1. Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan
2. Department of Electrical Engineering, National Tsing Hua University, Hsinchu, Taiwan
3. Department of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan
4. Department of Physics and the Center of Crystal Research, National Sun Yat-sen University, Kaohsiung, Taiwan