1. BCO Technologies (NI) Limited, Belfast BT11 8BU, Northern Ireland
2. Microelectronics Center, Technical University of Denmark, DK‐2800 Lyngby, Denmark
3. Department of Solid State Electronics, Chalmers University of Technology, S‐412 96 Göteborg, Sweden
4. SIMS‐laboratory, Department of Physics, Chalmers University of Technology, S‐412 96 Göteborg, Sweden