Influence of electrostatic forces on the investigation of dopant atoms in layered semiconductors by scanning tunneling microscopy/spectroscopy and atomic force microscopy

Abstract

Investigation of the atomic scale topography and electronic structure of dopant sites in semiconductor materials is a promising application of scanning probe microscopies. Dopants have been imaged with scanning tunneling microscopy (STM) on and near the surface of conventional semiconductor materials as well as on layered compounds. On both kinds of materials, dopants are detected as either protrusions or depressions in the STM image. The comparison of the measured heights between the materials shows that the values on layered materials are considerably larger than those on the conventional (three-dimensional) semiconductors. We interpret this as the influence of dopant induced electrostatic forces between the tip and sample leading to a structural deformation of the surface around dopant atoms. In order to investigate the influence of electrostatic forces, we performed STM measurements on p-type MoS2 at different bias voltages. The bias dependence of the images indicates the presence of electrostatic forces and demonstrates the influence of screening due to the surrounding electron density. Additional measurements with current imaging tunneling spectroscopy show that changes in the density of states at dopant sites plays only a minor role and cannot account for the large protrusions observed. Atomic force microscopy measurements, with an applied dc voltage between cantilever and sample, also confirm the role of electrostatic forces since voltage dependent changes in the topograpy were observed.