Zinc oxide

as one of novel photoelectric materials

has been drawn much attention recently due to its many advantages.For example

its large band gap of 3.37 eV and high excitonic binding energy of 60 meV make ZnO a good candidate for developing short wavelength optoelectronic devices.Furthermore

the p-doping ZnO can be produced and the p-n structure of ZnO can be formed on the silicon substrate

their characteristics make ZnO to be compatible with Si-based integrated circuits.Hence

ZnO film has wider application in optical and electronic industries.ZnO films can be easily prepared by several techniques such as reactive sputtering

chemical vapor deposition

and molecular beam epitaxy.The film prepared by sputtering has good adhesive to the substrate and its composition can be easily controlled in the deposition process.However

as compared to the films prepared by other methods

the stress of sputtering film is more obvious

the grain size is smaller and the defect density of grain interface is rather larger accordingly.Such disadvantages of sputtering film may degrade its physical properties

such as the intensity of photoluminescence

quantum efficiency of emission and respondence of current-voltage(I-V)behavior.Many studies have demonstrated that the thermal annealing process is a conventional way to improve the quality of sputtering film.For example

after annealing ZnO film in high temperature

it was found from the deep level transient spectroscopy(DLTS)and I-V measurements that some deep energy levels

which might depress the intensity of luminescence of the film

could be removed from the film and the electrical properties of the film were also optimized.As to further understand the thermal annealing effect on the properties of ZnO film

in this paper we utilize atomic force microscopy(AFM)and ellipsometry to study the annealing dependence of morphology and ellipsometric of the ZnO film.The results demonstrated that the morphology varies little when annealing temperature

T

a

<

850℃.However

it is found the grain size to grow larger apparently after annealing at

T

a

>

850℃

and the corresponding distribution of grain size is irregular.Accordingly

the ellipsometric parameters

such as △ and index n

are also changed with

T

a

.The change of △ and n when

T

a

<

850℃ can be attributed to improvement of the grain structures

variation of the ratio of oxygen and zinc

decrease of the adsorbated oxygen on the film and void in the film

and increase of the film density.On the other hand

when

T

a

>

850℃

decrease of the defect density

especially the decrease of the defects of grain interface should be mainly responsible for the variation of △ and n.