1. Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, 195, Sec. 4, Chung Hsing Road, Chutung 310, Taiwan
2. Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, Hsinchu 300, Taiwan
3. Department of Electronic Engineering, Chung Yuan Christian University, 200 Chung Pei Road, Chung Li 320, Taiwan