Explanation of low efficiency droop in semipolar $(20\bar 2\bar 1)$ InGaN/GaN LEDs through evaluation of carrier recombination coefficients

TL;DR

This study analyzes carrier recombination in semipolar $(20\bar 2\bar 1)$ InGaN/GaN LEDs, revealing key differences in recombination coefficients and carrier densities that explain their high efficiency and low droop compared to c-plane LEDs.

Contribution

It provides the first detailed extraction of recombination coefficients in semipolar $(20\bar 2\bar 1)$ LEDs, highlighting their unique carrier dynamics and implications for efficiency.

Findings

Higher nonradiative ($A$) and radiative ($B$) recombination coefficients compared to c-plane.

Lower carrier density at given current density in semipolar LEDs.

Reduced Auger ($C$) coefficient correlating with lower efficiency droop.

Abstract

We report the carrier dynamics and recombination coefficients in single-quantum-well semipolar $(20\bar 2\bar 1)$ InGaN/GaN light-emitting diodes emitting at 440 nm with 93% peak internal quantum efficiency. The differential carrier lifetime is analyzed for various injection current densities from 5 $A/cm^2$ to 10 $kA/cm^2$, and the corresponding carrier densities are obtained. The coupling of internal quantum efficiency and differential carrier lifetime vs injected carrier density ($n$) enables the separation of the radiative and nonradiative recombination lifetimes and the extraction of the Shockley-Read-Hall (SRH) nonradiative ($A$), radiative ($B$), and Auger ($C$) recombination coefficients and their $n$-dependency considering the saturation of the SRH recombination rate and phase-space filling. The results indicate a three to four-fold higher $A$ and a nearly two-fold higher $B_0$ for this semipolar orientation compared to that of $c$-plane reported using a similar approach [A. David and M. J. Grundmann, Appl. Phys. Lett. 96, 103504 (2010)]. In addition, the carrier density in semipolar $(20\bar 2\bar 1)$ is found to be lower than the carrier density in $c$-plane for a given current density, which is important for suppressing efficiency droop. The semipolar LED also shows a two-fold lower $C_0$ compared to $c$-plane, which is consistent with the lower relative efficiency droop for the semipolar LED (57% vs. 69%). The lower carrier density, higher $B_0$ coefficient, and lower $C_0$ (Auger) coefficient are directly responsible for the high efficiency and low efficiency droop reported in semipolar $(20\bar 2\bar 1)$ LEDs.