Neutron Damage Mechanisms in Silicon at 10°k
Neutron-irradiation effects on the photoconductivity and electrical properties of arsenic-doped silicon were measured at 10K so that the degradation mechanisms in infrared detectors fabricated from doped silicon could be examined. The measurements were performed at reduced optical background so that both thermal and optical generation were minimized as they are in the conditions under which the detectors are operated. The primary effects of the neutrons on the measured properties were (l) the introduction of acceptors (at a rate to 16 cm-1) which decreased the electron (majority-carrier) lifetime and thus the optical response, and (2) the production of bandtailing, which may be associated with cluster damage. The bandtailing diminished significantly, or even disappeared, with annealing up to 100K, but all the other effects remained essentially unchanged with annealing as high as 673K. The possibility of the bandtailing remaining after 100K annealing appeared to be related to the amount of damage present. It was hypothesized that a certain defect concentration and/or cluster size was required for the bandtailing to be observed.
INFRARED ABSORPTION IN NEUTRON-IRRADIATED GaAs.
The effect of fast‐neutron irradiation on infrared attenuation in single‐crystal GaAs has been investigated. Both n‐ and p‐type materials of several dopant concentrations were exposed to seven fluences of fast neutrons (> 10 keV) from 5×1014 to 1.5×1018 neutrons/cm2. Infrared transmission was measured before and after exposure. At low fluences, the free‐carrier absorption of all samples was reduced by neutron irradiation. At the higher neutron fluences, an absorption process which showed no structure and varied approximately as the square of the photon energy was observed to dominate the free‐carrier absorption. This absorption increased linearly with fast neutron fluence and was the same for all samples studied independent of impurity type and initial concentration. The results are in agreement with a neutron‐produced spike absorption mechanism proposed by McNichols and Ginell.
Effect of Fast-Neutron Irradiation on Optical Attenuation in Compound Semiconductors
Recently a model was proposed to account for anomalous infrared attenuation noted experimentally for fast-neutron-irradiated, compound semiconductors. The model represents neutron damage by localized phase transitions to a high-pressure metallic-like state. It is shown that this model not only accounts for the anomalous, continuous optical attenuation at photon energies less than the band gap, but also can explain the absorption edge fuzziness noted for neutron-irradiated GaAs. The optical attenuation produced by embedded metallic zones at frequencies above the absorption edge is derived, and it is shown that the metallic resonance absorption band is altered from the usual Lorentzian shape. It is shown that in GaAs irradiated with fast neutrons at sufficiently high doses the resonant absorption by the metallic phase can dominate the host semiconductor absorption.