Professor Thorsten Hesjedal FInstP

Abstract:

With the increasing interest in nanostructures and thin films, the need for a quantitative measuring method for elastic constants on the nanometer scale has become more evident. The fundamental physical quantity characterizing the elastic constants is the acoustic phase velocity. Due to the strong localization of surface acoustic waves (SAWs) in the near-surface region, SAWs are particularly favored for such investigations. The velocity measurement is commonly performed by time delay and acoustic far-field methods. Therefore the lateral resolution of the velocity measurement is restricted by the wavelength involved to some tens of microns. Recently, we introduced the scanning acoustic force microscope (SAFM) for the measurement of SAW amplitude distributions with nanometer lateral resolution. The key to detecting high-frequency surface oscillations by the slowly responding force microscope cantilever is the nonlinear force curve. This nonlinearity can be exploited in a heterodynetype setup for high-frequency wave mixing of a probe and a reference wave, revealing the phase of the probe wave. The difference frequency can be chosen to be as low as 1 kHz. We present measurements of the phase velocity over a lateral distance of 19:9 nm. The phase velocity dispersion due to Au layers on a quartz substrate was measured over distances as small as 200 nm and compared with calculations. © 1998 Springer-Verlag.

Abstract:

Surface acoustic wave (SAW) devices are of great importance in mobile communication and signal processing applications. For their optimization second-order effects, like diffraction or mass loading, have to be studied. However, meeting today's demands of GHz operation new ways of wave field mapping have to be developed, since common methods, like laser optical or electron microscope probing, are resolution limited to the micron range. Scanning acoustic force microscopy (SAFM) allows the detection of the high-frequency surface oscillations having sub-Å amplitudes with the force microscope's typical lateral resolution. The key for measuring the high mechanical frequencies is the nonlinear force curve. Another approach is the force microscope mapping of rearranged fine particles, revealing the nodes and antinodes of the standing wave field. We present measurements in the near field of, and within, acoustic devices fabricated on piezoelectric substrates, such as LiNbO3 and quartz, and being operated at frequencies around 600MHz. By employing SAFM, the local influence of the electrodes on the wave field, leading to undesired performance losses, was investigated. © 1998 Springer-Verlag.

Abstract:

This paper reports measurements of acoustic wave amplitude distributions within SAW devices with high spatial resolution. A modified scanning force microscope transfers the high frequency surface oscillations of the SAW into detectable cantilever vibrations by exploiting a nonlinear coupling mechanism. The capabilities of our technique are demonstrated on conventional Rayleigh wave devices up to 3 GHz and on surface transverse wave resonator devices, where the amplitude in the reflector section was mapped. The demonstrated spatial resolution of the imaged SAW amplitude patterns considerably exceeds the results obtained by conventional techniques.

Abstract:

Achieving atomic resolution with an STM demands a noise-free environment, where mechanical vibrations especially must be damped out. Introducing such vibrations in the form of defined ultrasound consequently leads to image distortion. In particular, the topography is smeared out. By employing surface acoustic waves, which lead to an oscillation of surface atoms on elliptically polarized trajectories, this smearing-out is directed, thereby giving a projection of the ellipse on the sample plane. However, by employing a stroboscopic heterodyne technique (mixing the highfrequency tunneling current with a slightly detuned electrical signal which is applied across the tunneling gap) a snapshot of the surface oscillation is seen.We present phase and amplitude images exhibiting atomic resolution. The atomic contrast of phase and amplitude is explained by the superposition of the surface topography and the oscillation trajectory, which can be obtained from a continuum theory model. © 1998 Springer-Verlag.

Abstract:

In this paper we present a new method that allows the measurement of the phase velocity of in-plane polarized SAWs with high spatial resolution. The capabilities of the scanning acoustic force microscope had to be extended by the analysis of torsional cantilever motion. A nonlinear coupling mechanism between in-plane oscillations and this movement could be found, that allows an mechanical mixing of in-plane SAWs. Phase velocity measurements of Love waves on the system Au/SiO2/ST-quartz are presented. A good agreement with theoretical predictions for the velocities could be found.