Breakdown physics of low-temperature silicon epitaxy grown from silane radicals

We grow epitaxial silicon films on (100) Si wafers at low temperature $(<400°\mathrm{C})$, from silane radicals, to understand the mechanisms of sudden epitaxy breakdown and simultaneous growth of amorphous and crystalline silicon phases. Surface roughness is below $0.7\mathrm{nm}$ before breakdown and below $1\mathrm{nm}$ in epitaxial regions persisting after breakdown, in contrast to the roughness-induced breakdown observed in molecular beam epitaxy from atomic Si. Spherical caps of hydrogenated amorphous Si (a-Si:H) breakdown cones protrude above the crystal surface, with each sphere centered on its cone apex. This means that the a-Si:H grows isotropically from impinging radicals with low surface mobility and that the a-Si:H growth rate is higher than the epitaxial growth rate. Similar physical mechanisms likely apply to nanocrystalline silicon film growth.