A unique growth mode of Ge on Si(110) substrates was clarified by photoluminescence (PL) spectroscopy. A spectral redshift and an increase of the relative no-phonon intensity were found for PL from the two-dimensional Ge layer on Si(110) compared to that on Si(100). These results likely arise from nonuniformity in the Ge layer thickness owing to the step-bunched Si(110) surface and resultant exciton localization. The two-dimensional to three-dimensional growth mode changeover was observed as evidenced by emergence of broad PL from Ge islands. In contrast to Ge on Si(100) PL from the wetting layer was found to show continuous redshift with increasing Ge coverage even after Ge island formation. ©1997 American Institute of Physics.