1. Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
2. Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA
3. Laboratory of Semiconductor Materials, Department of Materials and Nano Physics, Royal Institute of Technology (KTH), Electrum 229, 164 40 Kista, Sweden