Titanium dioxide (TiO2) in the form of pellets (pressed powder) and thin films are investigated, revealing the presence of distinct phases: mainly anatase and rutile. Characterization of optical, structural and electrical properties were carried out on samples submitted to different sort of thermal annealing (TA), at distinct temperatures, 500 and 1000ºC, due to their influence on the obtained phases. TA temperature along with pressure application for sample conformation, determine the bands present in the photoluminescence (PL) spectra, being about 550nm, characteristic of anatase phase and 800nm, related to the presence of rutile phase. The bandgap of thin films is determined from optical absorbance data, yielding 3.4eV for anatase phase (indirect transition), and 2.9eV for rutile phase (direct transition). Besides, irradiation with monochromatic light strongly affects the thin film conductivity, but the energy range (above or below the bandgap energy) does not seem to affect the behavior, which is associated with the excitation of intrabandgap states or crystallites belonging to phases with distinct bandgaps.

Alternative materials for use in electronic devices have grown interest in the past recent years. In this paper, the heterojunction SnO2/Al2O3 is tested concerning its use as a transparent insulating layer for use in FETs. The alumina layer is obtained by thermal annealing of metallic Al layer, deposited by resistive evaporation technique. Combination of undoped SnO2, deposited by sol-gel-dip-coating technique, and Al thermally annealed in O2-rich atmosphere, leads to fair insulation when the number of aluminum oxide layers is 4, with 0.3% of the current lost through the gate terminal as leakage current. This insulation is not obtained for devices with alumina layer treated for long time, under room atmosphere, due to degradation of the insulating film and interfusion with the conduction channel even using Sb-doped SnO2. The annealing of Al deposited on soda-lime glass substrate leads also to the formation of a Si layer, crystallized at Substrate/Al2O3 interface. The conclusion is that for an efficient insulation the thermal annealing must be short and then, O2-rich atmospheres are preferred.