Analysis of Chemical Reactions between Radical Growth Precursors Adsorbed on Plasma-Deposited Silicon Thin-Film Surfaces

The dominant precursor in the plasma deposition of hydrogenated amorphous silicon (a-Si:H) thin films is the SiH$_{3}$ radical. In this presentation, we report results of first-principles density functional theory calculations on the crystalline Si(001)-(2$\times $1):H surface and molecular-dynamics simulations on a-Si:H surfaces for the interactions between SiH$_{3}$ radicals adsorbed on Si thin-film surfaces. The analysis reveals that two SiH$_{3}$ radicals may either form disilane (Si$_{2}$H$_{6})$ that desorbs from the surface or undergo a disproportionation reaction producing an SiH$_{2}$ radical that is incorporated in the film and a silane molecule that is released in the gas phase. The corresponding activation barriers depend on the local atomic coordination of the surface Si atoms; Si$_{2}$H$_{6}$ formation is barrierless if both radicals are bonded to overcoordinated surface Si atoms and exhibits barriers in excess of 1 eV for two chemisorbed SiH$_{3}$ radicals.