Radial etching of strongly confined crystal-phase defined quantum dots

TL;DR

This paper demonstrates a method to create strongly confined quantum dots in InAs nanowires using crystal-phase control and chemical etching, revealing insights into their charging energies and spin-orbit interactions.

Contribution

It introduces a novel combined approach of epitaxial crystal-phase control and isotropic etching to produce ultra-small quantum dots with enhanced confinement in nanowires.

Findings

Maximum charging energy > 30 meV in small QDs

Stray capacitances limit charging energy at very small diameters

Strong spin-orbit interaction possibly linked to WZ/ZB interfaces

Abstract

We realize strongly confined quantum dots (QDs) in InAs nanowires (NWs) by combining epitaxial crystal-phase control with chemical wet etching. A strong axial confinement is first introduced by growing closely spaced wurtzite (WZ) tunnel barriers in NWs to enclose a zinc blende (ZB) QD. The NW cross-section is then reduced by isotropic etching to obtain very small QDs, with a maximum observed charging energy > 30 meV. Using low-temperature electrical characterization and finite-element method simulations, we study how charging energies and the onset of electron filling scale with QD diameter. For extremely small diameters, we identify a regime where stray capacitances become non-negligible, limiting further increase in charging energy by diameter reduction alone. This approach to increasing confinement is particularly relevant for understanding the strong spin-orbit interaction observed in crystal-phase QDs, possibly linked to polarization charges at the WZ/ZB interfaces. Small diameter QDs allow considerably weaker interfering electric fields when studied, but the QDs cannot be realized with epitaxial growth alone due to a loss of crystal phase control.