Location of atoms in the first monolayer of GaAs on Si

The position of Ga and As atoms at monolayer coverages of heteroepitaxial GaAs on clean Si(111) have been measured by x-ray standing waves in UHV. Though both As and Ga are incident on the surface As atoms choose to occupy the upper half of the (111) double plane about 5% higher on the average, relative to the Si(111) d spacing, than the bulk silicon position. Ga atoms are exclusively located on the lower half of the (111) plane about 3% higher than the corresponding bulk positions.