Professor Thorsten Hesjedal FInstP

Abstract:

Starting from the conventional microprobe techniques, a scanning acoustic tunneling microscope (SATM) and a scanning acoustic force microscope (SAFM) have been developed to detect particle displacements at solid surfaces up to GHz frequencies. Based on the non-linear dependence of the tunneling current in SATM and of the forces in SAFM on the tip to surface distance, respectively, it is demonstrated that wave field parameters of surface acoustic waves can be measured with a lateral resolution in the submicrometer range.

Abstract:

Grating-like metal electrodes supplied with high- and low-frequency voltages were investigated by a Scanning Acoustic Force Microscope (SAFM). On piezoelectric LiNbO3, these electrodes form an interdigital transducer which generates surface acoustic waves according to the charge distribution on its electrodes when excited at a transducer's passband frequency. The influence of surface deformation and of surface charges on the cantilever deflection of the SAFM in contact and non-contact mode was systematically studied. Comparing the images of the electrodes which were prepared on piezoelectric and dielectric substrates, surface deformation and charge distributions were qualitatively separated. © 1995 Springer-Verlag.

Abstract:

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 field and thereby the investigation of the elastic properties of thin films. © 1995.