Professor Thorsten Hesjedal FInstP

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.

Abstract:

The intrinsic stress and morphology of the Stranski-Krastanow system Ge/Si(111) have been investigated at deposition temperatures of 700-950 K. In a broad range of intermediate temperatures, only one distinct decline of stress is observed at the onset of three-dimensional islanding. Supported by a recent transmission electron microscopy study, the results demonstrate that the strain of Ge/Si(111), where the substrate surface in contrast to Ge/Si(001) is the glide plane for dislocations, is relieved by incorporation and continuous rearrangement of dislocations during the island stage. © 1998 American Institute of Physics.

Abstract:

We present a scanning acoustic force microscope (SAFM) for the study of surface acoustic wave (SAW) phenomena on the submicron lateral scale. Until now, SAWs with in-plane oscillation components could only be studied effectively via nonvanishing out-of-plane oscillation contributions. By operating the microscope in lateral force mode, where both bending and torsion of the cantilever are detected, additional amplitude-dependent signals are found, which are due to the interaction with purely in-plane polarized surface oscillations. To demonstrate the capabilities of this type of SAFM, Love waves were studied on the surface of layers deposited on ST-cut quartz with SAW propagation perpendicular to the crystal X-axis. The phase velocity of the wave as well as the amplitude of a standing wave field was measured and compared to calculated values. © 1998 American Institute of Physics.