Ion-impact chemistry in the system titanium-oxygen (studies on bombardment-enhanced conductivity—III)

Abstract

The bombardment of TiO 2, whether poly- or single crystalline, with Kr ions leads to an altered surface layer having the following characteristics. It exhibits a high electrical conductivity, has the diffraction pattern of finely polycrystalline Ti 2O 3, is on the average 110 ± 20 Å thick (for 30 keV Kr), and is indefinitely stable in air at room temperature. The formation of the layer is favored by increasing the target temperature. Formation is half complete at (6 ± 2) × 10 16 ions/cm 2, hence at a dose substantially greater than that for the half completion of sputter equilibrium ([7 ± 2] × 10 15 ions/cm 2). One model which could lead to Ti 2O 3 can be excluded fairly readily: this is thermal-spike stimulated vaporization, as the relevant vapor pressures are too low. More satisfactory is a model in which, due to either preferential oxygen sputtering or internal precipitation of oxygen, Ti 2O 3 nuclei are formed and grow. The reason that the stoichiometry is precisely Ti 2O 3 can be rationalized by an argument based on surface binding energies ( E b ), in the sense that E b for TiO 2 to sputter congruently is 6.4 eV, to yield nuclei of Ti 3O 5 is 5.7, to yield nuclei of Ti 2O 3 is 5.1, and to yield TiO is 6.4. A similar rationalization holds also for impact-induced chemical changes observed or inferred with AgBr, CuO, Fe 2O 3, MoO 3, U 3O 8 and V 2O 5, except that here thermal-spike stimulated vaporization cannot be excluded.