Stabilizing the metastable superhard material wurtzite boron nitride by three-dimensional networks of planar defects

Phase Stability of Hexagonal/Cubic Boron Nitride Nanocomposites

Boron nitride (BN) is an exceptional material, and among its polymorphs, two-dimensional (2D) hexagonal and three-dimensional (3D) cubic BN (h-BN and c-BN) phases are most common. The phase stability regimes of these BN phases are still under debate, and phase transformations of h-BN/c-BN remain a topic of interest. Here, we investigate the phase stability of 2D/3D h-BN/c-BN nanocomposites and show that the coexistence of two phases can lead to strong nonlinear optical properties and low thermal conductivity at room temperature. Furthermore, spark-plasma sintering of the nanocomposite shows complete phase transformation to 2D h-BN with improved crystalline quality, where 3D c-BN possibly governs the nucleation and growth kinetics. Our demonstration might be insightful in phase engineering of BN polymorph-based nanocomposites with desirable properties for optoelectronics and thermal energy management applications.

Hexagonal Boron Nitride for Next-Generation Photonics and Electronics

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 structural and optical properties of h-BN have expanded potential applications into emerging electronics and photonics fields. h-BN shows a very efficient deep-ultraviolet band-edge emission despite its indirect-bandgap nature, as well as stable room-temperature single-photon emission over a wide wavelength range, showing a great potential for next-generation photonics. In addition, h-BN is extensively being adopted as active media for low-energy electronics, including nonvolatile resistive switching memory, radio-frequency devices, and low-dielectric-constant materials for next-generation electronics.

Direct Determination of Band Gap of Defects in a Wide Band Gap Semiconductor

Crystal defects play an important role in the degradation and failure of semiconductor materials and devices. Direct determination of band gap of defects is a critical step for clarifying how the defects affect the physical properties of semiconductors. Here, high-quality aluminum nitride (AlN) thin films were grown epitaxially on single-crystal Al₂O₃ substrates via pulsed laser deposition. The atomic structure and band gap of three types of inversion domain boundaries (IDBs) in AlN were determined using aberration-corrected transmission electron microscopy and atomic-resolution valence electron energy-loss spectroscopy. It was found that the band gap of all of the IDBs reduces evidently compared to that of the bulk AlN. The maximum band gap reduction of the IDBs is 1.0 eV. First-principles calculations revealed that the band gap reduction of the IDBs is mainly due to the rise of p_z orbital at the valence band maximum, which originates from the elongated Al-N bonds along the [0001] direction at the IDBs. The successful band gap determination of defects paves an avenue for quantitatively evaluating the effect of defects on the performance of semiconductor materials and devices.

Bias-Enhanced Formation of Metastable and Multiphase Boron Nitride Coating in Microwave Plasma Chemical Vapor Deposition

Boron nitride (BN) is primarily a synthetically produced advanced ceramic material. It is isoelectronic to carbon and, like carbon, can exist as several polymorphic modifications. Microwave plasma chemical vapor deposition (MPCVD) of metastable wurtzite boron nitride is reported for the first time and found to be facilitated by the application of direct current (DC) bias to the substrate. The applied negative DC bias was found to yield a higher content of sp³ bonded BN in both cubic and metastable wurtzite structural forms. This is confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). Nano-indentation measurements reveal an average coating hardness of 25 GPa with some measurements as high as 31 GPa, consistent with a substantial fraction of sp³ bonding mixed with the hexagonal sp² bonded BN phase.

Size-dependent fracture behavior of GaN pillars under room temperature compression

Gallium nitride (GaN) offers high electron mobility, breakdown voltage and saturation velocity, and is an ideal candidate for advanced electronic and power devices. Meanwhile, it can also be used for microelectromechanical systems (MEMS) and micro/nano-mechanical devices. These applications fundamentally rely on its mechanical properties and structural reliability, in particular at the micro/nanoscale. In this paper, single crystalline [0001]-oriented GaN pillars with diameters ranging from ∼200 nm to ∼1.5 μm were microfabricated and systematically characterized by in situ compression tests inside a SEM/TEM at room temperature. It showed that a crack would nucleate at the top of the pillars with diameters >800 nm and propagate axially during compression. However, pillars with diameters less than 700 nm would deform plastically without splitting, with maximum stress up to 10 GPa. The corresponding yield/fracture strengths show a strong size effect, which increases from ∼4 GPa to ∼11 GPa with the diameter decreasing from ∼1.5 μm to ∼400 nm. In situ TEM compression tests suggest that the formation of slip bands on the (01[combining macron]11) plane dominates the plastic deformation of the pillars with diameters of ∼200-700 nm, while both crack splitting and slip bands were observed in the pillars with diameters around 700 to 800 nm during the brittle-to-ductile transition. This work provides critical insights for developing robust GaN-based MEMS and power electronic applications.