# Ultrafast Hot Carrier Dynamics in GaN and its Impact on the Efficiency Droop

## TL;DR

This study uses first-principles calculations to analyze hot carrier dynamics in GaN, revealing asymmetries in electron and hole relaxation times and linking slow electron cooling to efficiency droop in GaN LEDs.

## Contribution

It provides detailed insights into the ultrafast carrier relaxation processes in GaN, highlighting the role of electron-phonon interactions and band structure in device efficiency.

## Key findings

- Holes relax faster than electrons due to valence band degeneracy and effective mass differences.
- Hot electrons exhibit slow cooling and long mean free paths, contributing to efficiency droop.
- Asymmetry in carrier dynamics explains the efficiency limitations in GaN light emitters.

## Abstract

GaN is a key material for lighting technology. Yet, the carrier transport and ultrafast dynamics that are central in GaN light emitting devices are not completely understood. We present first-principles calculations of carrier dynamics in GaN, focusing on electron-phonon (e-ph) scattering and the cooling and nanoscale dynamics of hot carriers. We find that e-ph scattering is significantly faster for holes compared to electrons, and that for hot carriers with an initial 0.5$-$1 eV excess energy, holes take a significantly shorter time ($\sim$0.1 ps) to relax to the band edge compared to electrons, which take $\sim$1 ps. The asymmetry in the hot carrier dynamics is shown to originate from the valence band degeneracy, the heavier effective mass of holes compared to electrons, and the details of the coupling to different phonon modes in the valence and conduction bands. We show that the slow cooling of hot electrons and their long ballistic mean free paths (over 3 nm) are a possible cause of efficiency droop in GaN light emitting diodes. Taken together, our work sheds light on the ultrafast dynamics of hot carriers in GaN and the nanoscale origin of efficiency droop.