1. Department of Electronic Materials Engineering, Australian National University, Canberra 0200, Australia
2. Nuclear Science and Technology Department, Brookhaven National Laboratory, Upton, New York 11973, USA
3. Laboratório de Implantação Iônica, Instituto de Física, Universidade Federal do Rio Grande do Sul, Porto Alegre, RS 91501-970, Brasil
4. Australian Synchrotron, 800 Blackburn Road, Clayton, Melbourne, Australia