(Invited) High Performance Deep Ultraviolet Nanowire Light Emitting Diodes and Lasers

Deep ultraviolet light sources are important for a broad range of applications including water purification, disinfection, and medical diagnostics. The currently reported AlGaN planar light emitting diodes (LEDs) and lasers generally exhibit poor performance, due to the presence of large densities of defects and dislocations and the inefficient p-type doping. In this context, we have investigated the molecular beam epitaxial growth and characterization of nearly defect-free AlGaN nanowire heterostructures grown directly on Si substrate. We have demonstrated AlGaN nanowire LEDs that can operate in the UV-B (280-320nm) and UV-C (<280 nm) bands, which exhibit excellent current current-voltage characteristics. We have further demonstrated electrically injected AlGaN nanowire lasers on Si that can exhibit relatively low threshold current densities (~ 100 A/cm²).

In this work, catalyst-free AlGaN nanowires were grown directly on Si substrate by radio-frequency plasma-assisted molecular beam epitaxy. Due to the diffusion controlled growth process, such self-organized AlGaN nanowires exhibit unique core-shell structures, with the presence of Al-rich AlGaN shell. As a consequence, the nonradiative surface recombination can be greatly suppressed. By varying the Al/Ga flux ratio during the growth process, the emission wavelengths can be controllably varied from ~ 210 nm to >340 nm. The nanowire heterostructures exhibit relatively high internal quantum efficiency (~ 80%) at room-temperature in the UV-C band. We have further identified that Mg-dopant incorporation can be significantly enhanced in AlGaN nanowires, compared to their planar counterparts, due to the reduced formation energy for Al(Ga)-substitutional Mg-doping. The resulting AlN nanowire LEDs can exhibit excellent current-voltage characteristics, with a turn-on voltage of 6 V.

We have also demonstrated that self-organized AlGaN nanowire arrays can function as a high Q optical cavity, due to the Anderson localization of light. The AlGaN nanowire lasers consist of n-AlGaN cladding layer, i-AlGaN active region, and p-AlGaN cladding layer. The devices were fabricated using standard photolithography and metallization techniques. Under continuous wave biasing conditions, the devices can exhibit very low threshold current density. For devices operating in the UV-AII (~ 320-340 nm) band, the threshold current density is ~ 12 A/cm2 at 6 K. The threshold current density increases slightly to ~ 200 A/cm2 for operation wavelengths in the UV-C band. The measured threshold current densities are nearly two to three orders of magnitude lower, compared to conventional AlGaN quantum well lasers under either optical pumping or electrical injection.