The Effect of Fast Neutron Bombardment on the Electrical Properties of Germanium

Abstract

Lattice disorder in bulk Ge produced by collisions of fast neutrons with lattice atoms introduces a net excess of electron traps or acceptor states, which appear to have an energy distribution in the forbidden band. Bombardment of N-type Ge causes the conductivity to decrease, initially at a uniform rate, to a minimum value and then to increase. Examination of the data shows that initially about 3.2 conduction electrons are removed per incident fast neutron. The minimum value of the conductivity is higher than the value calculated assuming complete homogeneity and thermal equilibrium. Hall effect measurements prove that after the minimum is passed the material has been converted to P-type by fast neutron bombardment. The conductivity of P-type Ge increases with bombardment. The rate of increase decreases monotonically indicating an approach to saturation. The initial rate of carrier introduction in P-type Ge appears to be temperature dependent, being greater at higher temperatures (∼0.8 carrier per incident neutron at 30°C), and smaller than the rate of carrier removal for N-type Ge which is apparently temperature independent in the temperature range investigated (-79°C to 45°C). The effect of lattice disordering on the electrical properties of Ge may be removed by careful vacuum annealing at 450°C while a portion of this effect readily anneals at room temperature.