Tailoring CuO _x Films via Open‐Air Spatial Atomic Layer Deposition and Their Application in Room‐Temperature Humidity Sensing

This study demonstrates the tunability of copper oxide (CuO_x) thin films synthesized using open-air atmospheric-pressure spatial atomic layer deposition (AP-SALD). By systematically varying the deposition temperature and copper precursor flow rate, as well as introducing nitrogen doping through in situ oxidant switching (from H₂O to ammonium hydroxide (NH₄OH)), the crystallinity, oxidation state, chemical composition, carrier concentration, and resistivity of CuO_x films are modulated. Under optimized conditions, CuO_x films achieve a very low resistivity of 0.178 Ω·cm and a high carrier concentration of 10¹⁹ cm^- ³-among the best reported for AP-SALD-deposited CuO_x. As proof of properties tailoring, these films are integrated into interdigitated-electrode devices and evaluated as chemiresistive humidity sensors at room temperature. While CuO_x films cannot typically sense humidity at room temperature, a film deposited at 225 °C and 350 sccm Cu flow exhibits a limit of detection of 99 ppm and a sensitivity of 0.244 Ω ppm^-1. This work establishes AP-SALD as a powerful tool for tuning CuO_x film properties via controlled deposition conditions and in situ doping-enabling application-specific optimization for future optoelectronic or sensing platforms.