Van Der Waals Epitaxy Research Articles

Self-Organized Growth of Nanowires on a Graphene Film

The self-organized growth of nanowires (NWs) on a graphene film is realized through the van der Waals epitaxy mechanism by metalorganic chemical vapor deposition. It is a fundamental phenomenon which is qualitatively different from the known growth mechanisms, such as self- and metal-catalyzed NW growth. We present a theoretical model that explains the self-organized nucleation of InGaAs NWs on graphene, starting from the formation of InAs NW segments. Then, newly formed InAs NWs act just like seeds to initiate the subsequent growth of InAs/InxGa1–xAs core–shell NWs with spontaneous phase segregation. The model shows that due to the introduction of the graphene film, the nucleation speed of InAs and InGaAs nanoislands in the horizontal direction far exceeds that of the vertical direction. It also elucidates why the radius of the InAs core is two times larger than the InGaAs shell thickness by considering the adatom collection ability of the InAs core and InGaAs shell, respectively, during the transformative process. Thus, the theoretical analysis of the self-organized NW growth on a graphene film is of vital importance for further optimizing the fabrication procedures of two-dimensional material-based NWs.

Multiple Shapes Micro‐LEDs with Defect Free Sidewalls and Simple Liftoff and Transfer Using Selective Area Growth on Hexagonal Boron Nitride Template

AbstractSeveral technological challenges have prevented GaN‐based micro‐LEDs from finding application in mass market displays, despite their unique properties such as very high brightness and the very fast response time of GaN‐based materials. The primary challenges are the cost and complexity of lift‐off and transfer of LEDs from sapphire substrates to suitable supports as well as the lowered performance of tiny micro‐LEDs caused by chemical etching that defines individual LEDs. Herein, this work reports demonstration of a complete process that solves these challenges with epitaxy and cleanroom technologies that are commercially available. The process begins with van der Waals epitaxy of 2D h‐BN on silica masks with square, triangular and hexagonal patterns on sapphire substrates which define the micro‐LED regions. Then selective area growth of MQW LED heterostructures, with ultra smooth crystalline sidewalls, down to ultra tiny size of 1.4 µm is performed. Because of the lack of vertical chemical bonds in the h‐BN layer, simple mechanical lift‐off and transfer is performed on an array of LEDs heterostructures down to size of 8 µm. Finally, transparent ITO p‐contacts are deposited on LEDs with uniform lift‐off, resulting in high brightness LEDs.

InGaN/GaN QWs on tetrahedral structures grown on graphene/SiC

In recent years, flexible nitride LEDs have been developed for applications in fields such as lighting, displays but also medicine and biology. Said nanowire-based flexible LEDs are made of InGaN/GaN core-shell nanowires encapsulated in a polymer matrix, which is peeled off and constitutes the final flexible LED structure. However, the current peel off process is complex and hardly compatible with industrial applications. One solution to this issue could be the Van Der Waals epitaxy of the LED structures on graphene, as this approach is known to ease the separation from the original substrate. However, to realize Van der Waals epitaxy and prevent nucleation of GaN on defects located within the graphene layer, the quality of the graphene needs to be optimized. In this work we employ graphene grown by Chemical Vapor Deposition on SiC, known to be notably less defective than graphene grown on metals or sapphire. In this work, tetrahedral structures of GaN have been grown on monolayer and multilayer graphene on SiC. X-Ray Diffraction measurements confirmed that the GaN followed the crystallographic directions of SiC, implying growth through remote-epitaxy mechanism thanks to the presence of graphene in both cases. Last, InGaN/GaN structures were realized by growth of a 5 period of InGaN/GaN quantum wells on these GaN tetrahedra. Cathodoluminescence as well as photoluminescence measurements revealed the presence of stacking faults and Zinc Blende inclusion, as well as a broad visible emission near 480 nm of quantum wells.

Interface-mediated ferroelectricity in PMN-PT/PZT flexible bilayer via pulsed laser deposition

Ferroelectric thin-film bilayers of Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT)/PbZr0.52Ti0.48O3 (PZT) were grown on a flexible substrate of mica using pulsed laser deposition. Growth of the bilayer was induced with a thin film of LaNiO3 (LNO) single crystal, which was deposited on a mica substrate through van der Waals epitaxy. The LNO thin film also serves as the electrode for the bilayer device. The growth of the LNO thin film along the ⟨ 100 ⟩ orientation adopts a “Stranski–Krastanov” mechanism, governed by the relaxation of elastic energy between LNO/mica. Compared with the single layers of PMN-PT or PZT, or the bilayer of PZT/PMN-PT, the PMN-PT/PZT bilayer exhibits enhanced ferroelectric properties, with remnant polarization up to 72 μC/cm2. In addition, polarization in the PMN-PT/PZT bilayer exhibits excellent resistance against mechanical bending fatigue over 108 switching cycles. Such improved performances are ascribed to spontaneous polarizations enhanced by the residual stress at the PMN-PT/PZT heterointerface, increased interfacial potential barrier against leakage, and suppressed diffusion of Nb or Mg across the interface.

Microstructure control and property switching in stress-free van der Waals epitaxial VO2 films on mica

Realizing stress-free inorganic epitaxial films on weakly bonding substrates is of importance for applications that require film transfer onto surfaces that do not seed epitaxy. Film-substrate bonding is usually weakened by harnessing natural van der Waals layers (e.g., graphene) on substrate surfaces, but this is difficult to achieve in non-layered materials. Here, we demonstrate van der Waals epitaxy of stress-free films of a non-layered material VO2 on mica. The films exhibit out-of-plane 010 texture with three in-plane orientations inherited from the crystallographic domains of the substrate. The lattice parameters are invariant with film thickness, indicating weak film-substrate bonding and complete interfacial stress relaxation. The out-of-plane domain size scales monotonically with film thickness, but the in-plane domain size exhibits a minimum, indicating that the nucleation of large in-plane domains supports subsequent island growth. Complementary ab initio investigations suggest that VO2 nucleation and van der Waals epitaxy involves subtle polarization effects around, and the active participation of, surface potassium atoms on the mica surface. The VO2 films show a narrow domain-size-sensitive electrical-conductivity-temperature hysteresis. These results offer promise for tuning the properties of stress-free van der Waals epitaxial films of non-layered materials such as VO2 through microstructure control.

Van der Waals Epitaxy Growth of 2D Single-Element Room-Temperature Ferromagnet.

2D single-element materials, which are pure and intrinsically homogeneous on the nanometer scale, can cut the time-consuming material-optimization process and circumvent the impure phase, bringing about opportunities to explore new physics and applications. Herein, for the first time, the synthesis of ultrathin cobalt single-crystalline nanosheets with a sub-millimeter scale via van der Waals epitaxy is demonstrated. The thickness can be as low as ≈6nm. Theoretical calculations reveal their intrinsic ferromagnetic nature and epitaxial mechanism: that is, the synergistic effect between van der Waals interactions and surface energy minimization dominates the growth process. Cobalt nanosheets exhibit ultrahighblocking temperatures above 710 K and in-plane magnetic anisotropy. Electrical transport measurements further reveal that cobalt nanosheets have significant magnetoresistance (MR) effect, and can realize a unique coexistence of positive MR and negative MR under different magnetic field configurations, which can be attributed to the competition and cooperation effect among ferromagnetic interaction, orbital scattering, and electronic correlation. These results provide a valuable case for synthesizing 2D elementary metal crystals with pure phase and room-temperature ferromagnetism and pave the way for investigating new physics and related applications in spintronics.

Design and fabrication process flow for high-efficiency and flexible InGaN solar cells

We present the design of high-efficiency and flexible InGaN solar cells based on nano-pyramid absorbers, which allows for high indium incorporation and reduced polarization effects while maintaining high crystalline quality. The process uses nanoscale selective area growth and van der Waals epitaxy on two-dimensional (2D) hexagonal boron nitride on sapphire substrates. The 2D layer enables mechanical release of solar cells via self-lift-off and its transfer to a flexible substrate. Through coupled optical and electrical simulations, we compare different designs of the p- and n-regions and propose two device architectures that combine mechanical flexibility and high power conversion efficiency (up to 16.4%). This approach can provide a solution for the fabrication of high-efficiency, low-cost and flexible InGaN-based solar cells.

Wafer‐scale single‐crystalline MoSe2 and WSe2 monolayers grown by molecular‐beam epitaxy at low‐temperature — the role of island‐substrate interaction and surface steps

AbstractUltrathin two‐dimensional transition‐metal dichalcogenides (TMDs) have been pursued extensively in recent years for interesting physics and application potentials. For the latter, it is essential to synthesize crystalline TMD monolayers at wafer‐scale. Here, we report growth of single‐crystalline MSe2 (M = Mo, W) monolayers at wafer‐scale by molecular‐beam epitaxy at low temperatures (200–400°C) on nominally flat Au(1 1 1) substrates. The epifilms have low intrinsic defect densities of low 1012 cm−2. The grown films have then been exfoliated and transferred onto SiO2/Si by a wet chemical process, on which some optical measurements are performed, revealing high spatial uniformity of the samples. We also establish that MSe2 grows on Au via the van der Waals epitaxy mechanism, where a continuous film extends across the whole surface, overhangs atomic‐layer steps on substrate. We identify that the growth of highly crystalline MSe2 is promoted by an enhanced interaction between Au substrate and MSe2 islands rather than by the guidance of surface steps on substrate. The latter only arrests MSe2 lateral growth if they are multilayer high.Key points MBE growth of wafer‐scale highly crystalline TMD monolayers at low‐temperature is achieved. Island‐substrate interaction is found to play a critical role in vdW epitaxy of single‐crystalline TMDs on on‐axis substates. The TMD monolayers are of high uniformity and low intrinsic defect density.

Van der Waals Epitaxy Enables Rollable Dielectric Superlattice for Record High Overall Energy Density

AbstractNanoengineered polar oxide films have attracted much attention for electric energy storage thanks to their high energy density, though they are all deposited on thick and rigid substrates, resulting in inferior overall energy density and poor manufacturability. Herein, an alternative strategy is developed for oxide dielectrics utilizing van der Waals epitaxy on ultrathin and flexible mica substrate, with a dielectric superlattice of Pb0.92La0.08(Zr0.95Ti0.05)O3‐SrTiO3 carefully engineered to break its long‐range antiferroelectric polar order. An ultrathin flexible capacitor is obtained as a result, with a record high overall energy density of 12.19 J cm−3 and an efficiency of 90.98%, and there is much room for further improvement since mica substrate can approach 2D limit. The superlattice can be easily rolled for large‐scale manufacturing, and the energy storage performances are well maintained under large bending deformation as well as extended bending cycling. The study thus establishes a viable route for dielectric oxide films, paving way for their practical applications in high‐energy density capacitors.

Vertical full-colour micro-LEDs via 2D materials-based layer transfer

Micro-LEDs (µLEDs) have been explored for augmented and virtual reality display applications that require extremely high pixels per inch and luminance1,2. However, conventional manufacturing processes based on the lateral assembly of red, green and blue (RGB) µLEDs have limitations in enhancing pixel density3-6. Recent demonstrations of vertical µLED displays have attempted to address this issue by stacking freestanding RGB LED membranes and fabricating top-down7-14, but minimization of the lateral dimensions of stacked µLEDs has been difficult. Here we report full-colour, vertically stacked µLEDs that achieve, to our knowledge, the highest array density (5,100 pixels per inch) and the smallest size (4 µm) reported to date. This is enabled by a two-dimensional materials-based layer transfer technique15-18 that allows the growth of RGB LEDs of near-submicron thickness on two-dimensional material-coated substrates via remote or van der Waals epitaxy, mechanical release and stacking of LEDs, followed by top-down fabrication. The smallest-ever stack height of around 9 µm is the key enabler for record high µLED array density. We also demonstrate vertical integration of blue µLEDs with silicon membrane transistors for active matrix operation. These results establish routes to creating full-colour µLED displays for augmented and virtual reality, whilealso offering a generalizable platform for broader classes of three-dimensional integrated devices.

氮掺杂单层石墨烯上无中间层沉积高质量氮化镓

GaN on graphene/Al2O3 substrates grown via van der Waals epitaxy compensates for the deficiencies and defects caused by metal-organic chemical vapor deposition (MOCVD) on substrates with significant mismatches to GaN. However, the absence of dangling bonds on graphene leads to insufficient nucleation sites; hence, a thin layer of AlN or ZnO nanowalls should be deposited on graphene as an intermediate layer. In this work, high-quality GaN crystals with a low biaxial compressive stress of 0.023 GPa and low screw dislocation density of 9.76 × 107 cm−2 were successfully synthesized by MOCVD on nitrogen-doped graphene without a buffer layer. First-principles calculations demonstrated significant improvement in the adsorption energy of the Ga atom on the surface of nitrogen-doped graphene compared with that of pristine graphene, in agreement with the experimental observations of nucleation. In most cases, GaN films were obtained by forming C—Ga—N and N—Ga—N configurations via atomic nitrogen pretreatment on monolayer graphene. Therefore, it is hoped that the efficient method of atomic modulation of high-quality GaN films grown on nitrogen-doped graphene via interface manipulation used in this work will promote the industrial development of innovative semiconductor devices.

Van der Waals Heteroepitaxy of GaSe and InSe, Quantum Wells, and Superlattices

AbstractBandgap engineering and quantum confinement in semiconductor heterostructures provide the means to fine‐tune material response to electromagnetic fields and light in a wide range of the spectrum. Nonetheless, forming semiconductor heterostructures on lattice‐mismatched substrates is a challenge for several decades, leading to restrictions for device integration and the lack of efficient devices in important wavelength bands. Here, it is shown that the van der Waals epitaxy of 2D GaSe and InSe heterostructures occur on substrates with substantially different lattice parameters, namely silicon and sapphire. The GaSe/InSe heteroepitaxy is applied in the growth of quantum wells and superlattices presenting photoluminescence and absorption related to interband transitions.

Tiling the Silicon for Added Functionality: PLD Growth of Highly Crystalline STO and PZT on Graphene Oxide-Buffered Silicon Surface.

The application of two-dimensional (2D) materials has alleviated a number of challenges of traditional epitaxy and pushed forward the integration of dissimilar materials. Besides acting as a seed layer for van der Waals epitaxy, the 2D materials─being atom(s) thick─have also enabled wetting transparency in which the potential field of the substrate, although partially screened, is still capable of imposing epitaxial overgrowth. One of the crucial steps in this technology is the preservation of the quality of 2D materials during and after their transfer to a substrate of interest. In the present study, we show that by honing the achievements of traditional epitaxy and wet chemistry a hybrid approach can be devised that offers a unique perspective for the integration of functional oxides with a silicon platform. It is based on SrO-assisted deoxidation and controllable coverage of silicon surface with a layer(s) of spin-coated graphene oxide, thus simultaneously allowing both direct and van der Waals epitaxy of SrTiO3 (STO). We were able to grow a high-quality STO pseudo-substrate suitable for further overgrowth of functional oxides, such as PbZr1-xTixO3 (PZT). Given that the quality of the films grown on a reduced graphene oxide-buffer layer was almost identical to that obtained on SiC-derived graphene, we believe that this approach may provide new routes for direct and "remote" epitaxy or layer-transfer techniques of dissimilar material systems.

Epitaxial growth of flexible 1T-VSe2 thin films on mica by pulsed laser deposition

Recently, vanadium diselenide (VSe2), a member of transition metal dichalcogenides, has attracted a great deal of interest in spintronic devices and memory devices due to its unique physical properties. However, it is still a challenge to prepare a continuous VSe2 thin film which is critical for its potential application. Here, we report a continuous single-crystalline 1T-VSe2 thin film grown on mica by pulsed laser deposition. Both x-ray diffraction and high-resolution transmission electron microscopy verify the van der Waals epitaxy of the VSe2/mica heterostructure. Free-standing and flexible VSe2 thin films can be obtained and combined with integrated circuit technology, which is of great significance for the application of two-dimensional materials in the field of multifunctional flexible electronic devices.

Epitaxial Growth of GaN/AlN on h-BN/Si(111) by Metal–Organic Chemical Vapor Deposition: An Interface Analysis

Among III-nitride semiconductors, hexagonal boron nitride (h-BN) has a two-dimensional crystal structure and can be used as an insulator for nanotransistors or a substrate release layer for flexible electronics, which heavily rely on its interface properties. In this study, h-BN films were successfully grown on 150 mm Si(111) substrates by metal–organic chemical vapor deposition using AlN as a nucleation layer, and van der Waals epitaxy of GaN on the h-BN films was investigated, including the challenges of exfoliation of epilayers. As a result of the accumulated stress in GaN films during growth, self-exfoliation of GaN films was observed on GaN of thickness 1 μm and more. This issue was resolved by decreasing the h-BN thickness from 8 to 3 nm and lower, which results in improved crystal quality as evidenced by the smaller linewidth of the (0002) GaN X-ray rocking curves. Based on the exfoliation behavior of a series of GaN grown with different thicknesses, a self-exfoliation mechanism was proposed and an interface binding energy of 54 meV was estimated for the h-BN/AlN interface.

Epitaxial growth and E-beam induced structural changes of single crystalline 2D antimonene

Two-dimensional antimonene has attracted much attention because of its excellent stability and remarkable physical properties. Understanding the growth mechanism of antimonene by investigating processing parameters and structural properties is necessary for developing reliable fabrication methods. In this study, NaCl-assisted van der Waals epitaxy, one of the methods for scale-up fabrication, is employed to achieve atomic layer antimonene with high coverage rate. Furthermore, high-resolution images of atomic structure are presented with the powerful transmission electron microscopy, helping us confirm the exact configuration of antimonene. Meanwhile, we studied the effect of electron beam irradiation on antimonene; it was found that the stacking structure of antimonene would change under electron beam irradiation, and the moiré patterns of twisted few-layered antimonene were also observed. These results provide comprehensive study of growth mechanisms and in-depth knowledge of antimonene nanostructure, which are expected to facilitate the development of controllable fabrication for future device applications.

AlGaN UV Detector with Largely Enhanced Heat Dissipation on Mo Substrate Enabled by van der Waals Epitaxy

The epitaxy of AlGaN on metallic substrates exhibits numerous advantages including flexibility, vertical carrier injection, and enhanced heat dissipation for optoelectronic devices; however, there are still many challenges for the growth of AlGaN according to the serious interfacial reaction and lattice mismatch by conventional epitaxial techniques. In this work, the c-oriented AlGaN is grown on polycrystalline Mo substrate by van der Waals (vdWs) epitaxy with graphene as the insertion layer. A high-temperature annealed AlN is deposited as the nucleation layer, which optimizes the crystalline quality of following AlGaN. The isolation effect of the graphene insertion layer results in a good epitaxial interface, which suppresses the atoms diffusion and chemical reaction, demonstrating by the theoretical calculation of high energy barriers for Al/Mo atoms penetrating through graphene insertion layer. The ultraviolet (UV) detector is fabricated by further growing a n-AlGaN layer as the photosensitive absorber, whose responsivity (1.7 × 10–3 A W–1) is comparable with that fabricated on conventional sapphire. Even better, the UV detector on the Mo substrate possesses enhanced heat dissipation ability due to its higher thermal conductivity, and the temperature elevation after consistently applying a drive voltage largely decreases by 50%. This work enlightens the growth of AlGaN materials and fabrication of high-power/high-voltage optoelectronic devices on polycrystalline metallic substrates by the mean of vdWs epitaxy.

Directional growth of quasi-2D Cu2O monocrystals on rGO membranes in aqueous environments.

The preparation technology of unconventional low-dimensional Cu2O monocrystals, which exhibit specific crystal planes and present significantly unique interfacial and physicochemical properties, is attracting increasing attention and interest. Herein, by integrating a high-temperature oxidation process under vacuum and a pure-water incubation process under ambient conditions, we propose the self-assembled growth and synthesis of quasi-two-dimensional Cu2O monocrystals on reduced graphene oxide (rGO) membranes. The prepared Cu2O crystals have a single (110) crystal plane, regular rectangular morphology, and potentially well conductivity. Experimental and theoretical results suggest that this assembly is attributed to the pre-nucleation clusters aggregation and directional attachment of Cu and O on the rGO membranes in aqueous environment and cation-π interactions between the (110) crystal plane of Cu2O and rGO surface. Our findings offer a potential avenue for the discovery and design of advanced low-dimensional single-crystal materials with specific interfacial properties in a pure aqueous environment.

The ultra-thin AlN epitaxy on monolayer WS2 by helicon sputtering at 400 °C

This work demonstrates the epitaxial growth of ultra-thin AlN films grown on monolayer WS2/Sapphire by helicon sputtering. In our work, WS2 acts as a template to facilitate the initial two-dimensional (2D) AlN growth via van der Waals epitaxy (VDWE), and this growth method provides a promising semiconductor manufacturing process with low thermal budget (<450 °C) than the required for the backend-of-the-line (BEOL). The ultrathin AlN epitaxy on WS2 is characterized by synchrotron-based grazing-incidence wide-angle x-ray scattering (GIWAXS). There are two stages in AlN epitaxy on the WS2; the first is the layered AlN growth aligned with the underlying WS2 lattice. The subsequent development of AlN transforms into 3D growth, which is similar to Stranski–Krastanov (SK) mode. The ultrathin AlN films achieve atomic level smoothness and significant piezoelectric properties as indicated by the technique of atomic- and piezo force microscopy (AFM/PFM). This research broadens the application of 2D materials in assisting AlN epitaxy and has a potential avenue for hetero-epitaxial integration in developing future electronic devices.

Direct growth of flexible GaN film via van der Waals epitaxy on mica

A flexible gallium nitride (GaN) thin film was obtained on fluorphlogopite-mica through van der Waals epitaxy using an optimized two-step growing process in a hydride vapor phase epitaxy system. The interfacial stress caused by the mismatches of thermal expansion coefficient was significantly reduced via van der Waals epitaxy. The proposed two-step growth allows such low-stressed GaN thin film forms on mica directly where a critical (minimum) growth pressure was defined and the temperatures chosen were optimized. The transmittance was controlled by surface roughness and growth temperature, as high density of defects near their interface caused by GaN decomposition leading the GaN film non-transparent. In the measurement region, the average transmittance decreased linearly as the GaN surface became rougher. The bending of the flexible GaN thin film caused only slight PL peak shift. Thus, such direct grown GaN film on fluorphlogopite-mica can be evaluated as a reliable optical device.