In this work, we report the growth, fabrication, and characterization of aluminum gallium nitride (AlGaN) nanowire deep ultraviolet light-emitting diodes with a polarization engineered tunnel junction (TJ) and p-AlGaN layer. The major takeaway from this study is: first, devices emitting at around 250 nm with a maximum external quantum efficiency of around 0.01% are demonstrated. Second, the effect of the electric polarization field in the n+-Al0.1Ga0.9N/GaN/p+-Al0.1Ga0.9N TJ due to the incorporation of the GaN layer is observed by comparing the current-voltage (I–V) characteristics of devices with different GaN thicknesses. The incorporation of the GaN layer improves the I–V characteristics due to the improved tunneling process originating from the band bending induced by the polarization charges at GaN and AlGaN heterointerfaces. Third, the role of the graded p-AlGaN layer on the device's electrical performance is also elucidated. It is found that the graded p-AlGaN layer plays a significant role in improving the device electrical performance. Finally, the improved device electrical performance also transfers to the device optical performance.

Despite graphene being an attractive transparent conductive electrode for semiconductor deep ultraviolet (UV) light emitting diodes (LEDs), there have been no experimental demonstrations of any kind of semiconductor deep UV LEDs using a graphene electrode. Moreover, although aluminum gallium nitride (AlGaN) alloys in the format of nanowires are an appealing platform for surface-emitting vertical semiconductor deep UV LEDs, in particular, at short wavelengths, there are few demonstrations of AlGaN nanowire UV LEDs with a graphene electrode. In this work, we show that transferred graphene can serve as the top electrode for AlGaN nanowire deep UV LEDs, and devices emitting down to around 240 nm are demonstrated. Compared to using metal, graphene improves both the light output power and external quantum efficiency. Nonetheless, devices with a graphene electrode show a more severe efficiency droop compared to devices with metal. Here, we attribute the heating effect associated with the large contact resistance to be the major reason for the severe efficiency droop in the devices with a graphene electrode. Detailed scanning electron microscopy and Raman scattering experiments suggest that the nanowire height nonuniformity is the main cause for the large contact resistance; this issue could be potentially alleviated by using nanowires grown by selective area epitaxy that is able to produce nanowires with uniform height. This work, therefore, not only demonstrates the shortest wavelength LEDs using a graphene electrode but also provides a viable path for surface-emitting vertical semiconductor deep UV LEDs at short wavelengths.

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Gallium nitride (GaN) nanowires by selective area epitaxy (SAE) are an emerging platform for nanophotonic devices. Nonetheless, the use of a high substrate temperature in SAE limits the development of this technology. In this work, we report the SAE of GaN nanowires at low substrate temperatures by radio frequency plasma-assisted molecular beam epitaxy. Excellent selectivity is obtained at low substrate temperatures; the area without patterning is nearly free of any growth. Furthermore, a delicate control on the nanowire top-surface morphology is enabled by the low temperature epitaxy from an irregular shape to a hexagonal shape with semipolar top planes to a hexagonal shape with polar c-planes on top with controlled polar c-plane size. Such a low temperature SAE of GaN nanowires, together with the elegant control of the nanowire top-surface morphology, will enable a fully controllable, epitaxial nanophotonic platform, benefiting the development of a wide range of photonic devices such as light-emitting diodes, lasers, single photon sources, and multifunctional photonic devices.

Aluminum (Al)-containing nitrides, including AlN, AlScN, and AlGaN, are emerging material platforms for futuregeneration photonic, electronic, ferroelectric, and multifunctional semiconductor devices. Nonetheless, the low Al migration mobility in a nitrogen environment is a notorious hindrance to obtaining high-quality Al-containing nitride epilayers. This is particularly the case for the epitaxy of Al-containing nitrides with radio frequency (RF) plasma-assisted molecular beam epitaxy (MBE), due to the use of relatively low substrate temperatures. As such, various approaches have been employed in promoting Al migration such as metal-modulated epitaxy (MME). Despite the progress, the effect of the relative position of the Al and N sources on the Al migration mobility has remained unclear. In this study, we show that MBE-grown AlGaN nanowires can be an effective probe to examine the effect of the Al and N source relative position on the Al migration mobility and demonstrate clearly that Al adatoms appear to be more mobile from the Al source that is far away from the N source, compared to those from the Al source that is near the N source. This study therefore can potentially impact the development of emerging Al-containing semiconductor devices by MBE. Moreover, the demonstrated correlated Al source dependent morphological and optical properties of AlGaN nanowires can naturally help the development of AlGaN-nanowire-based UV light-emitting diodes (LEDs) and lasers.