Influence of CO60 Gamma Irradiation on the Surface and Bulk Recombination Rates in Silicon

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Abstract
An experimental investigation has been conducted to determine the influence of Co60 gamma irradiation on the surface recombination velocity in 100 ohm-cm n- and p-type silicon. Utilization of the technique of photoconductive decay to measure the surface recombination velocity requires the knowledge of the bulk minority carrier lifetime in the material. For completeness, the effects of the irradiation on this material parameter are also presented. When etched samples of n-type float zone refined phosphorous doped silicon were irradiated with Co60 gamma rays, the surface recombination velocity exhibited a relative minimum as a function of the total gamma ray exposure dosage. In general, the surface recombination velocity was strongly influenced by the gamma irradiation for n-type material. The surface recombination velocity of irradiated samples of p-type float zone refined boron doped silicon seemed to be only slightly sensitive to gamma radiation. The velocity was a very slowly monotonically increasing function of total gamma ray flux. In order to explain these observed variations of surface recombination velocity, a more complete mathematical model was formulated. The model includes the contribution due to recombination in the space charge layer near the surface to the effective surface recombination velocity at the beginning of this space charge region. Use of this model allowed the correlation of experimental and theoretical values of surface recombination velocity. When bulk minority carrier lifetime versus reciprocal temperature data was curve-fitted to the Shockley-Read lifetime, the position of the radiation-induced bulk recombination center in the forbidden region could be determined.