Spectral Diffusion of Phosphorus Donors in Silicon at High Magnetic Field

Donor and defect electronic spins in solids are promising platforms for quantum technologies. Understanding how these potential electron spin qubit candidates decohere under different experimental conditions is key to enabling improved materials design and to identifying optimal operating conditions. The donor electrons in phosphorus-doped silicon (Si:P) have some of the longest coherence times observed in solid-state spin systems. Natural silicon consists of 3 isotopes - ²⁸Si, ²⁹Si and ³⁰Si. While the 4.7% abundant ²⁹Si is a spin-1/2 nucleus, ²⁸Si and ³⁰Si are spin-0 nuclei. Many-body magnetic dipolar interactions between the ²⁹Si nuclear spins induce a fluctuating nuclear magnetic field at the site of the donor electron spin resulting in a decay of the electron spin coherence. This spectral diffusion due to the ²⁹Si spins is the dominant source of spin echo decay in lightly-doped natural Si:P samples at and below 4 K in low magnetic fields (< 0.5 T). The elimination of the spin-1/2 ²⁹Si nuclei was seen to dramatically suppress spectral diffusion of the phosphorus donor electron resonance in low-field experiments.

Here, we characterize the phase memory time of phosphorus donor electron spins in lightly-doped natural silicon at high magnetic field (8.58 T) in the dark and under optical excitation. At 4.2 K, the spectral diffusion time (T_SD) measured in the dark is observed to be about a factor of 2 smaller than that measured at low magnetic fields (0.35 T) at some orientations of the silicon crystal in the external magnetic field. Using a tunable laser we also measured the echo decay as the wavelength of the optical excitation is swept across the band edge from 1050 nm to 1090 nm. At low optical powers, above-bandgap excitation is seen to increase the spectral diffusion time of the donor electron spin at these crystal orientations, while higher power optical excitation is seen to shorten both the T₁ and T₂ of the donors significantly. Electron-nuclear double resonance experiments suggest that low power optical excitation modulates the hyperfine interactions in the sample.