Strain building and correlation with grain nucleation during silicon growth

This work is dedicated to the grain structure formation in silicon ingots with a particular focus on the crystal structure strain building and its implication in new grain nucleation process. The implied mechanisms are investigated by advanced in situ X-ray imaging techniques during silicon directional solidification. It is shown that the grain structure formation is mainly driven by S3 twin nucleation. Grain competition phenomena occurring during the growth process lead to the creation of higher order twin boundaries, localised strained areas and associated crystal structure deformation. On the one hand, it is demonstrated that local strain building can be directly related to the characteristics of the twin boundaries created during silicon growth due to grain competition. On the other hand, space restriction due to competition during growth can be at the origin of local strain building as well. Finally, the accumulation of all these factors generating strain is responsible for spontaneous new grain nucleation. When occurring, both grain nucleation and subsequent grain structure reorganisation contribute to lower the strain in the growing ingot. It is demonstrated as well that the local distribution of the strained areas created during silicon growth is retrieved after cooling down, from melting temperature to room temperature, on top of an additional larger scale deformation of the sample due to the cooling down only

In situ investigation of the structural defect generation and evolution during the directional solidification of 〈110〉 seeded growth Si

This work is dedicated to the advanced in situ X-ray imaging and complementary ex situ investigations of the growth mechanisms when silicon solidifies on a monocrystalline seed oriented ⟨110⟩ in the solidification direction. It aims at deepening the fundamental understanding of the phenomena that occur throughout silicon crystal growth with a particular focus on mechanisms of formation of defects detrimental for photovoltaic applications. Namely, grain nucleation, grain boundary formation and evolution, grain competition, twining occurrence, dislocation generation and interaction with structural defects are explored and analysed. Nucleation of twin crystals preferentially occurs on {111} facets at the edge of the sample where solid e liquid e vapor triple point lines exist in interaction also with the crucible as well as, at grain boundary grooves at the solid e liquid interface (solid e solid e liquid triple lines), where two grains are in competition, either on the {111} facets of the groove or in the groove. Enhanced undercooling and/or stress accumulation levels are found to act as driving forces for grain nucleation. Additionally, it is demonstrated that twin formation has the property to relax stresses stored in the crystal during the growth process. However, grains formed initially in twin position can undergo severe distortion when they are in direct competition or when they are squeezed in e between grains. Moreover, we show by X-ray Bragg diffraction imaging that on the one hand, coherent S3 ⟨111⟩ grain boundaries efficiently block the propagation of growth dislocations during the solidification process, while on the other hand, dislocations are emitted at the level of incoherent and/or asymmetric S27a ⟨110⟩ at the encounter with either S3 ⟨111⟩ or S9 ⟨110⟩ grain boundaries. Indeed, grain boundaries that deviate from the ideal coincidence orientation act as dislocation sources that spread inside the surrounding crystals

On the Deformation of Dendrites During Directional Solidification of a Nickel-Based Superalloy

Abstract: Synchrotron X-ray imaging has been used to examine in situ the deformation of dendrites that takes place during the solidification of a nickel-based superalloy. By combining absorption and diffraction contrast imaging, deformation events could be classified by their localization and permanence. In particular, a deformation mechanism arising from thermal contraction in a temperature gradient was elucidated through digital image correlation. It was concluded that this mechanism may explain the small misorientations typically observed in single crystal castings

In situ Synchrotron X-ray Imaging of Extended Defects Generation and Evolution during Monocrystalline Si Seeded Growth

International audienc

In situ synchrotron based investigation of grain kinetics and defects during silicon crystal growth

International audienc

Investigation by in situ X-ray imaging of grain competition and defect formation during the growth of silicon for photovoltaic applications

International audienc

In situ synchrotron based investigation of grain kinetics and defects during silicon crystal growth

International audienc

Random angle grain boundary formation and evolution dynamics during Si directional solidification

International audienceThe growth of multicrystalline silicon and the formation of a random angle grain boundary, as well as the dislocation generation and expansion is observed dynamically in situ, by Synchrotron X-ray imaging techniques. The focus is kept on a random angle grain boundary since its behavior is particularly important to better understand the HP mc-Si (High Performance Multi-crystalline Silicon) photovoltaic properties. Due to the process conditions and to the grain competition that occurs during the solidification, a facetted {111} /facetted {111} groove is formed by this random angle grain boundary at the solid/liquid interface. It is shown how the shape of the solid/liquid interface allows the change of the preferential {111} growth facet and affects the grain boundary propagation direction. In one of the groove configurations, the two adjacent {111} facets do not have the same growth velocity and as a consequence the corresponding grain boundary does not follow the bisector of the angle between the two facets. Indeed, the direction of the grain boundary is determined by the growth velocities of the facets which control the grain competition. Moreover, under these experimental conditions a clear relationship is observed between the existence of random angle grain boundaries and the local generation of dislocations as well as their expansion. By comparison, dislocation emission is not observed at the level of Σ3 {111} grain boundaries