# Localization landscape theory of disorder in semiconductors II: Urbach tails of disordered quantum well layers

## TL;DR

This paper develops a 3D absorption model based on a localization theory to explain Urbach tails caused by compositional disorder in InGaN quantum wells, matching experimental observations and revealing effects of bias and disorder on absorption edges.

## Contribution

It introduces a novel 3D disorder-based absorption model that accurately predicts Urbach tails in disordered quantum wells, advancing understanding of disorder effects in nitride semiconductors.

## Key findings

- The model reproduces experimental Urbach tail broadening.
- Bias causes a red shift in well-to-barrier absorption and a blue shift in quantum well absorption.
- The localization theory effectively explains disorder-induced absorption features.

## Abstract

Urbach tails in semiconductors are often associated to effects of compositional disorder. The Urbach tail observed in InGaN alloy quantum wells of solar cells and LEDs by biased photocurrent spectroscopy is shown to be characteristic of the ternary alloy disorder. The broadening of the absorption edge observed for quantum wells emitting from violet to green (indium content ranging from 0 to 28\%) corresponds to a typical Urbach energy of 20~meV. A 3D absorption model is developed based on a recent theory of disorder-induced localization which provides the effective potential seen by the localized carriers without having to resort to the solution of the Schr\"odinger equation in a disordered potential. This model incorporating compositional disorder accounts well for the experimental broadening of the Urbach tail of the absorption edge. For energies below the Urbach tail of the InGaN quantum wells, type-II well-to-barrier transitions are observed and modeled. This contribution to the below bandgap absorption is particularly efficient in near-UV emitting quantum wells. When reverse biasing the device, the well-to-barrier below bandgap absorption exhibits a red shift, while the Urbach tail corresponding to the absorption within the quantum wells is blue shifted, due to the partial compensation of the internal piezoelectric fields by the external bias. The good agreement between the measured Urbach tail and its modeling by the new localization theory demonstrates the applicability of the latter to compositional disorder effects in nitride semiconductors.