IN-SITU SPECTROSCOPIC ELLIPSOMETRY AND REFLECTANCE DIFFERENCE SPECTROSCOPY OF GAAS(001) SURFACE RECONSTRUCTIONS

We have studied GaAs(001) surface reconstructions prepared by molecula r beam epitaxy in-situ by spectroscopic ellipsometry (SE) and reflecta nce difference spectroscopy (RDS or RAS). The simultaneous measurement of the dielectric function by SE allow us to identify surface and bul k related contributions to the RDS signal and to extract the optical a nisotropy Delta epsilon = <epsilon((1)over bar 10>) - epsilon(110). Fo r the (4 x 2), the c(4 x 4), and the (2 x 4) reconstructions we find r esonances in Im(Delta epsilon) at 2.75 eV, 2.6 eV and 2.85 eV at T = 8 0 degrees C. The resonance at 2.85 eV corresponds at growth temperatur es (T = 600 degrees C) to the 2.6 eV feature so far attributed to elec tronic transitions of the As dimers and coincides with the E(1) bulk t ransition. Measurements as a function of temperature additionally reve al that they both share the same temperature dependence. This close re lationship between the As dimer and the bulk optical response will be discussed.