[1] Quant.-Funct. Semiconduct. Res. Ctr., Dongguk University, 3-26, Pildong, Chungku, Seoul 100-715, Korea, Republic of
[2] Adv. Semiconductor Research Center, Division of Electrical Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-gu, Seoul 133-791, Korea, Republic of
[3] Department of Physics, Wuhan University, Wuhan 430072, China