1. Laboratoire des Matériaux Semiconducteurs, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
2. Catalan Institute of Nanoscience and Nanotechnology (ICN2),
CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain
3. UPC - Universitat Politècnica de Catalunya, Calle Jordi Girona, 1-3, 08034 Barcelona, Spain
4. ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain
5. Laboratory of Semiconductor Materials, Institute of Materials, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland
6. Institute of Physics, École Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland