Thin film quantum materials

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.

Abstract:

It is well established that the growth of Ge on Si(001) proceeds by Stranski-Krastanov mode, i.e. 3D islands ("hut" and macroscopic clusters) nucleate on top of a 3-4 ML thick pseudomorphic layer. Here, we present in-situ intrinsic stress measurements of Ge/Si(001) up to the film thicknesses at which the 3D islands percolate. From the film stress - and supported by AFM investigations - three stages of film growth characterised by different reliefs of the misfit strain can be discriminated: (1) the pseudomorphic layer-by-layer stage, (2) nucleation and growth and (3) coalescence of 3D islands. © 1998 Elsevier Science B.V. All rights reserved.

Abstract:

The intrinsic stress of the Stranski-Krastanov system Ge/Si(001) was investigated in the range 700 1050 K. Characteristic stress features indicate that the relief of the misfit strain proceeds mainly in two steps: (i) by the formation of 3D islands on top of the Ge wetting layer and (ii) via misfit dislocations in larger 3D islands and upon their percolation. The temperature dependence of strain relief by 3D islands as well as their nucleation and growth behavior support a kinetic pathway for 3D islanding. © 1998 The American Physical Society.

Abstract:

In this paper we report the observation of surface acoustic waves using a scanning tunneling microscope (STM). As the STM's control electronics has a bandwidth limit in the kHz range, SAWs at typical frequencies of MHz to GHz cause a loss of contrast which can be clearly seen on an atomic scale. In order to access the amplitude and phase of a SAW, we introduced a heterodyning type STM, the scanning acoustic tunneling microscope (SATM). Contrary to the STM technique, the SATM measures snapshots of the state of oscillation. On the nanometer scale, two contributions to the phase and amplitude contrast are discussed. First, the SAWs phase delay gives a mainly linear dependence on the distance of the source. Second, the atomic oscillation trajectories within the SAW lead to a signal contribution that is made up of the shape of the oscillation trajectory and the local topography. On an atomic scale where the influence of the phase delay on the contrast can be neglected the oscillation trajectories of single surface atoms are studied. Finally, the atomically resolved phase and amplitude images are compared to simulated data.