We report on the etching characteristics of GaAs and AlGaAs by gas source molecular beam epitaxy (GSMBE) using a new precursor bisdimethylaminochloroarsine (BDMAAsCl). The etching rate of GaAs is linearly dependent on the BDMAAsCl flow rate. The activation energy of GaAs etching was 0.25 eV. However, Al0.28Ga0.72As etching shows the negative activation energy of -0.29 eV at the substrate temperature below 500 degrees C. The etchings of GaAs and AlGaAs are probably attributed to the formation of GaCl and AlCl2 or AlCl3, respectively. Furthermore, we demonstrated that the BDMAAsCl etching remarkably reduced the interfacial impurity (carbon, oxygen, silicon) density of GSMBE-grown GaAs epilayers/epiready substrates by one order of magnitude compared with trisdimethylaminoarsine cleaning only.