Vacancy-type and electrical defects in amorphous silicon probed by positrons and electrons

In summary, we have studied structural and electrical defects in amorphous Si by positron annihilation spectroscopy employing slow, variable energy positrons, and by lifetime measurements of optically generated free carriers. The PAS data indicate that a-Si contains a number of vacancy-like defects, some of which can be annealed out by heating to 500 °C. However, other explanations of the data are possible and cannot be completely excluded. Concurrent with the apparent removal of vacancy-type defects from a-Si by the annealing, a tenfold increase in the lifetime of an optically generated free carrier plasma is observed, suggesting a significant reduction in the number of bandgap states. A large increase in the free carrier lifetime has also been obtained by a low temperature (150 °C) anneal, but with hydrogen co-implanted in the a-Si. Such a low temperature anneal (with co-implanted hydrogen) also leads to a reduction of the S-parameter for positron annihilation similar to that obtained by a high-temperature anneal without hydrogen. This suggests that the bandgap states acting as recombination centres for free carriers are associated with the vacancy-type defects. Finally, it has been found that both positron spectroscopy and carrier lifetime measurements can be a sensitive probe of ion radiation damage in 500 °C annealed a-Si. When the estimated density of displaced atoms due to nuclear collisions approaches 10-3, both methods clearly detect a change. This suggests that the defect density in a-Si annealed at 500 °C is less than 0.1 at. %.