1. Max Planck Institute for Chemical Physics of Solids, Noethnitzerstrasse 40, 01187 Dresden, Germany
2. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
3. Institute of Physics, Academia Sinica, Taipei 11529, Taiwan
4. Institut
fur Angewandte Photophysik, Technische Universitat Dresden, George-Bahr-Strasse 1, 01069 Dresden, Germany
5. Department of Electrical
Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan
6. Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan
7. Department of Physics, National Cheng Kung University, Tainan City 701, Taiwan
8. Department
of Physics, Tsinghua University, Beijing 100084, China