The doping mechanism of Se and Te in GaAs molecular-layer epitaxy (MLE) was examined by the timing of supply of the impurity precursor on each orientation of the substrate. On (0 0 1) GaAs, the impurity incorporation on the growing surface may be prevented by AsHx (x = 0, 1, 2, 3) adsorbed during the AsH3 supply, and is apt to be promoted on a Ga-terminated surface. For each orientation of the substrate, the doping characteristics differ noticeably. On the (1 1 1)A surface, Te is incorporated at the lowest temperature, while it is incorporated at the highest temperature on (1 1 1)B. It was clarified that Control of the adsorption of impurity precursor is an essential factor in the precise control of doping in MLE.