Effect of Irradiation on the Hole Lifetime of N-Type Germanium

Abstract

The minority carrier lifetime of germanium is very sensitive to certain types of irradiation. The results of irradiating large single crystal samples of n‐type germanium with fast neutrons and Co⁶⁰ gamma rays are presented and discussed. Lifetime was determined from a transient measurement of the decay of holes following an injection pulse. The effect of initial carrier concentration has been considered. According to the James‐Lark‐Horovitz model, four energy levels are introduced by irradiation into the forbidden band of germanium, one above and three below the center of the band. Using the upper level, a one level recombination model serves to explain the experimental results approximately. The determination of the recombination level agrees well with the position of the first ionization level of the interstitial produced by irradiation at about 0.2 ev below the conduction band. On this basis, the effective cross section for hole capture by these recombination centers is found to be about 4×10⁻¹⁵ cm² and 5×10⁻¹⁶ cm² for neutron and gamma‐irradiated germanium, respectively. A lower level may also be responsible for the recombination process.