Scanning photo-induced impedance microscopy—an impedance based imaging technique

Abstract

A.C. impedance spectroscopy is a valuable tool in the characterisation of electrochemical systems and new materials. However, the results obtained using this method are always surface averaged. A new technique, which is suitable for imaging the complex impedance of electrochemical and solid-state systems with good spatial resolution, has been developed. It is based on photocurrent measurements at field effect structures. A semiconductor–insulator structure serves as the substrate for the film under investigation. A pulsed and focused light beam scanned across the sample surface results in photocurrents, which provide information about the local distribution of the dielectric properties of the film. Thin films of poly methyl methacrylate were investigated as a model system. Absolute values of the impedance could be calculated from the photocurrent measurements using a simple calibration procedure. Good spatial resolution was achieved by using a thin epitaxial film of silicon as the semiconductor substrate.