1. Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan
2. Colleage of Semiconductor Research, National Tsing-Hua University, Hsinchu 30013, Taiwan
3. Department of Physics, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan
4. Institute of Electronics Engineering, National Tsing-Hua University, Hsinchu 30013, Taiwan
5. Department of Materials Science and Engineering, Korea University, Seoul 02841, Republic of Korea