Valley Splittings in Si/SiGe Heterostructures from First Principles

TL;DR

This study uses first-principles density functional theory to accurately compute valley splittings in Si/SiGe heterostructures, benchmarking against traditional models and revealing limitations of semi-empirical methods.

Contribution

It provides a first-principles approach to analyze valley splittings, benchmarking and highlighting limitations of existing semi-empirical models.

Findings

DFT supports main conclusions of 2k0 theory

Semi-empirical methods have limitations in describing atomistic disorder

Strong valley-orbit mixing affects large valley splittings

Abstract

We compute valley splittings in Si/SiGe superlattices using ab initio density functional theory (DFT). This first-principle approach is expected to provide an excellent description of interfaces, strains, and atomistic disorder without empirically fitted parameters. We benchmark atomistic tight-binding (TB) and the ``$2k_0$'' theory within the effective mass (EM) approximation against DFT. We show that DFT supports the main conclusions of the 2$k_0$ theory, but reveals some limitations of semi-empirical methods such as the EM and TB, in particular about the description of atomistic disorder. The DFT calculations also highlight the effects of strong valley-orbit mixing at large valley splittings. Nevertheless, TB and the 2$k_0$ theory shall provide reasonable valley splitting statistics in many heterostructures of interest for spin qubit devices.