Probing electrical transport, electron interference, and quantum size effects at surfaces with STM/STS

Abstract

We use scanning tunneling microscopy (STM) and spectroscopy (STS) to probe electrical transport through the dangling-bond surface states of semiconductors and electron scattering and electron confinement effects in metal surface states. Specifically, we use point contacts between the STM tip and the sample to show the existence of surface electrical transport in Si(111)-7 × 7. Point contacts to silicon islands provide further support for the existence of the surface transport channel and illustrate the role played by carrier scattering at the boundaries of the nanostructure in electrical transport. In contrast to the silicon case, electrons in Shockley-type metal surface states act like a quasi-two-dimensional free-electron gas (2DFEG). This 2DFEG is scattered by steps, adsorbates, and defects, and the interference between incident and reflected electron waves leads to an oscillatory local density of states (LDOS). This LDOS is imaged in STM spectroscopic maps, and analysis of the oscillations provides novel information regarding electron scattering by individual surface features. Steps are found to act as barriers for surface electrons, and this property is utilized to confine them and form structures of lower dimensionality. Quasi-1D structures (quantum wires) are generated at narrow terraces of stepped surfaces, while small metal islands behave as 0D structures (quantum dots). Confined states with discrete spectra are observed even at 300 K, and their probability distributions are imaged.