Remote epitaxy has drawn attention as it offers epitaxy of functional materials that can be released from the substrates with atomic precision, thus enabling production and heterointegration of ...flexible, transferrable, and stackable freestanding single-crystalline membranes. In addition, the remote interaction of atoms and adatoms through two-dimensional (2D) materials in remote epitaxy allows investigation and utilization of electrical/chemical/physical coupling of bulk (3D) materials via 2D materials (3D–2D–3D coupling). Here, we unveil the respective roles and impacts of the substrate material, graphene, substrate–graphene interface, and epitaxial material for electrostatic coupling of these materials, which governs cohesive ordering and can lead to single-crystal epitaxy in the overlying film. We show that simply coating a graphene layer on wafers does not guarantee successful implementation of remote epitaxy, since atomically precise control of the graphene-coated interface is required, and provides key considerations for maximizing the remote electrostatic interaction between the substrate and adatoms. This was enabled by exploring various material systems and processing conditions, and we demonstrate that the rules of remote epitaxy vary significantly depending on the ionicity of material systems as well as the graphene–substrate interface and the epitaxy environment. The general rule of thumb discovered here enables expanding 3D material libraries that can be stacked in freestanding form.
Van der Waals (vdW) 2D/3D heterostructures are extensively studied for high‐performance photodetector applications. Until now, the type of 2D materials has been the primary area of interest rather ...than the design of 3D semiconductors. In this study, high‐speed broadband photodiodes (PDs) based on vdW p‐WSe2/n‐Ge heterojunctions are reported, and the performance compared with different n‐Ge regions formed via the ion‐implantation process. The fabricated PD with a typical long n‐Ge region and low doping concentration responds to a broad spectral range from visible to infrared near 1550 nm with a response time of ≈3 µs and responsivity of 1.3 A W−1. The inferior responsivity of PDs with short n‐Ge regions can be improved as demonstrated by experimental results and process simulation. Density functional theory calculations are performed to estimate the variation of the energy band structures with the doping concentration of n‐Ge. Fast photoresponse and efficient carrier separation across the heterojunction can be expected regardless of the n‐Ge doping concentration. Based on the experimental results together with theoretical band structure and process simulation, it is shown that the heterojunction with an optimized n‐Ge design is a promising high‐speed broadband photodetector that can be implemented with complementary metal‐oxide‐semiconductor design and fabrication processes.
A high‐speed broadband photodetector based on vdW p‐WSe2/n‐Ge heterojunction is demonstrated, and the performance is compared in the design aspect of Ge semiconductors. The results not only show that the p‐WSe2/n‐Ge heterojunction can be a promising high‐speed broadband photodetector up to near‐infrared implementable in complementary metal‐oxide‐semiconductor design and process but also offers design strategies for photodetectors based on 2D/3D heterojunctions.
Complex-oxide materials exhibit a vast range of functional properties desirable for next-generation electronic, spintronic, magnetoelectric, neuromorphic, and energy conversion storage devices
. ...Their physical functionalities can be coupled by stacking layers of such materials to create heterostructures and can be further boosted by applying strain
. The predominant method for heterogeneous integration and application of strain has been through heteroepitaxy, which drastically limits the possible material combinations and the ability to integrate complex oxides with mature semiconductor technologies. Moreover, key physical properties of complex-oxide thin films, such as piezoelectricity and magnetostriction, are severely reduced by the substrate clamping effect. Here we demonstrate a universal mechanical exfoliation method of producing freestanding single-crystalline membranes made from a wide range of complex-oxide materials including perovskite, spinel and garnet crystal structures with varying crystallographic orientations. In addition, we create artificial heterostructures and hybridize their physical properties by directly stacking such freestanding membranes with different crystal structures and orientations, which is not possible using conventional methods. Our results establish a platform for stacking and coupling three-dimensional structures, akin to two-dimensional material-based heterostructures, for enhancing device functionalities
.
Recent advances in flexible and stretchable electronics have led to a surge of electronic skin (e-skin)–based health monitoring platforms. Conventional wireless e-skins rely on rigid integrated ...circuit chips that compromise the overall flexibility and consume considerable power. Chip-less wireless e-skins based on inductor-capacitor resonators are limited to mechanical sensors with low sensitivities. We report a chip-less wireless e-skin based on surface acoustic wave sensors made of freestanding ultrathin single-crystalline piezoelectric gallium nitride membranes. Surface acoustic wave–based e-skin offers highly sensitive, low-power, and long-term sensing of strain, ultraviolet light, and ion concentrations in sweat. We demonstrate weeklong monitoring of pulse. These results present routes to inexpensive and versatile low-power, high-sensitivity platforms for wireless health monitoring devices.
Chip-less electronic skin
Flexible electronic materials, or e-skins, can be limited by the need to include rigid components. A range of techniques have emerged to bypass this problem, including approaches for wireless communication and charging based on silicon, carbon nanotubes, or conducting polymers. Kim
et al
. show that epitaxially grown, single-crystalline gallium nitride films on flexible substrates can be used for chip-less, flexible e-skins. The main advantage is that the material is flexible and breathable, thus providing better comfort. The devices convert electrical energy into surface acoustic waves using a piezoelectric resonator. The resonator is sensitive to changes in strain, mass changes due to the absorption or loss of ions, and ultraviolet light, all of which can be used for different sensing measurements. —MSL
Single-crystalline gallium nitride nanomembranes enable high-sensitivity surface acoustic wave sensors for wireless electronic skin.
Micro-LEDs (µLEDs) have been explored for augmented and virtual reality display applications that require extremely high pixels per inch and luminance
. However, conventional manufacturing processes ...based on the lateral assembly of red, green and blue (RGB) µLEDs have limitations in enhancing pixel density
. Recent demonstrations of vertical µLED displays have attempted to address this issue by stacking freestanding RGB LED membranes and fabricating top-down
, 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 technique
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, while also offering a generalizable platform for broader classes of three-dimensional integrated devices.
Abstract
Complex-oxide materials are gaining a tremendous amount of interest in the semiconductor materials and device community as they hold many useful intrinsic physical properties such as ...ferro/piezoelectricity, pyroelectricity, ferromagnetism, as well as magnetostriction and other properties suitable for energy storage elements. Complex-oxides can also be complemented with conventional semiconductor-based devices or used by themselves to realize state-of-the-art electronic/photonic/quantum information devices. However, because complex-oxide materials have vastly different crystalline structures and lattice constant difference compared to conventional semiconductor devices (such as Si or III-V/III-N materials), integration of complex-oxides onto conventional semiconductor platforms has been difficult. Thus, there has been constant efforts to produce freestanding single-crystalline complex-oxide thin films such that these films can be transferred and integrated together with device platforms based on other materials. This review will provide a comprehensive review on single-crystalline complex-oxide membranes technology developed thus far: how they are synthesized, methods to release them from the substrate, and their outstanding properties and applications.
Two-dimensional (2D) materials and their heterostructures show a promising path for next-generation electronics
. Nevertheless, 2D-based electronics have not been commercialized, owing mainly to ...three critical challenges: i) precise kinetic control of layer-by-layer 2D material growth, ii) maintaining a single domain during the growth, and iii) wafer-scale controllability of layer numbers and crystallinity. Here we introduce a deterministic, confined-growth technique that can tackle these three issues simultaneously, thus obtaining wafer-scale single-domain 2D monolayer arrays and their heterostructures on arbitrary substrates. We geometrically confine the growth of the first set of nuclei by defining a selective growth area via patterning SiO
masks on two-inch substrates. Owing to substantial reduction of the growth duration at the micrometre-scale SiO
trenches, we obtain wafer-scale single-domain monolayer WSe
arrays on the arbitrary substrates by filling the trenches via short growth of the first set of nuclei, before the second set of nuclei is introduced, thus without requiring epitaxial seeding. Further growth of transition metal dichalcogenides with the same principle yields the formation of single-domain MoS
/WSe
heterostructures. Our achievement will lay a strong foundation for 2D materials to fit into industrial settings.
Heterogeneous integration of single-crystal materials offers great opportunities for advanced device platforms and functional systems1. Although substantial efforts have been made to co-integrate ...active device layers by heteroepitaxy, the mismatch in lattice polarity and lattice constants has been limiting the quality of the grown materials2. Layer transfer methods as an alternative approach, on the other hand, suffer from the limited availability of transferrable materials and transfer-process-related obstacles3. Here, we introduce graphene nanopatterns as an advanced heterointegration platform that allows the creation of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects, ranging from non-polar materials to polar materials and from low-bandgap to high-bandgap semiconductors. Additionally, we unveil unique mechanisms to substantially reduce crystallographic defects such as misfit dislocations, threading dislocations and antiphase boundaries in lattice- and polarity-mismatched heteroepitaxial systems, owing to the flexibility and chemical inertness of graphene nanopatterns. More importantly, we develop a comprehensive mechanics theory to precisely guide cracks through the graphene layer, and demonstrate the successful exfoliation of any epitaxial overlayers grown on the graphene nanopatterns. Thus, this approach has the potential to revolutionize the heterogeneous integration of dissimilar materials by widening the choice of materials and offering flexibility in designing heterointegrated systems.Epitaxy on nanopatterned graphene enables the realization of a broad spectrum of freestanding single-crystalline membranes with substantially reduced defects.
