J.C. Peruchetti

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.

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.

Metastable film growth of Cr on Cu(OO1)

The growth and structure of Cr films (0–30 nominal monolayers (ML)) on Cu(001) held at room temperature have been investigated by photoemission, photoelectron diffraction and low energy electron diffraction techniques. The films exhibit ordered body-centred cubic (bcc) domains typically ∼ 10 lattice parameters in size with Cr(110) ∥Cu(001) and Cr[1111] ∥ Cu[110] and equivalent epitaxial orientations. LEED and photoelectron diffraction reveal a substantial deviation from relaxed bcc structure in ultra-thin films ( < 3 ML) with an in-plane distortion and expansion and an interlayer contraction of the Cr(110) layers. The initial mode of growth is characterized by the formation of multilayer islands and 90% substrate coverage occurs by ∼ 4 ML.

Early stages of epitaxial CoSi2 formation on Si(111) surface as investigated by ARUPS, XPS, LEED and work function variation

Abstract We performed the CoSi 2 formation on a clean (7 × 7) Si(111) surface under UHV conditions. The used techniques were angle resolved UV photoemission, X-ray photoemission, work function change and low energy electron diffraction in order to study the electronic and structural properties of the interface during its formation. At room temperature, a small amount of Co reacts strongly with Si to form an interfacial and very thin cobalt suicide. The ultraviolet photoelectron spectrum displays already two features corresponding respectively to the Co 3d and Si 3p electron bonding states and the Co 3d non-bonding states. With increasing coverage ( θ ≳ 4 ML) it seems that the interfacial suicide prevents further Co−Si interdiffusion to achieve the suicide reaction and a metal rich film is found. At high temperature (∼ 600°C) and in the first monolayer range, several superstructures are found (√7 × √7, and 2 × 2). They are induced by a Co-Si bidimensional compound where the Co atoms are not yet completely surrounded by Si atoms as in their bulk CoSi 2 structure. With increasing coverage, a CoSi 2 -like photoemission spectrum is observed reflecting the formation of the disilicide. The LEED pattern testifies an epitaxial growth displaying a (1 × 1) CoSi 2 diagram. The work function change technique also reflects faithfully this growth.

Annealing studies of the Co/Si(1 1 1) interface

Abstract The behaviour of the Co/Si(1 1 1) interface upon annealing is investigated by low energy electron diffraction (LEED), angle resolved ultraviolet (ARUPS) and X-ray (XPS) photoemission spectroscopy. According to the Co thickness two regimes can be distinguished. At low coverages (≲ 8 monolayers ML) no well defined bulk silicides other than the silicon rich epitaxial CoSi2 phase can be identified. In contrast for larger Co thickness (≳ 15–100 ML) it is found that increasing progressively the annealing temperature (up to 600°C) and time (up to ∼ 30 min) leads to the successive arrival of the following silicides phases within the probing depth of our techniques (∼ 5–20 A): Co, Co2Si, CoSi, CoSi2.

Identification of three distinct CoSi2(111)(1×1) surface structures

Abstract An angle-resolved photoemission (ARPES), low energy electron diffraction (LEED), and work function measurements are used to study the surface structure of thin epitaxial CoSi 2 films grown on Si(111). We demonstrate that by deposition of carefully controlled amounts of Co or Si in the monolayer range and low temperature (300–600°C) annealing, three different and well characterized (1×1) surface structures, referred to as CoSi 2 (111)-Co, CoSi 2 (111) and CoSi 2 (111)-Si respectively, can be prepared. It appears that CoSi 2 (111)-Co is a truncated CoSi 2 crystal exposing a plane of Co atoms having four-fold coordination, whereas CoSi 2 (111) is the natural termination of CoSi 2 exposing a plane of Si atoms with seven-fold coordinated surface Co. The Co-rich surface structures identified in earlier work are basically intermediate forms between CoSi 2 (111) and CoSi 2 (111)-Co. Finally, the Si-rich CoSi 2 (111)-Si surface identified previously must have eight-fold Co coordination as in the bulk.

Characterisation by photoemission of the surface structure of epitaxial CoSi2 grown on Si(111) by various preparation techniques

Abstract The nature of the surface phases formed on epitaxial CoSi2 layers grown on Si(111) is investigated by angle-resolved X-ray (ARXPS) and ultra-violet (ARUPS) photoemission, low energy electron diffraction (LEED) and work function measurements. Various preparation techniques, such as solid phase epitaxy (SPE) and high temperature annealing (≥ 500°C), Co and Si codeposition in the CoSi2 ratio and simultaneous or subsequent annealing at 360°C or above, and sequential deposition of equal amounts (≤10 ML) of Co and Si and subsequent heating to 400°C, all invariably lead to the Si rich high temperature phase labeled CoSi2(111)-Si. Co-rich surface phases with peculiar structures generally intermediate between two limiting geometries labeled CoSi2(111)-Co and CoSi2(111) can be obtained either by a ∼ 2 ML Co deposit onto CoSi2 films prepared with the techniques mentioned above and annealing at low temperature (≤ 400°C) or directly on films grown by reactive molecular beam epitaxy (reactive MBE). In the latter method a template layer is first prepared by evaporating 3 ML of Co onto the Si substrate held at room temperature and subsequent annealing at 360°C for 5 min. Co is then deposited at a low rate (∼ 0.5 ML/min) onto this template layer, maintained at 360°C, up to the desired film thickness. The Co richest pure CoSi2(111)-Co geometry can be formed by a 2 ML Co deposit on films prepared by reactive MBE and subsequent short anneal (≤ 3 min) at 360°C. This structure is basically metastable and converts progressively into the low temperature stable CoSi2(111) form upon annealing at 400°C.

Abrupt versus diffuse nature of the Cr/Ag and Cr/Au interfaces

Abstract Valence band photoemission spectra from Cr deposited on Ag and Au surfaces in the submonolayer range show magnetic Cr3d virtual bound states located at 1.15 ± 0.05 eV and 0.75 ± 0.05 eV binding energy respectively with a full width at half maximum of 0.6 ± 0.10 eV. For Au this feature becomes broader, shifts to ∼ 1.0 eV binding energy but persists on increasing the Cr coverage up to 3 monolayers in contrast with Ag where a Cr3d bulk-like band is quickly developed. This result combined with core level photoemission data indicate that alloying with the substrate takes place at the Cr/Au interface, whereas the Cr/Ag interface remains sharp. The high initial local magnetic moment (4–5 μB) for isolated Cr species in (on) the Au (Ag) substrate apparently reduced drastically (⩽ 1 μB) for structures with interatomic Cr spacings comparable to the bulk one.

Crystallographic and electronic structure of ultra thin Ag/Cr/Ag(100) sandwiches

Abstract We have investigated the electronic and crystallographic structure of thin Ag/Cr/Ag(100) sandwiches using angle-resolved ultraviolet (ARUPS) and X-ray (ARXPS) photoemission. First an ordered two-dimensional (2D) Cr monolayer is formed by deposition of one layer equivalent (LE) Cr onto a clean Ag(100) single crystal kept at 440 K under UHV conditions. The relevant ARXPS curves for Cr 2 p 1 2 are structureless as expected for a single monolayer. Then Ag films (from the submonolayer range to 5 LE) are deposited at RT onto this Cr monolayer. Even after deposition of 3 LE Ag, the ARUPS spectra show the persistence of Cr3d induced features characteristic of the 2D Cr monolayer. However after Ag deposition the valence band spectra also reveal an additional sharp Cr3d peak at 1.2 eV binding energy, and the Cr 2 p 1 2 ARXPS curves are structured and exhibit strong forward scattering features characteristic of a face-centered cubic environment. It is concluded that only a part of the Cr monolayer is destroyed and dissolves in the deposited Ag which grows in the form of 3D islands.

Electronic structure of epitaxial erbium silicide films on Si(111)

Abstract The present study is mainly motivated by the recent interest in rare-earth/Si interfaces for technological applications. Furthermore there is by now a rather poor understanding of their electronic structure. We present the first spectroscopic investigaton of epitaxial erbium silicide using angle-resolved ultraviolet photoemission spectroscopy (ARUPS) and low-energy electron diffraction (LEED). Thermal treatment at 700°C of a 15 monolayers Er deposit results in epitaxial ErSi 1.7 formation characterized by sharp √3 × √3 R30° LEED pattern. The relevant ARUPS spectra exhibit essentially emission of Er4f multiplet states in the 4–11 eV binding energy window and structures at lower binding energies assigned to hybridized Er5d and Si 3sp states. Finally, we have shown that in contrast with pure Er, there is apparently no surface-related 4f component for monocrystalline ErSi 1.7 .