Fast-neutron irradiation of n- and p-type germanium at low temperature

A low-temperature irradiation facility installed in a fast-neutron-source reactor, YAYOI, enables irradiation at liquid-helium temperature with neutrons of relatively high average energy of about 500 keV. Using this facility, highly doped n (Sb or As) - and p (Ga)-type germanium specimens have been irradiated. The resistivity before irradiation ranges from 0.02 to 0.05 ohm cm at room temperature, being between 0.02 and 2 x 103 ohm cm at liquid-helium temperature. In almost all cases, the resistivity around liquid- helium temperature decreases after 2 to 5 x 1014 neutrons/cm2 at 4.6 K in both n-and p-type specimens. Annealing characteristics as monitored by resistivity at liquid-helium temperature have been studied mainly in Sb-doped samples. Most of the resistivity change remains stable up to about 100 K. A remarkable reverse-annealing stage is found at 160 to 220 K, followed by normal annealing stages at 240 to 250 and at 280 to 310 K. Differences and similarities between the present results and other published results are discussed in terms of ionization effects and the structure of a cascade. It is tentatively considered that the defects do not show long-range migration during irradiation, as they are liberated above 100 K from the cascade to form complexes or to cause structural modifications. (author)