Extrinsic Contributions to Piezoresponse Force Microscopy
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0902-T07-02.1
Extrinsic Contributions to Piezoresponse Force Microscopy Frank Peter1, Bernd Reichenberg2, Andreas Rüdiger1, Rainer Waser1, and Krzysztof Szot1,3 1 cni-Center of Nanoelectronic Systems for Information Technology and IFF-Institute for Solid State Reseach, Research Centre Juelich, 52425 Jülich, Germany 2 aixACCT Systems GmbH, 52068 Aachen, Germany 3 Institute of Physics, University of Silesia, 40-007 Katowice, Poland ABSTRACT Piezoresponse force microscopy (PFM) is the method of choice to investigate piezoactivity on a nanometer scale. A careful distinction between intrinsic and extrinsic effects are mandatory, especially when measuring ferroelectric nanostructures. We focus on two omnipresent extrinsic contributions with a substantial impact: firstly adsorbates on the surface of perovsike materials and secondly the dependence of the lateral piezoresponse on the topography. A thorough understanding of these extrinsic contributions is essential in order to avoid ambiguities in the analysis of PFM measurements. INTRODUCTION Electromechanical sensing of ferroelectric nanostructures by AFM-techniques has been tremendously successful in the last years [1–3]. In piezoresponse force microscopy (PFM) a conductive tip is used in an AFM. As shown in Fig. 1 this tip is brought into contact with the surface of a piezoelectric material and functions as a movable top electrode. An AC voltage is applied via the tip to the sample leading to a piezoelectric deformation. This response is mechanically passed on to the cantilever and optically detected by the movement of the reflected laser on the four quadrant photo diode. The piezoelectric response having the same frequency as the excitation frequency can be extracted from the signal of the photo diode by a lock-in amplifier. In order to analyze the response in the vertical and lateral directions the signal of the photo diode is grouped into t-b'=(a+b)-(c+d) for the vertical movement and l-r=(a+c)-(b+d) for the lateral movement (rf. Fig. 1).
Figure 1. Principle of PFM.
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As the measurement is mechanically and electrically based on the tip-sample interaction, this system deserves particular attention. The quantification of electromechanical processes relies on the knowledge of local fields. We identify an adsorbate layer and present a method how it can be reduced. Furthermore we will show that the lateral piezoresponse is dependent on the shape of the piezoelectric material. SURFACE ADSORBATES One aspect which is often neglected in AFM measurements is the contact between the tip and the sample. How good is this electrical contact? When scanning with a conducting cantilever over a metallic surface, one would expect to have a constantly small contact resistance. Experiments show that this is often not the case [4]. Apart from a poorly coated tip, a contamination layer on top of the sample can be the cause of a bad electrical contact. This raises the question if ferroelectric samples are generally covered by some sort of contamination layer. Answers to this
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