In this work the IIb-VI compound semiconductor ZnO is doped, via ion implantation of stable and radioactive isotopes, in order to investigate the chemical nature of exciton recombinations bound to previously unidentified defects. Photoluminescence (PL) is discussed and is used extensively as the primary investigative technique. A new defect emission feature, centred around 3.324 eV, is found to be related to Ge impurities occupying substitutional Zn sites in ZnO. This centre is investigated by temperature dependent PL, piezospectroscopy and Zeeman spectroscopy. The centre is donor-like in nature. Uniaxial stress measurements indicate that the defect centre has trigonal symmetry and applied magnetic field measurements reveal the neutral charge state of the centre and the donor-like binding mechanism. Subsequent to this, a study is undertaken of the isoelectronic defect Hg in ZnO studying the zero phonon feature at 3.279 eV and its associated phonon replica band. Temperature dependent measurements reveal two close lying excited states with a common ground state, and a large thermal stability is reported for the defect from temperature dependent measurements. Uniaxial stress measurements reveal an excited state and a non-degenerate ground state for the defect in a centre of trigonal symmetry. Finally, a previously studied donor binding centre related to In is observed after implantation of radioactive 117Ag, which decays to 117In, through 117Cd. The 117In then decays to 117Sn. This results in three lines in the near band edge spectra, two related to substitutional In and one which is tentatively proposed to be due to Sn impurities. This hypothesis is investigated further via a chemical doping technique and a positive correlation is found between Sn impurities and the appearance of the neutral bound exciton centre, I10.