1. MIND-IN2UB, Department of Electronics and Biomedical Engineering, Universitat de Barcelona, E-08028 Barcelona, Spain
2. IHT-LENA, Technische Universität Braunschweig, D-38106 Braunschweig, Germany
3. IMB-CNM (CSIC), Institut de Microelectrònica de Barcelona, Campus UAB, E-08193 Bellaterra, Spain
4. Epitaxy Competence Center (ec2), Technische Universität Braunschweig, D-38106 Braunschweig, Germany