Position control of low-temperature solid-phase crystallization in amorphous-GeSn (a-GeSn) films is investigated using a-GeSn films covering Sn/a-Ge stacked islands on insulator. Two-steps annealing has been performed, i.e., first-step annealing (similar to 250 degrees C) to crystallize Sn/a-Ge islands for crystalline-seed formation and second-step annealing (<= 250 degrees C) to proceed lateral-growth from the crystalline-seed. It is found that Sn thickness exceeding similar to 70% of Ge thickness is the essential condition to achieve crystalline GeSn islands (grain size > 10 nm) with the diamond structure. This acts as crystalline-seed enabling the lateral-growth of a-GeSn over 80 mu m. The present process below the softening-temperature (similar to 300 degrees C) of plastic substrates facilitates realization of advanced flexible electronics. (C) 2015 The Electrochemical Society. All rights reserved.