ZnO

a Ⅱ Ⅵ compound semiconductor with a wide direct band gap of 3.37eV at room temperature

is a multi functional material which can be used in varistors

phosphors

gas sensors

transparent conductive thin films and surface acoustic wave devices

etc. We prepared ZnO thin films on c plane sapphire substrates by metal organic chemical vapor deposition and investigated thoroughly the structure and optical properties by X ray diffraction (XRD)

Raman spectrum

X-ray photoelectron spectrum (XPS) and photoluminescence (PL) spectrum. ZnO thin films were fabricated on (0001) sapphire at optimized temperature of 580℃ under the reactor pressure of 480Pa by using a low pressure MOCVD apparatus. Diethylzinc (DEZn) and oxygen as the reactants were introduced into the reactor separately. The structure and crystallinity of the ZnO thin films were characterized by XRD. Only (0002) diffraction peak of ZnO with hexagonal wurtzite structure could be observed. The diffraction peak positioned at 2θ=34.48° and the value of the full width at half maximum (FWHM) was 0.184°

which implied a good crystalline quality of ZnO thin films. The lattice constant calculated were 0.3261nm and 0.5198nm for a and c axes respectively. It could be concluded that the tensile stress along the a axis exist in the ZnO thin film. The averaged grain size was 47.12nm. Raman scattering spectrum of the ZnO thin films was performed at room temperature. E

2

(high) mode and A

1

(LO) mode were at 435.32cm

-1

and 575.32cm

-1

respectively. The former corresponded to the band characteristic of wurtzite phase

while the latter was related to the formation of oxygen deficiency

interstitial Zn and free carrier. Both modes shifted to high frequency side due to the tensile stress along the a axis. In addition

X ray photoelectron spectrum showed that O

2

could adsorb in the surface of the sample. After etching for 10 minutes

the quantity of O

2

adsorbed was reduced and the binding energy of O

1s

was 530.2eV

which was close to that in Zn-O bond. Simultaneously

the PL spectrum of the ZnO thin film was measured. A strong near band edge (NBE) emission was obviously observed

which was at 3.28eV with the FWHM of 125 meV. The deep level emission (DLE) centered on 2.55eV was quite weak. The ratio of the intensity of the NBE to that of DLE was 40:1

which suggested better optical quality of the ZnO film grown by MOCVD at the present growth conditions.