Optical investigations on excitons bound to impurities and dislocations in ZnO

The optical properties of excitonic recombinations in bulk ZnO are investigated by photoluminescence (PL) measurements. At liquid helium temperature the neutral donor bound excitons are positioned at 3.364, 3.362 and 3.361 eV, the line at 3.364 eV dominates the PL spectra. Annealing of the crystals demonstrates that the 3.364 eV vanishes, it is most likely caused by the hydrogen related donor. Two-electron satellite transitions of the donor bound excitons allow to determine the donor binding energies to 43, 52 and 55 meV. These results are in line with the temperature dependent Hall effect measurements. In the as-grown crystals two donors with binding energies of 30 and 50 meV control the conductivity, whereas after annealing only one donor with a binding energy of about 50 meV is necessary to fit the data perfectly. In addition at 3.335 eV an excitonic recombination is observed, which supported by spatially resolved cathodoluminescence measurements, is attributed to excitons bound to structural defects. (C) 2003 Elsevier Science B.V. All rights reserved.