Impact of air exposure on growth rate and electrical properties of SnO₂ thin films by atmospheric pressure spatial atomic layer deposition

SnO2 thin film is one of the most studied transparent conductive materials that can be deposited using vacuum-free techniques such as atmospheric pressure spatial atomic layer deposition (AP-SALD). This work studies SnO2 thin films prepared from tin(II) acetylacetonate and water vapor, with a particular focus on the impact of air exposure during the AP-SALD process on the growth rate and electrical properties of the films. In-situ resistance measurements and ex-situ Hall effect characterization demonstrated that longer exposure time of the growing film surface to the open air (t air) at 240 degree celsius led to conductivity degradation, while the film thickness decreases. The theoretical calculations show that -OH and O-2(dm) (oxygen molecule adsorbed on the five-coordinated Sn atom, also called O-2 dimer) are the two most stable surface structures. The formation of O-2(dm) is shown as the most thermodynamically favorable oxygen-related species on SnO2(110) surface formed when the film is exposed to the open air, giving rise to both the decrease of film thickness (associated with the desorption of -OH surface groups) and the increase of film resistivity versus t(air). The optimized polycrystalline SnO2 sample demonstrated relatively good electrical performance, including an electrical resistivity of 9.3 x 10(-3) Omega.cm, carrier density of 9.2 x 10(19) cm(-3), and Hall mobility of 7.3 cm(2) V-1 s(-1) at a growth temperature as low as 240 degree celsius. Our findings reveal the critical impact of processing in the open air on the electrical conductivity of the obtained SnO2 films by AP-SALD coating technology.