1. Department of Physics, Bradley University, 1501 W. Bradley Avenue, IL 61625, USA
2. Department of Microelectronics, North China University of Technology, 5 Jinyuanzhuang Road, Shijingshan District, Beijing 100144, China
3. Department of Physics, Fu-Jen Catholic University, 510 Chung-Cheng Road, Hsin-Chuang 242, Taiwan
4. Graduate Institute of Applied Science and Engineering, Fu Jen Catholic University, 510 Chung-Cheng Road, Hsin-Chuang 242, Taiwan