Carrier Confinement in GaN/AlGaN Nanowire Heterostructures for 0 < x <= 1

TL;DR

This study investigates how carrier confinement in GaN/AlGaN nanowire heterostructures affects their photoluminescence, highlighting the role of structural features like the AlGaN shell and internal electric fields across varying aluminum concentrations.

Contribution

It provides a detailed analysis combining structural characterization and numerical simulations to explain the influence of Al concentration and internal fields on emission properties.

Findings

Maximum emission energy at ~30% Al due to minimal strain gradient

High Al concentrations induce electric fields that reduce emission intensity

Reduced quantum confined Stark effect compared to quantum well structures

Abstract

The three dimensional carrier confinement in GaN nanodiscs embedded in GaN/AlGaN nanowires and its effect on their photoluminescence properties is analyzed for Al concentrations between x = 0.08 and 1. Structural analysis by high resolution transmission electron microscopy reveals the presence of a lateral AlGaN shell due to a composition dependent lateral growth rate of the barrier material. The structural properties are used as input parameters for three dimensional numerical simulations of the confinement which show that the presence of the AlGaN shell has to be considered to explain the observed dependence of the emission energy on the Al concentration in the barrier. The simulations reveal that the maximum in the emission energy for x ~ 30% is assigned to the smallest lateral strain gradient and consequently the lowest radial internal electric fields in the nanodiscs. Higher Al-concentrations in the barrier cause high radial electric fields that can overcome the exciton binding energy and result in substantially reduced emission intensities. Effects of polarization-induced axial internal electric fields on the photoluminescence characteristics have been investigated using nanowire samples with nanodisc heights ranging between 1.2 nm and 3.5 nm at different Al concentrations. The influence of the quantum confined Stark effect is significantly reduced compared to GaN/AlGaN quantum well structures which is attributed to the formation of misfit dislocations at the heterointerfaces which weakens the internal electric polarization fields.