Controlling the balance between remote, pinhole, and van der Waals epitaxy of Heusler films on graphene/sapphire

TL;DR

This study investigates the growth mechanisms of Heusler films on graphene/sapphire, demonstrating conditions that favor remote epitaxy over other mechanisms and identifying a unique 30-degree rotated superstructure as a remote epitaxy fingerprint.

Contribution

It provides experimental evidence for remote epitaxy in Heusler films on graphene and clarifies growth conditions that influence the epitaxial mechanism, including temperature and lattice mismatch considerations.

Findings

30-degree rotated superstructure indicates remote epitaxy

Lower growth temperatures increase remote epitaxy fraction

Proper annealing and lattice mismatch control are crucial for epitaxy

Abstract

Remote epitaxy on monolayer graphene is promising for synthesis of highly lattice mismatched materials, exfoliation of free-standing membranes, and re-use of expensive substrates. However, clear experimental evidence of a remote mechanism remains elusive. In many cases, due to contaminants at the transferred graphene/substrate interface, alternative mechanisms such as pinhole-seeded lateral epitaxy or van der Waals epitaxy can explain the resulting exfoliatable single-crystalline films. Here, we find that growth of the Heusler compound GdPtSb on clean graphene on sapphire substrates produces a 30 degree rotated epitaxial superstructure that cannot be explained by pinhole or van der Waals epitaxy. With decreasing growth temperature the volume fraction of this 30 degree domain increases compared to the direct epitaxial 0 degree domain, which we attribute to slower surface diffusion at low temperature that favors remote epitaxy, compared to faster surface diffusion at high temperature that favors pinhole epitaxy. We further show that careful graphene/substrate annealing ($T\sim 700 ^\circ C$) and consideration of the film/substrate vs film/graphene lattice mismatch are required to obtain epitaxy to the underlying substrate for a variety of other Heusler films, including LaPtSb and GdAuGe. The 30 degree rotated superstructure provides a possible experimental fingerprint of remote epitaxy since it is inconsistent with the leading alternative mechanisms.