1. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, USA
2. Photon Sciences Directorate, Argonne National Laboratory, Argonne, Illinois 60439, USA
3. Physical Sciences and Engineering Directorate, Argonne National Laboratory, Argonne, Illinois 60439, USA
4. AIXTRON, Inc., Sunnyvale, California 94089, USA
5. Department of Physics, Northern Illinois University, DeKalb, Illinois 60115, USA