Investigation On The Electrical Properties Of Oxygen-deficient Titanium Oxide Thin Films And Their Varistors

In this paper, the effect of films deposition condition including substrate temperatures, sputtering powers, oxygen partial pressures, and annealing temperatures on thin film composition and electrical properties were systematically studied. Also, Ti Oy-Ti Ox-Ti Oy(y>x) sandwish structure varistors were designed.The Ti Ox films deposited at low substrate temperature were amorphous. As the temperature increased up to 600 °C, the films became crystalline and a Ti O2 anatase phase was identified. The electrical resistivity of the prepared Ti Ox thin films generally decreased with the raise of substrate temperatures for deposition. And resistivity-switching characteristics were observed in the Ti Ox films prepared at room temperature. With the sputtering power increased from 70 to 135 W, the proportion of oxygen in Ti Ox films was gradually decreased, O/Ti atomic ratios became from 1.09, to 1.01. Meanwhile, particle aggregation appears on the surface of the films, and the resistivity decreased. When the oxygen partial pressure gradually increased, the resistivity of the Ti Oy films gradually increased. O/Ti atomic ratio was 1.4 in the samples prepared under 20% oxygen partial pressure. The resistivity first increased and then decreased with increasing annealing temperature for Ti Ox films and Ti Oy films. Due to different thicknesses, the turning point for Ti Ox and Ti Oy film were 500-600 and 600-700 ℃, respectively.(1) Ti Oy-Ti Ox-Ti Oy(y>x) sandwich thin films varistors were prepared by reactive RF magnetron sputtering. Such multilayered thin films could present good nonlinear behavior, which is completely different from the linear behavior of pure single-layer titanium oxides thin films. The varistor behavior of the obtained sandwich thin films can be well explained by the trapping and detrapping of electrons. And the measured nonlinear coefficient and varistor voltage were about 6 and 0.02 V/nm, respectively. This effect of annealing temperature on varistor properties were also studied. After the samples annealed at 300 and 400 ℃, the varistor performances do not change obviously. After the samples annealed at 500 ℃, their nonlinear coefficient increased sharply. After the samples annealed at 600 and 700 ℃, they showed linear properties with low resistivity.(2) The nonlinear coefficient decreased sharply in the samples prepared under different sputtering powers on the middle layer. Therefore, Ti Oy-Ti Ox-Ti Oy(y>x) sandwich thin film deposited at 70 W shows higher nonlinear properties and lower leakage current. The highest nonlinear coefficient of the triple-layered thin film sample is 13.7.