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.

Ultrawide bandgap III-nitrides, including AlN and high Al content AlGaN alloys, are of great importance for applications to deep ultraviolet photonics and radio frequency electronics. However, lack of suitable substrates remains a challenge. In this context, we investigate the molecular beam epitaxy of AlN and high Al content AlGaN epilayers on low-cost Si substrates using a nanowire template as an intermediate layer.First, the role of the nanowire template on the quality of AlN is elucidated. By comparing AlN epilayers grown with and without the nanowire template over a wide range of growth conditions, it is found that using the nanowire template can relax the tensile strain in the AlN epilayers, as well as improve the overall crystalline quality. Second, the role of the quality of the AlN epilayer on the quality of the p-AlGaN layer grown on top is further discussed. It is found that the improved quality of AlN, due to the use of the nanowire template, also transfers to the improvement of the p-type doping in the AlGaN epilayers grown on top, which further contributes to a drastic improvement on the electrical performance of AlGaN p-i-n diodes, i.e., a factor of 10 4 reduction on the reverse bias leakage current, as well as a 250× improvement on the light output, comparing the structure with and without using the nanowire template. The study potentially impacts the development of ultrawide bandgap III-nitrides on foreign substrates as well as low-cost ultrawide bandgap III-nitride template technologies for a wide range of electronic and photonic device applications.