Relative reactivity of arsenic and gallium dimers and backbonds during theadsorption of molecular oxygen on GaAs(100)(6x6)

The chemisorption sites of molecular oxygen on the mixed GaAs(100)(6x6) sur face were imaged at room temperature using scanning tunneling microscopy (S TM). This surface is terminated by both gallium dimers and arsenic dimers, neither of which react with oxygen. Instead, the As-Ga backbonds are shown to react with O-2 with 100% chemical selectivity. The reason for this selec tivity is found in the interaction of the highly electronegative oxygen ato ms with the higher electron density at the arsenic atoms. One oxygen atom d isplaces the attacked arsenic atom while the other oxygen atom bonds to two nearby gallium atoms, resulting in the thermodynamically most stable react ion products: metallic arsenic clusters and gallium oxide. (C) 2000 America n Institute of Physics. [S0021-9606(00)71042-7].