Surface reconstructions of (001) after molecular-beam epitaxy were studied  in situ  by means of spectroscopic ellipsometry and reflectance difference or reflectance anisotropy spectroscopy. Simultaneous measurements, using spectroscopic ellipsometry and reflection difference methods, permitted the identification of surface and bulk-related contributions to the optical anisotropy. In the case of (2 x 4), c(4 x 4), and (4 x 2) reconstructions, it was found that there were resonances at 2.9, 2.6 and 2.25eV, at a temperature of 80C. Although the resonance at 2.9eV coincided with the E1 bulk transition, and exhibited the same temperature dependence, they could be distinguished because the surface contribution changed sign upon switching from the (2 x 4) to the c(4 x 4) reconstruction while the bulk contribution remained the same.

M.Wassermeier, J.Behrend, K.H.Ploog, J.T.Zettler, K.Stahrenberg, W.Richter: Physical Review B, 1996, 53[20], 13542-6