  We report a systematic study on 1.2 MeV Ar^8+ irradiated ZnO by x-ray
diffraction (XRD), room temperature photoluminescence (PL) and
ultraviolet-visible (UV-Vis) absorption measurements. ZnO retains its wurtzite
crystal structure up to maximum fluence of 5 x 10^16 ions/cm^2. Even, the width
of the XRD peaks changes little with irradiation. The UV-Vis absorption spectra
of the samples, unirradiated and irradiated with lowest fluence (1 x 10^15
ions/cm^2), are nearly same. However, the PL emission is largely quenched for
this irradiated sample. Red shift of the absorption edge has been noticed for
higher fluence. It has been found that red shift is due to at least two defect
centers. The PL emission is recovered for 5 x 10^15 ions/cm^2 fluence. The
sample colour is changed to orange and then to dark brown with increasing
irradiation fluence. Huge resistivity decrease is observed for the sample
irradiated with 5 x 10^15 ions/cm^2 fluence. Results altogether indicate the
evolution of stable oxygen vacancies and zinc interstitials as dominant defects
for high fluence irradiation.
