Fourier analysis of temporal and spatial oscillations of tunneling currentin scanning tunneling microscopy

Partially oxidized Si(111) surfaces and surfaces of highly oriented pyrolyt ic graphite (HOPG) were studied by two different ultrahigh vacuum scanning tunneling microscope (UHV-STM) systems and by an STM system working under a mbient conditions, respectively. The STM current images of partially oxidiz ed Si(111) surfaces and HOPG surfaces were analyzed by one/two-dimensional fast Fourier transformation (1D-FFT/2D-FFT). The phenomenon of temporal osc illations of tunneling current on the partially oxidized Si (111) surfaces was detected with both UHV-STM systems. Temporal as well as spatial oscilla tions of tunneling current appeared in highly resolved STM current images o f the Si(111) surfaces simultaneously, but both kinds of oscillations could be discriminated according to their different influence on the 2D-FFT spec tra of the current images, while varying the scanning range and rate. On cl ean HOPG surfaces only spatial oscillations of tunneling current induced by the surface structure were observed.