Recently much attention has been paid to short wavelength lasers for use in high density information storage.It is widely accepted that ZnO is one of the most promising materials for producing an ultraviolet laser at room temperature due to its wide direct band gap(E

g

=3.37eV) and large excitonic binding energy of 60meV.It has potential uses in optoelectronical systems such as light emitting diodes(LEDs)

photodetectors

electroluminecence devices and solar cells.To obtain high quality ZnO thin films a variety of techniques may be used such as molecular beam epitaxy(MBE)

metal organic chemical vapor deposition(MOCVD)

magnetron sputtering

pulsed laser deposition

and so on.Among them

MOCVD provides the advantage of growing high-quality films due to its versatility in controlling the various thermodynamic interactions.ZnO has not only the same crystal structure as GaN

but also a larger exciton binding energy of 60meV

which is 2.4 times that of GaN.This indicates that ZnO should be the most potential material to realize the next generation UV semiconductor laser.High quality ZnO films were deposited on c-Al

2

O

3

substrate by the plasma-assisted metal-organic chemical vapor deposition.Strong optically pumped ultraviolet(UV) emission was observed.PL spectra under different excitation powers were performed.The PL intensity as a function of the excitation power

it is obvious that the PL intensity is linearly dependent on the excitation power.This indicates that the dominant photoluminescence of the sample should be the excitonic radiative recombination at room temperature.Optically pumped UV lasing spectrum of ZnO film was observed.The samples were optically pumped by a frequency-tripled mode-locked Nd:YAG laser 355nm emission

with 10 Hz repetition rate

and 15 ps pulse width.The pump beam was focused to a spot with diameter of about 25μm on the surface of ZnO film.The threshold of lasing was as low as 0.28 μJ.From the lasing spectrum

we could find that the lasing peak has much narrow line-width less than 0.03nm.In this paper

high quality ZnO films were grown on c-Al

2

O

3

by MOCVD.Optimized growth condition was obtained by XRD and PL measurements.ZnO films were strongly c-oriented from XRD measurement.Optically pumped UV lasing was observed in all kinds of directions at room temperature.This was due to the self-formed resonance cavities.