Vanishing fine structure splitting in highly asymmetric InAs/InP quantum dots without wetting layer

Michał Zieliński

doi:10.1038/s41598-020-70156-1

Abstract

Contrary to simplified theoretical models, atomistic calculations presented here reveal that sufficiently large in-plane shape elongation of quantum dots can not only decrease, but even reverse the splitting of the two lowest optically active excitonic states. Such a surprising cancellation of bright-exciton splitting occurs for shape-anisotropic nanostructures with realistic elongation ratios, yet without a wetting layer, which plays here a vital role. However, this non-trivial effect due to shape-elongation is strongly diminished by alloy randomness resulting from intermixing of InAs quantum-dot material with the surrounding InP matrix. Alloying randomizes, and to some degree flattens the shape dependence of fine-structure splitting giving a practical justification for the application of simplified theories. Finally, we find that the dark-exciton spectra are rather weakly affected by alloying and are dominated by the effects of lateral elongation.

Highlights

Quantum dots[1,2] are man-made semiconductor nanostructures that come in a wide variety of types[3,4,5], and are extensively studied with interest driven by both basic scientific curiosity as well as promising applications in quantum information6, computing[7,8], and cryptography[9]
Shape anisotropy is applied by elongating the system along the [1 1 ̄0 ] axis and simultaneously narrowing it in the perpendicular [110] direction, with the quantum dash volume kept constant (Fig. 1). in such a manner a cylindrical disk-shape quantum dot undergoes a transition to an elongated quantum dash with an elliptic base
We considered here quantum dash systems studied as a function of lateral aspect ratio and chemical composition

Summary

Introduction

Quantum dots[1,2] are man-made semiconductor nanostructures that come in a wide variety of types[3,4,5], and are extensively studied with interest driven by both basic scientific curiosity as well as promising applications in quantum information6, computing[7,8], and cryptography[9]. Apart from the elementary excitations, electrons and holes, quantum dots can confine interacting electron-hole pairs, namely excitons.[10] An emission cascade from a two exciton (biexciton) state, through two indistinguishable exciton states should lead to the emission of polarization entangled photon pairs.[9,11,12,13] in realistic quantum dots the intermediate exciton state is often split by the electron-hole exchange interaction[14,15,16] hindering the efficiency of the entanglement generation This energetic difference between the two bright exciton states, known as the fine-structure splitting or the bright exciton splitting (BES) is typically (10–100 μeV ) much larger than the emission linewidth (∼ 1 μeV ). In this work we show that shape elongation may reduce the bright exciton splitting[63], but even lead to its vanishing, and subsequently to the reversal of bright excitonic spectral lines This is possible for large but realistic elongation ratios, and practical quantum dash dimensions, yet for the growth process that does not incorporate the wetting layer. We study dark-exciton s pectra[15,70,71,72], properties of which in quantum dashes, such as the non-zero optical activity[63,73] and splitting, appear to be dominated by shape deformation and rather immune to alloying effects

Methods

Results

Conclusion