1. Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA.
2. Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802, USA.
3. Institut für Schichten und Grenzflächen ISG1-IT, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany.
4. Institute for Crystal Growth, Max-Born-Straße 2, D-12489 Berlin, Germany.
5. Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109, USA.