AMaSiS 2018 Workshop: Abstracts

Simulation of internal quantum efficiency and carrier transport in III-Nitride based UV light emitting diodes

Friedhard Römer, Heiko Fülle, and Bernd Witzigmann,

University of Kassel, Electrical Engineering/Computer Science Department and CINSaT

A model for carrier injection efficiency of Aluminum-Gallium-Nitride (AlGaN) based light emitting diodes (LEDs) will be presented. It consists of a combination of drift-diffusion currents, and quantum transport in the active region. Incomplete activation of the Mg-based acceptor doping is included which depends on the electric field, carrier density and presence of other impurities. The internal quantum efficiency of AlGaN- and InGaN-based LEDs will be analyzed.

Model and Results

In efficient bimolecular light emitters, the electron and hole densities need to be balanced and distributed uniformly in the active region at all relevant operating conditions. This is particularly challenging in III-nitride based devices, as there is a substantial imbalance in the electron and hole effective masses, mobilities, and band offsets at hetero interfaces. In addition, polarization charges at hetero interfaces create electric fields that result in reduced electron-hole overlap in the optically active quantum wells, and potential barriers for carrier transport. In AlGaN-based LEDs emitting in the ultraviolet spectral range, internal quantum efficiencies are below $10\%$ [1]. Careful theoretical study of the injection mechanisms is a valuable tool to identify performance bottlenecks and develop better designs. The following aspects will be discussed: on the p-side the hole injection efficiency depends on the distance of the Mg dopants to the p-side quantum well. Also, electron leakage is suppressed at this location by ionized Mg atoms forming an electrostatic barrier, which however depends on the ionization rate [2]. The latter is a non-trivial function of the local quasi-Fermi level, and therefore depends on the operation current [3]. On the n-side, electron injection into the wells depends critically on the well depth and polarization. Barrier doping is shown to improve homogeneous injection into all wells of a multiple quantum-well region in UV-LEDs [4]. Impurities increase the monomolecular non-radiative recombination rate, which is especially dominant in regions of strong imbalance of the carrier types, which re-inforces the need for equal distribution of electrons and holes.