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International audienceGrain orientation and competition during growth has been analyzed in directionally solidified multi-crystalline silicon samples. In situ and real-time characterization of the evolution of the grain structure during growth has been performed using synchrotron X-ray imaging techniques (radiography and topography). In addition, Electron Backscattered Diffraction has been used to reveal the crystalline orientations of the grains and the twin relationships. New grains formed during growth have two main origins: random nucleation and twinning. It is demonstrated that the solidified samples are dominated by ∑3 twin boundaries showing that twinning on {111} facets is the dominant phenomenon. Moreover, thanks to the in situ characterization of the growth, it is shown that twins nucleate on {111} facets located at the sides of the sample and at grain boundary grooves. The occurrence of multiple ∑3 twins during growth prevents the initial grains from developing all along the sample, and twin boundaries with higher order coincidence site lattices can form at the encounter of two grains in twin position. The grain competition phenomenon following nucleation and twinning acts as a grain selection mechanism leading to the final grain structure

The impact of the thermal field in a directional solidification furnace on the generation and propagation of dislocations is investigated in intrinsic floating zone single crystal silicon.Seeds with different crystallographic orientations are wire-cut from mono-crystalline wafers and dislocation sources are solely left at the edges. Thermal annealing experiments are carried out in situ at the European synchrotron radiation facility and the evolution of the silicon crystalline quality is studied by X-ray diffraction imaging technique. At 1073 K, dislocations nucleate only at the edges and their strain field remains local. At higher temperature (1373 K), dislocations propagate throughout the entire width of the seed via the preferential activation of slip planes, related to the crystallographic orientation of the seed. These results confirm the high importance of seed preparation in mono -like silicon growth process. Both mechanical 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 2 and chemical polishing of all seed surfaces, including their edges, are mandatory to prevent dislocation expansion and multiplication. Revised Manuscript

Undercooling during the solidification of silicon is an essential parameter that plays a major role in grain nucleation and growth. In this study, the undercooling of the solid-liquid interface during growth of multi-crystalline silicon samples is measured for two types of silicon: pure, and containing light elements (carbon and oxygen) to assess and compare their impact on crystal growth. The solid-liquid interface undercooling is measured using in situ and real time X-ray synchrotron imaging during solidification. As a subsequent step, ex situ Electron Backscattered Diffraction (EBSD) is performed to obtain information about the crystalline structure, the grain orientation and the grain boundary character. Two main conclusions arise: i) the undercooling of the global solid-liquid front increases linearly with the growth rate which indicates uniform attachment, i.e. all atoms are equivalent, ii) the same trend is observed for pure silicon and silicon containing carbon and oxygen. Indeed, the growth law obtained is comparable in both cases, which suggests that the solutal effect is negligible as concern the undercooling in the case of a contamination with carbon (C) and oxygen (O). However, there is a clear effect of the impurity presence on the crystalline structure and grain boundary type distribution. Many grains nucleate during growth in samples containing C and O, which suggests the presence of precipitates on which grain nucleation is favored.

The ribbon on sacrificial template (RST) process is a ribbon direct-wafering technology with specific ability for high throughput and thin multicrystalline wafer production, in the range of 60-140 μm. Mechanical and electrical properties of the RST material were investigated. Ball on ring and four-point bending tests showed good fracture stress values up to 260 MPa. The conversion efficiency potential for passivated emitter and rear cells (PERC) made out from the RST material, around 16%, is shown to be limited by defects reducing minority carrier lifetime. The interaction between impurities, such as C and transitions metals, with structural defects such as dislocations, results in highly recombinative areas in RST wafers. A model is proposed which shows that the carbon substrate is an important source of carbon contamination in the silicon melt during the growth of the ribbon. This high C contamination can be accompanied by transition metal contamination and can have an influence on the growth stability and on the generation of structural defects, especially if C accumulates in a boundary layer just above the growth interface. The study of the segregation of Sb indicates that the process conditions are close to the case of the diffusive regime near the solid/liquid interface, with a boundary layer thickness of about 70 μm.

To control the final grain structure and the density of structural crystalline defects in silicon (Si) ingots is still a main issue for Si used in photovoltaic solar cells. It concerns both innovative and conventional fabrication processes. Due to the dynamic essence of the phenomena and to the coupling of mechanisms at different scales, the post-mortem study of the solidified ingots gives limited results. In the past years, we developed an original system named GaTSBI for Growth at high Temperature observed by Synchrotron Beam Imaging, to investigate in situ the mechanisms involved during solidification. X-ray radiography and X-ray Bragg diffraction imaging (topography) are combined and implemented together with the running of a high temperature (up to 2073 K) solidification furnace. The experiments are conducted at the European Synchrotron Radiation Facility (ESRF). Both imaging techniques provide in situ and real time information during growth on the morphology and kinetics of the solid/liquid (S/L) interface, as well as on the deformation of the crystal structure and on the dynamics of structural defects including dislocations. Essential features of twinning, grain nucleation, competition, strain building, and dislocations during Si solidification are characterized and allow a deeper understanding of the fundamental mechanisms of its growth.

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