Sensitivity of Reflectance Anisotropy Spectroscopy to the Orientation of Ge Dimers on Vicinal Si(001)

Reflectance anisotropy spectroscopy (RAS) is employed to follow the initial room temperature growth of Ge dimer layers on clean vicinal Si(001)- $(1\times 2)$. The experimental data show structure in the region of 2.5 $$eV which changes sign depending on Ge dimer orientation. Comparison with microscopic calculations for one monolayer and two monolayer Ge coverages reveals excellent agreement with experiment, demonstrating that surface states localized on Ge dimers are responsible for the RA response in the region of 2.5 eV. This provides the first clear and unambiguous proof that a dimer related transition can be responsible for the RAS sensitivity to dimer orientation.