1. Department of Physics, Sungkyunkwan University, Suwon 16419, Republic of Korea
2. Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea
3. Department of Physics, Sogang University, Seoul 04107, Republic of Korea
4. Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Republic of Korea
5. School of Advanced Materials and Engineering, Sungkyunkwan University, Suwon 16419, Republic of Korea
6. Graduate School of Semiconductor Materials and Devices Engineering, Ulsan National Institute of Science and Technology, Ulsan 44919, Republic of Korea