1. Physics Department, Sapienza University of Rome, Piazzale Aldo Moro 5, 00185 Rome, Italy
2. Department of Experimental Physics, Faculty of Fundamental Problems of Technology, Wroclaw University of Science and Technology, Wroclaw 50-370, Poland
3. IHP-Leibniz Institut fur Innovative Mikroelektronik, Im Technologiepark 25, 15236 Frankfurt (Oder), Germany
4. Institute for Photonics and Nanotechnologies (CNR-IFN), National Research Council, 00156 Rome, Italy
5. Center for Functional Sensor & Actuator (CFSN), National Institute for Materials Science (NIMS), Tsukuba, Ibaraki 305-0044, Japan