No abstract

Surface topographic (STM) and spectroscopic (STS) studies have been performed on the Si-terminated 6H-SiC(0001)(3×3) surface using a scanning tunneling microscope (STM) in ultrahigh vacuum. High-quality (3 × 3) overstructures have been prepared as observed by LEED and STM. The regular (3 × 3) surface sites revealed much weaker I(V ) dependences as compared to the defect sites when measured using the constant tip-surface gap technique. The normalized (dI/dV )/(I/V ) vs. V spectra exhibit distinct bands of empty and filled states, which are separated by 1.2 eV for both surface sites, respectively. The results thereby support a Mott-Hubbard-type model as used for the calculation of the density of states. However, the STS spectra become completely featureless in the range of small tip-surface distances and reveal a "metallic"-like Ohmic I-V dependence.

An atomically flat Si(111)/Ag(3×3)R30° surface has been modified using a scanning tunneling microscope in ultrahigh vacuum. Mesoscopic pits have been created by applying negative voltage pulses to the sample, while at opposite voltage polarity mounds were formed. Moreover, lines could be written by moving the scanner at elevated voltages. The threshold voltage for pit formation increases almost linearly with the distance of the tip to the surface and drops to a value below 2 V for the closest approach. At sufficiently high voltages the depth extends beyond the silver layer height. The lateral pit size is well below 8 nm and can be reduced to values between 2 nm and 5 nm for voltages slightly above the threshold. Even selective top layer Ag atom removal has been achieved.