1. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
2. Terahertz Materials Analysis Center and Competence Center for III-Nitride Technology C3NiT—Janzén, Department of Physics, Chemistry and Biology (IFM), Linköping University, Linköping SE 58183, Sweden
3. NanoLund and Solid State Physics, Lund University, Lund SE 221 00, Sweden