  Erbium implanted silicon as a quantum technology platform has both
telecommunications and integrated circuit processing compatibility. In Si
implanted with Er to a concentration of 3x10^17 cm^3 and O to a concentration
of 10^20 cm^3, the electron spin coherence time, T2, and the spin-lattice
relaxation time, T1, were measured to be 7.5 ls and ~1 ms, respectively, at 5
K. The spin echo decay profile displayed strong modulation, which was
consistent with the super-hyperfine interaction between Er3{\th} and a spin
bath of 29Si nuclei. The calculated spectral diffusion time was similar to the
measured T2, which indicated that T2 was limited by spectral diffusion due to
T1-induced flips of neighboring Er3{\th} spins. The origin of the echo is an Er
center surrounded by six O atoms with monoclinic C1h site symmetry.
