Nanoimpedance Microscopy and Spectroscopy
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NANOIMPEDANCE MICROSCOPY AND SPECTROSCOPY Rui Shao, *Sergei V. Kalinin, and Dawn A. Bonnell Department of Materials Science and Engineering, University of Pennsylvania 3231 Walnut St, Philadelphia, PA 19104 ABSTRACT One of the key limiting factors in current-based scanning probe microscopies (SPM) is the quality of tip-sample contact and stray capacitance in the probe-surface junction. We conduct impedance spectroscopy over a broad frequency range (40Hz~110MHz) through an AFM tip to quantify local electrical properties. Equivalent circuit for the tip-surface contact is constructed based on the impedance data and is used to study the mechanisms of relaxation in the near-tip region. Relative contributions of tip-surface contact and materials properties to the signal are discussed. This technique, referred to as Nanoimpedance Microscopy/Spectroscopy, is demonstrated in the imaging of an electronic ceramic: a ZnO varistor. Analysis of impedance spectra allows separation of tip-surface interactions and grain boundary behavior. INTRODUCTION The continuous miniaturization of integrated electrical devices and growing interest in submicron phenomena require reliable methods to characterize local electrical properties. Recently, many scanning probe microscopy (SPM) techniques based on contact mode AFM have been developed. They are implemented either by current or capacitance sensing via a nanoscale sharp AFM tip in contact with the sample surface [1-3]. One of the most significant factors affecting these SPM techniques is tip-surface contact. Here, we have applied impedance spectroscopy to the characterization of the contact between an AFM tip and a flat noble metal surface in order to determine electrical parameters of tip-surface junction, such as stray capacitance, contact resistance of different conductive coatings, inductance and the dc bias dependence of these parameters. The measurement of the frequency dependence of ceramic or semiconductor samples using a conductive tip is implemented and referred to as Nanoimpedance Microscopy/Spectroscopy (NIM). If contact resistance is small, the measured capacitance is due to relatively large microstructural elements, such as grain boundaries, etc, as demonstrated by impedance imaging and spectroscopy of a polycrystalline ZnO. THEORY Impedance Z is defined as the ratio of ac bias applied to the system, Vac to the current response, Iac, as Z(ω)=Vac/Iac, where Vac, Iac are in complex form and ω is frequency. In general, Z = Zr+i·Zi=|Z|exp(iθ), where Zr, Zi are the real and imaginary parts of Z, and |Z|, and θ are the modulus and phase of Z. The impedance, Z of an ideal resistor, R, is Z = R, *
Currently at Condensed Matter Sciences Division, Oak Ridge National Laboratory
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θ = 0o;o for an ideal capacitor, C, θ=+90 .
Z=1/(iωC), θ=-90o; for an ideal inductor, L, Z=iωL,
The system of a tip in contact with a metal surface is best modeled by a parallel R-C element corresponding to the tip-surface resistance in parallel with probe-surface capacitance. The cable inducta
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