1. Department of Energy and Materials Engineering; Dongguk University; 30 Pildong-ro, 1-gil Jung-gu Seoul 04620 Republic of Korea
2. Department of Physics; National Chung Hsing University; Taichung 40227 Taiwan
3. Key Laboratory of Polar Materials and Devices; Ministry of Education; Department of Electronic Engineering; East China Normal University; Shanghai 200241 China