Strain Engineering of the Band Gap of HgTe Quantum Wells using Superlattice Virtual Substrates

TL;DR

This study demonstrates strain engineering of HgTe quantum wells using superlattice virtual substrates to significantly increase the topological insulator band gap, enabling higher temperature observations of topological conductance.

Contribution

It introduces a method to control the band gap of HgTe QWs via strain from superlattice substrates, achieving a gap of 55 meV that surpasses previous limits.

Findings

Transition from semi-metallic to 2D-TI regime with strain change

Enhanced energy gap of 55 meV in compressively strained QWs

Topological conductance observable at higher temperatures

Abstract

The HgTe quantum well (QW) is a well-characterized two-dimensional topological insulator (2D-TI). Its band gap is relatively small (typically on the order of 10 meV), which restricts the observation of purely topological conductance to low temperatures. Here, we utilize the strain-dependence of the band structure of HgTe QWs to address this limitation. We use $\text{CdTe}-\text{Cd}_{0.5}\text{Zn}_{0.5}\text{Te}$ strained-layer superlattices on GaAs as virtual substrates with adjustable lattice constant to control the strain of the QW. We present magneto-transport measurements, which demonstrate a transition from a semi- metallic to a 2D-TI regime in wide QWs, when the strain is changed from tensile to compressive. Most notably, we demonstrate a much enhanced energy gap of 55 meV in heavily compressively strained QWs. This value exceeds the highest possible gap on common II-VI substrates by a factor of 2-3, and extends the regime where the topological conductance prevails to much higher temperatures.