In the present work, semi - insulating GaAs samples implanted with 50 keV silicon negative ions with fluences varying from 1 x 10¹⁴ to 1 x 10¹⁶ ions cm^-2 have been investigated using scanning electron microscopy, atomic force microscopy and X-ray diffraction techniques. Scanning electron microscopy showed presence of pores on the surface of implanted samples. Energy dispersive X-ray analysis showed the increase in silicon concentration in GaAs sample from 1.00 atomic % for sample implanted with 1 x 10¹⁵ ion cm^-2 to 1.30 atomic % for sample implanted with 1 x 10¹⁶ ion cm^-2. Atomic force microscopy showed Lorentzian distribution of nano - structured hillocks on the surface of sample implanted up to the fluence of 1 x 10¹⁵ ion cm^-2. X-ray diffraction spectra showed increase in intensity of GaAs (400) Bragg’s peak for sample implanted with the fluence of 1 x 10¹⁶ ion cm^-2 compared to un-implanted sample.