Near band edge emission of MBE grown ZnO epilayers: identification of donor impurities and O2 annealing effects

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the near band edge emission (NBE) spectra of unintentionally doped ZnO epilayers. It is shown that the different NBE transitions involve donor centres only. The lines can be divided into two groups. Those lying on the low energy side consist of recombinations of neutral-donor bound exciton complexes, while those lying just below the A free excitonic emission correspond to donor-to-valence band transitions. One of the donors is identified as indium. The emission energy of the In-donor bound exciton, the spectra of the bound exciton excited states, the In-related D°h transition energy, and the electronic spectrum of the In-donor are established. The binding energies of another three donor centres are evaluated. The occurrence of two of these centres is shown to depend on the stoechiometry, as revealed by annealing experiments in an O 2 atmosphere.