Search

We report the site-selective remote epitaxial growth of mechanically transferable ZnO microrod (MR) and microdisk (MD) arrays via hydrothermal growth. To designate the growth sites, a hole-patterned poly(methyl methacrylate) mask layer is formed on the graphene-coated GaN substrate. ZnO microarrays are exclusively grown to be either MR or MD on graphene-exposed patterned areas via the remote epitaxy. The remote heteroepitaxial relation between ZnO and GaN across graphene is observed by atomic resolution scanning transmission electron microscopy. The non-covalent remote epitaxial interface allows the mechanical lift-off of the ZnO microarrays and mass-transfer onto a surface of interest using a sticky tape as those arrays are well maintained. The donor substrate is refurbished for repetitive position-controlled remote epitaxy. This study provides a simple method of fabricating mass-transferable microarrays of semiconductors that can maintain the addressable spatial arrays of semiconductors to an arbitrary receiver substrate for ease of heterogeneous integration without an additional assembly process for position control.

Semiconductor epitaxy on two-dimensional materials is beneficial for transferrable and flexible device applications. Graphene, due to the absence of permanent electric dipoles, cannot screen the electric field coming from the opposite side surface, allowing remote epitaxy for heteroepitaxy. This study demonstrates remote heteroepitaxy of ZnO microrods (MRs) on the GaN substrate across graphene layers via hydrothermal growth. Even the use of tri-layer graphene yields the remote heteroepitaxial MR arrays. Transmission electron microscopy reveals the remote heteroepitaxial relation between ZnO MRs and the GaN substrate despite the existence of graphene interlayers in between them. Density-functional theory calculations show that charge transfer along the z-direction at graphene/c-GaN possibly attract adatoms leading to remote heteroepitaxy, implying the field permeability of graphene. The ability of graphene to be released from the host substrate is exploited to exfoliate the overlayer MRs and regenerate the substrate.

This study investigates the influence of voids on the electroluminescence (EL) emission color of ZnO microdisk/p-GaN heterojunction light-emitting diodes (LEDs). For this study, position-controlled microdisk arrays were fabricated on patterned p-GaN via wet chemical epitaxy of ZnO, and specifically, the use of trisodium citrate dihydrate (TCD) yielded high-density voids at the bottom of the microdisk. Greenish yellow or whitish blue EL was emitted from the microdisk LEDs formed with or without TCD, respectively, at reverse-bias voltages. Such different EL colors were found to be responsible for the relative EL intensity ratio between indigo and yellow emission peaks, which were originated from radiative recombination at p-GaN and ZnO, respectively. The relative EL intensity between dichromatic emissions is discussed in terms of (i) junction edge effect provoked by interfacial voids and (ii) electron tunneling probability depending on the depletion layer geometry.

Position-controlled n-ZnO microwire (MW) and nanowire-bundle (NW-B) arrays were fabricated using hydrothermal growth of ZnO on a patterned p-GaN film. Both the wire/film p–n heterojunctions showed electrical rectification features at reverse-bias (rb) voltages, analogous to backward diodes. Dichromatic electroluminescence (EL) emissions with 445- and 560-nm-wavelength peaks displayed whitish-blue and greenish-yellow light from MW- and NW-B-based heterojunctions at rb voltages, respectively. The different dichromatic EL emission colors were studied based on photoluminescence spectra and the dichromatic EL peak intensity ratios as a function of the rb voltage. The different EL colors are discussed with respect to depletion thickness and electron tunneling probability determined by wire/film junction geometry and size.