Study of carrier diffusion in InGaN/GaN quantum wells: Impact of quantum well thickness and substrate type

Abstract

In InGaN/GaN micro-light-emitting diodes (μLEDs), the size-dependent efficiency loss is commonly attributed to carrier diffusion within quantum wells (QWs). When the μLED size is sufficiently small, carriers can diffuse laterally to reach defective sidewalls, leading to non-radiative recombination. This challenges earlier assumptions of short-range carrier diffusion in InGaN/GaN QWs. However, recent studies have demonstrated the potential for long-range diffusion, prompting further investigation into how QW design and growth conditions influence carrier diffusion length and μLED efficiency. This paper contributes to this investigation by examining carrier diffusion in c-plane InGaN/GaN single QW samples using photoluminescence experiments. By varying the QW thickness, we observe an increase in diffusion length with thicker QWs, consistent with the increased radiative recombination lifetime due to the quantum confined Stark effect. This suggests that reducing QW thickness could mitigate the size-dependent efficiency loss in μLEDs. As the substrate type plays a crucial role in advancing the industrialization of μLEDs, we compare carrier diffusion in QWs grown on a substrate of different nature: sapphire, freestanding GaN, and Si (111). Our results demonstrate that the three types of substrates enable long-range diffusion. Finally, analyzing the evolution of carrier diffusion length with carrier density reveals two opposite regimes. In the high-excitation regime, carrier diffusion length decreases by increasing the excitation power, which is in agreement with previous studies and supported by a diffusion–recombination model. However, in the low-excitation regime, carrier diffusion length unexpectedly increases by increasing the excitation power.