Magnetron sputter deposition of thin VO2 films onto polycrystalline substrate

Vanadium dioxide is a crystalline compound which undergoes metal-insulator phase transition at 68°C. Grain boundaries in crystal structure of VO2 decrease contrast of resistivity of VO2 in metal state in comparison with... [ view full abstract ]

Vanadium dioxide is a crystalline compound which undergoes metal-insulator phase transition at 68°C. Grain boundaries in crystal structure of VO₂ decrease contrast of resistivity of VO₂ in metal state in comparison with insulator state. This is why growth of thin VO₂ films principally involves epitaxy.

We report on a reactive RF sputter deposition of polysrystalline VO₂ film onto polycrystalline non-oriented substrate of Al₂O₃. Appropriate sputtering conditions combined with relevant post-deposition annealing results in resistivity contrast of VO₂ of as much as R(30°C):R(90°C) ~ 1000 at the film thickness of 100 nm.

We found that temperature exposure in both steps, deposition and recrystallization, is necessary to form VO₂ film. On a deposition stage, high temperature of substrate enables VO₂ phase formation. In temperature range from 20°C to 400°C, the higher the temperature of the substrate, the easier VO₂ phase forms. Sputter conditions to be carefully matched in order to obtain V⁺⁴ is composition of gas mixture and substrate bias. We show that exact combination of these parameters varies with a distance from target to substrate, pumping rate, pumping deceleration, and substrate temperature value.

As-deposited films show weak change in resistivity when heated from 30°C to 90°C. Post-deposition annealing causes recrystallization of the VO₂ film. We optimized annealing continuance, temperature, heating rate and cycling. We proposed that cyclic heating and cooling between 500°C and 750°C can improve effectiveness of recrystallization. This proposition is based on an existence of peritectic reaction V₆O₁₃ ↔ VO₂ + Liquid, which occurs at ~700°C. However, this effect, if present, is negligible to the oxidation caused by insufficient grade of flowing inert gas during annealing. As a result of recrystallization, lateral crystallite size of VO₂ increases from 20 to 400 nm. Also, we found no texture formation of VO₂ crystals.

Thin films which are obtained via this procedure can be applied as electromagnetic interference shielding elements. When heated, these films changes microwave transmittance coefficient by 1 dB in a range from 6 to 20 GHz.

This research was financially supported by the Russian foundation for basic research (RFBR project No. 16-33-01089).