Imaging free Carriers in Electronic Material using a Scanning Probe Microscope: Scanning Capacitance Microscopy

Abstract

The development of methods to measure electrical properties, which are suitable to directly yield the desired carrier distributions on a nanometer scale has greatly benefited from the development of scanning probe technology over the last decade. Scanning Probe Microscopes (SPMs) offer inherent two-dimensionality and have been shown to have applications ranging from Magnet force to electro-chemistry. We have used an SPM in contact mode to simultaneously measure topography (and therefore physical structure) and capacitance variations (due to an applied bias) of various electronic materials such asdoped silicon, poly silicon, SiC, and III-V materials.The technique aptly named Scanning Capacitance Microscopy (SCM) takes advantage of the fact that electrical carrier response to an applied electric field is largely dependent upon the local carrier concentration. Using a high 'Q' GHz resonant circuit, SCM measures capacitance variations due to an applied bias between the metalized nano-probe tip and semiconductor sample during scanning. Since thesevariations are directly related to dopant (carrier) concentration, the SCM generates a two-dimensional image with contrast corresponding to near-surface variations in carrier density. Because the measurement is done with an extremely sharp probe, we have been able to resolve features as small as lOnm, corresponding to attofarad (le-18 farad) capacitance changes.