NTD germanium: a novel material for low-temperature bolometers

Six samples of ultra-pure (absolute value N/sub A/ - N/sub D/ absolute value less than or equal to 1011cm-3), single-crystal germanium have been neutron transmutation doped with neutron doses between 7.5 x 1016 and 1.88 x 1018cm-2. After thermal annealing at 4000C for six hours in a pure argon atmosphere, the samples have been characterized with Hall effect and resistivity measurements between 300 and 0.3 K. Our results show that the resistivity in the low temperature, hopping conduction regime can be approximated with rho = rho0exp(Δ/T). The three more heavily doped samples show values for rho0 and Δ ranging from 430 to 3.3 Ω cm and from 4.9 to 2.8 K, respectively. The excellent reproducibility of neutron transmutation doping and the values of rho0 and Δ make NTD Ge a prime candidate for the fabrication of low temperature, low noise bolometers. The large variation in the tabulated values of the thermal neutron cross sections for the different germanium isotopes makes it clear that accurate measurements of these cross-sections for well defined neutron energy spectra would be highly desirable