The high crystalline quality, large junction surface area, and insensitivity to c-axis oriented polarization fields make core-shell
doped GaN nanowire p-n junctions exciting prospects for use as LEDs. The LED external efficiency depends upon
the spatial distribution of optical recombination within the device, which may be controlled through the use of radial
heterojunctions, such as quantum wells and electron blocking layers. In this work, we explore the impact of an axially
varying doping profile on the spatial distribution of optical recombination in a GaN nanowire LED.
The numerical simulation of the nanowire LED is carried out using the TiberCAD simulation package. This package
provides a finite-element-based solution to the drift-diffusion model of a nanowire. Simulations of core-shell nanowire
LEDs are performed with various doping profiles to produce variations in the optical recombination distribution
throughout the device.
In a core-shell device with a uniformly doped n-type core, the current density tends to travel primarily along the core, as
the mobility of electrons is much greater than that of holes in GaN devices. The optical recombination is concentrated
beneath the p-contact, where most of the current crosses the p-n junction. By properly setting a tapered doping profile in
the n-type core, it is possible to increase the uniformity of the optical recombination along the junction. In certain
geometries this will increase the emission efficiency of the nanowire LED.