  We analyze the error in trapped-ion, hyperfine qubit, quantum gates due to
spontaneous scattering of photons from the gate laser beams. We investigate
single-qubit rotations that are based on stimulated Raman transitions and
two-qubit entangling phase-gates that are based on spin-dependent optical
dipole forces. This error is compared between different ion species currently
being investigated as possible quantum information carriers. For both gate
types we show that with realistic laser powers the scattering error can be
reduced to below current estimates of the fault-tolerance error threshold.
