Analysis of the interaction of an electron beam with a solar cell—III. The effect of spacial variations of the number density of recombination centers on SEM measurements

By means of an exactly soluble model the short circuit current I_SC generated by a scanning electron microscope in a P- N junction has been determined in cases where the trap density N_t is inhomogeneous. The diffusion length for minority carriers becomes then dependent on the spacial coordinates. It is shown that in this case the dependence of the I_SC on characteristic parameters as cell thickness, distance of the beam excitation spot from ohmic contacts etc., becomes very intricate. This fact precludes the determination of the local diffusion length in the usual manner. Although the model is somewhat simplified in order to make it amenable to exact solutions, it is nevertheless realistic enough to lead to the conclusion that SEM measurements of bulk transport parameters in inhomogeneous semiconductor material are impractical since they may lead to serious errors in the interpretation of the data by customary means.