Interstitial carbon-related defects in Si1−xGex alloys

The interstitial carbon (CI) impurity vibrational modes in monocrystalline Si-rich SiGe compounds were investigated by Fourier Transform Infra Red spectroscopy and density functional modeling. The two absorption bands of CI are found to be close to those in Si, but lines show shifts in opposite directions with increasing Ge content. The transversal mode band at 932 cm−1 shifts slightly to the high-frequency side, while the longitudinal mode at 922 cm−1 suffers a pronounced red-shift. Each CI-related band is found to consist of two components. Such two-component structure of bands is suppose most likely to correspond to different combinations of Si and Ge atoms in the neighborhood of the CI atom. CI in Si1−xGex was found to begin to anneal at lower temperature than in Si and occurs in two stages. During the first stage (210-250 K) the main components of bands anneals and revealed components grow in intensity. At T>250 K all components start to disappear. The CI defect was modeled by a supercell density-functional pseudopotential method (AIMPRO). From energetics, it has been found that each Ge-C bond costs at least 0.4 eV in excess of a Si-C bond. The vibrational mode frequencies taken from several randomly generated SiGe cells produce the observed opposite shifts for the transverse and longitudinal modes. However, structures where Ge atoms are second neighbors to the C atom are marginally bound, and may explain the two-component band structure in the absorption measurements.