1. Department of Electrical and Computer Engineering, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
2. Nick Holonyak, Jr. Micro and Nanotechnology Laboratory, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States
3. Department of Physics, University of Illinois at Urbana−Champaign, Urbana, Illinois 61801, United States