Increasing valley splitting in Si/SiGe by practically achievable heterostructure profiles

TL;DR

This paper proposes a novel approach to increase valley splitting in Si/SiGe quantum wells by designing heterostructure profiles that enhance intervalley backscattering through constructive interference, achievable with current epitaxy techniques.

Contribution

It introduces a new perspective on intervalley coupling as impurity backscattering and designs heterostructure profiles to significantly boost valley splitting.

Findings

Valley splitting can be increased beyond 1 meV with tailored Ge profiles.

Constructive interference of multiple scatterers enhances valley splitting.

Profiles within current MBE growth capabilities are sufficient for significant valley splitting.

Abstract

Silicon spin qubits are marred by the valley degeneracy of the conduction band. In a nanodevice, the degeneracy is lifted by interfaces and alloy disorder, but the arising valley splitting is small, of order 100 $\mu$eV in Si/SiGe quantum wells. Substantial efforts were invested both in theory and experiments to overcome the valley issue. Unfortunately, the existing recipes either rely on atomistic details of the interface that are beyond experimental control, or demand heterostructure profiles beyond current state-of-the-art heterostructure epitaxy. We revisit the valley splitting induced by non-trivial Ge profiles and advocate a novel view of the intervalley coupling as a backscattering on point-like impurities realized by crystal planes containing Ge atoms. This perspective reveals that enhancing the backscattering amplitude, which sets the valley splitting, requires constructive interference of multiple scatterers. % We arrive at a remarkable prediction, that the Ge content along the heterostructure growth direction does not have to have any specific periodicity, including the practically unreachable $2\pi/(2k_0)$ period, to significantly increase the valley splitting. This statement is corroborated with numerical evidence from tight-binding simulations and intuitive physical interpretations. We devise profiles that seem within the capabilities of current MBE growth techniques and boost the valley splitting beyond the 1\,meV scale.