The spectroscopic characterization of interstitial oxygen in bulk silicon: A quantum mechanical simulation

The vibrational Infrared and Raman spectra of six interstitial oxygen defects in silicon containing a Si-O-Si bridge between adjacent Si atoms are obtained from all-electron B3LYP calculations within a supercell scheme, as embodied in the CRYSTAL code. Two series of defects have been considered, starting from the single interstitial defect, O₁. The first consists of four defects, O_1,n, in which two O₁ defects are separated by (n - 1) Si atoms, up to n = 4. The second consists of four defects, O_n, in which nO₁ defects surround a single Si atom, with n = 1-4, where O₄ has the same local nearest neighbor structure as α-quartz. For both series of defects, the equilibrium geometries, charge distributions, and band structures are reported and analyzed. The addition of 1-4 oxygen atoms to the perfect lattice generates 3-12 new vibrational modes, which, as a result of the lighter atomic mass of O with respect to Si, are expected to occur at wavenumbers higher than 521 cm^-1, the highest frequency of pristine silicon, thereby generating a unique new Raman spectrum. However, only a small subset of these new modes is found in the spectrum. They appear at 1153 cm^-1 (O₁), at 1049 cm^-1 and 1100 cm^-1 (O_1,2), at 1108 cm^-1 (O_1,3), at 1130 cm^-1 and 1138 cm^-1 (O_1,4), and 773 cm^-1, 1057 cm^-1, and 1086 cm^-1 (O₄), and can be considered "fingerprints" of the respective defects, as they are sufficiently well separated from each other. Graphical animations indicate the nature and intensity of each of the observed modes which are not overtones or combinations.