Positron-lifetime studies of hydrogenated amorphous silicon

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Abstract
Positron-lifetime studies have been carried out on hydrogenated amorphous silicon (a-Si:H) films prepared by the glow-discharge method. Films deposited and thermally annealed at different temperatures have been investigated for determination of microstructural aspects such as hydrogen content, microvoids, vacancies, etc. The appearance of a long-lifetime component (τ3>3 ns) in the lifetime spectrum together with a narrow peak in the two-dimensional angular correlation of positron-annihilation radiation confirms existence of large microvoids in the films. The systematics of the variation of the intensity of the long-lifetime component (I3) as a function of the deposition and annealing temperature is studied in detail. This study clearly shows that molecular hydrogen exists at high pressure in the microvoids and it effuses out at elevated temperature, leaving behind empty microvoids in the film. Two stages of effusion of hydrogen at 275 and 600 °C have been clearly identified in films deposited at 25 °C. Interestingly, the films deposited at 300 °C exhibit only the high-temperature effusion stage, establishing thereby that the low-temperature stage relates to trapped molecular hydrogen, while the one corresponding to high temperature (600 °C) relates to bonded hydrogen. The positronium lifetime (τ3) shows an increase with annealing temperature, representing growth of microvoid dimensions, presumably due to ensuance of an agglomeration process concurrent with hydrogen effusion. Information concerning the presence of quadrivacancies and pentavacancies in the films and their response to thermal treatment is also obtained from the study of the intensity (I2) and lifetime (τ2) corresponding to the trapped positron state. An attempt has been made to correlate the positronium component with electron-spin-resonance results.

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