1. Thin Film Physics Division, Department of Physics, Chemistry and Biology IFM, Linköping University, 581 83 Linköping, Sweden
2. Terahertz Materials Analysis Center, THeMAC and Center for III-Nitride Technology, C3NiT–Janzén, Linköping University, 581 83 Linköping, Sweden
3. Center for Nanotechnology Innovation @NEST, Instituto Italiano di Tecnologia, Pisa, Italy
4. Department of Electrical and Computer Engineering and Center for Nanohybrid Functional Materials, University of Nebraska-Lincoln, Lincoln, Nebraska 68588, USA
5. Semiconductor Materials Division, Department of Physics, Chemistry and Biology IFM, Linköping University, 581 83 Linköping, Sweden