Charge Carrier Trapping Processes in RE₂O₂S (RE = La, Gd, Y, and Lu)

Two different charge carrier trapping processes have been investigated in RE₂O₂S:Ln³⁺ (RE = La, Gd, Y, and Lu; Ln = Ce, Pr, and Tb) and RE₂O₂S:M (M = Ti⁴⁺ and Eu³⁺). Cerium, praseodymium and terbium act as recombination centers and hole trapping centers while host intrinsic defects provide the electron trap. The captured electrons released from the intrinsic defects recombine at Ce⁴⁺, Pr⁴⁺, or Tb⁴⁺ via the conduction band. On the other hand, Ti⁴⁺ and Eu³⁺ act as recombination centers and electron trapping centers while host intrinsic defects act as hole trapping centers. For these codopants we find evidence that recombination is by means of hole release instead of electron release. The released holes recombine with the trapped electrons on Ti³⁺ or Eu²⁺ and yield broad Ti⁴⁺ yellow-red charge transfer (CT) emission or characteristic Eu³⁺ 4f-4f emission. We will conclude that the afterglow in Y₂O₂S:Ti⁴⁺, Eu³⁺ is due to hole release instead of more common electron release.