We have investigated the effect of the irradiation of atomic deuterium (D) on the initial growth of Sn and Ge1-xSnx on Ge(0 0 1) substrates by using scanning tunneling microscopy (STM) comparing to the effect of the irradiation of atomic hydrogen (H). We found that the surfactant effect appears and the surface roughness is reduced when the surface coverage of D or H is higher than 69%. The efficiency for reducing the surface roughness increased with the coverage of D and H. Moreover, we found that D atoms effectively work as a surfactant on the Ge surface compared to H atoms under the same irradiation condition. Utilizing D as a surfactant is expected to improve the termination process of Ge surface. (C) 2012 Elsevier B. V. All rights reserved.