It was noted that electron paramagnetic resonance measurements of Er and O, implanted into Si, had revealed centers having monoclinic and trigonal symmetry. These centers were tentatively attributed to differing atomic configurations of O and Si atoms surrounding a central Er3+ ion. As the result of calculations concerning the structure and stability of multi O-related defects in Si, more sophisticated models were developed. The structure of the monoclinic centers observed was now attributed to an Er atom sitting at the tetrahedral interstitial site, surrounded by six O atoms. In this model, the O atoms were arranged in the form of three O interstitial dimers with each of the O atoms located near to the Si–Si bond-centered positions. In the case of the prominent defect center which exhibited trigonal symmetry, various possible structures for this center were explored; including locating the Er3+ ion residing at the hexagonal interstitial site surrounded by six O atoms as well as a center consisting of an O-decorated ring hexa-vacancy defect.

Structure of Multi-Oxygen-Related Defects in Erbium-Implanted Silicon. J.D.Carey: Journal of Physics - Condensed Matter, 2002, 14[36], 8537-47