C. Pirri

Continuous germanene layer on Al(111).

Germanene, a 2D honeycomb structure similar to silicene, has been fabricated on Al(111). The 2D germanene layer covers uniformly the substrate with a large coherence over the Al(111) surface atomic plane. It is characterized by a (3 × 3) superstructure with respect to the substrate lattice, shown by low energy electron diffraction and scanning tunnelling microscopy. First-principles calculations indicate that the Ge atoms accommodate in a very regular atomic configuration with a buckled conformation.

Experimental band structure and Fermi surface of a two-dimensional Er silicide on Si(111)

High resolution angle resolved photoemission measurements on a monolayer of Er deposited on Si(111) and annealed at 400°C are presented. A series of two-dimensional energy bands attests to the formation of a surface silicide with a high degree of perfection. In particular, a prominent band with remarkably large hole lifetimes (∼200 meV) and a dispersion of ∼1.65 eV crosses the Fermi level near the Γ point of the surface Brillouin zone. The two-dimensional Fermi surface is typical of a semi-metal and consists of small hole and electron pockets about the Γ and M points respectively.

STM investigation of 2- and 3-dimensional Er disilicide grown epitaxially on Si(111)

Abstract The surface atomic structure of 2- and 3-dimensional (D) Er disilicide epitaxially grown on Si(111) has been investigated by scanning tunneling microscopy (STM) and angle-resolved photoemission. The STM images reveal that highly ordered 2D and 3D silicide islands can be grown on the flat Si(111)7 × 7 terraces and atomic resolution scans clearly confirm that both silicides are terminated by a Si bilayer without vacancies. In the 3D case the outermost Si atoms exhibit an additional small buckling with √3 × √3R30° periodicity. The STM data imply a specific registry of the surface Si layer with respect to the vacancy net underneath which is found to be in nice agreement with the symmetry of the dangling bond states at \ gG observed in polarization dependent photoemission.

Formation of Epitaxial CsCl-Type Iron Silicide on Si(111)

The epitaxial growth of a cubic Fe silicide phase on Si(111) has been confirmed by means of X-ray photoelectron diffraction (XPD) and surface-extended X-ray absorption fine-structure (SEXAFS) experiments. XPD experiments show that a 5-monolayer Fe film deposited on Si(111) and subsequently annealed at ~ 500 °C has a cubic structure. SEXAFS measured at the Fe K edge (7110 eV) reveals that Fe atoms are coordinated with eight Si atoms with bond length of (2.38 ± 0.04) A and with six Fe atoms with bond length of (2.71 ± 0.04) A. All measurements lead to the conclusion that this cubic silicide has a CsCl-type structure.

Epitaxial growth of CrSi and CrSi2 on Si(1 1 1)

Abstract The possibility of epitaxial growth of chromium silicides upon thermal processing of thin Cr deposits (⪅ 30 monolayers (ML)) on Si(1 1 1) is demonstrated using low energy electron diffraction (LEED) and angle resolved X-ray (XPS) and ultra-violet (UPS) photoemission. For coverages θ ⪆ 4 ML epitaxial CrSi with a lattice misfit of ≈ 1.6% can be grown upon annealing at 350°C. Thermal treatment at 450°C for θ ⪆ 6 ML results in epitaxial CrSi 2 formation with two kinds of domains rotated by 30° with respect to each other around the surface normal. LEED intensities indicate essentially equal formation probabilities for both orientations despite the large difference in lattice misfit of ≈ 0.1 and ≈ 3.8% respectively.

Epitaxy of CoSi2 on Si (111) at low temperature (≤400 °C)

The epitaxial growth of thin CoSi2 films on a Si(111) surface has been studied using surface techniques such as low‐energy electron diffraction and photoemission spectroscopy. Various preparation methods at low temperature (≤400 °C) are investigated. Both layer by layer growth and coevaporation invariably exhibit a bulk and surface excess of Si. In contrast a different preparation method where the Co atoms were evaporated onto the Si(111) substrate maintained at ∼360 °C produces CoSi2 films exposing a Co‐rich CoSi2 surface without any Si excess in bulk. It is concluded from these experiments that at ∼360 °C diffusion of Si from substrate through the CoSi2 layer is much easier than usually expected and quite sufficient to sustain further CoSi2 growth without any extra Si supply.

Er-induced 2√3 × 2√3R30° reconstruction on Si(111): influence on the very low Er coverage silicide growth

Abstract Interaction of Er with Si(111) is investigated by scanning tunneling microscopy. Er reaction at 500°C with Si(111)7 × 7 in the submonolayer range is characterized by the formation of numerous islands and holes exposing a 2√3 × 2√3R30° surface reconstruction. This is the first report on the formation of such superstructure upon reaction of a rare-earth element on Si(111). The 2√3 × 2√3R30° unit cell is composed of two inequivalent halves and exhibits protrusions with a shape dependent on the bias voltage. Taken with a negative bias voltage, each half-cell exhibits three dimers aligned with the 〈101〉 direction of the substrate. By using a positive sample bias voltage, the surface reconstruction is atomically resolved. The protrusions observed in this mode are accommodated in a hexagonal ring of T4 sites, with sizeable atomic displacements with respect to the perfect T4 sites. The size and number of such reconstructed holes and islands are found to strongly depend on the growth conditions. They are mainly observed below 0.5 monolayer coverage, coexisting with small hexagonal-shaped one-monolayer height islands. Upon increasing the annealing temperature up to 700°C, the 2√3 × 2√3R30° reconstructed areas disappear. At this stage, the one monolayer height silicide islands are still observed along with additional two-monolayer height silicide islands. This contrasts with what occurs in the 0.5–1 monolayer coverage range, i.e. the formation of one-monolayer silicide only. The most striking feature here is that the two-monolayer silicide islands, induced by the formation of 2√3 × 2√3R30° reconstructed islands, grow on the Si(111) terraces along one preferential orientation only. They grow along the close-packed 〈101〉 direction of the substrate, giving rise to rod-shaped islands, as opposed to those formed in the higher coverage regime (above one monolayer) which exhibit an hexagonal shape, consistent with the threefold symmetry of the two-layer silicide. These rod-shaped islands exhibit a 2 × 1 surface periodicity.

Room-temperature ferromagnetism in single crystal Fe1.7Ge thin films of high thermal stability grown on Ge(111)

We report on the epitaxial growth of ultrathin ferromagnetic Fe1.7Ge layers on Ge(111) wafer. These single crystal intermetallic layers adopt the InNi2 (B82) crystallographic structure. They are ferromagnetic with a Curie temperature well above room temperature. The interface between the ferromagnet layer and the Ge wafer is of high perfection. Interestingly, the annealing of the sample up to 300°C alters neither the crystallographic structure, nor the interface quality, nor the magnetic properties but leads to a nearly perfect smoothening of the germanide layer surface. This high thermal robustness should open the way for the growth of fully epitaxial iron germanide/Ge hybrid structures.

Epitaxial cubic iron silicide formation on Si(111)

Abstract Ultra-thin (3–10 monolayers (ML)) Fe films deposited on Si(111)7×7 surfaces were annealed in the 300–600°C temperature range and investigated by X-ray photoelectron diffraction (XPD), angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and low-energy electron diffraction (LEED). ARUPS spectra reveal the formation of an iron-silicide film showing an increasing Si content with increasing annealing temperature. The comparison of the XPD results of these silicides to those of epitaxial CoSi2 films and to the results of single-scattering calculations allows us to identify the formation of Fe silicides with a local cubic Fe environment. The orientation of the silicides with respect to the substrate is essentially of B-type. Thin-enough films (

X‐ray photoemission characterization of thin epitaxial Fe silicide phases on Si(111)

Depending on preparation conditions, Fe silicides grown on Si(111) by means of solid phase epitaxy and molecular beam epitaxy show the formation of the bulk e‐FeSi and β‐FeSi2 phases as well as epitaxially grown metastable CsCl‐ and CaF2‐type Fe silicides. The valence‐band of these Fe silicides are measured with monochromatized Al Kα x‐ray photoemission and angle resolved ultraviolet photoemission and are found to be in remarkable agreement with calculated densities of states.