Eu-Doped AlGaN/GaN Superlattice-Based Diode Structure for Red Lighting: Excitation Mechanisms and Active Sites

In this work, we have established the effects of postgrowth annealing and Eu implantation, followed by annealing on an AlGaN/GaN superlattice-based diode structure, containing Mg-doped GaN top p-cap layers. The study is based on the combined information from different optical techniques, such as Raman, photoluminescence, and photoluminescence excitation. We have shown that the diode structure exhibits a stable crystalline quality even after annealing under high temperature and high pressure (HTHP) conditions (1400 degrees C in 1 GPa N-2). Furthermore, we have demonstrated that the implanted Eu ions reached the first quantum wells of the diode structure and that the postimplantation thermal annealing partly removed the implantation defects, recovering some of the as-grown luminescence and optically activating the Eu3+ in the diode structure. An in-depth study of the Eu3+ population mechanisms was realized through room temperature photoluminescence excitation. A model was built based on the different excitation bands originated from the materials present in the diode structure, demonstrating that an energy transfer between the AlGaN/GaN superlattice excitons and the Eu3+ ions occurs, therefore enlarging the excitation pathways for the ion's red luminescence. In addition, Eu3+ luminescence was observed not only with above but also with below GaN bandgap excitation. The temperature dependent study of the D-s(J) -> F-7(J) transitions allowed to tentatively provide the Eu3+ intraionic assignments of the diode structure. We have demonstrated that at least three non-equivalent active sites are created by the Eu implantation in the diode structure: Eu1, Eu2, and Eu-Mg defect in both configurations Eu0 and Eul(Mg).