Submonolayer sensitivity of InSb on InP determined by friction-force microscopy

Abstract

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. @S0163-1829~97!51920-5# The understanding of friction at atomic and molecular levels is a subject of intensive research with relevant technological implications. Several experimental and computational techniques are being applied to obtain the fundamental basis of friction, wear, and lubrication. 1,2 From those studies a picture is emerging where the friction between two surfaces implies hysteresis in the adhesion forces between them. 3 It also shows the importance of phonons and electrons as mechanisms to dissipate energy during the sliding of two solid surfaces. 4 The measurement of lateral forces with an atomic force microscope has introduced a proximal probe technique called friction-force microscopy ~FFM!. 5,6 Its rapid development has allowed systematic studies of the friction between a single, nanometer-size asperity and several surfaces. 7,8 FFM studies have underlined the anisotropy of the friction in crystalline surfaces. 9,10 It has also been pointed out that lateral forces could be used to differentiate chemical species in heterogeneous samples. 11‐15 Those findings could establish the basis for a spatially resolved spectroscopy based on force microscopy that could combine high spatial and compositional resolution. However, in many cases the forces contributing to the torsion of the cantilever come from topographic features, capillary or mechanical properties, i.e., not from what could be thought a friction property between the tip and the bare sample surface. In a previous paper we demonstrated that variations of less than 10% of indium composition in InxGa12xAs structures produce measurable changes in the frictional force while the normal load remains constant. 15,16 Here we report on the sensitivity of the FFM to detect the presence of adsorbed monolayers of semiconductor compounds. We also discuss the potential of FFM to develop a spatially resolved friction spectroscopy. As model systems to perform FFM studies with stiff and heterogeneous surfaces, we deposited InSb and InAs on semi-insulating InP~001! substrates. Those are highly strained semiconducting materials. It is known that beyond a certain thickness threshold, the strain can be relieved through free surfaces and substrate deformation. The resulting surface is characterized by the formation of nanostructures and quantum dots. The small size of the structures has raised interest of this process to fabricate devices such as arrays of quantum dots. 17 In this paper, the presence in the same surface of three-dimensional islands ~dots! and flat regions in between is used to evaluate, among other properties, the thickness of the overlayer to give a friction signal different from the substrate. The structures were grown in ultrahigh vacuum using a solid-source molecular-beam epitaxy system ~MBE! in a