Direct growth of crystalline silicon on GaAs by low temperature PECVD: Towards hybrid tunnel junctions for III-V/Si tandem cells

Monolithical integration of III-V and Si is of strong interest to produce tandem solar cells reaching high conversion efficiencies. In the context of the French ANR research project IMPETUS, an innovative approach for III-V/Si multijunction solar cells is studied. The targeted device is a tandem cell composed of a III-V top cell (AlGaAs) and a IV bottom cell (Si_1-xGe_x). The choice of Al_yGa_1-yAs as the top material is justified because it provides the optimum bandgap combination with Si_1-xGe_x (1.63 eV/0.96 eV), with theoretical efficiencies in excess of 42% for such a tandem configuration. In our inverted metamorphic approach, we first use MOVPE to grow the AlGaAs top cell on a lattice matched GaAs substrate, and then perform low temperature PECVD heteroepitaxial SiGe on top. We show here the first structural and electrical characterizations of Si(PECVD)/III-V(MOVPE) interfaces. Furthermore, the epitaxial growth of highly doped crystalline Si by low-temperature PECVD on GaAs enables us to fabricate hybrid tunnel junctions with low resistivity and a high current, suitable to interconnect the two subcells in the tandem III-V/Si solar cell.