The effects of core structure on radiative and non-radiative recombinations at metal ion substituents in semiconductors and phosphors

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Abstract
Carrier binding and recombination processes at transition metal (TM) and post-transition metal impurities in solids are reviewed. The presence of low-lying empty core orbitals in these impurities introduces novel binding and recombination phenomena and leads naturally to a classification of impurity centres as ‘simple’ or ‘structured’. ‘Simple’ impurities, generally the main group elements, introduce only effective-mass-like states into the forbidden gap of the host lattice, whereas ‘structured’ impurities show localized atomic-like transitions below the lattice absorption edge. The fact that donor or acceptor centres generated by TM ions in semiconductors are usually deep is discussed with reference to a simple oxidationstate correlation diagram based on the variable chemical valency characteristic of these ions. Many metal ion impurities in solids generate iso-electronic centres however, and evidence is assembled to show how the normally accepted range of iso-electronic centres can be extended to include such impurities. It is emphasized that the formation of bound states at these ‘structured’ cationic iso-electronic centres is fairly common, in contrast to the situation normally found for anionic substituents in semiconductors. A possible explanation for this general property of structured iso-electronic centres is given in terms of the low-lying core levels which they may introduce in the band gap and their enhanced polarizability. Simple arguments then suggest that those centres showing electron ionization thresholds or atomic inter-configurational transitions just below E g should be hole-attractive, whereas those showing charge transfer transitions should be electron-attractive. It is shown that two kinds of defect Auger recombination (DAR) may be significant for excitons localized at structured impurities. The first involves hole ionization at a deep neutral acceptor, and the conditions which must be met to make this process significant at room temperature are reviewed. The second DAR process involves the transfer of carrier recombination energy to the core electrons of the structured impurity. A simple model developed for this energy transfer process predicts that the coupling between exciton and core states will be larger if the impurity shows strong, broad dipole-allowed transitions quasi-resonant with the lattice absorption edge. Throughout the paper we stress the importance of recombination processes at structured impurities to the problems of phosphor activation and of shunt recombination mechanisms in semiconductor LEDs. In the final sections this general framework of ideas is used to explain the relative cathodoluminescence efficiencies of different rare-earth activators in crystals of Y3Al5O12, with good agreement between the theoretical predictions and the experimental observations. Possible evidence for the occurrence of bound exciton states at structured TM impurities is found in the appearance of anomalously weak zero-phonon lines in the near-edge luminescence of GaP crystals prepared under special conditions.