Nb~+ Polaron and Ti~3+ Shallow Donor Jahn-Teler Centers in LiNbO_3 Systems

摘要

The position of the conduction band edge in LiNbO_3 strongly depends on the presence of antisite niobiums (Nb on Li site) lowering the cutoff energy. This property can also be used for hgih precision routine contorl of the L/Nb ratio. The Nb 4d orbitals can form Nb~5+/4+ donor levels at these antisites, but this is possible at regular niobium sites as well due to nearby impurities and/or the polaron effect at low temperatures. Such shallow electron traps at niobium sites become important in heavily Mg-doped or stoichiometric crystals where no antisite niobiums are present. Trapping processes have recently been observed also on the nanometer timescale. The incorporation of the Ti dopant, used in particular for wave-guiding applications, has been shown to change upon Mg co-doping from Li to Nb substitution. Ti introduces in both cases shallow trapping levels in the gap. The properties of these traps, together with charge transfer processes between Nb~5+/4+ polaron ant Ti~4+/3+ shallow donor levels are discussed, and the Jahn-Teller effects of the involved d~1 paramagnetic states observed by EPR in reduced LiNbO_3:Ti and LiNbO_3:Mg:Ti are compared.