Defects and microvoids in a-Si and a-Si:H

We report the first measurements of positron-annihilation spectra of samples of both pure and hydrogenated amorphous silicon. Comparison of these spectra with that of crystalline silicon indicates that the lowest-lifetime component can be identified as the contribution mainly from valence-band electrons. Both the pure (a-Si) and the hydrogenated (a-Si:H) samples exhibit a component with intermediate lifetime, which we attribute to small vacancies consisting of about 4 missing atoms. Finally, only a-Si:H shows a significant long-lived line (τ > 5 ns), which arises from large microvoids, with ～ 100 missing atoms. The existence of these microvoids in a-Si:H is consistent with recent reports of the presence of occluded H₂ gas under high pressure in such films.