Chemisorption and oxidation studies of the (110) surfaces of GaAs, GaSb, and InP

The surface chemistry of cleaved GaAs (110) (and, to a lesser extent, InP and GaSb) is studied as a function of oxygen exposure (both unexcited and excited) with soft-x-ray photoemission spectroscopy. When the cleaved GaAs (110) surface is exposed to molecular oxygen in the ground state, chemisorption to only the arsenics takes place. No back bonds are broken even for large exposures. Room-temperature oxidation of the surface can be induced by exciting the oxygen, e. g. , by an ionization gauge. The adsorption of excited oxygen is initially the same as for the unexcited, except 500 times faster. However, after > 20% of a monolayer has been adsorbed, further exposure to excited oxygen causes back bonds to be broken and As₂O₅ and Ga₂O₃ are formed. Larger doses of excited oxygen result in the formation of thicker oxides composed primarily of Ga₂O₃ with small amounts of elemental As (or As bound to only one Ga) and As₂O₃, most of which has subilimed from the surface. No As₂O₅ is seen in the thicker oxide because there is a deficiency of oxygen, and any partially oxidized Ga present will reduce the arsenic oxides. The escape depth for GaAs (110) was measured for electron kinetic energies between 20 and 200 eV. This range includes the minimum in the escape depth which is about 6 at 60 eV. No chemical shift in the core levels between the atoms on the surface and in the bulk was observed. GaSb (110) and InP (110) surfaces were also studied. InP behaves like GaAs, whereas the GaSb is oxidized immediately even when exposed to only unexcited oxygen. The oxygen uptake curves for GaSb and GaAs were compared and found to be quite different with a sticking coefficient, at zero coverage, of 2 10^-4 for GaSb and 8 10^-10 for GaAs.