Thru‐Hole Epitaxy: A Highway for Controllable and Transferable Epitaxial Growth

Abstract

Controllable growth and facile transferability of a crystalline film with desired characteristics, acquired by tuning composition and crystallographic orientation, become highly demanded for advanced flexible devices. Here the desired crystallographic orientations and facile transferability of a crystalline film can be achieved by “thru‐hole epitaxy” in a straightforward and undemanding manner with no limitation on the layer number and polarity of a 2D space layer and the surface characteristics. The crystallographic alignment can be established by the connectedness of the grown material to the substrate through a small net cross‐sectional area of thru‐holes, which also allows the straightforward detachment of the grown material. Thru‐hole epitaxy can be adopted for the realization of advanced flexible devices on large scale with desired crystallographic orientation and facile transferability.

Graphene assisted III-V layer epitaxy for transferable solar cells

“…At the onset of growth of GaAs on graphene/GaAs, there is an interplay between different nucleation mechanisms including remote epitaxy, pinhole-seeded epitaxy, nucleation at impurities, graphene defects or carbides, and possibly Van der Waals epitaxy. Remote and pinhole seeded epitaxy provide an epitaxial alignment with the substrate and it is provenly complicated to differentiate both experimentally 2,11 . On the other hand, XPS reproducibly showed no Ni or other transfer-related contaminants, nor carbides in the C 1s peak after annealing in UHV or H2.…”

Establishing Epitaxial Connectedness in Multi‐Stacking: The Survival of Thru‐Holes in Thru‐Hole Epitaxy

“…[ 12 ] We indeed showed that this can be possible without those stringent conditions required for remote epitaxy by demonstrating not only the growth of crystallographically aligned GaN domains over a 2D material, such as h ‐BN, but also the facile detachment of those grown and merged GaN domains. [ 13 ] This epitaxial approach was named thru‐hole epitaxy. Note that the term “thru‐hole” is used to refer to a hole connected all the way from the top‐most 2D material to the substrate resulting in the establishment of the epitaxial connectedness between the material grown and the substrate.…”

“…Unlike remote epitaxy, which stringently requires a defect‐free graphene monolayer, we observed that thru‐hole epitaxy can be successfully carried out with tens of nanometers thick (multiply‐stacked) h ‐BN layers or even thicker as long as thru‐holes are maintained. [ 13 ] Here, a “stack” represents a transfer unit consisting of multiple layers of h ‐BNs grown together at the same time. Crystallographically‐aligned GaN domains were successfully grown over multiply‐stacked h ‐BN transferred onto the sapphire substrate and easily detached with a thermal release tape.…”

“…In other words, if such holes are completely sealed in multiple stacking, thru‐hole epitaxy will not occur. However, the successful observation of the thru‐hole epitaxy of GaN on eight‐time stacked h ‐BN [ 13 ] remained puzzling enough that we had to look for convincing evidence that holes survive as thru‐holes even with multiple stacks. If the holes in each stack are isolated from each other, we can expect the number of thru‐holes to decrease exponentially with the number of stacks.…”

“…The h ‐BN samples used in our previous study were fabricated without a growth optimization process, so it is likely that structural defects such as cracks were present. [ 13 ] These cracks in particular can act as laterally connected holes rather than isolated ones. Furthermore, thru‐holes do not need to be vertically straight but can be meandering and crooked like a maze of connecting paths, as shown in Figure 1b, as long as they simply run all the way from the top to the bottom stack, and thus to the substrate, to establish epitaxial connectedness.…”