1. Key Laboratory of Advanced Materials (MOE)School of Materials Science and EngineeringTsinghua University Beijing 100084 P. R. China
2. Key Laboratory of Microelectronic Devices & Integrated TechnologyInstitute of MicroelectronicsChinese Academy of Sciences Beijing 100029 P. R. China
3. National Center for Electron Microscopy in BeijingKey Laboratory of Advanced Materials (MOE)State Key Laboratory of New Ceramics and Fine ProcessingSchool of Materials Science and EngineeringTsinghua University Beijing 100084 P. R. China
4. Institute of MicroelectronicsTsinghua University Beijing 100084 P. R. China