Growth Mechanism and Film Properties of Atomic-Layer-Deposited Titanium Oxysulfide

In this work, atomic layer deposition (ALD) has been employed to prepare titanium oxysulfide (TiOxSy) thin films. Compositional control was achieved through a supercycle approach, where the ALD processes of TiOx and TiSx were interleaved. ALD was performed at 100, 150, and 200 °C, and tetrakis(dimethylamino)titanium (TDMAT), H2S, and H2O were used as precursors. It is shown that the conductivity and transparency of the film are highly tunable by controlling the composition of TiOxSy between pure TiSx and TiOx. Specifically, the incorporation of S enables the film resistivity to be reduced by up to 6 orders of magnitude, while the visible light absorption is increased significantly. It is furthermore shown that the resulting films are significantly more oxygen-rich than would be expected on the basis of the supercycle ratio, an effect which is more pronounced at higher deposition temperatures. Through mass spectrometry and in situ ellipsometry studies, this low S content is shown to originate from exchange reactions where S is replaced by O on the growth (sub)surface during H2O exposure. This is contrary to the common finding for ALD zinc, indium, and tin oxysulfide films, for which exchange reactions are known to favor S-rich films. These exchange reactions are shown to be more prominent at higher deposition temperatures and to persist until approximately 3–5 Å of TiOx has formed.