Composition and interface stability of sputtered MoOx ultra-thin films on silicon for optoelectronic applications

Sub-stoichiometric molybdenum oxide (MoOx) has attracted significant attention as hole-transport/selective layer in optoelectronic devices. However, its integration on Si is hindered by interfacial redox-reactions that can alter oxide stoichiometry and electronic properties. In this work, MoOx films, 5 nm thick, were deposited by non-reactive RF sputtering from a stoichiometric MoO3 ceramic target onto bare and thermally-oxidized Si substrates. Morphological analyses confirmed the formation of continuous and dense coatings, Kelvin probe mapping revealed spatially uniform WF of 5.06 eV. Angle-resolved XPS showed sub-stoichiometric composition (x = 2.6) regardless of substrate, with no variations across the film thickness. These results were corroborated by HAADF-STEM, which revealed uniform Mo and O distributions, confirming the absence of interfacial oxide reactions. Compared to thermally evaporated counterparts, sputtered films exhibit superior compositional stability due to the higher kinetic energy of sputtered species involved in the film growth. Overall, non-reactive sputtering enables the growth of stable films suitable for optoelectronic and photovoltaic applications.