[1] Quant.-Funct. Semiconduct. Res. Ctr., Dongguk University, Seoul 100-715, Korea, Republic of
[2] Department of Applied Physics, Dankook University, Seoul 140-714, Korea, Republic of
[3] Department of Physics, Wuhan University, Wuhan 430072, China
[4] Nano Device Research Center, Korea Inst. of Sci. and Technology, Seoul 130-650, Korea, Republic of