The Diffusion Trapping Model has been used to obtain the positron annihilation Doppler broadening lineshape parameter in ZnO and O+, B+, N+, Al+ implanted ZnO films. The concentration of vacancy clusters is found to be related to the atomic number and the fluence of the implanted ion. The S-parameter is found to be largest in the case of implantation of Al+ ions and is minimum for the implantation of B+ ions. Thus, the vacancy clusters are found to be largest in the case of Al+ implantation. The calculated results have been compared with the experimental value.