Establishing Epitaxial Connectedness in Multi-Stacking The Survival of Thru-Holes in Thru-Hole Epitaxy Youngjun Lee1Seungjun Lee1Jaewu Choi2

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Establishing Epitaxial Connectedness in Multi-Stacking: The

Survival of Thru-Holes in Thru-Hole Epitaxy

Youngjun Lee,1Seungjun Lee,1, ∗Jaewu Choi,2

Chinkyo Kim,1, 2, †and Young-Kyun Kwon1, 2, ‡

1Department of Physics, and Research Institute for Basic Sciences,

Kyung Hee University, Seoul 02447, Korea

2Department of Information Display,

Kyung Hee University, Seoul 02447, Korea

(Dated: August 23, 2023)

Abstract

Thru-hole epitaxy has recently been reported to be able to grow readily detachable domains

crystallographically aligned with the underlying substrate over 2D mask material transferred onto

a substrate. [Jang et al.,Adv. Mater. Interfaces,2023 10, 4 2201406] While the experimental

demonstration of thru-hole epitaxy of GaN over multiple stacks of h-BN was evident, the detailed

mechanism of how small holes in each stack of h-BN survived as thru-holes during multiple stacking

of h-BN was not intuitively clear. Here, we use Monte Carlo simulations to investigate the condi-

tions under which holes in each stack of 2D mask layers can survive as thru-holes during multiple

stacking. If holes are highly anisotropic in shape by connecting smaller holes in a particular direc-

tion, thru-holes can be maintained with a high survival rate per stack, establishing more epitaxial

connectedness. Our work veriﬁes and supports that thru-hole epitaxy is attributed to the epitaxial

connectedness established by thru-holes surviving even through multiple stacks.

arXiv:2210.04753v2 [cond-mat.mtrl-sci] 22 Aug 2023

I. INTRODUCTION

Remote epitaxy has attracted much attention because of its fascinating epitaxial growth

of readily detachable crystalline domains, which are crystallographically aligned with the

underlying substrate over 2D material without any form of direct bonding between the

material grown and the substrate.1–3 It was argued that remote epitaxial growth was possible

because crystallographic information of the substrate is well transferred through ultrathin 2D

layers especially if the underlying substrate has a strong ionic or polar character. Crystalline

ﬁlms without direct chemical bonding with the substrate are consequently shown to be

readily detached. Although remote epitaxy opened a new possibility for the growth of 3D

materials with barely any constraints to the lattice match with the substrate, it is still

not straightforward to carry out remote epitaxy because it requires stringent conditions: (i)

defect-free 2D layers, (ii) precise controllability for the number of 2D layers, and (iii) polar

characters of materials to be grown, to name a few.1–3 While current 2D material growth

and transfer techniques have greatly improved, it is still diﬃcult or impossible to transfer

completely defect-free graphene onto a substrate.

It is evident that not only do 2D materials prepared by current techniques contain struc-

tural defects such as holes and small cracks but they can also be degraded. Thus, during its

transfer process, such structural defects would be extended in a speciﬁc direction.4,5 Because

the substrate is well exposed as a consequence, such a degraded 2D material will act as a

mask for epitaxial lateral growth rather than as a transparent overlayer of substrate poten-

tial for remote epitaxy.6–8. Furthermore, it can be inferred that even the ﬁlm grown through

holes in 2D material can also be readily detached since the 2D material mostly acts as a

mask over a crystalline substrate. 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.9This 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. The same

thru-hole epitaxy was also successfully applied to grow GaN and ZnO over graphene and

MoS2, respectively.10 There was another report showing epitaxial growth of GaSb ﬁlms on

Substrate

(a)

(b)

Substrate Substrate

Substrate

1st layer

2nd layer

2nd

1st

Nuclei

Graphene

2 layer overlap

Thru-hole Connecting

path

FIG. 1. Schematic illustration of (a) remote epitaxy and (b) thru-hole epitaxy occurring on a 2D

material transferred onto a substrate. In the former process, nucleation occurs on a defect-free 2D

monolayer, while in the latter process, nucleation occurs directly on the substrate after diﬀusion

via thru-holes in a bilayer with some defects. In (b), the areas colored in blue and red represent

defects or holes in the ﬁrst and second layers, respectively. This bilayer has a maze of thru-holes

from the top surface all the way down to the substrate .

graphene-terminated surface by a similar approach called pinhole-seeded lateral epitaxy.11

The growth mechanisms of remote and thru-hole epitaxy are completely diﬀerent. This

distinction stems from whether or not there are thru-holes, through which atomic species ad-

sorbed on the 2D material can diﬀuse into the substrate, as illustrated in Fig. 1. In remote

epitaxy, the substrate potential should be sneaked out through the 2D material because

there are no thru-holes (see Fig. 1(a)). In thru-hole epitaxy, on the other hand, the adsorbed

atoms on the 2D material can permeate into the substrate via thru-holes and form the nuclei

directly on the substrate, as displayed in Fig. 1(b). Unlike remote epitaxy, which stringently

requires a defect-free graphene monolayer, we observed that thru-hole epitaxy can be suc-

cessfully carried out with tens of nanometers thick (multiply-stacked) h-BN layers or even

thicker SiO2as long as thru-holes are maintained.9Here, 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. Despite this clear

and evident experimental demonstration of thru-hole epitaxy, there remains still one impor-

tant issue, which is not intuitively convincing. How can small holes in each layer or stack

of h-BN survive as thru-holes without being sealed during multiple stacking? This question

naturally arises because our intuition, at ﬁrst sight, would say that the probability of the

survival of holes as thru-holes in multiple stacking would quickly drop close to zero even by

a few times of stacking of 2D materials containing a moderate concentration of structural

defects.

This is a critical question to answer in order to validate thru-hole epitaxy in multiply-

stacked 2D materials because, in thru-hole epitaxy, the epitaxial connectedness between the

ﬁlm to be grown and the underlying substrate can only be established by thru-holes. 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-BN9remained 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. However, if the holes in each stack are not well isolated, the decrease

rate of thru-holes with the number of stacks can be reduced. 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.9These 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 Fig. 1(b), 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.

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EstablishingEpitaxialConnectednessinMulti-Stacking:TheSurvivalofThru-HolesinThru-HoleEpitaxyYoungjunLee,1SeungjunLee,1,∗JaewuChoi,2ChinkyoKim,1,2,†andYoung-KyunKwon1,2,‡1DepartmentofPhysics,andResearchInstituteforBasicSciences,KyungHeeUniversity,Seoul02447,Korea2DepartmentofInformationDisplay,KyungH...

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Establishing Epitaxial Connectedness in Multi-Stacking The Survival of Thru-Holes in Thru-Hole Epitaxy Youngjun Lee1Seungjun Lee1Jaewu Choi2

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