1. Department of Materials and Optoelectronic Science; National Sun Yat-Sen University; Kaohsiung 804 Taiwan
2. Department of Applied Science; R.O.C. Naval Academy; Kaohsiung 81345 Taiwan
3. Department of Physics; National Sun Yat-Sen University; Kaohsiung 804 Taiwan
4. School of Technical Physics; Xidian University; Xi'an 710071 P. R. China
5. Key Laboratory of Wide Band Gap Semiconductor Materials and Devices; School of Microelectronics; Xidian University; Xi'an 710071 P. R. China
6. Department of Physics; Military Academy; Kaohsiung 800 Taiwan
7. Department of Electronics Engineering and Institute of Electronics; National Chiao Tung University; Hsinchu 300 Taiwan