The aim of the project is to provide low-cost and high efficiency tandem cells grown on crystalline silicon (c-Si) substrates, with merging both the monocristalline Si approach with the high-efficiency monocristalline multijunction approach based on III-V materials. These CPV cells will be used under natural lighting and under low light concentrators (100 suns) developed by IRDEP-CNRS, and benchmarked under medium concentration by Heliotrop sas. The PV cells efficiency is one of the most important parameters for the final cost of electricity, since it impacts directly the ratio between produced energy and production cost. With 22% efficiency modules based on c-Si, the technology seems to reach its limits. To increase further the efficiency of c-Si cells and modules, going to multijunction devices (association of two different absorbing layers in the same cell) seems to be the obvious choice. While many projects tend to focus on all silicon technology, best high bandgap cells are yet based on III-V compounds. This project proposes to demonstrate the proof-of-concept for a monolithic integration of high efficiency multijunction CPV device on a low cost monocristalline silicon substrate upon which a III-V lattice-matched material will be grown using molecular beam Epitaxy (MBE). This Lattice-Matched heterostructure with its very low structural defect densities (Dislocations, AntiPhase Domains, point defects) will be capable of sustaining III-V high performing PV devices onto silicon with long life-time. This novel route overcomes the problems of high cost substrates (as compared to Ge or III-V substrates used currently for this kind of CPV), the killer structural defect formation and reliability issues of lattice mismatched systems (metamorphic approach) and the low reliability and low lifetime of hybrid techniques (such as wafer bonding). The integration of photovoltaic functions onto a single silicon substrate will also achieve a reduction in the use of III-V based semiconducting materials in high-efficiency multijunction CPVs. The two main scientific and technologic objectives of the project are : 1) The achievement of GaAsPN (1.7 eV) single cell on Si (with a 15% efficiency under low concentration, i.e. 100 sun). 2) The demonstration of a high efficiency and low cost multi-junction solar cell: GaAsPN pn cell at 1.7 eV on Si pn cell at 1.1 eV (25% efficiency under low concentration, i.e. 100 sun, as a first step towards very high-efficiencies >30%) Lattice-matched layers and slightly tilted substrates are used to overcome the two main difficulties faced by the growth of III-V materials on silicon substrates: misfit dislocations and antiphase lattice defects, in order to obtain defect-free III-V materials and to get large minority carrier diffusion lengths for the PV applications. The PV devices will consist in high efficiency tandem cells III-V/Si double-pn-junctions separated with a Buried Tunnel Junction. The final structure will include a first bottom Si pn (1.1eV low gap) grown on the Si substrate, then a thin GaP layers is grown by MBE to prevent structural defects formation, a top cell GaAsPN pn (1.7eV large gap) junction is then grown on top of it. The project relies on a high quality consortium which brings together six french partners, and an associated European partner, with high, established competence and complementary methodology and expertise in their fields and leading appropriate workpackages: FOTON (growth of III-V materials), INL (Si-based PV technology), CEMES-CNRS (structural characterizations), IRDEP-CNRS (research in PV development), EDF R&D (a European leader in the Energy sector), HELIOTROP (French manufacturer of high concentration photovoltaic modules (HCPV)) and AALTO (a Finnish associated academic partner specialised in point defects analysis). The partners are active in European research consortia and in networks of excellence and they drive many projects on the national and international level.

The INSCOOP project proposes an original strategy to integrate optical links on Si wafer for on-chip interconnects. The link consists of a Si optical waveguide fabricated from a SOI (Si on insulator) substrate. A compact optical source operating at 1.2 µm wavelength or above will be fabricated directly on the Si waveguide. This source is based on III-V materials deposited selectively on the Si surface in the form of vertical photonic wires standing on top of the waveguide. A regular array of such wires will form a resonant active photonic crystal. The project aims at demonstrating that under optical pumping, hybrid Bloch modes can be amplified in the III-V wire array and propagate in the passive Si waveguide. This demonstration will pave the way to fully integrated photonic links on Si chips. For the sake of compatibility with CMOS technology, we propose to fabricate this device with a monolithic approach which can be implemented with high yield at large scale and high density, conversely to wafer bonding techniques. The integration of III-V material is based on epitaxial growth of nanowires (NWs) by the catalyst-assisted vapor-liquid-solid (VLS) method. This approach is very efficient to obtain defect-free materials of high optical quality, despite the highly mismatched interface between III-V and Si. Regular arrays will be obtained by patterning a SiO2 mask with nano-sized openings containing the catalyst particles. Core-shell heterostructures will be formed to limit carrier surface recombination. We will focus on the InAsP/InP system which covers the targeted wavelengths for which Si is transparent. Core and shell will be formed with adequate dimensions in successive growth steps. The full compatibility with CMOS raises critical issues which are addressed in the project. Growth must be performed on (001) oriented Si surface and without Au as catalyst which is an impurity to be avoided in CMOS process lines. We rely on the experience of one partner as well as on the recent literature to investigate alternative catalysts. On (001) surfaces, VLS process results in inclined NWs growing along the directions of the substrate. To get vertically standing NWs, a very thin intermediate layer of SrTiO3 is proposed on the basis of results of one partner. These growth and patterning issues are addressed in task 2, 3 and 4 of the project. The former activities will be supported by task 5 dealing with material structural and optical characterization and modeling. Modeling tools will be developed to optimize the VLS growth conditions. Of particular importance for the project, the conditions which stabilize the NW crystalline phase will be predicted for the self-catalytic case. Tight-binding and k.p models will be implemented to calculate the electronic properties of NWs with core-shell heterostructures of wurzite phase. These calculations, not yet available in the literature, will evaluate the excitonic and piezo-electric effects of major importance for the efficiency of the optical source. Task 6 will be dedicated to the optimization and evaluation of the optical micro-source (resonant LED or lasers). Electromagnetic simulations will guide the design of the periodic array of III-V photonic NWs on Si. High quality factor resonances and efficient coupling between these micro-resonators and the SOI waveguide will be targeted. After the fabrication of the NW arrays, their planarization with a low-index insulating material will be tested. This technological step will anticipate a longer term objective (out of the scope of this project), i.e. electrically injected devices. The emission characteristics of the micro-source will be measured under optical pumping, with and without device planarization. This ultra-compact source of original architecture is expected to have enhanced global efficiency and reduced power consumption, which is of prime importance in the field of silicon photonics.