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.

A modification of the growth structure of Eu-doped GaN (GaN:Eu) from a monolayer to a multilayer structure (MLS) consisting of alternating GaN and GaN:Eu, was shown to enhance the emission properties. Similarly, lowering the growth temperature of the GaN:Eu to 960°C nearly doubled the photoluminescence emission intensity, and also enhanced device performance. Hence, to design a higher power GaN:Eu red LED, a multilayer structure consisting of 40 pairs of alternating GaN and GaN:Eu was grown at 960°C. This combination resulted in the fabrication of an LED with a maximum output power of 110 μW, which is 5.8 times more output power per GaN:Eu layer thickness as compared to the best previously reported device. Moreover, it was found that the MLS sample grown at 960°C maintained a high crystal quality with low surface roughness, which enabled an increase in the number of pairs from 40 pairs to 100 pairs. An MLS-LED consisting of 100 pairs of alternating GaN/GaN:Eu layers was successfully fabricated, and had a maximum output power of 375 μW with an external quantum efficiency of 4.6%. These are the highest values reported for this system.

The influence of growth temperature on the surface morphology and luminescence properties of Eu-doped GaN layers grown by organometallic vapor phase epitaxy was investigated. By using a Eu source that does not contain oxygen in its molecular structure, and varying the growth temperature, the local defect environment around the Eu3+ ions was manipulated, yielding a higher emission intensity from the Eu3+ ions and a smoother sample surface. The optimal growth temperature was determined to be 960 °C and was used to fabricate a GaN-based red light-emitting diode with a significantly higher output power.

The behavior of trivalent europium (Eu3+) ions doped into gallium nitride (GaN) was investigated under intense excitation conditions to explore the excitation energy transfer characteristics in the presence of large carrier densities. Under such conditions, strong emission from the higher excited 5D1 and 5D2 states of the Eu3+ ions was observed in highly efficient AlGaN/Eu-doped GaN multiple quantum wells grown by organometallic vapor phase epitaxy. This behavior was studied using a variety of excitation sources and conditions. Most notably, when a femtosecond-pulse laser was used, the excitation of the Eu3+ ions into the higher energy states became significant only with a second excitation pulse arriving within the lifetime of the 5D0 state. We propose that an already excited Eu3+ ion is promoted from its 5D0 excited state into the higher 5D J states where it relaxes and can emit from the 5D1 and 5D2 states.

The development of efficient electrically driven color-tunable solid-state light sources will enable new capabilities in lighting and display technologies. Although alternative light sources such as organic light emitting diodes (O-LEDs) have recently gained prominence, GaN-based LEDs remain the most efficient light sources available, making GaN the ideal platform for color-tunable devices. In its trivalent form, Europium is well-known for its red emission at ∼620 nm; however, transitions at ∼590 and ∼545 nm are also possible if additional excited states are exploited. Using intentional codoping and energy-transfer engineering, we show that it is possible to attain all three primary colors due to an emission originating from two different excited states of the same Eu3+ ion mixed with near band edge emission from GaN centered at ∼430 nm. The intensity ratios of these transitions can be controlled by choosing the current injection conditions, such as injection current density and duty cycle under pulsed current injection.