1. Solid State Physics, Lund University, Box 118, S-221 00 Lund, Sweden, Institut für Halbleiter- und Festkörperphysik, Johannes Kepler Universität Linz, Altenbergerstr. 69, A-4040 Linz, Austria, Synchrotron Radiation Research, Lund University, Box 118, S-221 00 Lund, Sweden, and National Centre for High-Resolution Electron Microscopy (nCHREM), Lund University, Box 124, S-221 00 Lund, Sweden