Straightforward hetero-epitaxy of Germanium at very low temperature : application to photovoltaics

Here we present a bottom up approach to grow crystalline Ge films by standard RF-PECVD at temperatures below 200°C. This epitaxial growth, already obtained on nearly lattice matched GaAs wafers [Johnson et al.], is here extended to metamorphic hetero-epitaxy on silicon: Ge is directly grown on (100) FZ silicon substrates. Both structural and electronic properties of the epitaxial layers are investigated. High crystalline quality is confirmed by HRTEM. The electronic properties of germanium are studied by formation of ultra-thin film Ge heterojunction solar cells, based on the wafer equivalent approach [Cariou et al.] architecture. We obtain photovoltaic effect with Voc as high as 300 mV. Particular attention has been paid to both interfaces (with the substrate and with top electrical contact) with the aim to reduce the recombination effect. Those results are a first step towards multijunction solar cells with low thermal budget and potential cost reduction. Ref : • Johnson, E.V., Patriarche, G. & Roca i Cabarrocas, P. Directional growth of Ge on GaAs at 175 °C using plasma-generated nanocrystals. Appl. Phys. Lett. 92, 103108 (2008). • Cariou, R., Labrune, M. & Roca i Cabarrocas, P. Thin crystalline silicon solar cells based on epitaxial films grown at 165 °C by RF-PECVD. Solar Energy Materials and Solar Cells 95, 2260-2263 (2011).