Carbon incorporation pathways and lattice sites in Si _1-yC _y alloys grown on Si(001) by molecular-beam epitaxy

We use a combination of in situ and postdeposition experimental probes together with ab initio calculations of strain coefficients and formation energies associated with specific C configurations in the Si lattice to determine C incorporation pathways and lattice site distributions in fully coherent Si _1-yC _y alloy layers grown by molecular-beam epitaxy on Si(001) as a function of deposition temperature T _s (380°C-680°C) and C fraction y (0-0.026). Lattice strain and Raman spectroscopy measurements demonstrate that all C, irrespective of y, is incorporated into substitutional lattice sites in Si _1-yC _y(001) layers grown at T _s≤480°C. Increasing T _s≥580°C leads to strong C surface segregation, as shown by in situ angle-resolved x-ray photoelectron spectroscopy, yielding additional pathways for C incorporation. Photoluminescence measurements indicate that an increasing fraction of the incorporated C in the higher-temperature layers resides in dicarbon complexes. Reflection high-energy electron diffraction and cross sectional transmission electron microscopy reveal surface roughening at T _s≥580°C with the formation of bulk planar structures, interconnected by 113 segments, that are periodic along [001] with a periodicity which decreases with increasing T _s. We interpret the planar structures as layers of C-rich Si _1-yC _y which form in the presence of excess surface C resulting from segregation. Our ab initio density functional calculations show that substitutional C arranged in an ordered Si ₄C phase is 0.34 eV per C atom more stable than isolated substitutional C atoms.