Non-Destructive Method of Observing Inhomogeneities in p-n Junctions with a Chopped Photon Beam

A flying-spot scanner is constructed to obtain scanning images of p-n junctions made upon Si wafers, having diameters of 50 to 60 mm. The specimen is set upon a metal stage, and the junction surface is entirely covered with a 15-µm thick mylar sheet. A wide transparent electrode is put upon the sheet. A photon beam whose chopping frequency and diameter are 2 kHz and nearly 100 µm, respectively, irradiates the specimen through the transparent electrode and the mylar sheet. Ac photovoltages are measured. Variations of junction depths of 1.1 to 1.4 µm, and those of diffusion lengths in the substrate wafer are easily revealed with the present method.