1. Key Laboratory of Microelectronic Devices and Integrated Technology Institute of Microelectronics Chinese Academy of Sciences Beijing 100029 China
2. University of Chinese Academy of Sciences Beijing 100049 China
3. School of Microelectronics University of Science and Technology of China Hefei Anhui 230026 China
4. Department of Electrical and Electronic Engineering The University of Hong Kong Pok Fu Lam Road Hong Kong
5. Department of Electrical and Computer Engineering University of Massachusetts Amherst MA 01003 USA