Oxidation potential control of VO2 thin films by metal oxide co-sputtering

For metal-to-insulator transition (MIT) in vanadium oxide thin film, a thermodynamically stable vanadium dioxide (VO2) phase is essential. In VO2 films sputter-deposited on a quartz substrate from a V2O5 target, a radio-frequency (RF) magnetron sputter system at working pressure of 7 mTorr is used. Due to the lower sputtering yield of oxygen compared to vanadium leading to oxygen-ion deficiency, the reduction of V ions is resulted to compensate charge with the oxygen ions. Under lower working pressures, the deposition rate increases, but a simultaneous oxygen-ion deficiency causes the destabilization of VO2. To prevent this, titanium oxide co-deposition is suggested to enrich the oxygen source. When TiO2 is used, it is found that the Ti ion has a stable +4 charge state so that the use of extra oxygen in sputtering prevents the destabilization of VO2. However, this is not the case for TiO. For the latter, Ti ions are oxidized from the +2 state to the +3 and +4 states, and V ions with less oxidation potential are reduced to +3 or so. Pure VO2 thin film exhibits MIT at 66 °C and a large resistivity ratio of four orders of magnitude from 30 to 90 °C. The (V2O5 + TiO2) system under working pressure as low as 5 mTorr yields fairly good films comparable to pure VO2 deposited at 7 mTorr, whereas the use of TiO yields films with MIT absent or considerably weakened.