Thermal relaxation of strain and twist in ferroelectric hexagonal boron nitride moir\'e interfaces

TL;DR

This study investigates how twist and strain in ferroelectric hBN/hBN moiré interfaces relax during thermal annealing, revealing temperature-dependent behaviors and the influence of structural irregularities on relaxation uniformity.

Contribution

It provides the first detailed characterization of interfacial twist and strain relaxation dynamics in hBN/hBN moiré interfaces during annealing at relevant temperatures.

Findings

Minimal relaxation at 170°C in air.

Significant twist relaxation at 400°C with decreased uniformity.

Heterostrain shows initial increase then decrease, becoming more non-uniform.

Abstract

New properties can arise at van der Waals (vdW) interfaces hosting a moir\'e pattern generated by interlayer twist and strain. However, achieving precise control of interlayer twist/strain remains an ongoing challenge in vdW heterostructure assembly, and even subtle variation in these structural parameters can create significant changes in the moir\'e period and emergent properties. Characterizing the rate of interlayer twist/strain relaxation during thermal annealing is critical to establish a thermal budget for vdW heterostructure construction and may provide a route to improve the homogeneity of the interface or to control its final state. Here, we characterize the spatial and temporal dependence of interfacial twist and strain relaxation in marginally-twisted hBN/hBN interfaces heated under conditions relevant to vdW heterostructure assembly and typical sample annealing. We find that the ferroelectric hBN/hBN moir\'e relaxes minimally during annealing in air at typical assembly temperatures of 170{\deg}C. However, at 400{\deg}C, twist angle relaxes significantly, accompanied by a decrease in spatial uniformity. Uniaxial heterostrain initially increases and then decreases over time, becoming increasingly non-uniform in direction. Structural irregularities such as step edges, contamination bubbles, or contact with the underlying substrate result in local inhomogeneity in the rate of relaxation.