Advances in semiconductor technology have been primarily driven by exponentially reducing the size of silicon transistors and pushing the quantum limit. However, continued scaling becomes extremely difficult in accordance with Moore's law. Conversely, recent advances in monolithic and heterogeneous integration by exploring non-group IV materials envision beyond CMOS scaling. This study entails the development of scalable van der Waals (vdW) integration technology by using all CMOS back-end-of-line-compatible processes: vertical 3D and lateral 2D integration of III-N devices, 2D materials (graphene and molybdenum disulfide), and CMOS. Advanced fluidic-assisted self-alignment transfer (FAST) provides a process accuracy of ≈ 32.6 nm as analyzed on a 200 mm wafer scale. The freestanding III-N chips are vdW integrated onto 2D materials, and the vdW interfaced multi-layer graphene successfully functioned as a back-gating interconnect line. Moreover, fidelity of the vdW interface is confirmed by conducting systematic yield, uniformity, and reliability analysis. The unique fourfold rotationally symmetric design of GaN transistors makes them compatible with massive and random FAST processing. GaN-based radio-frequency power and cascode GaN/Si transistors are integrated on silicon-on-insulator-CMOS. The proposed approach affords a remarkable advantage by surpassing the physical limits and facilitating functional diversification, thus advancing the concept of "More than Moore."

Recent advances in mass transfer technology are expected to bring next-generation micro light-emitting diodes (µLED) displays into reality, although reliable integration of the active-matrix backplane with the transferred µLEDs remains as a challenge. Here, the µLED display technology is innovated by demonstrating pixel circuit-integrated micro-LEDs (PIMLEDs) and integrating them onto a transparent glass substrate. The PIMLED comprises of low-temperature poly-silicon transistors and GaN µLED. The square-shaped PIMLED is designed to secure a larger process margin but to reduce misalignment in the subsequent bonding process. Its unique four-fold rotational symmetric design together with concentric circular pixel electrodes of the substrate makes their massive transfer compatible with intrinsic randomness of fluidic-based transfer and free from angular misalignment during wafer bonding. The vertically integrated pixels show similar optical and electrical properties regardless of four possible arrangements. It is demonstrated that a 96 × 96 PIMLED display on a transparent glass substrate, which is expected to open a door to novel form-factor free and transparent µLED displays.

Understanding the complexity of grain boundaries between domains is essential for controlling material properties. While grain boundaries in two-dimensional (2D) materials have revealed a few cases of unique features with chemical reactivity and electronic structures, the intriguing case of one-dimensional (1D) grain boundaries still remains relatively unexplored, in particular, for non-hexagonal structures. Here, sliding grain boundary formation in 1T’-WTe2 has been investigated at the atomic scale. We found that the grain boundary keeps W-Te zigzag atomic rows in one direction. The asymmetric 1D sliding grain boundary formations exhibited an angle of 38° relative to the W-Te zigzag atomic rows, strain near the sliding grain boundary formation, and fluctuations in the local density of states (LDOS). The electronic structure in the asymmetric 1D sliding grain boundary formation shows two-line features in LDOS mapping. The structural models under the directional constraint were constructed with two symmetry operations, sliding and 180°-rotation, which agree well with the experimental results. Calculation of formation energy for the models suggested that the grain boundary formation was formed by 180°-rotated domains meeting during their growth along with sliding. The understanding of the sliding grain boundary formation provides a promising path to chemical applications such as hydrogen evolution reactions.

The emergence of artificial intelligence, big data processing, electrical vehicle technologies and so on necessitates a new approach to address the scaling, power efficiency and performance challenges of silicon (Si)-based technology beyond Moore’s law. As a complementary technology, wide bandgap semiconductors, including GaN and SiC, have attracted great attention owing to their unique features of high carrier mobility and high breakdown voltages. However, there are still limitations for widespread applications of wide bandgap semiconductors including scalability, high production cost and thermal management. To overcome these barriers, remote epitaxy and 2D layer transfer technology have been introduced and are in the process of being industrialized to produce single-crystalline semiconductor-based freestanding membranes. In this Perspective, we present the status and challenges for manufacturing GaN and SiC membranes based on remote epitaxy technology that offers significant advantages via wafer reuse and high-quality freestanding epilayer production. We also discuss how industrialization of advanced membrane technology can benefit numerous applications, including heterogeneously integrated circuits, power and radiofrequency systems. This Perspective discusses the status and challenges of remote epitaxy technology towards industrialization of wide bandgap semiconductors for the future electronics.

Despite the widespread use of charge-trap flash (CTF) memory, the atomistic mechanism behind the exceptionally stable charge storage at the localized trap sites is still controversial. Herein, by combining first-principles calculations and orbital interaction analysis, a charge-dependent switchable chemical-bond reorganization is elucidated as the underpinning chemistry in the working mechanism of CTF. Especially, positively charged fourfold-coordinated nitrogen (dubbed N+ center), unappreciated until now, is the decisive component of the entire process; once an electron occupies this site, the N+ center disappears by breaking one N─Si bond, simultaneously forming a new Si─Si bond with a nearby Si atom which, in turn, creates fivefold coordinated Si. As a result, the electron is stored in a multi-center orbital belonging to multiple atoms including the newly formed Si─Si bond. It is also observed that hole trapping accompanies the creation of an N+ center by forming a new N─Si bond, which represents the reverse process. To further support and validate this model by means of core-level calculations, it is also shown that an N+ center's 1s core level is 1.0-2.5 eV deeper in energy than those of the threefold coordinated N atoms, in harmony with experimental X-ray photoelectron spectroscopy data.

Electrides are ionic compounds in which electrons behave as anions in the interior of a positively charged framework. As a layered electride, ${\mathrm{Gd}}_{2}\mathrm{C}$ receives attention because of its ferromagnetism. Although previous research has focused on the bulk properties of ${\mathrm{Gd}}_{2}\mathrm{C}$, few studies have focused on ultrathin layers or surfaces for two-dimensional (2D) characteristics. Here, we report a first-principles study of the electronic properties of few-layer ${\mathrm{Gd}}_{2}\mathrm{C}$ structures. ${\mathrm{Gd}}_{2}\mathrm{C}$ has a work function of 3.35 eV. When a layered electride is exfoliated, the interstitial layer becomes a surface and may be exposed to the outside. Because the interlayer region has changed to the surface, the properties of the electron gases once located in the interlayer in the past will also change. We found that the surface anionic electrons accounted for about 25% of the number of electrons in the interlayer region in the absence of an external electric field. When we applied an external electric field, the number of surface electrons increased, and the increase was proportional to the square of the field intensity. Since the electronic properties of 2D materials can be understood through scanning tunneling spectroscopy (STS), we also performed the STS simulations. At $\ensuremath{-}0.9\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, the STS image was blurred because of surface anionic electrons. In contrast to the spin-up electron, an interlayer band of the spin-down electron crossed the Fermi level in the ultrathin ${\mathrm{Gd}}_{2}\mathrm{C}$ layers. Our findings open a possibility that the spin-polarized electronic gas in the few-layer electride could be used for spintronics.

Robust multi-level spin memory with the ability to write information electrically is a long-sought capability in spintronics, with great promise for applications. Here, nonvolatile and highly energy-efficient magnetization switching is achieved in a single-material device formed of van-der-Waals (vdW) topological ferromagnet Fe3GeTe2, whose magnetic information can be readily controlled by a tiny current. Furthermore, the switching current density and power dissipation are about 400 and 4000 times smaller than those of the existing spin-orbit-torque magnetic random access memory based on conventional magnet/heavy-metal systems. Most importantly, multi-level states, switched by electrical current are also demonstrated, which can dramatically enhance the information capacity density and reduce computing costs. Thus, the observations combine both high energy efficiency and large information capacity density in one device, showcasing the potential applications of the emerging field of vdW magnets in the field of spin memory and spintronics.

Intrinsic defects in graphitic materials, like vacancies and edges, have been expected to possess magnetic states from the many-body interaction of localized electrons. However, charge screening from graphite bulk carriers significantly reduces the localization effect and hinders the observation of those magnetic states. Here, we use an ultra-low-temperature scanning tunneling microscope with a high magnetic field to observe the magnetic states of atomic vacancies in graphite generated by ion sputtering. Scanning tunneling spectroscopy reveals localized states at the vacancies, which exhibit splitting at a certain magnetic field whose separation increases with the field strength. The transition is well described by the “Anderson model,” which describes the emergence of localized magnetic states inside the metallic reservoir through electron–electron interaction. The interaction strength is estimated to be between 1 meV and 3 meV, which is supported by the density functional theory calculation. The observation provides an important foundation for application of intrinsic defects to carbon-based spintronic devices.

Using scanning tunneling microscopy/spectroscopy (STM/STS), we examine quasiparticle scattering and interference properties at the surface of WTe2. WTe2, layered transition metal dichalcogenide, is predicted to be a type-II Weyl semimetal. The Weyl fermion states in WTe2 emerge as topologically protected touching points of electron and hole pockets, and Fermi arcs connecting them can be visible in the spectral function on the surface. To probe the properties of surface states, we have conducted low-temperature STM/STS (at 2.7 K) on the surfaces of WTe2 single crystals. We visualize the surface states of WTe2 with atomic scale resolution. Clear surface states emerging from the bulk electron pocket have been identified and their connection with the bulk electronic states shows good agreement with calculations. We show the interesting double resonance peaks in the local density of states appearing at localized impurities. The low-energy resonant peak occurs near the Weyl point above the Fermi energy and it may be mixed with the surface state of Weyl points, which makes it difficult to observe the topological nature of the Weyl semimetal WTe2.

The one-dimensional form of silicon (Si) has been attracting significant scientific and industrial interest again in the field of Li-ion batteries as well as electronic devices, offering higher reversible capacity than carbon materials in an anode. In this work, a new method to grow a uniform Si nanowire mat without metallic nanoparticles was developed, and the growth conditions and conformational properties were also characterized. It is suggested that Si nanowire could be grown by the charge transfer through the hexagonal boron nitride (h-BN) film from the Cu foil as the previously reported graphene growth on the h-BN film on Cu. Web-like bundles of Si nanowires or nanoclusters could be selectively grown on this h-BN/Cu substrate by controlling the substrate temperature during Si deposition. The morphology and chemical composition of the Si nanowire mats, therefore, were systematically characterized using scanning tunneling microscopy, atomic force microscopy, and X-ray photo-emission spectroscopy. When decoupled from the metal by a thin h-BN film during the growth, this pristine Si nanowire mat can provide promising technical breakthroughs for anode applications in Li-ion batteries.

The widely-studied ferromagnetic van-der-Waals (vdW) metal Fe3GeTe2 has great promise for studies of quantum criticality in the 2D limit, but is limited by a relatively high Curie temperature in excess of 200 K. To help render the quantum critical point achievable in such a system within the reach of practically possible tuning methods, we have grown single crystals of a variant of (Fe,Co)3GeTe2 with useful physical properties for both this purpose and the wider study of low-dimensional magnetism and spin transport. (Fe,Co)3GeTe2 is found through x-ray diffraction and electron microscopy to have an equivalent crystal structure to Fe3GeTe2, with a random distribution of the cobalt dopant sites. It exhibits a sharp ferromagnetic transition at a value below 40 K, a stronger anisotropy and a coercive field ten times larger than that of Fe3GeTe2. The transport properties and specific heat show the electronic properties and strong correlations of Fe3GeTe2 to be near-unchanged in this doped material. We demonstrate that (Fe,Co)3GeTe2 can be cleanly exfoliated down to monolayer thickness. This unprecedented hard metallic vdW ferromagnet is a valuable new addition to the limited range of materials available for the study of 2D magnetism.

The direct growth of graphene on a semiconducting substrate opens a new avenue for future graphene-based applications. Understanding the structural and electronic properties of the graphene on a semiconducting surface is key for realizing such structures; however, these properties are poorly understood thus far. Here, we provide insight into the structural and electronic properties of graphene grown directly on a Ge(110) substrate. Our scanning tunneling microscopy (STM) study reveals that overlaying graphene on Ge(110) promotes the formation of a new Ge surface reconstruction, i.e., a (6 × 2) superstructure, which has been never observed for a bare Ge(110) surface. The electronic properties of the system exhibit the characteristics of both graphene and Ge. The differential conductance (d I/d V) spectrum from a scanning tunneling spectroscopy (STS) study bears a parabolic structure, corresponding to a reduction in the graphene Fermi velocity, exhibiting additional peaks stemming from the p-orbitals of Ge. The density functional theory (DFT) calculations confirm the existence of surface states due to the p-orbitals of Ge.

Electrides are ionic compounds in which electrons confined in the interstitial spaces serve as anions and are attractive owing to their exotic physical and chemical properties in terms of their low work function and efficient charge-transfer characteristics. Depending on the topology of the anionic electrons, the surface electronic structures of electrides can be significantly altered. In particular, the electronic structures of two-dimensional (2D) electride surfaces are of interest because the localized anionic electrons at the interlayer space can be naturally exposed to cleaved surfaces. In this paper, we report the electronic structure of 2D Y2C electride surface using scanning tunneling microscopy (STM) and first-principles calculations, which reveals that anionic electrons at a cleaved surface are absorbed by the surface and subsequently resurged onto the surface due to an applied electric field. We highlight that the estranged anionic electrons caused by the electric field occupy the slightly shifted crystallographic site compared with a bulk Y2C electride. We also measure the work function of the Y2C single crystal, and it shows a slightly lower value than the calculated one, which appears to be due to the electric field from the STM junction.

Silicene, a silicon allotrope with a buckled honeycomb lattice, has been extensively studied in the search for materials with graphene-like properties. Here, we study the domain boundaries of a silicene 4 × 4 superstructure on an Ag(111) surface at the atomic resolution using scanning tunneling microscopy (STM) and spectroscopy (STS) along with density functional theory calculations. The silicene domain boundaries (β-phases) are formed at the interface between misaligned domains (α-phases) and show a bias dependence, forming protrusions or depressions as the sample bias changes. In particular, the STM topographs of the silicene–substrate system at a bias of ∼2.0 V show brightly protruding domain boundaries, which can be explained by an energy state originating from the Si 3s and 3pz orbitals. In addition, the topographs depicting the vicinity of the domain boundaries show that the structure does not follow the buckled geometry of the atomic ball-and-stick model. Inside the domain, STS data showed a step-up at ∼0.4 V, which originated from the Si 3p orbitals. We found this step-up to have shifted, which may be attributed to the strain effect at the interface regions between silver and silicene and between the domain and its boundary upon performing spatially resolved STS measurements. The metastable characteristic of the domain boundary (β-phase) causes changes, such as creation or annihilation, in the buckling structures (switching behavior). The observed low activation energy for the buckling change between distinct states may find applications in the electronic control of properties related to domain boundary structures in silicene.

Despite recent efforts for the development of transition-metal-dichalcogenide-based high-performance thin-film transistors, device performance has not improved much, mainly because of the high contact resistance at the interface between the 2D semiconductor and the metal electrode. Edge contact has been proposed for the fabrication of a high-quality electrical contact; however, the complete electronic properties for the contact resistance have not been elucidated in detail. Using the scanning tunneling microscopy/spectroscopy and scanning transmission electron microscopy techniques, the edge contact, as well as the lateral boundary between the 2D semiconducting layer and the metalized interfacial layer, are investigated, and their electronic properties and the energy band profile across the boundary are shown. The results demonstrate a possible mechanism for the formation of an ohmic contact in homojunctions of the transition-metal dichalcogenides semiconductor-metal layers and suggest a new device scheme utilizing the low-resistance edge contact.

We report the local observation of the band structure of topological surface states in Bi1.5Sb0.5Te1.7Se1.3 using scanning tunneling microscopy/spectroscopy (STM/STS). The energy-momentum dispersion relation is locally deduced by extracting the Landau level (LL) energies, which are formed in a high magnetic field, from the STS data. Spatial variation of LLs revealed a shift of the Dirac point energy at the nanometer scale. The structure of the potential fluctuation was not correlated with the topography, which indicated that the Te/Se substitution did not induce the potential shift because of their same valence. The results show that disorders from the Te/Se substitution at the surface do not induce any localized charged states and do not affect topological surface states.

Phonons, which are collective excitations in a lattice of atoms or molecules, play a major role in determining various physical properties of condensed matter, such as thermal and electrical conductivities. In particular, phonons in graphene interact strongly with electrons; however, unlike in usual metals, these interactions between phonons and massless Dirac fermions appear to mirror the rather complicated physics of those between light and relativistic electrons. Therefore, a fundamental understanding of the underlying physics through systematic studies of phonon interactions and excitations in graphene is crucial for realising graphene-based devices. In this study, we demonstrate that the local phonon properties of graphene can be controlled at the nanoscale by tuning the interaction strength between graphene and an underlying Pt substrate. Using scanning probe methods, we determine that the reduced interaction due to embedded Ar atoms facilitates electron–phonon excitations, further influencing phonon-assisted inelastic electron tunnelling.

Pinning single molecules at desired positions can provide opportunities to fabricate bottom-up designed molecular machines. Using the combined approach of scanning tunneling microscopy and density functional theory, we report on tip-induced anchoring of Ni-phthalocyanine molecules on an Au(111) substrate. We demonstrate that the tip-induced current leads to the dehydrogenation of a benzene-like ligand in the molecule, which subsequently creates chemical bonds between the molecule and the substrate. It is also found that the diffusivity of Ni-phthalocyanine molecules is dramatically reduced when the molecules are anchored on the Au adatoms produced by bias pulsing. The tip-induced molecular anchoring would be readily applicable to other functional molecules that contain similar ligands.