A study of barium ultra-thin films on the SrTiO3(1Â 0Â 0) surface by soft X-ray photoelectron spectroscopy

► We performed Ba adsorption on the SrTiO3(1 0 0) surface at 500 K. ► Ultrathin films of BaO were grown on the substrate with coverage ≤2 ML. ► Ba reduces the substrate and sustains the insulating character of the interface. ► Ba-O interaction prevents the metallization of the barium overlayer.