1. Werkstoffe der Elektrotechnik and CENIDE, University of Duisburg-Essen, Duisburg 47057, Germany
2. Department of Electrical and Electronics Engineering, Department of Physics, UNAM-Institute of Materials Science and Nanotechnology and National Nanotechnology Research Center, Bilkent University, Ankara 06800, 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