1. Key Laboratory of Polar Materials and Devices, Ministry of Education; Department of Electronic Engineering; East China Normal University; Shanghai 200241 China
2. Department of Electrophysics; National Chiao Tung University; Hsinchu 300 Taiwan
3. Department of Electrical Engineering and Institute of Electronic Engineering; National Tsing Hua University; Hsinchu 300 Taiwan
4. Department of Physics; National Chung Hsing University; Taichung 40227 Taiwan
5. Graduate Institute of Applied Science and Technology; National Taiwan University of Science and Technology; Taipei 106 Taiwan
6. Department of Materials Science and Engineering; National Chung Hsing University; Taichung 40227 Taiwan