1. Department of Physics, National Sun Yat-sen University, Kaohsiung, Taiwan
2. Department of Materials and Optoelectronic Science, National Sun Yat-sen University, Kaohsiung, Taiwan
3. Institute of Electronics, National Yang Ming Chiao Tung University, Hsinchu, Taiwan
4. Department of Physics, the Center of Crystal Research, College of Semiconductor and Advanced Technology Research, National Sun Yat-sen University, Kaohsiung, Taiwan