(Invited) Toward GaN-Based Red LEDs: The Influence of Local and Extended Defect Environments on the Optical and Material Properties of GaN:Eu
Authors: Mitchell Brandon , Dierolf Volkmar , Fujiwara Yasufumi 
Published: February 2020

The doping of Europium (Eu) into gallium nitride (GaN) has a promising future for solid state lighting applications.1 It has been shown that the Eu ions incorporate themselves into the GaN host in a variety of different defect environments.2 Locally, the defect environments around the Eu ions play a large role in the optical and magnetic properties of the ions themselves. Two of the main centers have been associated with the common vacancy defects in GaN, the gallium vacancy (VGa) and the nitrogen vacancy (VN).1 Once this association has been made, it becomes necessary to consider the charge state of the local vacancy defect. It now appears that one Eu center is actually a charged derivative of another center that was associated with a VGa. The two centers exhibit a temperature dependent meta-stability. Furthermore, the effective g-factor of the 7F2 multiplet for the “charged” center is far above the theoretical expectation for an isolated Eu ion,3 and more than two times larger than that of the other Eu centers. This is thought to be related to the delocalization of a captured carrier over the Eu-VGa complex.
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.
[1] Y. Fujiwara and V. Dierolf, Jpn. J. Appl. Phys. 53, 05FA13 (2014).
[2] N. Woodward et al., Opt. Mater. 33, 1050 (2011).
[3] N. Woodward, et al., MRS Proceedings, 1342, mrss11-1342-v05-06 (2010).
[4] B. Mitchell et al., Scientific Reports 6,18808 (2016).
[5] A. Nishikawa et al., Appl. Phys. Lett. 97, 051113 (2010).