1. Institute of Quantum Matter/Semiconductor Physics Group, University of Ulm, Albert-Einstein-Allee 45, 89081 Ulm, Germany
2. FIB Center UUlm, Institute of Analytical and Bioanalytical Chemistry, University of Ulm, Albert-Einstein-Allee 11, 89081 Ulm, Germany
3. Department of Materials Science and Metallurgy, Cambridge Centre for Gallium Nitride, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, United Kingdom
4. Intitute of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, 89081 Ulm, Germany