Controlled Electron–Hole Trapping and Detrapping Process in GdAlO₃ by Valence Band Engineering

Abstract

Two different trapping and detrapping processes of charge carriers have been investigated in GdAlO₃:Ce³⁺,Ln³⁺ (Ln = Pr, Er, Nd, Ho, Dy, Tm, Eu, and Yb) and GdAlO₃:Ln³⁺,RE³⁺ (Ln = Sm, Eu, and Yb; RE = Ce, Pr, and Tb). Cerium is the recombination center and lanthanide codopants act as electron-trapping centers in GdAlO₃:Ce³⁺,Ln³⁺. Different lanthanide codopants generate different trap depths. The captured electrons released from the lanthanide recombine at cerium via the conduction band, eventually producing the broad 5d–4f emission centered at ∼360 nm from Ce³⁺. On the other hand, Sm³⁺, Eu³⁺, and Yb³⁺ act as recombination centers, while Ce³⁺, Pr³⁺, and Tb³⁺ act as hole-trapping centers in GdAlO₃: Ln³⁺,RE³⁺. In this situation, we find evidence that recombination is by means of hole release instead of the more commonly reported electron release. The trapped holes are released from Pr⁴⁺ or Tb⁴⁺ and recombine with the trapped electrons on Sm²⁺, Eu²⁺, or Yb²⁺ and yield characteristic trivalent emission from Sm³⁺, Eu³⁺, or Yb³⁺ at ∼600, ∼617, or ∼980 nm, respectively. Lanthanum was introduced to engineer the valence band energy and change the trap depth in Gd_1–<i>x</i>La_<i>x</i>AlO₃:Eu³⁺,Pr³⁺ and Gd_1–<i>x</i>La_<i>x</i>AlO₃:Eu³⁺,Tb³⁺. The results show that the valence band moves upward and the trap depth related to Pr³⁺ or Tb³⁺ decreases