F. Thibaudau

Solid-state graphitization mechanisms of silicon carbide 6H–SiC polar faces

Abstract Due to the higher vapour pressure of silicon, silicon carbide surfaces annealed at high temperature under vacuum tend to graphitize. The comparison of graphite formation on the silicon and carbon terminations of 6H–SiC reveals significant differences in the graphitization mechanisms involved. The conduction-band structure of these interfaces has been determined by angle-resolved inverse photoemission spectroscopy (KRIPES). Although the graphite layers grown on the C face are essentially polycrystalline, a small fraction of the film keeps a preferred orientation, where the graphite lattice basis vectors are rotated by 30° with respect to the basis vectors of the SiC lattice as in the case of the Si face. This in-plane disorder is in contrast with the growth of graphite on the Si face that takes place on a “passivated” adatom-terminated surface, leading to single-crystalline, heteroepitaxial graphite growth. The observation of unshifted π* states indicates a very small interaction of the first graphite monolayer with the Si face. In contrast, KRIPES reveals that the first graphite layer is strongly bound to the C face. A rehybridization of the graphite π* states with occupied orbitals of the substrate is inferred from an observed increase in the density of states in the vicinity of the Fermi level.

Nanometer‐scale lithography on Si surface by decomposition of ferrocene molecules using a scanning tunneling microscope

We describe lithography experiments on boron doped Si substrates using the decomposition of ferrocene molecules with a scanning tunneling microscope tip. On the basis of writing conditions we propose that field ionization of the molecules is the key to the writing process and is responsible for the nanometer definition of the pattern edge.

Cooperative Segregation of Boron at Si(111)

Interface segregation of boron at Si(111) surfaces has been studied at the atomic scale using a scanning tunnelling microscope (STM). During the segregation process (produced by thermal annealing), strong cooperative effects take place which cannot be explained by a simple nearest-neighbour pair interaction model. Although average surface concentrations of boron in the investigated temperature range could be calculated in terms of a Fowlers model, the comparison between STM and simulated images suggests that one must introduce longer-range interactions to describe the atomic-scale configurations.

√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.

Scanning tunneling microscopy study of Fe(CO)5 and Fe(C5H5)2 adsorption on Si(111)7×7 and B/Si(111)√3×√3

The adsorption at room temperature of ferrocene and iron pentacarbonyl on Si(111)7×7 and B/Si(111)√3×√3 R30° have been studied. On Si(111)7×7, the adsorption sites have been identified by means of scanning tunneling microscopy. We propose a ferrocene adsorption model on Si(111)7×7, i.e., a di-sigma bridging by the molecule between an adatom and a restatom site similar to that proposed for the ethylene. This process is in agreement with the lack of reactivity of this molecule on the B/Si(111)√3×√3 R30° surface. For the iron pentacarbonyl, we have found evidence of a dissociative adsorption on nucleophilic sites. At a higher temperature, an exposure to iron pentacarbonyl lead to the growth of good quality iron silicide. Whereas, silicide carbide is formed on exposure to ferrocene. The films obtained can be explained by means of the chemisorption process at room temperature.

STM study of low pressure adsorption of silane on Si(111) 7 × 7

Abstract We report a study of silane adsorption on the Si(111)7 × 7 surface. We have been interested in the first stages of chemisorption at room temperature. Reactive sites of the unit cell have been clearly identified on Scanning Tunneling Microscopy (STM) images: the reaction involves the rest atom and the adjacent adatom of the DAS structure with preferential adsorption on the center adatom. We propose an original chemisorption mechanism which leads to the formation of two SiH 2 species by chemisorption and involves the breaking of SiSi backbonds of the adatom.

Scanning tunnelling microscopy of B/Si(111) √3×√3 R(30°)

The √3 superstructure induced by boron outdiffusion at the surface of highly p doped Si(111) samples is studied by means of Auger, LEED and STM. Simultaneous double imaging at different voltages allows us to probe both filled and empty electronic states. A modification of the T4 structural model of (Al, In, Ga)/Si(111) √3 is proposed for the B/Si(111) √3 superstructure.

Surface reaction mechanisms on Si(111)-(7 × 7) during silane UHV-CVD

Abstract We report an STM study of the Si(111)-(7 × 7) surface after silane adsorption in the temperature range 700–800 K. At these temperatures, we show that the decomposition of the adsorbed species and their subsequent diffusion lead to the etching of Si adatoms from the dimer adatom structure (DAS) and to the formation of hydrogenated silicon-atom clusters on the faulted part of the unit cell. We have also studied the steady state of the surface under static silane pressure. We conclude that hydrogen desorption occurs from these clusters, leading to a first-order desorption process.

Early stages of Cu growth on boron segregated Si(111) surfaces: A scanning tunneling microscopy study

Out segregation of boron from the bulk of highly doped Si samples leads to surfaces which present a B–Si(111)√3×√3 R(30°) reconstruction (hereafter abbreviated B√3). Scanning tunneling microscopy investigations show that, depending on the annealing temperatures, different boron equilibrium surface concentrations are obtained in the B√3 substrates. Comparisons of Cu growth on such substrates show that copper nucleation is highly correlated to the density of dangling bonds at the surface.

Cooperative Phenomenon in B/Si(111) Segregation

In the frame of a phenomenological model we show that sound cooperative effect (filament structure of typical patterns) discovered in the B/Si(111) segregation (Thibaudau F., Roge T. P., Mathiez Ph., Dumas P. and Salvan F., Europhys. Lett., 25 (1994) 353) could be explained by elastic tension creating a non-local interaction of triplet type. As a criterium we use the fitting of concentration equilibrium curve in combination with histogram treatment of B/Si(111) patterns based on a linear stratification of images.