1. Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
2. ams AG, Tobelbader Strasse 30, A-8141 Premstaetten, Austria
3. Department of Applied Physics, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands and Philips Innovation Services, High Tech Campus 4, 5656 AE Eindhoven, The Netherlands