Carrier concentration dependence of the scanning capacitance microscopy signal in the vicinity of p–n junctions

Scanning capacitance microscopy (SCM) was used to image (1) boron dopant gradients in p-type silicon and (2) identical boron dopant gradients in n-type silicon. The bias voltage dependence of the apparent $\mathrm{p–n}$ junction location in the (SCM) images was measured. The theoretical bias voltage dependence of the apparent $\mathrm{p–n}$ junction location of the same structures was determined using a two-dimensional, numerical Poisson equation solver. The simulations confirm that, for symmetric step $\mathrm{p–n}$ junctions, the apparent junction coincides with the electrical junction when the bias voltage is midway between the voltage that produces the peak SCM response on the p-type side and the voltage that produces the peak response on the n-type side. This rule is only approximately true for asymmetrically doped junctions. We also specify the extent of the region on the junction high and low sides from which valid carrier profiles may be extracted with a simple model.