In-situ spectroscopic ellipsometry and reflectance difference spectroscopy of GaAs(001) surface reconstructions

We have studied GaAs(001) surface reconstructions prepared by molecular beam epitaxy in-situ by spectroscopic ellipsometry (SE) and reflectance difference spectroscopy (RDS or RAS). The simultaneous measurement of the dielectric function by SE allow us to identify surface and bulk related contributions to the RDS signal and to extract the optical anisotropy Δε = ε110 − ε110. For the (4 × 2), the c(4 × 4), and the (2 × 4) reco nstructions we find resonances in Im(Δε) at 2.25 eV, 2.6 eV and 2.85 eV at T = 80°C. The resonance at 2.85 eV corresponds at growth temperatures (T = 600°C) to the 2.6 eV feature so far attributed to electronic transitions of the As dimers and coincides with the E1 bulk transition. Measurements as a function of temperature additionally reveal that they both share the same temperature dependence. This close relationship between the As dimer and the bulk optical response will be discussed.