Microwave-mixing scanning capacitance microscopy of pn junctions

We describe an approach to scanning capacitance microscopy. A mixing techni que is employed for imaging local capacitance variations simultaneously wit h the sample topography using an atomic force microscope (AFM) with a condu ctive tip. A SiO2/Si sample with lateral pn junctions formed by ion implant ation has been investigated. Microwave signals incident on the metal-oxide- semiconductor (MOS) structure formed by the AFM tip and the sample give ris e to mixing signals due to the nonlinear voltage dependence of the space ch arge capacitance in the Si. In our experiments two microwave input signals with frequencies f(1) and f(2) and a variable dc bias voltage were applied to the tip-sample MOS structure. The dependence of the generated sum freque ncy and third harmonic signals on the dc sample voltage shows that the f(1) + f(2) and 3 f signals are proportional to dC/dV and d(2)C/dV(2), respecti vely. Images of the sum frequency and third harmonic signals delineating th e pn junctions on our model sample are presented and the dc bias voltage de pendence of the images is discussed. (C) 1999 American Institute of Physics . [S0021-8979(99)04424-2].