Decoherence mechanisms of 209Bi donor electron spins in isotopically pure 28Si

Bismuth (209Bi) is the deepest group V donor in silicon and possesses the most extreme characteristics such as a 9/2 nuclear spin and a 1.5 GHz hyperfine coupling. These lead to several potential advantages for a Si:Bi donor electron spin qubit compared to the more common phosphorus donor. Most previous studies on Si:Bi have been performed using natural silicon where linewidths and electron spin coherence times are limited by the presence of 29Si impurities. Here, we describe electron spin resonance (ESR) and electron nuclear double resonance (ENDOR) studies on 209Bi in isotopically pure 28Si. ESR and ENDOR linewidths, transition probabilities, and coherence times are understood in terms of the spin Hamiltonian parameters showing a dependence on field and m_I of the 209Bi nuclear spin. We explore various decoherence mechanisms applicable to the donor electron spin, measuring coherence times up to 700 ms at 1.7 K at X band, comparable with 28Si:P. Importantly, the coherence times we measure follow closely to the calculated field gradients of the transition frequencies (df/dB), providing a strong motivation to explore "clock" transitions where coherence lifetimes could be further enhanced.