P. Wetzel

Formation of epitaxial Fe3−xSi1+x (0≤x≤1) silicides on Si(111)

Epitaxial Fe3−xSi1+x films have been grown on Si(111) by codeposition at room temperature. Their structural and electronic properties have been investigated by means of low‐energy electron diffraction (LEED), x‐ray photoelectron diffraction (XPD), and x‐ray photoemission spectroscopy (XPS). These films, with compositions ranging from Fe3Si to FeSi, exhibit a (1×1) LEED pattern. Both XPD and core level binding energy measurements indicate that single Fe3−xSi1+x phases (with 0<x<1), without bulk counterpart, can be stabilized by epitaxy on Si(111). The XPD experiment clearly shows that these Fe3−xSi1+x (0≤x≤1) films adopt the same cubic structure. Furthermore, the Si 2p, Fe 2p3/2, and Fe 3s core levels are slightly shifted to higher binding energies resulting from chemical effects and differences in local coordination when going from Fe3Si (DO3) to FeSi (CsCl). Multiplet splittings ΔE3s are observed in Fe 3s core‐level XPS spectra for all Fe3−xSi1+x compounds except the FeSi (CsCl) one.

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.

High Spin Polarization at Ferromagnetic Metal–Organic Interfaces: A Generic Property

A high spin polarization of states around the Fermi level, EF, at room temperature has been measured in the past at the interface between a few molecular candidates and the ferromagnetic metal Co. Is this promising property for spintronics limited to these candidates? Previous reports suggested that certain conditions, such as strong ferromagnetism, i.e., a fully occupied spin-up d band of the ferromagnet, or the presence of π bonds on the molecule, i.e., molecular conjugation, needed to be met. What rules govern the presence of this property? We have performed spin-resolved photoemission spectroscopy measurements on a variety of such interfaces. We find that this property is robust against changes to the molecule and ferromagnetic metals electronic properties, including the aforementioned conditions. This affirms the generality of highly spin-polarized states at the interface between a ferromagnetic metal and a molecule and augurs bright prospects toward integrating these interfaces within organic spintronic devices.

Epitaxial magnetic Fe layers grown on Si(001) by means of a template method

We demonstrate the possibility of growing good-quality epitaxial Fe films on a Si(001) substrate, opening up new prospects to prepare ferromagnetic superlattices on this substrate. A template technique prevents the formation of disordered interfacial iron silicides. Transmission electron microscopy reveals that the Fe layers are fairly uniform in thickness with abrupt interfaces and in majority epitaxial relationship Fe(001)[100]//Si(001)[100]. Both the diffraction data and the observation of Moire fringes indicate that the Fe lattice is almost relaxed towards its bulk bcc phase in thick layers [≥40 monolayers (ML)] but is still strained by the substrate in the thinnest films. Magneto-optical Kerr effect measurements show that the films evaporated at normal incidence with a thickness above 4 ML are ferromagnetic at room temperature and exhibit in-plane biaxial anisotropy.

Strong element dependence of C 1s and Si 2p X-ray photoelectron diffraction profiles for identical C and Si local geometries in β-SiC

Abstract We have measured photoelectron diffraction polar profiles for a β-SiC film grown epitaxially on Si(001). Tha data reveal dominant single domain growth with a good crystallinity but the C 1s and Si 2p profiles exhibit remarkably strong differences in spite of the identical geometries of the sites occupied by these elements in the ZnS-type lattice. Most obvious are the large angular shifts and changes in intensity between expected and measured forward scattering peaks. Our results obtained at low angular resolution (∼ 5°) in the high kinetic energy range (∼ 1000 eV) provide a striking example of the limitations of the often invoked forward scattering picture. The measured profiles are rather well reproduced by single scattering cluster simulations. The observed elemental dependence can be traced back to the marked change in complex scattering amplitude between C and Si along with the very general fact, by no means restricted to the SiC(001) case, that a large number of scatterers, in particular out-of-chain atoms with a low scattering angle, make a substantial contribution to the photoelectron wave around forward scattering directions. The related energy and element dependent interference effects are particularly strong along the low density [001] C chains of the open ZnS-type structure and reflect in a drastic peak splitting due to the strong scattering at lateral Si atoms. In contrast, the [001] Si chains in β-SiC lead to an essentially structureless forward scattering peak due to the lower scattering amplitude of lateral C atoms.

Magnetization reversal mechanisms in epitaxial Fe/Si(001) layers with twofold and fourfold magnetic anisotropies

Epitaxially grown iron thin films (thickness: 8 A < d < 120 A) on Si(0 0 1) with body centred cubic structure have been structurally and magnetically studied at room temperature. All lilms are characterized by ferromagnetic properties as soon as the thickness reaches 4 ML. In-plane uniaxial and cubic magnetic anisotropy constants are measured on samples obtained by modifying the incidence angle of evaporating iron flux. The magnetization reversal mechanisms are analyzed by combining longitudinal magneto-optical Kerr effect of both parallel and transverse magnetization components. These mechanisms are strongly influenced by the nature of the magnetic anisotropies.