On the role of group I elements in ZnO

Diffusion of Li, Na and K into single-crystal substrates of ZnO was performed. We compare the results with ZnO epitaxial films doped with the respective elements during growth. The diffused and in-situ doped layers were studied using mass spectroscopy and low temperature photoluminescence spectroscopy. Li and Na are known to produce deep acceptor centers which give rise to shallow donor to deep acceptor recombinations in the visible spectral region. We will demonstrate that shallow acceptors are also introduced, having binding energies around 300 meV. A donor-acceptor pair recombination (zero phonon line at 3.05 eV) with LO phonon replica is observed. We further investigated bulk ZnO crystals which contained the deep Li acceptor by thermal treatments under H-2 atmospheres. With increasing annealing temperature shallow donors are introduced as monitored by EPR while the EPR signal of the neutral Li acceptors decreases. Quite unexpectedly, the shallow Li acceptor centre which is not present in the as-grown state is also created.