Application of thermal neutron irradiation for large scale production of homogeneous phosphorus doping of floatzone silicon

The limitations of conventional melt doping of phosphorus in silicon are discussed in relation to the obtainable homogeneity. Due to "theoretical-design" possibilities and increased manufacturing yield for power components based on n-type silicon, the method of thermal neutron irradiation doping has been developed for large scale production of floatzone silicon of homogeneous resistivity. It is shown that radioactivity problems do not interfere for resistivities above approximately 5Ω- cm and that lattice radiation defects can be annealed out to the extent that they appear harmless for all major applications. The doping homogeneity is discussed in view of the influencing nuclear reactor characteristics and the choice of starting material. Doping variations less than 1 percent across slices of up to 80-mm diameter are demonstrated to be obtainable with production results typically 3-10 percent. The minority carrier lifetime of neutron-doped silicon is shown to lie in the range of 100-1000 µs.