In recent years

ultraviolet(UV)photodetector has drawn a great deal of interest

due to their potential application such as solar astronomy

fire alarm and combustion monitoring

missile plume detection

space-to-space transmission.Zinc oxide(ZnO)has large exciton binding energy(60 meV)at room temperature

and a large band gap at room temperature(3.37 eV)

and low power threshold for optical pumping at room temperature

high electromechanical coupling factor and high stability under hard conditions

ZnO is recognized as promising material in the ultraviolet(UV)detectors.High quality ZnO films can be obtained by various deposition techniques such as radio frequency(rf)magnetron sputtering technique

pulsed laser deposition(PLD)

metalorganic chemical vapor deposition(MOCVD)

and also molecular beam epitaxy(MBE).Among these techniques

the operation of radio frequency magnetron sputtering technique is simply and lower cost.ZnO thin flms were grown on quartz substrates using radio frequency(rf)magnetron sputtering technique.Before sputtering

the quartz substrates were cleaned in an ultrasonic bath with acetone

ethanol and deionized water at room temperature and the zinc(99.999%)target was etched with diluted nitric acid to remove the contamination.The base pressure of sputtering chamber was evacuated to be below 5×10

-4

Pa

and then filled to 1.0 Pa with mixed gases of 99.99% pure Ar and O

2

which were introduced into the sputtering chamber though a set of mass flow controllers with the rates of 60 and 20 sccm(standard cubic centimeter perminute)

respectively.The target-substrate distance was maintained at 50 mm and the substrate temperature was controlled at 400℃.The rate of deposition was adjusted so as to have a film thickness of nearly 400 nm during the film growth.The MSM structure with interdigitated configuration on the ZnO thin films was obtained by lithography and wet etching.It consists of 12 fingers at each electrode 5 μm wide and 500 μm long and have an interelectrode spacing of 5 μm.The θ-2θ XRD patterns of the ZnO films indicated that the highly c-axis oriented film was grown.The PL spectrum of the ZnO thin film taken at room temperature shows a luminescence peak at 3.3 eV.With the applied bias below 3 V

the dark current of the ZnO MSM UV detector is below 250 nA.The typical responsivity peaked at around 370 nm

and had values of 0.34 A/W.In addition

the UV(360 nm)to visible(420 nm)rejection ratio of around four orders can be extracted from the spectra response.Furthermore

the transient response measurement revealed its fast photoresponse with a rise time of 20 ns.