Vacancy clustering and acceptor activation in nitrogen-implanted ZnO

The role of vacancy clustering and acceptor activation on resistivity evolution in N ion-implanted n-type hydrothermally grown bulk ZnO has been investigated by positron annihilation spectroscopy, resistivity measurements, and chemical profiling. Room temperature 2200.3em0exkeV N implantation using doses in the low 10¹⁵0.3em0excm^-2 range induces small and big vacancy clusters containing at least 2 and 3--4 Zn vacancies, respectively. The small clusters are present already in as-implanted samples and remain stable up to 10000.2em0ex^∘C with no significant effect on the resistivity evolution. In contrast, formation of the big clusters at 6000.2em0ex^∘C is associated with a significant increase in the free electron concentration attributed to gettering of amphoteric Li impurities by these clusters. Further annealing at 8000.2em0ex^∘C results in a dramatic decrease in the free electron concentration correlated with activation of 10¹⁶--10¹⁷0.3em0excm^-3 acceptors likely to be N and/or Li related. The samples remain n type, however, and further annealing at 10000.2em0ex^∘C results in passivation of the acceptor states while the big clusters dissociate.