Study On Preparation And Properties Of ZnO Thin Films By PLD

Zinc oxide(ZnO) is a kind ofⅡ-Ⅵwide band gap compound semiconductor of wurtzite structure with a direct band gap of 3.37 eV at room temperature,exciton binding energy of 60 meV,which makes it have many applications,such as transparent conducting films,surface acoustic wave devices,gas sensors and photo-electronic devices.The report of ultraviolet laser emission of high quality ZnO thin films at room temperature attracted a lot of attention in ZnO researching.Like GaN compound,the research about ZnO has burned white hot in the range of wide band gap.The high quality ZnO thin films deposited by femtosecond pulsed laser deposition(fsPLD) on Si substrates were investigated in this dissertation.First,the progress of preparation techniques for ZnO thin films was summarized,the characteristics of the ZnO films deposited by fsPLD was analyzed,and a method to deposit large area ZnO thin films was provided.Second,the effects of technology parameters,such as substrate temperature,single pulsed laser energy,oxygen pressure and annealing on the quality of ZnO thin films deposited by fsPLD were studied in detail.The differences of the ZnO thin films deposited by fsPLD and nanosecond pulsed laser deposition(nsPLD) were investigated.The structural properties of the thin films were analyzed by x-ray diffraction(XRD).The surface morphologies of the films were observed by field scan electron microscope (FESEM) and atom force microscope(AFM).The optical properties of the thin films were investigated by ultraviolet-visible(UV-vis) spectra,Fourier-transform infrared(FTIR) spectra and photoluminescence(PL) spectra.A series of new results were obtained.Highly c-axis orientation degrees of ZnO thin films were obtained on lower substrate temperature(80℃) deposited by fsPLD for the first time.The c-axis orientation degrees, (002) spacing,and the visible transmittances reached the max at 80℃.The mean grain size of the films increased with the increasing of substrate temperature.The(002) spacing decreased and the orientation degrees had no changes after annealing.The mean grain size deposited at different temperature increased and reached to the same size after annealing. Highly efficient luminescent ZnO thin films were prepared at lower oxygen pressure(2.0 mPa) deposited by fsPLD.The PL spectra of ZnO films mainly contain the UV emission band and the visible emission band.The UV emission band is due to the near band-edge emission,and the visible emission band is result in impurities or defects.The films contained UV emission peak at wavelength of 382 nm and violet emission peak at 410 nm growth below oxygen pressure 1.0 mPa.The violet emission peak of 410 nm disappeared while the oxygen pressure increasing to 2.0 mPa and the visible emission band become very weak at the same time.The visible emission band disappeared completely with the oxygen pressure increasing to 5.0mPa,and only a narrower and stronger UV emission peak was in existence at 376 nm.It indicated that the visible emission band was resulted from oxygen vacancy.By increasing the pulsed laser energy,we could obtain the smaller and denser crystalline ZnO films,which is helpful for the UV emission.After annealing in the air at 600℃for 10 hours,the transmittances of the ZnO films increased,and the Zn-O absorption peak in the FTIR spectra became narrower and stronger.Comparing with nsPLD method,the ZnO thin films deposited by nsPLD had smaller craystallinity,a smoother surface,higher visible transmitances in UV-vis spectra and narrower UV emission peak at 376 nm in PL spectra.