1. School of Materials Science and Engineering, Korea University, Seoul 02841, South Korea
2. National University of Science and Technology MISiS, Moscow, Leninskiy pr. 4, Moscow 119049, Russia
3. Korea Photonics Technology Institute, Gwangju 500-779, South Korea
4. Soft-Epi Inc., Opo-ro 240, Gwangju-si, Gyeonggi-do 464-892, South Korea
5. Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611, USA