The dark-spot method for measuring the diffusion constant and length of excess charge carriers in semiconductors

The well-known basic method of measuring diffusion length, introduced by Goucher, is here applied in its optimum form. This consists illuminating the semiconductor surface with interrupted light of uniform intensity and casting a circular shadow whose diameter can be varied. A detector is placed at the centre of the shadow and the variation of signal with the shadow radius can be interpreted theoretically with high precision. The method possesses advantages of circular symmetry, maximum signal strength, simplicity of optical apparatus, and speed of operation. The same apparatus may be employed to measure the diffusion coefficient by using repeated impulses of light, the results giving an exponential law independent of surface recombination. Full experimental details and discussions are given, together with complete mathematical theory which takes into account surface recombination, penetration of photons, boundary effects and shadow effects, and gives a method of measuring the rate of change of diffusion length across the sample surface. A number of experimental results are presented with a tabulated function for their interpretation.