Raphaël Lachaume

Plasma-enhanced chemical vapor deposition epitaxy of Si on GaAs for tunnel junction applications in tandem solar cells

Abstract. We fabricated (n) c-Si/ (p) GaAs heterojunctions, by combining low temperature (∼175°C) RF-PECVD for Si and metal organic vapor phase epitaxy for GaAs, aiming at producing hybrid tunnel junctions for Si/III-V tandem solar cells. The electrical properties of these heterojunctions were measured and compared to that of a reference III-V tunnel junction. Several challenges in the fabrication of such heterostructures were identified and we especially focused in this study on the impact of atomic hydrogen present in the plasma used for the deposition of silicon on p-doped GaAs doping level. The obtained results show that hydrogenation by H2 plasma strongly reduces the doping level at the surface of the GaAs:C grown film. Thirty seconds of H2 plasma exposition at 175°C are sufficient to reduce the GaAs film doping level from 1×1020u2009u2009cm−3 to <1×1019u2009u2009cm−3 at the surface and over a depth of about 20 nm. Such strong reduction of the doping level is critical for the performance of the tunnel junction. However, the doping level can be fully recovered after annealing at 350°C.

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 (Si1-xGex). The choice of AlyGa1-yAs as the top material is justified because it provides the optimum bandgap combination with Si1-xGex (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.