INFLUENCE OF ELECTROSTATIC FORCES ON THE IMAGING PROCESS IN SCANNING-TUNNELING-MICROSCOPY

We performed scanning tunneling microscopy (STM) experiments on layere d semiconductor compound surfaces which suggest a significant influenc e of electrostatic forces (EF) on the imaging process. We performed ST M experiments at varying tunneling biases on plain MoTe2 and ultrathin epitaxial WS2 films on MoTe2 substrates. We observed tunneling bias d ependent height changes of up to several nanometers of layer edges and layer terraces. Both of the samples showed height increases of up to 8 nm for step edges depending on the tunneling bias. Only at certain b iases the values known from X-ray diffraction for the layer thickness were approached. These observations cannot be explained solely by elec tronic effects and tunneling probability changes. Our evaluation of th e results shows that mechanical changes of the morphology caused by va rying EF interaction between tip and sample are likely to be the cause of these phenomena. In order to investigate the magnitude and influen ce of EF between tip and sample more closely we performed additional e xperiments with bias applied atomic force microscopy (BAAFM) which ind icate a strong influence of EF on the imaging process. Atomic force cu rves with additionally applied bias yielded that the EF are in the ran ge of several ten to several hundred nN depending on the tunneling bia s. The corresponding charge density on tip and sample suggests the pre sence off single electrons and holes at the interface instead of homog eneous charge densities which might result in a pulsed EF interaction between tip and sample. (C) 1998 Elsevier Science S.A. All rights rese rved.