Imaging ambipolar diffusion of photocarriers in GaAs thin films

TL;DR

This study visualizes how photocarriers diffuse in GaAs thin films, revealing the transition from unipolar to ambipolar diffusion with increasing light excitation power, and quantifies the ambipolar diffusion constant.

Contribution

It provides the first direct imaging and analysis of ambipolar diffusion of photocarriers in GaAs thin films, linking luminescence patterns to diffusion dynamics.

Findings

Unipolar diffusion dominates at low excitation power.

Ambipolar diffusion becomes significant at higher power.

The ambipolar diffusion constant varies with electron concentration.

Abstract

Images of the steady-state luminescence of passivated GaAs self-standing films under excitation by a tightly-focussed laser are analyzed as a function of light excitation power. While unipolar diffusion of photoelectrons is dominant at very low light excitation power, an increased power results in a decrease of the diffusion constant near the center of the image due to the onset of ambipolar diffusion. The results are in agreement with a numerical solution of the diffusion equations and with a physical analysis of the luminescence intensity at the centre of the image, which permits the determination of the ambipolar diffusion constant as a function of electron concentration.