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36th European Photovoltaic Solar Energy Conference and Exhibition; 753-756, The monolithic integration on silicon of GaAs, and more generally of III-V semiconductors, is a very attractive and promising route for the production of high efficiencies multijunctions devices in the manner of those developed for space applications on germanium, but at a much lower cost suitable for terrestrial PV applications. The purpose of this paper is to present the building blocks we have developed for defect-free growth of GaAs on silicon for tandem solar cell applications. This will be addressed from the technological point of view as well as from design, modelling and characterization perspectives. Preliminary work has allowed the identification of critical technological bottlenecks, following which solution routes have been developed as will be presented and discussed. The resulting design for a GaAs/Si tandem solar cell will then be described.

International audience

International audience

Interest in the heteroepitaxy of GaAs on Si has never failed in the last years due to the potential for monolithic integration of GaAs-based devices with Si integrated circuits. But in spite of this effort, devices fabricated from them still use homo-epitaxy only. Here we present an epitaxial technique based on the epitaxial lateral overgrowth of micrometer scale GaAs crystals on a thin SiO2 layer from nanoscale Si seeds. This method permits the integration of high quality and defect-free crystalline GaAs on Si substrate and provides active GaAs/Si heterojunctions with efficient carrier transport through the thin SiO2 layer. The nucleation from small width openings avoids the emission of misfit dislocations and the formation of antiphase domains. With this method, we have experimentally demonstrated for the first time a monolithically integrated GaAs/Si diode with high current densities of 10 kA.cm(-2) for a forward bias of 3.7 V. This epitaxial technique paves the way to hybrid III-V/Si devices that are free from lattice-matching restrictions, and where silicon not only behaves as a substrate but also as an active medium.

International audience; NANOMAX TEM (part of the “EQUIPEX” TEMPOS) has been developed with the aim of implementing original experiments to study in situ the crystal growth mechanisms of nanoobjects. The growth can be observed in real time at the atomic scale using a environmental electron microscope equipped with an image aberration corrector. The microscope can be implemented either with molecular beam epitaxy (MBE) sources or with a gas injector for chemical vapor deposition (CVD). For this last case, numerous precursors have been installed to enable the growth of various semiconductor heterostructures.Here we present the growth of GaAs/Ge core/shell structures. Our goal is to perform the growth of the hexagonal crystal phase Ge-2H which may offer a possible direct band-gap optical transition. GaAs nanowires are used as template to epitaxially transfer the wurtzite structure the Ge shell. The GaAs-w and Ge-2H structures take advantage to present almost the same lattice constants. Au catalyzed GaAs nanowires are grown directly in the microscope at 400°C using TMGa and TBAs precursors. By changing the III/V flux ratio, we manage to monitor the wurtzite/zing-blende polytytpism of the GaAs nanowire. Simultaneously to the shutdown of Ga and As precursors, Ge2H6 is injected to start the lateral overgrowth. We show the very first epitaxial growth of Ge-2H. The dependence of the formation of intrinsic stacking faults on the growth conditions is discussed.

Dingding Ren-NTNU, Norway Crystal phase engineering in GaAsSb/GaAs nanowires grown by MBE Due to the catalyst-assisted 1D-growth method, the crystal phase of III-V nanowires can be tuned between zinc blende and wurtzite by controlling the growth conditions. In this presentation, I will present the crystal phase transition mechanism in Au-and Ga-catalyzed GaAsSb/GaAs nanowires grown by molecular beam epitaxy. In addition, the optical properties in heterostructured GaAsSb (zinc blende)/GaAs (wurtzite) nanowires will be discussed including the lasing behavior in a single nanowire with a GaAsSb/GaAs-based superlattice in the active region., We report on a stress induced martensitic phase transformation in Ge and Si nanowires attributed to a size effect. The transformation results in quasi-periodic heterostructures constituted of standard cubic-diamond-3C and hexagonal-2H allotropes along the nanowires. This unprecedented nano-structuration may open new properties and functionalities to be explored. Nanowires are embedded in a Hydrogen silsesquioxane resist-HSQ. The transformation is thermally activated above 400°C and is induced by an external shear-stress produced by the resist matrix surrounding the nanowires. -oriented nanowires with standard diamond structure (3C) undergo plastic deformation under external shear stress leading to a phase transformation toward the 2H diamond-hexagonal (also called lonsdaleite phase). The obtained nanostructures exhibit 2H nano-domains heterogeneously distributed along the nanowire. This polytypism was successfully achieved in Ge and Si nanowires from both bottom-up and top-down fabrication approaches. First, understanding the origin of this phase formation is critical to evaluate and optimise the possible technological applications. The transformation is dependent upon various key parameters that must be tuned: i) the diameter of the nanowires and the crystallographic direction , ii) the stress that can be viewed as the driving force for the phase transformation, and iii) the temperature at which the transformation occurs. A strain-induced martensitic transformation could account for the transformation in nanowires with a threshold temperature below which the stress induces plastic deformation. This novel allotropic 3C/2H heterostructure is stable at room temperature and may have very interesting semiconductor and optical properties opening new possibilities of applications of group-IV materials in next-generation devices. Simulations have been performed to give insight into electronic properties and combined DRX, Raman and NIR absorption spectroscopies were carried out to get fundamental physical properties of the 2H phase. Finally, C-AFM and thermal scanning microscopy measurements are under progress on both 3C and anostructured 3C/2H Ge NWs to analyse the electronic and thermal conductivities. The main goal is to give an exhaustive insight into the thermoelectric properties of heterostructures 2H/3C in Si and Ge NWs.

International audience; Our goal is to synthesize the hexagonal crystal phase Ge-2H which may offer a possible direct band-gap optical transition. We investigate two approaches. 1) Stress induced phase transformation. Ge nanowires with standard diamond structure (3C) undergo plastic deformation under external shear stress leading to a phase transformation toward the 2H-hexagonal phase and resulting in a quasi-periodic 3C/2H heterostructures along the -oriented NW axis. The transformation is dependent upon various key parameters: the diameter of the nanowires and the crystallographic direction, the stress that can be viewed as the driving force for the phase transformation, and the temperature. 2) Growth of GaAs/Ge core/shell structures. GaAs nanowires are used as template to epitaxially transfer the wurtzite structure to the Ge shell. The GaAs-w and Ge-2H structures take advantage to present almost the same lattice constants. GaAs nanowires were grown directly in a E-TEM microscope (NANOMAX) at 400°C using TMGa and TBAs precursors. By changing the III/V flux ratio, we manage to monitor in-situ the wurtzite/zing-blende polytytpism of the GaAs nanowire. Lateral overgrowth is then performed with Ge 2 H 6. We observe, in real time at the atomic scale, the epitaxial growth of Ge-2H. The dependence of the formation of intrinsic stacking faults on the growth conditions can be discussed. Finally, those heterostructured allotrope nanowires were characterized by various complementary methods such as Raman, NIR absorption, thermal conductivity measurements to gain insight into the basic fundamental properties of this promising Ge-2H structure.