| Titanium dioxide is a wide band gap oxide semiconductor material with excellent properties. It has a very extensive potential application in photocatalysis, transparent conducting film, dye sensitized solar cells, insulated gate and spintronic devices. Currently, the TiO2 films with good conductivity and photopermeability have been prepared on various substrates, which can be used in gas sensors and transparent conducting electrodes. TiO2 has become one of the hot spots in the research of new type oxide semiconductor thin film materials. Rutile TiO2 is a direct band gap semiconductor (～3.0 eV) with a very stable physical and chemical properties, so it is also a promising optoelectronic material. TiO2 films prepared by sputtering and sol-gel methods are almost polycrystalline, thus suffering a poor performance stability. Therefore, the preparation of TiO2 epitaxial thin films with a complete structure, studying its crystal structure and optical property, not only in terms of production of high performance optoelectronic devices has practical application value, but also has important scientific significance. In this paper, TiO2 epitaxial films with different orientations have been prepared on a-cut, c-cut, m-cut sapphire and LSAT substrates by metalorganic chemical vapor deposition (MOCVD). The structural and optical properties were investigated systematically.High pure Ti(N(CH3)2)4, N2 and O2 were used as titanium organic source, carrier gas and oxidant, respectively in the experiment. TiO2 films were prepared successfully on the above substrates at the organic source molar flow rate of 2.5×10-7 mol/min and the substrate temperature at 500℃、550℃、 600℃、 and 650℃、 The key research work and results are as following:1. The preparation of TiO2 films on a-cut, c-cut and m-cut sapphire substrates(1) XRD analysis results indicated that the films grown on a-cut sapphires are rutile TiO2 (101) film with single orientation. The film prepared at 550℃ had the best crystallinity, and the crystallinity of the film degenerated obviously at 650 ℃. The in-plane epitaxial relationship between TiO2 film and a-cut sapphire is TiO2 [010]‖а-Al2o3 [0001] and TiO2 [101]‖а-Al2o3 [1100]. Theφ scans analysis also showed that the deposited TiO2 film contained (101) twins. The transmittance of the films was more than 73% in the visible range.(2) TiO2 films deposited on c-cut sapphires had a big structure difference. The films grown at 500 and 550℃ were polycrystalline with the mixture of anatase and rutile. TiO2 film prepared at 550℃ had the best crystal quality, and grown along single anatase [001] orientation. The XRD 0 scans analysis indicated there were three domains in the film, which was due to the threefold symmetry of the c-cut sapphire. Failured to observed the diffraction peaks of TiO2 film prepared at 650 ℃, showing that the crystalline quality of the films decreased significantly. The transmittance of the deposited films was more than 76% in the visible range.(3) Rutile TiO2 films were prepared on m-cut sapphires. All the films were grown along single [001] orientation. However, when substrate temperature increased to 650℃, the crystalline quality of the films decreased significantly. The film obtained at 550℃ had the best crystallinity. The out-plane epitaxial relationship between the rutile TiO2 film and m-cut sapphire is TiO2 (001)‖α-Al2O3 (1010), the in-plane epitaxial relationship between them is TiO2 [100]‖a-AL2O3 [1210] and TiO2 [010]‖α-Al2O3 [0001]. The average transmittance of the films exceeded 76% in the visible range. The optical band gap of Tio2 film grown at 550℃ was about 3.64 eV. In addition, TiO2 films also have been prepared at different organic source molar flow rate (2.5xl0-7～4.0×10-6 mol/min) with the substrate temperature of 550℃ to study the effect of growth rate on its structure. It was concluded by analysis that the TiO2 film prepared at small flow rate (2.5 xl0-7 mol/min) that is growth rate of 6.5 nm/h had the best crystallinity, and the crystallinity was decreased with the increasing of the growth rate. When the growth rate reached 401.5 nm/h, TiO2 films became polycrystalline structure with the mixture phase of anatase and rutile, and the anatase is dominant.2. Anatase TiO2 films prepared on LSAT (001) substratesAnatase TiO2 films were deposited on perovskite structure LSAT (001) substrates at different temperature by MOCVD. XRD analysis showed that all the prepared films were grown along a single orientation which is perpendicular to anatase TiO2 [001]. The film prepared at 550℃ had the best crystallinity. The XRD 0 scans analysis showed the films were complete single crystal without twins. The epitaxial relationship between the anatase TiO2 film deposited at 550℃ and LSAT is TiO2 (001)‖LSAT (001) and TiO2 [100]‖LSAT [100]. The lattice mismatch is about 2.2%. The average transmittance of the films exceeded 84% in the visible range. The optical band gap of TiO2 film prepared at 550℃ was about 3.27 eV. Besides, TiO2 films also have been prepared on LSAT (001) at different organic source molar flow rate (2.5×l0-7～4.0×10-6 mol/min) with the substrate temperature of 550 ℃ to study the effect of growth rate on its structure. Only the film grown at 2.5×l0-7 mol/min that is growth rate of 5.9 nm/h was anatase TiO2 with single orientation, the others obtained at big growth rate were polycrystal. |
PDF Full Text Request