1. Department of Semiconductor Science and Technology, Semiconductor Physics Research Center, Chonbuk National University, Jeonju 561-756, Korea, MRG, IESL, Foundation for Research and Technology-Hellas, Vassilika Vouton, P.O. Box 1527, 71110 Heraklion, Crete, Greece, and IMEP/INP Grenoble, MINATEC, 3, parvis Louis Neel-BP 257, 38016 Grenoble Cedex 01, France