1. Institute of Physics, NCTU, Hsinchu 30010, Taiwan, Institute of Semiconductor Physics, pr. Lavrentieva, 13, Novosibirsk 630090, Russia, Department of Chemistry, State University of New York at Binghamton, Binghamton, New York 13902-6000, United States, Department of Electrophysics, NCTU, Hsinchu 30010, Taiwan