On the macroscale, the overall concentration of oxygen, which can be detrimental to device performance, was shown to influence the incorporation of the Eu ions.4 The oxygen was introduced into the samples due its presence in the standard Eu precursor Eu(DPM)3; therefore, a new liquid Eu source, which does not contain oxygen in its molecular structure, was developed. It was discovered that the removal of oxygen led to the broadening of the photoluminescence emission spectra, and the appearance of Eu-N surface precipitation. This was due to poor incorporation of the Eu ions onto Ga sites in the GaN, which was confirmed by Rutherford backscattering/channeling measurements. By intentionally reintroducing oxygen during growth, this issue was resolved. However, this issue can be subverted altogether by using a delta-doping growth structure, with alternating layers of GaN and GaN:Eu. The mechanism for this will be discussed.
Moreover, it was recently found that the incorporation of Eu into the GaN matrix can drastically influence the material properties, even at dilute levels (< 1%). By using the delta-doping structure and lowering the growth temperature, the surface morphology, as well as the size and concentration of threading dislocations can be controlled. This control had a large influence on the optical and electrical properties of LEDs grown using these conditions. Not only was the output power per Eu layer thickness significantly enhanced, but an injection current of 20 mA was achieved under an applied voltage of <5V, which is the lowest value reported for this material.5 Lastly, the brightest LED had an output power of ~375µW, and an external quantum of efficiency of ~4.6%, which are the highest values reported for this system.
 Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014).
 N. Woodward et al., Opt. Mater. 33, 1050 (2011).
 N. Woodward, et al., MRS Proceedings, 1342, mrss11-1342-v05-06 (2010).
 B. Mitchell et al., Scientific Reports 6,18808 (2016).
 A. Nishikawa et al., Appl. Phys. Lett. 97, 051113 (2010).