states function for the bcc Co double layer is in agreement with theoretical predictions for bcc Co.     

Ag induced restructuring of the oxygen precovered Cu(1 1 0) surface

The interplay between chemisorbed oxygen and deposited Ag on the Cu(1 1 0) surface has been studied by scanning tunneling microscopy (STM) and photoelectron emission microscopy (PEEM). The Cu-CuO stripe phase formed on the clean Cu(1 1 0) surface upon oxygen chemisorption at 660 K is partly dissolved by Ag deposition at 300 K. Upon annealing, however, a phase separation is observed, where the Cu-O compounds agglomerate into large CuO islands and the Ag is located in between. Also a strong preference for the Ag to attach to step bunches is observed. Especially on the fully (2×1)O reconstructed Cu(1 1 0) surface, all the deposited Ag is found at the step bunches giving rise to a contrast in PEEM.     

Epitaxial C60 thin films on Bi(0 0 0 1)

We have used the Bi(0 0 0 1)/Si(1 1 1) template to grow highly ordered C₆₀ epitaxial thin films and analyzed them using scanning tunneling microscopy and low-energy electron microscopy. The in situ low-energy electron microscope investigations show that the initial nucleation of the C₆₀ islands on the surface takes place at surface defects, such as domain boundaries and multiple steps. The in-plane lattice parameters of this C₆₀ film turns out to be the same as that of the bulk fcc(1 1 1) C₆₀. The line-on-line epitaxial structure is realized in spite of a weak interaction between the C₆₀ molecules and Bi(0 0 0 1) surface, while scanning tunneling spectroscopy indicates that there is a negligible charge transfer between the molecules and the surface.     

Anti-ferromagnetic contrast in NiO (0 0 1) studied with threshold photoemission electron microscopy

 A detailed study is presented of the potential of threshold photoemission electron microscopy (PEEM) for the imaging of anti-ferromagnetic (AF) domains of NiO (0 0 1). Characteristic patterns with large asymmetry have been observed experimentally. Upon heating the sample to temperatures significantly above the Néel temperature, the patterns clearly remain visible and a magnetic origin can therefore be excluded. The patterns probably originate in polishing damage, with the bright areas corresponding to areas with a high oxygen deficiency (with enhanced electron emission). After sputter cleaning the sample, thereby removing the dominant patterns, no significant asymmetries in electron emission remain. Obviously, for our samples the AF asymmetries as measured with threshold PEEM are less than the detection limits of our setup of 0.5%. The conclusion is supported by model calculations, which show that the asymmetry should have a distinct angular dependence and which give an estimate of the maximum asymmetry of below 1%.     

A combined in situ AFM and SEM study of the interaction between celestite (0 0 1) surfaces and carbonate-bearing aqueous solutions

In this paper, we present in situ atomic force microscopy (AFM) observations of the interaction between celestite (SrSO₄) (0 0 1) surfaces and Na₂CO₃ aqueous solutions. The observations indicate that the interaction is characterized by a rapid alteration (carbonatation) and dissolution of the original surface, shortly followed by the formation of a new phase. EDX analyses indicate that the new phase is strontianite (SrCO₃). Its crystallization involves the formation and spreading of islands of about 2.75 nm in height, which chiefly occurs on the step edges of the dissolving celestite substrate. The thickness of the islands remains almost constant during their spreading, which occurs mainly parallel to the celestite [0 1 0] direction. As a result of the progressive coalescence of the islands, a fairly homogeneous epitaxial layer forms on the celestite (0 0 1) face. At the initial stages, the formation of islands on the celestite (0 0 1) faces enhances dissolution, indicating the existence of a coupling between dissolution and crystallization reactions. Our measurements on series of AFM images provided quantitative information about coupled dissolution-growth rates on a nanoscale. The effect of the coupled reactions on the celestite (0 0 1) surface on a microscopic scale was also studied by scanning electron microscopy (SEM).     

Initial oxidation kinetics of copper (1 1 0) film investigated by in situ UHV-TEM

Previous studies of the initial stage of oxidation on clean single crystal of Cu(1 0 0) have been extended to the case of the Cu(1 1 0) surface. The dynamic observation of the nucleation and growth of Cu oxide by means of in situ ultra high vacuum transmission electron microscopy (UHV-TEM) shows a highly enhanced oxidation rate on Cu(1 1 0) surface as compared to Cu(1 0 0). The kinetic data on the nucleation and growth of the three-dimensional oxide islands agree well with our heteroepitaxial model of surface diffusion of oxygen.     

Photoemission electron microscopy study of anthracene growth on Si(1 1 1)

The thin film growth of anthracene films on Si(1 1 1) surfaces is studied by photoemission electron microscopy (PEEM). The thin film growth of anthracene on Si(1 1 1) is similar to the growth of pentacene on silicon. Initially a layer of flat lying molecules chemisorbs on the surface. Subsequent growth of fractal islands with standing up molecules proceeds on top of this flat layer.     

Interaction of cesium with charged oxide under UV irradiation imaged by photoemission electron microscopy

We report the first observation of electron transfer from charged SiO₂/Si(1 0 0) by ion-implantation via internal photoemission from Si by photoemission electron microscopy (PEEM) for the purpose of the microscopic control of promotion of catalyst by electron transfer from oxide support. The contrast of the PEEM image varies with the amount and kind of the implanted ion and the deposition of Cs through the formation of electrical double layer consisting of Cs⁺ and trapped electrons at trapping centers created by the implantation. It is then firmly established that oxide charging can be microscopically tuned by ion-implantation.     

Spatio-temporal dynamics of the NO + NH3 reaction on polycrystalline platinum

Spontaneous reaction rate oscillations and spatio-temporal patterns have been observed by mass spectrometry and photoemission electron microscopy (PEEM) during the reduction of NO by NH₃ on polycrystalline platinum at 1 × 10⁻⁴ Torr and temperatures from 460-520 K. The appearance of both oscillations and patterns was found to be strongly dependent on the gas phase composition and the temperature. In addition, the overall dynamics of the catalyst were found to be dominated by the nonlinear behavior of Pt(1 0 0) type grains, while other types of grains did not participate. In contrast to previous studies, a large number of complex multimodal oscillations were observed, particularly as the coupling between the surface and the gas phase was increased. The appearance of these complex oscillations demonstrates the importance of gas phase coupling to understanding catalytic reactions, even in high vacuum systems.     

Two-dimensional emission patterns of secondary electrons from graphene layers formed on SiC(0 0 0 1)

We used spectroscopic photoemission and low-energy electron microscopy to measure two-dimensional (2D) emission patterns of secondary electrons (SEs) emitted from graphene layers formed on SiC(0 0 0 1). The 2D SE patterns measured at the SE energies of 0-50 eV show energy-dependent intensity distributions in the 6-fold symmetry. The SE patterns exhibit features ascribed to energy band structures of 2D free electrons, which would prove that electrons are partially confined in thin graphene layers even above the vacuum level.     

Formation of single layer h-BN on Pd(1 1 1)

The structure of hexagonal boron nitride (h-BN) on Pd(1 1 1) was studied with low energy electron diffraction (LEED), photoelectron spectroscopy and scanning tunnelling microscopy (STM). h-BN forms flat monolayers on the Pd(1 1 1) surface in contrast to Rh(1 1 1) where a complex self-assembled double layer structure, the nanomesh [M. Corso, W. Auwärter, M. Muntwiler, A. Tamai, T. Greber, J. Osterwalder, Science 303 (2004) 217], appears. The LEED patterns reveal a dominating 10 × 10 h-BN superstructure, with a second, distinct structure rotated by 30° and further azimuthally randomly oriented h-BN overlayers. This is consistent with STM images which show several different Moiré patterns associated with different rotation angles of the overlayer. Additionally the use of thin Pd(1 1 1) films instead of single crystal substrates was studied. No significant differences in the h-BN film quality were found.     

Non-contact atomic force microscopy studies of (2 × 4) InP(0 0 1) surface

A sputter-cleaned indium-rich (2 × 4) InP(0 0 1) surface was investigated by non-contact scanning atomic force microscopy (NCAFM). Atomically-resolved images of the surface exhibit two different patterns. The patterns can be interpreted within the mixed dimer model of (2 × 4) reconstructed InP(0 0 1) surface. It is shown that due to contrast formation mechanism in NCAFM the features resolved are in close correspondence to scanning tunnelling microscopy (STM) data. Due to chemical interaction a P-terminated tip gives the image similar to an empty-state STM image, whereas an In-terminated tip gives the image resembling a filled-state STM one. Moreover, it is shown that due to dipole-dipole interaction, NCAFM can be sensitive to orientation of In-P dimers.     

Morphology of Ni ultrathin films on Mo(1 1 0) and W(1 0 0) studied by LEED and STM

The morphology of ultrathin Ni films on Mo(1 1 0) and W(1 0 0) has been studied by low-energy electron diffraction and scanning tunneling microscopy. Ni films grow pseudomorphically on Mo(1 1 0) at 300 K for a coverage of 0.15 ML. A (8 × 1) structure is found at 0.4 ML, which develops into a (7 × 1) structure by 0.8 ML. The film undergoes a structural change to fcc Ni(1 1 1) at 6 ML. The growth mode switches from layer-by-layer to Stranski-Krastanov between 4 ML and 6 ML. Annealing at around 850 K results in alloying of submonolayer films with the substrate, while for higher coverages the Ni agglomerates into nanowedge islands. Ni films grow pseudomorphically on W(1 0 0) up to a coverage of around 2 ML at 300 K, above which there is a structural change from bcc to hcp Ni with the epitaxial relationship . This is accompanied by the formation of orthogonal domains of uniaxial strain-relieving dislocations from the third layer of the film. For coverages up to 1 ML the growth proceeds by formation of two-dimensional islands, but shifts to three-dimensional growth by 2 ML with rectangular islands aligned along the 〈0 1 1〉 substrate directions. Annealing at around 550 K results in agglomeration of Ni into larger islands and increasing film roughness.     

Variety of iron silicides grown on Si(0 0 1) surfaces by solid phase epitaxy: Schematic phase diagram

We systematically studied the formation of various iron-silicide phases, grown on Si(0 0 1) surfaces by solid phase epitaxy, with scanning tunneling microscopy, low-energy electron diffraction and reflection high-energy electron diffraction. We found and studied the phases of c(2 × 2) islands, rectangle-like islands, elongated islands, layered islands, dome-like islands, eddy and cracked structures, and small clusters. A schematic phase diagram of these phases is successfully summarized against iron coverage at room temperature and subsequent annealing temperature.     

Sulfur-induced mobilization of Au surface atoms on Au(1 1 1) studied by real-time STM

The interaction of sulfur with gold surfaces has attracted considerable interest due to numerous technological applications such as the formation of self-assembled monolayers and as a chemical sensor. Here, we report on the interaction of sulfur with Au(1 1 1) at two different temperatures (300 K and 420 K) studied by real-time scanning tunnelling microscopy, low energy electron diffraction and Auger electron spectroscopy. In the low coverage regime (<0.1 ML), S adsorption lifts the herringbone reconstruction of the clean Au(1 1 1) surface indicating a lateral expansion of the surface layer. An ordered (√3 × √3)R30° sulfur adlayer develops as the coverage reaches ∼0.3 ML. At higher S coverages (>0.3 ML) gold surface atoms are removed from regular terrace sites and incorporated into a growing gold sulfide phase. At 300 K this process leads to the formation of a rough pit and mound surface morphology. This gold sulfide exhibits short-range order and an incommensurate, long-range ordered AuS phase develops upon annealing at 450-525 K. In contrast, formation of an ordered AuS phase via rapid step-retraction rather than etch pit formation is observed during S-interaction with Au(1 1 1) surfaces at 420 K. Our results shed new light on the S-Au(1 1 1) interaction.     

Phase defect inspection of multilayer masks for 13.5 nm optical lithography using PEEM in a standing wave mode

We report on recent developments of an “at wavelength” full-field imaging technique for defect inspection of multilayer mask blanks for extreme ultraviolet lithography (EUVL). Our approach uses photoemission electron microscopy (PEEM) in a near normal incidence mode at 13.5 nm wavelength to image the photoemission induced by the EUV wave field on the multilayer blank surface. We analyze buried defects on Mo/Si multilayer samples down to a lateral size of 50 nm and report on first results obtained from a six inches mask blank prototype as prerequisite for industrial usage.