Annealing Effect of Sol-Gel TiO2 Thin Film on pH-EGFET Sensor

In this study, a sol-gel titanium dioxide (TiO₂) was spin-coated on indium tin oxide (ITO) substrate as a sensing membrane for the extended-gate field-effect transistor (EGFET). To improve its sensing properties, the resultant films were thermal annealed at various temperatures. It shows that, as TiO₂ was post-annealed at Ta=200°C for 30 min, a highest pH sensitivity of 79.9 μA/pH was derived. As Ta > 500°C, the pH sensitivity was declined substantial to 70.1 μA/pH. The sensitivity would further deteriorate to 61.8 μA/pH as the annealing time extended to 90 min under identical Ta. These phenomena could be attributed to the less surface hydroxyl sites of TiO₂ thin film at elevated Ta. X-ray diffractograms indicates that an amorphous titania resides as Ta <; 200°C which presents more surface hydroxyl sites than those of crystalline structures at elevated Ta. It reveals that the pH sensitivity of EGFET was relied on the density of the surface hydroxyl structures. To employ to the portable sensing devices with desirable pH sensitivity, the post annealing of the asdeposited TiO₂ film is a crucial process.