Phase transformation of amorphous-germanium thin film has been carried out on amorphous substrate byin situthermal-pulse annealing in a high vacuum chamber directly after evaporation. The microstructure of the resultant film was shown to depend markedly both on the annealing ambient and on the time of exposure(te)for the as-deposited films under full-powered(<102 W/cm2)incoherent broadband irradiation. The heating rate was estimated to be not less than102 K/s.The surface morphology of the sample was examined by atomic force microscopy (AFM), scanning electron microscopy, and optical microscopy. For samples annealed in air, hillock growth mode was observed, while for samples annealed in vacuum, a transition from the microgranular to dendritic grain growth, depending sensitively onte,was clearly evident. Surprisingly, the length of the crystallized dendrites could be as long as≃104 &mgr;m,being at least≃104times larger than the thickness of the film. The dendritic morphology, the implied growth rate, and the condition of crystallization lead us to suggest that the Ge film may exist in a supercooled semiconductive liquid phase just before crystallization. X-ray diffraction analysis revealed that grains were crystallized dominantly with a random orientation forte<2.84±0.05 s,while a sharp transition to a preferred ⟨110⟩ crystal orientation occurred at the criticalteof3.22±0.05 s,corresponding to a maximum temperature(Tm)of 577 °C reached by the system. This transition is consistent with the appearance of dendrites in AFM micrographs. More interestingly, an anomalous lateral size effect of the substrate on the misalignment of the ⟨110⟩ crystal direction of different grains with respect to the substrate normal was observed from the x-ray rocking curves. Careful inspection of the AFM images found that the giant dendrites broke up into individual columnar grains as the substrate width went down in size. ©1997 American Institute of Physics.