ESR measurements of phosphorus dimers in isotopically enriched 28Si silicon

TL;DR

This study uses ESR spectroscopy to investigate phosphorus dimers in isotopically enriched 28Si, revealing fine spectral structures and showing that exchange coupling does not significantly affect their spin relaxation times.

Contribution

It reports the first observation of a fine structure in ESR spectra of phosphorus dimers in 28Si and demonstrates that exchange coupling up to 60 GHz does not limit spin relaxation times.

Findings

Observation of a new fine structure in ESR spectra due to state mixing.

Measurement of T1 and T2 times showing no significant impact from exchange coupling.

Spectral diffusion limits T2 to 4 ms at 1.7 K.

Abstract

Dopants in silicon have been studied for many decades using optical and electron spin resonance (ESR) spectroscopy. Recently, new features have been observed in the spectra of dopants in isotopically enriched 28Si since the reduced inhomogeneous linewidth in this material improves spectral resolution. With this in mind, we measured ESR on exchange coupled phosphorus dimers in 28Si and report two results. First, a new fine structure is observed in the ESR spectrum arising from state mixing by the hyperfine coupling to the 31P nuclei, which is enhanced when the exchange energy is comparable to the Zeeman energy. This fine structure enables us to spectroscopically address two separate dimer sub-ensembles, the first with exchange (J) coupling ranging from 2 to 7 GHz and the second with J ranging from 6 to 60 GHz. Next, the average spin relaxation times, T1 and T2 of both dimer sub-ensembles were measured using pulsed ESR at 0.35 T. Both T1 and T2 for transitions between triplet states of the dimers were found to be identical to the relaxation times of isolated phosphorus donors in 28Si, with T2 = 4 ms at 1.7 K limited by spectral diffusion due to dipolar interactions with neighboring donor electron spins. This result, consistent with theoretical predictions, implies that an exchange coupling of 2 - 60 GHz does not limit the dimer T1 and T2 in bulk Si at the 10 ms timescale.