Remote Epitaxy of ZnO Microrods on Graphene-Coated Substrates
Authors: Jeong Junseok , Hong Suklyun , Hong Young Joon 
Published: February 2020

Two-dimensional nanomaterials are emerging for use as seed substrates toward transferrable, transparent, or flexible device applications.[1,2] In the van der Waals heteroepitaxy, graphene acted as seed for epitaxial overlayers of III–V or II–VI nanostructures or thin films.[3-5] Because the hydrophobic surface of graphene, given from lack of dangling bonds, substantially hinders the high-density nucleation in heteroepitaxy,[3] the use of graphene substrates was precluded for practical use as a substrate in the semiconductor epitaxy. Such surface properties of graphene have been known to be changeable by contacting other substances underneath graphene layer, because of the wetting transparency of graphene. More recently, the chemical property transparency was applied to the epitaxy technique to obtain single crystalline thin film overlayer whose crystal structure and orientation can be copied from underlying substrate across graphene. This is the so-called remote epitaxy, which was firstly demonstrated by Kim et al.[6,7]

Here, we report on remote homoepitaxy of ZnO microrods on graphene-coated ZnO substrates with different a- and c-planes. The remote epitaxy was carried out via a hydrothermal growth using a nutrient solution with zinc nitrate hexahydrate and hexamethylenetetramine at 95 °C. For preparation of a- and c-plane ZnO substrate, ZnO thin films were heteroepitaxially coated on r-plane Al2O3 and c-plane GaN, respectively. Then, monolayer graphene, which was synthesized via a chemical vapor deposition (CVD) on copper foil, was transferred onto these substrates using a poly(methyl-methacrylate)-supported etching-and-transfer technique.
ZnO epitaxial overlayers grew to be horizontally and vertically aligned microrod arrays on a-plane and c-plane ZnO substrates, respectively, across graphene layers, which were also found to show a well-defined long-range in-plane orientation over the entire surface of graphene-coated ZnO substrate, despite the presence of CVD-grown graphene with typical domain size of several micrometers. Cross-sectional transmission electron microscopic and selected area electron diffraction analyses revealed the homoepitaxial relationship with a remote gap of graphene. We further studied the successful remote homoepitaxy of ZnO across bilayer- and trilayer graphene, on which lower density of horizontal- and vertical ZnO microrods were obtained in comparison with that on monolayer graphene-coated substrate. This result strongly suggests that ZnO substrates played a crucial role in determining the crystallographic orientation and lattice arrangement of ZnO microrod overlayer across graphene layer, but as increasing the thickness of graphene, the impact of substrate is lesser for nucleation and growth of ZnO overlayer in the remote homoepitaxy. Such finding that electric field given from the substrate can be penetrated across graphene was simulated via density functional theory (DFT) calculations. The DFT calculations also indicated that the field penetration strength decreases with the increase of graphene thickness. The ability of graphene, which can be released from the host substrate without covalent bonds, was exploited to transfer the microrod arrays via a thermal release tape exfoliation technique. After the exfoliation, the host substrate was reused for repeating the remote epitaxy over again. This study opens a way for producing well aligned, transferrable epitaxial semiconductor microrod arrays while regenerating the substrate for cost-saving device manufacturing process.
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