EPITAXIAL-GROWTH OF SI1-YCY ALLOYS CHARACTERIZED AS SELF-ORGANIZED, ORDERED, NANOMETER-SIZED C-RICH AGGREGATES IN MONOCRYSTALLINE SI

Molecular-beam epitaxy deposition at 600 degrees C of Si in the presen ce of a C precursor (C2H4) allows us to identify, in specific kinetic conditions, a particular C accommodation mode in Si. By cross-sectiona l transmission electron microscopy we observe a precipitation of nanom etric, highly supersaturated C-rich aggregates (1-3 nm) excluding sili con carbide or graphite formation. More surprisingly, these zero-dimen sional aggregates are all self-organized in two-dimensional layers, pa rallel 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 epitax ied Si matrix, forming a heterogeneous Si1-yCy alloy, in spite of cons tant C and Si supplies all along the growth. The kinetic conditions go verning this particular self-organization are specified in terms of Si and C impinging rates at the growth surface. Moreover, by x-ray photo electron diffraction on the C is core level, we demonstrate that a loc al ordering, corresponding to that in the surrounding Si matrix, exist s between the carbon atoms and their first Si neighbors inside the agg regates. This result provides major arguments in favor of the existenc e of the SinC 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 precipit ation is explained on the basis of recently developed concepts of surf ace related C-solubility enhancements and sequential burying in C-enri ched SinC phases of the accumulated C-rich surface layers. Such phases could prove more stable than diluted carbon when forced to match sili con.