Inspection and Manipulation of Ferroelectrics on the Nanometer Scale
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L.M. ENG Institute of Applied Photophysics, University of Technology Dresden D-01062 Dresden, Germany, [email protected]
ABSTRACT The increasing interest in scanning probe instruments (SPM) stems from the outstanding possibilities in measuring electric, magnetic, optical, and structural properties of surfaces and surface layers down to the molecular and atomic scale. For the inspection of ferroelectric materials both the scanning force microscope (SFM) and the scanning near-field optical microscope (SNOM) are promising techniques revealing information on the polarization vector and the electric field induced stress within a crystal. Polarization sensitive modes are discussed as is friction force microscopy, dynamic force microscopy (DFM) and voltage modulated SFM. From these measurements, 180' domain walls (c-domains) are resolved down to 4 nm, while 3-dimensional polarization mapping in ferroelectric BaTiO 3 ceramics reveals a 25 nm resolution. On the other hand, non-contact DFM measurements in ultra-high vacuum are able to resolve ferroelectric surfaces down to the atomic scale. Then also the chemical heterogeneity at the sample surface is differentiated from ferroelectric domains down to a 5 nm lateral resolution, taking advantage of the short range chemical forces. SNOM in contrast probes the optical properties of ferroelectric crystals both in transmission and reflection. Here image contrast arises from changes in the refractive index between different domains as well as at domain walls. In addition, SPM instruments are used for the local modification of ferroic samples by applying a relatively high voltage pulse to the SPM tip. Domains with diameters down to
30 nm are thus created with the size depending on both the switching and material parameters.
INTRODUCTION Since the discovery and development of scanning probe microscopes (SPM) in 1981 [11 this technique has rapidly evolved into an easy to use and sensitive analytical tool measuring minute electrical, magnetic, or optical properties of surfaces and surface layers. SPM applications to ferroelectric materials are reported as early as 1990 using the scanning force microscope (SFM) [2] and the scanning tunneling microscope (STM) [3,4] for domain wall and thin film inspection, respectively. The latter method, STM, however, is not suitable for the broad and concerted nanoscopic investigation of ferroelectrics since most samples under investigation are either nonconductive or much too thick in order to allow proper electron tunneling [5]. Hence, STM investigations of ferroelectrics are restricted to polymeric materials [4,6] or liquid crystals [3,7].
65 Mat. Res. Soc. Symp. Proc. Vol. 574 © 1999 Materials Research Society
Nevertheless, many more SPM methods have since been applied to the analysis of ferroic materials, such as the scanning capacitance [8], the scanning Kelvin Probe [9], the scanning acoustic [10], and the scanning micro-wave near field microscope [ II]. Unfortunately, none of them showed to be as helpful and sensitive for ferroelectric surface
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