Direct insertion of SiH3 radicals into strained Si-Si surface bonds duringplasma deposition of hydrogenated amorphous silicon films

We investigate the interaction of silyl (SiH3) radicals with hydrogenated a morphous silicon (a-Si:H) films using real time in situ infrared spectrosco py in a mode that can detect as little as similar to 0.2 ML of Si-H bonds. The results are directly relevant to the growth of a-Si:H by plasma-chemica l vapor deposition. In this paper, a remote silane plasma source is used to generate a pure SiH3 beam, without any contribution of H or SiH2. Deuterat ed (a-Si:D) films are exposed to this beam and the change in the IR absorpt ion caused by the loss of SiD groups and the creation of SiH groups is meas ured in real time. At the beginning of exposure to SiH3 radicals, a hydroge n-rich layer is deposited on top of the deuterated sample, but no release o f deuterium from SiD surface groups can be observed. This is a surprising r esult, since it has been assumed in the literature that SiH3 radicals can e asily abstract surface-bonded H atoms, and that abstraction must occur in o rder to create dangling bond sites on which SiH3 radicals subsequently adso rb. Our results show that the reaction mechanism with the largest rate coef ficient must be the direct insertion of SiH3 radicals into bonding sites at the film surface, which leads to a hydrogen-rich top layer while preservin g the preexisting SiD bonds. After completion of one monolayer of surface S i-H bonds, deuterium atoms from the initial surface are released simultaneo usly with the creation of SiH bulk groups. We propose a reaction scheme bas ed on the direct insertion of SiH3 radicals into strained Si-Si bonds. This scheme also predicts that the surface-bonding configurations depend on a d ynamic balance between the rates of SiH3 adsorption and thermal desorption, which is confirmed experimentally as a function of SiH3 flux. We discuss t he implications of this reaction mechanism for the growth of a-Si:H from si lane discharges, and for the growth of microcrystalline Si in H-2 diluted S iH4 discharges. [S0163-1829(99)03508-0].