Study Of Porous Nanocrystalline TiO₂ Thin Film Deposited By Atmospheric Pressure Plasma-enhanced Chemical Vapor Deposition

In recent years,people have considerable interest in plasma-enhanced chemical vapor deposition(PE-CVD)of thin films by Dielectric Barrier Discharge at atmospheric pressure.It requires practically no vacuum devices and consumes low energy comparing with other deposition techniques such as the magnetron sputtering technique. So,this deposition technique presents a promising approach for finishing large-size and low-cost products in industrial applications.In our research porous nanocrystalline titania thin film were deposited on glass substrates through PE-CVD by DBD at atmospheric process using argon with small admixtures of titanium tetrachloride vapor and oxygen.Firstly,we analyzed the discharge process in the DBD through measuring the discharge voltage,current and power.Then the electron temperature was calculated in order to control the discharge and deposition process.The discharge voltage and current waveforms showed that the DBD was composed of filaments and therefore inhomogeneous. The optical emission spectrum of TiCl₄,O₂ and Ar indicated that Ti²⁺,O, Cl^- existed in the plasma,most of which were needed for the synthesis of titania films.The increase of discharge power caused the intensity enhancement of all the element emission lines.The electron temperature was about 0.67eV by analyzing and calculating the optical emission spectrum of argon plasma.Then the thickness of titania thin film,surface morphology,element composition,chemical bonding states and crystal structure of the deposited titania thin film were extensively studied.Several parameters such as discharge power,deposition time,the ratio of carrier gas to the precursor gas,substrate bias voltage and the effect of post annealing were considered.Thickness of thin films was measured by ellipsometer.The morphology of the thin films was investigated using high resolution transmission electron microscope(HRTEM)and scanning electron microscopy(SEM).The crystal structure and phase composition of the thin films were studied by x-ray diffraction(XRD)and polarizing microscope with hot stage(PMWHS).The element composition of the thin films was determined with an energy dispersive x-ray spectroscopy (EDS)analysis tool attached to the SEM and X-ray photoelectron spectroscopy(XPS).The chemical bonding states of the thin films were analysed by Fourier transform infrared spectroscopy(FTIR).From the experiment results,it is concerned that the thin film was composed of many small particles(most of them was with a size between 20 and 30 nm)uniformly deposited and intervein on the substrate surface. Some large particles were tightly embedded into the thin film.The deposition rate increased by increasing the precursor flow rate and the discharge power.With the increasing of the monomer the particles got dense and become unstable.It was estimated that the deposition process was a cluster mode at this stage.After increasing substrate bias voltage, the deposition rate increased and the deposition process changed into cumulate mode.The XRD feature of the thin films indicated one diffraction peak appeared at 2θ=25.3°,which was assumed to correspond to(101) reflection for the crystalline anatase titania material.The average diameter of the particles on the thin films was about 80nm by Bragg and Scherrer formula.When post annealing temperature was about 500℃,the corresponding crystalline peak value was enhanced.EDS indicated the existence of Ti,O,Cl elements in the thin films while the infrared spectra indicated the existence of Ti-O,C=O.Especially,UV/Vis curves indicated that the deposited thin films had strong absorbency in the ultraviolet region.