[1] Advanced Semiconductor Research Center, Division of Electronics and Computer Engineering, Hanyang University, Seoul 133-791, Korea, Republic of
[2] Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea, Republic of
[3] Department of Physics, Kyung Hee University, Seoul 130-701, Korea, Republic of
[4] Research Institute for Basic Science, Kyung Hee University, Seoul 130-701, Korea, Republic of