F. Salvan

Catalytic action of gold atoms on the oxidation of Si(111) surfaces

Auger spectroscopy, electron energy loss spectroscopy and ion depth profiling techniques, under ultra high vacuum conditions, have been used in a comparative study of the oxidation of clean and gold precovered silicon (111) surfaces. Exposure of a Si surface covered by a few Au monolayers to an oxygen partial pressure induces the formation of SiO4 tetrahedra even at room temperature. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the well known formation of a chemisorbed oxygen monolayer. In the case of the Au covered surfaces, the enhancement of the oxide growth is attributed to the presence of an AuSi alloy where the hybridization state of silicon atoms is modified as compared to bulk silicon. This Au catalytic action has been investigated with various parameters as the substrate temperature, oxygen partial pressure and Au coverage. The conclusions are two fold. At low temperature (T < 400°C), gold atoms enhance considerably the oxidation process. SiO4 tetrahedra are readily formed even at room temperature. Nevertheless, the SiO2 thickness saturates at about one monolayer, this effect being attributed to the lack of Si atoms alloyed with gold in the reaction area. By increasing the temperature (from 20°C to ∼400°C), silicon diffusion towards the surface is promoted and a thicker SiO2 layer can be grown on top of the substrate. In the case of the oxidation performed at temperature higher than 400°C, the results are similar to the one obtained on a clean surface. At these temperatures, the metallic film agglomerates into tridimensional crystallites on top of a very thin AuSi alloyed layer. The fact that the latter has no influence on the oxidation is attributed to the different local arrangement of atoms at the sample surface.

7 × 7 Si(111)Cu interfaces: Combined LEED, AES and EELS measurements

Abstract CuSi(111) interfaces have been examined as a function of substrate temperature and deposition time using different surface sensitive techniques. At room temperature a layer by layer growth was found. Strong intermixing of Cu and Si takes place and beyond 10 monolayers an ordered α Si phase in Cu was found. At high temperature, the growth follows the layer plus island or Stranski-Krastanov growth mode. Three-dimensional crystallites, probably consisting of Cu 3 Si, grow epitaxially on an ordered intermediate layer.

Au/Si(111) overlayer: Characterization by tunneling microscopy and spectroscopy

Abstract Together with LEED and Auger spectroscopy, Scanning Tunneling Microscopy (STM) and Tunneling Spectroscopy (TS) have been used to characterize about 1 monolayer of Au deposited on a 7 × 7 Si(111) surface and annealed at 600°C. A √3 × √3 R(30°) LEED pattern was observed, whereas STM revealed a 6 × 6 superstructure locally. TS showed strong characteristic resonances in the field-emission range together with a pronounced empty surface state 1 eV above the Fermi level.

Electronic structure of Ag adsorbed on Si(111); experiment and theory

Abstract Experimental results (low energy electron loss spectroscopy) and band structure calculations relating to the early stages of Ag growth on a Si(111) surface are presented. Crystallography and thermal desorption kinetics studies of this interface, previously published, gave rise to the following conclusions. At room temperature and below 200°C, two-dimensional (2D) (111) epitaxial layers develop on top of a first ordered layer (√3 × √3), while at higher temperatures three-dimensional (3D) clusters develop over this first layer. Low energy electron loss experiments were performed at various surface coverages θ. They display different evolutions according to the growth modes. For the 2D epitaxial growth, one observes the disappearance of the peaks characteristic of a Si surface and the onset of Ag induced peaks located at 7.1 and 4.6 eV at completion of the √3 layer. These peaks narrow and shift to the bulk Ag excitation energies at 7.5 and 4 eV when a second Ag layer is deposited. In order to explain these results, we present a theoretical calculation of the electronic density of states of the interface using a tight binding approximation. This calculation accounts for the development of the Ag d band from the √3 coverage range to the (111) epitaxial Ag planes. The evolution of the spectra when θ is increased is discussed in view of these results.

Combined AES, LEED, SEM and TEM characterizations of CuSi(100) interfaces

CuSi(100) interfaces prepared under UHV at different substrate temperatures (TS) have been characterized using in-situ Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) as well as ex-situ scanning and transmission electron microscopy (SEM, TEM). At room temperature (RT), the film grows in a layer by layer like mode. With increasing TS, the intensity of the Cu M2,3VV (61 eV) Auger transition decreases and at TS = 500°C no Cu Auger signal could be measured below θ ∼ 100. Yet SEM and TEM observations of these deposits show islands in epitaxial relation with the substrate. It can be determined from TEM images that these islands are covered with a Si skin (∼ 50 A; thick) and that they are deeply implanted in the Si substrate. This explains the AES measurements.

Surface science lettersEnhancement of the room temperature oxidation of silicon by very thin predeposited gold layers

The oxidation of Si(111) surfaces covered with very thin layers of gold is studied by Auger and electron energy loss spectroscopies under ultra high vacuum conditions. It is found that by exposing the Au covered surface to an oxidizing atmosphere, formation of silicon dioxide occurs at room temperature on top of the substrate and the presence of SiO4 tetrahedra is clearly seen on electron energy loss spectra. In contrast, oxidation under the same conditions of a clean Si(111) surface leads to the formation of an oxygen monolayer and no structure corresponding to Si-O bonds in SiO4 tetrahedra are observed. This enhancement of the oxidation is attributed to a change in the hybridization state of Si atoms in a gold environment.

CuSi(111) interfaces: Oxidation properties in relation with their structural properties

The oxidation properties of CuSi(111) interfaces prepared either at room temperature (RT) or at high temperatures starting from a 7 × 7−Si(111) surface, are investigated in relation with their structural properties. At low oxygen exposures (104 langmuirs), at room temperature, enhanced oxidation effects are observed for a RT Cu deposit while an annealed interface which displays a nearly 5 × 5 LEED structure is completely passivated as regard to oxidation under the same conditions. With a stronger oxidation (3 × 107 L), we always measure an enhanced oxidation at the RT CuSi interface as compared with the virgin Si surface while the passivity property of the annealed CuSi interface is weakened.

Characterization of Cu/Si(100) interfaces by different surface-sensitive techniques

Abstract Room-temperature (RT) and high-temperature (HT) Cu/Si(100) have been investigated by in-situ Auger and electron energy loss spectroscopies and ex-situ SEM and TEM. At RT, a quasi-layer-by-layer growth of a CuSi compound is observed in the early stages. Beyond θ = 10 ML, metallicity of Si in Cu environment is observed. Estimation of the stoichiometry of the film by the plateau of the Auger growth curve suggests a composition near Cu 3 Si in agreement with the position of the Cu-induced peaks in EELS. At high temperature (130°–500°C), the growth of 3D crystallites occurs while a wetting by a Si film is revealed by both SEM and TEM.

Laser-induced oxidation of the Si(111) surface

Pulsed laser induced oxidation of clean Si(111) surfaces has been studied by Auger electron spectroscopy and electron energy loss spectroscopy. The short duration time of the pulse has allowed a precise investigation of the first stages of the oxidation. About 1–2 oxide monolayers first grow in less than 10 μs. Their stoichiometry evolves from SiOx towards SiO2 with increasing beam energy densities. Once this superficial layer has formed, no evolution is seen with further irradiation, suggesting that oxygen diffusion during the pulse duration cannot sustain the oxide growth.

√3 structure of Boron enriched Si(111) surfaces investigated by Auger, leed and scanning tunneling microscopy

Abstract Boron enrichment of the Si(111) surface was realized by long time annealing at 900 ° C of highly boron doped Si samples ( p ≈10 19 cm −3 ). Boron Auger peak (179 eV) and LEED patterns of boron induced 3 × 3 R 30° reconstructions could be detected after such a treatment. Preparation and characterization of the samples by averaging surface sensitive techniques have been performed in an UHV chamber equipped with a STM unit. High resolution STM images of the 3 structure have been obtained. Simultaneous imaging in both polarities, thus probing filled or empty surface states, allowed us to observe spectroscopic effects which are discussed in this paper. On the basis of our data, a modification of the T 4 adatom model of the 3 structure is proposed.