The Cu _PL defect and the Cu _s1Cu _i3 complex

The Cu _PL defect is characterized by intense photoluminescence (PL) emission with a 1014 meV zero-phonon line and has since long been called the 'copper-pair' defect. However, recent PL studies in isotopically pure ²⁸Si samples show clear evidence for four Cu atoms in this defect. This defect is one of a family of complexes that contain four (sometimes five) metal impurities. No complex containing two or three metal impurities (at least one of which is a transition metal) has been reported. We have performed systematic calculations of a priori possible Cu ₄ complexes, including the Cu _s1Cu _i3 complex proposed by Shirai et al. [18]. This complex has the lowest formation energy of all the Cu ₄ defects. We have studied its properties using first-principle theory. In addition to the structures, binding energies, and vibrational spectra, we studied the formation pathway in an attempt to figure out why smaller (or larger) complexes do not form. We find that Cu _s1Cu _in with n=0, 1 or 2 continue to trap Cu _i if the Fermi level is midgap, and that the process stops when n=3.