Hexagonal boron nitride (h-BN), a member of 2D materials, has an analogous crystal structure to graphene, yet, is an insulator having an indirect bandgap of ≈6 eV. [16] h-BN has been explored as the "ideal" substrate and excellent encapsulant [17] for other 2D materials including graphene and transition metal dichalcogenides (TMDs) due to its excellent insulating property, atomically flat surface free of dangling bonds, [18] and charged impurities, and high thermal conductivity. [19,20] Encapsulating other 2D materials with h-BN effectively protects them from local electrical and chemical environment, enabling them to manifest their theoretically expected "intrinsic" properties. In addition, h-BN has been utilized as passive components in 2D vdW electronic devices, such as gate dielectrics and tunneling barriers.Recently, h-BN is attracting intensive research interests, motivated by the variety of outstanding properties, it shows across the fields of quantum optics, [21] electronics, and optoelectronics. [22] For example, despite its indirect-bandgap nature, h-BN showed deep-ultraviolet (DUV) emission with exceptionally high internal quantum efficiency (IQE) comparable to or even higher than single-crystalline direct-bandgap semiconductors. Defects in h-BN are efficient [21,23] and stable sources for single-photon emission over a wide range of wavelengths, from near-infrared (NIR) [24] to nearultraviolet (NUV), [25] at room temperature. The giant lightmatter interaction and strong band-edge absorption offered by h-BN enable the realization of solar-blind DUV photo detection by using h-BN as the active layer where DUV photons are effectively absorbed and create electrical carriers. h-BN has also been adopted as active media for low energy electronic devices, including nonvolatile resistive switching (RS) memory and radio-frequency (RF) devices, in addition to passive components for field-effect transistors (FETs), effective diffusion barriers, and low-k interlayer dielectrics (ILDs).This article aims to review the most recent discoveries of extraordinary properties of h-BN and advancements in emerging photonic and electronic applications. Although there are several well-written review articles on h-BN, [26][27][28][29] this review article focuses on the most recent theoretical discoveries, such as the stacking sequences of h-BN and related optical properties, and the elucidation on how an indirect-bandgap h-BN exhibits strong emission as efficient as a direct-bandgap Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of st...

We demonstrate wafer-scale growth of high-quality hexagonal boron nitride (h-BN) film on Ni(111) template using metal-organic chemical vapor deposition (MOCVD). Compared with inert sapphire substrate, the catalytic Ni(111) template facilitates a fast growth of high-quality h-BN film at the relatively low temperature of 1000 °C. Wafer-scale growth of a high-quality h-BN film with Raman E 2g peak full width at half maximum (FWHM) of 18~24 cm −1 is achieved, which is to the extent of our knowledge the best reported for MOCVD. Systematic investigation of the microstructural and chemical characteristics of the MOCVD-grown h-BN films reveals a substantial difference in catalytic capability between the Ni(111) and sapphire surfaces that enables the selective-area growth of h-BN at pre-defined locations over a whole 2-inch wafer. These achievement and findings have advanced our understanding of the growth mechanism of h-BN by MOCVD and will contribute an important step toward scalable and controllable production of high-quality h-BN films for practical integrated two-dimensional materials-based systems and devices.

We present resistive switching (RS) behavior of few-layer hexagonal boron nitride (h-BN) mediated by defects and interfacial charge transfer. Few-layer h-BN is grown by metal−organic chemical vapor deposition and used as active RS medium in Ti/h-BN/Au structure, exhibiting clear bipolar RS behavior and fast switching characteristics about ∼25 ns without an initial electroforming process. Systematic investigation on microstructural and chemical characteristics of the h-BN reveals that there are structural defects such as homoelemental B−B bonds at grain boundaries and nitrogen vacancies, which can provide preferential pathways for the penetration of Ti x+ ions through the h-BN film. In addition, the interfacial charge transfer from Ti to the h-BN is observed by in situ X-ray photoelectron spectroscopy. We suggest that the attractive Coulomb interaction between positively charged Ti x+ ions and the negatively charged h-BN surface as a result of the interfacial charge transfer facilitates the migration of Ti x+ ions at the Ti/h-BN interface, leading to the facile formation of conductive filaments. We believe that these findings can improve our understanding of the fundamental mechanisms involved in RS behavior of h-BN and contribute a significant step for the future development of h-BN-based nonvolatile memory applications.

Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.

While two-dimensional (2D) hexagonal boron nitride (h-BN) is emerging as an atomically thin and dangling bond-free insulating layer for nextgeneration electronics and optoelectronics, its practical implementation into miniaturized integrated circuits has been significantly limited due to difficulties in large-scale growth directly on epitaxial semiconductor wafers. Herein, the realization of a wafer-scale h-BN van der Waals heterostructure with a 2 in. AlGaN/GaN highelectron mobility transistor (HEMT) wafer using metal−organic chemical vapor deposition is presented. The combination of state-of-the-art microscopic and spectroscopic analyses and theoretical calculations reveals that the heterointerface between ∼2.5 nm-thick h-BN and AlGaN layers is atomically sharp and exhibits a very weak van der Waals interaction without formation of a ternary or quaternary alloy that can induce undesired degradation of device performance. The fabricated AlGaN/GaN HEMT with h-BN shows very promising performance including a cutoff frequency (f T ) and maximum oscillation frequency (f MAX ) as high as 28 and 88 GHz, respectively, enabled by an effective passivation of surface defects on the HEMT wafer to deliver accurate information with minimized power loss. These findings pave the way for practical implementation of 2D materials integrated with conventional microelectronic devices and the realization of future all-2D electronics.