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We present a novel approach for growth of surface-directed spinel ZnGa2O4 and β-Ga2O3 nanofins coated with a non-polar GaN shell. Our results show that the use of a binary compound such as core–shell nanostructures as a starting material is not necessary to promote the Kirkendall effect. Our starting materials include ZnO fins and gaseous Ga atoms under nitrogen plasma. The surface-directed ZnO fins provide a spatial confinement to the Kirkendall effect by which the Zn is exchanged with Ga via a vacancy-assisted mechanism. The results show that at a higher Ga concentration, the wurtzite ZnO fin is converted to a ZnGa2O4 cubic spinel, while a lower Ga concentration leads to monoclinic Ga2O3 fins. The fin transformation is followed by GaN shell overgrowth on the fin side walls with a non-polar surface. Within the newly formed fins, we observe uniform Kirkendall nanochannels that are laterally formed between their side walls. This method offers the opportunity of growing heterojunctions of a broad range of wide bandgap spinel materials with GaN. The predictability over surface registries of the core/shell fins and their tunable porosity are anticipated to be of significance in a wide range of applications in chemical- and electro-optical based sensing as well as high power electronics.

Monolayer hexagonal boron nitride (hBN) has been widely considered a fundamental building block for 2D heterostructures and devices. However, the controlled and scalable synthesis of hBN and its 2D heterostructures has remained a daunting challenge. Here, an hBN/graphene (hBN/G) interface-mediated growth process for the controlled synthesis of high-quality monolayer hBN is proposed and further demonstrated. It is discovered that the in-plane hBN/G interface can be precisely controlled, enabling the scalable epitaxy of unidirectional monolayer hBN on graphene, which exhibits a uniform moiré superlattice consistent with single-domain hBN, aligned to the underlying graphene lattice. Furthermore, it is identified that the deep-ultraviolet emission at 6.12 eV stems from the 1s-exciton state of monolayer hBN with a giant renormalized direct bandgap on graphene. This work provides a viable path for the controlled synthesis of ultraclean, wafer-scale, atomically ordered 2D quantum materials, as well as the fabrication of 2D quantum electronic and optoelectronic devices.

We have investigated the nature of Al0.56Ga0.44N/Al0.62Ga0.38N multiquantum wells grown by plasma-assisted molecular beam epitaxy for application in deep-ultraviolet light emitters. Excitation and temperature-dependent and time-resolved photoluminescence measurements and transmission and reflectance spectroscopy have been complemented by high-angle annular dark field scanning transmission electron microscopy. The 3 nm quantum wells are characterized by interface roughness having a height of 0.3–1 nm and the maximum value is in excellent agreement with values obtained from calculations done to analyze the measured photoluminescence lineshape. The radiative lifetime increases with temperature, suggesting the role of electron-hole scattering to cool photoexcited carriers to the ground state of the quantum wells.

We have performed a detailed investigation of the molecular beam epitaxy of AlGaN epilayers on AlN template on sapphire substrate. The effects of various growth parameters, including nitrogen flow rate, and Al and Ga fluxes on tuning the Al composition of these epilayers are studied. An atomically smooth surface with a root mean square roughness of ∼0.4 nm was measured for ∼0.5-μm thick Al0.6Ga0.4N when grown under slightly metal-rich conditions. It was observed that dislocation propagation originating from the substrate layers can effectively be reduced or terminated in AlGaN epilayers with the incorporation of sub-nm interfaces. A refractive index contrast of 0.09 with the underlying AlN template was measured at a wavelength of 400 nm, making it suitable for waveguiding applications.

We demonstrate high quality factor (Q) ring resonators near 780 nm wavelength (Q ~170000), and at 532 nm green wavelength (Q ~100,000) using aluminum nitride on sapphire platform.

A multi-functional photocatalyst, that can combine the catalytic functions of water oxidation and proton reduction together with light harvesting capacity, is highly desired for low cost, high efficiency, and highly stable solar fuel production. Monolayer WSe2, with a direct energy gap of ~ 1.65 eV is a nearly ideal light absorber to convert sunlight to hydrogen fuels through solar water splitting. To date, however, the controlled synthesis of monolayer WSe2 on a wafer scale and the realization of overall water splitting on WSe2 have remained elusive. Here, we report the van de Waals epitaxy of crystalline monolayer WSe2 on large area amorphous SiOx substrates. We have demonstrated, for the first time, the multi-functionality of monolayer WSe2 in solar water splitting, including extraordinary capacities for efficient light harvesting, water oxidation, and proton reduction. The absorbed photon conversion efficiency exceeds 12% for a single monolayer WSe2. This work provides a viable strategy for wafer-scale synthesis of multi-functional photocatalysts for the development of efficient, low cost, and scalable solar fuel devices and systems.

Aluminum-rich AlGaN is the ideal material system for emerging solid-state deep-ultraviolet (DUV) light sources. Devices operating in the near-UV spectral range have been realized; to date, however, the achievement of high-efficiency light-emitting diodes (LEDs) operating in the UV-C band (200–280 nm specifically) has been hindered by the extremely inefficient p-type conduction in AlGaN and the lack of DUV-transparent conductive electrodes. Here, we show that these critical challenges can be addressed by Mg dopant-free Al(Ga)N/h-BN nanowire heterostructures. By exploiting the acceptor-like boron vacancy formation, we have demonstrated that h-BN can function as a highly conductive, DUV-transparent electrode; the hole concentration is ∼1020 cm–3 at room temperature, which is 10 orders of magnitude higher than that previously measured for Mg-doped AlN epilayers. We have further demonstrated the first Al(Ga)N/h-BN LED, which exhibits strong emission at ∼210 nm. This work also reports the first achievement of M...

We demonstrate low-loss aluminum nitride waveguides and ring resonators on sapphire. The ring resonators on this platform shows a high quality factor above 2.8 million at 1550 nm and above 120,000 at 780 nm.

One-dimensional semiconductor monolayer crystals are optimized to deliver a nearly perfect absorption over a broad spectral range by combining two complementary design schemes.

Aluminum-nitride-on-sapphire has recently emerged as a novel low-loss photonics platform for a variety of on-chip electro-optics as well as linear and nonlinear optics applications. In this Letter, we demonstrate ultrahigh quality factor (Qint) microring resonators using single-crystal aluminum nitride grown on a sapphire substrate with an optimized design and fabrication process. A record high intrinsic Qint up to 2.8×106 at the wavelength of 1550 nm is achieved with a fully etched structure, indicating a low propagation loss less than 0.13 dB/cm. Such high Qint aluminum-nitride-on-sapphire resonators with their wide bandgap and electro-optical and nonlinear optical properties is promising for a wide range of low-power and high-power compact on-chip applications over a broad spectral range.

We report the demonstration of ultra-high Q (>2.5 ×106) ring resonators made of fully-etched single crystal aluminum nitride waveguides on sapphire substrate, at 1550nm wavelength, enabling a new class of compact classical/quantum photonic devices.

We demonstrate microring resonators with high-Q using single crystal aluminum nitride directly grown on sapphire substrate. Initial measurement results show a Q-factor of 75, 000 at 770 nm and up to 1,100,000 at 1550 nm.

Dislocation-free III-nitride nanocrystals have emerged as the fundamental building blocks of micro LEDs, low threshold ultraviolet laser diodes, and high efficiency artificial photosynthesis. We discuss the epitaxy, structural, electronic, and optical properties of III-nitride nanocrystals. We also report on the demonstration of electrically pumped low threshold ultraviolet laser diodes with the use of such dislocation-free nanocrystals. Moreover, we have demonstrated the first ever artificial photosynthesis system that is capable of direct solar water splitting with stability >500 hrs at an efficiency level >2%.

A fundamental understanding and control of impurity incorporation and charge carrier recombination are critical for emerging ScxAl1−xN electronics, optoelectronics, and photonics. We report on the photoluminescence properties of ScxAl1−xN grown by plasma-assisted molecular beam epitaxy with varying growth temperatures and Sc contents. Bright and broad emission comprising a dominant peak at ∼3.52 eV and a weak peak at ∼2.90 eV was observed in Sc0.05Al0.95N. The origin of the ∼3.52 eV emission line is attributed to charge carrier recombination from the localized excited state of (Vcation-ON)2−/− to its ground state, whereas the second peak at ∼2.90 eV results from charge carrier recombination of isolated V cation 3 − / 2 − to the valence band. We further show that oxygen defect-related emission can be significantly suppressed by increasing growth temperature. This work sheds light on the recombination dynamics of photoexcited carriers in ScxAl1−xN and further offers insight into how to improve the optical and electrical properties of ScxAl1−xN that are relevant for a broad range of applications.

Quantum many-body theory is applied to discover the design principles of broadband absorbers based on III-nitride or transition metal dichalcogenide monolayer crystals. A combination of color-chirped exciton absorption and continuum-absorption-based coupling strategies is outlined and demonstrated to enable hyperspectral absorption with several eV bandwidth. The found structures are only a few micrometers thick and dominantly support direct generation of free carriers at optimal photovoltages.

Hexagonal boron nitride (h-BN) has shown tremendous promise when used alongside other two-dimensional (2D) materials such as graphene, and as a wide-bandgap semiconductor for deep-ultraviolet optoelectronics and quantum photonics. Owing to its large bandgap energy comparable or higher than Al(Ga)N, h-BN can be used to form heterostructures to address some of the critical challenges of Al(Ga)N-based systems. To date, however, many fundamental material properties of h-BN remain unknown. For example, its bandgap energy remains under debate, with reports spanning from 5.9 to 6.5 eV. There is also strong controversy as to whether h-BN has a direct or indirect bandgap. In this context, We have studied the epitaxy of h-BN by using ultrahigh temperature (up to 1850 °C) plasma-assisted molecular beam epitaxy (MBE) on sapphire, Ni and HOPG substrates. We show that, when grown at sufficiently high temperature, h-BN can exhibit predominantly excitonic emission at ~220 nm. The measured luminescence intensity is orders of magnitude higher than AlN under identical conditions. By forming a p-i-n structure using this high-quality h-BN as the active region, the current-voltage (I-V) and electroluminescence characteristics of a first demonstration of a h-BN deep UV LED will be reported.

We report on the van der Waals epitaxy of high-quality single-crystalline AlN and the demonstration of AlGaN tunnel junction deep-ultraviolet light-emitting diodes directly on graphene, which were achieved by using plasma-assisted molecular beam epitaxy. It is observed that the substrate/template beneath graphene plays a critical role in governing the initial AlN nucleation. In situ reflection high energy electron diffraction and detailed scanning transmission electron microscopy studies confirm the epitaxial registry of the AlN epilayer with the underlying template. Detailed studies further suggest that the large-scale parallel epitaxial relationship for the AlN epilayer grown on graphene with the underlying template is driven by the strong surface electrostatic potential of AlN. The realization of high-quality AlN by van der Waals epitaxy is further confirmed through the demonstration of AlGaN deep-ultraviolet light-emitting diodes operating at ∼260 nm, which exhibit a maximum external quantum efficiency of 4% for an unpackaged device. This work provides a viable path for the van der Waals epitaxy of ultra-wide bandgap semiconductors, providing a path to achieve high performance deep-ultraviolet photonic and optoelectronic devices that were previously difficult.

We demonstrate the growth of pure wurtzite phase ScxAl1−xN with a Sc composition as high as x = 0.34 on GaN and AlN templates using plasma-assisted molecular beam epitaxy. The wurtzite structure is well maintained even at high growth temperatures up to 900 °C for Sc0.2Al0.8N. Smooth surface morphology (root mean square roughness less than 1 nm) and excellent crystal quality [(002) plane rocking curve full-width at half maximum below 450 arc sec] are achieved over the range of x ≤ 0.34. Optical absorption studies indicate a decreasing bandgap with increasing Sc with a linear relationship of Eg(x) = 6.1 − 3.39x, which is in good agreement with the theoretical prediction. A monotonically tunable refractive index between AlN and GaN is further measured for ScxAl1−xN with various Sc compositions. This work provides a viable path for the epitaxy of wurtzite ScxAl1−xN with high Sc compositions. The distinct effect of substitutional Sc on bandgap and refractive index could be used in designing high-performance optoelectronic, electronic, and piezoelectric devices, and III-nitride integrated photonics and optical cavities.

High-quality AlN ultrathin films on sapphire substrate were grown by molecular beam epitaxy using an in situ high-temperature annealing approach. From transmission electron microscopy studies, it was found that the AlN epilayers are strain relaxed within the first nm, thus growing nearly strain free. Many of the dislocations generated at the AlN/sapphire interface are reduced within the first 50 nm of growth. Epitaxial films grown directly on sapphire, which are ∼100 nm thick, show X-ray diffraction (002) and (102) rocking curve peaks with full widths at half maximum of less than 150 and 1400 arc sec, respectively, which are the narrowest linewidths reported for AlN of this thickness. Detailed photoluminescence studies further showed that such AlN epilayers exhibit relatively high luminescence efficiency and strong near-band edge emission without defect-related transitions.

In conventional superconductors an external magnetic field generally suppresses superconductivity. This results from a simple thermodynamic competition of the superconducting and magnetic free energies. In this study, we report the unconventional features in the superconducting epitaxial thin film tungsten telluride (WTe2). Measuring the electrical transport properties of Molecular Beam Epitaxy (MBE) grown WTe2 thin films with a high precision rotation stage, we map the upper critical field Hc2 at different temperatures T. We observe the superconducting transition temperature T c is enhanced by in-plane magnetic fields. The upper critical field Hc2 is observed to establish an unconventional non-monotonic dependence on temperature. We suggest that this unconventional feature is due to the lifting of inversion symmetry, which leads to the enhancement of Hc2 in Ising superconductors.

We have investigated the impact of surface recombination on the effective carrier injection efficiency and the Joule heating of axial InGaN/GaN nanowire light-emitting diodes (LEDs). The results reveal that the carrier injection efficiency of such devices is extremely low $({ , due to the severe carrier loss through nonradiative surface recombination. It is further observed that the thermal resistance of typical nanowire LEDs is comparable with, or lower than that of their planar counterparts, in spite of the reduced thermal conductivity of nanowires. The poor carrier injection efficiency, however, leads to significantly elevated junction temperatures for nanowire LEDs. We have further demonstrated, both theoretically and experimentally, that the carrier injection efficiency can be significantly improved in p-doped nanowires, due to the downward surface band bending, and in InGaN/GaN/AlGaN dot-in-a-wire core-shell nanoscale heterostructures, due to the superior carrier confinement offered by the large bandgap AlGaN shell. This paper offers important insight for the design and epitaxial growth of high-performance nanowire LEDs.

AlGaN nanocrystals have emerged as the building blocks of future optoelectronic devices operating in the ultraviolet (UV) spectral range. In this article, we describe the design and performance characteristics of AlGaN nanocrystal UV light-emitting diodes (LEDs) and surface-emitting UV laser diodes. The selective-area epitaxy and structural, optical, and electrical properties of AlGaN nanocrystals are presented. The recent experimental demonstrations of AlGaN nanocrystal LEDs and laser diodes are also discussed.

The authors have investigated the impact of nanowire geometry on the carrier transport in axial indium gallium nitride and gallium nitride (InGaN/GaN) nanowire light-emitting diodes (LEDs). The results reveal that hole transport depends critically on the nanowire geometry. With identical material parameters, the carrier transport process can be varied with different nanowire geometry designs, which lead to different overall device performance. This study offers important insight into the design and epitaxial growth of high-performance nanowire LEDs.

We have studied the epitaxy of few-layer hexagonal boron nitride (h-BN) by plasma-assisted molecular beam epitaxy (MBE) using a low growth rate and nitrogen-rich condition. It has been determined that under such conditions, the growth temperature is the factor having the most significant impact on the structural and optical quality of the material. When grown at temperatures