Oxygen and vacancy defects in silicon. A quantum mechanical characterization through the IR and Raman spectra

The Infrared (IR) and Raman spectra of various defects in silicon, containing both oxygen atoms (in the interstitial position, O_i) and a vacancy, are computed at the quantum mechanical level by using a periodic supercell approach based on a hybrid functional (B3LYP), an all-electron Gaussian-type basis set, and the Crystal code. The first of these defects is VO: the oxygen atom, twofold coordinated, saturates the unpaired electrons of two of the four carbon atoms on first neighbors of the vacancy. The two remaining unpaired electrons on the first neighbors of the vacancy can combine to give a triplet (S_z = 1) or a singlet (S_z = 0) state; both states are investigated for the neutral form of the defect, together with the doublet solution, the ground state of the negatively charged defect. Defects containing two, three, and four oxygen atoms, in conjunction with the vacancy V, are also investigated as reported in many experimental papers: VO₂ and VOO_i (two oxygen atoms inside the vacancy, or one in the vacancy and one in interstitial position between two Si atoms) and VO₂O_i and VO₂2O_i (containing three and four oxygen atoms). This study integrates and complements a recent investigation referring to O_i defects [Gentile et al., J. Chem. Phys. 152, 054502 (2020)]. A general good agreement is observed between the simulated IR spectra and experimental observations referring to VO_x (x = 1-4) defects.