Surface hydrogen release during the growth of a Si: H by reactive magnetron sputtering

We present a study of the hydrogen release step during growth of hydrogenated amorphous silicon (a Si:H) thin films. Hydrogen release must occur whenever the hydrogen fraction in the growth flux exceeds the hydrogen incorporation rate into the film, which is usually the case in a Si:H deposition. The films in our study are deposited by direct current magnetron sputtering of silicon in an argon hydrogen atmosphere. In our experiment we monitor the rate of HD production with a mass spectrometer when a deuterated amorphous silicon sample is exposed to a silicon hydrogen growth flux. Using mass balance arguments, these data, along with known bulk film hydrogen incorporation rates, allow a determination of a lower bound for the rate of reactive hydrogen impingement on the growing surface. Depending on the hydrogen partial pressure in the discharge, we find that about one reacted hydrogen per incorporated silicon arrives at the substrate for typical deposition conditions. In addition, our modeling predicts that the surface of the growing film contains approximately one monolayer of hydrogen. The variation of the reacted hydrogen flux with hydrogen partial pressure and substrate temperature are discussed.