Study On The Preparation And Properties Of ZnMgO Thin Films

As a â…¡-â…¥ compound semiconductor material, Zinc oxide （ZnO） possesses standardhexagonal wurtzite structure. ZnO, which is a typical luminescent material, has many obviousadvantages in short wavelength source such as blue-green light, UV light and so on. Therefore,ZnO can be used to develop many luminescent devices, such as planar optical waveguides,transparent electrodes, UV detectors, UV luminous devices and so on. However, with thedevelopment of science and technology and increase of demands of people for the applicationof devices, some disadvantages of ZnO in applications become obvious increasingly, forexample, the exciton binding energy and the band-gap are not large enough. Therefore, peoplepay attention to doped materials based on ZnO. The band-gap of MgO is7.8eV, which ismuch larger than that of3.37eV of ZnO. The radii of Mg²⁺and Zn2+are similar (Mg²⁺:0.057nm; Zn2+:0.060nm). The solid solubility of Mg is much smaller in ZnO. On the basis ofMg doping quantity, the band-gap of Zn1-xMgxO （0≤x≤1） alloy can be changedcontinuously from3.37to7.8eV. Nowadays, Mg doping ZnO has caused extensive attentions.On the other hand, the adjustability of Zn1-xMgxO pioneer a new research field for late-modelsemiconductor devices on the basis of “band-gap engineer”.Many different deposition methods have been employed to fabricate ZnMgO films, suchas metal organic chemical vapor deposition （MOCVD）, pulsed laser deposition （PLD）,molecular beam epitaxy （MBE）, e-beam evaporation （E-BED）, magnetron sputtering （MS）,sol-gel and so on. Each method has its own advantages and disadvantages. Though thedeposition method and the study for structure, bandgap and optical property of ZnMgO hasbeen perfect, there are still many problems to reveal, such as controling the change of Mgcontent in films, bandgap changing with the Mg content and so on. We have done researchesby changing fabrication and annealing conditions by using PLD and MS techniques so as toobtain ZnMgO films which have special properties, and then made corresponding analysis forthe structure, content, bandgap and optical property.The main experiment contents and results of the paper are as follows:（1） The ZnMgO target with a diameter of2.5cm was made by mixing MgO powder of purity99.999%and ZnO powder of purity99.999%（the molar ratios of Zn:Mg=9:1）. Then,ZnMgO films were fabricated by PLD technique at different Si substrate temperatures in N₂,and the N₂pressure was fixed at1.0Pa. We used X-ray diffraction （XRD）, scanning electronmicroscope （SEM）, infrared radiation （IR）, photoluminescence （PL） to characterize ZnMgOfilms. The results show that the ZnMgO thin film with the maximum crystal size and bestcrystal structure was obtained at400℃. The largest band-gap value was4.23eV throughcalculation, and the UV emission peak blue-shifted from398nm to383nm with increasingtemperature.（2） The ZnMgO films were prepared on Si substrates at300℃by PLD method. Then,the films were annealed in annealing furnace in air from400to800℃for15minutes. Theresults show that Zn2+could be substituted easily by Mg²⁺at annealing temperature of500℃,and the Mg concentration and the band-gap were26.2%and4.29eV, respectively. A strongerUV emission and weaker visible emission were observed, and the emission mechanisms areanalyzed.（3） The target with diameter of7.5cm was prepared with the molar ratios of Zn:Mg=1:4.ZnMgO thin films were deposited on Si（111） substrates by magnetron sputtering. Thetemperature and the power were set as200℃and110W, respectively. Then the films wereannealed at180、240、300、360、420℃for15min, respectively. We made detailed analysisand explaination for the structure, surface morphology and optical property of ZnMgO films.The results show that the structural property of ZnMgO thin film is affected by annealingtemperature under300℃. The strongest UV emission peak is achieved at annealingtemperature of240℃, and the mechnisms of visible emission were discussed as electrontransition.（4） We used magnetron sputtering technique to prepare ZnMgO films on Si substrates. Inthe experiment, the power was120W, and the ambient was O2and N₂with flow-ratio1:4. Allthe films were annealed at300℃in air for20,25and30min, respectively. In order to studythe effect of annealing temperature on the films, we made detailed analysis and explaination.The results of structural and optical measurement show that the optical property is obviouslyaffected by annealing tome. The blue shift and red shift were observed, and the machnisms ofpurple and green emission were analyzed further.