Interaction of Cs and O with GaAs(1 0 0) at the overlayer–substrate interface during negative electron affinity type activations

Different activation procedures on GaAs(1 0 0) leading to negative electron affinity (NEA) have been investigated with photoemission and photovoltage effect spectroscopy. Three different regimes of NEA surface activation have been identified and characterized by their bandbending properties and their chemical interface composition. The NEA surface can be presented as a simple function of Cs and O depositions. A fixed excess of Cs 0.55 (∼0.2) ML is necessary for NEA activation. For activations involving thin activation layers and only small amounts of O, the bands are flattened only to a small degree. No increase in bandbending is observed. Activations involving thick layers of Cs and sufficient oxidation results in a flattening of the bands. The degree of substrate interface oxidation depends on the degree of charge transfer from the Cs-overlayer to the substrate prior to oxygen exposure. The catalytic properties of the adsorbed Cs depend on the Cs-layer metalicity and therefore the Cs-coverage prior to oxidation.