1. Tianjin Key Laboratory of Film Electronic & Communication Devices, School of Electronical and Engineering, Tianjin University of Technology, No. 391, Binshuixi Road, Xiqing District, Tianjin 300384, China
2. Department of Materials Science and Engineering and Inter-University Semiconductor Research Center, Seoul National University, Gwanak-ro 1, Daehag-dong, Gwanak-gu, Seoul 151-744, Republic of Korea