Conductivity Freeze-Out in Isotopically Pure Si-28 at milli-Kelvin Temperatures

TL;DR

This study reveals a sharp transition in electrical conductivity of isotopically pure Si-28 at around 1 Kelvin, linked to a change in conduction mechanism, with implications for quantum technology and semiconductor devices.

Contribution

It identifies a new low-temperature transition in silicon's conductivity and models the underlying phonon-less hopping conduction regime change.

Findings

Observation of a sharp conductivity transition at ~1 K

Transition attributed to change in conduction mechanism

Results enhance understanding of silicon's low-temperature physics

Abstract

Silicon is a key semiconducting material for electrical devices and hybrid quantum systems where low temperatures and zero-spin isotopic purity can enhance quantum coherence. Electrical conductivity in Si is characterised by carrier freeze out at around 40 K allowing microwave transmission which is a key component for addressing spins efficiently in silicon quantum technologies. In this work, we report an additional sharp transition of the electrical conductivity in a Si-28 cylindrical cavity at around 1 Kelvin. This is observed by measuring microwave resonator Whispering Gallery Mode frequencies and Q factors with changing temperature and comparing these results with finite element models. We attribute this change to a transition from a relaxation mechanism-dominated to a resonant phonon-less absorption-dominated hopping conduction regime. Characterising this regime change represents a deeper understanding of a physical phenomenon in a material of high interest to the quantum technology and semiconductor device community and the impact of these results is discussed.