Field Effect on an Illuminated Ge Surface and Investigation of the Surface Recombination Process

Surface conductance, photoconductance, and field effect were measured in the Brattain-Bardeen ambient cycle on germanium surfaces etched with CP-4. From the surface-conductance measurement, values of surface potential were deduced, and from the photoconductance measurement, relative changes of surface recombination velocity were obtained. By analyzing the change of surface recombination velocity as a function of surface potential, dominant recombination centers with discrete levels were found near the center of the gap with a ratio of hole to electron capture probabilities of 9. The energy levels of these centers were found to be consistent with the field-effect data. In addition, the effect of a normal ac field upon an illuminated sample was examined. The results can be interpreted as a superposition of the changes in the photoconductance and the surface conductance due to the field-induced change in surface potential. This effect can be used to determine the value of trap energy and the ratio of capture probabilities to a greater degree of accuracy than can the surface recombination velocity. Possible other applications of the effect are discussed.