Bombardment-Produced Defects inp-Type Germanium at Low Temperatures

Abstract

Lattice defects produced in nondegenerate (1-10 Q, cm) />-type germanium at 10°K by 1.0-MeV electrons cannot be detected electrically immediately following bombardment. Illumination by light having energy less than the band gap reveals the defects in the form of ionized donors. During ionization, the carrier concentration decreases as the sum of two exponentials, with time constants in the ratio «*6 to 1. Subsequent thermal-annealing investigations show two recovery stages in the temperature range 40 to 70°K. These stages appear to be intimately related to the time constants for decay during illumination, with the lowtemperature recovery associated with the fast component and the high-temperature recovery with the slow component. Both stages of recovery are independent of the type of impurity. Regardless of the extent of illumination, the electrical conductivity and the carrier concentration have essentially the same values they had prior to bombardment. When illuminated sufficiently long to ionize all the defects, the ionized donors become extremely susceptible to annihilation when the temperature is increased, and the situation after heating to 70 °K represents true annealing. Illumination for shorter times results in some un-ionized defects which do not anneal upon heating to 70°K. Isochronal heating to 150°K then causes a transformation of most of the unannealed defects into a configuration which the authors call a "two-state defect," the same defect observed following irradiation at 77°K. The transformed defects break up at about 200°K. Along with the higher-temperature effects, some impurity-dependent recovery stages are noted. A model to partially account for the results is briefly discussed. 790 T. M. FLANAGAN AND E. E. KLONTZ

Self-Interstitial in Germanium

“…Thus the true charge state of the 0 state cannot be neutral as has been assumed [8]. At about 200 K, the two-state donor defect disappears with an activation energy of 0.4 eV [8].…”

“…According to our calculations, the neutral interstitial forms a dumbbell configuration, which like V, does not possess a donor level. This immediately explains why compensation of chemical acceptors at low temperatures does not occur [2,8]. A barrier of about 0.5 eV prevents I 0 from switching sites to form the more stable caged sited ionized defects.…”

“…Between 100 and 200 K, the free hole concentration decreases and a strange ''two-state'' defect is observed [6 -8]. This defect, which seems to be an intrinsic donor [7,8], is a long-lived electron trap that can be cycled between two charge states, denoted '''' and ''0'', with a transition level around E c ÿ 0:2 eV [7,8]. The stable state in p-Ge is but a burst of free electrons from ionizing radiation puts the defect into the 0 state.…”

“…The latter state is correlated with the S 1=2 P1 electron paramagnetic resonance (EPR) center and must therefore have an odd number of electrons [7]. Thus the true charge state of the 0 state cannot be neutral as has been assumed [8]. At about 200 K, the two-state donor defect disappears with an activation energy of 0.4 eV [8].…”

“…The interstitial then can lose up to two electrons to become I 2 T . This transformation occurs in a time scale of minutes [8]. Injecting electrons into the conduction band leads to the conversion of I 2 T into I and I 0 .…”