Evidence for As lattice location and Ge bound exciton luminescence in ZnO implanted with $^{73}$As and $^{73}$Ge

The results of photoluminescence (PL) measurements performed on high quality single crystal ZnO implanted with radioactive (73)Ga and (73)As, both of which decay to stable (73)Ge, are presented. Identical effects are observed in the two cases, with a sharp line at 3.3225(5) eV found to grow in intensity in accordance with the growth rate of the Ge daughter atom populations. On the strength of the well-established result that Ga occupies Zn sites, we conclude from the identical outcomes for (73)Ga and (73)As implantations that implanted As also preferentially occupies Zn sites. This result supports the findings of others that As preferentially occupies the Zn rather than the O site in ZnO. The thermal quenching energy of the 3.3225(5) eV line is found to be only 2.9(1) meV in contrast to its large spectral shift of 53.4(1) meV with respect to the lowest energy free exciton. The PL is attributed to exciton recombination at neutral Ge double donors on Zn sites involving transitions that leave the donor in an excited state.