1. Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan 48109, United States
2. Department of Electrical and Systems Engineering, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States
3. Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, United States
4. AIXTRON Ltd, Swavesey, Cambridge CB24 4FQ, United Kingdom
5. Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109, United States