a b s t r a c t Nanopatterning of semi-insulating GaAs (1 0 0) by 50 keV Ar + ion irradiation at an angle of 50 • with respect to surface normal is investigated systematically. The samples were analyzed using atomic force microscopy and Raman spectroscopy as a function of fluence. Irradiation with fluence of 1 × 10 17 ions/cm 2 results in the formation of nano-sized dots on the surface of GaAs which ripens at a fluence of 3 × 10 17 ions/cm 2 and shows fragmentations at a fluence of 7 × 10 17 ions/cm 2 . The average diameter of dots varies between 20 and 35 nm in the fluence range. It is also observed that the density of dots first decreases with increase in fluence and later increases due to fragmentation of dots. Non-linear behavior of surface roughness is also observed with increase in fluence. Raman spectroscopy shows that the ion irradiation leads to amorphization of the irradiated surfaces in this range of fluence and relaxations and smoothing of the surface via decreased strain.