The capture of thermal electrons by gold centres in N-type germanium at 20°K

Abstract

Measurements of the capture rate of thermal electrons at Au − and Au − centres at 20°K in n-type germanium yielded the values 2.6 × 10 −11 cm 3 sec −1 and 2.0 × 10 −12 cm 3 sec −1 for the singly and doubly charged centres respectively. The method of measurement involved observation of the escape of electrons previously driven into the centres by an electric field. An analysis of the trapping kinetics for the system—conduction band, donor level, and two gold levels—is given, but this is found to be inadequate to explain the variation of time-constant with photo-excitation level which is observed, and the existence of a further trapping level has to be postulated. The origin of this trap is unknown, but a rough estimate suggests that a trap density of about 10 12 cm −3 and a level about 0.03 eV below the conduction band would account for the results. Changes in the electron population of the gold levels occur at moderate-to-high excitation-levels, and it is found that quantitative results for the population shifts could be indirectly inferred from experiment. Evidence is found for the existence of a highly photosensitive surface strip, about ten Debye lengths thick, within the space-charge region of an accumulation layer. This strip is due to the complete filling of the gold level 0.20 eV from the conduction band by the additional electrons which form the space-charge so that only Au − centres are present.