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

Two different charge carrier trapping processes have been investigated in RE2O2S:Ln(3+) (RE = La, Gd, Y, and Lu; Ln = Ce, Pr, and Tb) and RE2O2S:M (M = Ti(4+) and Eu(3+)). 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 Ce(4+), Pr(4+), or Tb(4+) via the conduction band. On the other hand, Ti(4+) and Eu(3+) 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 Ti(3+) or Eu(2+) and yield broad Ti(4+) yellow-red charge transfer (CT) emission or characteristic Eu(3+) 4f-4f emission. We will conclude that the afterglow in Y2O2S:Ti(4+), Eu(3+) is due to hole release instead of more common electron release.