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Rare Earth (RE) doped III-nitrides are being widely investigated for potential applications in optical communication and displays, due to the wide and direct energy bandgap of GaN resulting in low thermal quenching of RE ion sharp emission from ultraviolet (UV) through visible to infrared (IR) region. The UC Nanolab has been conducting RE doped GaN research for more than 10 years and many achievements were obtained, ranging from material growth to device fabrication. This dissertation studied RE emission in GaN material, focusing on the effects of electronic impurity (Si) co-doping on RE luminescence. Advanced RE doped GaN electroluminescent devices (ELDs) were also designed and fabricated. Detailed device characterization was carried out and the effect of co-dopant was investigated. Eu-doped GaN thin films were grown on sapphire wafers by molecular beam epitaxy (MBE) technique and the growth conditions were optimized for the strongest Eu luminescence. It was found that GaN thin film quality and Eu doping concentration mutually affected Eu luminescence. High quality GaN:Eu thin films were grown under Ga rich condition (III/V>1), but the strongest Eu luminescence was obtained under slightly N rich condition (III/V1). The optimum Eu doping concentration is ~0.1-1.0at.%, depending on the GaN:Eu thin film quality. Higher growth temperature (750°C) was also found to enhance Eu luminescence intensity (~10x) and efficiency (~30x). The effect of Si co-doping in GaN:RE thin films was investigated. Eu photoluminescence (PL) was enhanced ~5-10x by moderate Si co-doping (~0.05at.%) mostly due to the increase of Eu PL lifetime, but decreased very fast at high Si co-doping concentration (>0.08at.%). The increase of Eu PL lifetime is possibly due to the incorporation of Si uniformly distributing Eu ions and shielding Eu-Eu interactions. Combined with the increase in excitation cross section and carrier flux, there is a significant enhancement on Eu PL intensity. The electrical properties of GaN:RE thin films were changed from high resistive to weakly n-type due to increased electron concentration introduced by Si co-doping. GaN:RE ELDs were fabricated and the electrical and optical properties were studied by I-V and electroluminescence (EL) measurements. A hetero-junction PIN structure was designed on n-GaN:Si/GaN:RE/p-Si, employing p-Si substrates as p-type conductive layer. RE ions EL emission was found to be much stronger under forward bias than under reverse bias. The Si co-doping was also studied in GaN:RE ELDs. It was found that Er EL had strong visible & IR emission under forward bias, while there is little or no emission under reverse bias. A pn hetero-junction structure formed between p-Si and n-GaN:(Si, Er) layers was proposed to be responsible for the emission control. GaN:(Si, Eu) AC thin film ELDs were also fabricated and shown that the Si co-doping increased the Eu ions emission intensity and efficiency.