[1] Univ Oulu, Microelect Lab, FI-90014 Oulu, Finland
[2] Univ Oulu, Phys Mat Lab, FI-90014 Oulu, Finland
[3] Univ Oulu, Infotech Oulu, Elect Mat Packaging & Reliabil Tech Res Grp, FI-90014 Oulu, Finland
[4] Univ Latvia, Inst Solid State Phys, Ctr Excellence, CAMART, LV-1063 Riga, Latvia