In this study, the atomic layer etching characteristics and the etch mechanism of ͑100͒ InP as functions of Cl 2 pressure and Ne neutral beam irradiation dose were investigated. When Cl 2 pressure and Ne neutral beam irradiation dose were lower than the critical values of 0.4 mTorr and 7.2 ϫ 10 15 at./ cm 2 cycle, respectively, the InP etch rate ͑Å/cycle͒ and the InP surface roughness varied with Cl 2 pressure and Ne neutral beam irradiation dose. However, when the Cl 2 pressure and Ne neutral beam irradiation dose were higher than the critical values, the InP etch rate remained as 1.47 Å / cycle, corresponding to one monolayer per cycle, and the surface roughness and the surface stoichiometry remained similar to those of InP before etching.