1. Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 08826, South Korea
2. Power Semiconductor Research Center, Korea Electrotechnology Research Institute (KERI), Changwon 51543, South Korea
3. Department of Electronic Materials Engineering, Kwangwoon University, Seoul 01897, South Korea
4. Samsung Electronics, 1, Samsungjeonja-ro, Hwaseong-si, Gyeonggi-do 445-701, South Korea
5. Department of Physics, Dongguk University, Seoul 100-715, South Korea
6. Department of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, Gwangju 61186, South Korea