Gwenaëlle Hamon

Influence of deposition rate on the structural properties of plasma-enhanced CVD epitaxial silicon

Solar cells based on epitaxial silicon layers as the absorber attract increasing attention because of the potential cost reduction. In this work, we studied the influence of the deposition rate on the structural properties of epitaxial silicon layers produced by plasma-enhanced chemical vapor deposition (epi-PECVD) using silane as a precursor and hydrogen as a carrier gas. We found that the crystalline quality of epi-PECVD layers depends on their thickness and deposition rate. Moreover, increasing the deposition rate may lead to epitaxy breakdown. In that case, we observe the formation of embedded amorphous silicon cones in the epi-PECVD layer. To explain this phenomenon, we develop a model based on the coupling of hydrogen and built-in strain. By optimizing the deposition conditions to avoid epitaxy breakdown, including substrate temperatures and plasma potential, we have been able to synthesize epi-PECVD layers up to a deposition rate of 8.3 Å/s. In such case, we found that the incorporation of hydrogen in the hydrogenated crystalline silicon can reach 4 at. % at a substrate temperature of 350 °C.

Investigation of Hybrid Tunnel Junction Architectures for III-V/Si Tandem Solar Cells

We fabricated n-Si/p-GaAs and p-Si/n-GaAs junctions, by combining low temperature (under 200°C) RF-PECVD for Si and MOVPE for GaAs. In particular, we focused on low-resistance Si/GaAs tunnel junctions (< 1 mΩ.cm 2) suitable for the interconnection of two subcells in tandem III-V/Si solar cells. We first demonstrate the growth of highly doped epitaxial silicon films on GaAs despite the 4% lattice-match between these two materials. Spectroscopic ellipsometry measurements were used to confirm the quality of the epitaxial Si layers. The electrical properties of the grown junctions were measured based on four-point probes method and analyzed using TCAD simulations on Silvaco. We demonstrate a very low resistance for the p-Si/n-GaAs junction, down to 3.10-5 .cm 2 , with current densities above 10.000 A/cm 2 , suitable for ultra-high concentration photovoltaics, largely exceeding the requirement for our low concentration targeted conditions (below 20 suns).

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 1×1020  cm−3 to <1×1019  cm−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.