Investigation of surface acoustic wave fields by scanning tunneling microscopy

The modulation of the distance between the solid surface and the tip of a scanning tunneling microscope (STM) by a high frequency surface acoustic wave (SAW) is used to detect its amplitude and phase. Due to the nonlinearity of the tunneling process the distance modulation leads to an increase of the dc tunneling current and to a mixing of the SAW frequency with another electrical or acoustical rf signal applied to the tunneling gap. A STM used in this way is termed a scanning acoustic tunneling microscope (SATM). The current component at the difference frequency which can be chosen as low as a few kHz contains the full wave field information. The method utilized in our experimental setup applies a high frequency electrical signal which is fed as an ac modulation voltage to the tip. The STM signal is fed to the signal input channel of a two phase lock-in amplifier. As reference frequency for the lock-in a signal is supplied generated by external mixing of the SAW frequency with the modulation frequency. It has been found that the amplitude and the phase output signals of the lock-in are influenced by the topography of the surface. This influence has been modelled. The model leads to a separation of the SAW parameters (amplitude and phase) from contributions of the topography. On the other hand the topography is imaged in the phase domain. The physical background of the method and the experimental setup are described in detail. Measurements made with SAWs propagating in LiNbO₃, are presented and discussed in relation to the developed model. They demonstrate the ability of our method to determine velocity and propagation direction of a SAW in an area with dimensions much smaller than the SAW wavelength