1. Nanyang Technological University, School of Materials Science and Engineering, Singapore 639798, SingaporebNanyang Technological University, School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Singapore 637371, Singapore
2. Nanyang Technological University, School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Singapore 637371, Singapore
3. Chinese Academy of Sciences, State Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Beijing 100083, China
4. National Center for Nanoscience and Technology, CAS Key Laboratory of Standardization and Measurement for Nanotechnology, Beijing 100190, China
5. Nanyang Technological University, School of Materials Science and Engineering, Singapore 639798, Singapore
6. Nanyang Technological University, School of Physical and Mathematical Sciences, Division of Physics and Applied Physics, Singapore 637371, SingaporeeNanyang Technological University, School of Electrical and Electronic Engineering, NOVITAS, Nanoelectronics Centre of Excellence, Singapore 639798, Singapore