This paper analyzes the effect of 100[Formula: see text]keV silicon negative ion implantation in semi-insulating gallium arsenide sample for the fluences varying between [Formula: see text] and [Formula: see text][Formula: see text]ion[Formula: see text]cm[Formula: see text] using Raman spectroscopy, Rutherford backscattering spectroscopy and Electron spin resonance spectroscopy. The gallium arsenide sample implanted with silicon negative ion for different fluences showed shift in the TO peak position with respect to unimplanted gallium arsenide sample. Increase in the broadening of TO peak was observed in the as-implanted samples, indicating development of stress and phonon confinement due to the incorporation of silicon in gallium arsenide crystal lattice. Annealing of as-implanted samples showed stress relaxation. Increase in RBS backscattering yield was observed in the as-implanted samples. Annealing of as-implanted (with high fluence) sample showed flat RBS yield response. ESR measurement study revealed restructuring of defects in the gallium arsenide sample implanted with fluence of [Formula: see text][Formula: see text]ion[Formula: see text]cm[Formula: see text] after annealing to the temperature of [Formula: see text]C.