The carrier recombination processes in ZnO quantum dots ($\sim 4\mathrm{nm}$ in diameter), ZnO nanocrystals ($\sim 20\mathrm{nm}$ in diameter) and bulk ZnO crystal have been studied using photoluminescence (PL) spectroscopy in the temperature range from $8.5\text{to}300\mathrm{K}$. The obtained experimental data suggest that the ultraviolet PL in ZnO quantum dots originates from recombination of the acceptor-bound excitons for all temperatures. In the larger size ZnO nanocrystals, the recombination of the acceptor-bound excitons is the dominant contribution to PL only at low temperature $(T<150\mathrm{K})$. For higher temperatures $(T>150\mathrm{K})$, PL is mostly due to recombination of the donor-bound excitons. Recombination processes in ZnO quantum dots and nanocrystals differ from those in bulk ZnO mainly because of the large surface-to-volume ratio in both types of nanoparticles and, consequently, a large number of acceptor defects near the surface. No strong inhomogeneous broadening has been observed in ultraviolet PL from ZnO quantum dots. Our results shed light on the carrier-recombination processes in ZnO quantum dots and nanocrystals, and can be used for the ZnO nanostructure optimization for the proposed optoelectronic and spintronic applications.

• Received 15 November 2005

DOI:https://doi.org/10.1103/PhysRevB.73.165317