Chemical vapour deposition of epitaxial WO 3 films

Abstract

Thin films of WO 3 have been deposited from a gas mixture of WF 6 and H 2O in the temperature range 200–700°C. The microstructure of the films could be controlled by the total pressure and linear gas flow velocity. A homogeneous nucleation leading to a porous oxide film was observed at 50 Torr and low gas flow velocities. In contrast, depositions carried out at 10 Torr on sapphire (011̄2) substrates resulted in a wide deposition window for monoclinic WO 3 films. These films exhibited a microstructure with columnar grains, with each grain exhibiting an epitaxial relation towards the substrate. The observed epitaxial orientations were determined to be (100) WO 3 //(011̄2) substrate, (010) WO 3 //(011̄2) substrate and (001) WO 3 //(011̄2) substrate with the corresponding in-plane relationships [110] WO 3 //[100] substrate, [101] WO 3 //[100] substrate and [011] WO 3 //[100] substrate. The deposition rate of the WO 3 was higher than 1 μm/h also at 300°C. A kinetic investigation showed that the deposition process was controlled by mass transport.