We show the structural and optical properties of non-polar a-plane GaN epitaxial films modified by Si ion implantation. Upon gradually raising Si fluences from 5 10¹³ cm⁻² to 5 10¹⁵ cm⁻², the n-type dopant concentration gradually increases from 4.6 10¹⁸ cm⁻² to 4.5 10²⁰ cm⁻², while the generated vacancy density accordingly raises from 3.7 10¹³ cm⁻² to 3.8 10¹⁵ cm⁻². Moreover, despite that the implantation enhances structural disorder, the epitaxial structure of the implanted region is still well preserved which is confirmed by Rutherford backscattering channeling spectrometry measurements. The monotonical uniaxial lattice expansion along the a direction (out-of-plane direction) is observed as a function of fluences till 1 10¹⁵ cm⁻², which ceases at the overdose of 5 10¹⁵ cm⁻² due to the partial amorphization in the surface region. Upon raising irradiation dose, a yellow emission in the as-grown sample is gradually quenched, probably due to the irradiation-induced generation of non-radiative recombination centers.