Correlation between structure and photoluminescence of c-axis oriented nanocrystalline ZnO films and evolution of photo-generated excitons

The structural and photoluminescent properties as well as the evolution of photo-generated excitons were studied in combination with the effects of thermal annealing for c-axis oriented nanocrystalline zinc oxide (nc-ZnO) films on Si (100) prepared by plasma assisted reactive pulsed laser deposition. The extra energy and the reactive oxygen species provided by ECR oxygen plasma result in the low-temperature c-axis oriented growth of nc-ZnO with wurtzite structure and low density of oxygen vacancies in the grown ZnO. The annealing process leads to the improvement of crystallinity and the growth of crystallites at the expense of the introduction of oxygen vacancies. The prepared nc-ZnO films emit strong near band edge (NBE) photoluminescence (PL) associated with longitudinal optical (LO) phonon replicas of free and bound excitons at 325 nm light excitation at 7 K. The temperature-dependent PL features reveal the evolution of radiative recombination processes with temperature and indicate that the room-temperature NBE luminescence is dominated by the LO phonon replicas of free excitons, whereas inappreciable visible emission in the room-temperature PL from as-deposited nc-ZnO suggests low-density oxygen vacancies in the samples.