1. Department of Electrical and Electronics Engineering; Department of Physics; UNAM - Institute of Materials Science and Nanotechnology; Bilkent University; Ankara 06800 Turkey
2. Department of Physics; Abant Izzet Baysal University; Bolu 14280 Turkey
3. Luminous! Center of Excellence for Semiconductor Lighting and Displays; School of Electrical and Electronic Engineering; School of Physical and Materials Sciences; School of Materials Science and Nanotechnology; Nanyang Technological University; Singapore 639798 Singapore