1. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2. Leibniz-Institut für Kristallzüchtung, 12489 Berlin, Germany
3. Terahertz Materials Analysis Center and Center for III-N Technology, C3NiT–Janzèn, Department of Physics, Chemistry and Biology (IFM), Linköping University, 58183 Linköping, Sweden
4. Leibniz Institut für Polymerforschung e.V., 01069 Dresden, Germany