1. Luminous! Center of Excellence for Semiconductor Lighting and Displays, School of Electrical and Electronic Engineering, Division of Physics and Applied Physics, School of Physical and Mathematical Sciences Nanyang Technological University; Singapore 637371 Singapore
2. Department of Electrical and Electronics Engineering; Department of Physics; UNAM-Institute of Materials Science and Nanotechnology, Bilkent University; Ankara 06800 Turkey
3. Department of Physics and Astronomy; Ohio Univeristy; Athens Ohio 45701 United States