Holes in silicon are heavier than expected: transport properties of extremely high mobility electrons and holes in silicon MOSFETs

TL;DR

This study measures and compares high mobility electron and hole transport in silicon MOSFETs, revealing that holes are heavier than expected due to valence band nonparabolicity, with implications for silicon spin qubit fabrication.

Contribution

It provides the highest recorded hole mobility in silicon MOSFETs and explains the mobility discrepancy through valence band nonparabolicity and density-dependent effective mass.

Findings

Peak electron mobility ~40,000 cm²/Vs

Peak hole mobility ~2,000 cm²/Vs

Holes are heavier than expected due to nonparabolicity

Abstract

The quality of the silicon-oxide interface plays a crucial role in fabricating reproducible silicon spin qubits. In this work we characterize interface quality by performing mobility measurements on silicon Hall bars. We find a peak electron mobility of nearly $40,000\,\text{cm}^2/\text{Vs}$ in a device with a $21\,\text{nm}$ oxide layer, and a peak hole mobility of about $2,000\,\text{cm}^2/\text{Vs}$ in a device with $8\,\text{nm}$ oxide, the latter being the highest recorded mobility for a p-type silicon MOSFET. Despite the high device quality, we note an order-of-magnitude difference in mobility between electrons and holes. By studying additional n-type and p-type devices with identical oxides, and fitting to transport theory, we show that this mobility discrepancy is due to valence band nonparabolicity. The nonparabolicity endows holes with a density-dependent transverse effective mass ranging from $0.6m_0$ to $0.7m_0$, significantly larger than the usually quoted bend-edge mass of $0.22m_0$. Finally, we perform magnetotransport measurements to extract momentum and quantum scattering lifetimes.