1. Institute of Functional NanosystemsUlm University Albert-Einstein-Allee 45 89081 Ulm Germany
2. Central Facility of Electron MicroscopyUlm University Albert-Einstein-Allee 11 89081 Ulm Germany
3. Ferdinand-Braun-InstitutLeibniz-Institut für Höchstfrequenztechnik Gustav-Kirchhoff-Str. 4 12489 Berlin Germany
4. Institute of Quantum Matter/Semiconductor Physics GroupUlm University Albert-Einstein-Allee 45 89081 Ulm Germany