Subgrains, micro-twins and dislocations characterization in monolike Si using TEM and in-situ TEM

Abstract

A critical aspect in the development of mono-like Silicon for photovoltaic (PV) application lies in the control of its defect microstructure. While micro-twins do not seem to represent a major concern regarding electrical activity, some dislocations and low angle grain boundaries need to be avoided in the growing process because of their recombination properties. Here, we have studied how these defects grow and interact together in order to understand what are the mechanisms responsible for their presence in the cast ingots. We show that, in our specific convex liquid-solid interface growth mode, micro-twins probably originate from this interface while dislocations and subgrains build up both by epitaxy at the solid liquid interface and through dislocation-boundary interactions in the solidified region. Post-growth TEM characterization allowed identifying constitutive dislocations of some subgrain boundaries for the case of [001] seeds. In-situ TEM straining of Si at high temperature also allowed measuring dislocation mobility at several temperatures. Those are compared to the dislocation velocities in pure Czochralski Si found in literature.

Investigation of subgrains in directionally solidified cast mono-seeded silicon and their interactions with twin boundaries

“…Since the rotation axis is principally along the growth direction, many dislocations have an edge character. This is consistent with the analysis of Lantreibecq et al [18] who found that the SGBs in the source cast mono ingot are mainly composed of dislocations with a [001] line character and a Burgers vector a/2 [11 � 0]. They proposed that these dislocations grow by epitaxy on the solid-liquid interface.…”

“…Using b = 0.384 nm for the Burgers' vector magnitude in silicon along a <110> direction, the distance between each dislocation is d1 = 7.3 nm and d2 = 22 nm for θ of 3° and 1°, respectively. These misorientation angles are in the same order of magnitude as what was observed by Lantreibecq et al in the source cast mono ingot at a height of about 160 mm from the melt-back surface [18], which is above the area where electrically active subgrains do appear on the PL maps (cf. Fig.…”

“…However, the sample is not oversaturated in carbon so that only a small number of precipitates should be present. It was reported that precipitates in the source cast mono seed do not immobilize dislocations, but that they can create dislocation cross-slip, if they are exposed to sufficient stress [18]. A strong influence of C-and N-based impurities on subgrain formation is therefore not suspected.…”

“…Besides, dislocations in cast mono-Si are mainly generated at the junctions of the seed pavement [16,17]. Additionally, subgrains originate from dislocation alignments already present in the seed crystals that can propagate into the new grown crystal during growth [18][19][20]. For the latter mechanism, the initial dislocation arrangement in cells in the seed is a main factor [19,21] and depends on the stress in the seed [22][23][24].…”

X-ray Based in Situ Investigation of Silicon Growth Mechanism Dynamics—Application to Grain and Defect Formation

“…Indeed, if the displacement vector is perpendicular to the diffraction vector, the defect is not visible on the topograph. This is the case of dislocations whose image is extinct for the diffraction spots corresponding to diffraction vectors perpendicular to the Burgers vectors as also seen in TEM investigations [68,69]. On the one hand, this means that the absence of a dislocation strain field on a single diffraction spot does not mean that no dislocations are present.…”