  Selenium impurities in silicon are deep double donors and their optical and
electronic properties have been recently investigated due to their application
for infrared detection. However, a singly-ionised selenium donor (Se$^{+}$)
possesses an electron spin which makes it a potential candidate as a
silicon-based spin qubit, with significant potential advantages compared to the
more commonly studied group V donors. Here we study the electron spin
relaxation ($T_1$) and coherence ($T_2$) times of Se$^{+}$ in isotopically
purified 28-silicon, and find them to be up to two orders of magnitude longer
than shallow group V donors at temperatures above $\sim 15$ K. We further study
the dynamics of donor-acceptor recombination between selenium and boron,
demonstrating that it is possible to control the donor charge state through
optical excitation of neutral Se$^0$.
