1. Phone: +49 731 50 26192, Fax: +49 731 50 26049
2. Optoelectronics Department, University of Ulm, 89069 Ulm, Germany
3. OSRAM Opto Semiconductors, 93049 Regensburg, Germany
4. Institute of Physics, University of Neuchâtel, 2000 Neuchâtel, Switzerland