Strong ultraviolet emission and rectifying behavior of nanocrystalline ZnO films

Nanocrystalline ZnO thin films on p-type Si(1 0 0) substrates have been prepared by a sol-gel method. X-ray diffraction results showed polycrystalline wurtzite with a preferential (0 0 2) orientation. Morphology studies revealed that the films consisted of uniform grains in size of ∼33 nm. A strong ultraviolet emission with no distinct visible emissions was observed in room temperature photoluminescence spectrum. The ultraviolet emission results from the recombination of free exciton with stronger longitudinal optical (LO) phonon replica. Stronger LO phonon replica may be ascribed to the strong interaction between the exciton and LO phonon in nanostructure. Nanostructure of films is also responsible for the blueshift of free exciton emission compared with single-crystal ZnO. The absence of E1 (LO) mode in Raman spectrum further confirmed low defect density in as-grown nanocrystalline ZnO films. The electrical junction properties characterized by current–voltage measurement showed a rectifying behavior.