Time-dependent oscillations of tunneling current on partially oxidized Si(111) surfaces

Clean and partially oxidized Si (111) surfaces were investigated by scanning tunneling microscopy and spectroscopy (STM/STS). In contrast to STS spectra measured on clean Si surfaces, time-dependent periodic oscillations of tunneling current were detected on partially oxidized Si surfaces. The STM current images of the clean Si surface with atomic resolution and the partially oxidized Si surfaces were analyzed by two-dimensional fast Fourier transformation (2D-FFT). The oscillation frequencies determined by STS and Fourier analysis are roughly multiples of a basic frequency of 30 Hz. The oscillations disappear at small tip sample distance.