1. Electrical and Information Technology, Lund University, Lund, Sweden
2. NanoLund, Lund University, Box 118, Lund, SE-221 00, Sweden
3. School of Information Technology, Halmstad University, Box 823, Halmstad, SE-301 18, Sweden
4. Solid State Physics, Lund University, Box 118, Lund, SE-221 00, Sweden
5. Laboratory of Semiconductor Materials, Ecole Polytechnique Federale de Lausanne, Lausanne, Switzerland