1. CNR-IMM, Institute for Microelectronics and Microsystems 1 , Via Monteroni, 73100 Lecce, Italy
2. Department of Mathematics and Physics, CEDAD—Centre of Applied Physics Dating and Diagnostics, University of Salento 2 , Via Monteroni, 73100 Lecce, Italy
3. INFN Section of Lecce 3 , Via Monteroni, 73100 Lecce, Italy