4.3. Titanium dioxide (TiO2)

Titanium dioxide (TiO2) is another gate dielectric, which is explained in this chapter. The electronic bandgap energy of this material is relatively small (3.5 eV), but dielectric constant can be varied from 40 to 110. TiO2 exist in two important phases, Anatase and Rutile, which depends on growth process. Rutile phase of TiO2 is the thermally stable phase that presents the higher dielectric constant around 80. Other form i.e. Anatase is a thermally unstable phase, which shows a lower dielectric constant. The Anatase form can be transforming to Rutile phase by annealing the deposited material at temperatures more than 600° C. A high leakage current values and higher interface density are the most drawback of this material, which is unacceptable in the fabrication of transistor structure. In order to minimize these problems it is interesting to employ a stack layer of thin SiO2 and TiO2 on SiC substrate. In this way, the interface quality can be improved and the other problems may be minimized, turning this material viable and very attractive to substitute the current dielectric material on SiC surface. Variable-energy positron annihilation spectroscopy (VEPAS) was employed to investigate the atomic scale structure of TiO2/SiO2T gate dielectric stack on 4H-SiC surface (Coleman P.G., 2007). In this study a vacancy type defects was observed. Thin film of TiO2 film can be deposited with many techniques likes chemical vapour deposition (CVD), RF sputtering, e-beam evaporation, metal-organic chemical vapour deposition (MOCVD) and so on. The dielectric constant of TiO2 was reported to be 31, which is stable in the frequency range from 100 Hz to 1 MH. The critical breakdown field is 3 MV/cm. TiO2 is seems to of be very promising material in the development of gas sensors particularly Hydrogen sensors (Weng M-H, 2006; Shafiei M., 2008)
