Inhomogeneous broadening of optical transitions observed in photoluminescence and modulated reflectance of polar and non-polar InGaN quantum wells

TL;DR

This study investigates how structural inhomogeneities and polarization-induced electric fields cause broadening of optical transitions in InGaN/GaN quantum wells, affecting their photoluminescence and reflectance spectra, especially in polar structures.

Contribution

The paper introduces a detailed simulation model that accounts for inhomogeneities and electric fields to interpret optical spectra of InGaN quantum wells, clarifying the impact of these factors.

Findings

Built-in electric fields cause redshift and broadening of optical transitions.

QW inhomogeneities significantly influence the spectral features.

ER spectra can overestimate the Stokes shift compared to PL spectra.

Abstract

In this work the broadening of interband transitions in InGaN/GaN quantum wells (QWs) resulting from structural inhomogeneities is analyzed. The role of polarization-induced electric field in the mechanism behind the inhomogeneous broadening observed in photoluminescence (PL) and electromodulated reflectance (ER) spectra of InGaN QWs dedicated to green/blue lasers is explained. Spectra of both polar and non-polar QWs are simulated within the random QW model distinguishing contributions of individual transitions taking into account QW inhomogeneities (QW width and indium content fluctuations). On this basis we interpret the ER and PL spectra measured for a polar multiple QW InGaN/GaN structure. The built-in electric field shifts the emission wavelength to red, and enhances the broadening of optical transitions. It is clearly shown that for polar QWs the Stokes shift can be easily overestimated if PL spectra are compared with ER spectra since the intensity of the fundamental transition observed in ER spectra significantly decreases with the increase in QW width. In this way ER signal related to excited states start to dominate. This effect is strongly enhanced by QW inhomogeneities.