Er³⁺ induced point defects in ZnO and impact of Li⁺/Na⁺/K⁺ on the vacancy defects in ZnO:Er studied by positron annihilation spectroscopy

The defects species in as-prepared ZnO, Er-doped, and co-doped (with Li/Na/K) nanocrystallites synthesized by combustion method were examined by Positron Annihilation Lifetime (PAL) and Doppler Broadening (DB) spectroscopy. The S and W parameters were derived from positron trapping of valence and core electrons extracted from the central and wing regions of DB spectra to identify the vacancy-type defects. For as-prepared Er-doped ZnO samples, the observed high value of S indicated that the vacancies and vacancy cluster defects were located at the grain surface and two-grain junction. With co-doping of Li/Na/K, the S value was decreased due to the dopant occupying the vacancy clusters. On annealing, the major recovery of such defects was observed with enormous decreasing S for Li co-doped sample. Positron lifetime variations signified the presence of defects compared with bulk ZnO. Er doping in ZnO evidenced a small increase of defects with a gradual decrease when co-doped with Li/Na/K indicating the co-dopant impurity occupied the vacancies in the grain boundary. The first lifetime component of 193 ps owing to the Zn-type vacancies in as-prepared ZnO were increased on Er doping, evidencing the replacement of the Zn²⁺ by Er³⁺ ion and thereby created point defects due to charge compensation. On annealing to 800 °C, Li co-doped ZnO:Er resulted to enormous reduction of second lifetime τ₂, mean lifetime τ_ave, and S value signifying the Li ion localised in Zn vacancy site. Annealed ZnO:Er + Na/K resulted in considerable intensification in defect concentrations than ZnO:Er + Li due to the vacancy migration to form as clusters. The defect type, defect modification, the influence of Er together with co-doped Li/Na/K in ZnO, and their effect on annealing are elucidated in detail from the positron annihilation characteristics.