Bulk electronic structure ofAlB2-type erbium disilicide with and without Si vacancies

The bulk electronic structure of ${\mathrm{AlB}}_2$-type Er disilicide is investigated by means of extended Hückel band calculations. The formation of the silicide bands is discussed for both stoichiometric ${\mathrm{ErSi}}_2$ and experimentally observed nonstoichiometric ${\mathrm{ErSi}}_{1.7}$ exhibiting an ordered array of Si vacancies. For both cases, the bands near the Fermi level show a dominant Er 5d character. Mixed Er 5d–Si 3p bonding states are located in the 2–4-eV binding energy (BE) range whereas lower-lying bands are composed of almost pure Si 3s,3p orbitals. The presence of the Si vacancies significantly affects all the bands and produces two additional bands in the gap observed for stoichiometric ${\mathrm{ErSi}}_2$ along ΓA in the 1.1–2.6-eV BE range. These theoretical results are compared to photoemission data collected at normal emission from ${\mathrm{ErSi}}_{1.7}$ films grown epitaxially on Si(111). The agreement is surprisingly good when the emission from surface states is taken into account. In particular, the experiments indicate a Si vacancy-induced band located near 1.2 eV BE as well as the band folding resulting from the specific ordered arrangement of the Si vacancies (period doubling along c) as assumed in the calculations.