1. Department of Electrical and Electronic Engineering Southern University of Science and Technology Shenzhen 518055 P. R. China
2. Key Laboratory of Optoelectronic Devices and Systems of Ministry of Education and Guangdong Province, College of Physics and Optoelectronic Engineering Shenzhen University Shenzhen 518060 P. R. China
3. School of Materials Science and Engineering Hubei University Wuhan 430062 P. R. China