Advances in direct CVD growth of twisted two-dimensional materials

Adapting optical microscopy methods for nanoscale characterization of defects\nin two-dimensional (2D) materials is a vital step for photonic on-chip devices.\nTo increase the analysis throughput, waveguide-based on-chip imaging platforms\nhave been recently developed. Their inherent disadvantage, however, is the\nnecessity to transfer the 2D material from the growth substrate to the imaging\nchip which introduces contamination, potentially altering the characterization\nresults. Here we present a unique approach to circumvent these shortfalls by\ndirectly growing a widely-used 2D material (hexagonal boron nitride, hBN) on\nsilicon nitride chips, and optically characterizing the defects in the intact\nas-grown material. We compare the direct growth approach to the standard wet\ntransfer method, and confirm the clear advantages of the direct growth. While\ndemonstrated with hBN in the current work, the method is easily extendable to\nother 2D materials.\n

Direct growth and patterning of atomically thin two-dimensional (2D) materials on various substrates are essential steps towards enabling their potential for use in the next generation of electronic and optoelectronic devices. The conventional gas-phase growth techniques, however, are not compatible with direct patterning processes. Similarly, the condensed-phase methods, based on metal oxide deposition and chalcogenization processes, require lengthy processing times and high temperatures. Here, a novel self-limiting laser crystallization process for direct crystallization and patterning of 2D materials is demonstrated. It takes advantage of significant differences between the optical properties of the amorphous and crystalline phases. Pulsed laser deposition is used to deposit a thin layer of stoichiometric amorphous molybdenum disulfide (MoS2) film (∼3 nm) onto the fused silica substrates. A tunable nanosecond infrared (IR) laser (1064 nm) is then employed to couple a precise amount of power and number of pulses into the amorphous materials for controlled crystallization and direct writing processes. The IR laser interaction with the amorphous layer results in fast heating, crystallization, and/or evaporation of the materials within a narrow processing window. However, reduction of the midgap and defect states in the as crystallized layers decreases the laser coupling efficiency leading to higher tolerance to process parameters. The deliberate design of such laser 2D material interactions allows the self-limiting crystallization phenomena to occur with increased quality and a much broader processing window. This unique laser processing approach allows high-quality crystallization, direct writing, patterning, and the integration of various 2D materials into future functional devices.

The integration of CVD diamond is seen as an important technology for the passive thermal management of high power GaN devices. Diamond films are often attached to GaN devices either through direct growth on the GaN or through a plasma activated attachment process. Regardless of the method used, a thermal resistance between the GaN and diamond will exist at the interface either through the nucleation and growth of low thermal conductivity diamond at the interface or through a large phonon mismatch at the GaN diamond interace for the bonded approach. While recent demonstrations have shown the benefits of using CVD diamond in lowering the temperature of GaN HEMTs, the theoretical benefits of CVD diamond have not been achieved due to the need to control thermal boundary resistance and the elimination of low conductivity diamond near the interace with GaN. In this work, we will present data on the thermal performance of GaN on Diamond HEMTs fabricated using bonding as well as CVD growth on the GaN. To better understand the thermal response of these devices, the thermal conductivity of the diamond and quality of the diamond measured using Raman spectroscopy and microscopy approaches will be presented. In addition, the thermal boundary resistance between the GaN and diamond will be presented. A link between the thermal conductivity, thermal boundary resistance, and the bonding or growth approach will be used to elucidate their impact on the total thermal resistance of the GaN on Diamond system. Furthermore, the mechanical integrity of the GaN on diamond interface which can lead to a degradation in TBR measured using a transient thermoreflectance method will be shown. Finally, the temperature distribution in GaN on Diamond HEMTs will be measured and correlated to models of the devices employind properties measured in our experiments. Finally, the propspects for GaN on Diamond based on CVD growth and direct bonding approaches will be discussed.

Strain is a proven technique for modifying the bandgap and enhancing carrier mobility in 2D materials. Most current strain engineering techniques rely on the post-growth transfer of these atomically thin materials from growth substrates to target surfaces, limiting their integration into nanoelectronics. Here, we present a new approach where strain in 2D materials is already introduced directly during their growth on grayscale-patterned topographies instead of flat surfaces. Both strain levels and orientations are deterministically engineered by controlling grayscale surface contour lengths through thermal expansion mismatches in nanostructured stacks, where the conformally grown and firmly attached 2D material is forced to match the underlying morphology change during cooling. With this method, we experimentally demonstrate precise control of localized tensile strain from 0 to 0.5% in grown MoS2 monolayer along both uni- and multiaxial directions, while higher strain levels are shown to be theoretically possible. This strain-engineered growth of 2D material films directly on the target substrates is a generic and adaptable approach to various combinations of grayscale-thin-film/substrates and eliminates all the transfer-related limitations of previous approaches, thus paving the way for integrating strained 2D materials into next-generationnanoelectronics.

Although flakes of two-dimensional (2D) heterostructures at the micrometer scale can be formed with adhesive-tape exfoliation methods, isolation of 2D flakes into monolayers is extremely time consuming because it is a trial-and-error process. Controlling the number of 2D layers through direct growth also presents difficulty because of the high nucleation barrier on 2D materials. We demonstrate a layer-resolved 2D material splitting technique that permits high-throughput production of multiple monolayers of wafer-scale (5-centimeter diameter) 2D materials by splitting single stacks of thick 2D materials grown on a single wafer. Wafer-scale uniformity of hexagonal boron nitride, tungsten disulfide, tungsten diselenide, molybdenum disulfide, and molybdenum diselenide monolayers was verified by photoluminescence response and by substantial retention of electronic conductivity. We fabricated wafer-scale van der Waals heterostructures, including field-effect transistors, with single-atom thickness resolution.

This chapter considers new opportunities and accompanying challenges, examining various aspects of potential device applications of graphene and atom‐thick 2D materials, including optoelectronic devices, new type of transistors, and possible CMOS integration. Direct growth is the key technology to make all these applications realistic, so the chapter addresses the prospects of wafer‐scale graphene and 2D materials growth. Newly emergent 2D materials including graphene and transition metal dichalcogenides (TMDCs) began as playgrounds for observing exotic physics, such as unconventional quantum Hall effect and quantum transport phenomena. Just as the fundamental understanding and exploration of conventional two‐dimensional electron gas (2DEG) in an earlier era became the cornerstone of modern IT industry, these atom‐thick 2D materials may lead to an industrial revolution in the post‐Si era. The chapter discusses Cu‐graphene hybrid interconnects and graphene‐inserted source‐drain contacts that may help with the critical challenges in further scaling of Si technology.

Raman thermometry is a powerful technique for sub-microscale thermal measurements on semiconductor-based devices, provided that the active region remains accessible and is not obscured by metallization. Since pure metals do not exhibit Raman scattering, traditional Raman thermometry becomes ineffective in such cases. To overcome this limitation, we propose the use of atomically thin Two-Dimensional materials as local temperature sensors. These materials generate Raman spectra at the nanoscale, enabling highly precise absolute surface temperature measurements. In this study, we investigate the feasibility and effectiveness of this approach by applying it to power devices, including a calibrated gold resistor and an SiC Junction Barrier Schottky (JBS) diode. We assess the processing challenges and measurement reliability of 2D materials for thermal characterization. To validate our findings, we complement Raman thermometry with thermoreflectance measurements, which are well suited for metallized surfaces. For example, on the serpentine resistor, Raman thermometry applied to the 2D material yielded a thermal resistance of 22.099 °C/W, while thermoreflectance on the metallic surface measured 21.898 °C/W. This close agreement suggests good thermal conductance at the metal/2D material interface. The results demonstrate the potential of integrating 2D materials as effective nanoscale temperature probes, offering new insights into thermal management strategies for advanced electronic components. Additionally, thermal simulations are conducted to further analyze the thermal response of these devices under operational conditions. Furthermore, we investigate two 2D material integration methods, transfer and direct growth, and evaluate them through measured thermal resistances for the SiC JBS diode, highlighting the influence of the deposition technique on thermal performance.

Fabrication of van der Waals heterostructures through direct growth of rhenium disulfide on van der Waals surfaces

The Review Article by Mishra et al. (pp. 2277–2289) provides detailed insight into the graphene growth on SiC surfaces, its properties and technological relevance. The outstanding properties of graphene and the leading graphene growth techniques such as micromechanical exfoliation, CVD growth on metals and thermal decomposition of SiC are summarized. From the perspective of electronic device fabrication, thermal decomposition of SiC appears the most promising, thanks to its direct–growth process on a semiconductor surface, and the extent of control on number of layers, quality, and uniformity obtained. However, bulk SiC substrates present limitations in terms of costs, sizes and difficulty in micromachining. Direct growth of graphene on heteroepitaxial 3C–SiC on Si substrates is a promising alternative, fully compatible with established silicon fabrication technologies and allowing seamless integration. The cover image shows STM atomicscale snapshots of the sequence of surface transformations on 3C–SiC(111)/Si(111) leading from SiC(111) to monolayer graphene in UHV by high–temperature annealing (1250 °C). The sequence of surface reconstructions in the three insets occurs from bottom right to top left. Background image: bilayer graphene. (Dr. Bharati Gupta is kindly acknowledged for the images.)

The great interest in graphene applications in optics and optoelectronics still requires alternative methods for obtaining high-quality graphene. Currently used methods are based on graphene transfer onto a target substrate. As a consequence, the structural quality of graphene is lowered and so are its the electrical and optical properties. What is more, the transferred graphene layer is often contaminated with Cu, Ge or chemical residue. As a result, such a graphene layer can only be used in limited applications. Better quality is required when the 2D material is to be applied as a transparent conductive electrode (TCE) in a GaN-based light emitter or as an active layer in an Au-containing optical fibre. Therefore, alternative methods for graphene deposition are still in demand. In this work, a plasma-enhanced chemical vapour deposition (PECVD) process was used for direct graphene growth on GaN and Au substrates in the Aixtron Black Magic system. This method does not require chemical isolation, graphene transfer or a metallic catalyst. The entire process can be conducted in stable and repeatable conditions. Structural homogeneity of obtained graphene was observed using scanning electron microscopy. Its presence and overall optical characteristics were investigated with Raman spectroscopy over a large area (mapping mode). They confirmed the formation of graphene and its dependence on growth time through the presence of the 2D mode. The results prove that PECVD is a promising method for obtaining graphene in the form of flakes.

ConspectusThe rise of van der Waals layered materials such as graphene, hexagonal boron nitride, and transition-metal dichalcogenides opens up enormous opportunities for exploring novel quantum phenomena at the two-dimensional (2D) atomic limit. The physical properties of van der Waals materials are often affected by the symmetry of the crystal lattices. For example, breaking the inversion symmetry can lead to a variety of phenomena, such as second-harmonic generation, valley polarization, and ferroelectricity. The symmetry and symmetry breaking of layered materials are determined by the atomic arrangements within the layer and the layer stackings. The screw-dislocation-driven growth mechanism is general to 2D materials and can provide effective kinetic pathways to influence the layer stacking and generate diverse and complex layer stackings with different symmetry.Furthermore, different van der Waals layered materials can also be stacked vertically to create artificial new structures. In such stacked structures, the interlayer twist angle between stacked layers results in the formation of large-scale moiré superlattices that manipulate the electronic structures of van der Waals materials and provide an additional degree of freedom for tuning their physical properties. The stacking and twisting of layered materials lead to observations of new quantum phenomena, such as unconventional superconductivity, tunable Mott insulators, moiré excitons, and various magnetic and ferroelectric orderings. Previously, such twisted 2D structures were often fabricated by mechanically stacking exfoliated layers, but it was recently demonstrated that twisted van der Waals structures can form via direct growth. Moreover, continuously twisted structures of 2D materials can be realized through two distinct mechanisms that involve screw dislocations. The Eshelby twist mechanism induced by the strain of screw dislocation gives rise to twisted van der Waals nanowires with small interlayer twists. Beyond the Eshelby twist mechanism, supertwisted spirals have recently been enabled by a non-Euclidean twist mechanism which arises from the mismatched geometry between van der Waals crystals and non-Euclidean (curved) surfaces.In this Account, we start with reviewing the stacking configurations in the diverse polytypic structures of layered materials using transition-metal dichalcogenides as examples. We further discuss the twisted structures of layered materials from a structural point of view. After introducing screw-dislocation-driven growth and showing its generality in the crystal growth of layered materials, we further discuss how the unique structures of screw dislocations influence the stacking and symmetry of layered materials. Moreover, we highlight how screw dislocations enable interlayer twisting through the Eshelby twist mechanism and the non-Euclidean twist mechanism. In the end, the challenges and future perspectives are discussed for the study of screw-dislocated layered materials to control the stacking and twist for exotic physical properties.

Large-area, 2D, anisotropic, direct growth of nanostructures is considered an effective and straightforward way to readily fulfill transparent, flexible technology requirements. In addition, formation of thin hybrid structures by combining with another 2D material brings about dimensional advantages, such as intimate heterostructure functionalities, large specific area, and optical transparency. Here, we demonstrate 2D planar growth of thin Ni(OH)2 nanosheets on arbitrary rigid and soft supports, by exploiting the growth strategies of oriented attachment induced by interfacial chemistry and the intrinsic driving force of layered structure constitution. Moreover, large-scale 2D heterohybrids have successfully been prepared by direct conformal growth of Ni(OH)2 nanosheets overlying MoO3 nanobelts. Unlike the exfoliation and transfer of 2D materials technique, this approach minimizes multiple process contamination and physical-handling structural defects. Accordingly, proof-of-concept flexible electrochromism is demonstrated in view of its prerequisite to the access of a large homogeneous material coating. The as-synthesized 2D layered structure affirms its optical and electrochemical superiority through the display of wide optical modulation, high coloration efficiency, good cyclic stability, and flexibility.

Free‐standing gallium nitride has been prepared using various methods; however, the removal of the original substrate is still challenging with low success rates. In this work, 2‐inch free‐standing GaN films are obtained by direct growth on a fluoro phlogopite mica by hydride vapor‐phase epitaxy. Depending on the van der Waals (vdW) interaction between GaN and mica, the effect of the significant lattice mismatch is effectively reduced; thus, enabling the production of a high‐quality wafer‐scale GaN film on mica. The vdW‐induced cracks at GaN–mica interface are found to be initiated near the interface so that GaN can easily separate from mica during rapid cooling. Owing to the hydrophilic nature of mica, the residual GaN on the mica can be lifted off by following deionized water treatment, and the mica substrate can be repeatedly used to grow free‐standing GaN films. The self‐separated GaN films grown on both pristine and used mica substrates are single crystallinity and strain‐free. Additionally, a fully functional ultraviolet light‐emitting diode is demonstrated to show that the self‐separated GaN films are of device quality. The proposed approach achieves epitaxial growth of wafer‐scale single‐crystalline GaN on 2D materials and provides a new substrate option in the technology of III‐V materials.

Review: 147 refs.

Graphene is the most broadly discussed and studied two-dimensional material because of its preeminent physical, mechanical, optical, and thermal properties. Until now, metal-catalyzed chemical vapor deposition (CVD) has been widely employed for the scalable production of high-quality graphene. However, in order to incorporate the graphene into electronic devices, a transfer process from metal substrates to targeted substrates is inevitable. This process usually results in contamination, wrinkling, and breakage of graphene samples - undesirable in graphene-based technology and not compatible with industrial production. Therefore, direct graphene growth on desired semiconductor and dielectric substrates is considered as an effective alternative. Over the past years, there have been intensive investigations to realize direct graphene growth using CVD methods without the catalytic role of metals. Owing to the low catalytic activity of non-metal substrates for carbon precursor decomposition and graphene growth, several strategies have been designed to facilitate and engineer graphene fabrication on semiconductors and insulators. Here, those developed strategies for direct CVD graphene growth on semiconductors and dielectrics for transfer-free fabrication of electronic devices are reviewed. By employing these methods, various graphene-related structures can be directly prepared on desired substrates and exhibit excellent performance, providing versatile routes for varied graphene-based materials fabrication.