1. a Dept. of Electrical & Electronic Eng. , Dongshin University , 252 Daeho-dong, Naju, Chonnam , 520-714 , Korea
2. b Dept. of Electronics , Chosun College of Science & Technology , 290 SeoSuk-dong Dong-gu, Kwangju , 510-759 , Korea
3. c Institute of Functional Material Chemistry, Northeast Normal University , 2-12-1 O-okayama, Meguro-ku, Tokyo , 152-8552 , Japan
4. d Dept. of Physical Electronics , Tokyo Institute of Technology , 2-12-1 O-okayama, Meguro-ku, Tokyo , 152-8552 , Japan