1. Department of Physics, National Sun Yat‐Sen University, Taiwan
2. National Nano Device Laboratory, Hsin‐Chu 300, Taiwan
3. Department of Electronics Engineering and Institute of Electronics, National Chiao Tung University, Hsin‐Chu 300, Taiwan
4. Institute of Materials Science and Engineering, National Chiao Tung University, Hsin‐Chu 300, Taiwan