Using molecular-beam-epitaxy-grown InAs and InSb on InP(001) surfaces, we show that the friction-force microscope is sensitive to monolayer coverage. Those surfaces are characterized by three-dimensional islands separated by flat regions. For a constant load, the frictional forces measured on the InAs island and on the substrate are the same. This is due to the formation of a two-dimensional wetting layer (1.5 ML) of InAs covering the InP(001). The frictional force is controlled by the interaction of this layer and the tip. In contrast, the deposition of 2 ML of InSb on InP(001) produces a different behavior. The frictional force changes when the tip moves from the island to the flat region. Photoluminescence and atomic-force-microscopy experiments show the formation of an InSb submonolayer. The sensitivity of the friction-force microscope to monolayer coverage illustrates its usefulness for wetting-layer analysis. Based on these results we discuss the potential of the friction-force microscope to develop a spatially resolved friction spectroscopy.
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Tamayo et al. (1997) studied this question.