Reactive multi-pulse HiPIMS deposition of oxygen-deficient TiOx thin films

The reactive high power impulse magnetron sputtering operated in multi-pulse mode (m-HiPIMS) of a pure Ti target in Ar/O2 gas mixture (0.2% O2 of the total mass flow rate) has been used for the deposition of amorphous titanium oxide (TiOx) thin films with variable stoichiometry (x < 2). The m-HiPIMS employed a pulsing scheme consisting in repetition with low frequency (400-800 Hz) of sequences of three consecutive very short individual micropulses (5 μs in width) spaced by a delay time of 50 μs. At the repetition frequency of 800 Hz, the m-HiPIMS deposition produced TiOx thin films with a high deficit of oxygen, enhanced electrical conductivity and strong absorption of visible light. Then, the decrease of the m-HiPIMS repetition frequency determined depositions of films with lower deficit of oxygen. This result was explained by the measurements of deposition rate and ion mass spectrometry, which indicated lower sputtering rate and higher content of oxygen in the gas phase for low repetition frequency values. For higher repetition frequency values, the off-time between micropulse sequences was shorter and the target surface was maintained in a less oxidized state, causing in this way a higher sputtering rate and deposition of films with higher deficit of oxygen. The as-deposited oxygen-deficient thin films showed a weak photocatalytic activity. However, after isothermal treatment at 723 K, in argon, the photocatalytic activity has been considerably improved. This improvement may be attributed to formation of TiO2 anatase and Ti4O7 phases in the deposited films. The annealing treatment determined also a decrease of the film conductivity by two orders of magnitude and a slight decrease of the visible light absorption.