| 摘要 |
972, 549. Transmutation of elements. WESTERN ELECTRIC CO. Inc. Dec. 7, 1960 [Dec. 15, 1959], No. 42098/60. Drawings to Specification. Heading G6P. [Also in Division H1] Uniformly N type monocrystalline silicon is made by bombarding silicon, having an uncompensated impurity concentration less than 10 times that required, with thermal neutrons to convert Si 30 atoms to unstable Si 3i atoms and thence to P 31 atoms, and subsequently annealing to remove bombardment damage. If the bombardment is directional the crystal may be slowly rotated, during bombardment, about one or more axis. An arrangement for producing simultaneous rotation about three mutually perpendicular axis is described (see Fig. 1, not shown). The concentration of P 31 atoms produced varies only Π5% with depth even when using a cube of silicon 25 cm. across, so that the principal cause of resistivity variation in the product is non-uniformity of the initial doping. Other reactions produced in the silicon by the thermal neutrons, for instance of transmutation of Si 28 atoms to the unstable Si 27 which decays to AI 27 , and of Si 30 to unstable Mg 27 which also decays to AI 27 , reduce the effective cross-section of the main reaction but do not affect the reproducibility of the process. Oxygen, which is normally present in silicon crystals, particularly those produced by crystal pulling is converted by the bombardment into different isotopes and to C 14 and F 19 . C 14 has a long half-life but is produced in insufficient quantities to be dangerous. The doping levels obtainable are increased if the starting material is doped with an excess of the isotope Si 30 by recrystallising from a melt containing the latter. A number of bombardment processes are described in detail and the effect of annealing for different periods and at different temperatures is discussed. Specification 932,427 is referred to.
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