[1] Dongguk Univ, Qauntum Funct Semicond Res Ctr, Seoul 100715, South Korea
[2] Dongguk Univ, Dept Phys, Seoul 100715, South Korea
[3] Hanyang Univ, Div Elect & Comp Engn, Seoul 133791, South Korea
[4] Univ Calif San Diego, Mat Sci & Engn Program, Dept Mech & Aerosp Engn, La Jolla, CA 92093 USA