Professor Thorsten Hesjedal FInstP

Abstract:

A method is presented for the universal probing of surface acoustic waves (SAWs). For measuring high frequency SAWs by scanning tunneling microscopy (STM) a stroboscopic snapshot technique was employed, named scanning acoustic tunneling microscopy. The amplitude and the phase of a state of oscillation within a SAW field are a superposition of the surface topography and the oscillation trajectory. Measuring with atomic resolution the observed contrast can be understood by a spherical tunneling model. A STM based system is proposed that reaches submicron resolution for the quantitative evaluation of elastic constants. With this system the velocity dispersion is obtained from the detection of laser generated SAW pulses by a broadband STM. Scanning acoustic tunneling spectroscopy using different acoustic modes opens a door for quantitative studies of nanoscale structures. © 1997 American Vacuum Society.

Abstract:

This paper reports about the first determination of the phase velocity of surface acoustic waves (SAWs) on the nanometer scale. With a scanning acoustic force microscope (SAFM) a maximum lateral resolution of 19.9 nm has been achieved. This is almost two orders of magnitude better than the resolution of standard quantitative acoustic microscopy. The key of measuring the phase of high frequency signals with a slowly responding SAFM cantilever is frequency mixing at its non-linear force curve. For demonstrating its abilities SAW dispersion was studied on Au layers of different thicknesses by SAFM over a lateral distance of down to 200 nm.

Abstract:

The distributions of surface oscillation and surface charges were probed within an interdigital transducer (IDT). The IDT was driven at a frequency of 39.5 MHz. The measurements with sub-μm spatial resolution were performed with a scanning acoustic force microscope. It utilizes the nonlinear interaction of the sample with the tip of a scanning force microscope. In the case of surface oscillation detection, this nonlinearity leads to a shift of the mean position of the cantilever due to varying oscillation amplitudes. The surface charges are mapped through the additional cantilever deflection caused by the attraction of the plates of the capacitor formed by the cantilever and the sample. Spatial distributions of the amplitude of surface oscillations and of surface charges at the end of a 39.5 MHz splitfinger IDT are presented. The obtained experimental results may lead to a deeper understanding in modelling of IDTs in the future.

Abstract:

We report about a new technique for the investigation of SAW fields within SAW devices reaching submicron lateral resolution. The scanning acoustic force microscope (SAFM) is based on a standard force microscope and utilizes the nonlinear force curve in the sense of a mechanical diode. Varying wave amplitudes therefore lead to different shifts of the cantilever's rest position. With SAFM we investigated SAW devices with center frequencies above 600 MHz. We found a local effect of massloading on the standing wave amplitude within IDTs. Furthermore, we measured the dynamic behavior of the IDT's wave pattern when sweeping the frequency.

Abstract:

A high frequency oscillating Au (111) surface was measured with atomic resolution using a modified scanning tunneling microscope. On the atomic scale propagating surface acoustic waves lead to oscillations of atoms on elliptical trajectories, with the axes being determined by the material parameters of the surface. Since those oscillation frequencies are much higher than the scan frequencies the topography contrast is reduced. This basic problem is solved by measuring a stroboscopic snapshot seeing a defined state of oscillation. The atomic resolution of the phase and the amplitude contrast is explained by the superposition of the surface topography and the oscillation trajectory. © 1996 American Institute of Physics.