ZnO thin films with strong (002) preferred orientation have been deposited on Si(100) substrates using two-step AP-CVD. In the two-step growth process

a ZnO buffer layer was firstly grown at 450℃ for 5 min

and then a top layer was grown at 350℃ for 20 min. Arelatively high temperature buffer layer of ZnO provided a stable crystalline template for subsequent low temperature top layer to grow upon. In lattice-mismatched growth

ZnO thin film has a tensile built-in strain. The tensile strain in ZnO can be relaxed by providing sufficient thermal energy and meanwhile this relaxation process can give rise to more defects. Strong UV emission located at 381 nm is observed when excited with 325 nm light. The concentration of oxygen vacancies in the thin films increased after annealing at 800℃

and the 450~600 nm green emission band is observed. PLspectra for ZnO thin films grown by one-step method show typical UV emission located at 389 nm as well as blue emission centered at 437 nm. Likewise

the band of green emission is observed in PLspectra of thin films annealed at 800℃. The intensity of UV emission was dramatically decreased after annealing

which could be due to the increase of the size of ZnO particles and defect concentration. For the ZnO thin film grown by two-step method

the intensity of green luminescence after annealing in oxygen became weaker than that after annealing in air

so the corresponding mechanism for green emission is attributed to VO. For the ZnO thin film grown by one-step method

the intensity of green luminescence after annealed at oxygen became stronger than that after annealing in air. The relaxation process of tensile strain preferred to give rise to more O

Zn

defects. Therefore

the corresponding mechanism for green emission is attributed to O

Zn

. I-V properties also indicated that the emission mechanism of O

Zn

was reasonable.