ZnO is a wide band-gap semiconductor with good electrical and optical properties. ZnO has higher exciton binding energy of 59 meV at room temperature

leading to a lower threshold

and is favorable for efficient operation of optical devices. Short wavelength devices based on ZnO have become even more interesting. On the other hand

band gap devices based on ZnO/ZnMgO superlattices or quantum wells can confine both excitons and photons in the low dimensions

making the stimulated exciton-related emission process more efficient. Therefore

keeping focus on the Mg

x

Zn

1-x

O films for purpose of exploring its potential applications in ultraviolet optoelectronics is more and more important.High quality Mg

x

Zn

1-x

O alloy films have been grown by plasma-assisted molecular beam epitaxy on c-sapphire (c-Al

2

O

3

) substrate. The growth temperature was 800℃

the temperature of the zinc source is fixed at 245℃

and the flow rate of oxygen is 0.8 sccm. The Mg

x

Zn

1-x

O films were obtained with different Mg contents by changing the temperature of the Mg source. The quality of the Mg

x

Zn

1-x

O films was improved by growing ZnO buffer layers at low temperature. Their crystal structures are characterized by X-ray diffraction spectroscopy (XRD). The XRD patterns indicate all the Mg

x

Zn

1-x

O films with the (002) preference orientation of hexagonal wurtzite structure. When x value is varied from 0 to 0.15

the (002) diffraction peak of Mg

x

Zn

1-x

O shifts to the large angle side with increasing Mg contents

and the full wide at half maximum (FWHM) of the diffraction peak is widen with increasing Mg contents. The lattice constant of c-axis decreases from 0.5205 nm to 0.5189 nm as the Mg content increased from 0 to 0.15. The FWHM is only 0.145° for the Mg

0.15

Zn

0.85

O film

which exhibited the high quality of the Mg

x

Zn

1-x

O films. The intense ultraviolet emission was shown in photoluminescence spectra at room temperature

which shifts from 3.29 eV(x=0) to 3.54 eV(x=0.15) with increasing x values. The bandgaps of the films were evaluated by using the squared absorption coefficient (α

2

) of Mg

x

Zn

1-x

O films as a function of photon energy. The origin of the ultraviolet emission is studied by the PL spectra measured at the temperature from 80 K to 280 K. The emission peaks show a redshift

the FWHM of the emission peak widen and the intensities of the emission peak decreased with increasing the temperature. The temperature-dependent PL-integrated intensity of Mg

x

Zn

1-x

O were fitting by the equation:

I

=

I

0

/[1+

A

exp(-

E

/

k

B

T

)] (where E is the activation energy of the thermal quenching process

kB is Boltzmann constant

I

0

is the emission intensity at 0 K

T

is the thermodynamic temperature

and A is a constant). A fit of the experimental data to the equation yields

E

=54 meV

which agrees well with the exci-ton binding energy of 59 meV for bulk ZnO. Therefore

the ultraviolet emission peak in PL spectra of the Mg

x

Zn

1-x

O alloy films is attributed to the free exciton emission

indicating the high quality of Mg

x

Zn

1-x

O film.