Complex Nanotwin Substructure of an AsymmetricΣ9Tilt Grain Boundary in a Silicon Polycrystal

Abstract

Grain boundaries in materials have substantial influences on device properties, for instance on mechanical stability or electronic minority carrier lifetime in multicrystalline silicon solar cells. This applies especially to asymmetric, less ordered or faceted interface portions. Here, we present the complex atomic interface structure of an asymmetric Σ9 tilt grain boundary in silicon, observed by high resolution scanning transmission electron microscopy (HR-STEM) and explained by atomistic modeling and computer simulation. Structural optimization of interface models for the asymmetric Σ9 and related symmetrical Σ9 and Σ3 tilt grain boundaries, by means of molecular-statics simulations with empirical silicon potentials in combination with first-principles calculations, results in a faceted asymmetric interface structure, whose grain-boundary energy is so low that it is likely to exist. The simulated local atomic structures match the observed HR-STEM images very well.

Strain building and correlation with grain nucleation during silicon growth

“…In this case, the Σ9 twin boundary adopts the asymmetric {111}/{115} configuration (common poles are highlighted with the green circles in Figure 8.b) which corresponds to a higher grain boundary energy [43]. This structure has been revealed by TEM (Transmission Electron Microscopy) and its energy has been calculated by DFT (Density Functional Theory) [44]. Figure 5.c) is created just before the nucleation of the grain VI (purple grain in Figure 5.a).…”

“…Figure 5.c) is created just before the nucleation of the grain VI (purple grain in Figure 5.a). In general, non-symmetrical grain boundaries are deformed at the atomic scale [44] and offer greater resistance for dislocation crossing, thereby creating higher strain [37] and structure deformation, promoting dislocation emission. Concomitantly, an increasing strain is observed.…”

Strain-Induced Asymmetric Line Segregation at Faceted Si Grain Boundaries

“…Typically, these atomic scale segregation patterns are established at planar interfaces but in real materials grain boundaries often possess complex 3D topologies, consisting of a sequence of small facets with alternating local plane normal as drawn in Fig. 1(a), each with a specific atomic arrangement [14][15][16]. Faceting transitions at grain boundaries are commonly observed in a multitude of material systems impacting their properties such as grain boundary mobility, which is a crucial parameter for material processing [17][18][19].…”

Atomic scale displacement field induced by a near-Σ9 twin boundary in silicon

“…For other complex twin structures, the construction of the ASUs from undeformed crystals as detailed in Fig. (a and b) might not be possible because of the complexity of the atomic arrangements, see e.g., the asymmetric Σ9(111)||(115) boundary , the length T of which reaches 1.995 nm.…”