1. Institute of Solid-State Physics, University of Bremen 1 , Otto-Hahn-Allee 1, 28359 Bremen, Germany
2. School of Electrical and Computer Engineering, Cornell University 2 , Ithaca, New York 14853, USA
3. MAPEX Center for Materials and Processes, University of Bremen 3 , 28359 Bremen, Germany
4. Department of Material Science and Engineering, Cornell University 4 , Ithaca, New York 14853, USA
5. Kavli Institute at Cornell for Nanoscale Science, Cornell University 5 , Ithaca, New York 14853, USA
6. Leibniz-Institut fur Kristallzüchtung 6 , Max-Born-Str. 2, 12489 Berlin, Germany