Light ions in the range 11-20 amu have been implanted at low energies into hydrogenated amorphous silicon. Based on Schottky barrier measurements it is shown that for low doses the residual disorder following annealing above the equilibration temperature is approximately proportional to the energy dissipated per unit volume by the ion beam. This is manifest as a decrease in the effective Schottky barrier height arising from an increase in the strength of the conduction band tail and the Urbach energy and the associated decrease in mobility gap. At high doses the change in band gap saturates when the damage reaches a well defined value independent of ion mass. Chemical effects are detected following N and O implants both of which introduce a small number of donor-like states. Flourine has the remarkable ability to passivate defect states remaining after bombardment and annealing and improve the quality of the material. (C) 2003 Elsevier Ltd. All rights reserved.