C.L. Nicklin

Interface structure of Si(111)-(√3 × √3)R30°-ErSi2 − x

The interface structure of epitaxial ErSi2 − x (x ≈ 0.3) on Si(111) has been measured by means of surface X-ray diffraction. The silicide consists of a stack of alternating Si and Er planes. The Si planes resemble compressed substrate bilayers, in which a regular network of vacancies releases the compressive strain. This vacancy network gives rise to a (√3 × √3)R30° reconstruction of the silicide, in which Si atoms are displaced towards the vacancies, and Er atoms are displaced away from the vacancies. The Si-Si bond length in the silicide is 2.36±0.02 A, and the nearest-neighbour Erue5f8Si distance is 2.95±0.02 A. The positions of the ato the interface region are identical to those found for two-dimensional erbium silicide on Si(111), and first-layer Er atoms are located on T4 sites on the substrate. Our preparation method (reactive deposition epitaxy) leads to rough silicide films. Nevertheless, we are able to extract the atomic occupancies of eight individual layers. All silicide layers are reconstructed, but only the silicide layer at the interface shows long-range order of the vacancy network.

Electronic properties of low-dimensional Sm films adsorbed on Cr(211) and Cr(110)

Abstract The electronic properties of samarium layers adsorbed on Cr(211) and Cr(110) surfaces have been studied by XPS and EELS measurements and correlated to previous structural investigations by LEED and AES. The valence state of samarium has been discussed in terms of substrate-adsorbate interactions. For strong Cr-Sm interactions (ordered adlayer structures in registry with the substrate or disordered adsorption in the grooves of the Cr(211) face) only the trivalent Sm state is observed. When Sm-Sm interactions are dominant (dense disordered or incommensurate layers) a mixed-valence character appears and an intermediate value of 2.75 ± 0.05 is observed for the dense monolayer. Preliminary photodiffraction experiments on Cr(12) suggest that the intermediate valence observed at monolayer coverage could be homogeneous.

An X-ray diffraction study of oxide removal from InSb(001) substrates

Abstract A study of the removal of the native oxide from InSb(001) substrates using X-ray diffraction and Auger electron spectroscopy is reported. Several methods have been investigated to produce atomically flat, oxide free, surfaces. These include thermal annealing, argon ion bombardment at both room temperature and elevated temperature, and irradiation of the surface with atomic hydrogen. The quality of the resulting c(8 × 2) surface gave a good indication of the relative success of each technique. The reflected X-ray intensity was measured as a function of perpendicular momentum transfer l along the specular (00 l ) rod and gives a clear indication of the roughness of each surface. The lateral order was determined from the width of the in-plane fractional order reflections. The results show a marked improvement in surface order when using hydrogen irradiation/annealing as opposed to thermal annealing alone. A more significant improvement in surface quality, however, was noted when sputtering at elevated temperature.

An ultrahigh-vacuum chamber for surface X-ray diffraction

An ultrahigh-vacuum environmental chamber for surface X-ray diffraction on Station 9.4 at the Synchrotron Radiation Source, Daresbury Laboratory, is described. Film growth can be monitored by simultaneously recording the Auger signal and the X-ray intensity at a particular point in reciprocal space. Such in situ measurements are essential for understanding the dynamic processes that occur during adsorption. An example is given in which the specularly reflected X-ray signal is correlated with Auger plots, during growth of Tl on Cu(001). In addition, the diffractometer and chamber combination allow large reconstructions to be investigated as shown by the in-plane structural analysis of the c(4x4) InSb surface. A study of the layer structure of Cr on Ag(001), in which an extended out-of-plane detector assembly was used, is also presented.

Atomic structure of the InSb(001)-c(4 × 4) reconstruction determined by X-ray diffraction

Abstract We report a surface X-ray diffraction study of the indium-rich InSb(001)-c(8×2) reconstruction, which gives for the first time a detailed picture of the atomic structure. A total of 96 in-plane ( hk ) reflections were used to determine the structure in the surface plane. The out-of-plane atomic positions were found by measuring along three crystal truncation rods (CTRs) where the total intensity distribution is the result of interference between bulk and surface contributions. The detailed analysis disproves the missing dimer model proposed from earlier PES and STM measurements. It shows that the reconstruction is deeper than the Sb-rich c(4×4) and is composed of chains of In atoms running along the [110] axis, separated by pairs of Sb dimers on top of the Sb-terminated bulk. The proposed structure is consistent with recent STM images and represents a significant departure from the models previously suggested for the c(8×2) reconstruction on any of the III–V (001) surfaces.

Oxygen modified growth of Gd on Mo(110)

X-ray diffraction measurements have been made in real time during deposition of the rare earth Gd on the non-magnetic substrate Mo(110). For the clean surface at 300 K, the variation of the specularly reflected X-ray intensity with deposition time is consistent with the completion of a close-packed monolayer followed by increasingly disordered, multilayer, growth. Exposure of the Mo surface to 0.25 langmuir of oxygen prior to Gd deposition produced better defined intensity oscillations characteristic of improved layerwise growth. It is suggested that the oxygen creates nucleation sites which increases interlayer mass transport.

X-Ray Diffraction Studies of the InSb(001) Surface

The (001) surface of the III-V alloy semiconductors exhibits many reconstructions with large unit meshes. The appearance of a given reconstruction depends on the processing conditions and, in particular on the stoichiometry at the surface. A surface X-ray diffraction study of the atomic structure of the Sb-rich c(4×4) and the In-rich c(8×2) surfaces is described which allows for the first time, a detailed atomic model of these important reconstructions to be derived. The basic units consist of groups of Sb dimers and rows of In atoms on top of an Sb terminated bulk. The local bonding is similar to that found in rhombahedral Sb and tetragonal In bulk structures. The results explain why the Sb rich surface is used as a starting point for the growth and are relevant to other III-V (001) surfaces.

Valence state of low-dimensional thulium structures grown on molybdenum (110)

Abstract Valence band photoemission spectra of well-defined structures of the rare earth Tm deposited onto a Mo(110) surface have been measured using synchrotron radiation. The results correlate with the overlayer structure; mixed valence is observed at low coverages, in areas of island formation and beyond one monolayer. The presence of divalent features is associated with low coordinated sites, which provide insufficient transfer of energy to promote a 4f electron into the valence 5d6s states. This is consistent with theoretical predictions.

STRUCTURAL STUDY OF TM ON MO(110)

Abstract Rare earth metals and alloys have been the focus of considerable interest as a consequence of their unusual electronic and structural behaviour, associated with partially filled and highly localised 4f states. Adsorbed layers of RE metals on non-alloying substrates present a valuable opportunity for studying the effects of strong atomic-like correlations perturbed by the presence of a non-localised conduction band, in well-defined atomic configurations. These include intermediate or mixed valence and heavy fermion behaviour. We report an initial study of the growth and atomic structure of Tm adsorbed on Mo(110), at room temperature, using Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), work function changes (Δφ) and secondary electron emission crystal current (SEECC). After initial formation of a two-dimensional overlayer gas, an ordered arrangement with a 10 × 2 unit mesh is seen at 0.6 monolayer coverage. At one monolayer the structure is hexagonal; it persists up to three monolayers effective coverage. This behaviour is similar to that observed for Yb on this surface.