1. Department of Physics, Chemistry and Biology, Linköping University, 581 83Linköping, Sweden
2. Center for III-Nitride Technology, C3NiT-Janzén, Linköping University, 581 83Linköping, Sweden
3. THz Materials Analysis Center (TheMAC), Linköping University, 581 83Linköping, Sweden
4. Solid State Physics and Nanolund, Lund University, 22100Lund, Sweden
5. Hitachi Energy, Forskargrand 7, 722 26Vaesteras, Sweden