1. Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 300, Taiwan
2. Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan
3. Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan
4. Department of Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California 90095-1595