[1] Harvard Univ, Harvard John A Paulson Sch Engn & Appl Sci, Cambridge, MA 02138 USA
[2] TU Wien, Inst Solid State Elect, Vienna, Austria
[3] Texas A&M Univ, Dept Phys & Astron, College Stn, TX USA
[4] Politecn Torino, Dipartimento Elettron & Telecomunicaz, I-10129 Turin, Italy