1. College of Physical Science and Technology, Guangxi University, Nanning 530004, China
2. Laboratory of Optoelectronic Materials and Detection Technology, Guangxi Key Laboratory for the Relativistic Astrophysics, Guangxi University, Nanning 530004, China
3. Graduate Institute of Electronics and Department of Electrical Engineering, National Taiwan University, Taipei 106-17, Taiwan
4. Department of Physics, Indiana University of Pennsylvania, Indiana, PA 15705-1087, USA
5. Department of Physics, University of North Florida, Jacksonville, FL 32224, USA
6. National Synchrotron Radiation Research Center, Hsinchu 300-76, Taiwan
7. Department of Electrical and Computer Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA