1. Department of Physics, Chemistry and Biology (IFM), Linköping University 1 , 581 83 Linköping, Sweden
2. Center for III-Nitride Technology, C3NiT–Janzén, Linköping University 2 , 581 83 Linköping, Sweden
3. Terahertz Materials Analysis Center (THeMAC), Linköping University 3 , 581 83 Linköping, Sweden
4. NanoLund and Solid State Physics, Lund University 4 , 22100 Lund, Sweden
5. Department of Electrical and Computer Engineering, University of Nebraska-Lincoln 5 , Lincoln, Nebraska 68588, USA