We report the electronic passivation of a silicon surface by iodine te
rmination. The resulting surface recombination velocity on Si(100) is
less than 1 cm/s which is better than that obtained in concentrated hy
drofluoric acid (HF). We have produced a surface recombination velocit
y of 20 cm/s using bromine. We present a simple model for these phenom
ena of a surface coverage of Si-X where X is a monovalently bonded hal
ogen atom. The effectiveness of the passivation by halogens is shown t
o be limited by the oxidation of halogens by dissolved oxygen in solut
ion. We demonstrate the use of halogen:methanol solutions as an altern
ative to HF for the control of silicon surface chemistry.