Effect of thermal annealing and exposure to oxygen plasma on the properties of TiO$_2$–Si structures

The effect of thermal annealing and exposure to oxygen plasma on the phase composition, structure, and microprofile of titanium-dioxide films deposited by high-frequency magnetron sputtering on silicon substrates is studied. The influence exerted by processing modes on the capacitance-voltage and conductance-voltage characteristics of Me–TiO$_2$–Si–Ме structures and on the density of surface states at the semiconductor-insulator interface is examined. It is shown that TiO$_2$ films are amorphous upon their fabrication. Upon the annealing of films at 500$^\circ$C in an argon atmosphere, crystallites of anatase and rutile appear in the amorphous matrix. The treatment of a titanium-dioxide film in oxygen plasma gives rise to rutile crystallites with new crystallographic planes. As a result of annealing at 750$^\circ$C, the anatase phase disappears and the film becomes polycrystalline, containing only rutile crystallites. The capacitance of Me–TiO$_2$–Si–Ме structures in the accumulation mode reaches the maximum value upon annealing at 750$^\circ$C, which is due to the transformation of titanium dioxide to the rutile phase. The specific capacitance is 5.9 $\times$ 10$^{-2}$ F/cm$^3$. The decrease in the capacitance of the structures and in the amount of fixed charge in the insulator upon exposure to oxygen plasma is due to the diffusion of oxygen atoms across the titanium-dioxide layer to give a SiO$_2$ film at the TiO$_2$–Si interface. As a result of the annealing and treatment of a titanium-dioxide film in oxygen plasma, the energy density of surface states decreases by more than an order of magnitude as compared with the unannealed samples.