Gallium arsenide (GaAs) implanted with silicon forming intersubband of SiGaAs is a promising material for making novel electronic and optoelectronic devices. This paper is focused on finding optimum fluence condition for formation of  ntersubband of SiGaAs in GaAs sample after implantation with 50 keV silicon negative ions with fluences varying between 1 x 10¹⁴ and 1x 10¹⁶ ions cm^-2. The GaAs samples were investigated using X-ray photoelectron spectroscopy (XPS), UV-Vis.-NIR spectroscopy and X-ray diffraction (XRD) techniques. The X-ray photoelectron spectra for unimplanted sample showed peaks at binding energy of 18.74 eV and 40.74 eV indicating Ga3d and As3d core level, whereas the corresponding core level peaks for implanted sample were observed at binding energy of 19.25 eV and 41.32 eV. The shift in Ga3d and As3d core levels towards higher binding energy side in the implanted sample with respect to unimplanted sample were indicative of change in chemical state environment of Ga–As bond. The relative atomic percentage concentration of elemental composition measured using casa XPS software showed change in As/Ga ratio from 0.89 for unimplanted sample to 1.13 for sample implanted with the fluence of 1x 10¹⁶ ion cm^-2. The UV-Vis-NIR spectra showed absorption band between 1.365 eV and 1.375 eV due to the formation of intersubband of SiGaAs for fluences greater than 1 x 10¹⁵ ion cm^-2. The GaAs crystallite size calculated using Brus formula was found to vary between 162 nm and 540 nm, respectively. The XRD spectra showed the presence of Bragg's peak at 53.98° indicating (311) silicon reflection. The silicon crystallite size determined from full width at half maxima (FWHM) of (311) XRD peak was found to vary between 110 nm and 161 nm, respectively.