Origin and Evolution of I3 defects in Hexagonal Silicon and Germanium

Recently, hexagonal diamond (2H) SiGe has been synthesized by exploiting core/shell nanowires [1] and demonstrating the direct electronic bandgap for the Ge-rich 2H-SiGe shells [2]. However, because of the metastability of the 2H crystal phase, the inclusion of the most stable diamond cubic phase via defects appearing during the growth is easily understandable. Of particular importance is the I3 defect, very often present in the 2H-SiGe shell and has been analyzed in recent works [3]. Here, we propose a general atomistic mechanism, supported by ab initio Density Functional Theory (DFT) calculations and Machine Learning (ML)-based Molecular Dynamics (MD) simulations, describing the formation of the I3 defect in different configurations, including the triangular and non-triangular shapes for the extended defects. In particular, we link its formation to the surface reconstruction during the growth, thus identifying its formation mechanism with a line-like origin, explaining all experimental evidence [4]. Furthermore, we propose a model for the atomistic evolution of such I3 defects based on Nudged Elastic Band Calculations. We evaluated the kinetic barriers for the kink formation and migration in silicon by exploiting an established ML interatomic potential [5], enabling an estimate of the total transition rate for the defect propagation. Preliminary results have also been obtained for germanium [6], highlighting the higher mobility of such defects in germanium shells with respect to the silicon case. These findings may suggest a procedure to minimize the extension of I3 defects by playing with alloying composition and growth conditions. [1] H. Hauge et al., Nano Lett. 15 5855-5860 (2015); H. Hauge et al., Nano Letters 17, 85-90 (2017) [2] E. Fadaly et al., Nature 580, 205-209 (2020) [3] E. Fadaly et al., Nano Lett. 21 3619-3625 (2021); L. Vincent et al., Adv. Mater. Int. 9, 2102340 (2022) [4] F. Rovaris et al. ACS Appl. Nano Mater. 7 9396−9402 (2024) [5] J. Dellevoet et al., in preparation (2025) [6] A. Fantasia et al., J. Chem. Phys. 161, 014110 (2024)