Charge carrier trapping processes in lanthanide doped LaPO4, GdPO4, YPO4, and LuPO4

Abstract

Various methods for deliberate design of electron and hole trapping materials were explored with a study on double lanthanide doped rare earth ortho phosphates. Cerium acts as a recombination center, while lanthanide codopants act as electron trapping centers in LaPO₄:0.005Ce³⁺,0.005Ln³⁺. The electron trap depth generated by lanthanide codopants can be tailored by the choice of lanthanide, and for a fixed set of lanthanide dopants like in Gd_1−xLa_xPO₄:0.005Ce³⁺,0.005Ho³⁺ solid solutions, by changing x, leading to conduction band (CB) engineering. Herein, the electrons liberated from Ho²⁺ recombined through the conduction band at Ce⁴⁺ to yield Ce³⁺ 5d–4f emission. In contrast, samarium, europium and ytterbium act as recombination centers, while Tb³⁺ and Pr³⁺ act as hole trapping centers in double lanthanide doped YPO₄, as Tb³⁺ and Pr³⁺ codopant recombination is realized via hole release rather than the more commonly reported electron release. The holes recombine via the valence band with the electrons trapped in Yb²⁺, Sm²⁺, or Eu²⁺ to generate 4f–4f luminescence from Yb³⁺, Sm³⁺, or Eu³⁺. Lu³⁺ was introduced in YPO₄ to tailor the valence band (VB) energy and to tune the hole trap depths of Tb³⁺ and Pr³⁺ in Y_1−xLu_xPO₄:0.005Ln³⁺ solid solutions. Our results shed light on the deliberate design of electron and hole trapping materials based on a deep understanding of trap level locations and on the transport and trapping processes of charge carriers.