# Electron Spin Resonance of P Donors in Isotopically Purified Si Detected by Contactless Photoconductivity

## TL;DR

This paper introduces a contactless optical method for detecting phosphorus donor electron spins in isotopically purified silicon, achieving record coherence times up to 350 ms, surpassing traditional ESR detection limits.

## Contribution

The authors develop and validate an optical donor bound exciton transition technique for ESR detection, enabling measurement of longer coherence times in ultra-pure silicon samples.

## Key findings

- Achieved a coherence time of 130 ms in highly purified silicon.
- Measured a record coherence time of 350 ms using optical polarization.
- Demonstrated a contactless detection method surpassing traditional ESR sensitivity.

## Abstract

Coherence times of electron spins bound to phosphorus donors have been measured, using a standard Hahn echo technique, to be up to 20 ms in isotopically pure silicon with [P]$ = 10^{14}$ cm$^{-3}$ and at temperatures $\leq 4 $K. Although such times are exceptionally long for electron spins in the solid state, they are nevertheless limited by donor electron spin-spin interactions. Suppressing such interactions requires even lower donor concentrations, which lie below the detection limit for typical electron spin resonance (ESR) spectrometers. Here we describe an alternative method for phosphorus donor ESR detection, exploiting the spin-to-charge conversion provided by the optical donor bound exciton transition. We characterise the method and its dependence on laser power and use it to measure a coherence time of $T_2 = 130 $ms for one of the purest silicon samples grown to-date ([P]$ = 5\times 10^{11} $cm$^{-3}$). We then benchmark this result using an alternative application of the donor bound exciton transition: optically polarising the donor spins before using conventional ESR detection at 1.7~K for a sample with [P]$ = 4\times10^{12} $cm$^{-3}$, and measuring in this case a $T_2$ of 350 ms.