LED-lighting is at the verge of replacing conventional incandescent light sources. These white LEDs are based on nitride technology which produces the blue emission, that is subsequently converted in a separate phosphorescent layer to provide the additional required colors. The latter often consists of an insulating material doped with rare earth ions. In order to facilitate further integration, the possibility of introducing rare earth ions directly into the nitride material has been explored, with considerable success. Doping with europium ions (Eu) is of particular interest since they can produce the red color, which remains a challenge for nitride based materials. In the talk, it will be demonstrated that indeed gallium nitride is an excellent host for Eu ions exhibiting, under UV light excitation, high efficient emission such that optical gain and laser operation is possible. However, for direct electrical excitation and the realization of a highly efficient LED, additional challenges arise: (1) Common rare earth sources for OMVPE growth contain oxygen, which needs to be avoided in a LED device to ensure long device lifetimes. (2) In optimizing the rare-earth doped active region not only the emission properties but also the carrier injection has to be considered. Since some of the optimization strategies for the former involve co-doping with donors and acceptors and consequently modify the electrical properties of the layer, a comprehensive optimization approach is needed. First steps towards overcoming these challenges will be presented and discussed.
Prof. Volkmar Dierolf is a Distinguished University Professor of Physics and the current Chair of the Physics Department. He was a Mercator Visiting Professor at the University of Bonn in 2008 and holds a Private Lectureship (Habilitation) from the University of Paderborn, Germany. He is also a joint faculty with the Materials Science & Engineering department at Lehigh. His research interest lies in defects in ferroelectrics and semiconductors, in particular in terms of their optical and nonlinear optical properties. He has developed a variety of high resolution spectroscopy tools for their study. Among them, combined excitation emission spectroscopy in a confocal microscope and a combination of electron microscopy and confocal optical spectroscopy.