We present an innovative approach for the growth of crystalline silicon on GaAs using plasma-enhanced chemical vapor deposition (PECVD). In this process the substrate is kept at low temperature (175 °C) and epitaxial growth is obtained via the impact of charged silicon clusters which are accelerated towards the substrate by the plasma-potential and melt upon impact. Therefore, this is a nanometer size epitaxial process where the local temperature (nm scale) rises above the melting temperature of silicon for extremely short times (in the range from ps to ns). This allows obtaining epitaxial growth even on relatively rough GaAs films, which have been cleaned in-situ using a SiF4 plasma etching. We present in-plane X-Ray Diffraction (XRD) measurements which are consistent with the hypothesis that the epitaxial growth happens at a local high temperature. Indeed, the tetragonal structure observed and the low in-plane lattice parameter determined from XRD can only be explained by the thermal mismatch induced by a high growth temperature. The effect of the plasma on the underlying GaAs properties, in particular the formation of hydrogen complexes with GaAs dopants (C, Si, Te) is studied in view of the integration of the c-Si epi-layers into devices.
Silicon based multi-junction solar cells are a promising option to overcome the theoretical efficiency limit of a silicon solar cell (29.4%). With III-V semiconductors, high bandgap materials applicable for top cells are available. For the application of such silicon based multi-junction devices, a full integration of all solar cell layers in one 2-terminal device is of great advantage. We realized a triple-junction device by wafer-bonding two III-V-based top cells onto the silicon bottom cell. However, in such a series connected solar cell system, the currents of all sub-cells need to be matched in order to achieve highest efficiencies. To fulfil the current matching condition and maximise the power output, photonic structures were investigated. The reference system without photonic structures, a triple-junction cell with identical GaInP/GaAs top cells, suffered from a current limitation by the weakly absorbing indirect semiconductor silicon bottom cell. Therefore rear side diffraction gratings manufactured by nanoimprint lithography were implemented to trap the infrared light and boost the solar cell current by more than 1 mA/cm2. Since planar passivated surfaces with an additional photonic structure (i.e. electrically planar but optically structured) were used, the optical gain could be realized without deterioration of the electrical cell properties, leading to a strong efficiency increase of 1.9% absolute. With this technology, an efficiency of 33.3% could be achieved.
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.
Low temperature plasma processes provide a toolbox for etching, texturing and deposition of a wide range of materials. Here we present a bottom up approach to grow epitaxial crystalline silicon films (epi-Si) by standard RFPECVD at temperatures below 200°C. Booth structural and electronic properties of the epitaxial layers are investigated. Proof of high crystalline quality is deduced from spectroscopic ellipsometry and HRTEM measurements. Moreover, we build heterojunction solar cells with intrinsic epitaxial absorber thickness in the range of a few microns, grown at 175°C on highly doped (100) substrates, in the wafer equivalent approach. Achievement of a fill factor as high as 80 % is a proof that excellent quality of epitaxial layers can be produced at such low temperatures. While 8.5 % conversion efficiency has already been achieved for a 3.4 μm epitaxial silicon absorber, the possibility of reaching 15 % conversion efficiency with few microns epi-Si is discussed based on a detailed opto-electrical modeling of current devices.