Applications of remote epitaxy and van der Waals epitaxy

Abstract

Epitaxy technology produces high-quality material building blocks that underpin various fields of applications. However, fundamental limitations exist for conventional epitaxy, such as the lattice matching constraints that have greatly narrowed down the choices of available epitaxial material combinations. Recent emerging epitaxy techniques such as remote and van der Waals epitaxy have shown exciting perspectives to overcome these limitations and provide freestanding nanomembranes for massive novel applications. Here, we review the mechanism and fundamentals for van der Waals and remote epitaxy to produce freestanding nanomembranes. Key benefits that are exclusive to these two growth strategies are comprehensively summarized. A number of original applications have also been discussed, highlighting the advantages of these freestanding films-based designs. Finally, we discuss the current limitations with possible solutions and potential future directions towards nanomembranes-based advanced heterogeneous integration.

Keywords: 2DLT; Freestanding membrane; Heterogeneous integration; Remote epitaxy; Van der Waals epitaxy.

Fig. 1

a. Schematic illustration explaining the principle of the exfoliation of β-Ga₂O₃ from the graphene on SiC and photograph of the exfoliated β-Ga₂O₃ nanomembrane; the inset indicates a flip of 180° for the nanomembrane [34], b. Photograph image of flexible GaN film transferred from graphene/sapphire substrate [35], c. Schematic of various vdW oxide heteroepitaxy of ZnO, MoO₂, CoFe₂O₄, Fe₃O₄, PbZrTiO₃, VO₂ [, –41], d. Schematic views of remote epitaxy of III-V on graphene/III-V substrate and exfoliation of grown III-V layer, e. Photographs of single-crystalline GaAs(001) exfoliated from graphene GaAs(001) substrate [42], f. Schematic illustration depicting fabrication procedures for flexible GaN-LED array encapsulated with polyimide filler, g. Photograph of polyimide encapsulated GaN-LED exfoliated from substrate [43], h. Schematics depicting the exfoliation of the heteroepitaxial ZnO microrods process using the thermal release tape technique, and corresponding result sample photographs of ZnO microrods coated by PI [44]

Fig. 2

a. Schematics of epitaxial lift-off techniques using a chemically etched sacrificial layer, b. An optically induced separation between the epilayer and substrate, c. Brule-force mechanical spalling using a metal stressor layer, d. 2D material assisted layer transfer [64], e. Schematic illustration explaining the 2DLT and recycling substrate, f. Atomic force microscopy (AFM) image of the surface of the GaN grown on the graphene buffer layer. The RMS roughness is 0.18 nm, g. Photograph of the three-times-reused substrate and the exfoliated GaN epilayers [67], h. Schematic of successive multiple membranes production by 2DLT and wafer recycling, i. Photographs of exfoliated membrane surface (top) and EBSD map (bottom) [68], j. X-ray rocking curves of AlN grown on sapphire (black line) and plasma treated-graphene/sapphire substrates (red line), k. Atomically resolved STEM image of the interface of AlN/graphene/sapphire [72], l. Strain-relaxation efficiency of InGaP grown on graphene/GaAs and on GaAs [73], m. Schematic diagram of strain relaxation via the dislocation, the T indicates where a dislocation forma, and the schematic diagram below is strain via spontaneous relaxation, n. Cross-sectional view of GaP grown on bare GaAs, o. Cross-sectional view of GaP grown on graphene/GaAs [58]

Fig. 3

a. Schematic views of process sequence for the transfer of an AlGaN/GaN HEMT from a sapphire substrate to a copper plate, b. I_d-V_ds characteristics of AlGaN/GaN HEMTs before release from sapphire (blue line) and after transfer to a copper plate (red line), c. temperature map of the samples, taken with IR camera Neo Therno 700 (up-left and -right show the off state of the unreleased and transferred devices, respectively, down-left and -right show the on state oof the unreleased and transferred devices, respectively) [15], d. the flexibility of MoO₂/muscovite sample(left) and removal of MoO₂ film from muscovite substrate, e. carrier concentration of MoO₂ films do not show any obvious temperature dependence, f. mobility in the MoO₂ films do not show any obvious temperature dependence [22], g. Chip-less wireless e-skin based on surface acoustic wave (SAW) devices made of GaN freestanding membranes, h. Calculated electromechanical coupling coefficient (K²) of GaN SAW devices by the function of GaN thickness [74], i. The device structure of CFO/PMN-PT magnetoelectric coupling device. In the case of clamped device, the substrate and the bottom electrode are STO and SRO, respectively. In the case of the freestanding device, the substrate and the bottom electrode are PDMS and Ti, respectively, j. The voltage generated across the PMN-PT film in response to the magnetic field input [75], k. Schematic illustration of the BTO film growth, exfoliation, and transfer process onto a flexible PI substrate, l. Illustration of the experimental configuration of the c-AFM test, m. I-V curve of the Pt/BTO junction, n. Evolution of the current as a function of loading force, o. COMSOL FEM calculation with a tip-force model of the BTO film under applied force 28nN(up) and 196nN(down). The colors of blue to red correspond to lower to higher strain values [77]

Fig. 4

a. Light output power curve of violet LED which was grown by the vdW epitaxy [80], b. EL spectra of the vdW epitaxial DUV-LED [72], c. Optical image of flexible solar-blind photodetectors using β-Ga₂O₃ epilayer [34], d. UV light on/off cyclic tests of flexible Ga₂O₃ photodetector [81], e. I-V_ds characteristics of WS₂ epilayer-based photodetector according to V_gs [82], f. Fabrication procedure of GaN microrod/micropyramid arrays by using the vdW epitaxy [83], g. Cross-sectional high-resolution TEM image and SAED patterns of AlN thin-film grown on the graphene/NPSS [30], h. Cross-sectional SEM image and red light-emitting images of the remote epitaxial AlGaAs LED [90], i. Schematic image of DUV-LED on the h-BN/sapphire wafer. The inset image shows a magnified microscopic image of DUV-LED array, j. Normalized EL spectra of DUV-LED by various injection currents [91], k. Fabrication procedure of the remote epitaxial microrod LED array, l. Optical image of microcrystal-based LEDs emitting white light. The right inset displays a magnified photograph of microcrystal-based LEDs, composed of orange, yellow, green, blue, violet, and white-emitting nanocrystals, m. I-V characteristic of flexible microrod LED during 1000 bending cycles [101], n. Cross-sectional TEM image of the remote epitaxial perovskite thin-film on a polar NaCl wafer. The inset presents FFT patterns of the CPbBr₃ film, o. Steady-state PL curve of the CsPbBr₃ flakes on the NaCl wafer and the graphene/NaCl substrate. [102]

Fig. 5

a. Structural schematic of the Fe₂O₃/ZnO/mica heteroepitaxy, b. Cross-sectional TEM image of the ZnO/mica and Fe₂O₃/ZnO interface, c. Linear-sweep voltammograms of pure Fe₂O₃, ZnO, and the Fe₂O₃/ZnO electrodes with visible-light illumination and the inset photograph of flexible Fe₂O₃/ZnO/mica [114], d. The atomic structure of monolayer AlN and four possible adsorption sites, e. Charge density difference plots for CO₂/AlN, H₂/AlN, CO/AlN, O₂/AlN, NO/AlN [120], f. Photograph of an e-skin attached on the back of hand (top) and Schematic illustrations and wireless ion sensors based on GaN SAW device coated with Na⁺ ion–selective membranes, g. Resonant frequency shift in the wireless signals obtained from a GaN SAW ion sensor in response to changes in Na⁺ ion concentration, h. Continuous wireless recordings collected from a SAW ion sensor during a series of alternating injections of 0.86 mM NaCl solution and distilled water over the e-skin [74]