1. Laboratory of Semiconductor Materials Institute of Materials Ecole Polytechnique Fédérale de Lausanne Lausanne 1015 Switzerland
2. Department of Materials Science and Engineering Stanford University Stanford CA 94305 USA
3. Photovoltaics and Thin Film Electronics Laboratory Institute of Microengineering Ecole Polytechnique Fédérale de Lausanne Neuchâtel 2000 Switzerland