1. Max-Planck Institute for Metals Research, Heisenbergstr. 3, 70569 Stuttgart, Germany, Institute of Photo-electronic Thin Film Devices and Technology, Nankai University, No. 94, Weijin Road, 300071 Tianjin, P.R. China, Materials Science Institute of Barcelona (CSIC), 08193 Bellaterra, Spain, and German Electron Synchrotron (DESY), Notkestr. 85, 22607 Hamburg, Germany