Photoluminescence of deep defects involving transition metals in silicon - new insights from highly enriched 28Si

The fundamental properties of deep luminescence centres in Si associated with transition metals such as Cu, Ag, Au, and Pt have been studied for decades, both as markers for these deleterious contaminants, as well as for the possibility of efficient Si-based light emission. Due to the high diffusivity and solubility of these metals, these are among the most ubiquitous luminescence centres observed in Si, and have thus served as testbeds for elucidating the physics of isoelectronic bound excitons and for testing ab-initio calculations of defect properties. While these deep isoelectronic bound exciton centres have been studied extensively with many different methods, the actual composition of most centres could not be determined with certainty. Only the recent availability of high quality, highly enriched 28Si made it possible to advance the knowledge of the constituents of these complexes. The greatly improved spectral resolution resulting from the elimination of inhomogeneous isotope broadening in isotopically enriched 28Si enabled the extension of the established technique of observing isotope shifts to the measurement of isotopic fingerprints. These isotopic fingerprints reveal not only the presence of a specific element, but also the number of atoms of that element involved in the formation of a given luminescence centre. This technique has revealed that the detailed constituents of all of the centres previously studied had been identified incorrectly. In this work, the results of ultra-high resolution photoluminescence studies of these centres in specially prepared 28Si samples are discussed. In addition, new centres were discovered revealing the existence of several different families of impurity complexes containing either four or five atoms chosen from Li, Cu, Ag, Au, and Pt. The constituents of all these centres have been determined, together with no-phonon transition energies, no-phonon isotope shifts, local vibrational mode energies, and the isotope shifts of the local vibrational mode energies. The data presented here for these deep centres should prove useful for the still-needed theoretical explanations of their formation, stability, and properties.