Van Der Waals Epitaxial Growth Research Articles

Transferrable GaN-based Micro-LED heterostructures grown on h-BN for optogenetic applications

Individually transferrable GaN-based MQWs micro-LED structures grown on two-dimensional (2D) hexagonal boron nitride (h-BN) have been demonstrated. Different sizes (60 × 30 µm, 60 × 60 µm, 90 × 60 µm and 90 × 90 µm) of GaN-based LED heterostructures on layered h-BN were grown on dielectric SiN patterned sapphire substrates using Metal Organic Vapor Phase Epitaxy (MOVPE). Scanning electron microscope (SEM) images show micro-LEDs were grown with high selectivity within the mask opening areas. Consistent MQWs emission from micro-LEDs was observed at 452 ± 2 nm by cathodoluminescence (CL) spectroscopy for all micro-LED sizes. The structural and optical quality were similar to the GaN-based-LEDs grown on unpatterned substrates. In addition, we have successfully demonstrated the lift-off and transfer of individual micro-LEDs to arbitrary flexible templates using a transfer printing machine, which can be addressed by full front-end process. These studies highlight the role of combination of van der Waals (vdW) epitaxy and selective area growth (SAG) in facilitating the direct realization of dimensionally defined III-Nitride micro-LED structures and the mechanical release and transfer of these micro-LEDs to templates suitable for flexible display and optogenetics applications.

Van der Waals epitaxial growth of few layers WSe2 on GaP(111)B

2D material epitaxy offers the promise of new 2D/2D and 2D/3D heterostructures with their own specific electronic and optical properties. In this work, we demonstrate the epitaxial growth of few layers WSe2 on GaP(111)B by molecular beam epitaxy. Using a combination of experimental techniques, we emphasize the role of the growth temperature and of a subsequent annealing of the grown layers under a selenium flux on the polytype formed and on its structural and morphological properties. We show that a low growth temperature promotes the formation of the 1T′ and 3R phases depending on the layer thickness whereas a higher growth temperature favours the stable 2H phase. The resulting layers exhibit clear epitaxial relationships with the GaP(111)B substrate with an optimum grain disorientation and mean size of 1.1° and around 30 nm respectively for the 2H phase. Bilayer 2H WSe2/GaP(111)B heterostructures exhibit a staggered type II band alignment and p-doped character of the epi-layer on both p and n-type GaP substrates. This first realisation of stable p-type WSe2 epi-layer on a large-area GaP(111)B substrate paves the way to new 2D/3D heterostructures with great interests in nanoelectronic and optoelectronic applications, especially in the development of new 2D-material p-n junctions.

Van der Waals Epitaxially Grown Molecular Crystal Dielectric Sb2O3 for 2D Electronics.

Two-dimensional (2D) semiconducting materials have attracted significant interest as promising candidates for channel materials owing to their high mobility and gate tunability at atomic-layer thickness. However, the development of 2D electronics is impeded due to the difficulty in formation of high-quality dielectrics with a clean and nondestructive interface. Here, we report the direct van der Waals epitaxial growth of a molecular crystal dielectric, Sb2O3, on 2D materials by physical vapor deposition. The grown Sb2O3 nanosheets showed epitaxial relations of 0 and 180° with the 2D template, maintaining high crystallinity and an ultrasharp vdW interface with the 2D materials. As a result, the Sb2O3 nanosheets exhibited a high breakdown field of 18.6 MV/cm for 2L Sb2O3 with a thickness of 1.3 nm and a very low leakage current of 2.47 × 10-7 A/cm2 for 3L Sb2O3 with a thickness of 1.96 nm. We also observed two types of grain boundaries (GBs) with misorientation angles of 0 and 60°. The 0°-GB with a well-stitched boundary showed higher electrical and thermal stabilities than those of the 60°-GB with a disordered boundary. Our work demonstrates a method to epitaxially grow molecular crystal dielectrics on 2D materials without causing any damage, a requirement for high-performance 2D electronics.

Epitaxial Growth of GaN Films on Chemical-Vapor-Deposited 2D MoS2 Layers by Plasma-Assisted Molecular Beam Epitaxy.

The van der Waals epitaxy of wafer-scale GaN on 2D MoS2 and the integration of GaN/MoS2 heterostructures were investigated in this report. GaN films have been successfully grown on 2D MoS2 layers using three different Ga fluxes via a plasma-assisted molecular beam epitaxy (PA-MBE) system. The substrate for the growth was a few-layer 2D MoS2 deposited on sapphire using chemical vapor deposition (CVD). Three different Ga fluxes were provided by the gallium source of the K-cell at temperatures of 825, 875, and 925 °C, respectively. After the growth, RHEED, HR-XRD, and TEM were conducted to study the crystal structure of GaN films. The surface morphology was obtained using FE-SEM and AFM. Chemical composition was confirmed by XPS and EDS. Raman and PL spectra were carried out to investigate the optical properties of GaN films. According to the characterizations of GaN films, the van der Waals epitaxial growth mechanism of GaN films changed from 3D to 2D with the increase in Ga flux, provided by higher temperatures of the K-cell. GaN films grown at 750 °C for 3 h with a K-cell temperature of 925 °C demonstrated the greatest crystal quality, chemical composition, and optical properties. The heterostructure of 3D GaN on 2D MoS2 was integrated successfully using the low-temperature PA-MBE technique, which could be applied to novel electronics and optoelectronics.

Controlled growth of a 2D/2D heterojunction for high-performance sodium ion storage

A Van der Waals epitaxial growth method is developed for constructing large-scale 2D/2D heterostructure based on graphdiyne and Ti2C3Tx. The heterojunction shows enhanced performance in storing the Na+.

Improved epitaxy of CsI(Tl) film on Si substrate buffered by graphene for X-ray detection

The growth of the CsI(Tl) scintillators deposited on glass substrates with proper wavelength matching the mainstream semiconductor circuits is crucial for X-ray imaging industry. But the coupling loss between the scintillators and silicon based photo sensors has limited the performance. So fabricating CsI(Tl) films on monocrystalline silicon is greatly important for next-generation high performance X-ray image devices. However, the lattice and thermal mismatch between CsI(Tl) and Si always deteriorates the morphology and structure, and thus degrades the property of the CsI(Tl) scintillation converters. Here, by utilizing crystalline graphene(Gr) as a buffer layer, the van der Waals epitaxial growth of CsI(Tl) film on Si(111) substrate is reported and its advanced applications are demonstrated. The introduced graphene buffer layer extremely frees the stress between CsI(Tl) and Si substrate. Therefore, the CsI(Tl) films show perfect micro-columnar and crystalline structures, and then, exhibit much higher light output and signal to noise ratio, and better modulation transfer functions and a bar pattern X-ray image compared to that on bare Si(111) substrate. This facile method not only lays the foundation for the epitaxy of the alkali halide crystals, including CsI(Tl), onto Si-based detectors and its applications in radiation detection, but also may broaden a new perspective of deposition of CsI(Tl) scintillators into flexible substrates.

Epitaxial growth of atomically thin Ga2Se2 films on c-plane sapphire substrates

Broadening the variety of two-dimensional (2D) materials and improving the synthesis of ultrathin films are crucial to the development of semiconductor industry. As a state-of-the-art 2D material, Ga2Se2 has attractive optoelectronic properties when it reaches the atomically thin regime. However, its van der Waals epitaxial growth, especially for atomically thin films, has seldom been studied. In this paper, we used molecular beam epitaxy to synthesize Ga2Se2 single-crystal films with a surface roughness down to 1.82 nm on c-plane sapphire substrates by optimizing the substrate temperature, Se:Ga flux ratio, and growth rate. Then, we used a three-step mode to grow Ga2Se2 films with a thickness as low as three tetralayers and a surface roughness as low as 0.61 nm, far exceeding the performance of direct growth. Finally, we found that surface morphology strongly depends on the Se:Ga flux ratio, and higher growth rates widened the suitable flux ratio window for growing Ga2Se2. Overall, this work advances the understanding of the vdW epitaxy growth mechanism for post-transition metal monochalcogenides on sapphire substrates.

Two-dimensional single crystal monoclinic gallium telluride on silicon substrate via transformation of epitaxial hexagonal phase

Van der Waals (vdW) epitaxial growth of large-area and stable two-dimensional (2D) materials of high structural quality on crystalline substrates is crucial for the development of novel device technologies. 2D gallium monochalcogenides with low in-plane symmetry stand out among the layered semiconductor materials family for next-generation optoelectronic and energy conversion applications. Here, we demonstrate the formation of large-area, single crystal and optically active 2D monoclinic gallium telluride (m-GaTe) on silicon substrate via rapid thermal annealing induced phase transformation of vdW epitaxial metastable hexagonal gallium telluride (h-GaTe). Stabilization of multilayer h-GaTe on Si occurs due to the role of the first layer symmetry together with efficient GaTe surface passivation. Moreover, we show that the phase transformation of h-GaTe to m-GaTe is accompanied by the strain relaxation between Si substrate and GaTe. This work opens the way to the fabrication of single-crystal 2D anisotropic semiconductors on standard crystalline wafers that are difficult to be obtained by epitaxial methods.

Temperature−Dependent Raman Scattering Investigation on vdW Epitaxial PbI2/CrOCl Heterostructure

Van der Waals (vdW) epitaxial growth provides an efficient strategy to prepare heterostructures with atomically and electronically sharp interfaces. Herein, PbI2 was in situ thermally deposited onto exfoliated thin−layered CrOCl nanoflakes in high vacuum to fabricate vdW PbI2/CrOCl heterostructures. Optical microscopy, atomic force microscopy, X−ray diffraction, and temperature−dependent Raman spectroscopy were used to investigate the structural properties and phonon behaviors of the heterostructures. The morphology of PbI2 films on the CrOCl substrate obviously depended on the substrate temperature, changing from hemispherical granules to 2D nanoflakes with flat top surfaces. In addition, anomalous blueshift of the Ag1 and Au2 modes as the temperature increased in PbI2/CrOCl heterostructure was observed for the first time. Our results provide a novel material platform for the vdW heterostructure and a possible method for optimizing heterostructure growth behaviors.

Van der Waals epitaxial growth and optoelectronics of a vertical MoS2/WSe2 p–n junction

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted extensive attention due to their unique electronic and optical properties. In particular, TMDs can be flexibly combined to form diverse vertical van der Waals (vdWs) heterostructures without the limitation of lattice matching, which creates vast opportunities for fundamental investigation of novel optoelectronic applications. Here, we report an atomically thin vertical p–n junction WSe2/MoS2 produced by a chemical vapor deposition method. Transmission electron microscopy and steady-state photoluminescence experiments reveal its high quality and excellent optical properties. Back gate field effect transistor (FET) constructed using this p–n junction exhibits bipolar behaviors and a mobility of 9 cm2/(V·s). In addition, the photodetector based on MoS2/WSe2 heterostructures displays outstanding optoelectronic properties (R = 8 A/W, D* = 2.93 × 1011 Jones, on/off ratio of 104), which benefited from the built-in electric field across the interface. The direct growth of TMDs p–n vertical heterostructures may offer a novel platform for future optoelectronic applications.Graphical

Carrier properties of Bi(111) grown on mica and Si(111)

A comparison is presented between properties of Bi(111) films grown by Stranski-Krastanov epitaxy on Si(111) 7 $\ifmmode\times\else\texttimes\fi{}$ 7 and by van der Waals epitaxy on mica substrates. Thin film morphologies and electronic transport properties of Bi(111) films of variable thickness are investigated for each growth method. Atomic force micrographs for Bi(111) films on mica reveal clearly defined triangular regions consisting of layered steps with height 0.4 nm, corresponding to the Bi(111) bilayer height. Variable-temperature electronic transport measurements show the existence of a quantum confinement-induced energy gap in the film interiors, resulting in a semimetal-to-semiconductor transition. Magnetotransport analysis in a three-carrier model including metallic electrons in surface states and electrons and holes in the films' interiors provides a detailed study of densities, mobilities, and mean-free paths of the three carrier types. Improved electronic transport properties are found in Bi(111) films of higher thickness on mica compared to on Si(111), a likely result of the largely strain-free van der Waals epitaxial growth.

Van der Waals Epitaxial Growth of 2D Layered Room‐Temperature Ferromagnetic CrS2

Abstract2D magnetic materials have been attracting enormous interest for both fundamental research and potential spintronic applications. Here, this work demonstrates the van der Waals epitaxial growth of air‐stable 2D magnetic CrS2 on mica substrate by chemical vapor deposition (CVD). The layered nature, high crystallinity, stoichiometry, and high‐purity 1T phase of the individual CrS2 crystallites are verified by a series of complementary characterization techniques including atomic force microscope, scanning transmission electron microscope and energy dispersive X‐ray spectroscopy. The single‐crystalline 1T‐phase CrS2 crystallites exhibit a metallic nature with a moderate conductivity of 1.78 × 102 S m−1 and a room‐temperature ferromagnetism with a coercive field of 28.7 Oe. The CVD growth of the air‐stable CrS2 crystallites with 1T phase and high crystallinity offer a new platform for the further physics research and spintronic device applications.

Two-dimensional material templates for van der Waals epitaxy, remote epitaxy, and intercalation growth

Epitaxial growth, a crystallographically oriented growth induced by the chemical bonding between crystalline substrate and atomic building blocks, has been a key technique in the thin-film and heterostructure applications of semiconductors. However, the epitaxial growth technique is limited by different lattice mismatch and thermal expansion coefficients of dissimilar crystals. Two-dimensional (2D) materials with dangling bond-free van der Waals surfaces have been used as growth templates for the hetero-integration of highly mismatched materials. Moreover, the ultrathin nature of 2D materials also allows for remote epitaxial growth and confinement growth of quasi-2D materials via intercalation. Here, we review the hetero-dimensional growth on 2D substrates: van der Waals epitaxy (vdWE), quasi vdWE, and intercalation growth. We discuss the growth mechanism and fundamental challenges for vdWE on 2D substrates. We also examine emerging vdWE techniques that use epitaxial liftoff and confinement epitaxial growth in detail. Finally, we give a brief review of radiation effects in 2D materials and contrast the damage induced with their 3D counterparts.

Spin-Sensitive Epitaxial In2Se3 Tunnel Barrier in In2Se3/Bi2Se3 Topological van der Waals Heterostructure.

Current-generated spin arising from spin-momentum locking in topological insulator (TI) surface states has been shown to switch the magnetization of an adjacent ferromagnet (FM) via spin-orbit torque (SOT) with a much higher efficiency than heavy metals. However, in such FM/TI heterostructures, most of the current is shunted through the FM metal due to its lower resistance, and recent calculations have also shown that topological surface states can be significantly impacted when interfaced with an FM metal such as Ni and Co. Hence, placing an insulating layer between the TI and FM will not only prevent current shunting, therefore minimizing overall power consumption, but may also help preserve the topological surface states at the interface. Here, we report the van der Waals epitaxial growth of β-phase In2Se3 on Bi2Se3 by molecular beam epitaxy and demonstrate its spin sensitivity by the electrical detection of current-generated spin in Bi2Se3 surface states using a Fe/In2Se3 detector contact. Our density functional calculations further confirm that the linear dispersion and spin texture of the Bi2Se3 surface states are indeed preserved at the In2Se3/Bi2Se3 interface. This demonstration of an epitaxial crystalline spin-sensitive barrier that can be grown directly on Bi2Se3, and verification that it preserves the topological surface state, is electrically insulating and spin-sensitive, is an important step toward minimizing overall power consumption in SOT switching in TI/FM heterostructures in fully epitaxial topological spintronic devices.

Van der Waals epitaxial growth of two-dimensional PbSe and its high-performance heterostructure devices

Inspired by the great success of ultrathin two-dimensional (2D) layered crystals, more and more attention is being paid to preparing 2D nanostructures from non-layered materials. They can significantly enrich the 2D materials and 2D heterostructures family, extend their application prospects, and bring us distinct properties from their bulk counterparts due to the strong 2D confinement effect. However, the realization of 2D non-layered semiconductors with strong light-harvesting capability and the ability to construct high-performance 2D heterostructures is still a critical challenge. Herein, we successfully synthesized 2D PbSe semiconductors with a large lateral dimension and ultrathin thickness via van der Waals epitaxy. The fabricated 2D PbSe device exhibits good electrical conductivity and superior multi-wavelength photoresponse performance with high responsivity (∼103 A/W) and impressive detectivity (∼2 × 1011 Jones). Furthermore, we demonstrate that 2D PbSe nanosheets can serve as component units for constructing high-performance heterostructure devices. With our strategy, ultrahigh current on/off ratio (∼108) and rectification ratio (∼106), as well as high responsivity (∼3 × 103 A/W) and detectivity (∼7 × 1012 Jones), can be achieved in PbSe/MoS2 back-gated transistors. These results indicate that 2D PbSe nanosheets and their heterostructures have tremendous applications potential in electrical and optoelectronic devices.

Van der Waals Epitaxial Growth for High Performance Organic‐Free Perovskite Solar Cell: Experimental and Theoretical Insights

AbstractAll‐inorganic perovskite solar cells (PSCs) are most promising in yielding stabilized power conversion efficiencies (PCE) as compared to their hybrid organic–inorganic counterparts. However, all‐inorganic PSCs have suffered from low efficiency due to significant charge carrier recombination within perovskite active layer (PAL) bulk and at the carrier selective interfaces (CSI). Boosting the PCE through the remedy to both issues seems to be the only Achilles‐Heal on their way to commercialization. Here, a synergistic approach of liquid‐exfoliated molybdenum disulfide (MoS2) at the PAL (CsPbIBr2)/hole transport layer (NiO) interface to enhance the perovskite grain growth and stabilize the hole transport layer (HTL)/PAL interface against the carrier recombination within the PAL bulk and at the interface is reported. The in‐plane coupling between MoS2 crystal lattice and CsPbIBr2 is strongly revealed through transmission electron microscopy (TEM) results, which favored the van der Waals epitaxial growth of CsPbIBr2 PAL toward the larger crystalline grain sizes, preferential growth along (110), i.e., normal to MoS2, and low trap‐density. Therefore, the PCE of organic‐free CsPbIBr2 PSC is increased to as high as 10.98% under 1‐sun illumination, which is 48% higher than the pristine device (7.38%), which concludes that this strategy can be integrated to other analogous perovskite compositions.

Crack‐Free Monolayer Graphene Interlayer for Improving Perovskite Crystallinity and Energy Level Alignment in Efficient Inverted Perovskite Solar Cells

Inverted planar perovskite solar cells (PSCs) have intrigued great promise in negative hysteresis, simple fabrication process, and flexible substrate implementation, in which the shared hole transport materiel is NiOx. However, the low‐temperature solution processing of the NiOx film is usually accompanied by defect formation, which deteriorates the perovskite quality and device performance. Meanwhile, the energy‐level offset between the NiOx and perovskite films is relatively large, limiting the interfacial charge transport. To suppress those setbacks, a defect‐free monolayer graphene sheet is transferred onto the NiOx film surface as a template for van der Waals epitaxial growth of perovskite films for the first time, leading to enhancing crystallinity with a large grain size of perovskite layer, 0.20 eV energy level offset drop, and accelerating charge transfer for the devices. Finally, the power conversion efficiency of 19.21% without hysteresis is achieved, exceeding 18.35% of the control device.

Van der Waals Epitaxial Growth of ZnO Films on Mica Substrates in Low-Temperature Aqueous Solution

In this article, we demonstrate the van der Waals (vdW) epitaxial growth of ZnO layers on mica substrates through a low-temperature hydrothermal process. The thermal pretreatment of mica substrates prior to the hydrothermal growth of ZnO is essential for growing ZnO crystals in epitaxy with the mica substrates. The addition of sodium citrate into the growth solution significantly promotes the growth of ZnO crystallites in a lateral direction to achieve fully coalesced, continuous ZnO epitaxial layers. As confirmed through transmission electron microscopy, the epitaxial paradigm of the ZnO layer on the mica substrate was regarded as an incommensurate van der Waals epitaxy. Furthermore, through the association of the Mist-CVD process, the high-density and uniform distribution of ZnO seeds preferentially occurred on mica substrates, leading to greatly improving the epitaxial qualities of the hydrothermally grown ZnO layers and obtaining flat surface morphologies. The electrical and optoelectrical properties of the vdW epitaxial ZnO layer grown on mica substrates were comparable with those grown on sapphire substrates through conventional solution-based epitaxy techniques.

Layered Graphene Growth Directly on Sapphire Substrates for Applications.

Layer-by-layer graphene growth is demonstrated by repeating CVD growth cycles directly on sapphire substrates. Improved field-effect mobility values are observed for the bottom-gate transistors fabricated by using the bilayer graphene channel, which indicates an improved crystallinity is obtained after the second CVD growth cycle. Despite the poor wettability of copper on graphene surfaces, graphene may act as a thin and effective diffusion barrier for copper atoms. The low resistivity values of thin copper films deposited on thin monolayer MoS2/monolayer graphene heterostructures have demonstrated its potential to replace current thick liner/barrier stacks in back-end interconnects. The unique van der Waals epitaxy growth mode will be helpful for both homo- and heteroepitaxy on 2D material surfaces.

Van der Waals Epitaxial Deposition of CsPbBr3 Films for Flexible Optoelectronic Applications

Growth of high-quality inorganic halide perovskite films has attracted increasing attention in recent years for their important applications in solar cells, photodetectors, and communications. In this study, a centimeter-sized inorganic perovskite film of CsPbBr3 was prepared on an atomically flat flexible muscovite substrate by using pulsed laser deposition. Van der Waals epitaxial growth of pure cubic phase CsPbBr3 film is achieved by optimizing deposition conditions. In order to explore its potential in optoelectronic applications, the CsPbBr3 perovskite film was assembled into a planar photodetector. Benefitting from the high crystallinity of the film, the photodetector demonstrates an ultralow dark current and a high sensitivity to light. Under 10 V bias voltage and 405 nm irradiation, the device presents a high photocurrent on/off ratio of 1.1 × 105, a specific detectivity of 2.41 × 1014 Jones, and an ultrafast response time (the rising and decaying times are 42.16 μs and 44.88 μs, respectively). Moreover, the CsPbBr3-based photodetectors without encapsulation show good long-term stability (30 days with 6% attenuation) and bending stability. All these results indicate that a CsPbBr3 film epitaxially grown on flexible muscovite is a promising candidate for flexible optoelectronic applications.