[1] Tsinghua Univ, Tsinghua Natl Lab Informat Sci & Technol, Beijing 100084, Peoples R China
[2] Tsinghua Univ, Inst Microelect, Beijing 100084, Peoples R China
[3] Yale Univ, Dept Appl Phys, New Haven, CT 06520 USA
[4] SW Univ Sci & Technol, State Key Lab Cultivat Base Nonmet Composites & F, Mianyang 621010, Peoples R China