THE ROLE OF A SUPEROXO-LIKE SPECIES IN THE OXIDATION OF ALKALI METAL-PRECOVERED GAAS(100) SURFACES

The interaction of oxygen with a Li-, K-, and Cs-covered (1 monolayer) GaAs(100) surface is studied by means of AES, work function and UPS between 100 and 300 K. The initial sticking probabilities of oxygen (0.4(Li) and 0.7(K, Cs)) are enhanced by approximately 10(4) with respect to the clean surface. However, an induction period is always seen to precede the onset of the oxidation of the substrate. In the case of O2 + Cs/GaAs(100) at 100 K, a peroxo (O2(2-)) and a superoxo (O2-) species successively appear, the formation of the latter being seen to parallel the oxidation of the substrate. The increasing efficiency from Li to Cs in enhancing the oxidation of the substrate is discussed in this connection.