PROBING OF OSCILLATING SURFACES BY A SCANNING ACOUSTIC TUNNELING MICROSCOPE

The scanning acoustic tunneling microscope (SATM) which is based on a scanning tunneling microscope, is capable of detecting the amplitude and the phase of high-frequency surface acoustic waves (SAWs) as well as the surface topography. For our experiments the SAWs have been excited by interdigital transducers on YZ-LiNbO3 samples. A thin gold film with a thickness of about 100 nm has been deposited on its surface. The read-out of the high-frequency acoustic wave field is performed by a mixing technique. Owing to the non-linear dependence of the tunneling current on the tip-to-sample distance an additional slightly-shifted high-frequency modulation of the gap voltage leads to an increase of the d.c. tunneling current and to a low frequency signal at the difference frequency. The a.c, tunneling current contains the information on the elastic properties of the solid. Thin films deposited on the surface cause a dispersion of the phase velocity, which then reveals the elastic parameters. By measuring the phase and the amplitude of an acoustic wave field with high spatial resolution the SATM technique allows the mapping of the acoustic wave held and thereby the investigation of the elastic properties of thin films.