In the low energy plasma doping process, the contribution of not only ionised species but also neutral species to the doping process should be considered. In order to investigate such a contribution, experiments of gas phase doping combined with Ar plasma pre-treatment were carried out. Gas phase impurity absorption should be affected by the Si crystalline disorder caused by the plasma doping. Ar plasma was used to simulate this effect. As a result, significant increase of boron dose from the neutral gas phase was observed when the substrate surface was pre-treated by Ar plasma prior to exposure to neutral B2H6/He gas. The boron was considered to be absorbed in the amorphous layer. Understanding and control of this phenomenon are important for plasma doping technology, in which ion irradiation and absorption of neutral species proceed simultaneously.