Substitutional carbon defects in silicon: A quantum mechanical characterization through the infrared and Raman...

The infrared (IR) and Raman spectra of eight substitutional carbon defects in silicon are computed at the quantum mechanical level by using a periodic supercell approach based on hybrid functionals, an all electron Gaussian type basis set and the CRYSTAL code. The single substitutional C _s case and its combination with a vacancy (C _s V and C _s SiV) are considered first. The progressive saturation of the four bonds of a Si atom with C is then examined. The last set of defects consists of a chain of adjacent carbon atoms C $$ , with i = 1-3. The simple substitutional case, C _s , is the common first member of the three sets. All these defects show important, very characteristic features in their IR spectrum. One or two C related peaks dominate the spectra: at 596 cm^-1 for C _s (and C _s SiV, the second neighbor vacancy is not shifting the C _s peak), at 705 and 716 cm^-1 for C _s V, at 537 cm^-1 for C $$ and C $$ (with additional peaks at 522, 655 and 689 for the latter only), at 607 and 624 cm^-1 , 601 and 643 cm^-1 , and 629 cm^-1 for SiC $$ , SiC $$ , and SiC $$ , respectively. Comparison with experiment allows to attribute many observed peaks to one of the C substitutional defects. Observed peaks above 720 cm^-1 must be attributed to interstitial C or more complicated defects.