G. Gewinner

Si-H bond production by NH3 adsorption on Si(111): An UPS study

The adsorption of NH3 on Si(111) has been examined using essentially ultraviolet photoemission spectroscopy (UPS) between room temperature (RT) and 400°C. In this domain NH3 molecules chemisorb dissociatively on some surface sites as deduced from the observation of Si-H monohydride units at 5.4 eV below EF. Other species labeled NHX (X=1, 2 or 3), characterized by two NH3 induced orbitals at 4.9 and 10.6 eV, are also adsorbed at RT with a saturation coverage ≤1/3 monolayer for a 10 L (1 L=10−6 Torr s) exposure. A strong S2 surface state decrease results from adsorptions. With increasing substrate temperature the adsorption of the nitrided NHX species gradually decreases until 300°C where mainly Si-H bonds are observable. No direct conclusive assignment could be given by UPS about the exact nature of the NHX units, but XPS N 1s binding energy (BE) data give arguments for partly dissociated species (NH2 or NH). The nitride formation starts to develop only above 300°C as evidenced by both a rapid XPS nitrogen coverage increase and new Si-N UPS features in this domain.

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.

STRONG CORRELATION SATELLITES IN CORE LEVEL PHOTOEMISSION FROM MN IN THE MONOLAYER RANGE ON AG(001)

Abstract We observe a complex shape of the Mn2p core level photoemission spectra for Mn in structures of reduced dimensionality such as an ideal monolayer on Ag(001). The Mn2p 3/2 and Mn2p 1/2 spectral intensity is found to split into doublets with separation about 4 and 5 eV, respectively. The data indicate an interpretation in terms of a poorly screened and a well screened final state of the Mn with a core hole. In the monolayer range these two final states have comparable intensities and dominant contributions from atomic 2p 5 3d 5 and 2p 5 3d 6 configurations, respectively. Their intensity ratio is found to be extremely sensitive to the Mn atomic environment and directly reflects the degree of the Mn3d localization, electron correlation and reduction in charge fluctuations. Additional fine structure is clearly visible in high-resolution data for both final state 3d electrons counts and can be assigned to direct and exchange Coulomb interactions of the unfilled core and 3d shells. Slow 3d-charge fluctuations also result in a remarkable three-peak structure of the Mn3s line.

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.

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.