1. Photonic and Electronic Thin Film Lab., Dept. of Mater. Sci. & Eng., Chonnam National University, 300 Puk‐gu, Yongbong‐dong, Gwangju 500‐757, South Korea
2. Dept. of Mater. Sci. & Eng., Pohang University of Sci. & Tech., Pohang 790‐784, South Korea