E. G. Michel

Mechanism of alkali‐promoted oxidation of silicon

Photoelectron spectroscopy data suggest that the mechanism of enhanced oxidation of Si promoted by multilayers of K deposited on its surface is based on the formation of potassium oxides, identified as K2O2 and KO2, that transfer oxygen efficiently to the Si substrate upon annealing at 900 K.

Surface characterization of epitaxial, semiconducting, FeSi2 grown on Si(100)

We have identified the composition and range of thermal stability of FeSi and FeSi2 films grown on Si(100) by solid phase epitaxy and reactive deposition epitaxy. Evidence for the semiconducting character of FeSi2 is obtained from photoemission measurements giving a low density of states at the Fermi level. Si enrichment at the outer surface of the silicides at temperatures much lower than previously thought has been found by depth profiling. Scanning tunneling microscopy reveals a rather inhomogeneous growth with a tendency towards epitaxial growth favored by the presence of surface steps on the Si substrate.

Alkali-induced oxidation of silicon

Photoelectron and high resolution Auger spectroscopy indicate that multilayers of K deposited on Si(100)2×1 promote the oxidation of Si under subsequent exposure to O 2 . The oxygen uptake is enhanced by four orders of magnitude. The alkali can be completely desorbed from the surface at 900 K leaving behind a layer of SiO 2 , whose thickness is proportional to the amount of K deposited. The process can be further repeated. We propose a mechanism for this promoted oxidation supported by spectroscopic evidence.

Iron silicides grown on Si(100): metastable and stable phases

Abstract The growth of iron silicides on Si(100) by solid phase epitaxy has been investigated by photoelectron spectroscopies. The different iron silicide phases appearing and their stability ranges have been determined. We have found three different ranges depending on the initial Fe coverage deposited. In the case of large initial Fe coverages (above 16 ML), we have found the subsequent formation of ϵ-FeSi and β-FeSi 2 upon annealing. For initial Fe coverages below 3.4 ML, a phase of 1:1 stoichiometry is formed upon annealing, followed by the formation of a well-ordered metastable phase which we identify with FeSi 2 (CsCl). In the intermediate coverage range, while a poorly ordered phase of 1:1 stoichiometry is found for T

Epitaxial iron silicides: geometry, electronic structure and applications

Abstract A simultaneous understanding of geometrical and electronic aspects of epitaxial films is crucial whenever a deep knowledge of their fundamental properties is required. A conspicuous example are the epitaxial phases formed by the reaction of Fe and Si. These two elements give rise to a rich variety of bulk binary compounds. In addition to them, metastable phases of different stoichiometries can be epitaxially grown on Si(111) and Si(100). These are compounds where the delicate energetic balance giving rise to a particular structure has been altered by epitaxy and phases not formed spontaneously are found. A study on the electronic and geometric structures, covering both stable and metastable phases is presented here. In particular, the properties of metastable FeSi(CsCl) are discussed, when grown both on Si(111) and Fe 3 Si(100). The potential applications of this and other iron silicides is discussed in the context of their novel properties.

Observation of a Mott Insulating Ground State for Sn/Ge(111) at Low Temperature

We report an investigation on the properties of 0.33 ML of Sn on Ge(111) at temperatures down to 5 K. Low-energy electron diffraction and scanning tunneling microscopy show that the (3x3) phase formed at approximately 200 K, reverts to a new ((square root 3)x(square root 3))R30 degrees phase below 30 K. The vertical distortion characteristic of the (3x3) phase is lost across the phase transition, which is fully reversible. Angle-resolved photoemission experiments show that, concomitantly with the structural phase transition, a metal-insulator phase transition takes place. The ((square root 3)x(square root 3))R30 degrees ground state is interpreted as the formation of a Mott insulator for a narrow half-filled band in a two-dimensional triangular lattice.

The growth and characterization of iron silicides on Si(100)

Abstract We have studied and characterized different iron silicides grown in situ on a Si(100)-2 × 1 surface. Silicide films have been produced both by solid-phase epitaxy, and by reactive deposition epitaxy, keeping the substrate at several hundreds of degrees during the growth. The grown silicide layer was characterized by different surface sensitive techniques. Two different types of silicides (FeSi and FeSi 2 ) were identified by directly determining the atomic stoichiometry of the compound, as well as by studying the evolution of the characteristic plasmon loss structure of the compounds. Finally, ISS has revealed the existence of a Si-enriched layer on top of the silicide grown by solid-phase epitaxy.

Geometric and electronic structure of epitaxial iron silicides

The geometric and electronic structure of several iron silicide phases epitaxially grown on Si(111)7×7 have been characterized by means of surface sensitive techniques including scanning tunneling microscopy (STM), x‐ray photoelectron spectroscopy, ion scattering spectroscopy (ISS), and ultraviolet photoelectron spectroscopy. The silicides were grown by solid‐phase and reactive‐deposition epitaxy, and their stability range was determined in situ as a function of iron coverage and annealing temperature. In particular, we have studied the phases appearing in the low‐coverage low‐temperature region. Additionally, the crystallites of the most important FeSi2 phases (γ‐FeSi2 and β‐FeSi2) have been characterized at atomic level with STM, while the surface termination was analyzed with ISS.

A structural study of the K adsorption site on a Si(001)2 × 1 surface: Dimer, caves or both

Abstract The atomic structure of the clean Si(100) and K covered surfaces has been investigated by Auger electron diffraction (AED) monitoring the intensities along polar scans. This technique is sensitive to the asymmetric-dimer nature of the 2 × 1 reconstruction of the Si(001) surface. Data taken at room temperature for submonolayer coverages are consistent with adsorption of K on the troughs (cave position) existing between two consecutive dimer chains along the [110] direction. At 110 K both dimer and cave sites are occupied. A mild annealing to 300 K produces an overlayer redistribution in favor of the “cave” site further indicating that this site is energetically favoured as found in some recent calculations.

K/Si(100) 2 × 1: A Case Study for the Transfer of Charge between Alkali Metals and Semiconductor Surfaces

Experimental data and theoretical calculations for K/Si(100) 2 × 1 provide a picture of the interface interaction essentially similar to the one established for adsorption of alkali metals on metals in contradiction with recent calculations. In particular, the transfer of charge from K to Si decreases with coverage and it is far from being complete at saturation coverage. The K-Si bond length and the charge transfer indicate that the bonding has a substantial covalent character.