Photoluminescence of deep defects involving transition metals in Si: New insights from highly enriched ²⁸Si

Deep luminescence centers in Si associated with transition metals have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. They are among the most ubiquitous luminescence centers observed in Si, and have served as testbeds for elucidating the physics of isoelectronic bound excitons, and for testing ab-initio calculations of defect properties. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in the recently available highly enriched ²⁸Si enabled the extension of the established technique of isotope shifts to the measurement of isotopic fingerprints, which reveal not only the presence of a given element in a luminescence center, but also the number of atoms of that element. This has resulted in many surprises regarding the actual constituents of what were thought to be well-understood deep luminescence centers. Here we summarize the available information for four families of centers containing either four or five atoms chosen from (Li, Cu, Ag, Au, Pt). The no-phonon transition energies, their isotope shifts, and the local vibrational mode energies presented here for these deep centers should prove useful for the still-needed theoretical explanations of their formation, stability and properties.