Adsorption of oxygen on clean cleaved (110) gallium-arsenide surfaces

The adsorption of oxygen on clean cleaved (110) GaAs surfaces of $n$- and $p$-type material was studied by means of Auger electron spectroscopy, ellipsometry, and low-energy-electron diffraction. For both types of material a stable state of adsorption is found with an absolute coverage of approximately half a monolayer of oxygen. The sticking coefficient, which is dependent on the coverage, has different initial values $S_0$ for $p$- and $n$-type material ($p:S_0\simeq 1.4\times {10}^{-6\mathrm{}}, n:S_0\simeq 3\times {10}^{-5\mathrm{}}$). The coverage dependence of the sticking coefficient for both $p$- and $n$-type crystals is described in terms of an Elovich equation which can be derived for a thermally activated adsorption. Different bonding models for the adsorption of oxygen on the (110) surface are discussed and a qualitative interpretation of the different behavior of $p$- and $n$-type materials is given in terms of the different positions of the Fermi level at the surface.