Controlled electron and hole trapping in YPO4:Ce3+,Ln3+ and LuPO4:Ce3+,Ln3+ (Ln=Sm, Dy, Ho, Er, Tm)

A detailed and systematic electron trapping study was conducted in the scientifically important wide band-gap materials LuPO4 and YPO4 doubly doped with Ce3+ and Ln(3+) (Ln=Sm, Dy, Ho, Er, Tm). By using vacuum-ultraviolet luminescence spectroscopy and detailed thermoluminescence spectroscopy it was possible to establish by two independent methods that the observed electron traps can be assigned to the Ln(3+) codoping ions. It is shown that the associated electron trap depths are determined by the energy separation between the Ln(2+) ground states and the bottom of the conduction band. Both methods reveal a systematic behavior of electron trap depths as a function of the type of Ln(3+) codoping ion that can be explained by recently developed empirical models. Small differences in trap depths obtained by the two methods are discussed in the context of charge-transfer induced relaxation processes and uncertainties in glow peak analysis. Our experiments provide valuable information on 4f(n)<-> 4f(n), 4f(n)-> 4f(n-1)5d, O2--> Ln(3+) charge transfer and the lowest-energy PO43- group transitions as well as electron trap depths. These transition energies allowed us to construct a complete energy-level diagram for LuPO4:Ln(3+/2+) and YPO4:Ln(3+/2+).