Preparation And Analysis Of Physical Properties Of TiO₂-doped ZnO Thin Films And Varistor Ceramics

The TiO₂-doped ZnO varistor ceramics and target materials were prepared by solid state reaction method,respectively.The TZO transparent conductive thin films,n-TZO/p-Bi₂O₃ and n-TZO/p-Bi₂O₃/n-TZO heterojunctions were prepared by a RF magnetron sputtering system.Their microstructures were characherized by positron annihilation techniques,XRD and SEM.The varistor voltages,leakage currents and nonlinear coefficients of TiO₂-doped ZnO varistor ceramics were measured.The effect of TiO₂ content on the microstructure and electrical properties of ZnO-based varistor ceramics was discussed.The concentration of the carriers,the Hall coefficients and the specific resistance of the TZO thin films were measured by a Hall Effect Analysis System and a Physical Property Measurement System?PPMS?.The transmission and the emission spectra of the TZO thin films were measured by a UV-VIS spectrophotometry and a PL spectrometer.The transmittances and the optical band gaps of the TZO thin films were calculated.The effects of the fabrication process of target materials,the sputtering and the annealing process of the TZO thin films on the microstructure and the photoelectric properties of the TZO thin films were investigated.The microstructures and electric properties of n-TZO/p-Bi₂O₃ and n-TZO/p-Bi₂O₃/n-TZO films were investigated.The main experimental results were listed as following:?1?Positron lifetime spectrum and electrical property measurements were performed on ZnO-based ceramics doped with different contents of TiO₂.For ZnO-based ceramics with TiO₂ content less than 1.8 mol%,the mean positron lifetime of the ZnO-based ceramic decreases with increasing in TiO₂ content,and reaches a minimum value at 1.8 mol%TiO₂.As the TiO₂ content higher than 1.8 mol%,the mean positron lifetime increases with TiO₂ content.The ZnO-based varistor with 1.8 mol%TiO₂ exhibites an optimized varistor property;it has a relatively low leakage current IL,a relatively low breakdown voltage VB,and a relatively high nonlinear coefficient ?.?2?The content of TiO₂ in the target,the sintering temperature and the heat preservation time of the target have important influence on the photoelectric properties of TZO transparent conductive film.The experimental results showed that all films with different target preparation process had a?002?preferential orientation along the c axis at 2??34°in XRD patterns.The film sputtered from targets with 2.0 wt%TiO₂ sintered at 1300?-1450? for 6 hours soaking time has better optoelectronic property.This sample had superior crystal properties,high average optical transmittance?91%?,a high carrier concentration and a larger carrier mobility,the resistivity as low as 8.47×10-4?·cm.Furthermore,the resistivity of all the films changed with temperature between 10 and 350K,they experienced an initial decrease followed by an increase as the temperature was raised.The minimum value of resistivity near 150k indicates that the carrier concentration generated from the impurity excitation at this temperature reach saturation.?3?The microstructures and the optoelectronic properties of the TZO transparent conducting thin films were strongly affected by sputtering pressure and sputtering atmosphere.The results show that all samples with different sputtering process are polycrystalline with a hexagonal structure and have a preferred orientation perpendicular to the substrate along the c-axis.When the sputtering pressure is 0.5Pa,and the sputtering atmosphere is high purity argon,it is obtained for TZO film that the lowest electrical resistivity is 4.13×10-4?·cm with the higher average transmittance of above 91%in the visible range,the better crystallinity of the film,the larger average grain size.But for the sample sputtered in gas mixed with oxygen,the average transmittance of the film in the visible region decreased to 75%,the optical bandgap became smaller and the resistivity increased remarkably?41.72×10-4?·cm?.The resistivity of all the films changed with temperature,but the change rules of the films prepared by different sputtering atmospheres were very different.When the temperature rises from 10K to 350K,the TZO film sputtered in high purity argon experienced an initial decrease followed by an increase as the temperature was raised.However the film sputtered in gas mixed with oxygen decreased monotonically with the increase of temperature.?4?The photoelectric properties of the TZO films were strongly affected by the annealing atmosphere.After annealed at 400? for 40 minutes in argon,nitrogen and vacuum,respectively,all the annealed films still remained polycrystalline with a hexagonal structure and had a preferred orientation perpendicular to the substrate along the c-axis,and the crystalline properties were improved,the grain size became larger,the film surface became more compact and smooth.The obtained TZO film showed a relatively low electrical resistivity of 3.32×10-4?· cm,a rather high average transmittance of 92%in the visible range.The enhancement of conductivity was mainly due to the decrease of defect concentration in the film after annealing process,the decrease of defect concentration will weakened the scattering of carriers,which greatly improved the carrier mobility.However,the carrier concentration of the TZO film did not change when they were annealed at 400? for 40 minutes in argon,nitrogen and vacuum,respectively.The film annealed in argon exhibited the optimized photoelectric property.The resistivity of all the annealed TZO films still decreased initially then increased with the increase of temperature from 10 to 350K.?5?A single layer of TZO thin film and Bi₂O₃ thin film,n-TZO/p-Bi₂O₃ and n-TZO/p-Bi₂O₃/n-TZO films were prepared by RF magnetron sputtering on glass substrates with a Au film?or ITO film?on surface as electrode.The TZO thin film was an excellent n-type polycrystalline nano-film,which exhibited a preferential orientation of?002?planes.The Bi₂O₃ film was a p-type thin film with low conductivity,and has preferential orientation of?201?crystal plane.The I-V characteristic curves of n-TZO/p-Bi₂O₃ films showed that the junction was a normal PN junction.The leakage current of the n-TZO/p-Bi₂O₃ film on the ITO substrate was smaller than that on the Au film.The I-V characteristic curve of n-TZO/p-Bi₂O₃/n-TZO films indicated that it is a typical low voltage varistor.Compared the TZO-Bi₂O₃?45min?-TZO film with the TZO-Bi₂O₃?30min?-TZO film,It was found that the Bi₂O₃ layer in the former was thicker than that in the latter,the breakdown voltage and the nonlinear coefficient of the former was larger than that of the latter,and the leakage current of the former was less than that of the latter.