Understanding disorder in Silicon quantum computing platforms: Scattering mechanisms in Si/SiGe quantum wells

TL;DR

This paper theoretically investigates the scattering mechanisms affecting mobility in Si/SiGe quantum wells, identifying dominant sources at different densities and estimating the critical density for metal-insulator transition, with implications for quantum computing platforms.

Contribution

It provides a comprehensive theoretical analysis of disorder effects on mobility and the metal-insulator transition in Si/SiGe quantum wells, including the role of various scattering mechanisms and magnetic field effects.

Findings

Remote Coulomb impurities limit mobility at low densities.

Interface roughness limits mobility at higher densities.

Quantum mobility is mainly limited by background impurities.

Abstract

Motivated by recent experiments on Si/SiGe quantum wells with a co-design of high electron mobility and large valley splitting [B. Paquelet Wuetz, et al., Nature Communications 14, 1385 (2023); D. D. Esposti, et al., arXiv:2309.02832], suitable for a Si-based spin qubit quantum computing platform, we examine the role of disorder by theoretically calculating mobility and quantum mobility from various scattering mechanisms and their dependence on the electron density. At low electron densities $n_e < 4 \times 10^{11}$ cm$^{-2}$, we find that mobility is limited by remote Coulomb impurities in the capping layer, whereas interface roughness becomes the significant limiting factor at higher densities. We also find that alloy disorder scattering is not a limiting mechanism in the reported high-mobility structures. We estimate the critical density of the disorder-driven low-density metal-insulator transition using the Anderson-Ioffe-Regel localization criterion and qualitatively explain the breakdown of the Boltzmann-Born theory at low densities. We also estimate the critical density by considering inhomogeneous density fluctuations induced by long-range Coulomb disorder in the system, and find a larger critical density compared to the one obtained from the Anderson-Ioffe-Regel criterion. For quantum mobility, our calculation suggests that remote and distant background impurities are likely the limiting scattering sources across all density. Future measurements of quantum mobility should provide more information on the distribution of background impurities inside the SiGe barriers. Moreover, we extend our theoretical analysis to the effect of quantum degeneracy on transport properties and predict the mobility and the critical density for the metal-insulator transition in spin-polarized high-mobility structures under an external parallel magnetic field.