1. Institute for Materials Research, Tohoku University, 2‐1‐1 Katahira, Aoba‐ku, Sendai 980‐8577, Japan
2. Division of Technology, Korea Photonics Technology Institute, Bonchon‐dong 459‐3, Buk‐gu, Gwangju 500‐210, Korea
3. Department of Semiconductor Physics, Korea Maritime University, 1 Dongsam‐dong Yeongdo‐ku, Busan 606‐791, Korea
4. Center for Interdisciplinary Research, Tohoku University, Aramaki‐Aza‐Aoba, Aoba‐ku, Sendai 980‐8578, Japan