1. Department of Physics, National Sun Yat-Sen University, Kaohsiung 804, Taiwan R. O. C
2. Advanced Optoelectronics Technology Center, National Cheng Kung University, Tainan 701, Taiwan R. O. C.
3. Department of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung 804, Taiwan R. O. C.
4. Institute of Microelectronics, Tsinghua University, Beijing 100084, China
5. Department of Electronics Engineering and Institute of
Electronics, National Chiao Tung University, Hsinchu 300, Taiwan R. O. C.