Recombination lifetimes in gamma-irradiated silicon
The small-signal recombination lifetimes of minority carriers were measured as a function of temperature in silicon before and after irradiation at room temperature by cobalt 60 gamma rays. The resistivity of the boron doped samples ranged between 10 and 5000 ohm-cm, and the resistivity of the phosphorus doped samples was between 20 and 220 ohm-cm. Both crucible and floating zone grown materials were investigated. The radiation induced lifetime changes were interpreted by the theory of Hall, and Shockley and Read. The lifetime changes in n-type material were interpreted to be due to two energy levels, one 0.17 ev from the conduction band edge, which was assumed to be the substitutional oxygen (A center) defect, and the other at 0.4 ev from the conduction band edge. On the basis of annealing data, it appeared that the 0.4 ev level could be due to several defects whose energy levels coincided, but that most of the recombination in the float~zoned material occurred through the donor-vacancy complex (E center). The introduction rates of the levels were not strongly influenced by differences in resistivity, but the introduction rate of the 0.4 ev level was much less in material containing large oxygen concentrations. Defects controlling the lifetime in irradiated p-type material were placed at 0.18 ev above the valence band edge and at 0.3 ev below the conduction band edge. The former level could possibly be located 0.18 ev below the conduction band edge, but it is most unlikely that it is the A center because of the ratio of the hole and electron Qapture cross-sections. The 0.18 ev level controlled the lifetime in the room temperature region in the pulled material, and the 0.3 ev level controlled the lifetime at all measured temperatures in float-zoned material whose resistivity was 70 ohm-em or greater. The ratio of the hole to electron capture cross-section for the 0.3 ev level was about 20. No strong effect of resistivity was seen in either of the p-type silicon, but the effect of the larger oxygen content of the pulled material was to reduce the introduction rate of the 0.3 ev level.
recombination lifetimes, gamma-irradiated silicon, radiation induced lifetime changes
University of Illinois - Urbana-Champaign
Networked Digital Library of Theses and Dissertations