(Invited) Electrically Pumped Lasers and Light Emitting Diodes in the Ultraviolet-C Band with AlGaN Nanowires
Authors: Mi Zetian , Zhao Songrui 
Published: February 2020

Ultraviolet (UV) light sources, including light emitting diodes (LEDs) and lasers, with emission wavelengths less than 280 nm (UV-C band) are critically important for a wide range of applications including water purification, disinfection, data storage, medical diagnosis, and bio/chemical sensing. To date, the development of such light sources with aluminum gallium nitride (AlGaN) has been severely limited by the presence of large dislocation densities and the extremely inefficient p-type doping. In addition, the intrinsic TM light polarization of Al-rich AlGaN prohibits the light extraction from conventional c-plane wurtzite structures. In this context, we have investigated the molecular beam epitaxial (MBE) growth and characterization of nearly defect-free AlGaN nanowires on Si. We have demonstrated, for the first time, electrically injected lasers in the UV-C band, which exhibit a threshold current density of 0.2 kA/cm2. By exploiting the efficient light scattering among nanowire arrays, we have also demonstrated high efficiency AlN nanowire LEDs with emission wavelength ~ 207 nm.

In this experiment, the AlGaN nanowires were spontaneously formed on Si substrate by radio-frequency plasma-assisted MBE without using any foreign metal catalyst. The laser device structure consisted of n-GaN contact layer, n-AlGaN cladding layer, AlGaN active region, p-AlGaN cladding layer, and p-GaN contact layer. The optical cavity was formed by the Anderson localization of light, due to the recurrent multiple scattering of photons in random nanowire arrays. The fabrication process involved standard optical lithography and contact metallization techniques. The lasers operated under continuous wave (CW) biasing conditions. The electroluminescence emission was collected from the device top surface by a CCD camera coupled to a high-resolution spectrometer. A lasing peak around 262 nm was clearly measured at 77 K, with a threshold current density of 0.2 kA/cm2. This is the first demonstration of electrically injected semiconductor lasers in the deep UV wavelength range. Detailed studies further showed that the self-organized AlGaN nanowires exhibited extensive atomic scale compositional modulation. The extremely low threshold current is directly related to the three-dimensional quantum confinement of charge carriers by the quantum dot/dash-like nanostructures in nearly defect-free AlGaN nanowires.

We have further investigated deep UV LEDs based on self-organized AlGaN nanowires. In this study, the device structure consisted of n-GaN contact layer, n-Al(Ga)N, i-Al(Ga)N, p-Al(Ga)N, and p-(Al)GaN contact layers. Strong electroluminescence at 207 nm was measured at room-temperature from LEDs fabricated with AlN. Furthermore, detailed angle dependent electroluminescence studies indicated that the dominant light emission direction was from the c-axially grown nanowire top surface, due to the efficient light scattering amongst nanowires, in spite of TM light polarization in AlN. Moreover, an electrical efficiency of > 85% was realized from such AlN nanowire LEDs, which can be largely ascribed to the hole hopping conduction in the impurity band hopping. In addition, our detailed studies further indicated that the performance of AlN nanowire LEDs was largely limited by electron overflow.