REFLECTANCE-DIFFERENCE SPECTROSCOPY OF (001) GAAS-SURFACES IN ULTRAHIGH-VACUUM

Reflectance-difference spectroscopy (RDS) is employed to study in situ the (4 X 2), (1 X 6), (4 x 6), (3 X 1), (2 X 4)-alpha, (2 X 4)-beta, (2 X 4)-gamma, c (4 X 4), and d (4 X 4) reconstructions of (001) GRAS surfaces prepared in ultrahigh vacuum (UHV) by molecular-beam epitaxy and simultaneously characterized by reflection high-energy electron diffraction (RHEED). Reproducibility of the data is excellent. With the aid of previous theoretical calculations, we interpret characteristic spectral features at 1.9, 2.6, and 4.2 eV in terms of electronic excitations involving surface dimers of Ga, As, and As, respectively. Because RD couples to local electronic structure rather than to long-range order, RD spectra not only determine surface reconstructions but also provide details not accessible by RHEED, such as the existence of As dimers in the (1 X 6), (4 X 6), and (3 X 1) reconstructions and of the fractional coverage within a given reconstruction. Our data show that the (3 X 1), (1 X 6), and (4 X 6) reconstructions are at least partly determined by kinetics, since they can only be obtained by following specific heating or cooling procedures under very low AS4 flux. More generally, it is possible to employ this optical technique to determine surface atomic and electronic structure. Because RD spectra can be obtained with the surface in any transparent ambient, the database that we have established here provides a new approach for elucidating surface reconstructions of (001) GaAs and hence the dynamics of surface reactions in non-UHV environments.