Formation of metastable epitaxial CoSix (x<2) layers by reactive codeposition on CoSi2(111)

Abstract

The reaction of Co deposited at 350 °C on epitaxial CoSi2 is investigated by means of low-energy electron diffraction, core-level and angle-resolved valence-band photoemission, and ion scattering spectroscopy. Co is deposited onto ∼100-Å-thick CoSi2(111) films epitaxially grown on Si(111) which exhibit a Si-rich surface. For one Co monolayer equivalent, the Si-rich surface labeled CoSi2(111)–Si is converted into a bulklike terminated one, labeled CoSi2(111). The latter is characterized by a specific surface state located at ∼2.8 eV binding energy at the center of the surface Brillouin zone, which derives from dangling bonds on Co and Si in the topmost planes. Upon increasing the amount of Co, the latter is found to react with the silicide layer up to much larger depths comparable to the photoelectron mean free path (15–20 Å). A thin metastable epitaxial CoSix (x≤2) overlayer with a cubic structure is formed for Co amounts up to the equivalent of 15 monolayers, typically. The electronic as well as crystallographic structure of this thin metastable CoSix layer is compared to that of recently discovered metastable CoSix layers, epitaxially grown on Si(111) by codeposition and found to be closely related. The data strongly suggest a simple picture where the additional Co merely diffuses into the initial CaF2-type CoSi2 layer and occupies part of the interstitial octahedral sites of the face centered cubic Co sublattice. Moreover, it is shown that upon reaction of 6–8 Co monolayers the surface state observed on the ideal-bulk–CoSi2(111) termination still persists but is shifted toward higher binding energies due to changes in the composition of the silicide underneath. This indicates that the surface structure of the CoSix (x≤2) layer is also similar to that of bulk-terminated CoSi2(111). For higher Co coverages, an epitaxial ε-CoSi layer with the stable B20 type structure is achieved, as attested by a ∛×∛ R30° low-energy electron diffraction pattern and specific valence-band features.