1. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln, Lincoln, Nebraska 68588-0511, USA
2. J. A. Woollam Co., Inc., Lincoln, Nebraska 68508-2243, USA
3. Terahertz Materials Analysis Center, THeMAC and Center for III-Nitride Technology, C3NiT–Janzén, Department of Physics, Chemistry and Biology, Linköping University, SE 58183 Linköping, Sweden
4. Department of Physics and Optical Science, University of North Carolina at Charlotte, Charlotte, North Carolina 28223, USA
5. Leibniz Institute for Polymer Research, 01069 Dresden, Germany