A. Cros

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 Auue5f8Si 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 Auue5f8Si 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.

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.

Atomic bonding at the SiAu and SiCu interfaces

The first stages of the Si(111)7×7-Au and Si(111)7×7-Cu interface formation have been investigated under ultrahigh-vacuum conditions. The use of a differential Auger spectra analysis has shown that for thin metal layers (θ<5 ML) the Si(LVV) line can be considered as the superposition of a non-reacted Si(LVV) peak and a split Si(LVV) peak usually observed for higher coverages and characteristic of metal-rich metal-Si compounds. These results show that the same hybridization between the metal (d) and the Si (p) states is present in both regimes, small and large coverages. The formation of the long-range ordered 3×3 (Au) and quasi 5×5 (Cu) structures upon annealing does not modify the overall features of the metal-silicon bonding. We suggest that the interface reaction starts at the very beginning of the metal deposition and that a critical metal thickness is not necessary to break the Si-Si covalent bonds and to form an alloyed metal-Si phase.

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.

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.

Electron irradiation and adhesion at silicon-gold interfaces prepared under ultra high vacuum conditions

Abstract Silicon-gold interfaces have been grown and exposed to electron beam irradiation under ultra high vacuu, conditions. The adhesion of the gold film is good when Au is evaporated on an atomically clean Si(111) surface. In contrast, the presence of a thin SiO 2 native oxide between Au and Si decreases strongly the adhesion of the metallic layer. Electron irradiation of the clean Siue5f8Au interface provokes the agglomeration of the metallic layer. Direct determination of the sample temperature shows that thermal effects induced by the electron beam cannot be neglected.

Al reaction with SiO2 an auger electron spectroscopy and energy loss spectroscopy study

Evidence for a reaction between aluminium and SiO2 film is presented using Auger electron spectroscopy (AES) and low-energy electron-loss spectroscopy (ELS) techniques. This reaction is studied “in situ” during the manufacture of metal insulator semiconductor devices (MIS), under ultra-high vacuum conditions (UHV). A reduction of the SiO2 film upon aluminization occurs, even at room temperature, giving rise to a complex interface.

Mechanism of growth of ultrathin SiO2 layers on silicide substrates

Abstract The room temperature oxidation of near noble metal silicides and Auue5f8Si alloys has been studied with ESCA. The analysis of metal and silicon core levels shows that oxygens bonds to Si atoms and not to metal atoms. The oxidized Si atoms form a homogeneous SiO 2 layer at the surface and leave underneath a metal rich phase. The generality of the model is discussed.

Thermally-induced reactions at Pt-GaAs junctions

Abstract The solid-state reaction between a Pt layer and a GaAs substrate has been studied with Rutherford backscattering and X-ray photoemission spectroscopies. At 380°C, there is formation of an outer layer of PtGa covered by a few layers of unbound As and an inner layer mostly composed of PtAs 2 . At 550°C, PtGa 2 forms at the surface. Examination of the PtGa and PtGa 2 valence bands reveals a strong interaction between Pt and Ga atoms which appears as a shift of the emission maximum to lower binding energy. This behaviour is very similar to what is observed in Pt 2 Si and PtSi.

Laser annealing of the Si-Au interface

Abstract The laser annealing of the Si-Au interface was investigated by Auger and electron energy loss spectroscopies and scanning electron micrography. Two regimes were found. 1. (i)For low beam energy densities we observe diffusion of silicon atoms into the gold film. 2. (ii)For higher densities agglomeration of the metallic layer occurs. These results are similar to those obtained with conventional thermal annealings. An estimation of the diffusion coefficient of silicon gives a high value (approximately 10 -6 cm 2 s -1 ) which is thought to be characteristic of amorphous metallic alloys.