Scanning Impedance Microscopy: From Impedance Spectra to Impedance Images
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Scanning Impedance Microscopy: From Impedance Spectra to Impedance Images Sergei V. Kalinin and Dawn A. Bonnell Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut St, Philadelphia, PA 19104 ABSTRACT Impedance spectroscopy has long been recognized as one of the major techniques for the characterization of ac transport in materials. The primary limitation of this technique is the lack of spatial resolution that precludes the equivalent circuit elements from being unambiguously associated with individual microstructural features. Here we present a scanning probe microscopy technique for quantitative imaging of ac and dc transport properties of electrically inhomogeneous materials. This technique, referred to as Scanning Impedance Microscopy (SIM), maps the phase and amplitude of local potential with respect to an electric field applied across the sample. Amplitude and phase behavior of individual defects can be correlated with their transport properties. The frequency dependence of the voltage phase shift across an interface yields capacitance and resistance. SIM of single interfaces is demonstrated on a model metal-semiconductor junction. The local interface capacitance and resistance obtained from SIM measurements agrees quantitatively with macroscopic impedance spectroscopy. Superposition of a dc sample bias during SIM probes the C-V characteristics of the interface. When combined with Scanning Surface Potential Microscopy (SSPM), which can be used to determine interface I-V characteristic, local transport properties are completely determined. SIM and SSPM of polycrystalline materials are demonstrated on BiFeO3 and p-doped silicon. An excellent agreement between the properties of a single interface determined by SIM and traditional impedance spectra is demonstrated. Finally, the applicability of this technique for imaging transport behavior in nanoelectronic devices is illustrated with carbon nanotube circuit. INTRODUCTION Properties and performance of electronic devices are crucially dependent on interfacerelated phenomena. The most versatile tools for semiconductor characterization are impedance spectroscopy and dc transport measurements. The typical applications of impedance spectroscopy differentiate grain boundary, grain interior and electrode impedances by fitting the impedance data to corresponding equivalent circuit models. This approach addresses the average properties of a polycrystalline material and little or no information is obtained about the properties of the individual elements. Moreover, for complex (e.g. multiphase) systems the equivalent circuits rapidly become complex and non-unique. A number of approaches have been suggested to separate the impedance response of individual structural elements, such as microimpedance spectroscopy using patterned contact arrays1,2,3 or studies of bicrystal samples.4,5 However, the correlation of average transport properties with individual microstructural elements requires spatially resolved imaging of dc and ac transport pro
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