Recombination Centers and Fast States on Unstable Germanium Surfaces

Abstract

Measurements of surface conductance, photoconductance, dark field effect, and field effect under illumination were made on single crystals of $n$-type and $p$-type germanium. The samples, freshly etched with CP-4, were exposed to the Brattain-Bardeen ambient cycles. During early cycles, the measurements were irreproducible. The irreproducibility was due to changes in the density of the dominant surface recombination centers. The density of these centers varied by an order of magnitude during the ambient cycle, whereas the energy levels and the capture probabilities remained constant. With these assumptions, quantitative agreement between theory and experiment was obtained. From the changes in the photoconductance and the dark-field effect during repeated ambient cycles, the density of the dominant recombination centers was found to change at the same rate as the density of the "fast" states near the center of the gap. In addition, the recombination centers and the "fast" states have the same energy levels. Therefore, the recombination centers and "fast" states are identical.