1. Department of Physics and Earth Sciences; University of Ferrara; via Saragat, 1 44122 Ferrara Italy
2. Department of Physics and Astronomy; University of Padova; via Marzolo 8 35131 Padova Italy
3. Department of Information Engineering CNR-INO, Sensor Laboratory; University of Brescia; via Valotti 9 25123 Brescia Italy
4. CNR-IMM Institute for Microelectronics and Microsystems; Viale Gobetti 4 40129 Bologna Italy