Nitrogen ions at doses of 1 × 1013 - 1 × 10 15 cm-2 have been implanted in semi-insulating (SI) InP (100) single-crystal substrates. The surface topography of as-grown and implanted samples has been investigated using atomic force microscopy (AFM). The as-grown InP surface has minimum roughness and appears to be smooth. Angstrom-level pits-type defects are seen on the InP surface after the implantation. Root-mean-square roughness values measured for the as-grown and the implanted InP samples. The surface roughness increased with increasing the nitrogen-ion dose monotonically up to 5 × 1014 cm-2 and decreased at a dose of 1 × 1015 cm-2 the increase in the roughness was due to implantation-induced damages, and the decrease at higher dose 1015 cm-2 might have been be due to plastic deformation on the surface associated with a surface amorphisation. Post-implantation annealing smoothened the surface. Roughness values decreased relative to the implanted samples up to a dose 5 × 1014 cm -2, which indicates that the defects were partially annealed. At a higher dose of 1015 cm-2, there was no change in roughness value in spite of annealing. This indicates that the InP surface had been plastically deformed leading to amorphization at higher dose. Further, Raman characterization of these implanted samples clearly supports the surface amorphization at higher doses.