1. Key Laboratory of Microelectronics and Energy of Henan Province
2. College of Physics and Electronic Engineering
3. Xinyang Normal University
4. Xinyang 464000
5. P. R. China
6. Department of Physical Sciences
7. Jaramogi Oginga Odinga University of Science and Technology
8. Bondo
9. Kenya
10. School of Physics, Huazhong University of Science and Technology
11. Wuhan 430074