Interstitial carbon defects in silicon. A quantum mechanical characterization through the infrared and Raman spectra

The Infrared (IR) and Raman spectra of various interstitial carbon defects in silicon are computed at the quantum mechanical level by using an all electron Gaussian type basis set, the hybrid B3LYP functional and the supercell approach, as implemented in the CRYSTAL code (Dovesi et al. J. Chem. Phys. 2020, 152, 204111). The list includes two 〈100〉 split interstitial I_XY defects, namely I_CC and I_CSi , a couple of related defects that we indicate as I_X I_Y , the so called $$ in its A and B form, as well as SiC_i Si and C_s C_i C_s , in which the interstitial carbon atom is twofold coordinated. The second undergoes a large relaxation, and the final configuration is close to I_CC C_s . Geometries, relative stabilities, electronic, and vibrational properties are analysed. All these defects show characteristic features in their IR spectrum (above 730 cm^{- 1} ), whereas the Raman spectrum is dominated, in most of the cases, by the pristine silicon peak at 530 cm^-1 , that hides the defect peaks.