1. Communications Department, School of Electrical and Computer Engineering, University of Campinas, Campinas, SP 13083-852, Brazil
2. Center for Semiconductor Components and Nanotechnologies, University of Campinas, Campinas, SP 13083-570, Brazil
3. Institute of Physics Gleb Wataghin, University of Campinas, Campinas, SP 13083-859, Brazil
4. Center for Semiconductor Components and Nanotechnologies, University of Campinas, Campinas, SP 13083-870, Brazil