Self-organized AlGaN nanowires by molecular beam epitaxy have attracted significant attention for deep ultraviolet optoelectronics. However, due to the strong compositional modulations under conventional nitrogen rich growth conditions, emission wavelengths less than 250 nm have remained inaccessible. Here we show that Al-rich AlGaN nanowires with much improved compositional uniformity can be achieved in a new growth paradigm, wherein a precise control on the optical bandgap of ternary AlGaN nanowires can be achieved by varying the substrate temperature. AlGaN nanowire LEDs, with emission wavelengths spanning from 236 to 280 nm, are also demonstrated.

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 ∼10 cm 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 Mg-free III-nitride LEDs that can exhibit high electrical efficiency (80% at 20 A/cm).

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.

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 Tc 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.

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