Energy transfer and relaxation dynamics of europium-doped gallium nitride

This thesis describes a study of energy transfer mechanisms and relaxation dynamics of Europium(Eu)-doped GaN. Eu-doped GaN has been recently highlighted for its bright red visible electroluminescence and excellent spectral features. Doping with other rare earth elements, such as Erbium and Thulium, into a Nitride-based wide bandgap semiconductor allows fabrication of Red-Green-Blue full color display devices without using multiple host materials. The internal quantum efficiency of rare earth elements-doped semiconductor is still lower than 50%, so that the basic research for the energy transfer mechanism is necessary.; In this thesis, optical characterization under various experimental conditions such as excitation method (continuous or pulsed), excitation energy, and temperature is used to study energy transfer mechanisms and relaxation dynamics of the Eu-doped GaN. The main purpose of the experimental studies is to clarify: (1) the role of the dopant sites on the energy transfer mechanisms and (2) the excitation and relaxation dynamics of the Eu dopants. Both energy-resolved and time-resolved spectroscopies are performed to analyze complicated spectra. In order to manifest the underlying physics, bi-exponential fitting or a systematic construction of rate equations is carried on experimental data of time-dependent photoluminescence.; Four different sites of Eu dopants could be identified in the optical spectra under continuous-wave excitation, and one of them could not be excited under pulsed excitation. A very efficient energy transfer pathway through the I2 bound exciton formation is revealed. It is found that the excitation mechanism of the 5D0 state of Eu dopants, which is the excited state responsible for Europium's characteristic red emission, consists of both Auger energy transfer from the GaN host and the sequential relaxation from higher lying states. A 16 meV activation energy is obtained from the temperature dependence of the luminescence decay and from the rate equation modeling. This energy is correlated with the energy deficits required for the energy exchange among Eu dopants at each site. A hypothesis based on the cross-relaxation mechanism between Eu dopants is proposed in order to explain the excitation energy dependence on the luminescence decay and the reason why the Eu dopants at certain sites can be excited only under continuous-wave source.