No abstract

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.

No abstract

Despite the technological importance of developing AlGaN deep ultraviolet light-emitting diodes (UV LEDs) on Si, there are only a few reports about AlGaN deep UV LEDs on Si based on AlGaN epilayers. Herein, we show vertical AlGaN deep UV LEDs on Si with polarization enhanced p-AlGaN epilayers. The devices emit at 278 nm with uniform current injection. Compared to devices using standard p-AlGaN epilayer, the series resistance of devices with polarization enhanced p-AlGaN epilayer is reduced by a factor of five. This work represents the first report of AlGaN deep UV LEDs on Si with polarization enhanced p-AlGaN epilayers.

No abstract

Over the past decades, the aluminum gallium nitride (AlGaN) alloy system has received wide interest for the development of semiconductor deep ultraviolet (DUV) lasers due to its direct, tunable, and ultrawide bandgap energies (3.4-6.2 eV). The progress, nonetheless, has been remained slow, which is ascribed to a few major challenges, including large dislocation and defect densities, difficulty in obtaining p-type high-Al-content AlGaN layers with a sufficient p-type conduction, the large electric polarization fields, and the unfavorable optical polarization. In recent years, with AlGaN alloys grown by molecular beam epitaxy (MBE), including both thin films and nanowire structures, remarkable advancements have been made, such as highly conductive p-type high-Al-content AlGaN epilayers with resistivities as low as 0.7 Ω cm and DUV lasing down to 239 nm with nanowire structures under a direct current injection. Herein, the recent progress on the DUV lasers by the MBE-grown AlGaN is reviewed. The challenges and prospects of the MBE-grown AlGaN for DUV lasers are also discussed.

Aluminum gallium nitride (AlGaN) nanowires by molecular beam epitaxy (MBE) have become an emerging platform for semiconductor deep ultraviolet (UV) light-emitting diodes (LEDs). Despite of the progress, much less attention has been paid to the effect of substrate rotation speed on the device performance. Herein, we investigate the effect of the substrate rotation speed on the nanowire height and diameter uniformity, as well as the electrical and optical performance of MBE-grown AlGaN nanowire deep UV LED structures with low and high substrate rotation speeds. It is found that by increasing the substrate rotation speed from 4 revolutions per minute (rpm) to 15 rpm, the statistical variation of the nanowire height and diameter is reduced significantly. Increasing the substrate rotation speed also improves the device electrical performance, with a factor of 4 reduction on the device series resistance. This improved electrical performance further transfers to the improved optical performance. The underlying mechanisms for these improvements are also discussed.