1. Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, P. O. Box 912, 100083 Beijing, China
2. Division of Solid State Physics, Lund University, P. O. Box 118, SE-22100 Lund, Sweden
3. Photonics and Semiconductor Nanophysics, COBRA Inter-University Research Institute, Department of Applied Physics, Eindhoven University of Technology, P. O. Box 513, 5600 MB Eindhoven, The Netherlands