Growth Mechanism and Film Properties of Atomic-Layer-Deposited Titanium Oxysulfide
Authors: Macco Bart , van Kasteren Jeroen , Basuvalingam Saravana Balaji , Mattinen Miika , Bracesco Andrea , Kessels Wilhelmus M. M. (Erwin) , Bol Ageeth 
Published: February 2024

Titaniumoxysulfide (TiOxSy) is a technologically-relevant material for batteries, catalysis and, the area of our focus: photovoltaics. One of the key merits of TiOxSy is the wide range of electrical conductivity and optical absorption that can be achieved by tuning its composition. In this contribution, we present a supercycle-based ALD approach for TiOxSy which combines ALD of amorphous TiO2 and semimetallic two-dimensional (2D) TiS2, and uses tetrakis(dimethylamido)titanium (TDMATi), H2O and H2S as precursors. While ALD with supercycles is generally acknowledged to give a high level of control over film composition, in this case of TiOxSy we show through a variety of in-situ techniques that many effects occur during the ALD supercycles that strongly affect the film growth and resulting film properties. Yet, by understanding and taking these effects into account, the supercycle process allows for accurate tailoring between pure TiOx and TiSx. The effects of study focus mainly on exchange reactions and the film crystal morphology and its effect on the growth rate.

Exchange reactions: QMS measurements show that during supercycle ALD, strong exchange reactions occur during the H2O dosing step, where exchange occurs between underlying TiSx and H2O, resulting in the formation of TiOx and release of H2S. The exchange reactions and their effect on the film growth were studied in-depth using in-situ SE. From these studies, it was found that during ALD TiO2 cycles on TiSx, the exchange reactions behave similarly at 100 and 150 °C, while being more pronounced at 200 °C. Specifically, at 100 and 150 °C, the reactions persist for about seven cycles and stop once approximately 3−4 Å of TiO2 has grown, while at 200 °C, the reactions are more prominent and persist for about nine cycles and stop once approximately 5 Å of TiO2 has grown. This TiO2 forms both through back-oxidation of underlying TiSx and by ALD of TiO2 on top. These exchange effects are thus less prominent at lower growth temperatures, facilitating more accurate control over composition.

Film crystallinity: Interestingly, the pure ALD TiO2 and TiS2 processes yield films of very different morphology. Whereas ALD TiO2 at these temperatures results in smooth, amorphous films that grow with a constant growth-per-cycle (GPC), in the case of pure ALD 2D-TiSx the growth process is more intricate: Electron microscopy shows that these 2D films initially have crystal growth that occurs laterally, up to the point of coalescence where the 2D layers start to grow out of plane. As ALD growth on the sideplanes of the 2D material is much faster, the GPC of these films keeps increasing as the film thickness progresses. When combining ALD TiO2 and TiS2 in a supercycle, the TiO2 cycles interrupt the growth of the 2D TiS2 crystals. This not only leads to a lower crystallinity of the film, but due to suppression of the reactive 2D-TiS2 edges, also the GPC is strongly reduced. As such, this effect has a strong influence on the resulting film thickness and composition, which is markedly different than would be expected than on the basis of the constituent ALD cycles in the supercycle.
Based on the insights, we present a model that can explain the relation between supercycle ratio, deposition temperature and the resulting film properties. This enables the control over film conductivity over six orders of magnitude, where Hall measurements show that S-rich films exhibit both higher carrier densities and carrier mobility. Similary, controlling the composition allows tuning the optical properties between opaque, semimetallic 2D-TiS2 and transparent, dielectric TiO2 with a bandgap a little over 3 eV.
Since exchange reactions and film crystallinity are common factors in ALD films, we expect these results to be of broader interest to the ALD community, for those preparing multicomponent materials using supercycles

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