Zinc-oxide (ZnO) is an Ⅱ-Ⅵ semiconductor with a wide direct band gap of 3.37eV at room temperature (RT) and a high exciton binding energy of 60meV. Because of this feature

ZnO is a promising candidate for use in ultraviolet wavelengths light emitting devices and lasers. ZnO thin films have been grown on a (400) Si substrate by plasma molecular beam epitaxy (P MBE). Before growth

the Si substrate was chemically cleaned by a standard RCA process and then was thermally treated at 650℃ in ultrahigh vacuum for about 5min. During growth

the Si substrate temperature was at 450℃.The plasma power was kept up 300W. The zinc-flux of purity zinc(6N) source and oxygen(5N) overpressure in the growth chamber were kept at around 4×10

-5

Pa and 6×10

-3

Pa

respectively. Obtained thickness of the sample is about 100nm(Sample S1). To improve the crystal quality

the samples were annealed in oxygen for two hours at temperature of 700℃(Sample S2)

800℃(Sample S3)

and 900℃(Sample S4)

respectively. The ZnO thin films were characterized by X-ray using a rotating anode XRD with CuKα radiation wavelength of 0.154nm. PL spectra were excited by 325nm line of a He Cd laser. The X-ray diffraction spectra show a strong (002) diffraction peak and weaker other orientation peaks of the ZnO thin films. This indicates that the grown samples are polycrystalline structure. The full width at half maximum(FWHM) of S1

S2

S3 and S4 samples are 0.257

0.244

0.236

0.206°

respectively. Above result proves that the crystal quality is improved with increasing annealing temperature. In the PL spectra

an ultraviolet (UV) emission is observed at RT for all samples. In order to verify the origin of this UV emission

temperature dependences of the PL intensities are measured. This UV band at RT comes from the emission of free exciton and radiative recombination of bound exciton associated with one LO phonon

in which bound exciton is considered to be related to deep centers.