Remote epitaxial interaction through graphene

Van der Waals Epitaxy of 2D Gallium Telluride on Graphene: Growth Dynamics and Principal Component Analysis

A scalable epitaxy of 2D layered materials and heterostructures constitutes a crucial step in developing novel optoelectronic applications based on high-crystalline quality 2D materials. Here, the formation of continuous, strain-free, high-crystalline quality 2D hexagonal gallium telluride (h-GaTe) directly on epitaxial graphene using molecular beam epitaxy is demonstrated. Morphological and structural characterizations evidence a coherent layer at the heterostructure interface having an in-plane lattice constant of 4.05 ± 0.01 A ̇ $\dot{\textrm {A}}$ . The few-layer thick graphene determines the epitaxial registry of the h-GaTe with grains of sixfold symmetry and a multilayer-type homoepitaxial growth. Deposition temperature- and time-dependent surface topography indicate that the interlayer diffusion of adatoms plays a crucial role in achieving smooth GaTe films. Contrastive principal component analysis allows for screening large in situ diffraction data as a function of growth parameters. In this way, the trajectory of the 2D h-GaTe growth is mapped through phase space. These results are relevant for integrating epitaxial material in the fabrication of high-performance multifunctional devices.

Freestanding Wide-Bandgap Semiconductors Nanomembrane from 2D to 3D Materials and Their Applications

Wide-bandgap semiconductors (WBGS) with energy bandgaps larger than 3.4 eV for GaN and 3.2 eV for SiC have gained attention for their superior electrical and thermal properties, which enable high-power, high-frequency, and harsh-environment devices beyond the capabilities of conventional semiconductors. Pushing the potential of WBGS boundaries, current research is redefining the field by broadening the material landscape and pioneering sophisticated synthesis techniques tailored for state-of-the-art device architectures. Efforts include the growth of freestanding nanomembranes, the leveraging of unique interfaces such as van der Waals (vdW) heterostructure, and the integration of 2D with 3D materials. This review covers recent advances in the synthesis and applications of freestanding WBGS nanomembranes, from 2D to 3D materials. Growth techniques for WBGS, such as liquid metal and epitaxial methods with vdW interfaces, are discussed, and the role of layer lift-off processes for producing freestanding nanomembranes is investigated. The review further delves into electronic devices, including field-effect transistors and high-electron-mobility transistors, and optoelectronic devices, such as photodetectors and light-emitting diodes, enabled by freestanding WBGS nanomembranes. Finally, this review explores new avenues for research, highlighting emerging opportunities and addressing key challenges that will shape the future of the field.

Interface phenomena and emerging functionalities in ferroelectric oxide based heterostructures

Capitalizing on the nonvolatile, nanoscale controllable polarization, ferroelectric perovskite oxides can be integrated with various functional materials for designing emergent phenomena enabled by charge, lattice, and polar symmetry mediated interfacial coupling, as well as for constructing novel energy-efficient electronics and nanophotonics with programmable functionalities. When prepared in thin film or membrane forms, the ferroelectric instability of these materials is highly susceptible to the interfacial electrostatic and mechanical boundary conditions, resulting in tunable polarization fields and Curie temperatures and domain formation. This review focuses on two types of ferroelectric oxide-based heterostructures: the epitaxial perovskite oxide heterostructures and the ferroelectric oxides interfaced with two-dimensional van der Waals materials. The topics covered include the basic synthesis methods for ferroelectric oxide thin films, membranes, and heterostructures, characterization of their properties, and various emergent phenomena hosted by the heterostructures, including the polarization-controlled metal-insulator transition and magnetic anisotropy, negative capacitance effect, domain-imposed one-dimensional graphene superlattices, programmable second harmonic generation, and interface-enhanced polar alignment and piezoelectric response, as well as their applications in nonvolatile memory, logic, and reconfigurable optical devices. Possible future research directions are also outlined, encompassing the synthesis via remote epitaxy and oxide moiré engineering, incorporation of binary ferroelectric oxides, realization of topological properties, and functional design of oxygen octahedral rotation.

Tradeoff between Mechanical Strength and Electrical Conductivity of MXene Films by Nacre-Inspired Subtractive Manufacturing

Traditional strategies, by additive manufacturing, for integrating monolayer Ti3C2Tx nanosheets into macroscopic films with binders can effectively improve their mechanical strength, but the electrical conductivity is often sacrificed. Herein, inspired by the aligned nano-compacted feature of nacre, a flexible subtractive manufacturing strategy is reported to squeeze the interlayer 2D spacings by removing the nanoconfined water and interface terminations, leading to the improvement of mechanical strength and stability of Ti3C2Tx layered films without sacrificing the electrical conductivity. After the vacuum annealing of Ti3C2Tx films at 300 °C (A300), the interlayer 2D spacing decreased ≈0.1 nm with the surface functional groups (═O, ─OH, ─F) and interlayer water molecules greatly removed. The tensile strength (95.59 MPa) and Young's modulus (9.59 GPa) of A300 are ≈3 and ≈2 times improved, respectively. Moreover, the A300 films maintain a metallic electrical conductivity (2276 S cm-1) and show greatly enhanced stability. Compared to the original films, the mechanical strength of the A300 films is enhanced by increasing the interlayer friction and energy dissipation with the decrease of interlayer 2D spacings. This work provides a new way for engineering the self-assembled films with more functions for broad applications.

In-Plane Adaptive Heteroepitaxy of 2D Cesium Bismuth Halides with Engineered Bandgaps on c-Sapphire

The heteroepitaxy of 2D materials with engineered bandgaps are crucial to broaden the spectral response for their integrated optoelectronic devices. However, it is a challenge to achieve the high-oriented epitaxy and integration of multicomponent 2D materials with varying lattice constants on the same substrate due to the limitation of lattice matching. Here, in-plane adaptive heteroepitaxy of a series of high-oriented 2D cesium bismuth halide (Cs3Bi2X9, X = I, Br, Cl) single crystals with varying lattice constants from 8.41 to 7.71 Å is achieved on c-plane sapphire with distinct lattice constant of 4.76 Å at a low temperature of 160 °C in an air ambient, benefiting from tolerable interfacial strain by switching compressive stress to tensile stress during a 30° rotation of crystal orientation. First-principles calculation demonstrates that those are all thermodynamically stable phases, deriving from multiple minima of interfacial energy between single crystals and sapphire substrate. The detectivity of Cs3Bi2I9 photodetector reaches up to 3.7 × 1012 Jones, deriving from high single-crystal quality. This work provides a promising experimental strategy and basic theory to boost the heteroepitaxy and integration of 2D single crystals with varying lattice constants on low-cost dielectric substrate, paving the way for their applications in integrated optoelectronics.

Constructing Two-Dimensional, Ordered Networks of Carbon-Carbon Bonds with Precision

Organic semiconducting nanomembranes (OSNMs), particularly carbon-based ones, are at the forefront of next-generation two-dimensional (2D) semiconductor research. These materials offer remarkable promise due to their diverse chemical properties and unique functionalities, paving the way for innovative applications across advanced semiconductor material sectors. Graphene stands out for its extraordinary mechanical strength, thermal conductivity, and superior charge transport capabilities, inspiring extensive research into other 2D carbon allotropes like graphyne and graphdiyne. With its high electron mobility and tunable bandgap, graphdiyne is particularly attractive for power-efficient electronic devices. However, synthesizing graphdiyne presents significant challenges, primarily due to the difficulty in achieving precise and deterministic control over the coupling of its monomers. This precision is crucial for determining the material's porosity, periodicity, and overall functionality. Innovative approaches have been developed to address these challenges, such as the strategic assembly of molecular building blocks at heterogeneous interfaces. Furthermore, data-driven techniques, such as machine learning and artificial intelligence (AI), are proving invaluable in this field, assisting in screening precursors, optimizing structural configurations, and predicting novel properties of these materials. These advancements are essential for producing durable monolayer sheets that can be integrated into existing electronic components. Despite these advancements, the integration of graphdiyne into semiconductor technology remains complex. Achieving long-range coherence in bonding configurations and enhancing charge transport characteristics are significant hurdles. Continued research into robust and controllable synthesis techniques is essential for unlocking the full potential of graphdiyne and other 2D materials, leading to more efficient, faster, and mechanically robust electronics.

Remote epitaxy and exfoliation of vanadium dioxide via sub-nanometer thick amorphous interlayer

The recently emerged remote epitaxy technique, utilizing 2D materials (mostly graphene) as interlayers between the epilayer and the substrate, enables the exfoliation of crystalline nanomembranes from the substrate, expanding the range of potential device applications. However, remote epitaxy has been so far applied to a limited range of material systems, owing to the need of stringent growth conditions to avoid graphene damaging, and has therefore remained challenging for the synthesis of oxide nanomembranes. Here, we demonstrate the remote epitaxial growth of an oxide nanomembrane (vanadium dioxide, VO2) with a sub-nanometer thick amorphous interlayer, which can withstand potential sputtering-induced damage and oxidation. By removing the amorphous interlayer, a 4-inch wafer-scale freestanding VO2 nanomembrane can be obtained, exhibiting intact crystalline structure and physical properties. In addition, multi-shaped freestanding infrared bolometers are fabricated based on the epitaxial VO2 nanomembranes, showing high detectivity and low current noise. Our strategy provides a promising way to explore various freestanding heteroepitaxial oxide materials for future large-scale integrated circuits and functional devices.

From oxide epitaxy to freestanding membranes: Opportunities and challenges

Motivated by the growing demand to integrate functional oxides with dissimilar materials, numerous studies have been undertaken to detach a functional oxide film from its original substrate, effectively forming a membrane, which can then be affixed to the desired host material. This review article is centered on the synthesis of functional oxide membranes, encompassing various approaches to their synthesis, exfoliation, and transfer techniques. First, we explore the characteristics of thin-film growth techniques with emphasis on molecular beam epitaxy. We then examine the fundamental principles and pivotal factors underlying three key approaches of creating membranes: (i) chemical lift-off, (ii) the two-dimensional layer-assisted lift-off, and (iii) spalling. We review the methods of exfoliation and transfer for each approach. Last, we provide an outlook into the future of oxide membranes, highlighting their applications and emerging properties.

Route to Enhancing Remote Epitaxy of Perovskite Complex Oxide Thin Films

Remote epitaxy is taking center stage in creating freestanding complex oxide thin films with high crystallinity that could serve as an ideal building block for stacking artificial heterostructures with distinctive functionalities. However, there exist technical challenges, particularly in the remote epitaxy of perovskite oxides associated with their harsh growth environments, making the graphene interlayer difficult to survive. Transferred graphene, typically used for creating a remote epitaxy template, poses limitations in ensuring the yield of perovskite films, especially when pulsed laser deposition (PLD) growth is carried out, since graphene degradation can be easily observed. Here, we employ spectroscopic ellipsometry to determine the critical factors that damage the integrity of graphene during PLD by tracking the change in optical properties of graphene in situ. To mitigate the issues observed in the PLD process, we propose an alternative growth strategy based on molecular beam epitaxy to produce single-crystalline perovskite membranes.

Advancements and Challenges in the Integration of Indium Arsenide and Van der Waals Heterostructures

The strategic integration of low-dimensional InAs-based materials and emerging van der Waals systems is advancing in various scientific fields, including electronics, optics, and magnetics. With their unique properties, these InAs-based van der Waals materials and devices promise further miniaturization of semiconductor devices in line with Moore's Law. However, progress in this area lags behind other 2D materials like graphene and boron nitride. Challenges include synthesizing pure crystalline phase InAs nanostructures and single-atomic-layer 2D InAs films, both vital for advanced van der Waals heterostructures. Also, diverse surface state effects on InAs-based van der Waals devices complicate their performance evaluation. This review discusses the experimental advances in the van der Waals epitaxy of InAs-based materials and the working principles of InAs-based van der Waals devices. Theoretical achievements in understanding and guiding the design of InAs-based van der Waals systems are highlighted. Focusing on advancing novel selective area growth and remote epitaxy, exploring multi-functional applications, and incorporating deep learning into first-principles calculations are proposed. These initiatives aim to overcome existing bottlenecks and accelerate transformative advancements in integrating InAs and van der Waals heterostructures.

Influence of Synthesis Parameters on Structure and Characteristics of the Graphene Grown Using PECVD on Sapphire Substrate

The high surface area and transfer-less growth of graphene on dielectric materials is still a challenge in the production of novel sensing devices. We demonstrate a novel approach to graphene synthesis on a C-plane sapphire substrate, involving the microwave plasma-enhanced chemical vapor deposition (MW-PECVD) technique. The decomposition of methane, which is used as a precursor gas, is achieved without the need for remote plasma. Raman spectroscopy, atomic force microscopy and resistance characteristic measurements were performed to investigate the potential of graphene for use in sensing applications. We show that the thickness and quality of graphene film greatly depend on the CH4/H2 flow ratio, as well as on chamber pressure during the synthesis. By varying these parameters, the intensity ratio of Raman D and G bands of graphene varied between ~1 and ~4, while the 2D to G band intensity ratio was found to be 0.05-0.5. Boundary defects are the most prominent defect type in PECVD graphene, giving it a grainy texture. Despite this, the samples exhibited sheet resistance values as low as 1.87 kΩ/□. This reveals great potential for PECVD methods and could contribute toward efficient and straightforward graphene growth on various substrates.

Cold Seeded Epitaxy and Flexomagnetism in GdAuGe Membranes Exfoliated From Graphene/Ge(111)

Remote and van der Waals epitaxy are promising approaches for synthesizing single crystalline membranes for flexible electronics and discovery of new properties via extreme strain; however, a fundamental challenge is that most materials do not wet the graphene surface. We develop a cold seed approach for synthesizing smooth intermetallic films on graphene that can be exfoliated to form few nanometer thick single crystalline membranes. Our seeded GdAuGe films have narrow X-ray rocking curve widths of 9-24 arc seconds, which is 2 orders of magnitude lower than their counterparts grown by typical high temperature methods, and have atomically sharp interfaces observed by transmission electron microscopy. Upon exfoliation and rippling, strain gradients in GdAuGe membranes induce an antiferromagnetic to ferri/ferromagnetic transition. Our smooth, ultrathin membranes provide a clean platform for discovering new flexomagnetic effects in quantum materials.

Mixed-Dimensional Integration of 3D-on-2D Heterostructures for Advanced Electronics

Two-dimensional (2D) materials have garnered significant attention due to their exceptional properties requisite for next-generation electronics, including ultrahigh carrier mobility, superior mechanical flexibility, and unusual optical characteristics. Despite their great potential, one of the major technical difficulties toward lab-to-fab transition exists in the seamless integration of 2D materials with classic material systems, typically composed of three-dimensional (3D) materials. Owing to the self-passivated nature of 2D surfaces, it is particularly challenging to achieve well-defined interfaces when forming 3D materials on 2D materials (3D-on-2D) heterostructures. Here, we comprehensively review recent progress in 3D-on-2D incorporation strategies, ranging from direct-growth- to layer-transfer-based approaches and from non-epitaxial to epitaxial integration methods. Their technological advances and obstacles are rigorously discussed to explore optimal, yet viable, integration strategies of 3D-on-2D heterostructures. We conclude with an outlook on mixed-dimensional integration processes, identifying key challenges in state-of-the-art technology and suggesting potential opportunities for future innovation.

Ultrastable 3D Heterogeneous Integration via N-Heterocyclic Carbene Self-Assembled Nanolayers

The commercialization of 3D heterogeneous integration through hybrid bonding has accelerated, and accordingly, Cu-polymer bonding has gained significant attention as a means of overcoming the limitations of conventional Cu-SiO2 hybrid bonding, offering high compatibility with other fabrication processes. Polymers offer robust bonding strength and a low dielectric constant, enabling high-speed signal transmission with high reliability, but suffer from low thermomechanical stability. Thermomechanical stability of polymers was not achieved previously because of thermal degradation and unstable anchoring. To overcome these limitations, wafer-scale Cu-polymer bonding via N-heterocyclic carbene (NHC) nanolayers was presented for 3D heterogeneous integration, affording ultrastable packing density, crystallinity, and thermal properties. NHC nanolayers were deposited on copper electrodes via electrochemical deposition, and wafer-scale 3D heterogeneous integration was achieved by adhesive bonding at 170 °C for 1 min. Ultrastable conductivity and thermomechanical properties were observed by the spatial mapping of conductivity, work function, and force-distance curves. With regard to the characterization of NHC nanolayers, low-temperature bonding, robust corrosion inhibition, enhanced electrical conductivity, back-end-of-line process compatibility, and fabrication process reduction, NHC Cu/polymer bonding provides versatile advances in 3D heterogeneous integration, indicating that NHC Cu/polymer bonding can be utilized as a platform for future 3D vertical chip architectures.

Epitaxial Metal Electrodeposition Controlled by Graphene Layer Thickness

Control over material structure and morphology during electrodeposition is necessary for material synthesis and energy applications. One approach to guide crystallite formation is to take advantage of epitaxy on a current collector to facilitate crystallographic control. Single-layer graphene on metal foils can promote "remote epitaxy" during Cu and Zn electrodeposition, resulting in growth of metal that is crystallographically aligned to the substrate beneath graphene. However, the substrate-graphene-deposit interactions that allow for epitaxial electrodeposition are not well understood. Here, we investigate how different graphene layer thicknesses (monolayer, bilayer, trilayer, and graphite) influence the electrodeposition of Zn and Cu. Scanning transmission electron microscopy and electron backscatter diffraction are leveraged to understand metal morphology and structure, demonstrating that remote epitaxy occurs on mono- and bilayer graphene but not trilayer or thicker. Density functional theory (DFT) simulations reveal the spatial electronic interactions through thin graphene that promote remote epitaxy. This work advances our understanding of electrochemical remote epitaxy and provides strategies for improving control over electrodeposition.

Remote Epitaxy: Fundamentals, Challenges, and Opportunities

Advanced heterogeneous integration technologies are pivotal for next-generation electronics. Single-crystalline materials are one of the key building blocks for heterogeneous integration, although it is challenging to produce and integrate these materials. Remote epitaxy is recently introduced as a solution for growing single-crystalline thin films that can be exfoliated from host wafers and then transferred onto foreign platforms. This technology has quickly gained attention, as it can be applied to a wide variety of materials and can realize new functionalities and novel application platforms. Nevertheless, remote epitaxy is a delicate process, and thus, successful execution of remote epitaxy is often challenging. Here, we elucidate the mechanisms of remote epitaxy, summarize recent breakthroughs, and discuss the challenges and solutions in the remote epitaxy of various material systems. We also provide a vision for the future of remote epitaxy for studying fundamental materials science, as well as for functional applications.