X-ray standing-wave study of alkali-metal/Si(111) 7×7 interfaces

The x-ray standing-wave technique was applied to study the interfaces formed between alkali metals (Rb,Cs) and Si(111)7×7. Through the use of two Bragg reflections [(111) and (220)], the distance of the adsorbate atoms to two bulk lattice planes was measured, which facilitated the determination of the adsorption site for different adsorbate coverages and annealing temperatures. Both alkali metals behaved in a similar way. At room-temperature saturation coverage, a rather disordered adsorption was observed, having at least on-top and threefold-hollow sites (normal and eclipsed). After an anneal of the adlayer and coverage reduction, the occupation of hollow sites of normal type was promoted. At very low coverages (Θ