COADSORPTION OF CESIUM AND OXYGEN ON GAAS(001) SURFACES STUDIED BY METASTABLE DEEXCITATION SPECTROSCOPY

Metastable de-excitation spectroscopy (MDS) has been employed for moni toring the electronic structure variation in Cs- and oxygen-adsorption processes at a Ga rich p-GaAs(001)-(4 x 2) surface. MDS spectra at lo w Cs coverages showed a peak at 2.6 eV below E-F due to filling of Ga dangling bonds by charges from Cs 6s. At moderate coverages a Ca 6s-in duced peak (6 (s) over tilde) appeared just below E-F with an abrupt i ncrease at around thetasimilar to 0.5, whereas contribution from subs trate valence states remained on the MDS spectrum. This suggests that the Cs-induced electronic states are not fully delocalized. Upon admis sion of oxygen onto the surface, O 2p-induced states (2 (p) over tilde ) appeared with multiple peak structures, implying direct bonding of o xygen with substrate atoms even at the initial oxygenation stage. Vari ations in the Cs 6 (s) over tilde and O 2 (p) over tilde emissions wit h oxygen supply showed enhanced oxygen uptake induced by the charge tr ansfer from the Cs 6 (s) over tilde states. (C) 1998 Elsevier Science B.V. All rights reserved.