1. Department of Electrical and Electronic Engineering, Imperial College London, South Kensington, London SW7 2AZ, United Kingdom
2. Institute of Micro- and Nanoelectronics, Department of Micro- and Nanoelectronic Systems (MNES), Ilmenau University of Technology, Gustav-Kirchhoff-Str.1, 98693 Ilmenau, Germany
3. Hitachi Cambridge Laboratory, J. J. Thomson Avenue, CB3 0HE Cambridge, United Kingdom