We report a study on the Eu luminescence properties of Eu-doped GaN grown on a GaN substrate by organometallic vapor phase epitaxy. The site-selective excitation of Eu ions revealed the concentration of each luminescent site using the luminescence properties under resonant excitation. The quantitative evaluation of the Eu luminescent sites showed that more than 80% of Eu ions are incorporated into a high-symmetry site. However, the photoluminescence spectrum under indirect excitation is markedly different from that under resonant excitation, which indicates that the luminescent site with high symmetry exhibits low-efficiency energy transfer from the GaN host to the luminescent site.

The detrimental influence of oxygen on the performance and reliability of V/III nitride based devices is well known. However, the influence of oxygen on the nature of the incorporation of other co-dopants, such as rare earth ions, has been largely overlooked in GaN. Here, we report the first comprehensive study of the critical role that oxygen has on Eu in GaN, as well as atomic scale observation of diffusion and local concentration of both atoms in the crystal lattice. We find that oxygen plays an integral role in the location, stability, and local defect structure around the Eu ions that were doped into the GaN host. Although the availability of oxygen is essential for these properties, it renders the material incompatible with GaN-based devices. However, the utilization of the normally occurring oxygen in GaN is promoted through structural manipulation, reducing its concentration by 2 orders of magnitude, while maintaining both the material quality and the favorable optical properties of the Eu ions. These findings open the way for full integration of RE dopants for optoelectronic functionalities in the existing GaN platform.

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The effects of thermal annealing on Eu,Mg-codoped GaN (GaN:Eu,Mg) grown by organometallic vapor phase epitaxy were investigated. After annealing in nitrogen ambient, Eu-Mg related photoluminescence emission was quenched to 13% without a change in the spectral shape. The quenched emission recovered to 65% of the original intensity after a subsequent annealing in ammonia ambient. Combined excitation emission spectroscopy and time-resolved photoluminescence results revealed that the quenching behavior is attributed to a nonradiative process induced by unpassivated Mg acceptors in the relaxation of excited 4f electrons of Eu ions.

The effects of Mg and Si codoping on Eu luminescence properties have been investigated in Eu-doped GaN (GaN:Eu). The Mg codoping into GaN:Eu produced novel luminescence centers consisting of Eu and Mg, and increased photoluminescence (PL) intensity in Eu,Mg-codoped GaN (GaN:Eu,Mg). However, this increased PL intensity was quenched by thermal annealing in N2 ambient, which is due to activation of Mg acceptors. In GaN:Eu,Mg codoped additionally with Si (GaN:Eu,Mg,Si), on the other hand, the Eu–Mg centers disappeared, while an additional luminescence center appeared. Furthermore, the additional luminescence center showed no quenching under N2 annealing because Si donors compensated for the Mg acceptors in GaN. Thermal quenching of the luminescence center was also approximately half of that in GaN:Eu. These results indicate that the codoping with additional impurities in GaN:Eu is a powerful technique to control Eu luminescence centers for realization of improved device performance in red light-emitting diodes using GaN:Eu.

We report a study on Eu the luminescence properties of Eu-doped GaN (GaN:Eu) grown on a GaN substrate by organometallic vapor phase epitaxy (OMVPE). Thermal quenching of the Eu luminescence was suppressed using the GaN substrate, which is due to the preferential formation of a Eu luminescent site with a local structure with high symmetry. The preferential formation of this luminescent site was supported by the observation of strong near-band-edge emission. The strong near band-edge emission occurred as a result of suppressed formation of the dominant Eu luminescent site in GaN:Eu on a sapphire substrate, which is known to be coupled with a defect and to have a large capture cross section of photogenerated carriers.

We investigate resonantly excited photoluminescence from a Eu,O-codoped GaN layer embedded into a microcavity, consisting of an AlGaN/GaN distributed Bragg reflector and a Ag reflecting mirror. The microcavity is responsible for a 18.6-fold increase of the Eu emission intensity at ∼10K, and a 21-fold increase at room temperature. We systematically investigate the origin of this enhancement, and we conclude that it is due to the combination of several effects including, the lifetime shortening of the Eu emission, the strain-induced piezoelectric effect, and the increased extraction and excitation field efficiencies. This study paves the way for an alternative method to enhance the photoluminescence intensity in rare-earth doped semiconductor structures.

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