Hydrogen release kinetics during reactive magnetron sputter deposition of a-Si:H: An isotope labeling study

Abstract

The release of molecular hydrogen from the growing surface of hydrogenated amorphous silicon films is determined using an isotope labelling technique. The results demonstrate that surface-bonded H atoms are readily abstracted by atomic hydrogen arriving from the gas phase. The films are deposited by dc reactive magnetron sputtering of a silicon target in an argon-hydrogen atmosphere. To achieve isotope labeling, we first deposit a deuterated amorphous silicon film, then commence growth of hydrogenated amorphous silicon and measure the transient release of HD and D2 from the growing surface using mass spectrometry. Release occurs when the supply of reactive hydrogen in the growth flux exceeds the incorporation rate into the film, and is observed under all experimental conditions. The net rate of H incorporation is known from ex situ measurements of film growth rate and hydrogen content. We combine the H release and incorporation data in a mass balance argument to determine the H-surface kinetics. Under conditions which produce electronically useful films, (i) 0.5–1.0 hydrogen atoms react with the growing surface per incorporated silicon atom, (ii) the near surface of the growing film contains 1–3×1015/cm2 of excess hydrogen, (iii) the dominant hydrogen release mechanism is by direct abstraction to form H2 molecules, and (iv) the kinetics of H release and incorporation can be described by constant rate coefficients. These data are supported by studies of H interactions with single-crystal silicon and amorphous carbon surfaces.