C. Norris

X-ray diffraction from rough, relaxed and reconstructed surfaces

A general formalism is presented for computing the scattered X-ray intensity from surfaces showing disorder in the form of atomic-scale roughness or in the form of reconstructed domains of finite size. Special attention is paid to the interference between bulk and surface contributions, since this is important in the determination of relaxation parameters or of the registry of a surface unit cell.

The influence of Sb as a surfactant on the strain relaxation of Ge/Si(001)

We have investigated by the use of surface x‐ray diffraction the initial strain relaxation of Ge on Si(001) when a ‘‘surfactant’’ layer of Sb [0.7 monolayers (ML)] is present. Due to the high sensitivity of the technique to lateral strain in the overlayer, we have been able to observe directly the onset of strain relaxation at a coverage of 9–10 ML. This strain relief proceeds gradually as a function of coverage, but unlike the case without an Sb surfactant, it was not possible to relax the overlayer fully. No bulklike Ge was seen even up to a coverage of ≊55 ML. Concurrent specular reflectivity measurements also showed that the overlayer formed in a layer‐by‐layer mode up to the same level. These results differ quite markedly from those obtained without the surfactant layer, where several stages of island formation and strain relaxation are seen. They show dramatically how the modification of surface energies by the presence of a surfactant can affect the morphology of, and strain in, a lattice mismatched overlayer. In addition, they are direct confirmation that the initial strain relaxation mechanism in operation when a surfactant is not present is the formation of coherent Ge islands, rather than that of dislocations.

The growth of indium on the Si(111) surface studied by X-ray reflectivity and Auger electron spectroscopy

Abstract The growth of indium on the 7 × 7 reconstructed (111) surface of silicon has been studied as a function of substrate temperature by X-ray reflectivity and Auger electron spectroscopy. A correlation is seen between features in the X-ray reflectivity curves and breaks in the Auger signal versus time plot. The oscillation of the X-ray signal indicates that below 400°C two consecutive pseudomorphic indium layers are formed before three-dimensional islanding occurs. Kinematic analysis predicts the shape of the X-ray signal versus time curve and yields the perpendicular displacement of the indium layers from the substrate. It is proposed that the first layer of indium atoms are bonded vertically above the top most layer of silicon atoms in T1 sites.

Design and performance of a focused beam line for surface x‐ray diffraction

Focusing optics have been installed on the 5.0 T Wiggler beam line of the SRS at Daresbury Laboratory for use with x‐ray diffraction measurements of surfaces and interfaces. A significant increase in the flux has been achieved without excessive degradation of the resolution in the vertical plane. The full width at half‐maximum of the focused image compares well with the predictions of ray tracing analysis.

Strain relaxation during the surfactant modified epitaxial growth of Ge/Si(001)

The initial strain relaxation of Ge on Si(001) has been investigated during epitaxial growth modified by a “surfactant” layer of Sb. Grazing-incidence X-ray diffraction was used to measure the strain relaxation due to its high sensitivity to the in-plane distribution of lattice spacings. We have observed the critical thickness for strain relaxation in the Ge overlayer to be ~ 11 monolayers (ML), with further relaxation developing gradually and in stages. A meta-stable, partially relaxed layer forms up to a coverage of ~ 30 ML, after which a more pronounced relaxation takes place. Even at a coverage of ~ 55 ML, complete strain relief has not been reached, and was only achieved after a 700°C thermal anneal. Concurrent specular reflectivity measurements also reveal that the Ge overlayer grows in a layer-by-layer fashion, and not in the Stranski-Krastanow mode expected for the Ge/Si system. These data show dramatically how the modification of surface energies through the presence of a “surfactant” can affect the morphology of, and strain in, a lattice-mismatched heteroepitaxial system.

The growth and atomic structure of the Si(1 1 1)-indium interface studied by surface X-ray diffraction

Surface X-ray diffraction has been used to monitor the growth of indium on silicon (1 1 1) as a function of temperature and to determine the atomic structure of the Si(1 1 1) 4 × 1-In reconstruction. The results indicate there are four indium atoms per 4 × 1 unit mesh with an average near neighbour separation which is reduced from that of the indium bulk.

High-resolution photoemission study of Peierls instability in thallium chains on copper(100)

The E-k dispersion of the 6p derived valence band of Tl chains adsorbed on Cu(100) has been measured as a function of density and temperature using high-resolution angle-resolved photoelectron spectroscopy. The most dense chain structure, at a coverage of theta =0.67 undergoes a Peierls distortion with a corresponding band gap at the Fermi level estimated to be 0.25 eV. At temperatures below about 225 K, LEED shows two-dimensional order with an alignment of the distortion between neighbouring chains. The measured dispersion at room temperature indicates that the overlayer still retains a distorted chain structure and the observed loss of overlayer spots in the LEED pattern is due to the loss of inter-chain registry. At the lower coverage, theta =0.6, a less dense chain structure occurs which shows no evidence for a Peierls distortion. The suppression of any superstructure is ascribed to the greater commensurability between the overlayer and the substrate.

Photoemission from liquid germanium and silver-germanium alloys

Abstract Low-energy photoemission spectra for liquid and solid germanium are presented and discussed in terms of the structural changes that accompany the crystalline-amorphous and crystalline-liquid transitions. The derived optical density of states of liquid germanium is compared with the results of theoretical calculations. Photoelectron spectra of liquid silver-germanium alloys are also presented and discussed with reference to structural properties and models for glass formation.

The growth of Sm on Mo(110) studied by surface X-ray diffraction

Abstract X-ray diffraction measurements have been made in real time during the deposition of the rare earth Sm on the non-alloying substrate Mo(110). For growth on 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 trivalent monolayer with an atomic density of 96% of a (0001) layer in bulk Sm. The second layer has a separation which agrees, within error, with the formation of a divalent layer on top of the trivalent first layer. Further growth is suggested to be increasingly disordered with the formation of multilayer structures.

X-ray scattering from a vicinal Ge(001) surface

High-resolution X-ray diffraction measurements have been made on a Ge(001) surface that had been misoriented by 1.3 degrees . It is shown that the atomic steps formed on the surface are mainly bilayer in height and that the surface reconstructions have domain sizes that are limited by the terrace length between the steps, with both (2*1) and (1*2) domains present.