The nature of carrier localisation in polar and nonpolar InGaN/GaN quantum wells

P. Dawson,S. Schulz,C. J. Humphreys,M. J. Kappers,R. A. Oliver

doi:10.1063/1.4948237

Abstract

In this paper, we compare and contrast the experimental data and the theoretical predictions of the low temperature optical properties of polar and nonpolar InGaN/GaN quantum well structures. In both types of structure, the optical properties at low temperatures are governed by the effects of carrier localisation. In polar structures, the effect of the in-built electric field leads to electrons being mainly localised at well width fluctuations, whereas holes are localised at regions within the quantum wells, where the random In distribution leads to local minima in potential energy. This leads to a system of independently localised electrons and holes. In nonpolar quantum wells, the nature of the hole localisation is essentially the same as the polar case but the electrons are now coulombically bound to the holes forming localised excitons. These localisation mechanisms are compatible with the large photoluminescence linewidths of the polar and nonpolar quantum wells as well as the different time scales and form of the radiative recombination decay curves.

Highlights

Since the ground breaking work performed[1] by Akasaki, Amano, and Nakamura on InGaN/GaN heterostructures, InGaN light emitting diodes (LEDs) have found widespread applications in many areas such as high brightness displays and solid state lighting
the optical properties at low temperatures are governed by the effects of carrier localisation
the effect of the in-built electric field leads to electrons being mainly localised at

Summary

INTRODUCTION

Since the ground breaking work performed[1] by Akasaki, Amano, and Nakamura on InGaN/GaN heterostructures, InGaN light emitting diodes (LEDs) have found widespread applications in many areas such as high brightness displays and solid state lighting. At the heart of these devices are polar InGaN/GaN quantum well (QW) structures grown on sapphire substrates that are capable of generating visible light with high internal quantum efficiencies (IQE).[2–4] For example, blue light emitting LEDs have been fabricated that can exhibit IQE values as high as 90% at room temperature.[5] This remarkable success has been achieved despite the fact that the growth of these high efficiency device substructures occurs on a sapphire substrate, where the lattice mismatch with GaN is 16%.6. One possible explanation for this behaviour is that green emitters contain significantly larger fractions of In in the QWs than their blue emitting counter parts This leads to large elastic strains and in polar materials, much greater radiative lifetimes that can, in the presence of non-radiative recombination paths, lead to reduced values of IQE. Since the IQE is a function of carrier density[36] and the carrier density depends on the overall recombination rate and on the radiative recombination times, which inevitably differ between the polar and nonpolar samples, these values cannot be directly compared

POLAR QUANTUM WELLS

NONPOLAR QUANTUM WELLS

SUMMARY