1. Photophysics and Optoelectronics Group, Zernike Institute for Advanced Materials; University of Groningen; Nijenborgh 4 Groningen 9747 AG The Netherlands
2. Dipartimento di Scienze e Metodi dell'Ingegneria, Università degli Studi di Modena e Reggio Emilia; Via Amendola 2 Padiglione Morselli, Reggio Emilia I-42100 Italy
3. ETSF and Dipartimento di Fisica; Università degli Studi di Roma Tor Vergata; Via della Ricerca Scientifica 1 Rome I-00133 Italy
4. Nanostructured Materials and Interfaces Group, Zernike Institute for Advanced Materials, University of Groningen; Nijenborgh 4 Groningen 9747 AG The Netherlands
5. National Institute for Materials Physics; PO Box MG-7 Bucharest-Magurele 0770125 Romania
6. Institute for Semiconductor and Solid State Physics; University of Linz; Allenbergerstrasse 69 Linz 4040 Austria