1. Quantum Semiconductor International Co. Ltd., Chungnam 331-836, Republic of Korea
2. Korea Electronics Technology Institute, Gyeonggi 463-816, Republic of Korea
3. Department of Physics, Kongju National University, Chungnam 314-701, Republic of Korea
4. Department of Electrical Engineering, Korea University, Seoul 136-713, Republic of Korea
5. Department of Chemical Engineering, Dankook University, Gyeonggi 448-701, Republic of Korea
6. School of Materials Science and Engineering, Hongik University, Sejong 339-701, Republic of Korea