1. College of Engineering and Design, Hunan Normal University, Changsha City 410081, P. R. China
2. Engineering Research Center of Advanced Semiconductor Technology and Application of Ministry of Education, Changsha Semiconductor Technology and Application Innovation Research Institute, College of Semiconductors (College of Integrated Circuits), Hunan University, Changsha, Hunan 410082, P. R. China
3. School of Engineering, University of Edinburgh, Edinburgh EH9 3JW, UK