Formation of Nanotwin Networks during High-Temperature Crystallization of Amorphous Germanium

TL;DR

This study uses atomistic simulations to reveal how nanotwin networks form during high-temperature crystallization of amorphous germanium, highlighting a unique growth mechanism involving twin boundaries and providing experimental validation methods.

Contribution

It uncovers a novel crystallization mechanism in amorphous germanium leading to nanotwin network formation, supported by thermodynamic and kinetic analysis and X-ray diffraction predictions.

Findings

Crystallization involves atom transfer from amorphous to crystalline layers.

Twin networks develop during growth along the <111> orientation.

X-ray diffraction patterns of nanotwin networks are predicted for experimental validation.

Abstract

Germanium is an extremely important material used for numerous functional applications in many fields of nanotechnology. In this paper, we study the crystallization of amorphous Ge using atomistic simulations of critical nano-metric nuclei at high temperatures. We find that crystallization occurs by the recurrent transfer of atoms via a diffusive process from the amorphous phase into suitably-oriented crystalline layers. We accompany our simulations with a comprehensive thermodynamic and kinetic analysis of the growth process, which explains the energy balance and the interfacial growth velocities governing grain growth. For the $\langle111\rangle$ crystallographic orientation, we find a degenerate atomic rearrangement process, with two zero-energy modes corresponding to a perfect crystalline structure and the formation of a $\Sigma3$ twin boundary. Continued growth in this direction results in the development a twin network, in contrast with all other growth orientations, where the crystal grows defect-free. This particular mechanism of crystallization from amorphous phases is also observed during solid-phase epitaxial growth of $\langle111\rangle$ semiconductor crystals, where growth is restrained to one dimension. We calculate the equivalent X-ray diffraction pattern of the obtained nanotwin networks, providing grounds for experimental validation.