Epitaxial growth of Si1-yCy alloys characterized as self-organized, ordered, nanometer-sized C-rich aggregates in monocrystalline Si.

Abstract

Molecular-beam epitaxy deposition at 600 \ifmmode^\circ\else\textdegree\fi{}C of Si in the presence of a C precursor (${\mathrm{C}}_{2}$${\mathrm{H}}_{4}$) allows us to identify, in specific kinetic conditions, a particular C accommodation mode in Si. By cross-sectional transmission electron microscopy we observe a precipitation of nanometric, highly supersaturated C-rich aggregates (1--3 nm) excluding silicon carbide or graphite formation. More surprisingly, these zero-dimensional aggregates are all self-organized in two-dimensional layers, parallel to the growth surface, and reveal a periodicity of about 9 nm, like in a ``natural'' superlattice. This indicates the occurrence of a cyclic, growth-induced carbon precipitation into a defect-free epitaxied Si matrix, forming a heterogeneous ${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{y}}$${\mathrm{C}}_{\mathit{y}}$ alloy, in spite of constant C and Si supplies all along the growth. The kinetic conditions governing this particular self-organization are specified in terms of Si and C impinging rates at the growth surface. Moreover, by x-ray photoelectron diffraction on the C 1s core level, we demonstrate that a local ordering, corresponding to that in the surrounding Si matrix, exists between the carbon atoms and their first Si neighbors inside the aggregates. This result provides major arguments in favor of the existence of the ${\mathrm{Si}}_{\mathit{n}}$C phases recently predicted by ab initio calculations even if the observation of structured electron, forward-scattering events for next-nearest neighbors is hindered by probable distortions around the C atoms due to high local strain. Finally, the periodic C precipitation is explained on the basis of recently developed concepts of surface related C-solubility enhancements and sequential burying in C-enriched ${\mathrm{Si}}_{\mathit{n}}$C phases of the accumulated C-rich surface layers. Such phases could prove more stable than diluted carbon when forced to match silicon. \textcopyright{} 1996 The American Physical Society.