Hydrogen-induced modifications of GaAs(001) surfaces probed by reflectance anisotropy spectroscopy

GaAs(001) surfaces reconstructed c(4 x 4), (2 x 4) and (4 x 2) were prepared from As-2-capped samples grown by molecular beam epitaxy (MBE). The exposure of these surfaces to atomic hydrogen was monitored mainly with Reflectance Anisotropy Spectroscopy (RAS) but complemented in addition by LEED, HREELS and AES studies. The results show that the H exposure leads firstly to breaking of As dimers and with a slower rate also breaking of Ga dimers. This process leaves As-II and Ga-H bonds on the surface. While the latter seem to remain on the surface (nonvolatile GaH3 compound) sufficient H exposures induce an etching process for As (volatile AsH3 compound). All three reconstructions, therefore, evolve through different intermediate stages, which call be under stood from their starting dimer arrangement, to the same surface at large exposures (10(4) to 10(5) L): a surface of anisotropic rough morphology which is dominated and probably largely passivated by Ga-H bonds. This study, moreover, demonstrates the useful complementary propel ties of RAS with respect to the more classical electron surface science techniques.