Despite broad interest in aluminum gallium nitride (AlGaN) optoelectronic devices for deep ultraviolet (DUV) applications, the performance of conventional Al(Ga)N planar devices drastically decays when approaching the AlN end, including low internal quantum efficiencies (IQEs) and high device operation voltages. Here we show that these challenges can be addressed by utilizing nitrogen (N) polar Al(Ga)N nanowires grown directly on Si substrate. By carefully tuning the synthesis conditions, a record IQE of 80% can be realized with N-polar AlN nanowires, which is nearly ten times higher compared to high quality planar AlN. The first 210 nm emitting AlN nanowire light emitting diodes (LEDs) were achieved, with a turn on voltage of about 6 V, which is significantly lower than the commonly observed 20 – 40 V. This can be ascribed to both efficient Mg doping by controlling the nanowire growth rate and N-polarity induced internal electrical field that favors hole injection. In the end, high performance N-polar AlGaN nanowire LEDs with emission wavelengths covering the UV-B/C bands were also demonstrated.

We have examined the carrier injection process of axial nanowire light-emitting diode (LED) structures and identified that poor carrier injection efficiency, due to the large surface recombination, is the primary cause for the extremely low output power of phosphor-free nanowire white LEDs. We have further developed InGaN/GaN/AlGaN dot-in-a-wire core-shell white LEDs on Si substrate, which can break the carrier injection efficiency bottleneck, leading to a massive enhancement in the output power. At room temperature, the devices can exhibit an output power of ~1.5 mW, which is more than 2 orders of magnitude stronger than nanowire LEDs without shell coverage. Additionally, such phosphor-free nanowire white LEDs can deliver an unprecedentedly high color rendering index of ~92-98 in both the warm and cool white regions, with the color rendering capability approaching that of an ideal light source, i.e. a blackbody.

We report on the demonstration of a new type of axial nanowire LED heterostructures, with the use of self-organized InGaN/AlGaN dot-in-a-wire core-shell nanowire arrays. The large bandgap AlGaN shell is spontaneously formed on the sidewall of the nanowire during the growth of AlGaN barrier of the quantum dot active region. As such, nonradiative surface recombination, that dominates the carrier dynamics of conventional axial nanowire LED structures, can be largely eliminated, leading to significantly increased carrier lifetime from ~0.3 ns to 4.5 ns. The luminescence emission is also enhanced by orders of magnitude. Moreover, the p-doped AlGaN barrier layers can function as distributed electron blocking layers (EBLs), which is found to be more effective in reducing electron overflow, compared to the conventional AlGaN EBL. The device displays strong white-light emission, with a color rendering index of ~95. An output power of >5 mW is measured for a 1 mm × 1 mm device, which is more than 500 times stronger than the conventional InGaN axial nanowire LEDs without AlGaN distributed EBLs.

High crystal quality, vertically aligned AlxGa1-xN nanowire based double heterojunction light emitting diodes (LEDs) are grown on Si substrate by molecular beam epitaxy. Such AlxGa1-xN nanowires exhibit unique core-shell structures, which can significantly suppress surface nonradiative recombination. We successfully demonstrate highly efficient AlxGa1-xN nanowire array based LEDs operating at ∼340 nm. Such nanowire devices exhibit superior electrical and optical performance, including an internal quantum efficiency of ∼59% at room temperature, a relatively small series resistance, highly stable emission characteristics, and the absence of efficiency droop under pulsed biasing conditions.

In this paper, the optical and electrical properties of Mg-doped AlN nanowires are discussed. At room temperature, with the increase of Mg-doping concentration, the Mg-acceptor energy level related optical transition can be clearly measured, which is separated about 0.6 eV from the band-edge transition, consistent with the Mg activation energy in AlN. The electrical conduction measurements indicate an activation energy of 23 meV at 300 K-450 K temperature range, which is significantly smaller than the Mg-ionization energy in AlN, suggesting the p-type conduction being mostly related to hopping conduction. The free hole concentration of AlN:Mg nanowires is estimated to be on the order of 10 16 cm À3 , or higher. V