1. Department of Electronics and Nanoengineering; Aalto University; P.O. Box 13500 ,FI-0076 Aalto Finland
2. Department of Materials Science and Engineering; Center for Photonics and Nanoelectronics; 5 East Packer Ave Bethlehem PA 18015 USA
3. Leibniz Institute for Crystal Growth; 12489 Berlin Germany
4. Department of Applied Physics; Aalto University; P.O. Box 15100 ,FI-0076 Aalto Finland