1. Dept of Physics, SUPA, University of Strathclyde, Glasgow G4 0NG, UK
2. CIMAP UMR 6252 CNRS-ENSICAEN-CEA-UCBN 14050 Caen Cedex, France
3. Dipartimento di Fisica Astronomia, Università di Bologna, 40127 Bologna, Italy
4. Institute of Solid State Physics, Technical University Berlin, 10623 Berlin, Germany
5. AIXTRON SE, Kaiserstr. 98, 52134 Herzogenrath, Germany
6. Thales Research and Technology, III-V Lab, 91460 Marcoussis, France
7. LPN, Route de Nozay, 91460 Marcoussis, France
8. Institut des NanoSciences, Université Pierre et Marie Curie, 75015 Paris, France