Crystal-field analysis and models of Eu-emission centers with C3v symmetry in insitu Eu- and Mg-codoping GaN layers

Abstract

Gallium nitride active layers co-doped in situ with magnesium and europium, GaN(Mg):Eu, show strong red photoluminescence (PL), promising for red light emitting diode (LED) application (Eu@621 nm). Polarized PL lines excited by either above or below band-gap energy for the GaN(Mg):Eu layers are assigned to the 5D0→7FJ (J = 0, 1, 2) transitions lines from several Eu centers with C3v symmetry, which consist of single line, double lines and triple lines related to splitting of 7FJ (J = 0, 1, 2) by the C3v crystal-field, respectively. Cascade luminescence line-pattern of the 5D1→7FJ (J = 0, 1, 2) transitions is a complete replica of the PL line-pattern and gives evidence of the correct assignment of the polarized PL lines. Crystal-field splitting and Zeeman splitting of the PL lines have been analysed using the C3v crystal-field equivalent operator Hamiltonian. Structural Eu models with the C3v symmetry have been proposed on the basis of the analysis: the dominant Eu center is an Eu complex with defect-free perfect surrounding: the minor Eu centers are Eu complexes accompanied by nitrogen vacancies with different charge states on the c-axis in the closest vicinity of Eu. An important role of Mg atoms in the GaN(Mg):Eu active layers has been proposed to suppress creation of gallium vacancies, giving rise to enhancement of the PL emitted from the defect-free Eu centers.