1. Physics Division, Institute of Nuclear Energy Research 1 , Taoyuan City 32546, Taiwan
2. Institute of Electronics Engineering, National Tsing Hua University 2 , Hsinchu 30013, Taiwan
3. Department of Materials and Optoelectronic Science National Sun Yat-Sen University 3 , Kaohsiung 80424, Taiwan
4. Department of Physics, National Sun Yat-Sen University 4 , Kaohsiung 80424, Taiwan