1. School of Engineering, University of Glasgow, Oakfield Avenue, G12 8LT Glasgow, United Kingdom
2. Department of Electronic and Electrical Engineering, EPSRC National Centre for III-V Technologies, University of Sheffield, North Campus, Broad Lane, Sheffield S37HQ, United Kingdom
3. Sensing Technology R&D Project, R&D Headquarters LED Division, RohmCo. Ltd., 21 Saiin Mizosaki-cho, Ukyo-ku, Kyoto 615-8585, Japan