Preparation of Uniform Resistivity n-Type Silicon by Nuclear Transmutation

By the capture of a thermal neutron, the natural isotope Si 30 can be transmuted into the unstable isotope Si 31 which decays by beta emission with a 2.62 hr half‐life to the stable isotope P 31 . By means of this reaction, donors can be produced in a silicon crystal. Because of the uniformity of flux which can be obtained in large nuclear reactors, this technique lends itself to the preparation of uniformly doped n‐type silicon. After irradiation the silicon crystal is heavily damaged by the emitted betas, by the recoil of the decaying Si 31 , and by the high energy neutrons that are unavoidably present in the reactor. However, this damage can be removed by annealing at temperatures near 600°C. The technique has been used to produce n‐type silicon with nominal resistivities between 0.1 and 20 ohm‐cm. The resistivity of the resulting crystals was uniform over dimensions of 5 cm to within ±5%.