1. National Renewable Energy Laboratory 1 , Golden Colorado 80401, USA
2. Department of Materials Science and Engineering, University of California 2 , Berkeley California 94720, USA
3. Department of Semiconductor Materials Engineering, Wroclaw University of Science and Technology 3 , Wybrzeże Wyspiańskiego 27, 50-370, Wrocław, Poland
4. Research Organization of Science and Technology, Ritsumeikan University 4 , Kusatsu, Shiga 525-8577, Japan