PROBING ELECTRICAL-TRANSPORT, ELECTRON INTERFERENCE, AND QUANTUM-SIZEEFFECTS AT SURFACES WITH STM STS/

We use scanning tunneling microscopy (STM) and spectroscopy (STS) to p robe electrical transport through the dangling-bond surface states of semiconductors and electron scattering and electron confinement effect s in metal surface states. Specifically, we use point contacts between the STM tip and the sample to show the existence of surface electrica l transport in Si(111)-7x7. 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 quas i-two-dimensional free-electron gas (2DFEG). This 2DFEG is scattered b y steps, adsorbates, and defects, and the interference between inciden t and reflected electron waves leads to an oscillatory local density o f states (LDOS). This LDOS is imaged in STM spectroscopic maps, and an alysis of the oscillations provides novel information regarding electr on scattering by individual surface features. Steps are found to act a s barriers for surface electrons, and this property is utilized to con fine them and form structures of lower dimensionality. Quasi-1D struct ures (quantum wires) are generated at narrow terraces of stepped surfa ces, while small metal islands behave as OD structures (quantum dots). Confined states with discrete spectra are observed even at 300 K, and their probability distributions are imaged.