1. Thin Film Physics Division, Department of Physics (IFM), Linköping University, Linköping SE-58183, Sweden
2. Applied Nuclear Physics, Department of Physics and Astronomy, Uppsala University, Uppsala SE-75120, Sweden
3. Materials Research Laboratory and Department of Materials Science, University of Illinois, Urbana, Illinois 61801
4. Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan