Remote epitaxial interaction through graphene

Abstract

The concept of remote epitaxy involves a two-dimensional van der Waals layer covering the substrate surface, which still enable adatoms to follow the atomic motif of the underlying substrate. The mode of growth must be carefully defined as defects, e.g., pinholes, in two-dimensional materials can allow direct epitaxy from the substrate, which, in combination with lateral epitaxial overgrowth, could also form an epilayer. Here, we show several unique cases that can only be observed for remote epitaxy, distinguishable from other two-dimensional material-based epitaxy mechanisms. We first grow BaTiO 3 on patterned graphene to establish a condition for minimizing epitaxial lateral overgrowth. By observing entire nanometer-scale nuclei grown aligned to the substrate on pinhole-free graphene confirmed by high-resolution scanning transmission electron microscopy, we visually confirm that remote epitaxy is operative at the atomic scale. Macroscopically, we also show variations in the density of GaN microcrystal arrays that depend on the ionicity of substrates and the number of graphene layers.

Remote Epitaxy: Fundamentals, Challenges, and Opportunities

“…c, Confirming the remote epitaxial growth of BTO on graphene/STO by making a TEM specimen containing the entire nuclei (left) and then confirming a full coverage of graphene by STEM (right) . d, The effect of growth temperatures and substrate ionicity on the nucleation density in remote epitaxy of GaN . (c,d) Reprinted with permission of AAAS from ref .…”

“…For directly probing remote epitaxy mode in TEM, therefore, it is necessary to stop the growth at a nucleation stage with the nuclei size smaller than a few tens of nanometers, so that TEM specimen can contain the entire nucleus. 47 This completely eliminates the possibility of pinholes residing outside of the lamella made by focused ion beam (FIB) milling as a direct and irrefutable proof of remote epitaxy (Figure 5c). In other words, with the lamella containing the entire nucleus can the growth mode be unambiguously verified by adjusting the focus during TEM imaging and confirming the existence (pinhole-based epitaxy) or nonexistence (remote epitaxy) of pinholes.…”

“…In remote epitaxy, on the other hand, the diffusion length of adatoms determined by underlying substrates and growth conditions governs the nucleation density. The nucleation density increases in remote epitaxy by (a) changing the substrate material to have larger ionicity 19,47 and (b) decreasing the growth temperature to reduce the diffusion length, as summarized in Figure 5d. 9,47 In the heteroepitaxy of lattice-mismatched materials, strain properties are also important indicators in studying the growth mode.…”

“…The nucleation density increases in remote epitaxy by (a) changing the substrate material to have larger ionicity 19,47 and (b) decreasing the growth temperature to reduce the diffusion length, as summarized in Figure 5d. 9,47 In the heteroepitaxy of lattice-mismatched materials, strain properties are also important indicators in studying the growth mode. Spontaneous relaxation occurs in remote heteroepitaxy, resulting in quick relaxation of entire epilayers at an early stage of growth.…”

“…Second, screened electrostatic interaction from the substrate promotes Volmer–Weber growth mode, making it often difficult to form fully merged and planarized thin films . Therefore, the growth conditions need to be modified from standard conditions for enhanced nucleation density. ,, Intentional formation of dangling bonds on 2D materials by plasma treatment can also be employed in remote epitaxy to enhance the nucleation density, , although the conditions used for plasma treatment have to be carefully controlled to prevent the occurrence of other growth modes, such as quasi-vdW (qvdW) epitaxy and pinhole-based selective epitaxy.…”