Quantitative imaging of semiconductor doping distributions using a scanning electron microscope

The incorporation of impurity (i.e. dopant) atoms in semiconducting materials is fundamental to all semiconductor technology since the fabrication of (opto)electronic device structures requires precise, multi-dimensional control of the concentration and depth distribution of electrically active dopant species. Accordingly, a wide range of diagnostic techniques for dopant characterisation have been developed,(1) each with characteristic limitations. Most standard techniques are destructive, time-intensive and limited to one-dimensional profiling or require concentration-dependent chemical etching to delineate dopant distributions. Here we describe the emergence of a simple, robust technique based on low-energy secondary electron imaging in a scanning electron microscope. The efficiency of secondary electron emission in doped heterostructures is associated with surface electronic band structure variations between adjacent regions of different dopant type and/or concentration. We demonstrate that two-dimensional dopant distributions can be intuitively interpreted and quantitatively characterised at nanometre scale spatial resolutions and ppm concentration levels. Existing characterisation techniques are incapable of obtaining such high resolution and sensitivity over a wide dynamic range (10(15)-10(22) atoms cm(-3)). The ability quantitatively to map electrically active dopant distributions, without the need for specimen preparation, holds great promise for the routine analysis of semiconducting devices.