Origin of giant valley splitting in silicon quantum wells induced by superlattice barriers

TL;DR

This paper demonstrates a simplified silicon quantum well structure with superlattice barriers that achieve significant valley splitting, crucial for silicon spin qubits, and explains the underlying physics with an effective Hamiltonian model.

Contribution

It introduces a simplified SiGe heterostructure design with superlattice barriers that attain large valley splitting, supported by a microscopic physics model.

Findings

Superlattice barriers can reduce valley splitting unless strongly coupled.

Even minimal superlattice periods yield sizable valley splitting (~1.6 meV).

Strong coupling in superlattice barriers enhances valley splitting significantly.

Abstract

Enhancing valley splitting in SiGe heterostructures is a crucial task for developing silicon spin qubits. Complex SiGe heterostructures, sharing a common feature of four-monolayer (4ML) Ge layer next to the silicon quantum well (QW), have been computationally designed to have giant valley splitting approaching 9 meV. However, none of them has been fabricated may due to their complexity. Here, we remarkably simplify the original designed complex SiGe heterostructures by laying out the Si QW directly on the Ge substrate followed by capping a (Ge4Si4)n superlattice(SL) barrier with a small sacrifice on VS as it is reduced from a maximum of 8.7 meV to 5.2 meV. Even the smallest number of periods (n = 1) will also give a sizable VS of 1.6 meV, which is large enough for developing stable spin qubits. We also develop an effective Hamiltonian model to reveal the underlying microscopic physics of enhanced valley splitting by (Ge4Si4)n SL barriers. We find that the presence of the SL barrier will reduce the VS instead of enhancing it. Only the (Ge4Si4)n SL barriers with an extremely strong coupling with Si QW valley states provide a remarkable enhancement in VS. These findings lay a solid theoretical foundation for the realization of sufficiently large VS for Si qubits.