Spectroscoping Imaging of Surfaces with Atomic Resolution

Abstract

With its ability to probe surfaces with atomic resolution, the scanning tunneling microscope (STM) has attracted wide attention. Since tunneling depends on the overlap of charge densities of sample and tip, the ‘topograph’ determined with the STM also reflects the density of filled and empty states of the tip and surface, respectively, which are involved in tunneling [1,2]. For biases, of 1–2 eV several different surface states can be involved, particularly for semiconductor surfaces. Since the tip is presumably a point which one attempts to maintain the same while scanning, the ‘topograph’also reflects variations associated with the local surface density of states (DOS) of the sample. The surface states on semiconductors can dominate tunneling and produce variations in ‘topographic’ images up to 1A for homogeneous surfaces, e.g. Si [3], ~ 2A for inhomogeneous systems, e.g. Ag/Si [4], and up to 10A where strong charge transfer can occur, e.g. 0/GaAs [5]. Understanding the role of these electronic states and delineating DOS effects from geometric features on the surface is important for using the STM. Furthermore, delineating DOS features from geometric features can allow the spatial imaging of surface states [3,6], Relating the locations and energies of these states to the different atoms on the surface can provide new insight to the bonding interaction arising at semiconductor surfaces. This paper summarizes some recent methods and issues regarding spatially resolved surface spectroscopy with the STM as well as some recent findings.