Germanium thin films have been epitaxially regrown on (001) GaAs byin situthermal pulse annealing of evaporated amorphous germanium under<102Watt/cm2broad-band irradiation in high vacuum. Epitaxial regrowth was found to occur only when the duration of the thermal pulse(te)was greater than a critical valuetc(≃3.20 s). The crystal quality of the resultant film was examined by high resolution x-ray diffraction technique (HRXRD) and grazing-incidence x-ray diffraction technique (GIXRD). All rocking curves were found to have a full width at half-maximum of about 0.02°. Both HRXRD and GIXRD confirmed the Ge overlayer was grown epitaxially as well as pseudomorphically on the substrate. Scanning electron microscopy and atomic force microscopy revealed the very different surface morphologies resulting from differentte.Forte<tc,columnar germanium grains with a four-fold symmetry and a high uniformity in size were found, while forte⩾tc,epitaxial regrowth was observed. It is suggested that epitaxial regrowth takes place via a temporary formation of liquid phase Ge. ©1997 American Institute of Physics.