Van Der Waals Epitaxy Research Articles

Graphene-assisted molecular beam epitaxy of AlN for AlGaN deep-ultraviolet light-emitting diodes

We report on the van der Waals epitaxy of high-quality single-crystalline AlN and the demonstration of AlGaN tunnel junction deep-ultraviolet light-emitting diodes directly on graphene, which were achieved by using plasma-assisted molecular beam epitaxy. It is observed that the substrate/template beneath graphene plays a critical role in governing the initial AlN nucleation. In situ reflection high energy electron diffraction and detailed scanning transmission electron microscopy studies confirm the epitaxial registry of the AlN epilayer with the underlying template. Detailed studies further suggest that the large-scale parallel epitaxial relationship for the AlN epilayer grown on graphene with the underlying template is driven by the strong surface electrostatic potential of AlN. The realization of high-quality AlN by van der Waals epitaxy is further confirmed through the demonstration of AlGaN deep-ultraviolet light-emitting diodes operating at ∼260 nm, which exhibit a maximum external quantum efficiency of 4% for an unpackaged device. This work provides a viable path for the van der Waals epitaxy of ultra-wide bandgap semiconductors, providing a path to achieve high performance deep-ultraviolet photonic and optoelectronic devices that were previously difficult.

Flexible graphene-assisted van der Waals epitaxy growth of crack-free AlN epilayer on SiC by lattice engineering

Graphene has attracted much attention as novel buffer layer to grow III-nitrides. Here direct van der Waals epitaxy (vdWE) of high quality AlN films on graphene/SiC is demonstrated by effective lattice engineering and detailed nucleation mechanism is presented. The AlN films on graphene show high single crystalline quality, with the full width at half maximum (FWHM) of 79 arcsec for (1 0 2) asymmetry plane and 148 arcsec for (0 0 2) symmetry plane, respectively. First-principles calculations demonstrate the introduction of graphene enables the tunable lattice constant for the heteroepitaxy system and effectively releases the biaxial strains in AlN epilayers. The lattice engineering to minimize the strain of the epilayers is further confirmed by Raman spectra shift of graphene 2D peak. This provides a feasible way to grow high-quality AlN films with no cracks to meet the strict demands of devices, especially in ultraviolet optoelectronic devices.

Van der Waals Epitaxial Formation of Atomic Layered α-MoO3 on MoS2 by Oxidation.

The electronic, catalytic, and optical properties of transition metal dichalcogenides (TMDs) are significantly affected by oxidation, and using oxidation to tune the properties of TMDs has been actively explored. In particular, because transition metal oxides (TMOs) are promising hole injection layers, a TMD-TMO heterostructure can be potentially applied as a p-type semiconductor. However, the oxidation of TMDs has not been clearly elucidated because of the structural instability and the extremely small quantity of oxides formed. Here, we reveal the phases and morphologies of oxides formed on two-dimensional molybdenum disulfide (MoS2) using transmission electron microscopy analysis. We find that MoS2 starts to oxidize around 400 °C to form orthorhombic-phase molybdenum trioxide (α-MoO3) nanosheets. The α-MoO3 nanosheets so formed are stacked layer-by-layer on the underlying MoS2 via van der Waals interaction and the nanosheets are aligned epitaxially with six possible orientations. Furthermore, the band gap of MoS2 is increased from 1.27 to 3.0 eV through oxidation. Our study can be extended to most TMDs to form TMO-TMD heterostructures, which are potentially interesting as p-type transistors, gas sensors, or photocatalysts.

Van der Waals Epitaxy of Earth-Abundant Zn3P2 on Graphene for Photovoltaics

Earth-abundant semiconducting materials are a potential solution for large-scale deployment of solar cells at a lower cost. Zinc phosphide (Zn3P2) is one such Earth-abundant material with optoelectronic properties suitable for photovoltaics. Herein, we report the van der Waals epitaxy of tetragonal Zn3P2 (α-Zn3P2) on graphene using molecular beam epitaxy. The growth on graphene progresses by the formation of Zn3P2 triangular flakes, which merge to form a thin film with a strong (101) crystallographic texture. Photoluminescence from the Zn3P2 thin films is consistent with previously reported Zn3P2. This work demonstrates that the need for a lattice-matched substrate can be circumvented by the use of graphene as a substrate. Moreover, the synthesis of high-quality Zn3P2 flakes and films on graphene brings new material choices for low-cost photovoltaic applications.

Tungsten Diselenide Top-gate Transistors with Multilayer Antimonene Electrodes: Gate Stacks and Epitaxially Grown 2D Material Heterostructures

We have demonstrated that with e-beam deposition of a thin Al2O3 layer before atomic layer deposition, a uniform Al2O3 film can be obtained on WSe2/sapphire samples. Device performances are observed for WSe2 top-gate transistors by using oxide stacks as the gate dielectric. By using thermal evaporation, epitaxially grown multilayer antimonene can be prepared on both MoS2 and WSe2 surfaces. With multilayer antimonene as the contact metal, a significant increase in drain currents and ON/OFF ratios is observed for the device, which indicates that high contact resistance between metal/2D material interfaces is a critical issue for 2D devices. The observation of multilayer antimonene grown on different 2D material surfaces has demonstrated less dependence on the substrate lattice constant of the unique van der Waals epitaxy for 2D materials. The results have also demonstrated that stacking 2D materials with different materials plays an important role in the practical applications of 2D devices.

Superconductivity in Single-Quintuple-Layer Bi2Te3 Grown on Epitaxial FeTe.

How an interfacial superconductivity emerges during the nucleation and epitaxy is of great importance not only for unveiling the physical insights but also for finding a feasible way to tune the superconductivity via interfacial engineering. In this work, we report the nanoscale creation of a robust and relatively homogeneous interfacial superconductivity (TC ≈ 13 K) on the epitaxial FeTe surface, by van der Waals epitaxy of single-quintuple-layer topological insulator Bi2Te3. Our study suggests that the superconductivity in the Bi2Te3/FeTe heterostructure is generated at the interface and that the superconductivity at the interface does not enhance or weaken with the increase of the Bi2Te3 thickness beyond 1 quintuple layer (QL). The observation of the topological surface states crossing Fermi energy in the Bi2Te3/FeTe heterostructure with the average Bi2Te3 thickness of about 20 QL provides further evidence that this heterostructure may potentially host Majorana zero modes.

Growth mechanism of highly oriented layered Sb2Te3 thin films on various materials

Sb2Te3 is a layered material with outstanding properties leading to applications in interfacial phase-change memories, spintronic and thermoelectric devices. For successful integration in devices, controlling the orientation of the atomic planes of Sb2Te3 deposited by sputtering on various materials used for electrodes and on dielectric layers is required. We have succeeded in depositing Sb2Te3 thin films (thickness in the range 10–100 nm) by sputtering in industrial deposition equipments on WSi, TiN, amorphous Si as well as on native and thermal silicon oxide layers. The structure and orientation of the films were studied by x-ray diffraction. The Sb and Te planes are found parallel to the substrate, whatever the nature of the bottom material, provided that the sputtering conditions avoid a Te deficiency in the deposited film. These results show that deposition of Sb2Te3 with out-of-plane orientation on silicon oxide is actually possible, in contrast with previous literature results. Scanning transmission electron microscopy images of the interface between the Sb2Te3 film and the bottom material allow to elucidate the growth mechanism. The formation of a surface layer containing a few Te planes on top of the bottom material is mandatory for the subsequent growth of an out-of-plane oriented Sb2Te3 film by van der Waals epitaxy.

Temperature dependence of Raman scattering in defect-free AlN nanorods grown on multilayer graphene by van der Waals epitaxy

The crystalline quality of traditional epitaxy is hampered by the lattice and thermal mismatch of epilayer and substrate. Van der Waals epitaxy (vdWE) allows the epilayer to show no excessive strain and results in low defects density. Here, the multilayer graphene as a substrate for c-axis-oriented growth of vertically aligned AlN nanorods by vdWE have been presented. Defect-free of the AlN nanorods was evidenced through transmission electron microscopy (TEM). The strain of AlN nanorods was reduced with the exponential of height, which was characterized by micro-Raman spectroscopy. Moreover, the temperature dependence of Raman scattering of AlN has been further studied for clarifying the relationship of optical phonons and temperature. This temperature dependence was well matched by an empirical relationship which has proved to be applicable for other III-Nitride (such as GaN, InN) semiconductors.

Van der Waals epitaxy of ferroelectric ε-gallium oxide thin film on flexible synthetic mica

This study demonstrates the formation of the van der Waals epitaxy of the ε-gallium oxide (Ga2O3) thin film on cleaved synthetic mica via mist chemical vapor deposition. Orthorhombic ε-Ga2O3 (001) was epitaxially grown on synthetic mica (001). The analysis using transmission electron microscopy revealed an in-plane orientation of ε-Ga2O3 [010] ∣∣ synthetic mica [010]. However, the most thermodynamically stable β-Ga2O3 was grown at the film-substrate interface. The optical direct bandgap of the ε-Ga2O3 thin film grown by van der Waals epitaxy was estimated to be 5.0 eV, which was the same as for the heteroepitaxially grown ε-Ga2O3 thin film on other substrates. Besides, after ε-Ga2O3 thin film was grown on the synthetic mica substrates that are cleaved sufficiently thin, the sample could be bent or cut with scissors. These results denote that the epitaxial ε-Ga2O3 thin films grown by van der Waals epitaxy can be applied to flexible electronics.

Influence of Proximity to Supporting Substrate on van der Waals Epitaxy of Atomically Thin Graphene/Hexagonal Boron Nitride Heterostructures.

Combining graphene and the insulating hexagonal boron nitride (h-BN) into two-dimensional heterostructures is promising for novel, atomically thin electronic nanodevices. A heteroepitaxial growth, in which these materials are grown on top of each other, will be crucial for their scalable device integration. However, during this so-called van der Waals epitaxy, not only the atomically thin substrate itself must be considered but also the influences from the supporting substrate below it. Here, we reportnot only a substantial difference between the formation of h-BN on single- (SLG) and on bi-layer epitaxial graphene (BLG) on SiC, but also vice versa, that the van der Waals epitaxy of h-BN at growth temperatures well below 1000 °C affects the varying number of graphene layers differently. Our results clearly demonstrate that the additional graphene layer in BLG enhances the distance to the corrugated, carbon-rich interface of the supporting SiC substrate and thereby diminishes its influence on the van der Waals epitaxy, leading to a homogeneous formation of a smooth, atomically thin heterostructure, which will be required for a scalable device integration of 2D heterostructures.

Direct Growth of Antimonene on C-Plane Sapphire by Molecular Beam Epitaxy

Monolayer antimony (antimonene) has been reported for its excellent properties, such as tuneable band gap, stability in the air, and high mobility. However, growing high quality, especially large-area antimonene, remains challenging. In this study, we report the direct growth of antimonene on c-plane sapphire substrate while using molecular beam epitaxy (MBE). We explore the effect of growth temperature on antimonene formation and present a growth phase diagram of antimony. The effect of antimony sources (Sb2 or Sb4) and a competing mechanism between the two-dimensional (2D) and three-dimensional (3D) growth processes and the effects of adsorption and cracking of the source molecules are also discussed. This work offers a new method for growing antimonene and it provides ideas for promoting van der Waals epitaxy.

Influence of van der Waals epitaxy on phase transformation behaviors in 2D heterostructure

Despite exhibiting their attractive properties and performances, the interlayer interaction in two-dimensional van der Waals epitaxy heterostructures is not well understood. Here, we demonstrate the growth of two dimensional vertically stacked multilayer β-In2Se3/monolayer WS2 heterostructures via the chemical vapor deposition method. Despite a large lattice misfit (29.9%), the vertically stacked β-In2Se3/WS2 heterostructures exhibit van der Waals epitaxy with well-aligned lattice orientation of WS2(100)[001]//In2Se3(100)[001], forming a periodic superlattice. Interestingly, a reversible phase transformation of epitaxial β-In2Se3 has been observed by temperature-dependent Raman spectroscopy and electron diffraction conducted from liquid N2 to room temperature. Notably, the phase transformation of epitaxial β-In2Se3 can only be observed when its layer number is larger than 4 and the transformation temperature increases with the increase in the layer number, indicating a layer number dependent phase transformation of epitaxial β-In2Se3. These results indicate that the confinement effect exists between monolayer WS2 and epitaxial β-In2Se3, strongly constraining the lattice change of adjacent few layers of β-In2Se3.

Van der Waals Heteroepitaxy of Te Crystallites/2H-MoTe2 Atomically Thin Films on GaAs Substrates by Using Metal-Organic Chemical-Vapor Deposition

Te crystallites/few atomic-layer MoTe2 films grown on (100) GaAs substrates by using metal-organic chemical — vapor deposition (MOCVD) were investigated. MOCVD provides high-quality films, controllable film thickness, composition uniformity, and scalability to wafer scale size via the pyrolysis process of metal-organic sources. We observed several tens of nanometers of Te crystallites/three atomic layers 2H phase MoTe2 on the GaAs substrates. This observation was confirmed by using Raman spectroscopy, X-ray photoemission spectroscopy (XPS), atomic force microscopy, cross-sectional scanning transmission electron microscopy and energy dispersive X-ray spectroscopy. A two-dimensional transition-metal dichalcogenide deposited on GaAs by using the van der Waals epitaxy is a promising method for the development of future high-performance optoelectronic and electronic devices.

A comprehensive study on the effects of gamma radiation on the physical properties of a two-dimensional WS2monolayer semiconductor

This article reports the effects of gamma radiation on the structural, optical and magnetic properties of monolayer tungsten disulfide (WS2) grown by a scalable van der Waals epitaxial (VdWE) process on a SiO2coated Si substrate.

Universal Oriented van der Waals Epitaxy of 1D Cyanide Chains on Hexagonal 2D Crystals.

The atomic or molecular assembly on 2D materials through the relatively weak van der Waals interaction is quite different from the conventional heteroepitaxy and may result in unique growth behaviors. Here, it is shown that straight 1D cyanide chains display universal epitaxy on hexagonal 2D materials. A universal oriented assembly of cyanide crystals (AgCN, AuCN, and Cu0.5Au0.5CN) is observed, where the chains are aligned along the three zigzag lattice directions of various 2D hexagonal crystals (graphene, h‐BN, WS2, MoS2, WSe2, MoSe2, and MoTe2). The potential energy landscape of the hexagonal lattice induces this preferred alignment of 1D chains along the zigzag lattice directions, regardless of the lattice parameter and surface elements as demonstrated by first‐principles calculations and parameterized surface potential calculations. Furthermore, the oriented microwires can serve as crystal orientation markers, and stacking‐angle‐controlled vertical 2D heterostructures are successfully fabricated by using them as markers. The oriented van der Waals epitaxy can be generalized to any hexagonal 2D crystals and will serve as a unique growth process to form crystals with orientations along the zigzag directions by epitaxy.

Grafting Methyl Groups on the Si(111) Surface as a Buffer Layer for van der Waals Epitaxial Growth of ZnO Nanorods in Chemical Bath Deposition

The absence of an epitaxial relation of material growth with silicon substrates usually results from the formation of native oxide on the substrate surface, especially in the lengthy solution process at elevated temperature. An effective method to impede surface oxidation and the consequent inheritance of the lattice structure of the substrate are beneficial for the epitaxial growth. In this study, we use the two-step halogenation/methylation process to create a methylated Si(111) surface for the growth of ZnO nanorods in a chemical bath deposition (CBD) process. The grafted methyl groups effectively passivate the Si(111) surface, and the deposited ZnO nanorods exhibit an epitaxial relation with the silicon substrate—ZnO[0001]//Si[111]; ZnO{11̅00}//Si{22̅0}. Because no chemical bonding is formed between the ZnO nanorods and the Si(111) substrate, this solution process exhibits the characteristics of the van der Waals epitaxial (vdWE) growth. The approach developed in this study paves a way for vdWE growth of nanostructures on Si substrates by the CBD process.

Emergence of electron-phonon coupling in a dual topological insulator BiTe

A versatile van der Waals epitaxy of BiTe, a newly discovered dual TI and a predicted higher order topological insulator (HOTI) thin film on muscovite mica is demonstrated using pulsed laser deposition. Topographic, structural and XPS analyses confirm the chemical homogeneity and high crystalline quality with large-area coverage with atomically smooth surface. The magneto transport data reveals weak anti-localization and electron-electron interaction driven insulating ground state with n-type character. An elaborate thickness, temperature and magnetic field dependence of transport data indicates a transition from coupled, partially coupled and fully decoupled surface states wherein 3D electron-electron and electron-phonon scatterings play significant role in dephasing mechanism, unlike 2D electron-electron dephasing in most TIs. Also from the fitting of the resistivity upturn we found that the coulomb screening factor (F) in thicker samples turns out to be negative which indicates the presence of the electron phonon coupling in those samples.

Ultra-fast and high flexibility near-infrared photodetectors based on Bi2Se3 nanobelts grown via catalyst-free van der Waals epitaxy

van der Waals epitaxy (vdWe) method has recently attracted considerable interest due to its extensive application in the growth of layered structure materials. However, the growth process of vdWe is not completely understood. Here, we report the controlled growth process of Bi2Se3 nanobelts and study their photoresponse behaviour. The average length of nanobelts increases from 7 μm to 17 μm by adjusting the temperature of Bi2Se3 powder from 520 °C to 530 °C, however the average length becomes saturated with further increasing the source material temperature over 530 °C. Such a change can be attributed to the competition between the process of Bi2Se3 molecule diffusion and the process of crystal formation-related chemical reaction, leading to a symmetrically studying for the growth process of catalyst-free vdWe growth of Bi2Se3 nanobelts. The photodetectors based on these Bi2Se3 nanobelts show excellent device performance in the near-infrared light range, including an ultra-fast photoresponse (trsing ≈ 37 ms, tdecay ≈ 62 ms), a high responsivity of 10.1 mA/W and a high detectivity of 4.63 × 108 Jones. This high device performance could be related to the excellent carrier transport properties as Bi2Se3 nanobelt photodetectors also demonstrate a great potential for fabricating flexible and wearable electronics by still showing stable photoresponse after bending the device for 200 times.

Van der Waals epitaxy of continuous thin films of 2D materials using atomic layer deposition in low temperature and low vacuum conditions

Van der Waals epitaxy holds great promise in producing high-quality films of 2D materials. However, scalable van der Waals epitaxy processes operating at low temperatures and low vacuum conditions are lacking. Herein, atomic layer deposition is used for van der Waals epitaxy of continuous multilayer films of 2D materials HfS2, MoS2, SnS2, and ZrS2 on muscovite mica and PbI2 on sapphire at temperatures between 75 °C and 400 °C. For the metal sulfides on mica, the main epitaxial relation is MS2 ‖ mica. Some domains rotated by 30° are also observed corresponding to the MS2 ‖ mica alignment. In both cases, the presence of domains rotated by 60° (mirror twins) is also expected. For PbI2 on sapphire, the epitaxial relation is PbI2 ‖ Al2O3 with no evidence of 30° domains. For all of the studied systems there is relatively large in-plane mosaicity and in the PbI2/Al2O3 system some non-epitaxial domains are also observed. The study presents first steps of an approach towards a scalable and semiconductor industry compatible van der Waals epitaxy method.

Synthesis of V2O3 nanoplates for the exploration of the correlated supercritical state

Peculiar features exist in the stress-temperature phase stability diagram of V$_{2}$O$_{3}$ such as a first order phase transition between the paramagnetic insulating and metallic phases that ends with a critical point, quantum phase transition, and a triple point. These features remain largely unexplored and the exact nature of the phase transitions is not clear due to very limited control over the stress in bulk or film samples. Here, we show the synthesis of single-crystal V$_{2}$O$_{3}$ nanoplates for the first time using chemical vapor deposition via van der Waals epitaxy. Thickness of the V$_{2}$O$_{3}$ nanoplates range from a few to hundreds of nanometres and they can be mechanically exfoliated from the growth substrate. Using Raman spectroscopy on the nanoplates, we reveal that upon heating, V$_{2}$O$_{3}$ enters supercritical state for both tensiley strained and relaxed crystals with similar out-of-plane response. Transmission electron microscopy on V2O3 nanoplates hints the existence of a structural change when the crystals are heated. Our results show that V$_{2}$O$_{3}$ nanoplates should be useful for studying the physics of supercritical state and the phase stability of V$_{2}$O$_{3}$ to enable new horizons in applications.